An impinging shock wave/turbulent boundary layer interaction over a compliant panel was experimentally examined in a Mach 4 Ludwieg Tube through simultaneous measurements of both the fluid and structure response. Synchronized measurements were obtained using colinear focused laser differential interferometry and high-speed schlieren together with single-camera stereo-photogrammetry. The static and dynamic behavior of the compliant panel itself was also characterized through response to pressurization, roving hammer, and panel flutter experiments.
\r\n\r\nWhen the compliant panel was subjected to shock impingement at the mid-chord and leading-edge locations, the initial panel response was dominated by oscillations at the fundamental vibration mode and was primarily governed by one-way uncoupled fluid-structure behavior. However, under leading-edge impingement, oscillations at the fundamental vibration mode eventually dampened, and a transition to a two-way coupled fluid-structure response was observed. Under two-way coupling, the interaction was characterized by a reduction in oscillation amplitudes and an increase in relative strength of high-order vibration modes.
\r\n\r\nPhase analysis at the fundamental vibration mode between the separation and reattachment shock motion revealed that the one-way uncoupled mid-chord impingement case exhibited a repeatable phase alignment, where the phase of the separation shock tended to lead that of the reattachment shock by approximately one quarter of a panel oscillation. However, consistent with the behavior in rigid wall shock wave/boundary layer interactions, the instantaneous reattachment motion was anti-correlated and led that of the separation shock in all compliant cases. The transition to a two-way coupled response strengthened the synchronization between the separation shock and panel motion compared to that of the one-way uncoupled response.
\r\n\r\nSpectral analysis of the mid- and high-frequency content of density fluctuations in the separation bubble and boundary layer showed that the spectral distributions were not significantly modified due to surface compliance. Instead, the magnitude of the density fluctuations inside the separation bubble were attenuated under mid-chord impingement compared to that of leading edge impingement. Moreover, in all compliant cases, surface compliance amplified the spectral energy in the boundary layer downstream of flow reattachment by over a factor two compared to that of the rigid case.
" }, { "name": "Davis, Branson William", "degree": "PhD", "year": "2026", "title": "Autoignition Modeling and a Generalized Hot Surface Ignition Criterion", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:08262025-221106238", "creators": [ { "name": { "family": "Davis", "given": "Branson William" }, "id": "Davis-Branson-William", "orcid": "0000-0002-5288-7913", "display_name": "Davis, Branson William" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/meyv-1317", "abstract": "This work investigates the fundamental physics and predictive modeling of thermal ignition in heated volumes and near hot surfaces. Three-dimensional simulations of the ASTM-E659 apparatus revealed how natural convection and fuel stratification influence ignition timing and location, highlighting key limitations in standardized AIT testing. A one-dimensional analog further demonstrated the impact of radial temperature gradients on ignition behavior.
\r\n\r\nTo isolate the core mechanisms of thermal runaway, a canonical hot surface ignition problem was analyzed, showing that inclusion of low-temperature chemistry induces two-stage ignition and lowers critical surface temperatures. Building on insights from classical theory, a novel ignition model was developed based on chemical and thermal length scales. The model collapses ignition data across a wide range of configurations and defines a critical Damk\u00f6hler number. Despite some limitations for NTC fuels and catalytic effects, this unified framework represents a major advance over existing models.
" }, { "name": "Devey, Sean Patrick", "degree": "PhD", "year": "2026", "title": "Part I: A Novel Compact Water Tunnel. Part II: Evolution of Delta Wing Surface Contour from Flat Plates to Boxfish at Low Reynolds Numbers", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:09012025-210751150", "creators": [ { "name": { "family": "Devey", "given": "Sean Patrick" }, "id": "Devey-Sean-Patrick", "orcid": "0000-0002-8937-939X", "display_name": "Devey, Sean Patrick" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T-E", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "Sader", "given": "John E." }, "id": "Sader-J-E", "orcid": "0000-0002-7096-0627", "role": "member", "display_name": "Sader, John E." }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/rv8x-jm07", "abstract": "This thesis is presented in two parts.
\r\n\r\nPart I (Chapters 1\u20132) addresses a common challenge for experimentalists: the scarcity of laboratory space. Inspired by recent advances in fan-array wind tunnels, a novel, ultra-compact water tunnel was developed that uses an array of submerged thrusters to drive flow within a rectangular tank. Prioritizing space efficiency over power efficiency, the design achieves flow quality comparable to traditional facilities while occupying just 8% of the typical footprint. Flow characterization using particle image velocimetry (PIV), hot-film anemometry, and laser Doppler velocimetry (LDV) demonstrates the tunnel\u2019s effectiveness and suitability for fluid dynamic research.
\r\n\r\nPart II (Chapters 3\u20135) uses this new facility to investigate the aerodynamics of delta wings with extreme leeward surface contours in low Reynolds number subsonic flow. While the canonical delta wing flow field is dominated by a pair of counter-rotating leading-edge vortices (LEVs), similar vortex structures have been observed over a range of geometries\u2014including biological forms such as boxfish. However, the influence of non-uniform thickness and large leeward surface deformations on LEV behavior remains poorly understood.
\r\n\r\nTo address this, a family of 70\u00b0 sweep delta wing shapes was defined using B\u00e9zier splines, spanning a continuum from flat plates to forms approaching bluff bodies. Eight 3D-printed wing models were fabricated, with systematic variation in cross-sectional curvature and thickness, including a conical apex section with a thickness-to-span ratio of 0.5. Dye visualization, stereoscopic PIV (SPIV), and load cell measurements were used to examine LEV strength, position, and wing performance across multiple angles of attack. Results show that increasing the leeward surface height, effectively confining the LEV, leads to substantial reductions in vortex strength and shifts in vortex position. Experimental evidence of a dual primary vortex structure over thick delta wings is reported for the first time. Tomographic dye reconstructions reveal complex three-dimensional vortex behavior in the aft section of the wings, including flow separation patterns reminiscent of those seen in hatchbacks and Ahmed bodies. Force measurements reflect the observed flow field, with highly non-linear force evolution with angle of attack depending on model cross-sectional and longitudinal profiles. These findings provide new insight into the interplay between surface shape and vortex structure, and further demonstrate the capabilities of the compact flow facility.
" }, { "name": "Luo, Ying", "degree": "PhD", "year": "2026", "title": "Near-Wake Structure and Dynamics of a Cylinder in Hypervelocity Flows", "advisor": "Austin, Joanna M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:03122026-190459583", "creators": [ { "name": { "family": "Luo", "given": "Ying" }, "id": "Luo-Ying", "orcid": "0009-0004-2832-7715", "display_name": "Luo, Ying" } ], "advisors": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "advisor", "display_name": "Austin, Joanna M." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/v935-3c96", "abstract": "The near-wake of a circular cylinder at hypersonic conditions was investigated experimentally to assess the influence of high stagnation enthalpy thermochemical effects on mean wake structure and near-wake dynamics. Experiments were conducted in the T5 Free-Piston Reflected Shock Tunnel and the Caltech Ludwieg Tube using high-speed schlieren and simultaneous focused laser differential interferometry and high-speed schlieren.
\r\n\r\nMean flow quantities, including the separation point, separation region size, shock angles, and neck width, were extracted from schlieren data. Relative to perfect-gas conditions, high stagnation enthalpy flow exhibited a downstream shift of the separation point and a reduction in the separation region size. These features retained a strong dependence on Reynolds number at high stagnation enthalpy. The recompression shock angle was found to be largely insensitive to key flow parameters, remaining approximately constant within experimental uncertainty, while the neck width decreased with increasing stagnation enthalpy and followed a Re-1/2 scaling consistent with laminar perfect gas behavior.
\r\n\r\nNear-wake dynamics were examined using power spectral density (PSD) analysis and Spectral Proper Orthogonal Decomposition (SPOD). A dominant shear layer frequency and a secondary frequency were identified in both facilities, with both remaining approximately constant along the shear layer. Although this frequency in high stagnation enthalpy flow was higher than that observed in perfect-gas experiments at similar Reynolds numbers, scaling by the shear layer length collapsed the data onto a Strouhal number consistent with previous studies.
\r\n \r\nPSD contours and SPOD modes at the dominant frequency revealed standing wave structures in the separation region and banded structures between the shear layer and the separation shock. SPOD, correlation analysis, and direct schlieren visualization revealed downstream-propagating disturbances consistent with the Kelvin Helmholtz instability, accompanied by waves indicative of Mach wave radiation; upstream-propagating waves between the shear layer and the separation shock, originating near the neck and hypothesized to be associated with upstream-traveling acoustic disturbances within the recirculation region; and stationary standing wave structures. These observations are consistent with a resonant aeroacoustic feedback mechanism in which downstream-propagating instabilities generate acoustic waves that travel upstream through the recirculation region and perturb the shear layer at separation. The physical origin of the secondary frequency remains uncertain; the data are analyzed in the context of existing theories to evaluate their consistency with current observations, although further data are required for conclusive identification.
" }, { "name": "Mandralis, Ioannis M.", "degree": "PhD", "year": "2026", "title": "Leveraging Aerial Transformation for Enhanced Air\u2013Ground Robotic Mobility", "advisor": "Gharib, Morteza; Murray, Richard M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:11032025-192332480", "creators": [ { "name": { "family": "Mandralis", "given": "Ioannis M." }, "id": "Mandralis-Ioannis-M", "orcid": "0000-0001-5270-0672", "display_name": "Mandralis, Ioannis M." } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "advisor", "display_name": "Murray, Richard M." } ], "committee": [ { "name": { "family": "Ames", "given": "Aaron D." }, "id": "Ames-A-D", "orcid": "0000-0003-0848-3177", "role": "chair", "display_name": "Ames, Aaron D." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/srg8-sx98", "abstract": "Ground-aerial robots can extend endurance, versatility, and robustness by combining wheeled motion with flight, yet many flying-rolling robot designs add actuators that increase weight and reduce efficiency. Morphobots mitigate this by using multi-purpose actuators and body shape change to switch modes on the ground, but unpredictable vehicle-ground interactions can be an obstacle to robust operation. This dissertation develops the Aerially Transforming Morphobot (ATMO), a quadcopter that reconfigures in flight to land on wheels, enabling reliable air-ground transitions, mode switching without the hindrances of ground-morphing, and improved agility. We present ATMO\u2019s design and performance characterization, analyze its dynamics\u2013revealing transformation-induced couplings incompatible with standard quadcopter control\u2013and introduce a model-predictive control framework that stabilizes ATMO through aerial transformation to execute dynamic transitions. We then compare this approach with a learning-based controller that uses deep reinforcement learning for end-to-end morpho-transition, validating both experimentally. Finally, we revisit ATMO\u2019s design using aerodynamic principles to expand morphing flight through wake vectoring, showing that passive structures in the rotor wake substantially increase available thrust authority. Overall, we demonstrate that aerial shape change improves agility and reliability, highlighting a new direction for research in ground-aerial robotics." }, { "name": "Aller, Brayden Gieschen", "degree": "PhD", "year": "2025", "title": "Strain Sensing in Thin Composite Laminates with Embedded Fiber Bragg Grating Sensors", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05012025-194857973", "creators": [ { "name": { "family": "Aller", "given": "Brayden Gieschen" }, "id": "Aller-Brayden-Gieschen", "orcid": "0000-0002-5258-4154", "display_name": "Aller, Brayden Gieschen" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Watkins", "given": "Michael M." }, "id": "Watkins-M-M", "role": "member", "display_name": "Watkins, Michael M." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/zj2k-h305", "abstract": "Deployable structures are popular for space applications as they enable large, complex spacecraft structures to overcome the size constraints of launch vehicle fairings. Such structures are increasingly manufactured out of thin (< 200 \u03bcm thick) composite laminates as they have a high stiffness-to-weight ratio, the ability to withstand high curvatures during stowage, and the potential for self-deployment using stored strain energy. To ensure the reliability of these thin composite spacecraft structures in operation, it is of interest to be able to continuously monitor their internal strain state to detect potential changes or damage that may compromise their integrity.
\r\n \r\nAlthough there are a number of potential sensors that could be used for this, fiber Bragg grating (FBG) sensors are especially well suited for this task and have a track record of successfully monitoring both composite materials and large aerospace structures. However standard size FBG sensors, which have a cladding diameter of 125 \u03bcm, are too large to be integrated into the thin composite structures of interest. To overcome this, we worked with several suppliers to develop and manufacture ultra-thin FBG sensors (< 30 \u03bcm cladding diameter) for this work that are able to be successfully embedded into thin composite laminates.
\r\n\r\nThe primary objective of this thesis was to investigate the suitability of ultra-thin FBG sensors for the monitoring of strain changes in thin composite spacecraft structures. To this end, the work in this thesis first investigated how to best embed ultra-thin FBG sensors to be able to measure the internal strain changes of interest while minimizing their disruptions to the surrounding laminates. Second, mechanical testing was performed to assess the effect that the embedded ultra-thin FBG sensors have on the mechanical properties of thin laminates. Third, the ability of these sensors to detect and monitor for strain changes in thin composite laminates was assessed through further mechanical testing. Finally, the effects of temperature on ultra-thin FBG sensors were studied experimentally.
\r\n\r\nThrough this work, which was done at the coupon level, we sought to demonstrate the ability of these ultra-thin FBG sensors to monitor for strain changes in thin composite laminates and their potential for the health monitoring of thin composite spacecraft structures. It is our hope that our findings in this thesis help lay the groundwork for the future implementation of these sensors in not only thin composite spacecraft structures, but to many other composite materials and aerospace structures as well.
" }, { "name": "Arun, Rahul", "degree": "PhD", "year": "2025", "title": "Beyond Symmetry: Normality-Based Analysis of Velocity Gradients in Turbulent Flows", "advisor": "Colonius, Tim", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302025-232647443", "creators": [ { "name": { "family": "Arun", "given": "Rahul" }, "id": "Arun-Rahul", "orcid": "0000-0002-5942-169X", "display_name": "Arun, Rahul" } ], "advisors": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "advisor", "display_name": "Colonius, Tim" } ], "committee": [ { "name": { "family": "Lozano-Dur\u00e1n", "given": "Adri\u00e1n" }, "id": "Lozano-Duran-A", "orcid": "0000-0001-9306-0261", "role": "chair", "display_name": "Lozano-Dur\u00e1n, Adri\u00e1n" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "orcid": "0009-0007-5991-2863", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3py1-wj85", "abstract": "Small-scale turbulence is a hallmark of countless natural and engineered flows. Its features are often described and modeled using the velocity gradient tensor (VGT), which is conventionally decomposed into the (symmetric) strain-rate tensor and the (antisymmetric) vorticity tensor. Although this symmetry-based decomposition has found use in areas such as vortex identification and closure modeling, it provides limited insight into local flow structure. A more refined description can be obtained by further distinguishing the normal and non-normal parts of the VGT. The resulting normality-based decomposition identifies contributions associated with normal straining (symmetric/normal), rigid rotation (antisymmetric/normal), and pure shearing (non-normal). We use this decomposition to identify flow features that are obscured by symmetry-based analyses yet have significant implications for efforts to understand and model turbulent flows.
\r\n\r\nWe first demonstrate that partitioning the strength of velocity gradients using our normality-based approach can distinguish between different regimes in various turbulent flows. In wall-bounded flows, the near-wall partitioning is dominated by shearing whereas the partitioning far from the wall collapses onto the partitioning associated with isotropic turbulence. In an unbounded vortex ring collision, our analysis distinguishes the initial vortex rings, which have a strong imprint from rigid rotation, from the decaying turbulent cloud produced by their collision, for which the partitioning is similar to that of isotropic turbulence. It also identifies enhanced shear\u2013rotation correlations as a distinctive fingerprint of the elliptic instability during transition, which can be interpreted using relevant geometric features of local streamlines. By deriving algebraic expressions for the partitioning constituents in terms of the invariants of the VGT and an additional parameter, which represents the alignment of shear vorticity with the local rotation axis, we identify a key facet of our analysis that goes beyond previous analyses of the VGT.
\r\n\r\nWe then apply our normality-based framework to filtered velocity gradients in direct and large-eddy simulations of isotropic turbulence. Our analysis enables shear layers, which are associated with shear vorticity, to be distinguished from vortex cores, which are associated with rigid rotation, in a multiscale setting. It reveals that filtering mitigates the relative contribution of shear layers in the subinertial range of the energy cascade. Moreover, it identifies crucial (yet perhaps overlooked) contributions from shear layers to fundamental energy transfer mechanisms, including strain self-amplification, vortex stretching, and backscatter associated with strain\u2013vorticity covariance. The dominant role of shear layers in the backscatter mechanism suggests that they contribute significantly to the bottleneck effect in the subinertial range of the cascade. Our analysis of large-eddy simulation data shows that they also amplify the artificial bottleneck effect produced by an eddy viscosity model in the inertial range. This reflects that the eddy viscosity model mimics an unfiltered direct numerical simulation at a lower Reynolds number. A mixed model can be used to mitigate the artificial bottleneck effect since it more accurately mimics a filtered direct numerical simulation.
" }, { "name": "Chan, Miles J.", "degree": "PhD", "year": "2025", "title": "Reduced Order Modeling of Near-Wall and Roughness Sublayer Turbulence Using Resolvent Analysis", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05222025-012553215", "creators": [ { "name": { "family": "Chan", "given": "Miles J." }, "id": "Chan-Miles-J", "orcid": "0009-0007-7898-3739", "display_name": "Chan, Miles J." } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Piomelli", "given": "Ugo" }, "id": "Piomelli-Ugo", "orcid": "0000-0002-7834-7894", "role": "member", "display_name": "Piomelli, Ugo" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5ycd-9x89", "abstract": "Modeling near-wall and roughness sublayer turbulence using physics-based methods remains a topic of paramount importance, since most engineering-relevant flows are turbulent and most surfaces are not smooth. While today there exists a wide range of empirical, data-driven modeling approaches for turbulence, these methods are limited because fully resolved turbulence data remains expensive to generate and burdensome to store and analyze. Therefore, the ability to predict out-of-sample is important, and since data-driven methods struggle to extrapolate, developing physics-based approximations that give useful, inexpensive predictions remains necessary. Yet the complexity of near-wall turbulence makes developing theoretical models difficult. This thesis tackles two main challenges. First, methods for reduced order modeling of the sensitivity of turbulence to multiscale, engineering-relevant roughness geometries are developed. In particular, a physics-based method for incorporating a drag-scaled, Reynolds-decomposed volume penalization into resolvent analysis yields a linear reduced order model that gives computationally inexpensive estimates for roughness sublayer fluctuations and dispersive stresses given a surface geometry and the mean flow profile in a rough wall channel flow. Then, an iterative method is developed to predict the mean flow profile, equivalent sand grain roughness, and Hama roughness function that utilizes the discovered relationship between the fluctuations and the mean flow. That model yields a closed-loop system for predicting roughness sublayer turbulence and the mean response given only a scan of the roughness geometry and a bulk Reynolds number in a rough wall channel flow. Second, a methodology for generating spatiotemporal representations of near-wall turbulence with very few degrees of freedom is developed. It utilizes a coarse-graining approach to reduce the number of modes required to describe a turbulent flow, selection criteria for picking descriptive modes, and Reynolds number scaling to provide predictions for an out-of-sample, higher Reynolds number flow. A spatiotemporal representation is generated, and results from Piomelli et al. that incorporate the modal representation into the wall layer of a wall modeled large eddy simulation are presented. Overall, this thesis contributes new reduced order modeling approaches that make use of physics-based insights to tackle outstanding problems in the prediction of near-wall and roughness sublayer turbulence." }, { "name": "Dassanayake, D. M. Sahangi Pulsarani", "degree": "PhD", "year": "2025", "title": "Space Legos: A Concept for In-Space Assembly of Large Structures with a Stationary Robot", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:11072024-033128289", "creators": [ { "name": { "family": "Dassanayake", "given": "D. M. Sahangi Pulsarani" }, "id": "Dassanayake-D-M-Sahangi-Pulsarani", "orcid": "0000-0002-1363-5764", "display_name": "Dassanayake, D. M. Sahangi Pulsarani" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "member", "display_name": "Asimaki, Domniki" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/0pn5-3w42", "abstract": "Human nature is inherently driven by the desire to build; advancing from primitive shelters to skyscrapers, and extending this relentless pursuit of progress to space through technological innovations. As space missions require larger and more complex structures, traditional deployable systems face challenges due to constraints on launch mass, volume, and complex deployment mechanisms. In-space assembly (ISA) offers a promising solution for constructing large structures, such as telescopes and satellites, directly in space.
\r\n\r\nThis thesis introduces a novel ISA concept with a centralized `truss builder' for autonomous assembly of polygonal-ring structures, using simple, repetitive operations and focusing on scalable mesh reflectors for communication and imaging. Utilizing the standard AstroMesh architecture, a rapid generalized design method is developed. Through the analysis of reflector geometry, optimized cable prestress, structural design, and a high-fidelity finite element model, analytical scaling laws are derived for mass, stowed envelope, and natural frequency based on aperture diameter. A semi-analytical homogenization model is introduced to efficiently predict fundamental natural frequencies. Stowed volume is a key limitation for large deployable reflectors, approaching current and future launch capacity limits, while the proposed ISA reflectors face no such constraints for apertures up to 200 meters.
\r\n\r\nA two-dimensional finite element model simulates the assembly kinematics of large ring-like structures with the proposed ISA concept, enhancing understanding of the process and evaluating key design aspects of a stationary robot assembling scalable ring-like trusses. The model provides insights for optimizing autonomous assembly systems and underscores the need for advanced numerical simulations to ensure smooth assembly and stability during ISA, especially as structures scale.
\r\n\r\nLab-scale prototype testing validates the ISA concept, with results aligning qualitatively with simulations. Both experiments and simulations reveal a range of viable solutions, demonstrating flexibility for future mission designs. This research offers crucial insights into the design and scaling of mesh reflectors, setting the stage for comparing ISA with traditional deployable systems. The proposed ISA concept presents a practical solution for building high-precision, large-scale structures in space, advancing the field of space construction and supporting future extended space missions.
" }, { "name": "Harms, Tanner David", "degree": "PhD", "year": "2025", "title": "Chasing After the Wind: Flow Structure Detection Strategies for Autonomous Mobile Flow Field Measurements", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:09142024-001942971", "creators": [ { "name": { "family": "Harms", "given": "Tanner David" }, "id": "Harms-Tanner-David", "orcid": "0009-0003-2913-7414", "display_name": "Harms, Tanner David" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "chair", "display_name": "Dabiri, John O." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Brunton", "given": "Steven L." }, "id": "Brunton-S-L", "orcid": "0000-0002-6565-5118", "role": "member", "display_name": "Brunton, Steven L." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aerospace" ], "doi": "10.7907/vjvv-vb21", "abstract": "Modern flow measurement technology enables studies of fluid motion that, half a century ago, would have seemed unfathomable. However, despite staggering capabilities, measuring many natural flows in the field remains challenging. In particular, resolving coherent flow structures within physical scales ranging from meters to kilometers is not readily achieved. This dissertation proposes autonomous mobile flow field measurements (AMFM) as a paradigm for expanding flow field measurement capabilities into this range of scales. In the AMFM framework, a mobile platform such as a drone would identify critical flow structures and follow them autonomously as they evolve; the device would be taught, in a sense, to chase after the wind for the sake of measuring it. The greatest theoretical challenge to AMFM is that of flow structure detection: what, after all, should be identified in the flow? How is it to be measured? Answering these questions is the overarching motivation of this dissertation. In response, two principal contributions are developed. The first is a theoretical approach to gradient estimation labeled Lagrangian gradient regression (LGR), which enables instantaneous and finite-time flow gradients to be approximated from sparse flow observations. The second is a semantic approach to flow measurement, which provides the ability to discern fluid motion from complex natural images using arbitrarily defined flow tracers. Together, these tools enable a range of studies which would be difficult to conduct otherwise. To demonstrate their combined ability, two experiments are performed. The first examines the motion of imperfect surface tracers measured by the proposed methods relative to sub-surface flows measured by conventional techniques. The second experiment analyzes flow features in the Caltech turtle ponds using only tracers naturally occurring on its surface. While it is demonstrated that the methods and results obtained in this work are meritorious in their own right, they also provide a framework from which future AMFM technologies can be built." }, { "name": "Heidt, Liam Frank Raven", "degree": "PhD", "year": "2025", "title": "Modal Analysis of Harmonically Forced Turbulent Flows with Application to Jets", "advisor": "Colonius, Tim", "url": "https://resolver.caltech.edu/CaltechTHESIS:08072024-203148023", "creators": [ { "name": { "family": "Heidt", "given": "Liam Frank Raven" }, "id": "Heidt-Liam-Frank-Raven", "orcid": "0000-0003-1967-6847", "display_name": "Heidt, Liam Frank Raven" } ], "advisors": [ { "name": { "family": "Colonius", "given": "Tim" }, "orcid": "0000-0003-0326-3909", "role": "advisor", "display_name": "Colonius, Tim" } ], "committee": [ { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "chair", "display_name": "Bae, H. Jane" }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Sader", "given": "John E." }, "id": "Sader-J-E", "orcid": "0000-0002-7096-0627", "role": "member", "display_name": "Sader, John E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/e6fe-kz94", "abstract": "Many turbulent flows exhibit time-periodic statistics. These include flows in turbomachinery, the wakes of bluff bodies, and flows exposed to harmonic actuation. However, many existing techniques for identifying and modeling coherent structures, most notably spectral proper orthogonal decomposition (SPOD) and resolvent analysis, assume statistical stationarity. In this thesis, we develop extensions to study turbulent flows with periodic statistics. We focus on the application of turbulent jets and jet noise reduction through harmonic actuation, which is of interest for both commercial and military aviation due to its success in reducing noise by up to 5dB.
\r\n\r\nTo analyze the coherent structures in harmonically forced flows, we develop the cyclostationary spectral proper orthogonal decomposition (CS-SPOD). We examine the resulting properties of CS-SPOD and develop a theoretical connection between CS-SPOD and harmonic resolvent analysis (HRA), thereby providing the theoretical basis for HRA to be used as a model for coherent structures of cyclostationary flows. We develop and validate a computationally efficient algorithm and then illustrate its efficacy using the linearized (complex) Ginzburg-Landau equation.
\r\n\r\nWe next employ cyclostationary analysis to investigate the impact of an axisymmetric acoustic harmonic forcing on the mean, turbulence, and coherent structures of a round turbulent jet with a Mach number of 0.4 and a Reynolds number of 450000. We perform large-eddy simulations for four cases at two forcing frequencies and amplitudes. Both low-frequency (Strouhal number of 0.3) and high-frequency (Strouhal number of 1.5) forcing is found to generate an energetic, nonlinear, tonal response consisting of the rollup of vortices via the Kelvin-Helmholtz mechanism. However, the impact of forcing on the broadband turbulence and coherent structures is limited, particularly at the low forcing amplitude associated with jet-noise-reduction devices. Additionally, the dominant coherent structures for the forced jets are similar in their energy, structure, and mechanism. At high forcing amplitudes, phase-dependent features arise in the dominant coherent structures and are associated with coupling to the high-velocity/shear regions of the mean. Overall, our results support the existing hypotheses that jet noise reduction can be associated with the deformation of the mean flow field rather than through direct interaction between the forcing and the turbulence. Lastly, we find that HRA predicts the dominant coherent structures well. This shows that HRA can be used to develop models of forced jets in a similar manner to how resolvent is employed for natural jets, which may be useful to guide future sound-source models of jets subjected to active control.
" }, { "name": "Hooper, Meredith Leigh", "degree": "PhD", "year": "2025", "title": "Machine-Learned Propulsion Strategies: From Adaptive Damage Compensation to Advanced Aeromobility", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06042025-002343274", "creators": [ { "name": { "family": "Hooper", "given": "Meredith Leigh" }, "id": "Hooper-Meredith-Leigh", "orcid": "0009-0004-4819-9941", "display_name": "Hooper, Meredith Leigh" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "chair", "display_name": "Dabiri, John O." }, { "name": { "family": "Ames", "given": "Aaron D." }, "id": "Ames-A-D", "orcid": "0000-0003-0848-3177", "role": "member", "display_name": "Ames, Aaron D." }, { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "member", "display_name": "Yue, Yisong" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/79aa-ja50", "abstract": "Autonomous vehicles are regularly sent into \"dull, dirty, and dangerous\" environments where the risk of damage is high. Avoidance or mitigation of such damage is therefore paramount to maintain effective autonomy. In this thesis, we use machine learning to investigate two different propulsive strategies that may be used by autonomous vehicles. The first, flapping propulsion, shows remarkable ability in nature to recover from damage simply by altering stroke kinematics. Using machine learning, we ask whether and how such mitigation of damage would be possible for a robotic autonomous vehicle. The second propulsive strategy we investigate is single-rotor propulsion, most commonly seen in helicopters. With this system, we seek to avoid damage before it occurs by improving mobility and control authority via thrust vectoring.
\r\n\r\nIn Part I, we use an evolutionary strategy (CMA-ES) with hardware-in-the-loop to explore optimal machine-learned adaptations to propulsor damage. Experimental function evaluations are performed by a flexible propulsor actuated by a spherical parallel manipulator (SPM). The machine-learned forces and trajectory parameters are compared to in vivo observations in order to determine whether bio-inspired strategies to adapt to significant propulsor damage are the most efficient, or whether they may be affected by irrelevant evolutionary pressures. With amputation of approximately 50% of the propulsor, we find that a complete recovery in thrust production and fitness is made. Some characteristics of the recovered trajectory are similar to natural swimmers, while others differ. Recovery when producing side-force is even more complex. Not all trials are able to recover force production and fitness, and no clear strategy to modify amplitude or frequency is seen. We conclude Part I by using PIV measurements to detail the effect of compensatory strategies on hydrodynamics. Both amputated and intact trajectories clearly show utilization of a drag-based paddling strategy, but the hydrodynamics of the intact and amputated fins differ significantly. This suggests that the machine-learned trajectories are not simply reestablishing the same wake as the intact fin to achieve the same thrust and fitness.
\r\n\r\nGiven the success in applying machine learning in-the-loop to a complex propulsive system where fluid-structure interactions are significant, we utilize the same strategy in Part II to begin to explore helicopter aeromechanics. We built an independent blade control (IBC) system that interfaces with the CMA-ES algorithm to explore optimal blade pitch trajectories. Using this platform, we explore two preliminary optimizations designed to vector thrust; the first, for sustained thrust vectoring that might be utilized upon takeoff or landing, and the second, for short-time thrust vectoring that could be used for enhanced maneuverability. We present some preliminary results from these optimizations and lay out a foundation for future applications of this experimental system.
" }, { "name": "Lupu, Elena Sorina", "degree": "PhD", "year": "2025", "title": "Perception-Driven Autonomy and Learning Control for Ground Vehicles", "advisor": "Chung, Soon-Jo", "url": "https://resolver.caltech.edu/CaltechTHESIS:06092025-020707222", "creators": [ { "name": { "family": "Lupu", "given": "Elena Sorina" }, "id": "Lupu-Elena-Sorina", "orcid": "0000-0002-3968-2630", "display_name": "Lupu, Elena Sorina" } ], "advisors": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "advisor", "display_name": "Chung, Soon-Jo" } ], "committee": [ { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "chair", "display_name": "Yue, Yisong" }, { "name": { "family": "Hadaegh", "given": "Fred" }, "id": "Fred-Hadaegh", "orcid": "0000-0002-0992-6323", "role": "member", "display_name": "Hadaegh, Fred" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." } ], "option_major": [ "aerospace" ], "doi": "10.7907/79tk-eg16", "abstract": "Autonomous robots are widely recognized as highly valuable and are expected to become increasingly prevalent. They will play a critical role across a wide range of terrestrial applications in complex, unstructured environments, as well as in space, supporting infrastructure and exploration on various bodies throughout the solar system and beyond. Looking ahead, autonomous robots will play a crucial role in the search for extraterrestrial life by enabling exploration of remote and extreme environments beyond Earth.\r\nAs robots need to approach more complex tasks, the ability to rapidly perceive, understand, make real-time decisions, and operate at speed requires advances in perception-driven controls, improved predictability, and robustness to disturbances. \r\nTo enable these capabilities, the first part of this thesis proposes an innovative approach to enhancing ground vehicle mobility by integrating a vision-based control algorithm that adapts to changes in real-time. \r\nOur approach improves the vehicle's ability to assess and respond to complex terrains in real-time by leveraging visual information through visual foundation models and meta-learning.\r\nOur controller has provable guarantees of exponential stability and was validated on board two ground vehicles.\r\nNext, an extension of the previously mentioned method applied to detecting objects in space using a visual foundation model is presented. Our method was successfully demonstrated in space in early 2025 aboard the EdgeNode Lite spacecraft.\r\nEfficient operation comes from the synergy of suitable autonomy and control with a suitable robot body.\r\nFollowing this consideration, the second part of the thesis presents the design and control of multi-degrees of freedom robots designed for mobility in complex environments.\r\nIt presents a nonlinear tracking controller with adaptation to improve the walking performance of walking-flying robots. This is illustrated by our implementation on Leonardo, the first robot to combine walking with flying to create a new type of locomotion, which we showcase in complex acrobatic movements such as slacklining and skateboarding.\r\nIn a second case study, we aim to further understand and improve biped walking by introducing a bipedal robot designed to be lightweight, easily manufactured, and easily repaired, serving as a platform for testing learning-based controllers.\r\nWe introduce and demonstrate the performance of two controllers: a model-based and a learning-based control.\r\nThis work highlights the importance of tightly integrated perception, control, and electromechanical design in achieving robust autonomy: on Earth, in orbit, and beyond." }, { "name": "Mohebbi, Nina", "degree": "PhD", "year": "2025", "title": "Flow Induced by Collective Vertical Migration: Impact of Swimmer Distribution, Buoyancy, and Wake Interactions", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05082025-223940273", "creators": [ { "name": { "family": "Mohebbi", "given": "Nina" }, "id": "Mohebbi-Nina", "orcid": "0000-0003-4014-6111", "display_name": "Mohebbi, Nina" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "member", "display_name": "Dickinson, Michael H." }, { "name": { "family": "Lozano-Duran", "given": "Adrian" }, "id": "Lozano-Duran-A", "orcid": "0000-0001-9306-0261", "role": "member", "display_name": "Lozano-Duran, Adrian" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/hz5j-g795", "abstract": "Various animal species exhibit collective motion, characterized by coordinated movement within groups of organisms. A prominent oceanic example is diel vertical migration (DVM), wherein zooplankton migrate vertically from deeper waters during the day to shallower regions at night, often covering distances of approximately 1 kilometer. Despite numerous field measurements, laboratory observations, and theoretical studies of biogenic mixing resulting from collective swimming, the scale of fluid mixing induced by DVM remains unresolved. A key challenge is linking the behavior and flows created by large numbers of individual organisms to collective-scale fluid dynamics. Since most swimmers involved in DVM operate at intermediate Reynolds numbers, the dynamics of these systems are nonlinear and span a wide range of spatial and temporal scales.
\r\n\r\nThis thesis investigates flow scaling generated by vertical migration of brine shrimp (Artemia salina) aggregates, using laboratory measurements complemented by semi-analytical modeling. A volumetric laser scanning system first measured swimmer behaviors and flow interactions during laboratory-induced vertical migrations. Swimmers consistently maintained vertical swimming velocities under varying environmental conditions, showed a Gaussian horizontal distribution within the tank cross-section, and exhibited a pronounced tendency toward the tank center, where illumination was brightest. A scaling relationship between swimmer buoyancy, ascent speeds, and resulting flow velocities was developed to contextualize these results.
\r\n\r\nA semi-analytical model was then developed to estimate the flow generated by wakes of multiple swimmers in proximity. Individual swimmer behaviors were informed by empirical observations and combined through an iterative approach that conserves mass and momentum, providing an aggregation-scale flow solution. Numerical results indicated that induced upstream flows within the aggregation were relatively insensitive to downstream swimmer presence, that average flow speeds approached a plateau beyond a critical aggregation length, and that closer swimmer spacing significantly enhanced induced flow velocities.
" }, { "name": "Pederson, John Monroe Jr.", "degree": "PhD", "year": "2025", "title": "Thermoelastic Deflections of Thin-Shell Composite Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:12172024-163805389", "creators": [ { "name": { "family": "Pederson", "given": "John Monroe Jr." }, "id": "Pederson-John-Monroe-Jr", "orcid": "0009-0002-8500-2285", "display_name": "Pederson, John Monroe Jr." } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Sader", "given": "John E." }, "id": "Sader-J-E", "orcid": "0000-0002-7096-0627", "role": "member", "display_name": "Sader, John E." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/xyry-w852", "abstract": "As space structures become larger, lighter, and deployable, thermal deflections induced by sunlight become a significant source of structural inaccuracy and even spacecraft vibration. Studying these deflections is notoriously difficult: analytical solutions rapidly become intractable, experiments under vacuum and cooling are low-visibility and expensive, and multiphysics finite-element simulations are computationally demanding and usually don\u2019t account for coupled thermo-structural analyses and/or changing radiation view factors.
\r\n\r\nThis work demonstrates key improvements in experimental methods and thermo-structural simulation of these thermal deflections. First, simultaneous full-field measurements of structural temperatures and deflections are achieved by constructing and using a custom vacuum chamber and heating setup; significant thermal gradients and repeatable thermal deformations are measured and analyzed, forming a ground truth for succeeding simulations. Second, multiphysics models of the experimental chamber are created in COMSOL Multiphysics and characterized, even accounting for residual convection, and used to inform prototype improvements and more advanced simulations. Third, based off such predictions, the unit structure prototype composite is improved by adding a layer of graphitized polymer film, with further experimentation showing a dramatic reduction in deflections.
\r\n\r\nFinally, the accumulated knowledge is used to simulate a satellite slew maneuver with realistic orbital heating; a custom technique to couple thermal (Thermal Desktop) and structural (Abaqus) finite-element software via a MATLAB script allows for the recalculation of radiation view factors during simulations, a feat necessary for accurate heating calculations on deployable structures. These results have immediate applicability in predicting structural temperatures and deflections during the satellite maneuvers proposed for the Caltech Space Solar Power Project, as well as suggesting critical improvements to ensure reliability and mission success.
" }, { "name": "Schoeffler, Donner Thomas", "degree": "PhD", "year": "2025", "title": "Measurement and Modeling of Detonation-Driven Shock Tube Flows", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272025-230901725", "creators": [ { "name": { "family": "Schoeffler", "given": "Donner Thomas" }, "id": "Schoeffler-Donner-Thomas", "orcid": "0000-0002-1932-5986", "display_name": "Schoeffler, Donner Thomas" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T-E", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/vedz-t661", "abstract": "The detonation driver is a device for generating the strong shock waves used in high-enthalpy hypersonic flow research facilities. The dynamic production of high-pressure and high-temperature driver gas has several advantages for shock-tube performance, however the unsteady gas dynamics of detonation waves also introduces several challenges. These are investigated here analytically and experimentally.
\r\n\r\nFor forward-mode operation, where the detonation propagates into the shock-tube diaphragm, the detonation Taylor wave attenuates the driven shock, and a model is needed to predict the resulting shock dynamics. This is accomplished by first analyzing the problem of plane shock decay generally. A new approximate solution is formulated for the classic piston start-stop problem and shown to be a significant advancement over predecessors. This result is applied to the shock decay from a detonation driver, and a two-parameter model is fit to simulation data, yielding a method for predicting shock trajectories from shock-tube initial conditions.
\r\n\r\nA small-scale shock tube is designed and constructed using a detonation driver that is operable in both the forward and reverse mode. A transparent driven section is used with large field-of-view shadowgraphy to perform novel time-resolved shock speed measurements. These are used to calibrate the decay model for a forward-mode driver and enable unique observations of shock-speed oscillations, resulting from diaphragm rupture and detonation initiation processes. Results are also obtained for shock tube operation with a conventional high-pressure helium driver.
\r\n\r\nThe gradients and fluctuations in post-shock flows are characterized using a heterodyne focused laser interferometer, a new instrument with advanced capabilities for measuring large phase changes with high resolution. As a development upon the FLDI, spatial filtering characteristics are preserved, and both differential and absolute phase data are acquired simultaneously, enabling a new technique for measurement of gas densities. The instrument is developed, experimentally validated, and then used to probe detonation-driven shock tube flows, achieving phase measurements of over 100 radians with milliradian resolution in a 10 MHz bandwidth. Results from forward-mode operation find that a hydrogen-oxygen driver produces remarkably disturbance-free flows. For reverse-mode operation, the amplitude of flow oscillations is found to be positively correlated with the contact-surface sound-speed ratio, and frequencies are consistent with first-order lateral acoustic waves.
" }, { "name": "Tawney, Jacqueline Rose", "degree": "PhD", "year": "2025", "title": "Aqueous Metallo-Megasupramolecules: From Stability to Extensional Flow Properties", "advisor": "Kornfield, Julia A.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06022025-131020183", "creators": [ { "name": { "family": "Tawney", "given": "Jacqueline Rose" }, "id": "Tawney-Jacqueline-Rose", "orcid": "0000-0002-4276-0652", "display_name": "Tawney, Jacqueline Rose" } ], "advisors": [ { "name": { "family": "Kornfield", "given": "Julia A." }, "id": "Kornfield-J-A", "role": "advisor", "display_name": "Kornfield, Julia A." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "member", "display_name": "Bae, H. Jane" }, { "name": { "family": "Nelson", "given": "Chris W." }, "id": "Nelson-Chris-W", "role": "member", "display_name": "Nelson, Chris W." }, { "name": { "family": "Kornfield", "given": "Julia A." }, "id": "Kornfield-J-A", "orcid": "0000-0001-6746-8634", "role": "member", "display_name": "Kornfield, Julia A." } ], "option_major": [ "aerospace" ], "doi": "10.7907/98fw-fx90", "abstract": "The addition of long, flexible polymers (> 1 Mg/mol) to a fluid is known to reduce turbulent drag and control droplet behavior, which has the potential to significantly enhance the efficiency of engineering flows across various industries, from agriculture to aviation. However, hydrodynamic forces can break the polymers and diminish their effectiveness, which is presently a major roadblock to their practical utilization in both applications and research. To address this challenge, the Kornfield group developed end-associative, self-healing polymers for use in fuel and, more recently, for use in water\u2014aqueous terpyridine-ended polyacrylamide (TPAM) supramolecules. This thesis examines the relationships between the molecular structure of TPAM, the amount of metal provided to link pairs of chain ends, and kinetic processes of the resulting supramolecules and the rheological properties and performance they provide. The most useful polymers for reducing turbulent drag, controlling mist, and tailoring droplet impact behavior combine high efficacy at low concentration (< 0.1 wt%), minimal impact on shear viscosity (< 2x), and long extensional relaxation time (> 1 ms), enabling them to stretch and resist elongational flow in turbulent eddies or fluid filaments. This thesis explores the fundamental nature of TPAM supramolecules and their potential utility as a rheological modifier, using measurements of molecular weight distributions and extensional relaxation times to illuminate the relationship between supramolecular structure and flow behavior.
\r\n\r\nFirst, we examine chemical degradation (desirable in the environment, but not during use), revealing that its rate can be controlled by limiting air exposure, avoiding an excess of metal ions relative to ligands, and storing samples in refrigerated conditions (4℃). Next, we assess how changes in metal-to-ligand ratios (M:L) and unimer lengths influence TPAM\u2019s megasupramolecular size, equilibration, and decay dynamics, showing that the presence of supramolecules comprising over 10 unimers gives rise to a relaxation time around 2 ms at 0.04 wt%\u2014long and dilute enough to cause drag reduction. In pursuit of even longer supramolecules (and thus longer relaxation times) with the same amount of TPAM, we modified the solution preparation protocol by introducing metal ions to a more concentrated TPAM solution prior to dilution. This exposed new and intriguing topologies with molecular weights extending beyond our measurable limit (10 Mg/mol), expanding the envelope of the longest accessible relaxation times (from ~2 to ~6 ms with M:L = 1:2 for Ni(II):terpyridine). We evaluated their potential as chain scission-resistant, turbulent drag-reducing agents. Initially, they reduce drag while maintaining backbone integrity; however, their supramolecular structure and extended relaxation time are not retained after multiple passes through contraction, turbulent, and expansion flows. The preservation of backbone integrity, along with the broad range of relaxation times achieved using more conventional linear topologies (up to ~3 ms), suggests that TPAM is a promising and robust rheological modifier worthy of continued investigation. Our findings enhance understanding of TPAM\u2019s structural and rheological properties under a range of conditions and lay the groundwork for further study of aqueous megasupramolecule dynamics and applications.
" }, { "name": "Gomez De La Cruz, Salvador Rey", "degree": "PhD", "year": "2024", "title": "Linear Amplification in Nonequilibrium Turbulent Boundary Layers", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:08312023-005517217", "creators": [ { "name": { "family": "Gomez De La Cruz", "given": "Salvador Rey" }, "id": "Gomez-De-La-Cruz-Salvador-Rey", "orcid": "0000-0002-7568-721X", "display_name": "Gomez De La Cruz, Salvador Rey" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Di Renzo", "given": "Mario" }, "id": "Di Renzo-Mario", "orcid": "0000-0003-0468-3606", "role": "member", "display_name": "Di Renzo, Mario" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/hn98-c285", "abstract": "Resolvent analysis is applied to nonequilibrium incompressible adverse pressure gradient (APG) turbulent boundary layers (TBL) and hypersonic boundary layers with high temperature real gas effects, including chemical nonequilibrium. Resolvent analysis is an equation-based, scale-dependent decomposition of the Navier Stokes equations, linearized about a known mean flow field. The decomposition identifies the optimal response and forcing modes, ranked by their linear amplification. To treat the nonequilibrium APG TBL, a biglobal resolvent analysis approach is used to account for the streamwise and wall-normal inhomogeneities in the streamwise developing flow. For the hypersonic boundary layer in chemical nonequilibrium, the resolvent analysis is constructed using a parallel flow assumption, incorporating N\u2082, O\u2082, NO, N, and O as a mixture of chemically reacting gases.
\r\n \r\nBiglobal resolvent analysis is first applied to the zero pressure gradient (ZPG) TBL. Scaling relationships are determined for the spanwise wavenumber and temporal frequency that admit self-similar resolvent modes in the inner layer, mesolayer, and outer layer regions of the ZPG TBL. The APG effects on the inner scaling of the biglobal modes are shown to diminish as their self-similarity improves with increased Reynolds number. An increase in APG strength is shown to increase the linear amplification of the large-scale biglobal modes in the outer region, similar to the energization of large scale modes observed in simulation. The linear amplification of these modes grows linearly with the APG history, measured as the streamwise averaged APG strength, and relates to a novel pressure-based velocity scale.
\r\n \r\nResolvent analysis is then used to identify the length scales most affected by the high-temperature gas effects in hypersonic TBLs. It is shown that the high-temperature gas effects primarily affect modes localized near the peak mean temperature. Due to the chemical nonequilibrium effects, the modes can be linearly amplified through changes in chemical concentration, which have non-negligible effects on the higher order modes. Correlations in the components of the small-scale resolvent modes agree qualitatively with similar correlations in simulation data.
\r\n\r\nFinally, efficient strategies for resolvent analysis are presented. These include an algorithm to autonomously sample the large amplification regions using a Bayesian Optimization-like approach and a projection-based method to approximate resolvent analysis through a reduced eigenvalue problem, derived from calculus of variations.
" }, { "name": "Graebener, Josefine Berta Marie", "degree": "PhD", "year": "2024", "title": "Formal Methods for Test and Evaluation: Reasoning over Tests, Automated Test Synthesis, and System Diagnostics", "advisor": "Murray, Richard M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312024-094443866", "creators": [ { "name": { "family": "Graebener", "given": "Josefine Berta Marie" }, "id": "Graebener-Josefine-Berta-Marie", "orcid": "0000-0002-1376-0741", "display_name": "Graebener, Josefine Berta Marie" } ], "advisors": [ { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "advisor", "display_name": "Murray, Richard M." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." } ], "option_major": [ "aerospace" ], "doi": "10.7907/4xdc-b988", "abstract": "With the integration of autonomous systems into our everyday lives edging closer to reality, ensuring the safety of these systems is paramount. Part of the safety verification process is a rigorous testing procedure, which currently does not exist for autonomous vehicles. In this thesis, we aim to provide approaches using formal methods to increase the efficiency of testing campaigns.\r\nFirst, we provide a framework based on assume-guarantee contracts to specify tests in the form of a test structure. Using these test structures, we then show how to combine, split, and compare tests. Additionally, we characterize when tests can be combined and when the resulting test requires temporal constraints. Next, we demonstrate the approach on examples and find a strategy for a test agent using winning sets and Monte Carlo tree search.
\r\n\r\nSecond, we present a framework to automatically synthesize a test environment, consisting of static and reactive obstacles, and dynamic test agents. We characterize the desired test behavior in a system and a test objective in the form of a linear temporal logic specification, consisting of sub-tasks commonly used for robotic missions. This test environment must ensure that the test is not impossible (i.e. a correct system can pass the test), but also that every test execution that satisfies the system objective also satisfies the test objective. We use tools from automata theory to construct the virtual product graph that represents all possible test executions, and the virtual system graph, which corresponds to the system's perspective.\r\nWe formulate this routing problem as a network flow optimization on the virtual product graph in the form of a mixed integer linear program for different test environments. We show that this routing problem is NP-hard. We propose a counterexample-guided search using GR(1) synthesis to find a strategy for a test agent. This framework is demonstrated in several examples in simulation and hardware.
\r\n\r\nLastly, we present a framework to diagnose a system-level fault by identifying the component responsible for the failure. We make use of assume-guarantee contracts and Pacti, a tool for compositional system analysis and design, to construct a diagnostics map, which allows us to trace a system-level guarantee to possible causes. We show that this framework can reduce the number of statements that need to be checked in the diagnostics process. We illustrate this framework on several abstract examples and two examples inspired by a real-world autonomous system.
" }, { "name": "Gunnarson, Peter John", "degree": "PhD", "year": "2024", "title": "Autonomous Flow-Based Navigation in Unsteady Underwater Environments", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06052024-052757779", "creators": [ { "name": { "family": "Gunnarson", "given": "Peter John" }, "id": "Gunnarson-Peter-John", "orcid": "0000-0002-4437-5379", "display_name": "Gunnarson, Peter John" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "chair", "display_name": "Bae, H. Jane" }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/vnh6-3t44", "abstract": "Autonomous ocean-exploring robots promise to significantly enhance the rate at which we can explore ocean environments. However, the limited range and speed of existing autonomous underwater vehicles (AUVs) are barriers to comprehensive ocean exploration. To address these limitations, the work in this thesis investigates strategies for improving the capabilities of existing AUVs, such as targeted sampling and efficient navigation through background flows. Inspired by the ability of aquatic animals to navigate via flow sensing, hydrodynamic cues are investigated as a sensory input for accomplishing these feats of autonomous navigation using only onboard sensors. First, reinforcement learning (RL) is investigated as an algorithm for accomplishing efficient point-to-point navigation in simulated cylinder flow. The algorithm entails inputting point measurements of flow quantities such as velocity and vorticity into a deep neural network, which then determines a swimmer's actions. Using point velocity as the sensory input, the RL algorithm achieved a near 100 percent success rate in reaching the target locations while approaching the time-efficiency of optimal navigation trajectories. To test RL and flow-based navigation in a physical setting, we next developed the Caltech autonomous reinforcement learning robot (CARL), a palm-sized underwater robotic platform. As proof-of-concept analogy for tracking hydrothermal vent plumes in the ocean, the robot was tasked with locating the center of turbulent jet flows in a 13,000-liter water tank using data from onboard pressure sensors. Using a navigation policy trained with RL in a simulated flow environment, CARL successfully located the turbulent plumes at more than twice the rate of random searching by detecting mean flow gradients with the onboard pressure sensors. Lastly, combing both flow sensing and efficient navigation, the accelerometer onboard CARL was used to sense and exploit the flow from a passing vortex ring for energy-efficient propulsion. Body acceleration and rotation were shown to be effective methods of indirect flow sensing, which enabled the energy-efficient vortex ring surfing strategy. Throughout this work, efforts are made to understand the governing physics behind the discovered navigation strategies to generalize the results beyond a specific navigation problem, sensor type, or robotic implementation." }, { "name": "Rivi\u00e8re, Benjamin Pierre", "degree": "PhD", "year": "2024", "title": "Do Robots Dream of Random Trees? Monte Carlo Tree Search for Dynamical, Partially Observable, and Multi-Agent Systems", "advisor": "Chung, Soon-Jo", "url": "https://resolver.caltech.edu/CaltechTHESIS:06032024-152357240", "creators": [ { "name": { "family": "Rivi\u00e8re", "given": "Benjamin Pierre" }, "id": "Rivi\u00e8re-Benjamin-Pierre", "orcid": "0000-0002-0597-5400", "display_name": "Rivi\u00e8re, Benjamin Pierre" } ], "advisors": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "advisor", "display_name": "Chung, Soon-Jo" } ], "committee": [ { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "chair", "display_name": "Yue, Yisong" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Hadaegh", "given": "Fred" }, "id": "Fred-Hadaegh", "orcid": "0000-0002-0992-6323", "role": "member", "display_name": "Hadaegh, Fred" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/dbwa-we50", "abstract": "Autonomous robots are poised to transform various aspects of society, spanning transportation, labor, and scientific space exploration. A critical component to enable their capabilities is the algorithm that interprets sensor data to generate intelligent planned behavior. Although reinforcement learning methods that train parameterized policies offline from data have shown recent success, they are inherently limited when robots inevitably encounter situations outside their training domain. In contrast, optimal control techniques, which compute trajectories in real-time using numerical optimization, typically yield only locally optimal solutions.
\r\n\r\nThis research endeavors to bridge the gap by developing algorithms that compute trajectories in real-time while converging towards globally optimal solutions. Building upon the Monte Carlo Tree Search (MCTS) framework\u2014a stochastic tree search method that simulates future trajectories while balancing exploration and exploitation\u2014the research focus is twofold: (i) constructing an efficient discrete representation of continuous systems in a decision trees, and (ii) searching on the resulting tree while balancing exploration and exploitation to achieve global optimality.
\r\n\r\nThe study spans theoretical analysis, algorithmic design, and hardware demonstrations across dynamical, partially observable, and multi-agent systems. By addressing these critical questions, this research aims to advance the field of autonomous robotics, enabling the deployment of intelligent robots in complex and diverse environments.
" }, { "name": "Wen, Alexander Huai-Cheng", "degree": "PhD", "year": "2024", "title": "Vibration Damping of Coiled Structures Through Frictional Slip", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:12132023-225911259", "creators": [ { "name": { "family": "Wen", "given": "Alexander Huai-Cheng" }, "id": "Wen-Alexander-Huai-Cheng", "orcid": "0009-0008-9038-3039", "display_name": "Wen, Alexander Huai-Cheng" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "chair", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "member", "display_name": "Asimaki, Domniki" }, { "name": { "family": "Watkins", "given": "Michael M." }, "id": "Watkins-M-M", "role": "member", "display_name": "Watkins, Michael M." } ], "option_major": [ "aerospace" ], "doi": "10.7907/gvps-8x65", "abstract": "Vibration management is important for the survivability of structures. The response of a structure under vibration is dependent upon interaction between the excitation environment and the properties of the structure. If the input excitation cannot be adjusted, then the structure must be engineered to survive. One approach to engineering structures to reduce vibration response is through damping, which is achieved by adding damping devices or materials to covert kinetic energy into heat, where removing energy from the system reduces the amplitude of response. There are a variety of existing vibration damping concepts and techniques, however, conventional methods of these approaches are subject to limitations such as compromising stiffness for increased damping and performance that is excitation profile dependent.
\r\n\r\nThis research proposes a novel, passive vibration damping concept which is motivated by recent deployable structures for space that use coiling as a packaging architecture. The proposed concept, referred to as \"wound roll damping\", is a friction-based damping scheme for coiled structures, where the structure is wound around a mandrel with tension that allows interlayer slip during vibration. The friction between slipping layers provides an energy dissipation mechanism, which reduces the overall level of response. The concept was developed with the challenges of mitigating spacecraft launch vibration and the limitations of conventional damping techniques in mind.
\r\n\r\nUnderstanding of the working principle and performance of this damping concept is achieved using a combination of experiments, analysis, and FEA. A method for determining the locations of slip within a wound roll under vibration is presented. This consists of modeling the interlayer friction forces, using analytical expressions for the stress fields that arise during tension winding of wound rolls, and comparing these values against loading estimates obtained from analysis and FEA. The locations of slip for wound rolls supported by a cantilevered mandrel with bending vibration modes are towards the root of the wound roll structure, near the inner layers.
\r\n\r\nExperimental studies that demonstrate the performance and properties of this damping concept are presented in this work. A wound roll test sample is subjected to a range of excitation profiles including: sine sweep, sine dwell, random, and shock with varying levels of sample winding tension and excitation amplitude. Using these experiments, this concept is demonstrated to not be subject to the limitations of conventional damping schemes. This scheme is observed to be capable of significantly increasing the overall stiffness while providing elevated damping levels, with a performance that is tunable with winding tension, independent of excitation profile, and scales with excitation amplitude. The locations of slip are observed to be consistent with predictions from FEA and analysis.
\r\n\r\nTwo approaches to simulate and model the wound roll damper are developed to both better understand the physical mechanism of this concept and provide analysis tools. The first method is an FEA model, consisting of the base vibration of concentric shells and solids that have frictional contact interactions. The second method is a 2-DoF reduced order model that simulates the frictional contact between two mass-spring-damper systems. Both methods are demonstrated to have good correlation with experimental measurements.\r\n
\r\n\r\nA majority of this work demonstrates the performance of this concept, using both experiments and simulation at lab scales. This work also presents simulation studies that demonstrate the viability of this concept at realistic scales. Using simulations scaled to recent coilable space structures, both implemented and proposed, the wound roll damping concept is demonstrated to provide significant stiffness and damping.
" }, { "name": "Yu, Wesley Minlai", "degree": "PhD", "year": "2024", "title": "Experiments on Separation Shear Layer Instabilities in Hypervelocity Flows", "advisor": "Austin, Joanna M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05202024-181545768", "creators": [ { "name": { "family": "Yu", "given": "Wesley Minlai" }, "id": "Yu-Wesley-Minlai", "orcid": "0000-0002-1133-3199", "display_name": "Yu, Wesley Minlai" } ], "advisors": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "advisor", "display_name": "Austin, Joanna M." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1m4t-5n55", "abstract": "Shock-boundary layer interactions (SBLI) are complex fluid dynamic phenomena that occur when shocks are generated near corners and irregular geometries on vehicles flying near or above supersonic speeds, causing external flow distortion and possible boundary layer separation. Accurate prediction of the mean and unsteady SBLI surface interaction is imperative to avoid failure from highly localized aerodynamic and heating loads, and loss of authority near a control surface. For hypersonic flight-enthalpy matched conditions, current SBLI simulations tend to under-predict thermal loads and have significant disagreements with ground-based experiments in separation location, and location and magnitude of peak heating. These discrepancies are potentially largely due to uncertainties in the modeling and recreation of the coupled real-gas (thermochemical) molecular and gas dynamic processes. To address this issue, efforts have been placed on developing and validating thermochemical gas models, which presents a need for off-surface experimental data. The disagreement between simulations and high-enthalpy ground test experiments have also highlighted the need to better characterize the freestream thermodynamic, velocity, and noise conditions.
\r\n\r\nExtensive freestream characterization of the T5 Free-Piston Reflected Shock Tunnel was performed over the course of numerous experimental campaigns using high-speed shadowgraph/schlieren imaging, static and pitot pressure probes, tunable diode laser absorption spectroscopy (TDLAS), and focused laser differential interferometry (FLDI). Accurate time-resolved static pressure measurements are key to characterizing the operation and freestream thermodynamic state in hypervelocity reflected shock tunnels, through both direct measurement and for interpretation of TDLAS signals. A series of three static pressure probes were built for use in T5 at a range of conditions from 8-16 MJ/kg stagnation enthalpies, and measurements agreed well with TDLAS-inferred pressure and numerical simulations of the static probe response. At higher enthalpy conditions, TDLAS measurements showed a substantial decrease in freestream temperature (~1000 K) while velocity was constant. This finding motivated the need for a method to characterize the arrival time and degree of driver gas contamination in T5. An opposing-wedge detector was designed to leverage the sensitivity of the canonical Mach stem flow to the freestream \u03b3, such that the flow would choke at a prescribed increase in \u03b3 corresponding to the arrival of a specific mole fraction of monatomic driver gas. With high-speed schlieren/shadowgraph imaging, driver gas arrival times and mole fractions were obtained for the 8 MJ/kg test condition.
\r\n\r\nInformed by these freestream characterization experiments, near-surface FLDI measurements of instabilities in a separation shear layer on a 25\u00b0-55\u00b0 double-cone model were performed with simultaneous static pressure and freestream tunnel noise measurements in hypervelocity conditions. Three main frequency regimes were considered: i) low-frequency content associated with Kelvin-Helmholtz instabilities and streamwise acoustic disturbances along the shear layer, ii) a strong medium-frequency (peak ~370-450 kHz) signal associated with shear layer instabilities communicating with the model surface, and iii) high-frequency features associated with Mack (second-mode) disturbances. Length scaling arguments are discussed for each case, informed by axisymmetric simulations of the mean flow over the double-cone. A ray-tracing model was used to simulate the FLDI response to certain disturbances. The low-frequency Kelvin-Helmholtz and streamwise acoustic disturbance frequencies did not vary beyond uncertainty bounds along the shear layer. The medium-frequency content had a clear dependence on the local separation height, with the mean frequency decreasing with streamwise position. The high-frequency Mack mode disturbances were only observed in some experiments, suggesting the disturbance is limited only to within the shear layer, making detection difficult if any bulk shear layer motion occurs relative to the FLDI beam positions. This study provides the first known FLDI data on shear layers in hypervelocity flows, together with simultaneous freestream characterization, with the aim to inform future experiments in hypervelocity ground testing facilities and high-resolution numerical simulations.
" }, { "name": "Canales Escobedo, Fabricio Gianfranco", "degree": "PhD", "year": "2023", "title": "Numerical Analysis of Folding and Deployment Dynamics of Thin Shell Structures with Localized Folds", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012023-233806562", "creators": [ { "name": { "family": "Canales Escobedo", "given": "Fabricio Gianfranco" }, "id": "Canales-Escobedo-Fabricio-Gianfranco", "orcid": "0000-0001-9071-3263", "display_name": "Canales Escobedo, Fabricio Gianfranco" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "member", "display_name": "Bae, H. Jane" }, { "name": { "family": "Schroeder", "given": "Peter" }, "id": "Schr\u00f6der-P", "orcid": "0000-0002-0323-7674", "role": "member", "display_name": "Schroeder, Peter" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/gt81-0s18", "abstract": "This thesis focuses on the analysis of tape springs folded in the opposite sense and their dynamic deployment, and aims to use methods to reduce the computational cost of the analysis. The tape spring is a thin shell deployable structure that has features in common with other deployable structures. The deployment process of such structures can be difficult to predict, and the use of numerical models can be a more cost-effective alternative to experimental testing. Approaches to reduce the computational cost of the analysis of tape springs are investigated such as adaptive meshing and reduced order models. The thesis also presents an accurate analysis of tape spring deployment and a detailed study of the energies and the physics of the deployment. This is used to investigate the energy leak observed in previous tape spring deployment work. Overall, this thesis contributes to improving the efficiency and accuracy of the analysis of deployable structures, particularly tape springs, which can have significant applications in spacecraft technology." }, { "name": "Gandhi, Vatsa Bhupeshkumar", "degree": "PhD", "year": "2023", "title": "Shock Compression of Body-Centered Cubic Metals from the Atomistic to Continuum Scale: Iron and Molybdenum", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:05052023-185856720", "creators": [ { "name": { "family": "Gandhi", "given": "Vatsa Bhupeshkumar" }, "id": "Gandhi-Vatsa-Bhupeshkumar", "orcid": "0000-0002-6752-113X", "display_name": "Gandhi, Vatsa Bhupeshkumar" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Mello", "given": "Michael" }, "id": "Mello-Michael", "orcid": "0000-0003-2129-9235", "role": "member", "display_name": "Mello, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/kwf1-7y79", "abstract": "Fundamental understanding of material behavior under extreme conditions is crucial for designing high strength, light weight, and high temperature resistance materials, and for modeling planetary physics problems such as behavior of the core and impact phenomena. Under extreme conditions, materials not only exhibit a different mechanical, thermal, and failure response but can also undergo structural changes, such as phase transformations, which significantly alters their material properties. This motivates studying their dynamic response and developing constitutive models for applications such as hypersonics, high speed manufacturing, impact and blast of structures, aircraft and spacecraft shielding, meteorite impact, and collision of planets. Despite the importance, experimental investigations of shock induced phase transitions, inelastic material behavior, and elastic-plastic anisotropy under multi-axial stress states and at microscopic length scales of metals still remains largely unexplored. Thus, the focus of this thesis is on the shock compression behavior of body-centered cubic (BCC) metals, specifically iron and molybdenum, under compression-shear loading and at the atomistic-continuum spatial scales. In particular, the role of solid-solid phase transformation of body-centered cubic (BCC) iron on material strength and the orientation dependence of single crystal molybdenum on its elastic-plastic transition is investigated.
\r\n\t \r\nIron in its high pressure hexagonal close-packed (HCP) \u03f5-phase is critical in geological and planetary applications such as inner cores of rocky planets and hypervelocity impacts of asteroids, and meteorites. Thus, understanding plasticity behavior of iron under these condensed matter states is important to develop more accurate models for such applications and to understand deformation mechanisms of inner planetary cores. Because the \u03f5-phase is unstable, iron reverts to its ambient \u03b1-phase (BCC) upon release making it difficult to probe the strength behavior using conventional methods. Additionally, solid-solid phase transformations provide a unique opportunity to study material strength as they are crucial for expanding the design space for various load-bearing applications. In the first part of the thesis, the pressure dependent dynamic strength behavior of both the ambient BCC \u03b1-phase and high-pressure HCP \u03f5-phase of iron at strain rates on the order of 1 X 10\u2075 s\u207b\u00b9 and pressures up to 42 GPa is investigated. Pressure shear plate impact experiments are conducted using a sandwich configuration to decouple the effect of pressure and shear thereby allowing to probe shear strength once the sample reaches an equilibrated state of pressure but prior to release. The strength of the \u03f5-phase is observed to be more than double the strength of \u03b1-phase possibly due to microstructural evolution during phase transformation. Additionally, the evolution of yield properties with pressure, temperature, and strain is presented for the first time, enabling more accurate modeling of extreme deformation phenomena associated with iron-rich celestial bodies such as planetary collisions.
\r\n\t\r\nMolybdenum, its alloys, and other body-centered cubic (BCC) refractory metals are critical in geological and planetary applications such as structural properties of terrestrial planetary composition, formation of the earth-moon system, and hypervelocity impacts of rocky planets. Additionally, the high temperature specific strength, creep resistance, and ductility of BCC refractory metals make them ideal for aerospace and armor/anti-armor applications. Under high strain-rate inelastic loadings, the macroscopic response of these metals is often influenced by the atomistic mechanisms including dislocation motion and deformation twinning. Current material models rely on investigations that involve continuum measurements followed by postmortem microstructural analysis of recovered samples. However, these may not reflect the material behavior during the passage of the shock wave and, thus, requires real-time in-situ atomistic characterization to link the microstructure to macroscopic response. In the second part of the thesis, plate impact experiments coupled with both laser interferometry continuum measurements and in-situ dynamic Laue x-ray diffraction (XRD), at the Advanced Photon Source (APS), are conducted on single crystal molybdenum. Here, the role of crystal orientation, either [100] or [111], on deformation mechanisms during the elastic-plastic transition and the steady state response is explored at pressures ranging from 9-19 GPa. Complementary simulation methodology is developed to analyze the evolution of the Laue diffraction spots captured during impact. By extracting the lattice strain and stresses from XRD images, dislocation slip along [110]\u2329111\u232a and [112]\u2329111\u232a is found to be the probable deformation mechanism during compression with negligible anisotropy observed at the Hugoniot state. For the first time, real-time evidence of molybdenum undergoing deformation twinning along [112\u0305]\u2329111\u232a during shock release beyond a critical pressure of 16 GPa irrespective of the loading orientation is presented.
" }, { "name": "Martin, Conor Daniel", "degree": "PhD", "year": "2023", "title": "Experiments in Thermal Ignition: Influence of Natural Convection on Properties of Gaseous Explosions", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06022023-192522565", "creators": [ { "name": { "family": "Martin", "given": "Conor Daniel" }, "id": "Martin-Conor-Daniel", "orcid": "0000-0003-2332-7383", "display_name": "Martin, Conor Daniel" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/twcf-m219", "abstract": "Explosion hazards exist in many industrial sectors including chemical processing, mining, nuclear power, and aviation. Thermal ignition is the name given to the particular case where the initiation energy is supplied via thermal heating of a gas. The critical conditions leading to thermal ignition are in general highly configuration dependent and require a broad set of experimentation to investigate the influence of wide ranging physical processes on ignition. To aid this effort the present work comprises three main experiments covering a range of thermal ignition hazards. First, a heated atmosphere test with fuel injection (ASTM-E659) was implemented to enable the study of heavy hydrocarbon fuels such as Jet A and multicomponent surrogates. This approach showed the existence of cool flame ignition modes near the ignition thresholds for most fuels. The autoignition temperature (AIT) of commodity Jet A was found to be reasonably reproducible by most alkane fuels including n-hexane. Multicomponent surrogates were also able to match the cool flame ignition regimes reasonably well.
\r\n\r\nNext, ignition using a vertical heated surface in a cold reactive atmosphere was studied in the laminar flow regime. The effects of dilution with nitrogen and reduced pressure were explored for n-hexane/oxygen/nitrogen mixtures. Results found a modest dependence of minimum ignition temperatures on pressure and nitrogen fraction however, with a significant reduction in explosion severity as measured by the maximum overpressure and transient duration. At sufficiently reduced oxygen concentrations, localized weakly propagating flames were found to form in the thermal layer near the surface and produce sustained puffing flame instabilities. One-dimensional flame simulations with detailed kinetics were conducted to supplement and aid in interpretation of the experimental measurements for diluted mixtures. Correlation of ignition thresholds were found to be possible using simplified flame properties and laminar natural convection boundary layer theory.
\r\n\r\nFinally, a novel experiment was designed to explore the effects of turbulent transition and confinement of large heated surfaces on ignition thresholds. Modeling of the energy balance for resistive heating showed that cylinders up to 36 in. long could be heated using modest power supplies. Six cylinder sizes of varying length were chosen based on this analysis to explore laminar, transitional, and turbulent flow regimes. A large scale flow visualization system was created to study these flow regimes and found that turbulent transition occurred for cylinders as small as 10 in. long for wall temperatures of 1000 K. A study of the transitional dependence on temperatures for large temperature difference (T = 555--1140 K), highly non-Boussinesq conditions found that the transitional Rayleigh number decreased by two orders of magnitude in this regime. The thermal layer thickness at the transition height was estimated in order to obtain a relevant length scale to the boundary layer transition problem. Using this a more consistent transition criteria was obtained (Ra using the thermal thickness length scale) and found to vary by only a factor of two in the high temperature cases studied.
\r\n\r\nThe implementation of these cylinders in ignition testing revealed that there was a strong influence of heating rate due to confinement. The use of absorption spectroscopy showed that for low heating rates the fuel was mostly consumed in low temperature reactions prior to or in place of rapid ignition. This resulted in larger ignition temperatures and weak flames which propagate only in the thermal boundary layer. This effect was explained as a consequence of reduced flow recirculation times due to confinement. A strong influence of turbulence was also found for ignition thresholds when compared with other data for ignition by vertical hot surfaces in the laminar regime. Turbulence was also found to strongly influence the explosion properties due to turbulent flame acceleration. This resulted in larger explosion pressures, shorter transients, and faster flames.
" }, { "name": "Oshima, Emile Kazuo", "degree": "PhD", "year": "2023", "title": "Experimental Studies of Flow Control Techniques for Future Aircraft", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:02042023-015312785", "creators": [ { "name": { "family": "Oshima", "given": "Emile Kazuo" }, "id": "Oshima-Emile-Kazuo", "orcid": "0000-0002-1689-3726", "display_name": "Oshima, Emile Kazuo" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "member", "display_name": "Bae, H. Jane" }, { "name": { "family": "Wygnanski", "given": "Israel J." }, "id": "Wygnanski-I-J", "orcid": "0009-0001-5711-7029", "role": "member", "display_name": "Wygnanski, Israel J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/fpcj-w268", "abstract": "From the signing of the Paris Agreement to the COVID-19 outbreak, the past decade has truly challenged the aviation industry to adapt. New technologies need to be developed constantly to meet the increasing commercial and defense demands for more efficient, quiet, safe, and agile aircraft. To keep up with these rapidly changing times, an approach that marries a fundamental understanding of aerodynamics with systems design and optimization is necessary. This thesis explores two promising concepts for controlling flow over next-generation aircraft: active control on a swept wing for airplane applications, and passive control on a rotating blade for drone applications. In each, force measurements are combined with advanced flow visualization techniques to create a research framework that is both data-driven and physics-informed.
\r\n\r\nIn Part I, a comprehensive wind tunnel campaign is carried out on a swept wing model of modular geometry equipped with an array of sweeping jet actuators, which have demonstrated tremendous promise for flow control authority in both laboratory settings and full-scale flight tests. The flow physics and performance of the wing is investigated first without actuation, revealing separation behaviors at both the leading and trailing edges that are crucial to consider when flow control is applied. This paves the way for an optimization study in a newly proposed framework that relies on fluid power coefficients rather than the momentum coefficient that has been the accepted parameter of choice for characterizing blowing systems over the past seven decades of active flow control research.
\r\n\r\nPart II explores the feasibility of a \"prop-shroud\" concept for small-scale aerial vehicles, in which the shroud is directly attached to the blade tips and thus co-rotates with the propeller. Such a configuration has the potential to provide the various aerodynamic and engineering benefits of a shrouded propeller without the associated costs and complexities of its installation. The hover efficiency of a prop-shroud is shown to be comparable to commercially available drone propellers, even without a rigorous optimization of its geometry. The effect of the co-rotating shroud is then analyzed in detail on the time-averaged, phase-averaged, and unsteady features of the flow field. A model based on vortex formation time is developed, laying out a foundation for future research and understanding.
" }, { "name": "Palmer, Emily Hope", "degree": "PhD", "year": "2023", "title": "Locomotory Control Algorithms and Their Neuronal Implementation in Drosophila melanogaster", "advisor": "Dickinson, Michael H.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05192023-015643241", "creators": [ { "name": { "family": "Palmer", "given": "Emily Hope" }, "id": "Palmer-Emily-Hope", "orcid": "0009-0006-8370-4709", "display_name": "Palmer, Emily Hope" } ], "advisors": [ { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "advisor", "display_name": "Dickinson, Michael H." } ], "committee": [ { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "chair", "display_name": "Murray, Richard M." }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "member", "display_name": "Dickinson, Michael H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/yyjd-a554", "abstract": "Scientists and engineers alike have long looked to animals in their pursuit of understanding the natural world and how best to interact with it. While researchers have looked across diverse classes, insects have been extensively studied for their rich diversity of life histories and abilities to perform at spatial and temporal scales difficult for engineered systems. Within insects, the fruit fly, Drosophila melanogaster, is a particularly well-studied organism because of its experimental tractability and status as a genetic model organism, providing both detailed descriptions of a broad suite of behaviors and access to and control over specific sets of tissue. In this work, we make use of these tools to study two behaviors in Drosophila, local search, the behavior in which walking flies will search the area around a food site in search of other food sources nearby, and the optomotor response, wherein they will stabilize in response to visual motion during flight. In these studies, we will use modern techniques from both biology and engineering, to exhaustively characterize and describe the observed behaviors and attempt to untangle the underlying algorithms and their neuronal implementation.
\r\n\r\nFirst, we explore the algorithmic structure of local search in fruit flies. When flies encounter a piece of food, they will often perform walking searches nearby; as food tends to be patchy in natural settings, searches may allow flies to locate other food sites in the area. We induce local search using optogenetic stimulation of sugar-sensing neurons and constrain the flies to a dark, annular arena. These experimental details result in a simplified behavior, as the fly has access to a limited sensory environment, so that the search can be interpreted as an example of idiothetic path integration, and the search itself is one-dimensionalized and therefore more easily analyzed. Our experiments, in tandem with complementary modeling using a state transition diagram formalization of the behaviors, generate two principle findings. First, flies can integrate their location in two dimensions--after the optogenetic activation is disabled and the flies can no longer receive the food stimulus, they will continue to search over the former food site even after completing a full revolution of the annular arena. Second, when multiple food sites are present, they search over a center of the food sites, rather than over one distinct food site. These results both provide insights into the algorithmic structure of local search and an experimental and descriptive paradigm for further inquiries into the behavior.
\r\n\r\nSecond, we investigate the role of a population of neurons, the DNg02s, in the optomotor response. In response to visual patterns of wide-field motion, such that the entire world is moving in the fly-centric reference frame, the animal will attempt to steer to cancel the visual motion, as the most parsimonious explanation of the motion is that the fly itself is moving in the global reference frame. We demonstrate that the DNg02 neurons are a required component in the neural circuitry underlying the optomotor response, but that they are insufficient to induce steering behaviors. We conclude with a set of models that fully recapitulate the collected dataset. With current techniques, distinguishing between the two possible models of the downstream connectivity from the DNg02s to the motor neurons associated with wing motor output is not possible. However, as new datasets become available, particularly complete connectomes of the Drosophila nervous system, the neuronal pathways from the DNg02s to the motor systems may be elucidated.
" }, { "name": "Reddy, Narravula Harshavardhan", "degree": "PhD", "year": "2023", "title": "Folding and Dynamic Deployment of Ultralight Thin-Shell Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292023-160132013", "creators": [ { "name": { "family": "Reddy", "given": "Narravula Harshavardhan" }, "id": "Reddy-Narravula-Harshavardhan", "orcid": "0000-0003-3897-8162", "display_name": "Reddy, Narravula Harshavardhan" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Sader", "given": "John E." }, "id": "Sader-J-E", "orcid": "0000-0002-7096-0627", "role": "member", "display_name": "Sader, John E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/m7rd-6s86", "abstract": "Thin-shell structures are becoming increasingly popular for space missions due to their high stiffness-to-mass ratio, easy folding and coiling, and self-deployment using stored strain energy. Broadly, two deployment strategies exist: 1) controlled or deterministic, and 2) unconstrained. Controlled deployment involves carefully orchestrated events using control or guidance systems, while in unconstrained deployment, the structure is simply allowed to self-deploy with minimal guidance. Unconstrained deployment offers lighter deployment mechanisms and better packaging efficiency but the unpredictability of this process has been a significant obstacle to its adoption.
\r\n\r\nThis study focuses on demonstrating the predictability of unconstrained dynamic deployment of thin-shell structures, using the Caltech Space Solar Power Project (SSPP) structures as a case study. The Caltech SSPP uses composite triangular rollable and coilable longerons as the primary building blocks to create large bending-stiff structures. The specific objective is to improve the predictability and robustness of the unconstrained dynamic deployment of the Caltech SSPP structures. Deployment is influenced by the initial conditions and the interaction between the structure and the mechanism during the deployment. To understand these effects, high-fidelity numerical simulations are developed and validated against experiments. The study also examines the sensitivity of deployment characteristics to various design parameters and external influences to ensure the robustness of deployment.
\r\n\r\nThis research demonstrates that the interaction between the structure and the deployment mechanism must be minimal to ensure the predictability of deployment, as thin-shell structures can self-deploy using stored strain energy. This study's sensitivity analysis will inform the design of future SSPP deployment mechanisms and structures. Additionally, the numerical simulation techniques developed have broader applicability beyond this specific case study to any deployable thin-shell structure.
\r\n\r\nDue to the large aspect ratios of thin-shell structures, a very fine finite element mesh is required to model them accurately. A dense finite element mesh is also required to model the contact interactions between the structure and the rigid components of the deployment mechanism. As large spacecraft structures become increasingly complex, full-scale numerical modeling becomes impractical, necessitating the search for more computationally efficient finite element methods. In this study, NURBS-based isogeometric analysis is explored, and it is shown that it is not yet worth switching to NURBS-based elements for the analysis of thin-shell deployable structures. In addition, h-adaptive meshing for quadrilateral shell elements is investigated, and more efficient refinement indicators and solution mapping techniques for nonlinear analyses are proposed and their superior performance is demonstrated using a test case of quasi-static folding of a tape spring.
\r\n\r\nThis thesis fills a gap in the literature on unconstrained dynamic deployment of space structures, providing crucial insights and numerical modeling tools for further research. It establishes a knowledge and resource foundation to advance space structure design and promote more frequent use of unconstrained deployment, marking a pivotal contribution to the field and enabling safe and efficient space structure deployment. Furthermore, the study provides insights into more computationally efficient finite element methods, such as h-adaptive meshing. These insights are broadly applicable and can inform the design of future deployable structures beyond the tested cases.
" }, { "name": "Renn, Peter Ian James", "degree": "PhD", "year": "2023", "title": "Applied Machine Learning for Prediction and Control of Fluid Flows", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:02272023-230531120", "creators": [ { "name": { "family": "Renn", "given": "Peter Ian James" }, "id": "Renn-Peter-Ian-James", "orcid": "0000-0002-5735-3873", "display_name": "Renn, Peter Ian James" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Bae", "given": "H. Jane" }, "id": "Bae-H-J", "orcid": "0000-0001-6789-6209", "role": "chair", "display_name": "Bae, H. Jane" }, { "name": { "family": "Anandkumar", "given": "Anima" }, "id": "Anandkumar-A", "orcid": "0000-0002-6974-6797", "role": "member", "display_name": "Anandkumar, Anima" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/smnv-tz73", "abstract": "Modern aerodynamic technologies such as unmanned aerial systems and horizontal axis wind turbines must regularly contend with forces from highly stochastic and turbulent atmospheric gusts. Conventional methods for modeling and controlling fluid flows are limited in their ability to mitigate these aerodynamic forces in real-time. By applying modern machine learning techniques in an experimental setting, this thesis demonstrates the utility of machine learning in addressing these important problems. We follow two complementary approaches towards this goal.
\r\n\r\nFirst, we find an end-to-end solution for control in a gusty environment with model-free reinforcement learning. We deploy state-of-the-art reinforcement learning algorithms on a generalized aerodynamic test-bed consisting of an airfoil with motorized trailing edge flaps. The system features embedded flow sensors, enabling the inclusion of flow measurements in state observations. We place this system in a highly irregular wake behind a bluff-body, dynamically mounted on elastic bands and therefore free to oscillate, and train reinforcement learning agents to minimize the net lifting force on the system by controlling the position of the trailing edge flaps. We find that model-free reinforcement learning agents can outperform basic linear controllers in this gusty, turbulent environment. We also show that augmenting state observations with flow measurements can lead to more consistent learning of the system dynamics.
\r\n\r\nNext, we explore Fourier neural operators (FNOs) as a method for forecasting the time evolution of turbulent fluid flows. FNOs are capable of learning underlying operator solutions to families of partial differential equations and can be evaluated in just milliseconds. We specifically focus on training FNOs with experimentally measured velocity fields of bluff body wakes in the subcritical regime. To the best of our knowledge, this is the first application of operator learning for fluid mechanics that features experimental measurements. We find that FNOs can accurately predict the evolution of these turbulent wakes even when trained with imperfect measurements. We then show that FNOs can quickly adapt to unseen conditions with minimal data and training through transfer learning. Finally, we consider the performance of FNOs over longer prediction horizons. This approach could enable real-time gust prediction capabilities and monitoring for applied aerodynamic systems.
" }, { "name": "Tsukamoto, Hiroyasu", "degree": "PhD", "year": "2023", "title": "Contraction Theory for Robust Learning-Based Control: Toward Aerospace and Robotic Autonomy", "advisor": "Chung, Soon-Jo", "url": "https://resolver.caltech.edu/CaltechTHESIS:05262023-141116640", "creators": [ { "name": { "family": "Tsukamoto", "given": "Hiroyasu" }, "id": "Tsukamoto-Hiroyasu", "orcid": "0000-0002-6337-266", "display_name": "Tsukamoto, Hiroyasu" } ], "advisors": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "advisor", "display_name": "Chung, Soon-Jo" } ], "committee": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "chair", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Doyle", "given": "John C." }, "id": "Doyle-J-C", "orcid": "0000-0002-1828-2486", "role": "member", "display_name": "Doyle, John C." }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Watkins", "given": "Michael M." }, "id": "Watkins-M-M", "role": "member", "display_name": "Watkins, Michael M." }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" } ], "option_major": [ "space" ], "doi": "10.7907/rznp-g568", "abstract": "Machine learning and AI have been used for achieving autonomy in various aerospace and robotic systems. In next-generation research tasks, which could involve highly nonlinear, complicated, and large-scale decision-making problems in safety-critical situations, however, the existing performance guarantees of black-box AI approaches may not be sufficiently powerful. This thesis gives a mathematical overview of contraction theory, with some practical examples drawn from joint projects with NASA JPL, for enjoying formal guarantees of nonlinear control theory even with the use of machine learning-based and data-driven methods. This is not to argue that these methods are always better than conventional approaches, but to provide formal tools to investigate their performance for further discussion, so we can design and operate truly autonomous aerospace and robotic systems safely, robustly, adaptively, and intelligently in real-time.
\r\n\r\nContraction theory is an analytical tool to study differential dynamics of a non-autonomous (i.e., time-varying) nonlinear system under a contraction metric defined with a uniformly positive definite matrix, the existence of which results in a necessary and sufficient characterization of incremental exponential stability of multiple solution trajectories with respect to each other. Its nonlinear stability analysis boils down to finding a suitable contraction metric that satisfies a stability condition expressed as a linear matrix inequality, resulting in many parallels drawn between linear systems theory and contraction theory for nonlinear systems. This yields much-needed safety and stability guarantees for neural network-based control and estimation schemes, without resorting to a more involved method of using uniform asymptotic stability for input-to-state stability. Such distinctive features permit the systematic construction of a contraction metric via convex optimization, thereby obtaining an explicit exponential bound on the distance between a time-varying target trajectory and solution trajectories perturbed externally due to disturbances and learning errors. The first two parts of this thesis are about a theoretical overview of contraction theory and its advantages, with an emphasis on deriving formal robustness and stability guarantees for deep learning-based 1) feedback control, 2) state estimation, 3) motion planning, 4) multi-agent collision avoidance and robust tracking augmentation, 5) adaptive control, 6) neural net-based system identification and control, for nonlinear systems perturbed externally by deterministic and stochastic disturbances. In particular, we provide a detailed review of techniques for finding contraction metrics and associated control and estimation laws using deep neural networks.
\r\n\r\nIn the third part of the thesis, we present several numerical simulations and empirical validation of our proposed approaches to assess the impact of our findings on realizing aerospace and robotic autonomy. We mainly focus on the two joint projects with NASA JPL: 1) Science-Infused Spacecraft Autonomy for Interstellar Object Exploration and 2) Constellation Autonomous Space Technology Demonstration of Orbital Reconfiguration (CASTOR), where we also perform hardware demonstrations of our methods using our thruster-based spacecraft simulators (M-STAR) and in high-conflict, distributed, intelligent UAV swarm reconfiguration with up to 20 UAVs (crazyflies).
" }, { "name": "Ubamanyu, Kanthasamy (Uba)", "degree": "PhD", "year": "2023", "title": "Time-Dependent Failure of Thin-Ply Composite Laminates", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312023-210300139", "creators": [ { "name": { "family": "Ubamanyu", "given": "Kanthasamy (Uba)" }, "id": "Ubamanyu-Kanthasamy-Uba", "orcid": "0000-0002-3679-6173", "display_name": "Ubamanyu, Kanthasamy (Uba)" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/x286-g488", "abstract": "The demand for larger and lighter structures for next-generation space designs necessitates the use of deployable structures. Among the materials that hold promise for such applications, thin-laminate fiber composites with thicknesses less than 200 μm stand out due to their strength-to-weight ratio, packaging efficiency, and ability to deploy using stored strain energy. However, designing deployable structures with thin-laminate composites is challenging as they need to be stiff enough to withstand loads during deployment while also having a small volume in the packaged configuration. Complicating matters further, stress relaxation of the polymer matrix within the composite during long-term stowage in response to an imposed curvature can drastically impact both the deployment process and the performance of the structure in its deployed state, even leading to catastrophic failure in the stowed configuration.
\r\n\r\nThis thesis presents a comprehensive study of the time-dependent failure behavior of thin-laminate fiber composites under bending, with a focus on a fundamental material-level understanding. The work is divided into three main parts. First, a novel test method called Flattening to Rupture (FTR) test was developed to effectively load composite coupons under long-term bending, enabling the measurement of time-dependent rupture and identification of the underlying failure mechanisms. Second, numerical simulations using the Abaqus/Standard finite element software were developed to understand the sequence of rupture events and the influence of several parameters that affect time-dependent rupture. Finally, a statistical approach was proposed to model the stochastic nature of the failure of thin composite laminates.
\r\n\r\nThe contributions of this thesis extend the understanding of the microscale failure mechanisms involved in the time-dependent failure of fiber composites. These new insights pave the way for the efficient design of tightly and safely packaged deployable structures under long-term loading. The findings of this research can be utilized to optimize the design and performance of deployable space structures made of fiber composites, leading to new technologies that can advance space exploration.
" }, { "name": "Wei, Nathaniel James", "degree": "PhD", "year": "2023", "title": "Dynamics and Performance of Wind-Energy Systems in Unsteady Flow Conditions", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012023-233342281", "creators": [ { "name": { "family": "Wei", "given": "Nathaniel James" }, "id": "Wei-Nathaniel-James", "orcid": "0000-0001-5846-6485", "display_name": "Wei, Nathaniel James" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/d9wh-pj98", "abstract": "Wind energy is poised to play a considerable role in the global transition to clean-energy technologies within the next few decades. Modern wind turbines, like aircraft and other aerodynamic structures, are typically designed with the assumption that the flows they encounter will be uniform and steady. However, atmospheric flows are highly unsteady, and systems operating within them must contend with gust disturbances that can lead to performance losses and structural damage. Therefore, the next generation of wind-energy systems requires physics-informed design principles that effectively account for and even leverage these unsteady flow phenomena for enhanced power generation, robustness, and operational longevity. Accordingly, this work details experimental and analytical efforts to characterize unsteady aerodynamics in wind-turbine contexts. First, the effects of unsteady streamwise motion on turbine performance are studied, as recent work has suggested that these dynamics may enable time-averaged efficiencies that exceed the steady-flow Betz limit on turbine efficiency. The power production of and flow around a periodically surging wind turbine are thus investigated using wind-tunnel experiments, which suggest that turbines in these flow conditions could leverage unsteady surge motions for power-extraction gains of up to 6.4% over the stationary case. Linearized and nonlinear dynamical models of the response of the turbine to these time-varying flows are derived and validated against the experimental data. These models are also coupled with a potential-flow model of the upstream induction zone of the turbine in order to predict temporal variations in the flow velocities and pressures in this region. Unsteady contributions to the time-averaged efficiency are also considered through theoretical potential-flow derivations. Additionally, a novel three-dimensional particle-tracking velocimetry approach using artificial snow as seeding particles is deployed to obtain volumetric flow measurements in the wakes of full-scale vertical-axis wind turbines in field conditions. These measurements yield insights into the effects of unsteady vortex dynamics on the structure of the near wake, with implications for the performance of turbines in wind-farm arrays. These investigations provide the analytical and experimental foundations for future studies of unsteady atmospheric flows, and will lead to the development of principles and techniques for wind-farm siting, control, and optimization." }, { "name": "Dorn, Charles Jacob", "degree": "PhD", "year": "2022", "title": "Geometry Synthesis and Multi-Configuration Rigidity of Reconfigurable Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:09182021-045958776", "creators": [ { "name": { "family": "Dorn", "given": "Charles Jacob" }, "id": "Dorn-Charles-Jacob", "orcid": "0000-0001-6516-2586", "display_name": "Dorn, Charles Jacob" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "chair", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "space" ], "doi": "10.7907/ph2w-9a34", "abstract": "Reconfigurable structures are structures that can change their shapes to change their functionalities. Origami-inspired folding offers a path to achieving shape changes that enables multi-functional structures in electronics, robotics, architecture and beyond. Folding structures with many kinematic degrees of freedom are appealing because they are capable of achieving drastic shape changes, but are consequently highly flexible and therefore challenging to implement as load-bearing engineering structures. This thesis presents two contributions with the aim of enabling folding structures with many degrees of freedom to be load-bearing engineering structures.
\r\n\r\nThe first contribution is the synthesis of kirigami patterns capable of achieving multiple target surfaces. The inverse design problem of generating origami or kirigami patterns to achieve a single target shape has been extensively studied. However, the problem of designing a single fold pattern capable of achieving multiple target surfaces has received little attention. In this work, a constrained optimization framework is presented to generate kirigami fold patterns that can transform between several target surfaces with varying Gaussian curvature. The resulting fold patterns have many kinematic degrees of freedom to achieve these drastic geometric changes, complicating their use in the design of practical load-bearing structures.
\r\n\r\nTo address this challenge, the second part of this thesis introduces the concept of multi-configuration rigidity as a means of achieving load-bearing capabilities in structures with multiple degrees of freedom. By embedding springs and unilateral constraints, multiple configurations are rigidly held due to the prestress between the springs and unilateral constraints. This results in a structure capable of rigidly supporting finite loads in multiple configurations so long as the loads do not exceed some threshold magnitude. A theoretical framework for rigidity due to embedded springs and unilateral constraints is developed, followed by a systematic method for designing springs to maximize the load-bearing capacity in a set of target configurations. An experimental study then validates theoretical predictions for a linkage structure. Together, the application of geometry synthesis and multi-configuration rigidity constitute a path towards engineering reconfigurable load-bearing structures.
" }, { "name": "Dougherty, Christopher John", "degree": "PhD", "year": "2022", "title": "On the Experimental Simulation of Atmospheric-Like Disturbances Near the Surface", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272022-085410375", "creators": [ { "name": { "family": "Dougherty", "given": "Christopher John" }, "id": "Dougherty-Christopher-John", "orcid": "0000-0002-0974-5696", "display_name": "Dougherty, Christopher John" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Brown", "given": "Garry L." }, "id": "Brown-Garry-L", "role": "member", "display_name": "Brown, Garry L." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/293w-ev66", "abstract": "Any and every 'decision-maker'\u2019 gravity-bound to the planetary surface (or very nearly so) must contend with the frictional complexities confined to its relatively small surface layer. From the perspective of the near-surface bound small autonomous flyer, it is the microclimatic local set of atmospheric conditions (i.e. the weather), characterized by moisture, temperature, and the parameters describing wind, that determines the baseline flowfields within which these flyers must navigate and negotiate. Unlike their human-on-board counterparts, mission parameters relegate small (nearly) massless autonomous flyers to the lower regions of the atmospheric boundary layer, where they may not be fortuned enough to soar above the effects of friction or wait for clearer skies. Relatively little focus has been placed on the experimental strategies of how these machines might learn to function in challenging scenarios well-before encountering them in the real-world. To address such shortcomings, this work focuses on the experimental simulation of flight-relevant environments through the development of multi-source wind generating apparatuses (i.e. fan arrays) that can initialize velocity distributions discretely-individually or in-concert to produce appropriate mean and fluctuating velocities through an ample open-air test envelope that enables full-scale conventional statically-mounted aerodynamic-characterizations up through free-flight autonomous vehicle testing. Though outside the scope of current experimental work, as full of an environmental description (i.e. moisture, temperature, and wind) is given as possible, prior to ultimately reducing the scope to a neutrally stable atmosphere devoid of any major weather events other than a reasonably strong prevailing wind. Nearly always set amongst the backdrop of a high Reynolds number turbulent flowfield, two primary prototypical flowfields (continuous-gust and discrete-gust) are identified as meriting consideration for mainstay experimental simulation. The core features within the spectral overlap of these windy disturbance environments with the response characteristics of flyers of interest ensure that the turbulence of consideration is nearly always of the mechanical-type. Unlike air motions far above local effects in the inertial sublayer (ISL), the dominant flow mechanism within regions of interest near canopied surfaces is augmented by the presence of coherent structures due to the prevalence of locally initiated mixing layers and wakes such that the task becomes one of simulation of suitable forcing spectra in the physical domain for the regions of interest during anticipated times-of-flight.
\r\n\r\nLikely to prove challenging to the small autonomous flyer are encounters of a change in wind state that occur upon piercing the dividing streamline of air masses of two different velocities. From the view of the flyer navigating the built-up environment, intermittent free shear layers due to wind-interactions with surface roughness elements are unavoidable and are experienced discretely when the flyer and shear layer dynamics are decoupled. Fan array techniques for the generation of mixing layers, the basic building block of any such free shear layer, is explored as a candidate flowfield for the experimental simulation of a discrete gust forcing input for the flyer near the surface. Both initialized dual-stream and triple-stream mixing layers at flight-relevant freestream velocity differences are explored and found to principally behave like the mixing layers developed in a more conventional splitterplate experiment. The Reynolds number Reδω based on the velocity difference ΔU and vorticity thickness δω (both outer scale parameters) is shown to linearly increase with downstream development as the vorticity thickness increases commensurately. The spectral analysis along the centerline confirms local isotropy for every tested case.
\r\n\r\nThe continuous-gust flowfield (simply referred to as 'turbulence) is prevalent throughout the atmospheric boundary layer as are quasi-coherent flowfields of superimposed wakes within canopied environments. Because velocity fluctuations manifest as (predominantly) random deviations at any given instant, these flowfields are good candidates for statistical analysis. Generation techniques to produce such turbulent flowfields are introduced and compared against the uniform flow modality (i.e. all fan units set to produce nominally the same initial velocity condition to develop a well-mixed turbulent flowfield beyond x/L \u223c 0.5 with ReλT = 135). The random-phase (R-P) perturbation technique proves useful in increasing ReλT upwards of nearly sevenfold with only a slight further-loss-of-uniformity (to within 3.7% of the mean). The uniform flow modality with the (R-P) perturbation activated is shown, through the presence of a -5/3 slope power law region, to be locally isotropic at relevant freestream velocities. Significant increases in ReλT are made through a static-reconfiguring of the discrete source fan units into a so called quasi-grid (Q-G) configuration. The highest recorded Taylor microscale Reynolds number was found to be ReλT = 2700, likely accompanied by a non-negligible loss of uniformity at the fixed measurement location, though traverses were not undertaken during this campaign so no direct statement of homogeneity is put forth.
\r\n\r\nFor all the flow modalities presented (i.e uniform, pseudo-random, quasi-coherent, and mixing layer), the high-Re number criteria (Reδω \u2248 104 , ReλT \u2248 102) has been met. This serves, then, as a necessary minimum benchmark in the development of multi-source wind tunnels with intended use as environmental simulators for flyers near the surface and also provides the basis for a spectral framework of comparison to enable systematic development of flowfields in future work. Characteristics of the evolving flowfields can further be tuned through the introduction of perturbation techniques applied as initial conditions to both increase the standard deviation of the fluctuating velocities about a desired mean as well as to initiate, evolve, and combine flowfields in representative ways. A preliminary example of one such combination of flow modalities (pseudo-random and mixing layer) indicates significant alteration of flow development compared to a nominal mixing layer case.
" }, { "name": "Hooper, Morgan Louise", "degree": "PhD", "year": "2022", "title": "Characterization and Optimization of a Fully Passive Flapping Foil in an Unsteady Environment for Power Production and Propulsion", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312022-024822211", "creators": [ { "name": { "family": "Hooper", "given": "Morgan Louise" }, "id": "Hooper-Morgan-Louise", "display_name": "Hooper, Morgan Louise" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Brunton", "given": "Steven L." }, "id": "Brunton-S-L", "role": "member", "display_name": "Brunton, Steven L." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/tymc-1985", "abstract": "This thesis provides an experimental window into the duality between thrust production and energy harvesting by a flapping foil subject to unsteadiness in an oncoming flow. In particular, an airfoil is placed downstream of a circular cylinder, and allowed to interact with the vorticity shed in its wake to produce motions in both the transverse and streamwise directions. It is confirmed that under the right conditions, passive fluid-structure interactions arising from such a configuration give rise to simultaneous extraction of energy from the flow, coupled with net thrust larger than net drag experienced by the airfoil.
\r\n\r\nMeasurements of forces acting on the airfoil and the motion that arises are presented, for cases where the flapping motion is both active (the foil is driven through a pre-planned trajectory) and fully passive (the foil is allowed to react to the fluid forcing it experiences). These are coupled with simultaneous Particle Image Velocimetry (PIV) measurements of the flow field in the region of the airfoil. These measurements allow for direct observation of fluid-structure interactions which give rise to both thrust production and power extraction potential, illuminating the mechanisms driving each. The dynamics of a fully passive flapping foil are largely determined by the mounting system used to facilitate its motion. It is shown that by leveraging Cyber-Physical Fluid Dynamics (CPFD) capabilities to tune these mounting parameters, the behaviour of a fully passive flapping foil can be made similar to that of a representative driven system. A framework based on a simplified linear model for mounting system dynamics is presented, to allow for the optimization of such a system for power extraction potential subject to relevant engineering constraints. The effects of nonlinearity on airfoil behaviour, particularly those due to friction in the mechanism(s) permitting passive flapping, are also explored. Finally, two-dimensional motion of a fully passive flapping foil is demonstrated, allowing for the foil to travel upstream against the oncoming flow solely due to forces induced by interactions with oncoming unsteadiness.
" }, { "name": "Song, Yichuan", "degree": "PhD", "year": "2022", "title": "Rheological Measurements in Moderate Reynolds Number Liquid-Solid Flows", "advisor": "Hunt, Melany L.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06062022-033735914", "creators": [ { "name": { "family": "Song", "given": "Yichuan" }, "id": "Yichuan-Song", "orcid": "0000-0001-7276-2029", "display_name": "Song, Yichuan" } ], "advisors": [ { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "advisor", "display_name": "Hunt, Melany L." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Brady", "given": "John F." }, "id": "Brady-J-F", "orcid": "0000-0001-5817-9128", "role": "member", "display_name": "Brady, John F." }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "member", "display_name": "Hunt, Melany L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5ey8-v324", "abstract": "Liquid-solid flows with inertial and viscous effects are critical for many engineering and geophysical applications, such as the processing of biomass slurry and the control of debris flows. However, modeling the rheological behaviors of these complex flows remains a challenge. Prior investigations on the liquid-solid flows typically cover suspensions in which the particle Reynolds numbers (Re) based on the particle diameter and shear rate are less than 1. Limited prior study at Caltech focuses on particle Reynolds numbers above 10. This thesis focuses on rheological experiments for the moderate Reynolds number regime where both inertial and viscous effects are important, with particle Reynolds numbers from 0.5 to 800. The rheological experiments include torque measurements of mm scale-sized polystyrene and SAN particles with a range of solid fractions from 10% to 50%, considering both neutrally-buoyant and settling suspensions with density ratios of 1 and 1.05. This thesis discusses rheological measurements of three different fields: pure fluids, neutrally-buoyant suspensions, and non-neutrally-buoyant suspensions.
\r\n\r\nThe pure fluids measurements determine the flow starts to transition to turbulent flow for gap Reynolds numbers above 6500 in the Caltech Couette flow device. For suspensions with matched particle and fluid densities and solid fractions less than 40%, we find that the effective viscosity only depends on the particle solid fraction until we observe the shear-thickening behaviors for Re of approximately 10. For the intermediate Re from 10 to 100 and lower solid fractions, the effective viscosity not only depends on the particle solid fraction, but also shows increased dependence on Re. For Re greater than 100, the liquid-solid flows transition to the turbulent regime, similar to what we see for the pure fluids. At the maximum solid fraction of 50%, the magnitude of the effective viscosity has increased by a factor of 20 as compared to the results of the 10% solid fraction, but the effective viscosity is nearly independent of Re. A particle Reynolds number (Re') based on the maximum shear flow velocity and the particle diameter is introduced to examine the effective viscosity of the suspensions. Since the present studies use particles with different sizes, Re' is found to be a better way to correlate the effective viscosity than the traditional Re. For the analysis of liquid-solid flows with a density ratio of 1.05, the effective viscosity of the particulate flow increases with the Stokes number for loading fractions of 10% and 20%, while the dependence is reversed for higher solid fractions.
" }, { "name": "Jones, Silken Michelle", "degree": "PhD", "year": "2021", "title": "Thermal Ignition by Vertical Cylinders", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12182020-055522985", "creators": [ { "name": { "family": "Jones", "given": "Silken Michelle" }, "id": "Jones-Silken-Michelle", "orcid": "0000-0003-3496-7191", "display_name": "Jones, Silken Michelle" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverly J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverly J." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9g5j-2b97", "abstract": "Accidental thermal ignition events present a significant hazard to the aviation industry. There is scarcity of experimental data on ignition by external natural convection flows for surface areas larger than 10 cm\u00b2. In this work, thermal ignition of external natural convection flows by vertical cylinders is investigated. The effect of geometry is studied by resistively heating stainless steel cylinders of various sizes in a stoichiometric n-hexane and air mixture at 298 K and 1 bar. Cylinder lengths range from 12.7 to 25.4 cm, and cylinder surface areas vary from 25 to 200 cm\u00b2. Logistic regression is used to provide statistical information about the ignition threshold (50% probability of ignition). The maximum ignition threshold found is 1117 K for a cylinder 12.7 cm long and 50 cm\u00b2 in surface area. The minimum ignition threshold found is 1019 K for a cylinder 25.4 cm long and 200 cm\u00b2 in surface area. The maximum uncertainty on these ignition thresholds is \u00b129 K, which comes from the maximum uncertainty on the pyrometer measurement used to record cylinder surface temperatures. \r\nThe dependence of ignition threshold on both surface area and length of a cylinder is found to be minor. High speed visualizations of ignition indicated that ignition occurs near the top edge of all cylinders.
\r\n \r\nThe entire experimental setup is heated to allow for ignition tests with multi-component, heavy-hydrocarbon fuels including Jet A and two surrogate fuels, Aachen and JI. The cylinder used for all testing of heavier fuels is 25.4 cm long and 200 cm\u00b2 in surface area. Hexane is also tested with the heated vessel to investigate the effect of ambient temperature on ignition. At an ambient temperature of 393 K, the ignition threshold of hexane is 933 K. Aachen has an ignition threshold of 947 K at an ambient temperature of 373 K. JI has an ignition temperature of 984 K at an ambient temperature of 393 K. Jet A has an ignition temperature of 971 K at an ambient temperature of 333 K. The maximum uncertainty on these thresholds is \u00b129 K. JI is found to be the most appropriate surrogate for Jet A.
\r\n \r\nFrom the experiments, two main conclusions are reached. Ignition threshold temperatures in external natural convection flows are very weakly correlated with surface area. The observed ignition thresholds do not show the drastic transition of ignition temperature with surface area that is observed in internal natural convection situations. Observed ignition thresholds for comparable surface areas (100 to 200 cm\u00b2) are 500 to 600 K higher for external natural convection than internal natural convection. Hexane was found to be a reasonable surrogate for Jet A (38 K difference in ignition threshold) in external natural convection ignition testing. The more complex multi-component JI surrogate, while having an ignition threshold more comparable to Jet A (13 K difference in ignition threshold), requires heating the experimental apparatus and associated difficulties of fuel handling as well as the soot generation by combustion.
\r\n \r\nTwo simplified models of ignition are explored. The first is an investigation of ignition chemistry using a zero-dimensional reactor and a detailed kinetic mechanism for hexane. The temperature history of the reactor is prescribed by an artificial streamline whose rate of temperature increase is parametrically varied. The results from the zero-dimensional reactor computation reveal that a gradually heated streamline exhibits two-stage ignition behavior, while a rapidly heated streamline only experiences one ignition event. The second model of ignition is a one-dimensional simulation of ignition adjacent to a cylinder at a prescribed temperature. The formulation included diffusion of species and thermal energy as well as chemical reaction and employed Lagrangian coordinates. The chemistry is modeled with a reaction mechanism for hydrogen to reduce numerical demand. Heat flux and energy balance are analysed to gain insight into the ignition dynamics. Initially, heat transfer is from the wall into the gas, and a mostly nonreactive thermal boundary layer develops around the cylinder. As reaction in the gas near the surface begins to release energy, the heat transfer decreases, and, near the critical temperature for ignition, the direction of heat flux reverses and is from the gas into the wall. In a case where ignition takes place, there is rapid rise in temperature in the gas within the thermal layer, and a propagating flame is observed to emerge into surrounding cold gas. The heat transfer from the hot combustion products results in a continuous heat flux from the gas into the wall. In a case where ignition does not take place, no flame is observed and the heat flux at the wall is slightly positive. For the critical condition just below the ignition threshold, a balance between energy release and diffusion in the adjacent gas results in a small temperature rise in the thermal layer, but a propagating flame is not created. The Van't Hoff ignition criterion of vanishing heat flux at the ignition threshold is approximately but not exactly satisfied. Contrasting the two modeling ideas, we observe that modeling adiabatic flows along computed nonreactive streamlines is useful in examining the role of detailed chemistry but lacks important diffusion effects. Including mass and thermal transport provides more insight into important ignition dynamics but comes at the expense of increased computational complexity.
" }, { "name": "Lawson, Joel Michael", "degree": "PhD", "year": "2021", "title": "Focused Laser Differential Interferometry", "advisor": "Austin, Joanna M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05132021-180953405", "creators": [ { "name": { "family": "Lawson", "given": "Joel Michael" }, "id": "Lawson-Joel-Michael", "orcid": "0000-0002-3042-0909", "display_name": "Lawson, Joel Michael" } ], "advisors": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "advisor", "display_name": "Austin, Joanna M." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5thh-f652", "abstract": "The focused laser differential interferometer (FLDI) is a non-imaging optical diagnostic that is sensitive to density disturbances. A distinguishing feature is reduced sensitivity away from the focal plane of its beams. The spatial resolution is sub-mm, and the temporal resolution is restricted only by photodetector bandwidth, typically >10 MHz. These traits make FLDI particularly suited to measurements in hypervelocity ground-testing facilities, where the low densities, short time-scales, and harsh environments preclude the use of intrusive diagnostics. Line of sight integration issues associated with other optical techniques are therefore minimized, a distinct advantage for measurements in impulse facilities, where the core flow of interest is often surrounded by highly-turbulent shear layers.
\r\n\r\nThe systematic design principles for single and double FLDI systems are discussed, based on ray transfer matrix analysis combined with Gaussian optics. A detailed guide is presented for the practicalities of aligning, calibrating, and operating an FLDI.
\r\n\r\nA modular numerical implementation of Schmidt and Shepherd's FLDI ray-tracing model is developed, capable of accepting arbitrary flow-fields defined via analytical expressions, simulation coupling, or experimental datasets. This numerical implementation is used to perform the first comprehensive experimental validation of the model, using known static and dynamic phase objects. Quantitatively-accurate predictions of the response of real FLDI systems are obtained. Importantly, the spatial sensitivity of the instrument is found to be dependent on disturbance wavelength, with scaling matching that predicted analytically from the model. Propagating shock waves are used as another highly-dynamic test phase object, and it is shown that FLDI maintains its theoretical performance at sub-μs time-scales.
\r\n\r\nThe validated ray-tracing model is used to develop analytical expressions for the response of FLDI to propagating plane waves, extending on the results of Schmidt and Shepherd, and Settles and Fulghum. For the first time, the inverse problem is solved for this class of flow-field, allowing the density fluctuation spectrum to be recovered quantitatively from FLDI phase shift data. This approach is validated using synthetic flow-fields with the numerical ray-tracing scheme, and is also compared with the approximate approach introduced by Parziale et al.
\r\n\r\nFLDI is used to make freestream density fluctuation measurements on two facilities: a conventional blowdown tunnel, and an expansion tube. On the conventional tunnel, a comparison is made between pitot-probe and FLDI measurements after converting both to freestream pressure fluctuation spectra. A modification of Stainback and Wagner's theory, incorporating recent numerical results from Chaudhry et al., is used to interpret the pitot data, while the new inversion algorithm is applied to the FLDI data. Close agreement is found between the two sets of spectra, showing that accurate quantitative data can be obtained with FLDI, and used to extend spectra beyond the pitot bandwidth.
\r\n\r\nOn the expansion tube, the theory of Paull and Stalker for freestream noise originating in the driver gas is investigated. Their proposed relationship between freestream density fluctuations and the primary interface sound speed ratio is not observed. Spectral banding is also absent, however this is expected due to the relatively low secondary expansion strengths. The envelope of accessible conditions is somewhat restricted due to the low mean freestream densities that lead to signal-to-noise issues.
\r\n\r\nSignificant performance improvements can still be made to FLDI, in terms of its noise and bandwidth limitations, and to the spatial localization of its sensitive region; suggestions are given for possible approaches. With the ray-tracing model now validated, it can be used to optimize FLDI, or even to suggest derivative instruments based on similar principles.
" }, { "name": "Nakka, Yashwanth Kumar", "degree": "PhD", "year": "2021", "title": "Spacecraft Motion Planning and Control under Probabilistic Uncertainty for Coordinated Inspection and Safe Learning", "advisor": "Chung, Soon-Jo", "url": "https://resolver.caltech.edu/CaltechTHESIS:05142021-163257155", "creators": [ { "name": { "family": "Nakka", "given": "Yashwanth Kumar" }, "id": "Nakka-Yashwanth-Kumar", "orcid": "0000-0001-7897-3644", "display_name": "Nakka, Yashwanth Kumar" } ], "advisors": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "advisor", "display_name": "Chung, Soon-Jo" } ], "committee": [ { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "chair", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "member", "display_name": "Yue, Yisong" }, { "name": { "family": "Hadaegh", "given": "Fred" }, "id": "Fred-Hadaegh", "role": "member", "display_name": "Hadaegh, Fred" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" } ], "option_major": [ "space" ], "doi": "10.7907/6329-sf68", "abstract": "During a spacecraft mission design process, engineers often balance the following three criteria: science return, optimality in performance, and safety. Given a science criterion, engineers design the orbit parameters with predefined performance and safety. Often in this approach, the spacecraft has no understanding of the expected outcome or the knowledge of the mission safety criteria. Autonomous science-driven orbit (or goal) selection and planning for safety under uncertainty enable efficient and adaptable missions. To this end, we propose an architecture for information-based guidance and control for coordinated inspection, motion planning and control algorithms for safe and optimal guidance under uncertainty, and architecture for safe exploration.
\r\n\r\nIn the first part of this thesis, we present an architecture for inspection or mapping of a target spacecraft in a low Earth orbit using multiple observer spacecraft. We use an information gain approach to directly consider the trade-off between gathered data and fuel/energy cost. The estimated information gain is a crucial input to the motion planner, which computes orbits and reconfiguration strategies for each of the observers to maximize the information gain from distributed observations of the target spacecraft. The resulting motion trajectories jointly consider observational coverage of the target spacecraft and fuel/energy cost. We validate our architecture in a mission simulation to visually inspect the target spacecraft and on the three degree-of-freedom robotic spacecraft dynamics simulator testbed.
\r\n\r\nIn the second part of the thesis, we present gPC-SCP, Generalized Polynomial Chaos-based Sequential Convex Programming method, to compute a sub-optimal solution for a continuous-time chance-constrained stochastic nonlinear optimal control (SNOC) problem. The approach enables motion planning and control of robotic systems under uncertainty. The proposed method involves two steps. The first step is to derive a deterministic nonlinear optimal control problem (DNOC) with convex constraints that are surrogate to the SNOC by using gPC expansion and the distributionally-robust convex subset of the chance constraints. The second step is to solve the DNOC problem using sequential convex programming (SCP) for trajectory generation and control. We prove that in the unconstrained case, the optimal value of the DNOC converges to that of SNOC asymptotically and that any feasible solution of the constrained DNOC is a feasible solution of the chance-constrained SNOC. We derive a stable stochastic model predictive controller using the gPC-SCP for tracking a potentially unsafe trajectory in the presence of uncertainty. We empirically demonstrate the efficacy of the gPC-SCP method for the following three test cases: 1) collision checking under uncertainty in actuation, 2) collision checking with stochastic obstacles, and 3) safe trajectory tracking under uncertainty in the dynamics and obstacle location by using a receding horizon control approach. We validate the effectiveness of the gPC-SCP method on the robotic spacecraft testbed.
\r\n\r\n\r\nIn the third part of this thesis, we present a new approach for optimal motion planning for safe exploration that integrates the chance-constrained stochastic optimal control with dynamics learning and feedback control. We derive an iterative convex optimization algorithm that solves an Information-cost Stochastic Nonlinear Optimal Control problem (Info-SNOC). The optimization objective encodes control cost for performance and exploration cost for learning, and the safety is incorporated as distributionally robust chance constraints. The dynamics are predicted from a robust regression model that is learned from data. The Info-SNOC algorithm is used to compute a sub-optimal pool of safe motion plans that aid in exploration for learning unknown residual dynamics under safety constraints. A stable feedback controller is used to execute the motion plan and collect data for model learning. We prove the safety of rollout from our exploration method and reduction in uncertainty over epochs, thereby guaranteeing the consistency of our learning method. We validate the effectiveness of Info-SNOC by designing and implementing a pool of safe trajectories for a planar robot. We demonstrate that our approach has a higher success rate in ensuring safety when compared to a deterministic trajectory optimization approach.
" }, { "name": "Pastor, Daniel", "degree": "PhD", "year": "2021", "title": "Towards Learning Robotic Dynamics: Application to Multirotor Takeoff and Landing", "advisor": "Burdick, Joel Wakeman", "url": "https://resolver.caltech.edu/CaltechTHESIS:03152021-082447788", "creators": [ { "name": { "family": "Pastor", "given": "Daniel" }, "id": "Pastor-Daniel", "orcid": "0000-0003-3452-0605", "display_name": "Pastor, Daniel" } ], "advisors": [ { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "advisor", "display_name": "Burdick, Joel Wakeman" } ], "committee": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "chair", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "member", "display_name": "Yue, Yisong" }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "display_name": "Burdick, Joel Wakeman" } ], "option_major": [ "space" ], "doi": "10.7907/199j-dk87", "abstract": "Multirotors have become widespread but their usage is still limited. Ensuring safety during take-off and landing is still an open problem. Towards this goal this thesis proposes two different solutions to address this problem. The two approaches complement each other and they are tested on hardware.
\r\n \r\nThe first approach is to design a vehicle that is stable during take-off, despite hardware failures or unsteady take-off platforms. A solution is to use a ballistic launch to impose a deterministic path, preventing collisions with its environment. Following this approach led to the development of several SQUID (Streamlined Quick Unfolding Investigation Drone) vehicles. The main challenges are the ballistic initial flight, large accelerations during launch, and limited volume. A first prototype was developed, which is able to transition mid-flight from stable ballistic flight to a fully controllable multirotor. The system has been fabricated and field tested from a moving vehicle up to 50mph to successfully demonstrate the feasibility of the concept and experimentally validate the design's aerodynamic stability and deployment reliability. A second prototype expanded the first one's capabilities incorporating fully-autonomous vision-based navigation, while keeping the ballistic passive stability and stable transition abilities. The new design includes a more reliable plate-based structure and more effective folding fins.
\r\n \r\nThe second approach focuses on designing controllers that are safe regardless of the platform. For that purpose, a Model Predictive Control (MPC) is used to ensure state and input constraints. Given the highly non-linear dynamics platforms and fast dynamics that require a quick controller evaluation, the work in this thesis is built using Koopman Operator theory, which allows tools from linear analysis to be applied to systems with inherently non-linear dynamics. One of the main contributions is a novel method to find Koopman Eigenfunctions directly from data. Another key contribution is an episodic approach to model non-linear actuation dynamics. The proposed method is first tested on simulation and it outperforms comparable approaches. The method is also demonstrated on-board a multirotor for a fast landing application, where the nonlinear ground effect is learned and used to improve landing speed and quality. An additional extension considers model uncertainty in the MPC architecture, where an Ensemble Kalman Sampler is used to learn the uncertainty distribution.
" }, { "name": "Pedivellano, Antonio", "degree": "PhD", "year": "2021", "title": "Deployment Dynamics of Thin-Shell Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012021-002457442", "creators": [ { "name": { "family": "Pedivellano", "given": "Antonio" }, "id": "Pedivellano-Antonio", "orcid": "0000-0003-2321-7301", "display_name": "Pedivellano, Antonio" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "space" ], "doi": "10.7907/4zbq-g037", "abstract": "Thin-shell structures provide a lightweight solution for deployable structure applications. Despite being only few tens of microns thick, these structures provide excellent bending stiffness, thanks to their curved cross-section. Their thinness also allows them to be elastically packaged into small volumes to fit into a launch vehicle; once in space, they can be self-deployed by releasing their stored elastic energy.
\r\n\r\nMost space applications use thin-shell structures to deploy and tension thin membranes, such as solar sails, drag sails, and solar arrays. Recently, a novel space solar power architecture has been developed at Caltech, and it relies on distributed thin-shell components, connected in a space frame, to create large-area deployable structures. Thanks to the unique properties of thin shells, these structure provide superior stiffness-to-mass ratio and self-deployment capabilities. However, to demonstrate their reliability and enable their use on space missions, their deployment dynamics must be understood and predicted.
\r\n\r\nGround testing is the established approach to verify a structure throughout its design and qualification process. However, replicating the space environment in a laboratory setting is generally not possible, especially for lightweight structures, which are very sensitive to the effects of gravity and air. Numerical models are therefore the only tool to predict the behavior of a structure in space. However, validation with ground experiments is necessary to build confidence in the models, which must be able to capture the complexity of the interaction with air, gravity, and the suspension system that supports the weight of the structure.
\r\n\r\nThe goal of this thesis is to develop high-fidelity models for large space structures, where multiple thin-shell components are folded together and deploy by releasing their strain energy. This overall objective is achieved in 3 steps. First, a ladder-type rectangular strip is introduced, as a building block for more complex architectures. The strip is composed by two thin-shell longerons, symmetrically folded at two locations. The deployment dynamics of this structure is investigated through experiments on 1 m-scale prototypes, both in air and in vacuum. A detailed analysis of its elastic folds is performed using full-field displacement measurements from Digital Image Correlation. A finite element model of this strip is presented, and it is shown to accurately capture the dynamics of the strip for all tested conditions. Then, the implementation of the packaging and deployment scheme of a space solar power spacecraft, composed of multiple strips, is discussed. A kinematic model of the structure is proposed as a design tool to achieve systematic folding. A novel concept of a deployment mechanism to coil the structure in a robust and reliable way is proposed. Also, a staged deployment scheme is demonstrated, to reduce the uncertainty of strain-energy deployment for large space structures. Finally, the deployment dynamics of a 2 m-scale space structural prototype, based on the space solar power architecture, is investigated. A full-scale finite element model of the structure is implemented to replicate its complex folding scheme and capture the deployment process, including the interaction with the deployment mechanism and the suspension system. The simulations predict well the behavior of the structure observed in experiments through motion capture techniques.
\r\n\r\nThe work presented in this thesis advances previous studies on the deployment dynamics of simple thin-shell components, and demonstrates that even complex thin-shell architectures can be packaged and deployed in a controlled and predictable way. The solutions proposed in this thesis have guided the packaging process and the design of the deployment mechanism for DOLCE, an upcoming flight demonstration of the space solar power architecture described in this work. However, this research has much broader implications, as the experimental and numerical framework presented herein can be generalized to different shell-based architectures, and contributes to enabling a new generation of lightweight deployable structures for future space applications.
" }, { "name": "Royer, Fabien A.", "degree": "PhD", "year": "2021", "title": "Probing the Buckling of Thin-Shell Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312021-185024653", "creators": [ { "name": { "family": "Royer", "given": "Fabien A." }, "id": "Royer-Fabien-A", "orcid": "0000-0003-2452-2893", "display_name": "Royer, Fabien A." } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/ksn2-t598", "abstract": "The overarching goal of the research presented in this dissertation is to apply and extend a newly developed methodology to understand the buckling of complex thin shell structures. This methodology enables the determination of tighter buckling criteria and paves the way to the development of more efficient structures, used closer than ever to their buckling load and even beyond. It would result in dramatically lighter structures to be built and has the potential to enable new applications, such as extremely large aperture satellites.
\r\n\r\nWe first analyze the stability of open section thin shell structures under a pure bending moment, through simulations. These structures are composed of longitudinal thin-shell elements connected transversely by thin rods, and inspired by real spacecraft structures. The present study applies and extends recent work on the stability of cylindrical and spherical shells. The role of localization in the buckling of these structures is investigated and early transitions into the post-buckling regime are unveiled using a probe that locally displaces the structure. The probing method enables the computation of the energy input needed to transition early into a post-buckling state, which is central to determining the critical buckling mechanism for the structure. We show that the structure follows stability landscapes also found in cylindrical and spherical shell buckling problems. This initial computational study is the basis for the first ever probing experiment on a complex structure.
\r\n\r\nIn order to test these new structures under bending, a new bending apparatus is designed and implemented. The boundary conditions are chosen such that the apparatus is statically determinate (isostatic), and no state of self stress can develop in the sample during its mounting and testing. This feature is especially desirable in the study of thin shell structures and their elastic instabilities, for which imperfection sensitivity plays a crucial role in the buckling transition and the post-buckling regime. The accuracy of the isostatic bending machine is first assessed through the testing of rods, and its imperfection insensitive behavior is then highlighted in experiments on tape springs, and through numerical studies of the same structures.
\r\n\r\nThe new bending machine is complemented by a probing apparatus, and the stability of the open section thin-shell structures subjected to a pure bending moment is studied experimentally. The experiment confirms that localization of deformations plays a paramount role in the structure's nonlinear post-buckling regime and is extremely sensitive to imperfections. This characteristic is investigated through probing experiments. The range of moments for which the early buckling of the structure can be triggered using this probe perturbation is determined, as well as the energy barrier separating the pre-buckling and post-buckling states. The stability of the local buckling mode is then illustrated by an experimental stability landscape of shell buckling, and probing is then extended to the entire structure to reveal alternate buckling modes disconnected from the structure's fundamental path. These results can be used to elaborate efficient buckling criteria for this type of structures, through the use of transition diagrams determined experimentally.
\r\n\r\nFinally, the buckling and post-buckling behavior of ultralight ladder-type coilable structures is investigated. These specific structures are used in the Space Solar Power Project at Caltech and are referred to as strips. Similarly to the previous studies, the stability of strip structures loaded by normal pressure is computationally studied by applying controlled perturbations through localized probing. The probing technique is generalized to higher-order bifurcations along the post-buckling path, and low-energy escape paths into buckling that cannot be predicted by a classical eigenvalue formulation are identified. It is shown that the stability landscape for a pressure-loaded strip is similar to the landscape for classical shells, and the open section thin shell structure studied initially in this thesis. While classical shell structures buckle catastrophically, strip structures feature a large stable post-buckling range. Probing enables the full characterization of the structure's unstable behavior, which paves the way to extend its operation closer than ever to the buckling load, and even in the post-buckling regime. It would enable the design of more efficient structures by dramatically reducing their mass, therefore enabling new large spacecraft to be built.
" }, { "name": "Shamai, Maysam", "degree": "PhD", "year": "2021", "title": "Complexity Reduction of Fluid-Structure Systems at Low Forcing Frequencies", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282021-215050622", "creators": [ { "name": { "family": "Shamai", "given": "Maysam" }, "id": "Shamai-Maysam", "orcid": "0000-0002-1099-1456", "display_name": "Shamai, Maysam" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Plesniak", "given": "Michael W." }, "id": "Plesniak-M-W", "orcid": "0000-0001-6575-5074", "role": "member", "display_name": "Plesniak, Michael W." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/rhs5-yq49", "abstract": "This thesis addresses complexity reduction in periodic fluid-structure systems at low forcing frequencies. A novel quasi-steady time scaling framework is developed to relate the dynamics of a forced system to a corresponding unforced system.
\r\n\r\nParticle Image Velocimetry and dye flow visualization are used to study the streamwise-oscillating cylinder's wake at a mean Reynolds number of 900. Forcing frequencies both one and two orders of magnitude below the stationary shedding frequency are considered. Forcing amplitudes are such that the instantaneous Reynolds number remains above the critical value at all times. It is shown that this forcing regime is synonymous with the development of both frequency and amplitude modulation in the wake. While frequency modulation is linked to vortex shedding, amplitude modulation arises due to symmetric reorganization of the wake at certain phases in the forcing cycle. Furthermore, Dynamic Mode Decomposition is used to extract underlying flow structures and quasi-steady time scaling is employed to relate dynamics to the corresponding unforced system. Specifically, forcing regimes where quasi-steady shedding can develop are identified and time is scaled to transform the system to resemble the stationary cylinder at the same mean Reynolds number.
\r\n\r\nExperimental flowfields are also used to analyze the wake of a surface mounted hemisphere subject to a highly pulsatile freestream, characterized by a forcing amplitude equal to the mean. Although this flow sees regular shedding of hairpin vortices in the unforced case, pulsatile forcing leads to significant deviations. For a nominal mean Reynolds number of 1000, analysis of the wake shows that forcing at a frequency much smaller than that associated with hairpin shedding can lead to frequency modulated shedding. Consequently, time scaling is employed to reduce system complexity associated with hairpin shedding and to relate wake dynamics to the analogous unforced system.
" }, { "name": "Shen, Naijian", "degree": "PhD", "year": "2021", "title": "Part I: The Equations of Plasma Physics and the Richtmyer-Meshkov Instability in Magnetohydrodynamics. Part II: Evolution of Perturbed Planar Shockwaves.", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:10042020-174941725", "creators": [ { "name": { "family": "Shen", "given": "Naijian" }, "id": "Shen-Naijian", "orcid": "0000-0002-0533-8081", "display_name": "Shen, Naijian" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Bellan", "given": "Paul Murray" }, "id": "Bellan-P-M", "orcid": "0000-0002-0886-8782", "role": "member", "display_name": "Bellan, Paul Murray" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/e9b8-y318", "abstract": "Part I: Mitigating the Richtmyer-Meshkov instability (RMI) is critical for energy production in inertial confinement fusion. Suitable plasma models are required to study the hydrodynamic and electromagnetic interactions associated with the RMI in a conducting medium. First, a sequence of asymptotic expansions in several small parameters, as formal limits of the non-dissipative and non-resistive two-fluid plasma equations, leads to five simplified plasma/magnetohydrodynamics (MHD) systems. Each system is characterized by its own physical range of validity and dispersion relations, and includes the widely used magnetohydrodynamic (MHD) and Hall-MHD equations. Next we focus on the RMI in MHD. Using ideal MHD, it has been shown that the RMI is suppressed by the presence of an external magnetic field. We utilize the incompressible, Hall-MHD model to investigate the stabilization mechanism when the plasma ion skin depth and Larmor radius are nonzero. The evolution of an impulsively accelerated, sinusoidally perturbed density interface between two conducting fluids is solved as a linearized initial-value problem. An initially uniform background magnetic field of arbitrary orientation is applied. The incipient RMI is found suppressed through oscillatory motions of the interface due to the ion cyclotron effect. This suppression is most effective for near tangential magnetic fields but becomes less effective with increasing plasma length scales. The vorticity dynamics that facilitates the stabilization is discussed.
\r\n\r\nPart II: We consider the evolution of a planar gas-dynamic shock wave subject to smooth initial perturbations in both Mach number and shock shape profile. A complex variable formulation for the general shock motion is developed based on an expansion of the Euler equations proposed by Best [Shock Waves, {1}: 251-273, (1991)]. The zeroth-order truncation of Best's system is related to the well-known geometrical shock dynamics (GSD) equations while higher-order corrections provide a hierarchy of closed systems, as detailed initial flow conditions immediately behind the shock are prescribed. Solutions to Best's generalized GSD system for the evolution of two-dimensional perturbations are explored numerically up to second order in the weak and strong shock limits. Two specific problems are investigated: a shock generated by an impulsively accelerated piston with a corrugated surface, and a shock traversing a density gradient. For the piston-driven flow, it is shown that this approach allows full determination of derivative jump conditions across the shock required to specify initial conditions for the retained, higher-order correction equations. In both cases, spontaneous development of curvature singularity in the shock shape is detected. The critical time at which a singularity occurs follows a scaling inversely proportional to the initial perturbation size. This result agrees with the weakly nonlinear GSD analysis of Mostert et al. [J. Fluid Mech., {846}: 536-562, (2018)].
" }, { "name": "Shi, Xichen", "degree": "PhD", "year": "2021", "title": "Intelligent Control for Fixed-Wing eVTOL Aircraft", "advisor": "Chung, Soon-Jo", "url": "https://resolver.caltech.edu/CaltechTHESIS:02182021-040721884", "creators": [ { "name": { "family": "Shi", "given": "Xichen" }, "id": "Shi-Xichen", "orcid": "0000-0002-5366-9256", "display_name": "Shi, Xichen" } ], "advisors": [ { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "advisor", "display_name": "Chung, Soon-Jo" } ], "committee": [ { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "chair", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Yue", "given": "Yisong" }, "id": "Yue-Yisong", "orcid": "0000-0001-9127-1989", "role": "member", "display_name": "Yue, Yisong" } ], "option_major": [ "space" ], "doi": "10.7907/51c6-aa57", "abstract": "Urban Air Mobility (UAM) holds promise for personal air transportation by deploying \"flying cars\" over cities. As such, fixed-wing electric vertical take-off and landing (eVTOL) aircraft has gained popularity as they can swiftly traverse cluttered areas, while also efficiently covering longer distances. These modes of operation call for an enhanced level of precision, safety, and intelligence for flight control. The hybrid nature of these aircraft poses a unique challenge that stems from complex aerodynamic interactions between wings, rotors, and the environment. Thus accurate estimation of external forces is indispensable for a high performance flight. However, traditional methods that stitch together different control schemes often fall short during hybrid flight modes. On the other hand, learning-based approaches circumvent modeling complexities, but they often lack theoretical guarantees for stability.
\r\n\r\nIn the first part of this thesis, we study the theoretical benefits of these fixed-wing eVTOL aircraft, followed by the derivation of a novel unified control framework. It consists of nonlinear position and attitude controllers using forces and moments as inputs; and control allocation modules that determine desired attitudes and thruster signals. Next, we present a composite adaptation scheme for linear-in-parameter (LiP) dynamics models, which provides accurate realtime estimation for wing and rotor forces based on measurements from a three-dimensional airflow sensor. Then, we introduce a design method to optimize multirotor configuration that ensures a property of robustness against rotor failures.
\r\n\r\nIn the second part of the thesis, we use deep neural networks (DNN) to learn part of unmodeled dynamics of the flight vehicles. Spectral normalization that regulates the Lipschitz constants of the neural network is applied for better generalization outside the training domain. The resultant network is utilized in a nonlinear feedback controller with a contraction mapping update, solving the nonaffine-in-control issue that arises. Next, we formulate general methods for designing and training DNN-based dynamics, controller, and observer. The general framework can theoretically handle any nonlinear dynamics with prior knowledge of its structure. Finally, we establish a delay compensation technique that transforms nominal controllers for an undelayed system into a sample-based predictive controller with numerical integration. The proposed method handles both first-order and transport delays in actuators and balances between numerical accuracy and computational efficiency to guarantee stability under strict hardware limitations.
" }, { "name": "Toedtli, Simon Silvio", "degree": "PhD", "year": "2021", "title": "Control of Wall-Bounded Turbulence Through Closed-Loop Wall Transpiration", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272021-055610816", "creators": [ { "name": { "family": "Toedtli", "given": "Simon Silvio" }, "id": "Toedtli-Simon-Silvio", "orcid": "0000-0001-9371-9572", "display_name": "Toedtli, Simon Silvio" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Hutchins", "given": "Nicholas" }, "id": "Hutchins-Nicholas", "role": "member", "display_name": "Hutchins, Nicholas" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/me3y-te05", "abstract": "Many wall-bounded flows of practical relevance are turbulent, including the flows past airplanes and ships. The turbulent motions enhance momentum mixing and, as a result, the drag force on the engineering surface increases, for transportation vessels typically by at least a factor of two compared to laminar flow. Turbulent flow control aimed at drag reduction therefore has the potential to deliver enormous energetic and economic savings, but many challenges remain despite active research for well over a century. The present thesis aims to contribute towards two open questions of the field: first, what are suitable controller design tools for high Reynolds number flows? And second, how does actuation through closed-loop wall transpiration change the flow physics? We investigate aspects of these questions through direct numerical simulation (DNS) and modal analyses of an example control scheme, which is applied to a low Reynolds number turbulent channel flow. The controller is a generalization of the opposition control scheme, and introduces a phase shift between the Fourier transformed sensor measurement and actuator response.
\r\n\r\nThe first part of the thesis demonstrates that a low-order model based on the resolvent framework is able to approximate the drag reduction results of DNS over the entire parameter space considered. The model is about two orders of magnitude cheaper to evaluate than DNS at low Reynolds numbers, and we present a strategy based on subsampling of the wave number space and analytical scaling laws that enables model-based flow control design at technologically relevant Reynolds numbers. The second part of the thesis shows that the physics of the controlled flow can be understood from two distinct families of spatial scales, termed streamwise-elongated and spanwise-elongated scales, respectively. Wall transpiration with streamwise-elongated scales attenuates or amplifies the near-wall cycle and therefore leads to drag reduction or increase, depending on the phase shift. In contrast, wall transpiration with spanwise-elongated scales only leads to drag increase, which occurs at positive phase shifts and is due to the appearance of spanwise rollers which largely enhance momentum mixing. Both patterns are robust features of flows with closed-loop wall transpiration, and the present study offers a simple explanation of their origin in terms of phase relations at distinct spatial scales. The findings of this study may set the stage for a unifying framework for various forms of wall transpiration, and implications for future flow control design are discussed.
" }, { "name": "Ferraro, Serena", "degree": "PhD", "year": "2020", "title": "Topology Optimization and Failure Analysis of Deployable Thin Shells with Cutouts", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:02032020-164711057", "creators": [ { "name": { "family": "Ferraro", "given": "Serena" }, "id": "Ferraro-Serena", "orcid": "0000-0002-6038-7863", "display_name": "Ferraro, Serena" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/9VZ4-3E71", "abstract": "Shell structures with cutouts are widely used in architectural and engineering applications. For thin, lightweight, and deployable space structures, cutouts are cleverly positioned to fold and store the structure in a small volume. To maintain shape accuracy, these structures must fold without becoming damaged and must be stiff in their deployed configurations. Intuitive designs often fail to satisfy these two requirements. This research proposes solutions to the topology optimization of composite, thin shell structures with cutouts.
\r\n\r\nA novel optimization algorithm was developed that makes no assumptions on the initial number, shape, and location of cutouts on deployable thin shells. The algorithm uses a density-based approach, which distributes the material within the structure by assigning a density parameter to discretized locations. This parametrization of the design domain allows for the finding of new features and the connectivity of the domain, thus providing a completely general formulation to the optimization problem. The goal is to study the effects of volume and stress constraints imposed in a deformed configuration of thin shell structures. While classical topology optimization studies focus on finding solutions to linear problems, this method is applicable to geometrically nonlinear problems and implements stress constraints in the deformed, and hence most stressed, configuration of these shells. A mathematical formulation of the optimization problem and interpolation schemes for stiffness tensor, volume, and stress are presented. A sensitivity analysis of objective function, volume, and stress constraints is provided. Finally, solutions for a thin plate and a tape spring are proposed.
\r\n\r\nDensity-based methods are computationally expensive when applied to large structures and complex shapes because of the large number of design variables. To address these challenges, two optimization methods that provide more specific solutions to the problem of composite, deployable shells are proposed. The first method uses level sets to parametrize the cutouts, thereby restricting the design space and simultaneously limiting the number of design variables. This greatly reduces the computational cost. Using this approach, successful solutions are found for stiff, composite, thin shells with complex shapes that can fold without becoming damaged. The second method uses a spline representation of the contour of a single cutout on the shell, thus performing fine tuning of the shape of the cutout. Modeling techniques that simulate localized strain and experimental methods for studying the quasi-static folding of these composite shells are developed. A laminate failure criterion suitable for thin, plain-weave composites is used in simulations to predict the onset of failure in folded shells. Numerical results are validated with folding experiments that demonstrated good agreement with numerical solutions.
\r\n\r\nLastly, it was discovered that many of the best performing solutions have multiple closely spaced cutouts, as opposed to current designs for deployable space structures that have fewer large cutouts. This leads to the formation of small strips of material between cutouts. Hence, the behavior of thin, plain-weave composite material was characterized and the first study on size-scaling effects at small length scales (\u2264 15 mm) in this type of material was performed. Size-scaling effects on stiffness and strength shown in this study were introduced in numerical simulations of deployable thin shells. The study demonstrates that the prediction of the onset of failure in folded shells strongly depends on these size effects. Numerical predictions are corroborated by an experimental investigation of localized damage in thin strips of material forming between cutouts. Deployable shells resulting from the optimization studies are built and tested and localized damage is measured via digital volume correlation techniques.
" }, { "name": "Figueroa-Schibber, Erika", "degree": "PhD", "year": "2020", "title": "High-Cycle Dynamic Cell Fatigue with Applications on Oncotripsy", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:01202020-210729635", "creators": [ { "name": { "family": "Figueroa-Schibber", "given": "Erika" }, "id": "Figueroa-Schibber-Erika", "orcid": "0000-00002-6629-297X", "display_name": "Figueroa-Schibber, Erika" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "chair", "display_name": "Asimaki, Domniki" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "space" ], "doi": "10.7907/0425-SN62", "abstract": "The method of oncotripsy (from Greek, onco- meaning \"tumor\" and \u2013tripsy \"to break\") exploits aberrations in the material properties and morphology of cancerous cells to target them selectively using tuned low-intensity pulsed ultrasound. Compared to other noninvasive ultrasound treatments that ablate malignant tissue, oncotripsy has the capability of targeting unhealthy tissue with minimal damage to healthy cells in the ablation process.
\r\n\r\nWe propose a model of oncotripsy that follows as an application of cell dynamics, statistical mechanical theory of network elasticity and 'birth-death' kinetics to describe processes of damage and repair of the cytoskeleton. We also develop a reduced dynamical model that approximates the three-dimensional dynamics of the cell and facilitates parameter studies, including sensitivity analysis and process optimization.
\r\n\r\nThe dynamical system encompasses the relative motion of the nucleus to the cell membrane and a state variable measuring the extent of damage to the cytoskeleton. The dynamical system evolves in time as a result of structural dynamics and kinetics of cytoskeletal damage and repair. The resulting dynamics are complex and exhibits behavior on multiple time scales, including the period of vibration and attenuation, the characteristic time of cytoskeletal healing, the pulsing period and the time of exposure to the ultrasound. Damage on the cells develops in the order of millions of ultrasound cycles, and the failure mechanism is explained as a fatigue process. We also account for cell variability and estimate the attendant variance of the time-to-death of a cell population. We show that the dynamical model predicts \u2014 and provides a conceptual basis for understanding \u2014 the oncotripsy effect and other trends observed in experiments.
" }, { "name": "Garc\u00eda Su\u00e1rez, Antonio Joaqu\u00edn", "degree": "PhD", "year": "2020", "title": "Application of Path-Independent Integrals to Soil-Structure Interaction", "advisor": "Asimaki, Domniki", "url": "https://resolver.caltech.edu/CaltechTHESIS:11212019-100323260", "creators": [ { "name": { "family": "Garc\u00eda Su\u00e1rez", "given": "Antonio Joaqu\u00edn" }, "id": "Garc\u00eda-Su\u00e1rez-Antonio-Joaqu\u00edn", "orcid": "0000-0001-8830-4348", "display_name": "Garc\u00eda Su\u00e1rez, Antonio Joaqu\u00edn" } ], "advisors": [ { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "advisor", "display_name": "Asimaki, Domniki" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "member", "display_name": "Asimaki, Domniki" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MMWW-B046", "abstract": "Assessing seismic pressure increment on buried structures is a critical step in the design of infrastructure in earthquake-prone areas. Due to intrinsic complexities derived from the need to match the solution in the far-field to the localized solution around the structure, the near-field, researchers have aimed at finding simplified models focused on engineering variables as the seismic earth thrust. One such model is the so-called Younan-Veletsos model, which pivots on a stringent assumption on the stress tensor.
\r\n\r\nAt the same time, the might of the path-independent integrals of solid mechanics to deal with problems in Geotechnical Engineering at large, and Soil-Structure Interaction in particular, has remained unexplored, despite of a rich landscape of potential applications. The unbridled success of these path-independent integrals in Fracture Mechanics, a discipline which cannot be understood without them currently, may be mirrored in problems in Geotechnical Engineering, since the two fields, despite appearing very detached from each other at first glance, share deep traits: in both cases, the system under consideration can be conceptualized as a domain with simple, easy-to-assess regions (the areas where remote loading is applied and the far-field, respectively) and also with other complex, hard-to-understand regions (the crack tip, the near-field).
\r\n\r\nWe present the first derivation of the exact solution of the Younan-Veletsos problem, which is later analyzed to reveal phenomena not captured by previous approximate solutions. Then, we introduce a novel model which relies on the path-independent Rice\u2019s J-integral, a customary tool in Fracture Mechanics, which is applied here in the Soil-structure Interaction context for the first time. This novel model captures those features of the exact solution that were missed by prior approximations. The capabilities of the J-integral to, first, find an upper bound of the force induced by earthquakes over the walls of underground structures, under some conditions, and, second, to understand the soil-structure kinematic interaction phenomenon are also assessed.
\r\n\r\nAdditionally, the intermediate step of analyzing of the far-field yielded some results concerning Site Response Analysis which are also included in the text.
" }, { "name": "Leclerc, Christophe", "degree": "PhD", "year": "2020", "title": "Mechanics of Ultra-Thin Composite Coilable Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:01232020-134850757", "creators": [ { "name": { "family": "Leclerc", "given": "Christophe" }, "id": "Leclerc-Christophe", "orcid": "0000-0003-1999-4757", "display_name": "Leclerc, Christophe" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "chair", "display_name": "Daraio, Chiara" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/X60S-BR30", "abstract": "Coilable structures are thin-shell structures that can be coiled around a hub by flattening their cross-section. They are attractive for multiple space applications as they allow efficient packaging and deployment of large planar structures. Reducing the shell thickness enables smaller coiling radius and more efficient packaging.
\r\n\r\nThis thesis investigates TRAC structures, a type of coilable structure, made of ultra-thin composite materials. A design using a laminate made of glass fiber plainweave fabric and carbon fiber unidirectional tape is proposed, leading to a shell thickness of 0.08 mm. An in-autoclave, two-cure manufacturing process is presented, and a shape measurement method is used to mitigate post-cure shape changes due to residual stresses.
\r\n\r\nA study of the structure behavior in its deployed configuration is performed. First, the behavior when subjected to pure bending is investigated experimentally for structures with a length of 575 mm. Two regimes are observed, with a pre-buckling phase transitioning to a stable post-buckling phase after an initial buckling event. The ultimate buckling moment following the stable post-buckling regime can be as high as four times the initial buckling moment. A finite element model is developed and is able to reproduce all the features observed experimentally, except the ultimate buckling. This simulation model is used to study the effect of varying the structure length from 300 mm to 5000 mm on the initial buckling moment. Results show that nonlinearities in the pre-buckling deformations of the flanges under compression lead to a constant wavelength lateral-torsional buckling mode for which the critical moment is mostly constant across the range of length. The torsional behavior of the TRAC structure is also investigated. Good agreement is obtained between experiments and numerical simulations, and initial twist in the structure is shown to have little effect on the overall behavior due to the small torsional stiffness in the underformed configuration.
\r\n\r\nAn analytical method to predict the buckling load of a TRAC structure under pure bending is presented. It is achieved by considering only one flange of the structure and solving the problem of a cylindrical shell panel with a longitudinal free edge under non-uniform axial compression. Partially uncoupled stability equations for a balanced laminate are derived and are used in conjunction with the Rayleigh-Ritz method to approximate the buckling load. This method overestimates the buckling load by 44% in the case of a 500 mm TRAC structure made with ultra-thin composite materials.
\r\n\r\nA study of the coiling behavior is also presented. High localized curvature in the transition region between the coiled and deployed regions is observed in experiments, leading to material failure for a structure made only of carbon fiber unidirectional tape. A numerical framework is developed and reproduces the localized curvature observed in experiments, predicting stress concentration at this location. The study shows that changing the laminate to a a single ply of carbon fiber unidirectional tape sandwiched between plies of glass fiber plainweave fabrics reduces significantly the maximum stress in the transition region, to the extent that the highest stress is now in the fully coiled region and can be accurately predicted using simple equations based on the change of curvatures due to the coiling process.
" }, { "name": "Leibowitz, Matthew Gregory", "degree": "PhD", "year": "2020", "title": "Hypervelocity Shock Tunnel Studies of Blunt Body Aerothermodynamics in Carbon Dioxide for Mars Entry", "advisor": "Austin, Joanna M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272020-173051776", "creators": [ { "name": { "family": "Leibowitz", "given": "Matthew Gregory" }, "id": "Leibowitz-Matthew-Gregory", "orcid": "0000-0002-7297-2592", "display_name": "Leibowitz, Matthew Gregory" } ], "advisors": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "advisor", "display_name": "Austin, Joanna M." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/chyn-ea06", "abstract": "A low mass and reliable thermal protection system for Martian atmospheric entry requires an accurate prediction of the aerothermal environment encountered by the spacecraft. In order to move forward with predictive models for larger vehicles needed for manned and sample return missions, anomalous data needs to be resolved.\r\nThis work aims to address two critical problems relevant for Mars missions.
\r\n\r\nI) We investigate significant discrepancies between experimental and simulated blunt body bow shock standoff distance in ground test facilities. Experiments using high-speed and high-resolution schlieren imaging are conducted in the T5 reflected shock tunnel and the Hypervelocity Expansion Tube (HET) to examine facility\r\nindependence of the measurements. A recently-developed model for sphere and sphere-cone behavior is in good agreement with experiments, and with predictions from Navier-Stokes simulations with thermal and chemical nonequilibrium. The need to account for the divergence of the streamlines in conical nozzles is highlighted.\r\nThe contributions of vibrational and chemical nonequilibrium to the stagnation-line density profile are quantified using the simulation results in order to compare different reaction rate models.
\r\n\r\nII) We measure and characterize carbon dioxide mid-wave infrared radiation in hypervelocity flow. Initially assumed negligible in the design of the Mars Science Laboratory (MSL) mission heat shield, this mechanism of heating must be considered for accurate predictions of the heating environment. Specifically, carbon dioxide radiation can be a dominant source of heating in the afterbody, particularly later in the trajectory at lower velocities. Presented are spectral measurements of the 4.3 \u03bcm fundamental band of carbon dioxide radiation measured using fiber optics embedded on the surface of an MSL scaled heat shield model. When comparing experiments and simulations, good agreement is found when running the HET in shock tube mode where the shock layer is optically thick, while discrepancies are observed in expansion tube mode where the shock layer is optically thin. A thorough analysis of flow features in the line-of-sight including freestream uncertainties is performed to explore possible reasons for this discrepancy. After developing the spectroscopic calibration technique and obtaining forebody measurements in the expansion tube, an experimental campaign is completed in the T5 Reflected Shock Tunnel to measure spectral radiation in the forebody and afterbody. The accompanying T5 simulations needed for radiation predictions are being carried out by NASA Ames.
" }, { "name": "McMullen, Ryan Michael", "degree": "PhD", "year": "2020", "title": "Aspects of Reduced-Order Modeling of Turbulent Channel Flows: From Linear Mechanisms to Data-Driven Approaches", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282020-161209039", "creators": [ { "name": { "family": "McMullen", "given": "Ryan Michael" }, "id": "McMullen-Ryan-Michael", "orcid": "0000-0003-1371-7150", "display_name": "McMullen, Ryan Michael" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Graham", "given": "Michael D." }, "id": "Graham-Michael-D", "role": "member", "display_name": "Graham, Michael D." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/wayx-eh75", "abstract": "This thesis concerns three key aspects of reduced-order modeling for turbulent shear flows. They are linear mechanisms, nonlinear interactions, and data-driven techniques. Each aspect is explored by way of example through analysis of three different problems relevant to the broad area of turbulent channel flow.
\r\n\r\nFirst, linear analyses are used to both describe and better understand the dominant flow structures in elastoinertial turbulence of dilute polymer solutions. It is demonstrated that the most-amplified mode predicted by resolvent analysis (McKeon and Sharma, 2010) strongly resembles these features. Then, the origin of these\r\nstructures is investigated, and it is shown that they are likely linked to the classical Tollmien-Schichting waves.
\r\n\r\nSecond, resolvent analysis is again utilized to investigate nonlinear interactions in Newtonian turbulence. An alternative decomposition of the resolvent operator into Orr-Sommerfeld and Squire families (Rosenberg and McKeon, 2019b) enables a highly accurate low-order representation of the second-order turbulence statistics. The reason for its excellent performance is argued to result from the fact that the decomposition enables a competition mechanism between the Orr-Sommerfeld and Squire vorticity responses. This insight is then leveraged to make predictions about how resolvent mode weights belonging to several special classes scale with increasing Reynolds number.
\r\n\r\nThe final application concerns special solutions of the Navier-Stokes equations known as exact coherent states. Specifically, we detail a proof of concept for a data-driven method centered around a neural network to generate good initial guesses for upper-branch equilibria in Couette flow. It is demonstrated that the neural network is capable of producing upper-branch solution predictions that successfully converge to numerical solutions of the governing equations over a limited range of Reynolds numbers. These converged solutions are then analyzed, with a particular emphasis on symmetries. Interestingly, they do not share any symmetries with the known equilibria used to train the network. The implications of this finding, as well as broader outlook for the scope of the proposed method, are discussed.
" }, { "name": "Pajunen, Kirsti Mari", "degree": "PhD", "year": "2020", "title": "Dynamics of Lightweight Tensegrity-Inspired Metamaterials Fabricated with 3D-Printing", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012020-003455628", "creators": [ { "name": { "family": "Pajunen", "given": "Kirsti Mari" }, "id": "Pajunen-Kirsti-Mari", "orcid": "0000-0002-5366-1190", "display_name": "Pajunen, Kirsti Mari" } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Hofmann", "given": "Douglas C." }, "id": "Hofmann-D-C", "role": "member", "display_name": "Hofmann, Douglas C." } ], "option_major": [ "space" ], "doi": "10.7907/wm2f-4013", "abstract": "Tensegrity structures and lattices have been of interest in engineering applications for decades, with their dynamics becoming a thriving field of study. Tensegrities consist of structural members under purely axial loading, either tension or compression, and obtain their stability from prestress. They possess unique characteristics such as high strength-to-weight ratio, nonlinear behavior, and elastic response under severe deformation. Tensegrity lattices (or metamaterials) have been shown to exhibit appealing dynamic attributes such as continuous tunability with prestress, impact mitigation, energy trapping and lensing, and nonlinear wave propagation, to name a few. However, their pin-jointed and prestressed nature presents significant manufacturing limitations, especially in the formation of lattices with large numbers of tessellated unit cells. Therefore, experimental validation of the dynamics of tensegrity metamaterials has remained elusive. For lattices with tensegrity-like characteristics to be manifested for real-world applications, a method for producing tensegrity-like metamaterials at multiple length scales is needed.
\r\n\r\nIn this thesis, we present a design for a 3D-printable tensegrity-inspired structure with the equivalent strain energy capacity and stress-strain response as a pin-jointed tensegrity. Using this structure as a building block for multidimensional lattices, we subject them to a range of dynamic loading conditions to study their response. First, we perform experiments and simulations to obtain the dispersion relations for 1D and 3D lattices. We demonstrate the lattices\u2019 ability to continuously tune the dispersion characteristics (e.g., band gap and wave speed) under precompression. This trait shows potential for acoustic lensing and dispersive wave propagation. In 3D, we show that the lattice shows the same type of unique properties, such as faster shear speed than longitudinal speed, as pin-jointed tensegrity lattices. Next, we study the lattices under impact loading. Long-duration impact experiments on baseline unit cells and 1D lattices show their resilience to repeated deformation, elasticity, and load limitation behaviors. Short-duration impulse experiments and simulations exhibit a wealth of desirable properties, such as high force transmission reduction, highly dispersive wave propagation, tunable wave speeds, energy trapping, and redirection of energy. We demonstrate that these tensegrity-inspired metamaterials not only exhibit and experimentally demonstrate tensegrity-like characteristics, but open a new range of lightweight metamaterials with unprecedented dynamic properties.
" }, { "name": "Talon, Thibaud", "degree": "PhD", "year": "2020", "title": "Surface Reconstruction from Distributed Angle Measurements", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:02282020-192725947", "creators": [ { "name": { "family": "Talon", "given": "Thibaud" }, "id": "Talon-Thibaud", "orcid": "0000-0002-8240-1101", "display_name": "Talon, Thibaud" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Hajimiri", "given": "Ali" }, "id": "Hajimiri-A", "orcid": "0000-0001-6736-8019", "role": "chair", "display_name": "Hajimiri, Ali" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/ZG2D-2K77", "abstract": "This thesis presents an innovative solution to the shape measurement of large structures for space applications. The current state-of-the-art heavily relies on optical solutions such as cameras or lasers to recover the shape of a surface. Because of the impracticality of placing a system in front of a large structure flying in space, new solutions need to be developed. The proposed solution is to embed angular sensors (such as sun sensors) directly on the surface. The sensors provide a collection of distributed measurements that form a discrete map of the angular orientation of the structure. An integration scheme can then estimate the 3D shape of the surface.
\r\n\r\nA mathematical model to perform the integration from angle measurements to the shape of a 3D surface is presented first. This model is purely geometric and serves as a basis for similar concepts. The surface is known in a reference configuration and is assumed to have deformed inextensibly to its current shape. Inextensibility conditions are enforced through a discretization of the metric tensor generating a finite number of constraints. This model parameterizes the shape of the surface using a small number of unknowns, and thus requires a small number of sensors. We study the singularities of the equations and derive necessary conditions for the problem to be well-posed. The limitations of the algorithm are highlighted. Simulations are performed on developable surfaces to analyze the performance of the method and to show the influence of the parameters used in the algorithm. Optimal schemes which lower the RMS error between the reconstructed shape and the actual one are presented.
\r\n\r\nAn experimental validation of the proposed solution and algorithm is performed on a 1.3 x 0.25 m structure with 14 embedded sun sensors. The sensors measure the two local angles of the surface from a light source placed in front of the surface. A small, lightweight and expandable design of the sensors is shown in this thesis. A calibration procedure accurately correlates the output of the sensor with a 0.5\u00b0 precision. The procedure also highlights the limitations of the design. The structure was deformed in bending and torsion with amplitudes of a few centimeters, and its shape was reconstructed to an accuracy on the order of a millimeter.
\r\n\r\nThe accuracy of the initial algorithm is found to be limited by local shape deformations caused by the mechanical response of the structure. A new algorithm, replacing the discrete inextensibility conditions with the equilibrium equations derived from a finite-element model, is shown. This new algorithm is tested on the experimental structure and the accuracy of the reconstruction is increased by a factor of 2. The RMS error is under a millimeter on average over the different applied shapes and goes as low as 0.3 mm.
\r\n\r\nTo understand how this solution can apply to large space structures, simulations are performed on a model of a large planar spacecraft. A 25 x 25 m structure representing the current concept for the Caltech Space Solar Power Project satellite is used as an example. Sensors with similar noise properties as the ones built for the experiment are placed on the spacecraft. A finite-element model combining the vibration of the spacecraft with large rigid body rotations is presented. This model is used in a Kalman filter that estimates the shape of the structure by iterative prediction from the dynamic finite-element model and correction from the angle measurements. Simulations are performed around the thruster actuation applied at the corner of the structure to follow a specific guidance scheme that is optimal for space solar power satellites. The actuation creates both vibrations of the structure with amplitudes of few centimeters and large rotations of the spacecraft. The designed Kalman filter can accurately estimate both effects and it is shown that millimeter accuracy is achievable. The relationship between the number of sensors, the reconstructed shape error, as well as potential stiffness deviations in the FE model is studied. The results provide first order estimates of the performance of this measurement system, in order to enable the design of future space missions.
" }, { "name": "Yanes, Nelson Javier", "degree": "PhD", "year": "2020", "title": "Ultraviolet Radiation of Hypervelocity Stagnation Flows and Shock/Boundary-Layer Interactions", "advisor": "Austin, Joanna M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02112020-170613058", "creators": [ { "name": { "family": "Yanes", "given": "Nelson Javier" }, "id": "Yanes-Nelson-Javier", "orcid": "0000-0001-8423-6958", "display_name": "Yanes, Nelson Javier" } ], "advisors": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "advisor", "display_name": "Austin, Joanna M." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "orcid": "0000-0002-1225-4695", "role": "member", "display_name": "Polk, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WYHM-1218", "abstract": "Shock/boundary-layer interactions can induce flow distortion, create flow separation with loss of control authority, and result in high thermal loads. Correct prediction of the flow structure and heating loads is vital for vehicle survival. However, a recent NATO workshop revealed severe underprediction of thermal loads and discrepancies in the location of separation by simulations of high enthalpy air flows. Due to the coupling between thermochemistry and fluid mechanics, a substantial effort has been placed on the development and validation of thermochemical models. As a result, there is a need for experimental data that are more than mean flow surface measurements.
\r\n\r\nSpatially resolved emission spectra are collected in the post-shock regime of hypervelocity flow over a circular cylinder and a 30-55 degree double wedge. The Hypervelocity Expansion Tube (HET) is used to generate high Mach number, high enthalpy flow (Mach numbers 5 - 7, h\u2080 = 4 - 8 MJ/kg) with minimal freestream dissociation. The NO \u03b3 band (A\u00b2\u03a3\u207a - X\u00b2\u03a0) emission is measured in the ultraviolet range of 210-250 nm at downstream locations behind shock waves. Excitation temperatures are extracted from the NO \u03b3 emission from spectrum fitting. The result is a temperature relaxation profile that quantifies the state thermal non-equilibrium. Profiles of vibrational band intensity as a function of streamwise distance are used as direct measurements of chemical non-equilibrium in the flow.
\r\n\r\nCylinder experiments are performed with varying freestream total enthalpy, Mach number, and test gas O\u2082 mole fraction to examine changes in relaxation profile. Schlieren images are used to accurately measure standoff distance. Temperature measurements are compared against a zero-dimensional state-to-state model. Strategies for spectrum fitting are presented for cases where the gas is not optically thin and for radiation containing multiple electronic states. For freestream mixtures with reduced oxygen mole fraction, an electronic excitation temperature is required to describe the radiation of the NO \u03b3, \u03b2 (B\u00b2\u03a0 - X\u00b2\u03a0), and \u03b4 (C\u00b2\u03a0 - X\u00b2\u03a0) transitions. The creation of electronically excited NO is discussed in the context of measured vibrational band intensities and computed NO(A) number density profiles using a two-temperature reactive Landau-Teller model.
\r\n\r\nEmission spectra are collected in the post bow shock and reattachment shock region of hypervelocity flow over a double wedge. High speed schlieren imaging is performed to investigate facility startup effects and for tracking features in a shock/boundary-layer interaction. Detector exposures occur at select times throughout the flow development process to study temporal changes in thermal and chemical non-equilibrium. Time evolution of temperatures at strategic locations of the flow is obtained from spectrum fitting. Two-temperature calculations of the oblique shock system are compared against the emission results. Radiation data are discussed in the context of recent simulation efforts.
" }, { "name": "Bilgi, Pavaman", "degree": "PhD", "year": "2019", "title": "Optimization of CCD Charge Transfer for Ground and Space-Based Astronomy", "advisor": "Kulkarni, Shrinivas R.; Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302019-172558320", "creators": [ { "name": { "family": "Bilgi", "given": "Pavaman" }, "id": "Bilgi-Pavaman", "orcid": "0000-0002-2642-8553", "display_name": "Bilgi, Pavaman" } ], "advisors": [ { "name": { "family": "Kulkarni", "given": "Shrinivas R." }, "id": "Kulkarni-S-R", "role": "advisor", "display_name": "Kulkarni, Shrinivas R." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "co-advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Smith", "given": "Roger M." }, "id": "Smith-Roger-M", "role": "member", "display_name": "Smith, Roger M." }, { "name": { "family": "Kulkarni", "given": "Shrinivas R." }, "id": "Kulkarni-S-R", "role": "member", "display_name": "Kulkarni, Shrinivas R." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CNKG-8Y84", "abstract": "This thesis will be of particular interest to anyone integrating Charge-Coupled Devices (CCDs) into any precision scientific imaging instrument, especially so in space. The first part of the thesis concerns optimization of a CCD camera as a whole. CCDs for the WaSP imager at the Hale telescope are characterized using a minimal amount of data using just a flat-field illumination source. By measuring performance over the entire parameter space of (clock and bias) inputs and analyzing the multidimensional output (linearity, dynamic range, read noise etc), optimal operating conditions can be selected quickly (and possibly automatically). With ever growing sizes of detector arrays such as the recently launched Gaia mission, the upcoming Euclid mission and ground-based cameras such as the LSST (189 CCDs), the task of streamlining detector optimization will be increasingly important. In the second (larger) part, the optimization of Charge Transfer Efficiency (CTE) is explored in particular. In modern CCDs, CTE is caused by lattice defects in the bulk silicon and is significantly worsened by radiation exposure, which is unavoidable in space. As shown in the literature, just a year of exposure to high energy solar proton radiation at low earth orbit can result in CTE reducing to 0.9999 for a signal level of 10,000e- \u2014 problematic for most precision astronomical measurements. Here, CTE degrading traps are fully explored in an undamaged CCD to new levels of accuracy. Several unique species are identified, and their population statistics are analyzed by both wafer and sub-pixel location. Subsequently, easily applied CTE measurement techniques are presented, yielding results with new levels of accuracy, concluding in the presentation of a new trap mitigating readout clocking scheme. This scheme can be readily applied to any CCD employing a parallel transfer gate without readout speed penalty. It is proposed that the results herein may be used to construct a simple model to predict CTE given a temperature, readout timing and signal level. This model could then be used to automatically optimize CTE for any CCD, given only its trap parameter statistics.
" }, { "name": "Deffo Nde, Arnold Durel", "degree": "PhD", "year": "2019", "title": "A Line-Free Method of Monopoles for 3D Dislocation Dynamics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:08042018-083338014", "creators": [ { "name": { "family": "Deffo Nde", "given": "Arnold Durel" }, "id": "Deffo-Nde-Arnold-Durel", "orcid": "0000-0001-9077-8315", "display_name": "Deffo Nde, Arnold Durel" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ariza", "given": "Pilar" }, "id": "Ariza-Pilar", "role": "member", "display_name": "Ariza, Pilar" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/23YV-3312", "abstract": "Despite the emergence of architected materials for various applications, metals still play a key role in engineering in general and aeronautics in particular. Turbine blades in jets engines for instance are made from single-crystal Nickel superalloys. As a result, studying the failure mechanism of these crystalline materials would help understand the limits of their applications. At the core of this mechanism are line defects called dislocations. Indeed, the plastic deformation of metals is governed by the motion of dislocation ensembles inside the crystal. In this thesis, we propose a novel approach to dislocation dynamics through the method of monopoles. In this approach, we discretize the dislocation line as a collection of points (or monopoles), each of which carries a Burgers \"charge\" and an element of line. The fundamental difference between our method and current methods for dislocation dynamics lies in the fact that the latter discretize the dislocation as a collection of line segments from which spans a need to keep track of the connectivity of the nodes. In our approach, we propose a \"line-free\" discretization where a linear connectivity or sequence between monopoles need not be defined. This attribute of the formulation offers significant computational advantages in terms of simplicity and efficiency. Through verification examples, we show that our method is consistent with existing results for simple configurations. We then build on this success to investigate increasingly complex examples, this with the ultimate goal of simulating the plastic deformation of a BCC grain in an elastic matrix.
" }, { "name": "Dhandapani, Chandru", "degree": "PhD", "year": "2019", "title": "Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:06102019-185605511", "creators": [ { "name": { "family": "Dhandapani", "given": "Chandru" }, "id": "Dhandapani-Chandru", "orcid": "0000-0002-7319-557X", "display_name": "Dhandapani, Chandru" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FH31-4468", "abstract": "Most energy requirements of modern life can be fulfilled by renewable energy sources, but it is impossible in the near future to provide an alternative energy source to combustion for airplanes. That being said, combustion in aviation can be made more sustainable by using alternative jet fuels, which are made from renewable sources like agricultural wastes, solid wastes, oils, and sugars. These alternative fuels can be used in commercial flights only after a long certification process by the Federal Aviation Agency (FAA) and ASTM International. Unfortunately, in over 50 years of fuel research, only five fuels have been certified.\r\nThis research project aims to speed up the certification process with quicker testing of alternative fuels. Engine testing and even laboratory testing require large amounts of time and fuel. Simulations can make the process much more efficient, but accurately simulating highly turbulent flames in such complex geometries would need large amounts of computational resources. The goal of this thesis is to create an efficient computational framework, that can replicate different engine-like turbulent flow conditions in simple geometries with numerical tractability.
\r\n\r\nThe central idea is to decompose the flow field into ensemble mean and fluctuating quantities. The simulations then resolve only the fluctuations using simple computational domains, while emulating the effect of the mean flow using \"forcing\" terms. These forcing terms are calculated first for incompressible turbulence, and this method is later extended to turbulent reacting flows. In incompressible turbulence, Direct Numerical Simulations (DNS) performed on simple triply periodic cubic domains reasonably capture the statistically stationary shear turbulence, that is observed in free shear flows. The simulations are also performed in cuboidal domains, that are longer in one direction and with an inflow/outflow along it. Both changes are observed to not have a significant impact on the turbulence statistics. Finally, shear convection is applied to the turbulence simulations with inflow/outflow, which has a significant impact on the turbulence. These simulations accurately capture the turbulence anisotropy in free-shear flows.
\r\n\r\nThe study is extended to DNS of highly turbulent n-heptane-air flames performed under different flow conditions. Turbulent flames involve two-way coupling between fluid mechanics and combustion. The effects of the flame on the turbulence and the impact of the turbulent flow conditions on the flame behavior are analyzed. The focus is placed on the effects of turbulence production, shear convection, and pressure gradients. The anisotropy produced in the turbulence due to the different flow conditions and the flame are also compared and contrasted. While the global behavior and flow anisotropy were affected by these conditions, the local chemistry effects were unaffected, and depend only on the laminar flame properties and turbulence intensity. These findings can help predict turbulent flame behavior, and can expedite the search and testing of sustainable alternatives to conventional jet fuels.
" }, { "name": "Guerrero Vela, Pedro Pablo", "degree": "PhD", "year": "2019", "title": "Plasma Surface Interactions in LaB\u2086 Hollow Cathodes with Internal Xe Gas Discharge", "advisor": "Meiron, Daniel I.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06032019-100503451", "creators": [ { "name": { "family": "Guerrero Vela", "given": "Pedro Pablo" }, "id": "Guerrero-Vela-Pedro-Pablo", "orcid": "0000-0001-5766-2038", "display_name": "Guerrero Vela, Pedro Pablo" } ], "advisors": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "advisor", "display_name": "Meiron, Daniel I." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "orcid": "0000-0002-1225-4695", "role": "member", "display_name": "Polk, James E." } ], "option_major": [ "space" ], "doi": "10.7907/4CW7-2K35", "abstract": "The ultimate goals of space vehicles are to move faster, further, and more reliably in the space environment. Electric propulsion (EP) has proven to be a necessary technology in the exploration of our solar system ever since its working principle was empirically tested in space in 1964. Thanks to the high exhaust velocities of ionized propellant gases, EP enables efficient utilization of the limited supply of propellant aboard spacecrafts. This technology has opened the possibility of long distance autonomous space missions.
\r\n\r\nEP devices require electron sources to ionize the propellant gas and to neutralize charges that are leaving the spacecraft. In modern EP thrusters, this is achieved by the use of hollow cathodes -- complex devices that employ low work function materials to emit electrons. Hollow cathodes using polycrystalline LaB6 inserts are attractive candidates for long duration EP based space missions. However, the physics behind LaB6 hollow cathode operation has not been studied in detail, which limits the possibility of their optimization. This work presents an integrated experimental and computational approach to investigate LaB6 hollow cathode thermal behaviour and the interplay between LaB6 insert surface chemistry and xenon plasma.
\r\n\r\nOur investigation of the thermal behaviour of LaB6 cathodes led to the unexpected discovery of a thermal transient when a new insert is first used. Specifically, we observed that the cathode temperature decreases by approximately 300 degrees over 50 hours before reaching steady state. This finding suggests a beneficial dynamic evolution of the cathode's chemical state when it interacts with its own plasma. This evolution is intrinsic to cathode operation and can only be precisely understood when the multiphysic nature of the cathode is self-consistently simulated. Thus, we built a numerical platform capable of combining the plasma, thermal and chemical behavior of a discharging hollow cathode. Simulations incorporating different neutralization models, inelastic ion-surface interaction and heterogeneous chemical evolution led to two major conclusions. First, simulations predicted a significant reduction of the LaB6 work function (0.42~eV) compared to previously reported baseline values, which is of paramount importance for EP thruster efficiency and operational lifetimes. Second, simulations suggested that the interaction between xenon low energy ions (<\t50 eV) and the LaB6 surface occurs following a two step neutralization mechanism. The predicted work function reduction was experimentally confirmed by photoemission spectroscopy. Furthermore, using a combination of crystallographic analysis, scanning electron microscopy and profilometry, we demonstrated that work function reduction is caused by the creation of a crystallographic texture at the LaB6 surface upon interaction with Xe plasma. In addition, we postulated the existence of a work function enhancing mechanism of secondary importance, which can be explained by forced cationic termination of plasma exposed crystals.
\r\n\r\nOur results revealed the unexpected phenomenon of work function reduction upon plasma exposure of LaB6. These findings suggest that LaB6 hollow cathodes may outperform current technologies and become the component of choice in EP thrusters for future space missions.
" }, { "name": "Huertas-Cerdeira, Cecilia", "degree": "PhD", "year": "2019", "title": "On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-103225366", "creators": [ { "name": { "family": "Huertas-Cerdeira", "given": "Cecilia" }, "id": "Huertas-Cerdeira-Cecilia", "orcid": "0000-0003-4553-0470", "display_name": "Huertas-Cerdeira, Cecilia" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/326X-M576", "abstract": "Despite serving analogous functions, the mechanical designs conceived by human engineering and those that result from natural evolution often possess fundamentally differing properties. This thesis explores the use of principles that stem from natural evolution to improve the performance of engineered mechanisms, focusing on systems whose role is to interact with a fluid environment. Two different principles are considered: the use of compliance, abundant in nature's structures, and the use of flapping propulsion, prevalent among nature's swimmers.
\r\n\r\nThe first part of this thesis is dedicated to investigating the physics that govern the behavior of an inverted-flag energy harvester; an unactuated flexible cantilever plate that is clamped at its trailing edge and submerged in a flow. The resonance between solid motion and fluid forcing generates large-amplitude unsteady deformations of the structure that may be used for energy harvesting purposes. The effect of the flag's aspect ratio on its stability is first evaluated. Flags of very small aspect ratio are demonstrated to undergo a saddle-node bifurcation instead of a divergence instability. The angle of attack of the flag is then modified to reveal the existence of dynamical regimes additional to those present at zero angle of attack. A side-by-side flag configuration is finally explored, highlighting the presence of an energetically favorable symmetric flapping mode among other coupled dynamics.
\r\n\r\nThe second part of this thesis delves into the analysis of underwater flapping propellers and the optimization of their three-dimensional motion to generate desired maneuvering forces, with the objective of obtaining an appendage for use in autonomous underwater vehicles that can perform both fast maneuvering and efficient propulsion. An experimental optimization procedure is employed to obtain the most efficient trajectory that generates a specified side force. The effect of increasing the fin's aspect ratio is examined, and a highly efficient trajectory, that makes use of high three-dimensionality and rotation angles, is obtained for a fin of AR=4. The use of a flexible fin is then analyzed and shown to be detrimental to the maneuvering efficiency of the system.
" }, { "name": "Huynh, David Pham", "degree": "PhD", "year": "2019", "title": "Spatio-Temporal Response of a Compliant-Wall, Turbulent Boundary Layer System to Dynamic Roughness Forcing", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04232019-162807004", "creators": [ { "name": { "family": "Huynh", "given": "David Pham" }, "id": "Huynh-David-Pham", "orcid": "0000-0002-8430-6255", "display_name": "Huynh, David Pham" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/A5PS-GT54", "abstract": "This thesis investigates the interaction between an elastic compliant surface and a turbulent boundary layer exposed to dynamic roughness forcing. The goals are to explore a unique perspective of this fluid-structural problem through narrow-band forcing, and to further develop the understanding of dynamic roughness. Water tunnel experiments are designed with flow and surface measurements, both phase-locked to the roughness actuation. This enables a phase-averaged analysis, which leverages the deterministic input to isolate the temporally correlated components of the flow and surface response. Identifying the directly interacting velocity and deformation modes allows the complex, fluid-structural system to be studied in a more tractable, input-output manner.
\r\n\r\nThe first experiment is conducted with a smooth-wall turbulent boundary layer forced by dynamic roughness, and contributes to the knowledge of this type of forcing through structure-resolved particle image velocimetry. This allows for the streamwise-spatial nature and the wall-normal velocity component (v) of the roughness-forced flow to be explored, which had not been previously studied. A spatial amplitude modulation is observed in the synthetic structure and investigated directly through the spatial spectra. Through a parametric study and an empirical fit, the forcing frequency may now be selected to target a particular streamwise length scale.
\r\n\r\nThe second experiment implements a gelatin sample subject to an unforced turbulent boundary layer. The surface response is characterized and serves as a base case with which to identify the roughness-forced component of the deformations. This naturally leads to the third experiment, where the full compliant-wall, dynamic-roughness-forced turbulent boundary layer system is considered. The surface response to the synthetic flow structure is confirmed, which sets the stage for a comparison between the smooth-wall and compliant-wall data to study the effect of the compliant surface.
\r\n\r\nThe smooth/compliant comparison is guided by a resolvent analysis, which predicts a virtual wall feature in the v velocity mode for the elastic material under consideration. Using this prediction to inform a conditional average, the virtual wall is revealed in the experimental data. Thus, the action of the elastic surface is interpreted as opposing the v velocity near the wall, in a manner similar to wall-jet opposition control.\r\nPrevious experimental studies of viscoelastic compliant surfaces have demonstrated the potential for turbulent drag reduction, though either indirectly via the turbulence intensities or with relatively high skin friction measurement error. A common observation in these studies was the importance of the interaction between the surface and the coherent structures in the flow. To that end, this study has isolated and modeled the behavior of the fluid-structural system with a single spatio-temporal scale generated by dynamic roughness forcing. The results provide a physical interpretation of the effect of an elastic surface on turbulent boundary layer flow structures and informs the ongoing development of a reduced-order modeling tool in the resolvent analysis.
" }, { "name": "Kettenbeil, Christian", "degree": "PhD", "year": "2019", "title": "Dynamic Strength of Silica Glasses at High Pressures and Strain Rates", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:02202019-104738145", "creators": [ { "name": { "family": "Kettenbeil", "given": "Christian" }, "id": "Kettenbeil-Christian", "orcid": "0000-0003-0301-3678", "display_name": "Kettenbeil, Christian" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Mello", "given": "Michael" }, "id": "Mello-Michael", "orcid": "0000-0003-2129-9235", "role": "member", "display_name": "Mello, Michael" }, { "name": { "family": "Clifton", "given": "Rodney J." }, "id": "Clifton-Rodney-J", "role": "member", "display_name": "Clifton, Rodney J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "space" ], "doi": "10.7907/RZJW-MX30", "abstract": "Understanding the behavior of silica glasses at high pressures and strain rates is of great importance for geological processes and highly relevant to many technological applications including high-powered laser-matter interactions in optical elements and impact/blast damage in defense systems. Materials typically experience large inelastic deformations at high pressures, which are strongly affected by strength-related phenomena such as work hardening, damage and thermal softening. The pressure-shear plate impact experiment (PSPI) provides detailed information on the pressure and strain rate dependent strength properties of materials subjected to uniaxial compression. However, its range of attainable pressures has so far been limited and the assumptions required for its analysis become invalid at pressures beyond the Hugoniot elastic limit of the anvil materials. In this dissertation, a high-pressure PSPI (HP-PSPI) technique is developed that greatly extends the range of attainable experimental conditions by achieving higher terminal projectile velocities in a powder gun setup. A novel fiber-optic heterodyne transverse velocimeter (HTV) is developed to enable the use of robust frequency-based data reduction techniques, which reduce the effect of signal noise and light coupling losses. A forward analysis method, based on finite element simulations, is employed to match the experimentally observed material response during HP-PSPI experiments on soda-lime glass samples while considering the inelastic deformation of the utilized tungsten carbide anvils. Symmetric HP-PSPI experiments on tungsten carbide revealed a loss of strength at normal stresses exceeding 25 GPa, which hint at active damage or softening mechanisms under nominally uniaxial strain compression. A pressure-dependent strain softening model transitions soda-lime glass from an intact strength of 2.8 GPa, below strains of 10-30%, to a failed granular state following extensive inelastic shear deformation, which accurately predicts the measured response over a wide range of stresses (9-21 GPa) and strain rates (3\u2022105-2\u2022107s-1). Extending the range of previously attainable pressures and strain rates in PSPI experiments, combined with more robust diagnostics and analysis tools, will greatly benefit our understanding of material strength in extreme environments and enables the investigation of material behavior in a currently unexplored range of pressures and strain rates.
" }, { "name": "Kumar, Siddhant", "degree": "PhD", "year": "2019", "title": "An Enhanced Maximum-Entropy Based Meshfree Method: Theory and Applications", "advisor": "Kochmann, Dennis M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05062019-043913897", "creators": [ { "name": { "family": "Kumar", "given": "Siddhant" }, "id": "Kumar-Siddhant", "orcid": "0000-0003-1602-8641", "display_name": "Kumar, Siddhant" } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "advisor", "display_name": "Kochmann, Dennis M." } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Danas", "given": "Konstantinos" }, "id": "Danas-Konstantinos", "role": "member", "display_name": "Danas, Konstantinos" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0AP6-5F94", "abstract": "This thesis develops an enhanced meshfree method based on the local maximum-entropy (max-ent) approximation and explores its applications. The proposed method offers an adaptive approximation that addresses the tensile instability which arises in updated-Lagrangian meshfree methods during severe, finite deformations. The proposed method achieves robust stability in the updated-Lagrangian setting and fully realizes the potential of meshfree methods in simulating large-deformation mechanics, as shown for benchmark problems of severe elastic and elastoplastic deformations. The improved local maximum-entropy approximation method is of a general construct and has a wide variety of applications. This thesis presents an extensive study of two applications - the modeling of equal-channel angular extrusion (ECAE) based on high-fidelity plasticity models, and the numerical relaxation of nonconvex energy potentials. In ECAE, the aforementioned enhanced maximum-entropy scheme allows the stable simulation of large deformations at the macroscale. This scheme is especially suitable for ECAE as the latter falls into the category of severe plastic deformation processes where simulations using mesh-based methods (e.g. the finite element method (FEM)) are limited due to severe mesh distortions. In the second application, the aforementioned max-ent meshfree method outperforms FEM and FFT-based schemes in numerical relaxation of nonconvex energy potentials, which is essential in discovering the effective response and associated energy-minimizing microstructures and patterns. The results from both of these applications show that the proposed method brings new possibilities to the subject of computational solid mechanics that are not within the reach of traditional mesh-based and meshfree methods.
\r\n" }, { "name": "Morgan, Jonathan Philip", "degree": "PhD", "year": "2019", "title": "Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04112019-234812867", "creators": [ { "name": { "family": "Morgan", "given": "Jonathan Philip" }, "id": "Morgan-Jonathan-Philip", "orcid": "0000-0003-2898-4868", "display_name": "Morgan, Jonathan Philip" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7RSR-3277", "abstract": "This thesis explores the linear and non-linear interactions which take place in a rough-wall turbulent boundary through experiments and modeling. In order to derive physics-based models for the relation between roughness geometry and flow physics, two very simple periodic roughnesses were 3D printed and placed in a boundary layer wind tunnel for separate experiments. Hot-wire measurements were taken at a grid of points within a single period of the roughness in order to map the spatial variation of important flow statistics in way that allows correlation back to the roughness geometry. Time averaged streamwise velocity and the power spectrum of instantaneous streamwise velocity were both found to vary coherently with the roughness. The spatial variation of the time averaged velocity was identified as the linear result of the roughness, as it has identical wavenumber and frequency to the static roughness geometry. Modeling the time-averaged velocity field as a response mode of the linear resolvent operator was found to be reasonable for certain wavenumbers. The spatial distribution of the power spectrum was shown to be a non-linear effect of the roughness; the power spectrum only measures the energy of convecting modes, which necessarily have non-zero frequency and cannot correlate linearly to the static roughness. The spatial modulation of the power spectrum was found to be indicative of non-linear triadic interactions between the static velocity Fourier modes and pairs of convecting modes, as allowed by the Navier-Stokes equations. A low-order model for these interactions, and their effect on the power spectrum, was constructed using resolvent response modes to represent all velocity Fourier modes. The model was found to qualitatively predict the modulation of the power spectrum for several sets of wavenumbers. The success of such a simple model suggests that it presents a useful low-order understanding of non-linear forcing between scales in rough-wall boundary layers.
" }, { "name": "Phlipot, Gregory Paul", "degree": "PhD", "year": "2019", "title": "A Fully-Nonlocal Quasicontinuum Method to Model the Nonlinear Response of Periodic Truss Lattices", "advisor": "Kochmann, Dennis M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05242019-115317802", "creators": [ { "name": { "family": "Phlipot", "given": "Gregory Paul" }, "id": "Phlipot-Gregory-Paul", "orcid": "0000-0003-2721-8678", "display_name": "Phlipot, Gregory Paul" } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "advisor", "display_name": "Kochmann, Dennis M." } ], "committee": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "chair", "display_name": "Daraio, Chiara" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "member", "display_name": "Asimaki, Domniki" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." } ], "option_major": [ "space" ], "doi": "10.7907/3MPP-Q119", "abstract": "We present a framework for the efficient, yet accurate description of general periodic truss networks based on concepts of the quasicontinuum (QC) method. Previous research in coarse-grained truss models has focused either on simple bar trusses or on two-dimensional beam lattices undergoing small deformations. Here, we extend the truss QC methodology to nonlinear deformations, general periodic beam lattices, and three dimensions. We introduce geometric nonlinearity into the model by using a corotational beam description at the level of individual truss members. Coarse-graining is achieved by the introduction of representative unit cells and a polynomial interpolation analogous to traditional QC. General periodic lattices defined by the periodic assembly of a single unit cell are modeled by retaining all unique degrees of freedom of the unit cell (identified by a lattice decomposition into simple Bravais lattices) at each macroscopic point in the simulation, and interpolating each degree of freedom individually. We show that this interpolation scheme accurately captures the homogenized properties of periodic truss lattices for uniform deformations. In order to showcase the efficiency and accuracy of the method, we compare coarse-grained simulations to fully-resolved simulations for various test problems, including: brittle fracture toughness prediction, static and dynamic indentation with geometric and material nonlinearities, and uniaxial tension of a truss lattice plate with a cylindrical hole. We also discover the notion of stretch locking --- a phenomenon where certain lattice topologies are over-constrained, resulting in artificially stiff behavior similar to volumetric locking in finite elements --- and show that using higher-order interpolation instead of affine interpolation significantly reduces the error in the presence of stretch locking in 2D and 3D. Overall, the new technique shows convincing agreement with exact, discrete results for a wide variety of lattice architectures, and offers opportunities to reduce computational expenses in structural lattice simulations and thus to efficiently extract the effective mechanical performance of discrete networks." }, { "name": "Ratnaswamy, Vishagan", "degree": "PhD", "year": "2019", "title": "Constraining the Mantle's Rheology Using Methods in Uncertainty Quantification", "advisor": "Gurnis, Michael C.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05132019-143045769", "creators": [ { "name": { "family": "Ratnaswamy", "given": "Vishagan" }, "id": "Ratnaswamy-Vishagan", "orcid": "0000-0002-2371-807X", "display_name": "Ratnaswamy, Vishagan" } ], "advisors": [ { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "role": "advisor", "display_name": "Gurnis, Michael C." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Simons", "given": "Mark" }, "id": "Simons-M", "role": "member", "display_name": "Simons, Mark" }, { "name": { "family": "Gurnis", "given": "Michael C." }, "id": "Gurnis-M-C", "role": "member", "display_name": "Gurnis, Michael C." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F6FW-T648", "abstract": "An accurate estimation of the large-scale forces in the mantle has been difficult to obtain as numerical models either do not use an accurate rheology nor reproduce surface observations. While much work has been done in developing high-fidelity forward models that capture the salient physics of shear-thinning and dynamic weakening, they fail to reproduce observations such as plate motions and topography. In this thesis, we develop an optimization methodology that minimizes the misfit in surface observations such as plate motions and average effective viscosity for certain regions of the mantle. We utilize adjoints to calculate the gradient, while using second-order adjoints to construct the Hessian so as to infer the rheological parameters of the mantle's rheology. Furthermore, we build on this optimization scheme by constructing the Gaussian approximation of the posterior distribution for the inferred rheological parameters using the Hessian and establish the trade-offs between each parameter through their conditional distributions. We further extend this Gaussian approximation to infer extrinsic quantities such as the stresses in the fault zones and the average effective viscosity in the hinge zones to not only quantify the uncertainty, but also to see partitioning of the coupling of each subduction zone. " }, { "name": "Sridhar, Akshay", "degree": "PhD", "year": "2019", "title": "Large-Eddy Simulation of Turbulent Boundary Layers with Spatially Varying Roughness", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:09082018-212920204", "creators": [ { "name": { "family": "Sridhar", "given": "Akshay" }, "id": "Sridhar-Akshay", "orcid": "0000-0002-2642-8246", "display_name": "Sridhar, Akshay" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "orcid": "0009-0007-5991-2863", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "orcid": "0009-0007-5991-2863", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Callies", "given": "Joern" }, "id": "Callies-J", "orcid": "0000-0002-6815-1230", "role": "member", "display_name": "Callies, Joern" } ], "option_major": [ "space" ], "doi": "10.7907/8YWS-B862", "abstract": "This dissertation addresses high Reynolds number turbulent boundary layers flows with different inhomogeneous surface roughness distributions using large eddy simulations. The stretched vortex subgrid scale model for the outer flow LES is coupled with a virtual-wall model for the friction velocity with a correction accounting for local roughness effects.
\r\n\r\nA semi-empirical model that describes a fully developed rough-walled turbulent boundary layer with sand-grain roughness length-scale ks = \u03b1x that varies linearly with streamwise distance is first developed, with \u03b1 a dimensionless constant. For large Rex and a free-stream velocity U\u221e ~ xm, a simple log-wake model of the local turbulent mean-velocity profile is used that contains a standard mean-velocity correction for the asymptotic, fully rough regime. A two parameter (\u03b1; m) family of solutions is obtained for which U\u221e+ (or equivalently Cf) and boundary-layer measures can be calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable z/ks where z is the wall-normal co-ordinate. Results over a range of \u03b1 are discussed for cases including the zero-pressure gradient (m = 0) and sink-flow (m = -1) boundary layers. Model trends are supported by high Re wall-modeled LES. Linear streamwise growth of boundary layer measures is confirmed, while for each \u03b1, mean-velocity profiles and streamwise turbulent stresses are shown to collapse against z/(\u03b1x). Inner scaled velocity defects are shown to collapse against z/\u0394, where \u0394 is the Rotta-Clauser parameter. The present results suggest that these flows may be interpreted as the fully-rough limit for boundary layers in the presence of small-scale, linear roughness.
\r\n\r\nNext, an LES study of a flat-plate turbulent boundary layer at high Re under nonequilibrium flow conditions due to the presence of abrupt changes in surface roughness is presented. Two specific cases, smooth-rough (SR) and rough-smooth (RS) transition are examined in detail. Streamwise developing velocity and turbulent stress profiles are considered and sharp departures from equilibrium flow properties with subsequent relaxation are shown downstream. Relaxation trends are studied using integral parameters and higher-order mean flow statistics with emphasis on Re\u03c4 and ks+ dependence. Results are compared with RS experiments at matched Re\u03c4, and show good agreement in terms of recovery rates.
\r\n\r\nFinally, the case of static, impulsive wall-roughness in flows at high Re is addressed using the same LES framework. The initial perturbation from smooth-to-rough appears to dominate the flow behaviour with the length of the impulsive patch showing little effect on recovery rates at matched Re\u03c4 and ks+. The resulting trends show good agreement with low Re experiments and support the wall-modeled LES framework as a suitable method for analysing high Re flows in practical applications.
" }, { "name": "Tutcuoglu, Abbas Davud", "degree": "PhD", "year": "2019", "title": "Stochastic Multiscale Modeling of Dynamic Recrystallization", "advisor": "Kochmann, Dennis M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05242019-144233476", "creators": [ { "name": { "family": "Tutcuoglu", "given": "Abbas Davud" }, "id": "Tutcuoglu-Abbas-Davud", "orcid": "0000-0003-2360-706X", "display_name": "Tutcuoglu, Abbas Davud" } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "advisor", "display_name": "Kochmann, Dennis M." } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." } ], "option_major": [ "space" ], "doi": "10.7907/1VVP-T060", "abstract": "Materials by design is a core driver in enhancing sustainability and improving efficiency in a broad spectrum of industries. To this end, thermo-mechanical processes and many of the underlying phenomena were studied extensively in the context of specific cases. The goal of this thesis is threefold: First, we aim to establish a novel numerical model on the micro- and mesoscale that captures dynamic recrystallization in a generalized framework. Based on the inheritance of the idea of state switches, we term this scheme Field-Monte-Carlo Potts method. We employ a finite deformation framework in conjunction with a continuum-scale crystal plasticity formulation and extend the idea of state switches to cover both grain migration and nucleation. We introduce physically-motivated state-switch rules, based on which we achieve a natural marriage between the deterministic nature of crystal plasticity and the stochastic nature of dynamic recrystallization. Using a novel approach to undertake the states-switches in a transient manner, the new scheme benefits from enhanced stability and can, therefore, handle arbitrary levels of anisotropy. We demonstrate this functionality at the example of pure Mg at room temperature, which experiences strong anisotropy through the different hardening behavior on the \u2329c+a\u232a-pyramidal and prismatic slip systems as opposed to the basal slip systems as well as through the presence of twinning as an alternative strain accommodating mechanisms. Building on this generalized approach, we demonstrate spatial convergence of the scheme along with the ability to capture the transformation from single- to multi-peak stress-strain behavior.
\r\n\r\nSecond, motivated by the lack of transparency concerning the benefits of high-fidelity approaches in the modeling of dynamic recrystallization, we present two derivative models of the Field-Monte-Carlo Potts method, both of which afford reduced computational expense. One model preserves the spatial interpretation of grains, but imposes a Taylor assumption regarding the distribution of strain; the other reduces the spatial notion of a grain to a volume fraction in the idea of a Taylor model. In order to concentrate on the differences in accuracy between the various approaches, we fit all three schemes to experimental data for pure copper, which allows us to employ a well-understood crystal plasticity-based constitutive model and to simultaneously provide sufficient data for the analysis of the texture, stress and grain-size evolution. Owing to the large strains attained in these simulations, using the FFT-based scheme, we achieve capturing a precursor of continuous dynamic recrystallization. For low temperatures, the Taylor model fails to replicate the nucleation-dominated recrystallization process, whereas, at high temperatures, it shows compelling agreement with experiments and the two higher-fidelity models both in terms of the homogenized stress-evolution and the microstructural evolution.
\r\n\r\nFinally, we present a novel multiscale analysis of thermo-mechanical processes through coupling of the computationally efficient Taylor model for modeling dynamic recrystallization on the mesoscale to a max-ent based meshfree approach on the macroscale in the idea of vertical homogenization. We analyze the severe plastic deformation-based process of equal channel angular extrusion, which is intriguing from a numerical perspective due to the heavily localized zone of extensive shear deformation. By employing novel tools on the microscale regarding the stable update of internal variables as well as a careful interpretation of macroscale boundary conditions, we present the first multiscale analysis of a severe plastic deformation process informing simultaneously about the evolution of stress, texture and grain refinement. We attain convincing qualitative agreements for the evolution of the plunger force and texture. As an outlook on future investigations, we analyze multiple passes of the same billet in the form of route C with emphasis on the texture evolution after the second pass.
" }, { "name": "Veilleux, Jean-Christophe", "degree": "PhD", "year": "2019", "title": "Pressure and Stress Transients in Autoinjector Devices", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02172019-174051312", "creators": [ { "name": { "family": "Veilleux", "given": "Jean-Christophe" }, "id": "Veilleux-Jean-Christophe", "orcid": "0000-0002-5420-9411", "display_name": "Veilleux, Jean-Christophe" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VJSH-TF65", "abstract": "The viscosity of drug solutions delivered parenterally has been increasing over the years. Injecting viscous drug solutions using spring-actuated autoinjector devices is challenging due to a number of technical and human factor constraints. Some of the related challenges are investigated in this thesis.
\r\n\r\nActuation of autoinjector devices powered using stiff springs can create deleterious pressure and stress transients which are not needed to achieve the normal functions of the device. Experimental measurements have shown that peak pressures and stresses substantially larger than what is needed to achieve the normal device function can occur during the actuation phase, creating unnecessary potential for device failure.
\r\n\r\nThe acceleration of the syringe during actuation can be very large, often creating transient cavitation in the cone region. The occurrence or absence of cavitation is determined by the relative timing of syringe pressurization and syringe acceleration, which is affected by several factors such as the presence, location, and size of an air gap inside the syringe, and the friction between the plunger-stopper and the syringe.
\r\n\r\nExperiments and numerical simulations have shown that sharp pressure waves traveling inside the syringe can be amplified within the cone terminating the syringe. Despite the potential for shock focusing, the impulsive pressurization and the rapid deceleration of pre-filled syringes create a potential for failure which is localized in the syringe shoulder and at the junction between the flange and the barrel, not inside the cone. The cavitation events, on the other hand, create a potential for failure which is limited to a region in close proximity of the bubble upon collapse. The collapse of cavitation bubbles located within the syringe cone can be enhanced due to geometrical effects, and the resulting stresses can be large enough to cause syringe failure.
\r\n\r\nThis thesis demonstrates that static and quasi-static analyses do not provide accurate estimates of the peak pressures and stresses occurring within the device. The pressure and stresses created by the highly dynamic events occurring during actuation need to be accounted for during device design in order to improve device reliability, the user's experience, and patient's adherence to prescribed treatments. The findings discussed in this work provide insights and guidance as to how the transient events can be mitigated.
" }, { "name": "Vidyasagar, A.", "degree": "PhD", "year": "2019", "title": "Predicting Microstructural Pattern Formation Using Stabilized Spectral Homogenization", "advisor": "Kochmann, Dennis M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:03272019-170619076", "creators": [ { "name": { "family": "Vidyasagar", "given": "A." }, "id": "Vidyasagar-A", "orcid": "0000-0003-0262-5429", "display_name": "Vidyasagar, A." } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "advisor", "display_name": "Kochmann, Dennis M." } ], "committee": [ { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "chair", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F1VN-1X80", "abstract": "Instability-induced patterns are ubiquitous in nature, from phase transformations and ferroelectric switching to spinodal decomposition and cellular organization. While the mathematical basis for pattern formation has been well-established, autonomous numerical prediction of complex pattern formation has remained an open challenge. This work aims to simulate realistic pattern evolution in material systems exhibiting non-(quasi)convex energy landscapes. These simulations are performed using fast Fourier spectral techniques, developed for high-resolution numerical homogenization. In a departure from previous efforts, compositions of standard FFT-based spectral techniques with finite-difference schemes are used to overcome ringing artifacts while adding grid-dependent implicit regularization.
\r\n\r\nThe resulting spectral homogenization strategies are first validated using benchmark energy minimization examples involving non-convex energy landscapes. The first investigation involves the St. Venant-Kirchhoff model, and is followed by a novel phase transformation model and finally a finite-strain single-slip crystal plasticity model. In all these examples, numerical approximations of energy envelopes, computed through homogenization, are compared to laminate constructions and, where available, analytical quasiconvex hulls.
\r\n\r\nSubsequently, as an extension of single-slip plasticity, a finite-strain viscoplastic formulation for hexagonal-closed-packed magnesium is presented. Microscale intragranular inelastic behavior is captured through high-fidelity simulations, providing insight into the micromechanical deformation and failure mechanisms in magnesium. Studies of numerical homogenization in polycrystals, with varying numbers of grains and textures, are also performed to quantify convergence statistics for the macroscopic viscoplastic response.
\r\n\r\nIn order to simulate the kinetics of pattern evolution, stabilized spectral techniques are utilized to solve phase-field equations. As an example of conservative gradient-flow kinetics, phase separation by anisotropic spinodal decomposition is shown to result in cellular structures with tunable elastic properties and promise for metamaterial design. Finally, as an example of nonconservative kinetics, the study of domain wall motion in polycrystalline ferroelectric ceramics predicts electromechanical hysteresis behavior under large bias fields. A first-principles approach using DFT-informed model constants is outlined for lead zirconate titanate, producing results showing convincing qualitative agreement with in-house experiments. Overall, these examples demonstrate the promise of the stabilized spectral scheme in predicting pattern evolution as well as effective homogenized response in systems with non-quasiconvex energy landscapes.
" }, { "name": "Wang, Cong", "degree": "PhD", "year": "2019", "title": "On the Manipulation of a Turbulent Boundary Layer by Unsteady Boundary Conditions", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-114300433", "creators": [ { "name": { "family": "Wang", "given": "Cong" }, "id": "Wang-Cong", "orcid": "0000-0002-8271-5637", "display_name": "Wang, Cong" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Brown", "given": "Garry L." }, "id": "Brown-Garry-L", "role": "member", "display_name": "Brown, Garry L." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FT8M-PM75", "abstract": "Reducing the frictional drag generated by a turbulent boundary layer (TBL) is critical for many engineering applications. Motivated by existing turbulent drag reduction methods, this study explores the possibility of sustaining wall-attached air-films and manipulating the near-wall turbulence in hydrodynamic TBL. An innovative air-retaining system is designed to sustain and dynamically modulate the wall-attached air-films in TBL. In still water, the oscillating air-films induce vortical motions in the near-region of air-films. In TBL, phenomena such as Stokes-type oscillatory motion, zero- shear-stress layer, 'inactive' turbulence and reduced viscous shear stress are observed in the vicinity region of air-films. The analysis shows that TBL momentum transfer toward the wall is suppressed and a turbulence re-laminarization mechanism is induced in the near-wall region. One potential physical mechanism points to the process of vorticity generation in the near-region of oscillating air-films, which 'pushes' the TBL near-wall vortical structures away from the wall. With this viewpoint, the phenomena mentioned above can be explained. The modified momentum transfer mechanism and turbulence re-laminarization process are shown to be the potential cause of suppressed viscous shear stress in the near-wall region. Estimated using the Clauser chart method, the turbulent wall-skin friction shows a noticeable decrease in the presence of air-films.
" }, { "name": "Wei, Yuchen", "degree": "PhD", "year": "2019", "title": "Deployable Piezoelectric Thin Shell Structures: Concepts, Characterization and Vibration Control", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-114129662", "creators": [ { "name": { "family": "Wei", "given": "Yuchen" }, "id": "Wei-Yuchen", "orcid": "0000-0002-7669-020X", "display_name": "Wei, Yuchen" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "orcid": "0000-0002-6657-3907", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/4DRX-2X87", "abstract": "The thesis presents three interconnected technology paths to the design and realization of novel deployable active thin shell structures. The baseline concept envisioned is built upon a deployable ultra-thin piezoelectric active thin shell architecture, with segmented tessellations. This vision is motivated by the need to deploy and control large, curved and precise surfaces for a variety of applications including future space telescopes, and is made possible by recent progress in ultra-thin high-performance composites and active material technologies. The thesis uses a combination of heuristic design, theoretical analysis, numerical modeling and novel experimental techniques to construct and validate proposed concepts for deployable piezoelectric thin shells.
\r\n\r\nSpecifically, the thesis answers the following questions: i) How to design and manufacture precise, foldable and curved piezoelectric shells. ii) How to deploy these shells reliably and maintain shape correctability in the deployed state. iii) How to synthesize large, curved deployable surfaces with the aforementioned advantages. iv) How to characterize and predict the nonlinear behavior of piezoelectric materials and thin structures under high electric field actuation and large bending deformations. v) How to improve the shape stability of piezoelectric active thin shells under dynamic disturbances without introducing external sensors.
\r\n\r\nFirst, the thesis proposes new methodologies and design criteria to synthesize deployable, modular edge-supported thin shells based on a combination of origami-inspired folding patterns and spatial mechanisms. In contrast to traditional deployable surface designs, which attach rigid shells to deployable trusses, the proposed methodology enables concurrent folding of flat or curved shells along with the support structures. Starting from a basic module, a variety of deployable surface concepts are proposed through tessellations of the module.
\r\n\r\nA piezoelectric material unimorph architecture is further introduced, providing global curvature and shape correction capabilities. All components of the basic concept are validated through model prototyping and material folding tests, and it is discovered that both the ultra thin carbon fiber composites and piezoelectric ceramic materials can achieve a small folding radius without failure. A composite, doubly-curved foldable shell is also designed and manufactured while still maintaining low shape error. These efforts have led to a new family of deployable piezoelectric thin shell structures that integrate low areal density, high shape accuracy, and structural foldability to an unprecedented degree.
\r\n\r\nThe thesis then tackles the challenge of estimating the actuation response and residual structural deformation of unimorph active thin shells under high electric field and large bending motion. A rate-independent, full field phenomenological constitutive model for a polycrystalline piezoelectric material is characterized experimentally. It successfully captures both the observed ferroelectric and ferroelastic domain switching effects. To overcome the difficulty of testing ultra thin piezoelectric plates, a set of novel characterization techniques is developed and implemented to measure the dielectric and mechanical responses of this material. The characterized material constitutive relation is implemented in an efficient model for estimating the structural response of unimorph thin shells under general electric and mechanical loading. The complete set of governing equations is integrated with a Backward-Euler algorithm, reproducing the measured responses of both the material and the structure under complex loading sequences.
\r\n\r\nActive vibration damping based on self-sensing piezoelectric thin shells is then analyzed and demonstrated on testbed. The self-sensing architecture removes redundant external sensors by making dual use of the piezoelectric layer of the active shell. An adaptive identification method with the associated hardware to track the evolution of field dependent piezoelectric capacitance is implemented, and a new identification strategy is proposed. Closed loop damping with in-situ capacitance adaptation is conducted in bench tests on self-sensing cantilever beams and achieves -12~dB attenuation at the resonance frequency. A highly efficient modeling technique for general self-sensing piezoelectric thin shell structures is proposed which is able to construct closed loop dynamic models based on the vibration eigenmodes and actuation responses obtained from commercial finite element software. These validated modeling techniques are extended to a multi-electrode doubly curved thin shell, where the improvements of shape stability under closed loop damping are evaluated through simulations. It is discovered that the electrode pattern of the self-sensing piezoelectric layer determines the damping performance under the specific boundary conditions of the shell.
" }, { "name": "Braun, Noah Oakley", "degree": "PhD", "year": "2018", "title": "An LES and RANS Study of the Canonical Shock-Turbulence Interaction", "advisor": "Pullin, Dale Ian; Meiron, Daniel I.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05212018-165403115", "creators": [ { "name": { "family": "Braun", "given": "Noah Oakley" }, "id": "Braun-Noah-Oakley", "orcid": "0000-0002-9710-0686", "display_name": "Braun, Noah Oakley" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "co-advisor", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "co-advisor", "display_name": "Meiron, Daniel I." } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Gore", "given": "Robert A." }, "id": "Gore-Robert-A", "role": "member", "display_name": "Gore, Robert A." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BGX1-C128", "abstract": "The canonical problem of a nearly stationary, nearly planar shockwave passing through isotropic turbulence is investigated within high Reynolds number regimes. The subject flow contains a wide range of turbulent scales and is addressed in Large Eddy Simulation (LES) to relax the otherwise prohibitive computational cost of simulating these flows. Aliasing errors in the LES of the upstream isotropic turbulence are shown to interact with the mean compression of the shock in a problematic matter, and may result in nonphysical behavior such as a reduction in the dissipation rate as the flow crosses the shock. A method for the regularization of LES of shock-turbulence interactions is presented which is constructed to enforce that the energy content in the highest resolved wavenumbers decays as k-5/3, and is computed locally in physical space at low computational cost. The application of the regularization to an existing subgrid scale model is shown to remove high wavenumber errors while maintaining agreement with DNS of forced and decaying isotropic turbulence. Comparisons to analytical models suggest that the regularization significantly improves the ability of the LES to predict amplifications in subgrid terms over the modeled shockwave.
\r\n\r\nThe regularization method is then employed in high resolution LES intended to illustrate the physical behavior of the shocked, turbulent flow. Turbulent statistics downstream of the interaction are provided for a range of weakly compressible upstream turbulent Mach numbers Mt = 0.03 - 0.18, shock Mach numbers Ms = 1.2 - 3.0, and Taylor-based Reynolds numbers Re\u03bb = 20 - 2500. The LES displays minimal Reynolds number effects once an inertial range has developed for Re\u03bb > 100. The inertial range scales of the turbulence are shown to quickly return to isotropy, and downstream of sufficiently strong shocks this process generates a net transfer of energy from transverse into streamwise velocity fluctuations. The streamwise shock displacements are shown to approximately follow a k-11/3 decay with wavenumber as predicted by linear analysis. In conjunction with other statistics this suggests that the instantaneous interaction of the shock with the upstream turbulence proceeds in an approximately linear manner, but nonlinear effects immediately downstream of the shock significantly modify the flow even at the lowest considered turbulent Mach numbers.
\r\n\r\nLES allows consideration of high Re\u03bb flows, but remains expensive to compute relative to lower cost modeling approaches such as Reynolds-Averaged Navier Stokes (RANS). Conventional RANS models are often not well suited for simulations containing discontinuous features such as shocks and, in an effort to improve the performance of RANS, models for averaged shock corrugation effects and the impact of turbulent entropy or acoustic modes on the energy equation are presented. Unlike previous RANS work that has focused on the modification of turbulent statistics by the shock, the proposed models are introduced to capture the effects of the turbulence on the profiles of primitive variables --- mean density, velocity, and pressure. By producing accurate profiles for the primitive variables, it is shown that the proposed models improve numerical convergence behavior with mesh refinement about a shock, and introduce the physical effects of shock asphericity in a converging shock geometry. These effects are achieved by local closures to turbulent statistics in the averaged Navier-Stokes equations, and can be applied in conjunction with existing Reynolds stress closures that have been constructed for broader applications beyond shock-turbulence interactions.
" }, { "name": "Fragoso, Anthony Thomas", "degree": "PhD", "year": "2018", "title": "Egospace Motion Planning Representations for Micro Air Vehicles", "advisor": "Murray, Richard M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:10242017-193520989", "creators": [ { "name": { "family": "Fragoso", "given": "Anthony Thomas" }, "id": "Fragoso-Anthony-Thomas", "orcid": "0000-0002-5805-9668", "display_name": "Fragoso, Anthony Thomas" } ], "advisors": [ { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "display_name": "Murray, Richard M." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Chung", "given": "Soon-Jo" }, "id": "Chung-Soon-Jo", "role": "member", "display_name": "Chung, Soon-Jo" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Matthies", "given": "Larry H." }, "id": "Matthies-L-H", "role": "member", "display_name": "Matthies, Larry H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9GX48RJ", "abstract": "Navigation of micro air vehicles (MAVs) in unknown environments is a complex sensing and trajectory generation task, particularly at high velocities. In this work, we introduce an efficient sense-and-avoid pipeline that compactly represents range measurements from multiple sensors, trajectory generation, and motion planning in a 2.5\u2013dimensional projective data structure called an egospace representation. Egospace coordinates generalize depth image obstacle representations and are a particularly convenient choice for configuration flat mobile robots, which are differentially flat in their configuration variables and include a number of commonly used MAV plant models. After characterizing egospace obstacle avoidance for robots with trivial dynamics and establishing limits on applicability and performance, we generalize to motion planning over full configuration flat dynamics using motion primitives expressed directly in egospace coordinates. In comparison to approaches based on world coordinates, egospace uses the natural sensor geometry to combine the benefits of a multi-resolution and multi-sensor representation architecture into a single simple and efficient layer.\r\nWe also present an experimental implementation, based on perception with stereo vision and an egocylinder obstacle representation, that demonstrates the specialization of our theoretical results to particular mission scenarios. The natural pixel parameterization of the egocylinder is used to quickly identify dynamically feasible maneuvers onto radial paths, expressed directly in egocylinder coordinates, that enable finely detailed planning at extreme ranges within milliseconds. We have implemented our obstacle avoidance pipeline with an Asctec Pelican quadcopter, and demonstrate the efficiency of our approach experimentally with a set of challenging field scenarios. The scalability potential of our system is discussed in terms of sensor horizon, actuation, and computational limitations and the speed limits that each imposes, and its generality to more challenging environments with multiple moving obstacles is developed as an immediate extension to the static framework." }, { "name": "Gat, Ilana Batya", "degree": "PhD", "year": "2018", "title": "Investigations of Incompressible Variable-Density Turbulence in an External Acceleration Field", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12052017-154614667", "creators": [ { "name": { "family": "Gat", "given": "Ilana Batya" }, "id": "Gat-Ilana-Batya", "orcid": "0000-0003-0223-0507", "display_name": "Gat, Ilana Batya" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Matheou", "given": "Georgios" }, "id": "Matheou-Georgios", "role": "member", "display_name": "Matheou, Georgios" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9JM27V7", "abstract": "Of interest to turbulence modeling is the behavior of variable-density flow at high Reynolds numbers - a flow difficult to model. This thesis provides insight into variable-density flow behavior by examining the dynamics and mixing of variable-density turbulence subject to an externally imposed acceleration field. The flow is studied in the zero-Mach-number limit with a series of direct numerical simulations. The flow configuration consists of alternating slabs of high- and low-density fluid in a triply periodic domain. Density ratios in the range of 1.005 to 10 are investigated. The flow produces temporally evolving shear layers.
\r\n\r\nA perpendicular mean density\u2013pressure gradient is maintained as the flow evolves, with multi-scale baroclinic torques generated in the turbulent flow that ensues. For all density ratios studied, the simulations attain Reynolds numbers at the beginning of the fully developed turbulence regime.
\r\n\r\nAn empirical relation for the convection velocity predicts the observed entrainment-ratio and dominant mixed-fluid composition statistics. Two mixing-layer temporal evolution regimes are identified: an initial diffusion-dominated regime with a growth rate with the square-root of time followed by a turbulence-dominated regime with a cubic growth rate in time. In the turbulent regime, composition probability density functions within the shear layers exhibit a slightly tilted ('non-marching') hump, corresponding to the most probable mole fraction. The shear layers preferentially entrain low-density fluid by volume at all density ratios, which is reflected in the mixed-fluid composition.
\r\n\r\nThe mixed-fluid orientations of vorticity, baroclinic torques, density gradients, and pressure gradients are presented. Baroclinic torques, the cross product of the density and pressure gradients, tend to be aligned with positive or negative vorticity direction, with vorticity preferentially aligning with the intermediate eigenvector of the local strain-rate tensor, with some variance.
" }, { "name": "Gori, Marcello", "degree": "PhD", "year": "2018", "title": "Laboratory Investigation of Shear Ruptures: Supersonic Propagation and Nucleation by Fluid Injection", "advisor": "Rosakis, Ares J.; Lapusta, Nadia", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312018-150338327", "creators": [ { "name": { "family": "Gori", "given": "Marcello" }, "id": "Gori-Marcello", "orcid": "0000-0002-7380-3723", "display_name": "Gori, Marcello" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "co-advisor", "display_name": "Lapusta, Nadia" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Andrade", "given": "Jose E." }, "id": "Andrade-J-E", "role": "member", "display_name": "Andrade, Jose E." }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AH9X-V905", "abstract": "Shear rupture nucleation and dynamic propagation is a challenging, non- linear, highly interactive process with important practical implications. Here we focus on two aspects of this problem: propagation speeds and shock front radiation from the dynamic crack tip as well as nucleation of dynamic rupture due to fluid injection.
\r\n\r\nSpontaneously propagating cracks in solids emit pressure and shear waves and are, in part, driven by energy transfer due to them. When a shear crack propagates faster than the shear wave speed of the material, the coalescence of the shear wavelets emitted by the near-crack-tip region forms a shock front that significantly concentrates particle motion. The equivalent scenario involving a pressure shock front should not be possible, since cracks should not be able to exceed the pressure wave speed, at least in an isotropic linear-elastic solid. Here we present full-field experimental evidence of dynamic shear cracks in viscoelastic polymers that result in the formation of a pressure shock front, in addition to the shear one. In that sense, the crack appears to be supersonic. The apparent violation of classic theories is explained by the strain-rate-dependent material behavior of polymers: the increased wave speeds within the highly- strained region around the crack tip allow for supersonic crack propagation with respect to the (lower) wave speeds at short distances away from the interface, resulting in the formation of the pressure shock front. The crack speed remains below the pressure wave speed prevailing locally, about its tip, in agreement with basic physics and energy considerations of linear-elastic theories.
\r\n\r\nWe find that the shock fronts emitted by the shear cracks in the viscoelastic materials are curved and propose a novel method to quantify the viscoelastic wave speeds of the solids in the dynamic range of strain rates based on the curvature. Only kinematic relationships are used in the method, without the need for the constitutive relationship of the material. Measuring or inferring the material properties at elevated strain rates in viscoelastic solids is a difficult task, because of practical limitations of obtaining accurate measurements in that regime. Under the quasi-elastic solid approximation, in which the strain-rate history is neglected, we use the pressure-wave speed measurements to infer the associated value of the Young\u2019s modulus, estimated by assuming a constant value of the Poisson\u2019s ratio. We complement these results with the characterization of the Young\u2019s modulus at lower strain rates via canonical compressive tests. Our results not only confirm previous findings that the Young\u2019s modulus dependence on the strain rate in PMMA is significant but also demonstrate that its variation is more pronounced in the dynamic strain-rate range, with important consequences for the design of structures employing viscoelastic materials that are required to withstand elevated strain rates.
\r\n\r\nThe second part of the study concentrates on the nucleation of shear dynamic rupture due to fluid injection or, more broadly, on the interaction of frictional faulting with fluids. Fluid overpressure is recognized to play a fundamental role in promoting fault motion. A large number of observations has shed light on the interplay between fluids and faulting, both in natural events and in earth-quakes induced by human activities, such as wastewater disposal associated with oil and gas extraction. Fluids can induce a variety of earthquake source behaviors ranging from unstable, dynamic motions to stable, quasi-static ones, which a number of field studies suggests that can coexist on the same fault areas at different times, depending on the local conditions. In fact, a higher pore pres-sure plays the dual role of reducing the frictional strength of the fault and of increasing the nucleation size, e.g., the critical length for a shear crack to transition from quasi-static to dynamic motions. However, due to the complexity of the frictional problem at the fault interface, the understanding of which of these two effects prevails remains elusive. The assumption of a critical nucleation length represents a powerful, yet simplified concept, which currently does not include the dependence on the rate of the pore pressure increase.
\r\n\r\nHere, we explore the effect of the rate of the pore pressure increase on the rupture nucleation. We find that elevated injection rates induce triggering of the rupture at lower pressure values and minimal volumes of the injected fluid, if compared to slow injection rates. For the slow injection rates, we experimentally observe a much larger portion of interface wetted by the fluid and a phase of accelerated slip prior to the dynamic event (quasi-dynamic nucleation process). In some cases, we record much smaller foreshock-like events at the injection site. These findings suggest the presence of a prominent quasi-static nucleation process over the interface. In cases of rapid pore pressure increase, the nucleation process is much shorter in time and much more compact in space, being highly concentrated around the injection location. The dynamic events, once initiated, are qualitatively similar across different injection rates, but quantitatively different, with the slow-injection ones experiencing higher stress drops and higher slips, perhaps due to the effect of fluids on the friction properties.\r\nThese findings suggest the need to develop nucleation size estimates that include the rate of the pore pressure increase and motivate further investigation of how friction properties depend on the presence of fluids. The details of the obtained experimental findings, once analyzed through numerical modeling, will place important constrains on the forms of the acceptable friction laws, including the effects of pore fluid pressure and its rate of change.
" }, { "name": "Grivel, Morgane Anne Marie", "degree": "PhD", "year": "2018", "title": "On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:08182017-103752052", "creators": [ { "name": { "family": "Grivel", "given": "Morgane Anne Marie" }, "id": "Grivel-Morgane-Anne-Marie", "orcid": "0000-0002-4391-799X", "display_name": "Grivel, Morgane Anne Marie" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9736P2V", "abstract": "Numerous studies have investigated how liquid water behaves on solid surfaces with uniformly hydrophilic or uniformly hydrophobic wetting properties. In particular, uniformly hydrophobic surfaces have been widely studied for modifying flow behavior of rivulets and drops at smaller scales, as well as for drag reduction on ships or other free-surface-piercing bodies at larger scales. Despite the extensive body of work on surfaces with uniform wetting properties, minimal work has been done to investigate how combining hydrophilic and hydrophobic regions onto a single surface to create macroscopic non-uniform wetting properties affects flows. Research in this vein has predominantly focused on low Reynolds number flows, such as in microfluidic channels or droplet impacts.
\r\n\r\nThis thesis expands on the current literature by investigating contact line dynamics and global flow behavior on surfaces with larger-scale non-uniform wetting properties. Experiments were first carried out to study thin sheet flow down an inclined plate at Re ~ 50 - 1200. The plate's wetting condition was changed by introducing alternating hydrophilic and hydrophobic bands 2-25 mm wide oriented at different angles with respect to the flow direction. Results show that the contact line of such flows is heavily modified compared to the uniform cases. At low Reynolds numbers, large-scale wettability heterogeneities are observed to tune the fingering instability wavelength if the bands are parallel to the flow direction and to dampen finger oscillations if the bands are perpendicular to the flow direction. At higher Reynolds numbers, roller structures are introduced at every hydrophilic-to-hydrophobic junction, modifying the global flow morphology. Entrained air bubbles are also captured and observed to coalesce if the bands are perpendicular to the flow direction.
\r\n\r\nThese experiments were then extended to a surface-piercing hydrofoil coated with alternating hydrophilic and hydrophobic bands. Experiments were run in Caltech's Free Surface Laboratory water tunnel for Re on the order of 104 to 105. The experiments demonstrate that the contact line is modulated in this context, alternating from concave to convex over the different wettability regions. The modulation of the contact line propagates to the rest of the water free-surface via the generation of standing waves and further modifies the free-surface separation point's location and steadiness. In addition, changes in wettability are observed to generate side force, which is of interest for vessel maneuvers in naval applications.
" }, { "name": "Kirchdoerfer, Trenton Thomas", "degree": "PhD", "year": "2018", "title": "Data Driven Computing", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:09122017-092017294", "creators": [ { "name": { "family": "Kirchdoerfer", "given": "Trenton Thomas" }, "id": "Kirchdoerfer-Trenton-Thomas", "orcid": "0000-0003-2290-1857", "display_name": "Kirchdoerfer, Trenton Thomas" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "chair", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Asimaki", "given": "Domniki" }, "id": "Asimaki-D", "orcid": "0000-0002-3008-8088", "role": "member", "display_name": "Asimaki, Domniki" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9Z899MV", "abstract": "Data Driven Computing is a new field of computational analysis which uses provided data to directly produce predictive outcomes. This thesis first establishes definitions of Data-Driven solvers and working examples of static mechanics problems to demonstrate efficacy. Significant extensions are then explored to both accommodate noisy data sets and apply the deveoloped methods to dynamic problems within mechanics. Possible method improvements discuss incorporation of data quality metrics and adaptive data sampling, while new applications focus on multi-scale analysis and the need for public databases to support constitutive data collaboration.\r\n" }, { "name": "Maggi, Alessandro", "degree": "PhD", "year": "2018", "title": "Three-Dimensional Nano-Architected Materials as Platforms for Designing Effective Bone Implants", "advisor": "Greer, Julia R.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12242017-060345135", "creators": [ { "name": { "family": "Maggi", "given": "Alessandro" }, "id": "Maggi-Alessandro", "display_name": "Maggi, Alessandro" } ], "advisors": [ { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "advisor", "display_name": "Greer, Julia R." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Shapiro", "given": "Mikhail G." }, "id": "Shapiro-M-G", "role": "member", "display_name": "Shapiro, Mikhail G." }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "member", "display_name": "Greer, Julia R." } ], "option_major": [ "medeng" ], "doi": "10.7907/Z947482K", "abstract": "The growing world population coupled with longer human life expectancy warrants the need for better medical implant development. Recent advances in lithographic techniques have opened the door to a variety of approaches to tackle the aforementioned issue. However, several scientific hurdles must be overcome before patients can use fully synthetic and effective implants.
\r\n\r\nIdentifying the optimal material, porosity, and mechanical properties of the scaffold to induce cell functionality are key obstacles. Limitations in established fabrication techniques have hindered the ability to fully understand cell behavior on 3D substrates. 3D printing is limited to feature sizes that are at least one order of magnitude larger than a single cell (~10\u03bcm); electrospinning is able to yield features that are on the same scale as cells, but its stochastic nature leads to scaffolds with poor mechanical properties; salt leeching doesn\u2019t allow for control of pore size and distribution which have detrimental effects on nutrient diffusion and cell ingrowth, thereby thwarting the formation of functional tissue.
\r\n\r\nMuch effort has been made to create a suitable platform for regenerating a relatively less complex organ, such as bone, yet the inability to fully understand cell mechanics on 3D scaffolds has curbed the fabrication of effective bone implants.
\r\n\r\nThe first part of this thesis focuses on the suitability of nanoarchitected materials as 3D platforms for bone-tissue growth. We employed two-photon lithography to create polymeric and hydroxyapatite-coated 3D nanolattices to explore scaffold biocompatibility and material effects on osteoblast attachment and growth. Our experiments showed that the unit cell geometry, tetrakaidekahedron, and size, 25\u03bcm, were adequate for cell attachment and infiltration, which are hallmark signs of biocompatibility. Our study also corroborated previous findings that mammalian cells respond differently to different materials that they come in contact with. To isolate structural effects, we fabricated nanolattices coated with a uniform 20nm-thick outermost layer of TiO2. These nanolattices, which had fixed porosity and unit cell size (25\u03bcm) while they varied in structural stiffness (~2-9MPa) were used to explore the influence of scaffold properties on the viability of osteoblasts in a microenvironment similar to that of natural bone. Upon growing osteogenic cells on the nanolattices, significant cell attachment and presence of various calcium phosphate species, which are commonly found in natural bone, were observed. These findings suggest that 3-dimensional nano-architected materials can be used as effective scaffolds for bone cell growth and proliferation.
\r\n\r\nThe second part of the thesis investigates the effects of nanolattice structural stiffness and loading conditions on osteoblast behavior. We fabricated nanolattices with stiffness ranging from ~0.7MPa to 100MPa. Experiments done by seeding osteoblast-like cells on these nanolattices revealed that both stress fiber concentration and bioapatite deposition were higher on the most compliant nanolattice, (0.7 MPa) by ~20% and ~40% respectively. These results provide insights into cell behavior in 3D microenvironments which can lead to a better understanding of stress shielding at the cellular level. Preventing stress shielding by creating scaffolds with structural stiffness and porosity that enhances osteoblasts activity could lead to the creation of effective implants with improved mechanical stability which ultimately improves osteointegration.
\r\n\r\nIn addition to investigating static cell-scaffold interactions we took advantage of the nanolattices tunability to study the effects of dynamic loading on cell behavior. Bone adaptation is driven by dynamic, rather than static loading, however there is still wide controversy on whether stress, strain or loading frequency plays the most significant role in bone remodeling, which drives bone healing.
\r\n\r\nIn order to understand cell sensitivity to varying loads, displacements and frequencies, we fabricated hollow TiO2 nanolattices with stiffness ranging from ~0.7-35MPa which were populated with osteoblast-like cells and subjected to cyclic compression to either a constant stress or strain. After seeding SAOS-2 cells on the nanolattices for 12 days different dynamic loading conditions were tested: (1) cyclic uniaxial compressions to strains ranging from ~0.3-2% strain were carried out to investigate the effects of strain magnitude on cell behavior. (2) Cyclic uniaxial compressions to stresses spanning from ~0.02-1MPa were performed to explore the role of stress magnitude on the cells\u2019 stress fibers formation. (3) The nanolattices were cyclically loaded at different frequencies, ~0.1-3Hz, while maintaining stress and strain constant, which provided insights into how loading frequency affects osteoblasts behavior.
\r\n\r\nCell activity, which was measured by monitoring f-actin and vinculin fluorescence intensity, revealed increased fluorescence in those cells that were mechanically stimulated as opposed to those that were statically grown on the nanolattices regardless of loading condition. Cell response was most drastically affected by varying the loading frequency. A ~30% increase in f-actin fluorescence was observed in the cells grown on the nanolattices that were loaded at ~3Hz compared to those that were grown on the nanolattices that were cyclically compressed at ~0.1Hz.
\r\n\r\nThe last part of this thesis is focused on developing a three-dimensional architected capacitor that could be used as a strain gauge to further our understanding of cell mechanics in 3D. We took advantage of the mechanical tunability of the nanolattices to fabricate a 3D parallel-plate capacitor with a basal capacitance of ~280fF and able to sense forces as low as ~30\u03bcN. This work points to nano-architected materials as promising candidates for ideal platforms to investigate more realistic cellular conditions which can immensely benefit the field of tissue engineering.
" }, { "name": "Martin, Nathan Koon-Hung", "degree": "PhD", "year": "2018", "title": "Analysis of Flapping Propulsion: Comparison, Characterization, and Optimization", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072018-133402239", "creators": [ { "name": { "family": "Martin", "given": "Nathan Koon-Hung" }, "id": "Martin-Nathan-Koon-Hung", "orcid": "0000-0001-6038-6177", "display_name": "Martin, Nathan Koon-Hung" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "member", "display_name": "Dickinson, Michael H." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q6CG-QY57", "abstract": "In recent decades, the development of autonomous underwater vehicles (AUVs) has rapidly increased and inspiration for novel designs has recently come from nature, primarily based on the fast, efficient, and maneuverable flapping motion of fish. Due to its potential, flapping propulsion is investigated through three studies.
\r\n\r\nThe first study involves the comparison between swimming by flapping and by periodic contractions. A direct comparison is made between the two propulsion mechanisms by simplifying the motions, utilizing a machine that can operate in either mode of propulsion, and evaluating the average thrust generated and the average input power required per cycle between the two mechanisms when the overall kinematics are identical. The two propulsion mechanisms are tested using a variety of overall kinematics, flexible plates, and modified duty cycles, all of which suggest that flapping propulsion is the more efficient; however, periodic contractions with a modified duty cycle are shown to generate more thrust per cycle.
\r\n\r\nThe second study involves the characterization of the impact of chord-wise curvature on the hydrodynamic forces and torques, motivated by the dorso-ventral bending of a fish's caudal fin during locomotion. The impact of curvature is shown to depend on the planform area of the flapping plate. Plates with a smaller or an identical planform area compared with a baseline rigid flat rectangular plate either decrease or increase the generated thrust, respectively. These phenomena are utilized to develop an actuated plate for velocity modulation and a snap-buckling plate to provide a greater thrust and efficiency compared with a rigid plate propulsor.
\r\n\r\nThe third study involves the development and demonstration of a method to experimentally optimize an arbitrary three-dimensional trajectory for a flapping propulsor. The trajectory is parameterized by variables inspired by birds and fish, executed by a mechanism that can actuate an arbitrary motion in a hemisphere, and optimized using an adaptive evolutionary strategy. The trajectories are scored based upon their difference from a desired force set-point and their efficiency. All trajectory searches demonstrate good convergence properties and match the desired force set-point almost immediately. Additional generations primarily improve the efficiency. This novel approach finds optimal trajectories for generating side-forces, similar to how a fish's pectoral fin or a bird's wing functions, and for generating thrust, similar to how a fish's caudal fin operates.
" }, { "name": "Mateos Arrieta, Arturo Jos\u00e9", "degree": "PhD", "year": "2018", "title": "Tensile Failure and Fracture of Three-Dimensional Brittle Nanolattices", "advisor": "Greer, Julia R.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06122018-094549478", "creators": [ { "name": { "family": "Mateos Arrieta", "given": "Arturo Jos\u00e9" }, "id": "Mateos-Arrieta-Arturo-Jos\u00e9", "orcid": "0000-0002-9306-3531", "display_name": "Mateos Arrieta, Arturo Jos\u00e9" } ], "advisors": [ { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "advisor", "display_name": "Greer, Julia R." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Faber", "given": "Katherine T." }, "id": "Faber-K-T", "role": "member", "display_name": "Faber, Katherine T." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "member", "display_name": "Greer, Julia R." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AZXG-NB17", "abstract": "The emergence of a new class of cellular solids, i.e., nano- and micro-architected materials, poses the question of whether they can be characterized as a continuum solid. Extensive research has shown that these ultralight and strong structural metamaterials are particularly attractive for mechanically-demanding applications; yet their susceptibility to flaws, fracture behavior, and discrete-continuum duality remains relatively unexplored. In the course of this work, we report the fabrication and tensile-to-failure response of three-dimensional ceramic nanolattices, comprised of 50nm-thick alumina tubes that are arranged into periodic 5um-wide octet-truss unit cells, with and without pre-fabricated through-thickness center notches oriented at different angles to the loading direction. In-situ uniaxial tensile experiments revealed that for all notch orientations, failure always initiated at the notch root, as would be in a monolithic material, with the tube walls at nodal junctions fracturing first, followed by instantaneous crack propagation through the discrete lattice architecture along nodal planes orthogonal to the loading direction. Measured tensile strength of 27.4 MPa was highest for the unnotched samples and decreased systematically with the increase of notch orientation to its minimum of 7.2 MPa in the orthogonally-notched samples. We found the specific tensile strength of hollow-tube octet alumina nanolattices to be 4 times higher than what has been reported for architected and bulk materials at similar low densities. Three-dimensional finite element simulations closely reproduce the observed failure mechanism and trends in failure strength. A direct comparison is made between the experimental measurements, finite element simulations, and predictions of linear elastic fracture mechanics for a self-similar monolithic tensile samples made out of an ideally-brittle solid. Results are in good agreement with the scaling of failure strengths from classical mode I fracture criteria and suggest that trajectory of crack propagation can be adequately explained by considering the connectivity of the lattice architecture. These findings imply that the continuum nature of nano-architected materials offers predictability of failure stresses, which helps enable the development of advanced materials through informed architectural design." }, { "name": "Newman, Matthew Gregory", "degree": "PhD", "year": "2018", "title": "On the Kinetics of Materials of Geophysical Interest", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:08282017-105838669", "creators": [ { "name": { "family": "Newman", "given": "Matthew Gregory" }, "id": "Newman-Matthew-Gregory", "orcid": "0000-0003-2752-0121", "display_name": "Newman, Matthew Gregory" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Kraus", "given": "Richard" }, "id": "Kraus-Richard", "role": "member", "display_name": "Kraus, Richard" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9319T35", "abstract": "Knowledge of the equation of state and phase diagram of magnesium silicates and light iron alloys is important for understanding the thermal evolution and interior structure of terrestrial planets. Dynamic compression techniques are the primary viable methods to create the temperature and pressure conditions that are relevant to Earth and super-Earth (1-10 Earth mass) sized planets. However, due to the kinetic constraints imposed by the timescale of dynamic compression experiments, the nature of the state within the dynamically compressed sample (whether equilibrium or metastable) is uncertain. Here, we present the results of a series of dynamic compression experiments performed on both laser driven compression and plate impact facilities to study the nanosecond to microsecond response of forsterite and iron silicide. In situ x-ray diffraction measurements are used to probe the crystal structure of solid phases and test for the presence of melt, from which we investigate the decomposition of forsterite and iron silicide into compositionally distinct phases at high pressure. For forsterite, we do not observe chemical segregation in the solid phase, however the presence of melt speeds up the kinetics and allows chemical segregation to occur on nanosecond timescales. For iron silicide, our results show a textured solid phase upon shock compression to pressures ranging from 166(14) to 282(24) GPa consistent with cubic and hcp structures in coexistence. Above 313(29) GPa, the intense and textured solid diffraction peaks give way to a diffuse scattering feature and loss of texture, consistent with melting along the Hugoniot." }, { "name": "Rosenberg, Kevin Thomas", "degree": "PhD", "year": "2018", "title": "Resolvent-Based Modeling of Flows in a Channel", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012018-114927289", "creators": [ { "name": { "family": "Rosenberg", "given": "Kevin Thomas" }, "id": "Rosenberg-Kevin-Thomas", "orcid": "0000-0001-6101-3823", "display_name": "Rosenberg, Kevin Thomas" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PHDW-Z389", "abstract": "This thesis concerns the continued development of the resolvent framework (McKeon and Sharma, 2010) to model wall-bounded turbulent flows. Herein, we introduce novel modifications and extensions of the framework to improve the compact representation of flows in a channel. In particular, inspired by ideas rooted in classical linear stability theory, we introduce a decomposition of the velocity field into Orr-Sommerfeld (OS) and Squire (SQ) modes in a nonlinear context via the resolvent operator. We demonstrate through the analysis of a number of exact coherent states (ECS) of the Navier-Stokes equations (NSE) in Couette and Poiseuille flow that this decomposition offers a significant improvement in the low-dimensional representation of these flows. With this efficient basis, we are able to develop through the notion of interaction coefficients a method to compute accurate, self-consistent solutions of the NSE with knowledge of only the mean velocity profile. We also highlight the role of the solenoidal component of the nonlinear forcing in the solution process. In addition, the resolvent framework is extended to the analysis of 2D/3C flows. This approach, again applied to ECS, sheds light on the underlying scale interactions which sustain these solutions. Notably, it reveals that lower branch ECS can be effectively described in their entirety with a single resolvent response mode. This discovery is leveraged to construct a method to compute accurate approximations of ECS starting from a laminar profile using a single parameter model. This thesis also utilizes a constant time-step DNS of a turbulent channel to perform a direct characterization of the nonlinear forcing terms. We compute power spectra and confirm that the nonlinear forcing has a non-trivial signature in the wavenumber-frequency domain. We also compute and analyze spectra for the OS/SQ vorticity and discuss the potential benefit of this decomposition technique to the study of fully turbulent flows as well.
" }, { "name": "Sakovsky, Maria", "degree": "PhD", "year": "2018", "title": "Design and Characterization of Dual-Matrix Composite Deployable Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302018-165415595", "creators": [ { "name": { "family": "Sakovsky", "given": "Maria" }, "id": "Sakovsky-Maria", "orcid": "0000-0002-3683-8505", "display_name": "Sakovsky, Maria" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/CNPY-A883", "abstract": "Dual-matrix composites are a promising approach to deployable high performance antennas for small satellites. Several techniques exist for packaging large antenna apertures. Assemblies of rigid bars and hinges obtain high deployed precision but are heavy and mechanically complex. Thin shell structures deployed using stored strain energy are a lightweight alternative offering efficient packaging but reduced surface precision. Moreover, elastomer composites shells attain even smaller fold radii upon packaging but are limited by the deployed structure's stiffness. Dual-matrix composites combine the advantages of several of these approaches to enable larger antenna apertures. They consist of a continuous woven fiber reinforcement with an elastomer matrix embedded in localized hinge regions and a stiff epoxy resin elsewhere. Such structures can achieve small fold radii, are strain energy deployable, and promise high deployed stiffness.
\r\n\r\nThis research demonstrates the capabilities of the proposed dual-matrix structures through direct comparison to existing antenna designs. Analytic scaling relations between structural and electromagnetic performance of various deployable antenna designs are developed. These are used to rapidly predict achievable antenna performance as a function of a common set of antenna geometric parameters. Plotting of this data on a coordinated set of 2D design plots enables the direct comparison of antenna concepts and the selection of specific designs meeting all requirements. This methodology was used to design a deployable dual-matrix composite conical log spiral (CLS) antenna for use on CubeSats which outperformed existing off-the-shelf designs through higher gain, higher bandwidth, and more efficient packaging.
\r\n\r\nStarting from this initial design, the antenna is tuned to maximize performance and an assembly including the CubeSat, dual-matrix antenna, dual-matrix hinge for antenna deployment, and a flexible feeding network is developed. All portions of the assembly are prototyped and tested. The antenna electromagnetic performance is predicted using ANSYS HFSS and verified by testing in an anaechoic chamber with antenna gains predicted within 4% of measured values. Structural stiffness is characterized through the antenna's fundamental frequency with simulated performance in the Abaqus finite element software within 6% of measured values. Comparison of antenna performance before and after packaging and deployment shows the structural frequency, antenna gain, and antenna bandwidth are unaffected by folding, demonstrating that dual-matrix composites are appropriate for use as deployable structures.
\r\n\r\nTechniques for the quasi-static deployment of dual-matrix composites are presented. An analytic minimum energy method, which accounts for fiber microbuckling in regions of high curvature, is used to predict the folded shape and deployment moments of a dual-matrix hinge. The model shows excellent agreement with LS-Dyna finite element simulations for a variety of material properties. Comparison with experimental characterization demonstrates the capability of the models to predict folded radii and deployment moment of a prototype hinge withing 5% of measured values. The developed analysis tool-set enables a design of deployment restraints and mechanisms.
\r\n\r\nThe woven elastomer composites forming the fold regions in dual-matrix composites have been the subject of very few studies. Existing methods for predicting the stiffness of woven epoxy composites are applied to elastomer composites here and show poor agreement with measurements. A novel approach is presented for the prediction of tow stiffness in elastomer composites using a semi-empirical approach. The reinforcing efficiency parameter in the well-established Halpin-Tsai model for tow homogenization is estimated using experimental measurements of stiffnesses of several laminates. It is shown that for elastomer composites, the parameter values are orders of magnitude higher than the heuristic values used for epoxy composites. The method is used to predict the stiffness of woven epoxy and elastomer composites making up the dual-matrix structures studied in this work showing agreement withing 15% of experimental measurements for arbitrary layups. The method is extended to the prediction of viscoelastic behavior of dual-matrix structures to enable investigation of deployment reliability after long storage times.
" }, { "name": "Schlup, Jason Robert", "degree": "PhD", "year": "2018", "title": "Numerical Investigations of Transport and Chemistry Modeling for Lean Premixed Hydrogen Combustion", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312018-170312588", "creators": [ { "name": { "family": "Schlup", "given": "Jason Robert" }, "id": "Schlup-Jason-Robert", "orcid": "0000-0002-3121-3477", "display_name": "Schlup, Jason Robert" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KFN6-7K54", "abstract": "The use of hydrogen as a fuel for power generation applications has been suggested as an additive to, or replacement of, hydrocarbon fuels. The safety of hydrogen combustion has also received recent attention due to nuclear power plant disasters and the rise of hydrogen refuelling stations. In these uses and scenarios, lean hydrogen--air flames are prone to thermo-diffusive instabilities which can be dangerous to equipment and personnel. These instabilities are heavily influenced by two mechanisms: transport properties (e.g., diffusion) and chemical species production rates. This thesis investigates lean premixed hydrogen combustion using direct numerical simulations. A wide range of flame configurations are considered, spanning one-dimensional steady configurations to three-dimensional unsteady laminar and turbulent flames with high curvature. In particular, the two controlling mechanisms of thermo-diffusive instabilities are carefully investigated.
\r\n\t\r\nThe effects of transport properties, in particular the importance of thermal diffusion in these mixtures, are quantified through global and local evaluations. Thermal diffusion is found to change flame speeds in one-dimensional flat flames, and also modify species profiles due to the increased diffusivity of light reactants. The impact of thermal diffusion is greatly enhanced in the presence of flame curvature, resulting in higher flame speeds (20% to 30% for two- and three-dimensional laminar and turbulent flames), fuel consumption, and flame surface area relative to simulations neglecting thermal diffusion. The mixture-averaged thermal diffusion model proposed by Chapman and Cowling (1970) is found to accurately reproduce global and local flame statistics (including enhanced burning and local extinction) computed using multicomponent transport at significantly reduced costs. Further cost reductions of the mixture-averaged thermal diffusion method are undertaken, and a new model is developed with constant computational requirements for large (~100 species) chemical models. The resulting reduced thermal diffusion model additionally improves upon the accuracy of the mixture-averaged thermal diffusion technique.
\r\n\t\r\nThe effects of fluctuating chemical source terms on flame instabilities are then investigated using tabulated chemistry. One-dimensional unstretched flames including non-equal diffusion and thermal diffusion are incorporated into a chemistry table. This table successfully captures the interaction of differential diffusion and flame curvature. The chemistry tabulation approach is applied to a similar set of flame configurations, and accurate predictions of global and local statistics are found. The tabulated chemistry method reproduces flame curvature, local enhanced burning, and local extinction of unstable flames using one-dimensional, flat, burning flames in its construction. The proposed reduced-order thermal diffusion and chemistry tabulation models significantly reduce computational costs while simultaneously including physical properties necessary to predict lean premixed hydrogen--air flame instabilities.
" }, { "name": "Symon, Sean Pearson", "degree": "PhD", "year": "2018", "title": "Reconstruction and Estimation of Flows Using Resolvent Analysis and Data-Assimilation", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302018-181049042", "creators": [ { "name": { "family": "Symon", "given": "Sean Pearson" }, "id": "Symon-Sean-Pearson", "orcid": "0000-0001-9085-0778", "display_name": "Symon, Sean Pearson" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/B4K7-K876", "abstract": "A flow reconstruction methodology is presented for incompressible, statistically stationary flows using resolvent analysis and data-assimilation. The only inputs necessary for the procedure are a rough approximation of the mean profile and a single time-resolved measurement. The objective is to estimate both the mean and fluctuating states of experimental flows with limited measurements which do not include pressure. The input data may be incomplete, in the sense that measurements near a body are difficult to obtain with techniques such as particle image velocimetry (PIV), or contaminated by noise. The tools developed in this thesis are capable of filling in missing data and reducing the amount of measurement noise by leveraging the governing equations. The reconstructed flow is capable of estimating fluctuations where time-resolved data are not available and solving the flow on larger domains where the mean profile is not known.
\r\n\r\nThe first part of the thesis focuses on how resolvent analysis of the mean flow selects amplification mechanisms. Eigenspectra and pseudospectra of the mean linear Navier-Stokes (LNS) operator are used to characterize amplification mechanisms in flows where linear mechanisms are important. The real parts of the eigenvalues are responsible for resonant amplification and the resolvent operator is low-rank when the eigenvalues are sufficiently separated in the spectrum. Two test cases are studied: low Reynolds number cylinder flow and turbulent channel flow. The latter is studied by considering well-known turbulent structures while the former contains a marginally stable eigenvalue which drowns out the effect of other eigenvalues over a large range of temporal frequencies. There is a geometric manifestation of this dominant mode in the mean profile, suggesting that it leaves a significant footprint on the time-averaged flow that the resolvent can identify. The resolvent does not provide an efficient basis at temporal frequencies where there is no separation of singular values. It can still be leveraged, nevertheless, to identify coherent structures in the flow by approximating the nonlinear forcing from the interaction of highly amplified coherent structures.
\r\n\r\nThe second part of the thesis extends the framework of Foures et al. (2014), who data-assimilated the mean cylinder wake at very low Reynolds numbers. The contributions presented here are to assess the minimum domain for successfully reconstructing Reynolds stress gradients, modifying the algorithm to assimilate mean pressure, determining whether weighting input measurements contributes to improved performance, and adapting the method to experimental data at higher Reynolds numbers. The results from data-assimilating the mean cylinder wake at low Reynolds numbers suggest that the measurement domain needs to coincide with the spatial support of the Reynolds stress gradients while point weighting has a minimal impact on the performance. Finally, a smoothing procedure adapted from Foures et al. (2014) is proposed to cope with data-assimilating an experimental mean profile obtained from PIV data. The data-assimilated mean profiles for an idealized airfoil and NACA 0018 airfoil are solved on a large domain making the mean profile suitable for global resolvent analysis. Data-assimilation is also able to fill in missing or unreliable vectors near the airfoil surface.
\r\n\r\nThe final piece of the thesis is to synthesize the knowledge and techniques developed in the first two parts to reconstruct the experimental flow around a NACA 0018 airfoil. Preliminary results are presented for the case where \u03b1 = 0\u00b0 and Re = 10250. The mean profile is data-assimilated and used as an input to resolvent analysis to educe coherent structures in the flow. The resolvent operator for non- amplified temporal frequencies is forced by an approximated nonlinear forcing. The amplitude and phase of the modes are obtained from the discrete Fourier-transform of a time-resolved probe point measurement. The final reconstruction contains less measurement noise compared to the PIV snapshots and obeys the incompressible Navier-Stokes equations (NSE). The thesis concludes with a discussion of how elements of this methodology can be incorporated into the development of estimators for turbulent flows at high Reynolds numbers.
" }, { "name": "Delapierre, M\u00e9lanie", "degree": "PhD", "year": "2017", "title": "Dynamics and Stability of Spinning Membranes", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05222017-093718571", "creators": [ { "name": { "family": "Delapierre", "given": "M\u00e9lanie" }, "id": "Delapierre-M\u00e9lanie", "display_name": "Delapierre, M\u00e9lanie" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "chair", "display_name": "Daraio, Chiara" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/Z9HT2MC8", "abstract": "Many future space missions require large structures subject to stringent shape accuracy requirements. Spinning membrane-like structures are a cost effective solution for these applications. However, any small deflection of a spinning structure, due to maneuvers or solar radiation pressure, leads to geometrically nonlinear effects on its stability and dynamics. Accurate experiments, simulation tools, and models are required to ensure that buckling and vibrations will not affect mission objectives.
\r\n \r\nWe first focus on the influence of transverse uniform loads on the dynamics and stability of spinning isotropic uniform membranes. A transverse uniform load models the effect of a transverse light beam on flat membranes with small deflections. We present experimental measurements of the angular velocities at which various membranes become wrinkled, and of the wrinkling mode transitions that occur upon spin down. A theoretical formulation to predict the critical angular velocities and critical transverse loads is also presented. The transition between bending dominated and in-plane dominated behavior is identified, and the wrinkling modes are obtained. Next, deflected, non-buckled membranes are further analyzed. Axisymmetric nonlinear oscillations are studied analytically, and a reduced-order model is presented. This model predicts that the deflection of the membrane introduces a hardening behavior at low angular velocities and a softening behavior at high angular velocities. This model is validated through experiments and FEM simulations.
\r\n \r\nThen, we relax the assumption of uniform membranes loaded by transverse light beams. We present an Abaqus model of foldable membranes and show that for particular types of hinges and at high angular velocities, these structures behave like uniform membranes. Finally, we derive an FEM model for solar radiation pressure for quadrilateral surface elements and 3D problems and present its implementation in Abaqus. We show that this follower load introduces an unsymmetric stiffness matrix and that instabilities known as solarelastic flutter can develop. This new FEM capability enables equilibrium and frequency-based stability analyses for a wide range of spacecraft.
" }, { "name": "Hirsch, Damian George", "degree": "PhD", "year": "2017", "title": "An Experimental and Theoretical Study of Active Flow Control", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06092017-112408552", "creators": [ { "name": { "family": "Hirsch", "given": "Damian George" }, "id": "Hirsch-Damian-George", "display_name": "Hirsch, Damian George" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Wygnanski", "given": "Israel J." }, "id": "Wygnanski-I-J", "orcid": "0009-0001-5711-7029", "role": "member", "display_name": "Wygnanski, Israel J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9N014KR", "abstract": "The accelerating growth of environmental awareness has not stopped at the aerospace industry. The need for greener and more efficient airplanes threatens to outpace the flow of new technology. This has ignited development in several fields, one of which is active flow control (AFC). Active flow control has quickly proven its tremendous potential for real applications. Even though the roots of this technology date back a century, we still lack fundamental understanding. This thesis combines both modern and traditional approaches to lay out a new foundation for future research.
\r\n\r\nThe thesis first focuses on the rising stars of active flow control: the so-called fluidic oscillators or sweeping jet actuators. These devices consist of simple, rigid internal geometries that create a sweeping output jet motion. The fluid dynamic interactions with the internal geometry are studied in detail using high-speed Schlieren imaging. Additionally, the influence of adjacent sweeping jets is investigated. It is revealed that the internal driving mechanism is far stronger than the fluid dynamic interactions at the outlet, resulting in a completely independent jet behavior.
\r\n\r\nNext, a high-lift airfoil design is combined with active flow control, and an extensive wind tunnel study is carried out. It is shown that for the given wing design active flow control leads to much higher lift benefits when applied to the trailing edge. Applied to the leading edge active flow control disrupts the vortex lift of the high-lift airfoil, resulting in a deleterious lift effect; however, it shows potential for pitch moment control. This project also underlines the advantages of jet-like active flow control over steady blowing actuation at limited available mass flow rates.
\r\n\r\nThe momentum input coefficient as an important parameter in active flow control is discussed in detail, identifying common misconceptions and difficulties that hinder its proper calculation. An innovative, much simpler approach is introduced. This allows a detailed study of the underlying physics, unveiling unknown limitations of active flow control. The approach is then used as a model to derive the novel concept of thermal active flow control. Experimental studies, including a wind tunnel test campaign, are performed to confirm the viability of the concept for practical applications.
\r\n\r\nThe new calculation method of the input momentum coefficient emphasizes its weakness as a similarity parameter in active flow control studies. The extended mass flow coefficient is introduced as a new parameter. It is shown that it can overcome the deficiencies of the input momentum coefficient without suffering other disadvantages. Its further investigation leads to a deeper understanding of active flow control, which is supported by PIV experiments. The main findings of this investigation divide active flow control into three different \"states\": boundary layer thickening, separation control, and supercirculation.
" }, { "name": "Hogstrom, Kristina", "degree": "PhD", "year": "2017", "title": "Robotically Assembled Space Telescopes with Deployable Modules: Concepts and Design Methodologies", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:07182016-100030713", "creators": [ { "name": { "family": "Hogstrom", "given": "Kristina" }, "id": "Hogstrom-Kristina", "display_name": "Hogstrom, Kristina" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" } ], "option_major": [ "space" ], "doi": "10.7907/Z9T151NT", "abstract": "This thesis first presents a novel architecture for robotically assembled optical telescopes with apertures between 20 m and 100 m, that utilizes only currently available technology. In this architecture, the primary mirror consists of two layers: a reflective layer and a truss backplane layer. The reflective layer is divided into mirror modules, or groups of mirror segments and actuators. The truss backplane layer is divided into truss modules that fold compactly for launch and are deployed in space by the robot. In this thesis, the design methodology of the mirror modules and truss modules is detailed. The ability of the designed truss layer to maintain precision requirements in the presence of typical space environment loads is demonstrated.
\r\n \r\nThis architecture requires the deployment of many truss modules, and thus the deployment must be reliable despite errors introduced during manufacturing. In this thesis, a new simulation-based toolkit for estimating deployment reliability is described, including the experimental validation of the deployment simulation and the Monte Carlo-style method for repeating deployment simulations with different distributions of random fabrication errors to statistically estimate reliability. Using the toolkit, a set of reliability trade studies are then presented, revealing how different types of errors and design parameters affect reliability. Finally, the manufacturing tolerances and design modifications required to ensure high reliability are proposed.
\r\n \r\nEven if all modules deploy successfully, fabrication errors will still be present and may affect the assembly process. In this thesis, a new simulation method is presented that can model the step-by-step assembly of flexible modules with errors. The method is used to reveal that overall shape errors grow with the number of connections, resulting in significantly decreased surface precision and large-scale deformations from the nominal backplane shape as the size of the backplane increases. The misalignment at each individual connection does not increase as the backplane increases, but can still be much larger than the applied manufacturing tolerances simply due to random combinations. A simple design for the interconnects between modules is then tested, with simulation results demonstrating that it is unlikely to fully engage when the expected errors are present. With this information, a requirement on the complexity of the interconnect design is inferred, and potential modifications that may increase its efficacy are suggested.
" }, { "name": "Hufstedler, Esteban Antonio Lemus", "degree": "PhD", "year": "2017", "title": "Experimental Generation and Modeling of Vortical Gusts and Their Interactions with an Airfoil", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05232017-111820349", "creators": [ { "name": { "family": "Hufstedler", "given": "Esteban Antonio Lemus" }, "id": "Hufstedler-Esteban-Antonio-Lemus", "orcid": "0000-0001-7162-920X", "display_name": "Hufstedler, Esteban Antonio Lemus" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9Q52MN5", "abstract": "This thesis examines two methods of vortical gust generation and the interaction between these gusts and an airfoil. These flows were studied with both experiments at a Reynolds number of 20,000 and with potential-flow based simulations.
\r\n\r\nThe standard method of generating a vortical gust has been to rapidly pitch an airfoil. A novel approach is presented: heaving a plate across the tunnel, and changing direction rapidly to release a vortex. This method is motivated by the desire to limit a test article's exposure to the wake of the gust generator by moving it to the side of the tunnel.
\r\n\r\nA series of potential flow models were used to examine these flows: steady and unsteady thin airfoil theory, an extension of Tchieu and Leonard's unsteady airfoil model, and an unsteady vortex panel method.
\r\n\r\nExperiments characterized the generated gusts and verified that the strength of the shed vortices approximately matched the theoretical predictions. The inviscid simulations were unable to predict viscous effects like the wakes of the generators. The pitching airfoil resulted in a persistent wake in the test section, whereas the wake of the heaving plate only temporarily disturbed the flow.
\r\n\r\nThe vortex-wing interaction was examined using both mechanisms. When the wake of the generator was far from the wing, the unsteady simulations provided reasonable estimates for the early variation in lift. This demonstrated that the initial lift peak is due to inviscid effects. Each of the potential flow methods with wake models provided reasonable estimates of this lift. The simplicity of the unsteady thin airfoil theory model recommends its use for examining early vortex-wing interactions.
\r\n\r\nWith the test article mounted at the midline of the tunnel, the wakes had substantial effects when the pitching generator was near the midline of the tunnel, or when the heaving plate passed the midline. The simulations were not able to capture the effects of the wakes or predict the effects of the airfoil's angle of attack. This had the largest effect on the timescale of the post-gust approach to the final forces. With the airfoil at \u03b1=0\u00b0, this was 5-10 convective time units, which is characteristic of attached flows. The airfoil at \u03b1=10\u00b0 needed double the time to approach its final state after perturbations due to its separated flow. The heaving plate's withdrawal allowed for measurement of the resumption of vortex shedding, which was impossible with the pitching airfoil's persistent wake.
" }, { "name": "Nadkarni, Neel P.", "degree": "PhD", "year": "2017", "title": "Nonlinear Dynamics of Transition Waves in Multi-Stable Discrete and Continuous Media", "advisor": "Kochmann, Dennis M.; Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:01132017-035026894", "creators": [ { "name": { "family": "Nadkarni", "given": "Neel P." }, "id": "Nadkarni-Neel-P", "orcid": "0000-0002-4311-2817", "display_name": "Nadkarni, Neel P." } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "advisor", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Cross", "given": "Michael Clifford" }, "id": "Cross-M-C", "role": "member", "display_name": "Cross, Michael Clifford" }, { "name": { "family": "Abeyaratne", "given": "Rohan" }, "id": "Abeyaratne-R", "role": "member", "display_name": "Abeyaratne, Rohan" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9DR2SG2", "abstract": "The concept of phase transitions, i.e., switching between two or more different equilibrium states of a system, is commonly encountered in many physical, chemical and biological phenomena. The exact mechanism of this switching is a highly nonlinear dynamical process that is accommodated by the propagation of a localized wave. The characteristics of the nonlinear wave such as its profile, velocity, energy, and width of transition are governed by the type and specifics of the system that it is propagating through which may be conservative, dissipative, or diffusive in nature. The goal of this thesis is to develop a fundamental understanding of the dynamics of such processes in general nonlinear systems capable of undergoing phase transitions and the application of new theories to elucidate the kinetic and energetic properties of transition waves in different scenarios. In conservative systems, we show that there are three different modes of stable wave propagation that we analytically solve for and validate computationally. In contrast, dissipative and diffusive systems allow the stable propagation of only the strongly nonlinear kink mode whose kinetic energy and propagation velocity are linked through a linear relation. We further validate our results in dissipative systems experimentally by fabricating and testing a strongly nonlinear lattice and show that transition waves are unidirectional in nature, as predicted by theory. Finally, as an application, we devise a strategy of using the physics of dissipative phase transitions to propagate stable mechanical signals in highly dissipative media such as soft polymers which effectively damp out small-amplitude linear waves." }, { "name": "Palm, Steven Leslie", "degree": "PhD", "year": "2017", "title": "Thermo-Acoustic Coupling and Dynamic Response of a Premixed Methane-Air Flame", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechTHESIS:11302017-214955280", "creators": [ { "name": { "family": "Palm", "given": "Steven Leslie" }, "id": "Palm-Steven-Leslie", "orcid": "0000-0003-3095-0368", "display_name": "Palm, Steven Leslie" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Yang", "given": "Vigor" }, "id": "Yang-Vigor", "role": "member", "display_name": "Yang, Vigor" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9V12309", "abstract": "The work herein generally applies to the problem of combustion instability. Combustion instabilities first arose in engineering practice in the 1940s when they were experienced during the development of solid and liquid propellant rocket engines. Later, similar problems arose in gas turbine combustors and afterburners. However, the earliest technical case of the phenomenon dates back to Rijke in 1859 with his \"singing\" tube.
\r\n\r\nThe presented work focuses on the study of a simple, stagnation plane stabilized, laminar, flat-flame burner. In particular the dynamic response of the burner is examined under excitation by a driven acoustic field. After characterization of the burner\u2019s operational range, the response of the system is measured from 20 Hz to nearly 2000 Hz over the span of operating parameters using an optically filtered PMT and lens combination. A library of the collected and reduced data is generated.
\r\n\r\nA deeper investigation of the burner dynamics at a given reference operating condition is performed using phase-resolved PLIF. Fluctuations in the spatial distributions of the LIF signals for several target species (OH, CH, CH2O) under acoustic forcing are measured. In addition, visualization of the unsteady reactant flow using precision acetone seeding and PLIF at 277 nm is performed. Subsequent cinematographic sequences are produced along with spatially resolved plots of the combustion response function and the forced Rayleigh index for numerous drive frequencies. A library of the collected and reduced data is assembled.
\r\n\r\nAnalysis of the collected data reveals two principal mechanisms contributing to the unsteady response of the flame. Structure development in (and subsequent convention along) the unsteady shear layer of the laminar jet dominates the response at the outer reaches of the flame. The inner region of the flame is driven largely by the Helmholtz response of the burner nozzle cavity. These two operations mutually contribute to produce the general shape of the combustion response curve. Ultimately, the data is used to construct a simplified model for the combustion response function. The model is enhanced with two additional revisions guided by the improved understanding of the mechanisms involved.
\r\n\r\nThe document ends with numerous appendices describing, in detail, the equipment used, much of which was fabricated specifically for this work. These appendices, in combination with information presented in the chapters, provide substantial detail regarding the experimental configuration and operating conditions. Great effort was made to provide the necessary information to allow replication of the experiments as well as to support future modeling endeavors as a validation dataset.
\r\n" }, { "name": "Roh, Chris", "degree": "PhD", "year": "2017", "title": "Hydrodynamics of Insects. Part 1. Jetting of the Dragonfly Larvae. Part 2. Honeybee at the Air-water Interface: Surfing with the Capillary Wave", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06082017-183218154", "creators": [ { "name": { "family": "Roh", "given": "Chris" }, "id": "Roh-Chris", "orcid": "0000-0002-5681-0040", "display_name": "Roh, Chris" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "chair", "display_name": "Dickinson, Michael H." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z97P8WFW", "abstract": "This thesis presents the study on the hydrodynamics of two insects commonly known for their aerial adaptation: the dragonfly and the honeybee.
\r\n\r\nPart 1: Anisopteran dragonflies live underwater in their larval stages. The key factor for their aquatic adaptation is the modified hindgut chamber that is used as a pump. The two main functions of this biological pump are jet propulsion and respiration. Both functions involve jetting and refilling of the chamber through an orifice guard by a tri-leaflet anal valve. Despite it being a unique machinery among insects, associated hydrodynamic studies are limited thus far. In the first part of this thesis, various aspects of the hydrodynamics of the dragonfly larvae\u2019s ventilatory flow are studied. The flow visualization showed that the respiratory flow is laminar but the propulsion flow is turbulent. The hydrodynamic force analysis showed that jetting and refilling phase forces are dominated by quasi-steady momentum flux and unsteady acceleration, respectively. Finally, simultaneous measurement of the anal valve kinematics and jet flow showed that the larvae could influence the direction and magnitude of the jet by controlling the anal valve leaflets.
\r\n\r\nPart 2: Water-collecting honeybees often fall onto water surfaces. However, bees trapped by the \u201cstickiness\u201d of the water can propel by vibrating their wings, often making it to shore. In the second part of this thesis, the honeybee\u2019s propulsion mechanisms at the air\u2013water interface is studied. The result shows that the bees can achieve three body-lengths per second propulsion speed. High-speed video of their wing motion shows that honeybee\u2019s propulsion involves pulling blobs of water with the underside of the wing, while pushing on a surface wave with its trailing edge. This propulsion mechanism resembles surfing on a self-generated capillary wave. Moreover, their wing vibration generates complicated surface waves and flows, below which the deeper water flow shows a single jet stream. From the wave and flow field measurements, the average force imparted to the surrounding fluid is estimated and compared to the average force calculated from the bee\u2019s body motion. The resulting average forces are of the same order of magnitude, which means that generating wave and flow are both important for the bee\u2019s propulsion.
" }, { "name": "Sternberger, Zachary Martin Murphy", "degree": "PhD", "year": "2017", "title": "Determining Strength of Materials Under Dynamic Loading Conditions Using Hydrodynamic Instabilities", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:05182017-095600418", "creators": [ { "name": { "family": "Sternberger", "given": "Zachary Martin Murphy" }, "id": "Sternberger-Zachary-Martin-Murphy", "orcid": "0000-0002-7612-673X", "display_name": "Sternberger, Zachary Martin Murphy" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9N877T5", "abstract": "Hydrodynamic instability experiments allow access to material properties at extreme conditions where the pressure exceeds 100 GPa and the strain rate exceeds 106 1/s. Laser ablation dynamically loads a sample, causing a manufactured initial perturbation to grow due to hydrodynamic instability. The instability growth rate depends on the strength of the sample. Material strength can then be inferred from a measurement of the instability growth. Past experiments relied on in-flight diagnostics to measure the amplitude growth, which are not available at all facilities.
\r\n\r\nRecovery instability experiments, where the initial and final amplitude of the instability are measured before and after the sample is dynamically loaded, obviate the need for in-flight diagnostics. Recovery targets containing copper and tantalum samples coined with 2D (hill and valley) and 3D (eggcrate) initial perturbations were dynamically loaded using the Janus laser at the Jupiter Laser Facility, Lawrence Livermore National Laboratory. The energy of the laser pulse was varied to cover a range of conditions in the dynamically compressed sample with pressures in the range 10 GPa to 150 GPa and strain rates in the range 105 1/s to 108 1/s.
\r\n\r\nThe coupling of laser energy into a loading wave was studied with a combination of laser-matter interaction simulations (Hyades) and velocity interferometry data (VISAR). Laser ablation of the recovery targets generated a blast wave, loading the coined initial perturbations with a shock wave followed by a release wave. Different ablator materials and variations in the amount of laser energy deposited in the ablator lead to variations in the loading wave and consequently variations in instability growth.
\r\n\r\nThe growth of the initial perturbation amplitude from initial to final conditions was studied with hydrocode simulations (CTH). During dynamic loading of the sample, the shock wave caused amplitude growth due to hydrodynamic instability. The release wave accelerated the perturbed interface and slowed amplitude growth, in some cases reversing growth.
\r\n\r\nThe sensitivity of the instability growth to coarse changes in the strength model was demonstrated. However, uncertainty in modeling the laser ablation loading prevented a definitive comparison between simulation and experiment.
" }, { "name": "Summy, Dustin Phillip", "degree": "PhD", "year": "2017", "title": "Maximum Entropy Reconstruction for Gas Dynamics", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:05262017-215132894", "creators": [ { "name": { "family": "Summy", "given": "Dustin Phillip" }, "id": "Summy-Dustin-Phillip", "orcid": "0000-0002-6383-0621", "display_name": "Summy, Dustin Phillip" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9GT5K7W", "abstract": "We present a method for selecting a unique and natural probability distribution function (PDF) which satisfies a given number of known moments and apply it for use in the closure of moment-based schemes for approximately solving the Boltzmann equation in gas dynamics.
\r\n\r\nThe method used for determining the PDF is the Maximum Entropy Reconstruction (MER) procedure, which determines the PDF with maximum entropy which satisfies a given set of constraining moments. For the five-moment truncated Hamburger moment problem in one dimension, the MER takes the form of the exponential of a quartic polynomial. This implies a bimodal structure which gives rise to a small-amplitude packet of PDF-density sitting quite far from the mean. This is referred to as the Itinerant Moment Packet (IMP). It is shown by asymptotic analysis that the IMP gives rise to a solution that, in the space of constraining moments, is singular along a line emanating from, but not including, the point representing thermodynamic equilibrium. We use this analysis of the IMP to develop a numerical regularization of the MER, creating a procedure we call the Hybrid MER (HMER). Compared with the MER, the HMER is a significant improvement in terms of robustness and efficiency while preserving accuracy in its prediction of other important distribution features, such as higher order moments.
\r\n\r\nWe apply the one-dimensional HMER to close a fourth order moment system derived from the Boltzmann equation by using a specific set of moment constraints which allow the full, three-dimensional velocity PDF to be treated as a product of three independent, one-dimensional PDFs. From this system, we extract solutions to the problem of spatially homogeneous relaxation and find excellent agreement with a standard method of solution. We further apply this method to the problem of computing the profile within a normal shock wave, and find that solutions exist only within a finite shock Mach number interval. We examine the structure of this solution and find that it has interesting behavior connected to the singularity of the MER and the IMP. Comparison is made to standard solution methods. It is determined that the use of the MER in gas dynamics remains uncertain and possible avenues for further progress are discussed.
" }, { "name": "Wilson, Lee L.", "degree": "PhD", "year": "2017", "title": "Analysis of Packaging and Deployment of Ultralight Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05242017-230338904", "creators": [ { "name": { "family": "Wilson", "given": "Lee L." }, "id": "Wilson-Lee-L", "orcid": "0000-0002-5865-9903", "display_name": "Wilson, Lee L." } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Andrade", "given": "Jose E." }, "id": "Andrade-J-E", "role": "member", "display_name": "Andrade, Jose E." } ], "option_major": [ "space" ], "doi": "10.7907/Z9B27S96", "abstract": "This thesis presents a new approach to modeling in finite element analysis (FEA) creased thin-film sheets such as those used for drag sails, as well as modeling the packaging behavior of coilable deployable booms. This is highly advantageous because these deployable space structures are challenging to test on the ground due to their lightweight nature and the effects of gravity and air resistance. Such structures are utilized in the space industry due to their low mass and ability to be packaged into a small volume during their launch into space.
\r\n\r\nIt is shown that removing the crease bending stiffness in creased sheets still allows the deployment behavior of a benchmark problem to be captured, including deployment forces and equilibrium configurations. In addition, folding creased sheets from a flat state into a packaged configuration can result in crease crumpling and excessive wrinkling. To avoid this the Momentless Crease Force Folding (MCFF) technique is developed.
\r\n\r\nFurther presented is the behavior of tape springs and Tubular Rollable and Coilable (TRAC) booms when coiled to radii greater than their natural bend radius. Under these conditions the booms can form multiple localized folds which may jam during boom deployment. Understanding this behavior is important for extending the use of these booms to large scale space structures such as orbital solar power stations.
\r\n\r\nA useful analytical model is developed determining when the localized folds in a tape spring will bifurcate and is verified against simulation results. Additionally, a numerical model of the wrapping an isotropic tape spring around a hub with a radius greater than the localized fold radii is validated against physical experiments. This model is used to predict trends in the force required to fully wrap a tape spring around a given hub radii.
\r\n\r\nFinally, when examining the coiling and uncoiling behavior of TRAC booms it was found that the tension force required to keep a TRAC boom tightly coiled is significantly less than the force required to initially coil the boom.
" }, { "name": "Araya, Daniel Borsodi", "degree": "PhD", "year": "2016", "title": "Aerodynamics of Vertical-Axis Wind Turbines in Full-Scale and Laboratory-Scale Experiments", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12022015-023535926", "creators": [ { "name": { "family": "Araya", "given": "Daniel Borsodi" }, "id": "Araya-Daniel-Borsodi", "display_name": "Araya, Daniel Borsodi" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9VD6WC2 ", "abstract": "Within a wind farm, multiple turbine wakes can interact and have a substantial effect on the overall power production. This makes an understanding of the wake recovery process critically important to optimizing wind farm efficiency. Vertical-axis wind turbines (VAWTs) exhibit features that are amenable to dramatically improving this efficiency. However, the physics of the flow around VAWTs is not well understood, especially as it pertains to wake interactions, and it is the goal of this thesis to partially fill this void. This objective is approached from two broadly different perspectives: a low-order view of wind farm aerodynamics, and a detailed experimental analysis of the VAWT wake.
\r\n\r\nOne of the contributions of this thesis is the development of a semi-empirical model of wind farm aerodynamics, known as the LRB model, that is able to predict turbine array configurations to leading order accuracy. Another contribution is the characterization of the VAWT wake as a function of turbine solidity. It was found that three distinct regions of flow exist in the VAWT wake: (1) the near wake, where periodic blade shedding of vorticity dominates; (2) a transition region, where growth of a shear-layer instability occurs; (3) the far wake, where bluff-body oscillations dominate. The wake transition can be predicted using a new parameter, the dynamic solidity, which establishes a quantitative connection between the wake of a VAWT and that of a circular cylinder. The results provide insight into the mechanism of the VAWT wake recovery and the potential means to control it.
" }, { "name": "Arya, Manan", "degree": "PhD", "year": "2016", "title": "Packaging and Deployment of Large Planar Spacecraft Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05232016-115519723", "creators": [ { "name": { "family": "Arya", "given": "Manan" }, "id": "Arya-Manan", "orcid": "0000-0003-3522-6010", "display_name": "Arya, Manan" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "member", "display_name": "Hunt, Melany L." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/Z9Z60M0D", "abstract": "This thesis presents a set of novel methods to biaxially package planar structures by folding and wrapping. The structure is divided into strips connected by folds that can slip during wrapping to accommodate material thickness. These packaging schemes are highly efficient, with theoretical packaging efficiencies approaching 100%. Packaging tests on meter-scale physical models have demonstrated packaging efficiencies of up to 83%. These methods avoid permanent deformation of the structure, allowing an initially flat structure to be deployed to a flat state.
\r\n\r\nAlso presented are structural architectures and deployment schemes that are compatible with these packaging methods. These structural architectures use either in-plane pretension -- suitable for membrane structures -- or out-of-plane bending stiffness to resist loading. Physical models are constructed to realize these structural architectures. The deployment of these types of structures is shown to be controllable and repeatable by conducting experiments on lab-scale models.
\r\n\r\nThese packaging methods, structural architectures, and deployment schemes are applicable to a variety of spacecraft structures such as solar power arrays, solar sails, antenna arrays, and drag sails; they have the potential to enable larger variants of these structures while reducing the packaging volume required. In this thesis, these methods are applied to the preliminary structural design of a space solar power satellite. This deployable spacecraft, measuring 60 m x 60 m, can be packaged into a cylinder measuring 1.5 m in height and 1 m in diameter. It can be deployed to a flat configuration, where it acts as a stiff lightweight support framework for multifunctional tiles that collect sunlight, generate electric power, and transmit it to a ground station on Earth.
" }, { "name": "Burgoyne, Hayden Andrew", "degree": "PhD", "year": "2016", "title": "Dynamics of Granular Crystals with Elastic-Plastic Contacts", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:05182016-164150213", "creators": [ { "name": { "family": "Burgoyne", "given": "Hayden Andrew" }, "id": "Burgoyne-Hayden-Andrew", "orcid": "0000-0003-0891-6411", "display_name": "Burgoyne, Hayden Andrew" } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "chair", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Andrade", "given": "Jose E." }, "id": "Andrade-J-E", "role": "member", "display_name": "Andrade, Jose E." }, { "name": { "family": "Newman", "given": "John A." }, "id": "Newman-J-A", "role": "member", "display_name": "Newman, John A." }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" } ], "option_major": [ "space" ], "doi": "10.7907/Z9J38QG6", "abstract": "We study the behavior of granular crystals subjected to impact loading that creates plastic deformation at the contacts between constituent particles. Granular crystals are highly periodic arrangements of spherical particles, arranged into densely packed structures resembling crystals. This special class of granular materials has been shown to have unique dynamics with suggested applications in impact protection. However, previous work has focused on very low amplitude impacts where every contact point can be described using the Hertzian contact law, valid only for purely elastic deformation. In this thesis, we extend previous investigation of the dynamics of granular crystals to significantly higher impact energies more suitable for the majority of applications. Additionally, we demonstrate new properties specific to elastic-plastic granular crystals and discuss their potential applications as well. We first develop a new contact law to describe the interaction between particles for large amplitude compression of elastic-plastic spherical particles including a formulation for strain-rate dependent plasticity. We numerically and experimentally demonstrate the applicability of this contact law to a variety of materials typically used in granular crystals. We then extend our investigation to one-dimensional chains of elastic-plastic particles, including chains of alternating dissimilar materials. We show that, using the new elastic-plastic contact law, we can predict the speed at which impact waves with plastic dissipation propagate based on the material properties of the constituent particles. Finally, we experimentally and numerically investigate the dynamics of two-dimensional and three-dimensional granular crystals with elastic-plastic contacts. We first show that the predicted wave speeds for 1D granular crystals can be extended to 2D and 3D materials. We then investigate the behavior of waves propagating across oblique interfaces of dissimilar particles. We show that the character of the refracted wave can be predicted using an analog to Snell's law for elastic-plastic granular crystals and ultimately show how it can be used to design impact guiding \"lenses\" for mitigation applications." }, { "name": "Coronel, Stephanie Alexandra", "degree": "PhD", "year": "2016", "title": "Thermal Ignition Using Moving Hot Particles", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06032016-210051818", "creators": [ { "name": { "family": "Coronel", "given": "Stephanie Alexandra" }, "id": "Coronel-Stephanie-Alexandra", "orcid": "0000-0002-7088-7976", "display_name": "Coronel, Stephanie Alexandra" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9W37T9X", "abstract": "In this work, ignition of n-hexane-air mixtures was investigated using moving hot spheres of various diameters and surface temperatures. Alumina spheres of 1.8-6 mm diameter were heated using a high power CO2 laser and injected with an average velocity of 2.4 m/s into a premixed n-hexane-air mixture at a nominal initial temperature and pressure of 298 K and 100 kPa, respectively. The 90% probability of ignition using a 6 mm diameter sphere was 1224 K. High-speed experimental visualizations using interferometry indicated that ignition occurred in the vicinity of the separation point in the boundary layer of the sphere when the sphere surface temperature was near the ignition threshold. Additionally, the ignition threshold was found to be insensitive to the mixture composition and showed little variation with sphere diameter.
\r\n\r\nNumerical simulations of a transient one-dimensional boundary layer using detailed chemistry in a gas a layer adjacent to a hot wall indicated that ignition takes place away from the hot surface; the igniting gas that is a distance away from the surface can overcome diffusive heat losses back to the wall when there is heat release due to chemical activity. Finally, a simple approximation of the thermal and momentum boundary layer profiles indicated that the residence time within a boundary layer varies drastically, for example, a fluid parcel originating at very close to the wall has a residence time that is 65x longer than the residence time of a fluid parcel traveling along the edge of the momentum boundary layer.
\r\n\r\nA non-linear methodology was developed for the extraction of laminar flame properties from synthetic spherically expanding flames. The results indicated that for accurate measurements of the flame speed and Markstein length, a minimum of 50 points is needed in the data set (flame radius vs. time) and a minimum range of 48 mm in the flame radius. The non-linear methodology was applied to experimental n-hexane-air spherically expanding flames. The measured flame speed was insensitive to the mixture initial pressure from 50 to 100 kPa and increased with increasing mixture initial temperature. One-dimensional freely-propagating flame calculations showed excellent agreement with the experimental flame speeds using the JetSurF and CaltechMech chemical mechanisms.
" }, { "name": "Cymbalist, Niccolo", "degree": "PhD", "year": "2016", "title": "Mixing, Chemical Reactions, and Combustion in Supersonic Flows", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05242016-143905617", "creators": [ { "name": { "family": "Cymbalist", "given": "Niccolo" }, "id": "Cymbalist-Niccolo", "display_name": "Cymbalist, Niccolo" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9G73BNR", "abstract": "Experiments were conducted at the GALCIT supersonic shear-layer facility to investigate\r\naspects of reacting transverse jets in supersonic crossflow using chemiluminescence and schlieren\r\nimage-correlation velocimetry. In particular, experiments were designed to examine mixing-delay\r\nlength dependencies on jet-fluid molar mass, jet diameter, and jet inclination.
\r\n\r\nThe experimental results show that mixing-delay length depends on jet Reynolds number, when\r\nappropriately normalized, up to a jet Reynolds number of 500,000. Jet inclination increases the\r\nmixing-delay length, but causes less disturbance to the crossflow when compared to normal jet\r\ninjection. This can be explained, in part, in terms of a control-volume analysis that relates jet\r\ninclination to flow conditions downstream of injection.
\r\n\r\nIn the second part of this thesis, a combustion-modeling framework is proposed and developed\r\nthat is tailored to large-eddy simulations of turbulent combustion in high-speed flows. Scaling arguments place supersonic hydrocarbon combustion in a regime of autoignition-dominated distributed\r\nreaction zones (DRZ). The proposed evolution-variable manifold (EVM) framework incorporates an\r\nignition-delay data-driven induction model with a post-ignition manifold that uses a Lagrangian\r\nconvected 'balloon' reactor model for chemistry tabulation. A large-eddy simulation incorporating\r\nthe EVM framework captures several important reacting-flow features of a transverse hydrogen jet\r\nin heated-air crossflow experiment.
" }, { "name": "Duvvuri, Subrahmanyam", "degree": "PhD", "year": "2016", "title": "Non-Linear Scale Interactions in a Forced Turbulent Boundary Layer", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02292016-143116051", "creators": [ { "name": { "family": "Duvvuri", "given": "Subrahmanyam" }, "id": "Duvvuri-Subrahmanyam", "display_name": "Duvvuri, Subrahmanyam" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9Z31WKP ", "abstract": "This thesis explores the dynamics of scale interactions in a turbulent boundary layer through a forcing-response type experimental study. An emphasis is placed on the analysis of triadic wavenumber interactions since the governing Navier-Stokes equations for the flow necessitate a direct coupling between triadically consist scales. Two sets of experiments were performed in which deterministic disturbances were introduced into the flow using a spatially-impulsive dynamic wall perturbation. Hotwire anemometry was employed to measure the downstream turbulent velocity and study the flow response to the external forcing. In the first set of experiments, which were based on a recent investigation of dynamic forcing effects in a turbulent boundary layer, a 2D (spanwise constant) spatio-temporal normal mode was excited in the flow; the streamwise length and time scales of the synthetic mode roughly correspond to the very-large-scale-motions (VLSM) found naturally in canonical flows. Correlation studies between the large- and small-scale velocity signals reveal an alteration of the natural phase relations between scales by the synthetic mode. In particular, a strong phase-locking or organizing effect is seen on directly coupled small-scales through triadic interactions. Having characterized the bulk influence of a single energetic mode on the flow dynamics, a second set of experiments aimed at isolating specific triadic interactions was performed. Two distinct 2D large-scale normal modes were excited in the flow, and the response at the corresponding sum and difference wavenumbers was isolated from the turbulent signals. Results from this experiment serve as an unique demonstration of direct non-linear interactions in a fully turbulent wall-bounded flow, and allow for examination of phase relationships involving specific interacting scales. A direct connection is also made to the Navier-Stokes resolvent operator framework developed in recent literature. Results and analysis from the present work offer insights into the dynamical structure of wall turbulence, and have interesting implications for design of practical turbulence manipulation or control strategies." }, { "name": "Lapointe, Simon", "degree": "PhD", "year": "2016", "title": "Simulation of Premixed Hydrocarbon Flames at High Turbulence Intensities", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-105842881", "creators": [ { "name": { "family": "Lapointe", "given": "Simon" }, "id": "Lapointe-Simon", "display_name": "Lapointe, Simon" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z90V89SW", "abstract": "Turbulent premixed hydrocarbon flames in the thin and distributed reaction zones regimes are simulated using both Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES). A series of DNS is performed to study the transition from the thin reaction zones regime to the distributed reaction zones regime. Differential diffusion effects, distributed burning, and local extinctions are quantified. Different fuels, chemical mechanisms, and equivalence ratios are considered. The fuel Lewis number significantly influences the chemical source terms and turbulent flame speeds. More precisely, simulations with differential diffusion effects exhibit lower mean fuel consumption and heat release rates than their unity Lewis number counterparts. However, the differences are reduced as the reaction zone Karlovitz number is increased. The turbulent reaction zone surface areas increase with the turbulence intensity but aren't strongly affected by fuel, equivalence ratio, chemical mechanism, or differential diffusion. Unsurprisingly, changes in the integral length at a fixed Karlovitz number do not affect the chemical source terms but lead to an increase in flame surface area. Assumptions behind closure models for the filtered source term are then studied a priori using the DNS results. Using the concept of optimal estimators, it is shown that a tabulation approach using a progress variable and its variance can predict accurately the filtered progress variable source term. The filtered source terms are compared to predictions from two common presumed sub-filter Probability Density Functions (PDF) models. Both models show deviations from the filtered DNS source terms but predict accurately the mean turbulent flame speed. Finally, LES of experimentally-studied piloted premixed jet flames are performed using tabulated chemistry. Velocity and flame height measurements from simulations and experiments are compared. The LES are in good agreement with the experimental results for the four different hydrocarbon fuels and three different Reynolds numbers simulated. This corroborates that fuel and chemistry effects in turbulent flames are limited to effects present in laminar flames." }, { "name": "Liska, Sebastian", "degree": "PhD", "year": "2016", "title": "Fast Lattice Green's Function Methods for Viscous Incompressible Flows on Unbounded Domains", "advisor": "Colonius, Tim", "url": "https://resolver.caltech.edu/CaltechTHESIS:04062016-223108239", "creators": [ { "name": { "family": "Liska", "given": "Sebastian" }, "id": "Liska-Sebastian", "orcid": "0000-0003-4139-9364", "display_name": "Liska, Sebastian" } ], "advisors": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "advisor", "display_name": "Colonius, Tim" } ], "committee": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "chair", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9ZC80TG", "abstract": "In this thesis, a collection of novel numerical techniques culminating in a fast, parallel method for the direct numerical simulation of incompressible viscous flows around surfaces immersed in unbounded fluid domains is presented. At the core of all these techniques is the use of the fundamental solutions, or lattice Green\u2019s functions, of discrete operators to solve inhomogeneous elliptic difference equations arising in the discretization of the three-dimensional incompressible Navier-Stokes equations on unbounded regular grids. In addition to automatically enforcing the natural free-space boundary conditions, these new lattice Green\u2019s function techniques facilitate the implementation of robust staggered-Cartesian-grid flow solvers with efficient nodal distributions and fast multipole methods. The provable conservation and stability properties of the appropriately combined discretization and solution techniques ensure robust numerical solutions. Numerical experiments on thin vortex rings, low-aspect-ratio flat plates, and spheres are used verify the accuracy, physical fidelity, and computational efficiency of the present formulations." }, { "name": "Meng, Jomela Chen-Chen", "degree": "PhD", "year": "2016", "title": "Numerical Simulations of Droplet Aerobreakup", "advisor": "Colonius, Tim", "url": "https://resolver.caltech.edu/CaltechTHESIS:05262016-092840941", "creators": [ { "name": { "family": "Meng", "given": "Jomela Chen-Chen" }, "id": "Meng-Jomela-Chen-Chen", "display_name": "Meng, Jomela Chen-Chen" } ], "advisors": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "advisor", "display_name": "Colonius, Tim" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "member", "display_name": "Austin, Joanna M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9KW5D09", "abstract": "The work presented in this thesis aims to bridge an existing gap in the state of droplet aerobreakup knowledge associated with the fundamental flow physics that govern the experimentally observable droplet morphologies. Using direct numerical simulations of the aerobreakup of water cylinders and droplets in the flow behind shock waves in air, we investigate the behavior of the surrounding gas flow to gain insight into the droplet\u2019s deformation and evolution in the stripping breakup regime. The compressible multicomponent Navier-Stokes equations are solved using the Multicomponent Flow Code \u2014 a high-order accurate structured finite-volume flow solver with shock- and interface-capturing. Following qualitative descriptions of the aerobreakup process, comparisons are made with available experimental data. In 2D, accurate measurements of the cylinder\u2019s center-of-mass acceleration across a range of incident shock Mach numbers allow characterization of the unsteady drag coefficient. Additionally, mass loss measurements from viscous simulations refute a well-known boundary layer stripping theory. The results of a 3D nonaxisymmetric aerobreakup simulation are presented with an emphasis on describing the intricate flow phenomena observable in the wake region. Subsequent analyses of the surface instabilities and a Fourier decomposition of the flow field reveal asymmetrical azimuthal modulations and broadband instability growth that result in the devolution of the wake region into chaotic flow." }, { "name": "Mitchell, Stephanie Jane", "degree": "PhD", "year": "2016", "title": "Metaconcrete: Engineered Aggregates for Enhanced Dynamic Performance", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:07072015-124133131", "creators": [ { "name": { "family": "Mitchell", "given": "Stephanie Jane" }, "id": "Mitchell-Stephanie-Jane", "orcid": "0000-0002-7303-8216", "display_name": "Mitchell, Stephanie Jane" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Pandolfi", "given": "Anna" }, "id": "Pandolfi-A", "role": "member", "display_name": "Pandolfi, Anna" } ], "option_major": [ "space" ], "doi": "10.7907/Z9H12ZXN", "abstract": "This work presents the development and investigation of a new type of concrete for the attenuation of waves induced by dynamic excitation. Recent progress in the field of metamaterials science has led to a range of novel composites which display unusual properties when interacting with electromagnetic, acoustic, and elastic waves. A new structural metamaterial with enhanced properties for dynamic loading applications is presented, which is named metaconcrete. In this new composite material the standard stone and gravel aggregates of regular concrete are replaced with spherical engineered inclusions. Each metaconcrete aggregate has a layered structure, consisting of a heavy core and a thin compliant outer coating. This structure allows for resonance at or near the eigenfrequencies of the inclusions, and the aggregates can be tuned so that resonant oscillations will be activated by particular frequencies of an applied dynamic loading. The activation of resonance within the aggregates causes the overall system to exhibit negative effective mass, which leads to attenuation of the applied wave motion. To investigate the behavior of metaconcrete slabs under a variety of different loading conditions a finite element slab model containing a periodic array of aggregates is utilized. The frequency dependent nature of metaconcrete is investigated by considering the transmission of wave energy through a slab, which indicates the presence of large attenuation bands near the resonant frequencies of the aggregates. Applying a blast wave loading to both an elastic slab and a slab model that incorporates the fracture characteristics of the mortar matrix reveals that a significant portion of the supplied energy can be absorbed by aggregates which are activated by the chosen blast wave profile. The transfer of energy from the mortar matrix to the metaconcrete aggregates leads to a significant reduction in the maximum longitudinal stress, greatly improving the ability of the material to resist damage induced by a propagating shock wave. The various analyses presented in this work provide the theoretical and numerical background necessary for the informed design and development of metaconcrete aggregates for dynamic loading applications, such as blast shielding, impact protection, and seismic mitigation." }, { "name": "Montemayor, Lauren Christine", "degree": "PhD", "year": "2016", "title": "Fabrication, Characterization, And Deformation of 3D Structural Meta-Materials ", "advisor": "Greer, Julia R.", "url": "https://resolver.caltech.edu/CaltechTHESIS:07132015-150843708", "creators": [ { "name": { "family": "Montemayor", "given": "Lauren Christine" }, "id": "Montemayor-Lauren-Christine", "display_name": "Montemayor, Lauren Christine" } ], "advisors": [ { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "orcid": "0000-0002-9675-1508", "role": "advisor", "display_name": "Greer, Julia R." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "orcid": "0000-0002-9675-1508", "role": "member", "display_name": "Greer, Julia R." } ], "option_major": [ "space" ], "doi": "10.7907/Z9D21VH2", "abstract": "Current technological advances in fabrication methods have provided pathways to creating architected structural meta-materials similar to those found in natural organisms that are structurally robust and lightweight, such as diatoms. Structural meta-materials are materials with mechanical properties that are determined by material properties at various length scales, which range from the material microstructure (nm) to the macro-scale architecture (\u03bcm \u2013 mm). It is now possible to exploit material size effect, which emerge at the nanometer length scale, as well as structural effects to tune the material properties and failure mechanisms of small-scale cellular solids, such as nanolattices. \r\nThis work demonstrates the fabrication and mechanical properties of 3-dimensional hollow nanolattices in both tension and compression. Hollow gold nanolattices loaded in uniaxial compression demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. Structural effects were explored by increasing the unit cell angle from 30\u00b0 to 60\u00b0 while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200nm to 635nm, at a constant relative density and grain size. In-situ uniaxial compression experiments reveal an order-of-magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of 3-dimensional architected meta-materials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics. \r\nThis work also explores the flaw tolerance of 3D hollow-tube alumina kagome nanolattices with and without pre-fabricated notches, both in experiment and simulation. Experiments demonstrate that the hollow kagome nanolattices in uniaxial tension always fail at the same load when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. For notches with (a/w) > 1/3, the samples fail at lower peak loads and this is attributed to the increased compliance as fewer unit cells span the un-notched region. Finite element simulations of the kagome tension samples show that the failure is governed by tensile loading for (a/w) < 1/3 but as (a/w) increases, bending begins to play a significant role in the failure. This work explores the flaw sensitivity of hollow alumina kagome nanolattices in tension, using experiments and simulations, and demonstrates that the discrete-continuum duality of architected structural meta-materials gives rise to their flaw insensitivity even when made entirely of intrinsically brittle materials.\r\n" }, { "name": "Schmidt, Bryan Eric", "degree": "PhD", "year": "2016", "title": "On the Stability of Supersonic Boundary Layers with Injection", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05252016-141702166", "creators": [ { "name": { "family": "Schmidt", "given": "Bryan Eric" }, "id": "Schmidt-Bryan-Eric", "orcid": "0000-0001-9193-7760", "display_name": "Schmidt, Bryan Eric" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Austin", "given": "Joanna M." }, "id": "Austin-J-M", "orcid": "0000-0003-3129-5035", "role": "chair", "display_name": "Austin, Joanna M." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z93X84M6", "abstract": "The problem of supersonic flow over a 5 degree half-angle cone with injection of gas through a porous section on the body into the boundary layer is studied experimentally. Three injected gases are used: helium, nitrogen, and RC318 (octafluorocyclobutane). Experiments are performed in a Mach 4 Ludwieg tube with nitrogen as the free stream gas. Shaping of the injector section relative to the rest of the body is found to admit a \"tuned\" injection rate which minimizes the strength of shock waves formed by injection. A high-speed schlieren imaging system with a framing rate of 290 kHz is used to study the instability in the region of flow downstream of\r\ninjection, referred to as the injection layer. This work provides the first experimental data on the wavelength, convective speed, and frequency of the instability in such a flow. The stability characteristics of the injection layer are found to be very similar to those of a free shear layer. The findings of this work present a new paradigm for future stability analyses of supersonic flow with injection." }, { "name": "Bitter, Neal Phillip", "degree": "PhD", "year": "2015", "title": "Stability of Hypervelocity Boundary Layers", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06052015-111128842", "creators": [ { "name": { "family": "Bitter", "given": "Neal Phillip" }, "id": "Bitter-Neal-Phillip", "display_name": "Bitter, Neal Phillip" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9Q23X5Z", "abstract": "The early stage of laminar-turbulent transition in a hypervelocity boundary layer is studied using a combination of modal linear stability analysis, transient growth analysis, and direct numerical simulation. Modal stability analysis is used to clarify the behavior of first and second mode instabilities on flat plates and sharp cones for a wide range of high enthalpy flow conditions relevant to experiments in impulse facilities. Vibrational nonequilibrium is included in this analysis, its influence on the stability properties is investigated, and simple models for predicting when it is important are described.
\r\n\r\nTransient growth analysis is used to determine the optimal initial conditions that lead to the largest possible energy amplification within the flow. Such analysis is performed for both spatially and temporally evolving disturbances. The analysis again targets flows that have large stagnation enthalpy, such as those found in shock tunnels, expansion tubes, and atmospheric flight at high Mach numbers, and clarifies the effects of Mach number and wall temperature on the amplification achieved. Direct comparisons between modal and non-modal growth are made to determine the relative importance of these mechanisms under different flow regimes.
\r\n\r\nConventional stability analysis employs the assumption that disturbances evolve with either a fixed frequency (spatial analysis) or a fixed wavenumber (temporal analysis). Direct numerical simulations are employed to relax these assumptions and investigate the downstream propagation of wave packets that are localized in space and time, and hence contain a distribution of frequencies and wavenumbers. Such wave packets are commonly observed in experiments and hence their amplification is highly relevant to boundary layer transition prediction. It is demonstrated that such localized wave packets experience much less growth than is predicted by spatial stability analysis, and therefore it is essential that the bandwidth of localized noise sources that excite the instability be taken into account in making transition estimates. A simple model based on linear stability theory is also developed which yields comparable results with an enormous reduction in computational expense. This enables the amplification of finite-width wave packets to be taken into account in transition prediction.
" }, { "name": "Gabuchian, Vahe", "degree": "PhD", "year": "2015", "title": "Experimental Investigation of Thrust Fault Rupture Mechanics", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04222014-195027916", "creators": [ { "name": { "family": "Gabuchian", "given": "Vahe" }, "id": "Gabuchian-Vahe", "orcid": "0000-0001-7457-5921", "display_name": "Gabuchian, Vahe" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Bhat", "given": "Harsha S." }, "id": "Bhat-H-S", "role": "member", "display_name": "Bhat, Harsha S." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9J96497", "abstract": "Thrust fault earthquakes are investigated in the laboratory by generating dynamic shear ruptures along pre-existing frictional faults in rectangular plates. A considerable body of evidence suggests that dip-slip earthquakes exhibit enhanced ground motions in the acute hanging wall wedge as an outcome of broken symmetry between hanging and foot wall plates with respect to the earth surface. To understand the physical behavior of thrust fault earthquakes, particularly ground motions near the earth surface, ruptures are nucleated in analog laboratory experiments and guided up-dip towards the simulated earth surface. The transient slip event and emitted radiation mimic a natural thrust earthquake. High-speed photography and laser velocimeters capture the rupture evolution, outputting a full-field view of photo-elastic fringe contours proportional to maximum shearing stresses as well as continuous ground motion velocity records at discrete points on the specimen. Earth surface-normal measurements validate selective enhancement of hanging wall ground motions for both sub-Rayleigh and super-shear rupture speeds. The earth surface breaks upon rupture tip arrival to the fault trace, generating prominent Rayleigh surface waves. A rupture wave is sensed in the hanging wall but is, however, absent from the foot wall plate: a direct consequence of proximity from fault to seismometer. Signatures in earth surface-normal records attenuate with distance from the fault trace. Super-shear earthquakes feature greater amplitudes of ground shaking profiles, as expected from the increased tectonic pressures required to induce super-shear transition. Paired stations measure fault parallel and fault normal ground motions at various depths, which yield slip and opening rates through direct subtraction of like components. Peak fault slip and opening rates associated with the rupture tip increase with proximity to the fault trace, a result of selective ground motion amplification in the hanging wall. Fault opening rates indicate that the hanging and foot walls detach near the earth surface, a phenomenon promoted by a decrease in magnitude of far-field tectonic loads. Subsequent shutting of the fault sends an opening pulse back down-dip. In case of a sub-Rayleigh earthquake, feedback from the reflected S wave re-ruptures the locked fault at super-shear speeds, providing another mechanism of super-shear transition." }, { "name": "Mbengue, Cheikh Oumar", "degree": "PhD", "year": "2015", "title": "Storm Track Response to Perturbations in Climate", "advisor": "Schneider, Tapio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05112015-075223217", "creators": [ { "name": { "family": "Mbengue", "given": "Cheikh Oumar" }, "id": "Mbengue-Cheikh-Oumar", "display_name": "Mbengue, Cheikh Oumar" } ], "advisors": [ { "name": { "family": "Schneider", "given": "Tapio" }, "id": "Schneider-T", "orcid": "0000-0001-5687-2287", "role": "advisor", "display_name": "Schneider, Tapio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Teixeira", "given": "Joao" }, "id": "Teixeira-Joao", "role": "member", "display_name": "Teixeira, Joao" }, { "name": { "family": "Schneider", "given": "Tapio" }, "id": "Schneider-T", "orcid": "0000-0001-5687-2287", "role": "member", "display_name": "Schneider, Tapio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9FT8J05", "abstract": "This thesis advances our understanding of midlatitude storm tracks and how they respond to perturbations in the climate system. The midlatitude storm tracks are regions of maximal turbulent kinetic energy in the atmosphere. Through them, the bulk of the atmospheric transport of energy, water vapor, and angular momentum occurs in midlatitudes. Therefore, they are important regulators of climate, controlling basic features such as the distribution of surface temperatures, precipitation, and winds in midlatitudes. Storm tracks are robustly projected to shift poleward in global-warming simulations with current climate models. Yet the reasons for this shift have remained unclear. Here we show that this shift occurs even in extremely idealized (but still three-dimensional) simulations of dry atmospheres. We use these simulations to develop an understanding of the processes responsible for the shift and develop a conceptual model that accounts for it.
\r\n\r\nWe demonstrate that changes in the convective static stability in the deep tropics alone can drive remote shifts in the midlatitude storm tracks. Through simulations with a dry idealized general circulation model (GCM), midlatitude storm tracks are shown to be located where the mean available potential energy (MAPE, a measure of the potential energy available to be converted into kinetic energy) is maximal. As the climate varies, even if only driven by tropical static stability changes, the MAPE maximum shifts primarily because of shifts of the maximum of near-surface meridional temperature gradients. The temperature gradients shift in response to changes in the width of the tropical Hadley circulation, whose width is affected by the tropical static stability. Storm tracks generally shift in tandem with shifts of the subtropical terminus of the Hadley circulation.
\r\n\r\nWe develop a one-dimensional diffusive energy-balance model that links changes in the Hadley circulation to midlatitude temperature gradients and so to the storm tracks. It is the first conceptual model to incorporate a dynamical coupling between the tropical Hadley circulation and midlatitude turbulent energy transport. Numerical and analytical solutions of the model elucidate the circumstances of when and how the storm tracks shift in tandem with the terminus of the Hadley circulation. They illustrate how an increase of only the convective static stability in the deep tropics can lead to an expansion of the Hadley circulation and a poleward shift of storm tracks.
\r\n\r\nThe simulations with the idealized GCM and the conceptual energy-balance model demonstrate a clear link between Hadley circulation dynamics and midlatitude storm track position. With the help of the hierarchy of models presented in this thesis, we obtain a closed theory of storm track shifts in dry climates. The relevance of this theory for more realistic moist climates is discussed.
" }, { "name": "Ning, Xin", "degree": "PhD", "year": "2015", "title": "Imperfection Insensitive Thin Shells", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05212015-174045815", "creators": [ { "name": { "family": "Ning", "given": "Xin" }, "id": "Ning-Xin", "display_name": "Ning, Xin" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "chair", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." } ], "option_major": [ "space" ], "doi": "10.7907/Z91J97P9", "abstract": "The buckling of axially compressed cylindrical shells and externally pressurized spherical shells is extremely sensitive to even very small geometric imperfections. In practice this issue is addressed by either using overly conservative knockdown factors, while keeping perfect axial or spherical symmetry, or adding closely and equally spaced stiffeners on shell surface. The influence of imperfection-sensitivity is mitigated, but the shells designed from these approaches are either too heavy or very expensive and are still sensitive to imperfections. Despite their drawbacks, these approaches have been used for more than half a century.
\r\n\r\nThis thesis proposes a novel method to design imperfection-insensitive cylindrical shells subject to axial compression. Instead of following the classical paths, focused on axially symmetric or high-order rotationally symmetric cross-sections, the method in this thesis adopts optimal symmetry-breaking wavy cross-sections (wavy shells). The avoidance of imperfection sensitivity is achieved by searching with an evolutionary algorithm for smooth cross-sectional shapes that maximize the minimum among the buckling loads of geometrically perfect and imperfect wavy shells. It is found that the shells designed through this approach can achieve higher critical stresses and knockdown factors than any previously known monocoque cylindrical shells. It is also found that these shells have superior mass efficiency to almost all previously reported stiffened shells.
\r\n\r\nExperimental studies on a design of composite wavy shell obtained through the proposed method are presented in this thesis. A method of making composite wavy shells and a photogrametry technique of measuring full-field geometric imperfections have been developed. Numerical predictions based on the measured geometric imperfections match remarkably well with the experiments. Experimental results confirm that the wavy shells are not sensitive to imperfections and can carry axial compression with superior mass efficiency.
\r\n\r\nAn efficient computational method for the buckling analysis of corrugated and stiffened cylindrical shells subject to axial compression has been developed in this thesis. This method modifies the traditional Bloch wave method based on the stiffness matrix method of rotationally periodic structures. A highly efficient algorithm has been developed to implement the modified Bloch wave method. This method is applied in buckling analyses of a series of corrugated composite cylindrical shells and a large-scale orthogonally stiffened aluminum cylindrical shell. Numerical examples show that the modified Bloch wave method can achieve very high accuracy and require much less computational time than linear and nonlinear analyses of detailed full finite element models.
\r\n\r\nThis thesis presents parametric studies on a series of externally pressurized pseudo-spherical shells, i.e., polyhedral shells, including icosahedron, geodesic shells, and triambic icosahedra. Several optimization methods have been developed to further improve the performance of pseudo-spherical shells under external pressure. It has been shown that the buckling pressures of the shell designs obtained from the optimizations are much higher than the spherical shells and not sensitive to imperfections.
\r\n" }, { "name": "Savard, Bruno", "degree": "PhD", "year": "2015", "title": "Characterization and Modeling of Premixed Turbulent n-Heptane Flames in the Thin Reaction Zone Regime", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282015-154709861", "creators": [ { "name": { "family": "Savard", "given": "Bruno" }, "id": "Savard-Bruno", "display_name": "Savard, Bruno" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "space" ], "doi": "10.7907/Z9GM858F", "abstract": "n-heptane/air premixed turbulent flames in the high-Karlovitz portion of the thin reaction zone regime are characterized and modeled in this thesis using Direct Numerical Simulations (DNS) with detailed chemistry. In order to perform these simulations, a time-integration scheme that can efficiently handle the stiffness of the equations solved is developed first. A first simulation with unity Lewis number is considered in order to assess the effect of turbulence on the flame in the absence of differential diffusion. A second simulation with non-unity Lewis numbers is considered to study how turbulence affects differential diffusion. In the absence of differential diffusion, minimal departure from the 1D unstretched flame structure (species vs. temperature profiles) is observed. In the non-unity Lewis number case, the flame structure lies between that of 1D unstretched flames with \"laminar\" non-unity Lewis numbers and unity Lewis number. This is attributed to effective Lewis numbers resulting from intense turbulent mixing and a first model is proposed. The reaction zone is shown to be thin for both flames, yet large chemical source term fluctuations are observed. The fuel consumption rate is found to be only weakly correlated with stretch, although local extinctions in the non-unity Lewis number case are well correlated with high curvature. These results explain the apparent turbulent flame speeds. Other variables that better correlate with this fuel burning rate are identified through a coordinate transformation. It is shown that the unity Lewis number turbulent flames can be accurately described by a set of 1D (in progress variable space) flamelet equations parameterized by the dissipation rate of the progress variable. In the non-unity Lewis number flames, the flamelet equations suggest a dependence on a second parameter, the diffusion of the progress variable. A new tabulation approach is proposed for the simulation of such flames with these dimensionally-reduced manifolds." }, { "name": "Steeves, John Bradley", "degree": "PhD", "year": "2015", "title": "Multilayer Active Shell Mirrors", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282015-145339959", "creators": [ { "name": { "family": "Steeves", "given": "John Bradley" }, "id": "Steeves-John-Bradley", "display_name": "Steeves, John Bradley" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "space" ], "doi": "10.7907/Z99W0CFB", "abstract": "This thesis presents a novel active mirror technology based on carbon fiber composites and replication manufacturing processes. Multiple additional layers are implemented into the structure in order to provide the reflective layer, actuation capabilities and electrode routing. The mirror is thin, lightweight, and has large actuation capabilities. These features, along with the associated manufacturing processes, represent a significant change in design compared to traditional optics. Structural redundancy in the form of added material or support structures is replaced by thin, unsupported lightweight substrates with large actuation capabilities.
\r\n\r\nSeveral studies motivated by the desire to improve as-manufactured figure quality are performed. Firstly, imperfections in thin CFRP laminates and their effect on post-cure shape errors are studied. Numerical models are developed and compared to experimental measurements on flat laminates. Techniques to mitigate figure errors for thicker laminates are also identified. A method of properly integrating the reflective facesheet onto the front surface of the CFRP substrate is also presented. Finally, the effect of bonding multiple initially flat active plates to the backside of a curved CFRP substrate is studied. Figure deformations along with local surface defects are predicted and characterized experimentally. By understanding the mechanics behind these processes, significant improvements to the overall figure quality have been made.
\r\n\r\nStudies related to the actuation response of the mirror are also performed. The active properties of two materials are characterized and compared. Optimal active layer thicknesses for thin surface-parallel schemes are determined. Finite element simulations are used to make predictions on shape correction capabilities, demonstrating high correctabiliity and stroke over low-order modes. The effect of actuator saturation is studied and shown to significantly degrade shape correction performance.
\r\n\r\nThe initial figure as well as actuation capabilities of a fully-integrated active mirror prototype are characterized experimentally using a Projected Hartmann test. A description of the test apparatus is presented along with two verification measurements. The apparatus is shown to accurately capture both high-amplitude low spatial-frequency figure errors as well as those at lower amplitudes but higher spatial frequencies. A closed-loop figure correction is performed, reducing figure errors by 94%.
" }, { "name": "Wojnar, Charles Stanley", "degree": "PhD", "year": "2015", "title": "Exploring the Kinetics of Domain Switching in Ferroelectrics for Structural Applications", "advisor": "Kochmann, Dennis M.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302015-221358386", "creators": [ { "name": { "family": "Wojnar", "given": "Charles Stanley" }, "id": "Wojnar-Charles-Stanley", "display_name": "Wojnar, Charles Stanley" } ], "advisors": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "advisor", "display_name": "Kochmann, Dennis M." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9HD7SM7", "abstract": "The complex domain structure in ferroelectrics gives rise to electromechanical coupling, and its evolution (via domain switching) results in a time-dependent (i.e. viscoelastic) response. Although ferroelectrics are used in many technological applications, most do not attempt to exploit the viscoelastic response of ferroelectrics, mainly due to a lack of understanding and accurate models for their description and prediction. Thus, the aim of this thesis research is to gain better understanding of the influence of domain evolution in ferroelectrics on their dynamic mechanical response.\r\n\r\nThere have been few studies on the viscoelastic properties of ferroelectrics, mainly due to a lack of experimental methods. Therefore, an apparatus and method called Broadband Electromechanical Spectroscopy (BES) was designed and built. BES allows for the simultaneous application of dynamic mechanical and electrical loading in a vacuum environment. Using BES, the dynamic stiffness and loss tangent in bending and torsion of a particular ferroelectric, viz. lead zirconate titanate (PZT), was characterized for different combinations of electrical and mechanical loading frequencies throughout the entire electric displacement hysteresis. Experimental results showed significant increases in loss tangent (by nearly an order of magnitude) and compliance during domain switching, which shows promise as a new approach to structural damping.\r\n\r\nA continuum model of the viscoelasticity of ferroelectrics was developed, which incorporates microstructural evolution via internal variables and associated kinetic relations. For the first time, through a new linearization process, the incremental dynamic stiffness and loss tangent of materials were computed throughout the entire electric displacement hysteresis for different combinations of mechanical and electrical loading frequencies. The model accurately captured experimental results.\r\n\r\nUsing the understanding gained from the characterization and modeling of PZT, two applications of domain switching kinetics were explored by using Micro Fiber Composites (MFCs). Proofs of concept of set-and-hold actuation and structural damping using MFCs were demonstrated." }, { "name": "Coss\u00e9, Julia Theresa", "degree": "PhD", "year": "2014", "title": "On the Behavior of Pliable Plate Dynamics in Wind: Application to Vertical Axis Wind Turbines", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272014-160129404", "creators": [ { "name": { "family": "Coss\u00e9", "given": "Julia Theresa" }, "id": "Coss\u00e9-Julia-Theresa", "display_name": "Coss\u00e9, Julia Theresa" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "chair", "display_name": "Dabiri, John O." }, { "name": { "family": "Sader", "given": "John E." }, "id": "Sader-J-E", "orcid": "0000-0002-7096-0627", "role": "member", "display_name": "Sader, John E." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/X7S3-CS74", "abstract": "Numerous studies have shown that flexible materials improve resilience and durability of a structure. Several studies have investigated the behavior of elastic plates under the influence of a free stream, such as studies of the fluttering flag and others of shape reconfiguration, due to a free stream.
\r\n\r\nThe principle engineering contribution of this thesis is the design and development of a vertical axis wind turbine that features pliable blades which undergo various modes of behavior, ultimately leading to rotational propulsion of the turbine. The wind turbine design was tested in a wind tunnel and at the Caltech Laboratory for Optimized Wind Energy. Ultimately, the flexible blade vertical axis wind turbine proved to be an effective way of harnessing the power of the wind.
\r\n\r\nIn addition, this body of work builds on the current knowledge of elastic cantilever plates in a free stream flow by investigating the inverted flag. While previous studies have focused on the fluid structure interaction of a free stream on elastic cantilever plates, none had studied the plate configuration where the trailing edge was clamped, leaving the leading edge free to move. Furthermore, the studies presented in this thesis establish the geometric boundaries of where the large-amplitude flapping occurs.
" }, { "name": "Fokoua Djodom, Landry", "degree": "PhD", "year": "2014", "title": "Optimal Scaling in Ductile Fracture", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:10162013-221817628", "creators": [ { "name": { "family": "Fokoua Djodom", "given": "Landry" }, "id": "Fokoua-Djodom-Landry", "display_name": "Fokoua Djodom, Landry" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Weinberg", "given": "Kerstin" }, "id": "Weinberg-K", "role": "member", "display_name": "Weinberg, Kerstin" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/B1TW-2D81", "abstract": "This work is concerned with the derivation of optimal scaling laws, in the sense of matching lower and upper bounds on the energy, for a solid undergoing ductile fracture. The specific problem considered concerns a material sample in the form of an infinite slab of finite thickness subjected to prescribed opening displacements on its two surfaces. The solid is assumed to obey deformation-theory of plasticity and, in order to further simplify the analysis, we assume isotropic rigid-plastic deformations with zero plastic spin. When hardening exponents are given values consistent with observation, the energy is found to exhibit sublinear growth. We regularize the energy through the addition of nonlocal energy terms of the strain-gradient plasticity type. This nonlocal regularization has the effect of introducing an intrinsic length scale into the energy. We also put forth a physical argument that identifies the intrinsic length and suggests a linear growth of the nonlocal energy. Under these assumptions, ductile fracture emerges as the net result of two competing effects: whereas the sublinear growth of the local energy promotes localization of deformation to failure planes, the nonlocal regularization stabilizes this process, thus resulting in an orderly progression towards failure and a well-defined specific fracture energy. The optimal scaling laws derived here show that ductile fracture results from localization of deformations to void sheets, and that it requires a well-defined energy per unit fracture area. In particular, fractal modes of fracture are ruled out under the assumptions of the analysis. The optimal scaling laws additionally show that ductile fracture is cohesive in nature, i.e., it obeys a well-defined relation between tractions and opening displacements. Finally, the scaling laws supply a link between micromechanical properties and macroscopic fracture properties. In particular, they reveal the relative roles that surface energy and microplasticity play as contributors to the specific fracture energy of the material. Next, we present an experimental assessment of the optimal scaling laws. We show that when the specific fracture energy is renormalized in a manner suggested by the optimal scaling laws, the data falls within the bounds predicted by the analysis and, moreover, they ostensibly collapse---with allowances made for experimental scatter---on a master curve dependent on the hardening exponent, but otherwise material independent." }, { "name": "Jewell, Joseph Stephen", "degree": "PhD", "year": "2014", "title": "Boundary-Layer Transition on a Slender Cone in Hypervelocity Flow with Real Gas Effects", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292014-220110640", "creators": [ { "name": { "family": "Jewell", "given": "Joseph Stephen" }, "id": "Jewell-Joseph-Stephen", "orcid": "0000-0002-4047-9998", "display_name": "Jewell, Joseph Stephen" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "chair", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Leyva", "given": "Ivett A." }, "id": "Leyva-I-A", "role": "member", "display_name": "Leyva, Ivett A." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9H9935V", "abstract": "The laminar to turbulent transition process in boundary layer flows in thermochemical nonequilibrium at high enthalpy is measured and characterized. Experiments are performed in the T5 Hypervelocity Reflected Shock Tunnel at Caltech, using a 1 m length 5-degree half angle axisymmetric cone instrumented with 80 fast-response annular thermocouples, complemented by boundary layer stability computations using the STABL software suite. A new mixing tank is added to the shock tube fill apparatus for premixed freestream gas experiments, and a new cleaning procedure results in more consistent transition measurements. Transition location is nondimensionalized using a scaling with the boundary layer thickness, which is correlated with the acoustic properties of the boundary layer, and compared with parabolized stability equation (PSE) analysis. In these nondimensionalized terms, transition delay with increasing CO2 concentration is observed: tests in 100% and 50% CO2, by mass, transition up to 25% and 15% later, respectively, than air experiments. These results are consistent with previous work indicating that CO2 molecules at elevated temperatures absorb acoustic instabilities in the MHz range, which is the expected frequency of the Mack second-mode instability at these conditions, and also consistent with predictions from PSE analysis. A strong unit Reynolds number effect is observed, which is believed to arise from tunnel noise. NTr for air from 5.4 to 13.2 is computed, substantially higher than previously reported for noisy facilities. Time- and spatially-resolved heat transfer traces are used to track the propagation of turbulent spots, and convection rates at 90%, 76%, and 63% of the boundary layer edge velocity, respectively, are observed for the leading edge, centroid, and trailing edge of the spots. A model constructed with these spot propagation parameters is used to infer spot generation rates from measured transition onset to completion distance. Finally, a novel method to control transition location with boundary layer gas injection is investigated. An appropriate porous-metal injector section for the cone is designed and fabricated, and the efficacy of injected CO2 for delaying transition is gauged at various mass flow rates, and compared with both no injection and chemically inert argon injection cases. While CO2 injection seems to delay transition, and argon injection seems to promote it, the experimental results are inconclusive and matching computations do not predict a reduction in N factor from any CO2 injection condition computed.
" }, { "name": "John, Kristen Kathleen", "degree": "PhD", "year": "2014", "title": "Strength of Tantalum at High Pressures through Richtmyer-Meshkov Laser Compression Experiments and Simulations", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:08092014-195153430", "creators": [ { "name": { "family": "John", "given": "Kristen Kathleen" }, "id": "John-Kristen-Kathleen", "display_name": "John, Kristen Kathleen" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NE7Y-CK04", "abstract": "Strength at extreme pressures (>1 Mbar or 100 GPa) and high strain rates (106-108 s-1) of materials is not well characterized. The goal of the research outlined in this thesis is to study the strength of tantalum (Ta) at these conditions. The Omega Laser in the Laboratory for Laser Energetics in Rochester, New York is used to create such extreme conditions. Targets are designed with ripples or waves on the surface, and these samples are subjected to high pressures using Omega\u2019s high energy laser beams. In these experiments, the observational parameter is the Richtmyer-Meshkov (RM) instability in the form of ripple growth on single-mode ripples. The experimental platform used for these experiments is the \u201cride-along\u201d laser compression recovery experiments, which provide a way to recover the specimens having been subjected to high pressures. Six different experiments are performed on the Omega laser using single-mode tantalum targets at different laser energies. The energy indicates the amount of laser energy that impinges the target. For each target, values for growth factor are obtained by comparing the profile of ripples before and after the experiment. With increasing energy, the growth factor increased.
\r\n \r\nEngineering simulations are used to interpret and correlate the measurements of growth factor to a measure of strength. In order to validate the engineering constitutive model for tantalum, a series of simulations are performed using the code Eureka, based on the Optimal Transportation Meshfree (OTM) method. Two different configurations are studied in the simulations: RM instabilities in single and multimode ripples. Six different simulations are performed for the single ripple configuration of the RM instability experiment, with drives corresponding to laser energies used in the experiments. Each successive simulation is performed at higher drive energy, and it is observed that with increasing energy, the growth factor increases. Overall, there is favorable agreement between the data from the simulations and the experiments. The peak growth factors from the simulations and the experiments are within 10% agreement. For the multimode simulations, the goal is to assist in the design of the laser driven experiments using the Omega laser. A series of three-mode and four-mode patterns are simulated at various energies and the resulting growth of the RM instability is computed. Based on the results of the simulations, a configuration is selected for the multimode experiments. These simulations also serve as validation for the constitutive model and the material parameters for tantalum that are used in the simulations.
\r\n\r\nBy designing samples with initial perturbations in the form of single-mode and multimode ripples and subjecting these samples to high pressures, the Richtmyer-Meshkov instability is investigated in both laser compression experiments and simulations. By correlating the growth of these ripples to measures of strength, a better understanding of the strength of tantalum at high pressures is achieved.
\r\n" }, { "name": "Lyon, Bradley Joseph", "degree": "PhD", "year": "2014", "title": "A Multi-Scale Approach to Shaping Carbon Nanotube Structures for Hollow Microneedles", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302014-012121120", "creators": [ { "name": { "family": "Lyon", "given": "Bradley Joseph" }, "id": "Lyon-Bradley-Joseph", "display_name": "Lyon, Bradley Joseph" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Tai", "given": "Yu-Chong" }, "id": "Tai-Yu-Chong", "orcid": "0000-0001-8529-106X", "role": "chair", "display_name": "Tai, Yu-Chong" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BJGT-TB74", "abstract": "The concept of a carbon nanotube microneedle array is explored in this thesis from multiple perspectives including microneedle fabrication, physical aspects of transdermal delivery, and in vivo transdermal drug delivery experiments. Starting with standard techniques in carbon nanotube (CNT) fabrication, including catalyst patterning and chemical vapor deposition, vertically-aligned carbon nanotubes are utilized as a scaffold to define the shape of the hollow microneedle. Passive, scalable techniques based on capillary action and unique photolithographic methods are utilized to produce a CNT-polymer composite microneedle. Specific examples of CNT-polyimide and CNT-epoxy microneedles are investigated. Further analysis of the transport properties of polymer resins reveals general requirements for applying arbitrary polymers to the fabrication process.
\r\n\r\nThe bottom-up fabrication approach embodied by vertically-aligned carbon nanotubes allows for more direct construction of complex high-aspect ratio features than standard top-down fabrication approaches, making microneedles an ideal application for CNTs. However, current vertically-aligned CNT fabrication techniques only allow for the production of extruded geometries with a constant cross-sectional area, such as cylinders. To rectify this limitation, isotropic oxygen etching is introduced as a novel fabrication technique to create true 3D CNT geometry. Oxygen etching is utilized to create a conical geometry from a cylindrical CNT structure as well as create complex shape transformations in other CNT geometries.
\r\n\r\nCNT-polymer composite microneedles are anchored onto a common polymer base less than 50 \u00b5m thick, which allows for the microneedles to be incorporated into multiple drug delivery platforms, including modified hypodermic syringes and silicone skin patches. Cylindrical microneedles are fabricated with 100 \u00b5m outer diameter and height of 200-250 \u00b5m with a central cavity, or lumen, diameter of 30 \u00b5m to facilitate liquid drug flow. In vitro delivery experiments in swine skin demonstrate the ability of the microneedles to successfully penetrate the skin and deliver aqueous solutions.
\r\n\r\nAn in vivo study was performed to assess the ability of the CNT-polymer microneedles to deliver drugs transdermally. CNT-polymer microneedles are attached to a hand actuated silicone skin patch that holds a liquid reservoir of drugs. Fentanyl, a potent analgesic, was administered to New Zealand White Rabbits through 3 routes of delivery: topical patch, CNT-polymer microneedles, and subcutaneous hypodermic injection. Results demonstrate that the CNT-polymer microneedles have a similar onset of action as the topical patch. CNT-polymer microneedles were also vetted as a painless delivery approach compared to hypodermic injection. Comparative analysis with contemporary microneedle designs demonstrates that the delivery achieved through CNT-polymer microneedles is akin to current hollow microneedle architectures. The inherent advantage of applying a bottom-up fabrication approach alongside similar delivery performance to contemporary microneedle designs demonstrates that the CNT-polymer composite microneedle is a viable architecture in the emerging field of painless transdermal delivery.
\r\n" }, { "name": "Maqueda Jim\u00e9nez, Ignacio", "degree": "PhD", "year": "2014", "title": "High Strain Composites and Dual-Matrix Composite Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292014-191924394", "creators": [ { "name": { "family": "Maqueda Jim\u00e9nez", "given": "Ignacio" }, "id": "Maqueda-Jim\u00e9nez-Ignacio", "display_name": "Maqueda Jim\u00e9nez, Ignacio" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "chair", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Murphey", "given": "Thomas" }, "id": "Murphey-T", "role": "member", "display_name": "Murphey, Thomas" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z34C-NY82", "abstract": "Most space applications require deployable structures due to the limiting size of current launch vehicles. Specifically, payloads in nanosatellites such as CubeSats require very high compaction ratios due to the very limited space available in this typo of platform. Strain-energy-storing deployable structures can be suitable for these applications, but the curvature to which these structures can be folded is limited to the elastic range. Thanks to fiber microbuckling, high-strain composite materials can be folded into much higher curvatures without showing significant damage, which makes them suitable for very high compaction deployable structure applications. However, in applications that require carrying loads in compression, fiber microbuckling also dominates the strength of the material. A good understanding of the strength in compression of high-strain composites is then needed to determine how suitable they are for this type of application.
\r\n\r\nThe goal of this thesis is to investigate, experimentally and numerically, the microbuckling in compression of high-strain composites. Particularly, the behavior in compression of unidirectional carbon fiber reinforced silicone rods (CFRS) is studied. Experimental testing of the compression failure of CFRS rods showed a higher strength in compression than the strength estimated by analytical models, which is unusual in standard polymer composites. This effect, first discovered in the present research, was attributed to the variation in random carbon fiber angles respect to the nominal direction. This is an important effect, as it implies that microbuckling strength might be increased by controlling the fiber angles. With a higher microbuckling strength, high-strain materials could carry loads in compression without reaching microbuckling and therefore be suitable for several space applications.
\r\n\r\nA finite element model was developed to predict the homogenized stiffness of the CFRS, and the homogenization results were used in another finite element model that simulated a homogenized rod under axial compression. A statistical representation of the fiber angles was implemented in the model. The presence of fiber angles increased the longitudinal shear stiffness of the material, resulting in a higher strength in compression. The simulations showed a large increase of the strength in compression for lower values of the standard deviation of the fiber angle, and a slight decrease of strength in compression for lower values of the mean fiber angle. The strength observed in the experiments was achieved with the minimum local angle standard deviation observed in the CFRS rods, whereas the shear stiffness measured in torsion tests was achieved with the overall fiber angle distribution observed in the CFRS rods.
\r\n\r\nHigh strain composites exhibit good bending capabilities, but they tend to be soft out-of-plane. To achieve a higher out-of-plane stiffness, the concept of dual-matrix composites is introduced. Dual-matrix composites are foldable composites which are soft in the crease regions and stiff elsewhere. Previous attempts to fabricate continuous dual-matrix fiber composite shells had limited performance due to excessive resin flow and matrix mixing. An alternative method, presented in this thesis uses UV-cure silicone and fiberglass to avoid these problems. Preliminary experiments on the effect of folding on the out-of-plane stiffness are presented. An application to a conical log-periodic antenna for CubeSats is proposed, using origami-inspired stowing schemes, that allow a conical dual-matrix composite shell to reach very high compaction ratios.
" }, { "name": "Mohan, Nisha", "degree": "PhD", "year": "2014", "title": "Extracting Material Response from Simple Mechanical Tests on Hardening-Softening-Hardening Viscoplastic Solids", "advisor": "Greer, Julia R.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05142014-151151819", "creators": [ { "name": { "family": "Mohan", "given": "Nisha" }, "id": "Mohan-Nisha", "display_name": "Mohan, Nisha" } ], "advisors": [ { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "orcid": "0000-0002-9675-1508", "role": "advisor", "display_name": "Greer, Julia R." } ], "committee": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "chair", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "orcid": "0000-0002-9675-1508", "role": "member", "display_name": "Greer, Julia R." } ], "option_major": [ "space" ], "doi": "10.7907/MMTW-FF91", "abstract": "Compliant foams are usually characterized by a wide range of desirable mechanical properties. These properties include viscoelasticity at different temperatures, energy absorption, recoverability under cyclic loading, impact resistance, and thermal, electrical, acoustic and radiation-resistance. Some foams contain nano-sized features and are used in small-scale devices. This implies that the characteristic dimensions of foams span multiple length scales, rendering modeling their mechanical properties difficult. Continuum mechanics-based models capture some salient experimental features like the linear elastic regime, followed by non-linear plateau stress regime. However, they lack mesostructural physical details. This makes them incapable of accurately predicting local peaks in stress and strain distributions, which significantly affect the deformation paths. Atomistic methods are capable of capturing the physical origins of deformation at smaller scales, but suffer from impractical computational intensity. Capturing deformation at the so-called meso-scale, which is capable of describing the phenomenon at a continuum level, but with some physical insights, requires developing new theoretical approaches.
\r\n\r\nA fundamental question that motivates the modeling of foams is \u2018how to extract the intrinsic material response from simple mechanical test data, such as stress vs. strain response?\u2019 A 3D model was developed to simulate the mechanical response of foam-type materials. The novelty of this model includes unique features such as the hardening-softening-hardening material response, strain rate-dependence, and plastically compressible solids with plastic non-normality. Suggestive links from atomistic simulations of foams were borrowed to formulate a physically informed hardening material input function. Motivated by a model that qualitatively captured the response of foam-type vertically aligned carbon nanotube (VACNT) pillars under uniaxial compression [2011,\u201cAnalysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,\u201d J. Mech.Phys. Solids, 59, pp. 2227\u20132237, Erratum 60, 1753\u20131756 (2012)], the property space exploration was advanced to three types of simple mechanical tests: 1) uniaxial compression, 2) uniaxial tension, and 3) nanoindentation with a conical and a flat-punch tip. The simulations attempt to explain some of the salient features in experimental data, like
\r\n1) The initial linear elastic response.
\r\n2) One or more nonlinear instabilities, yielding, and hardening.
The model-inherent relationships between the material properties and the overall stress-strain behavior were validated against the available experimental data. The material properties include the gradient in stiffness along the height, plastic and elastic compressibility, and hardening. Each of these tests was evaluated in terms of their efficiency in extracting material properties. The uniaxial simulation results proved to be a combination of structural and material influences. Out of all deformation paths, flat-punch indentation proved to be superior since it is the most sensitive in capturing the material properties.
" }, { "name": "Patterson, Keith D.", "degree": "PhD", "year": "2014", "title": "Lightweight Deformable Mirrors for Future Space Telescopes", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:12182013-094108778", "creators": [ { "name": { "family": "Patterson", "given": "Keith D." }, "id": "Patterson-Keith-D", "display_name": "Patterson, Keith D." } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "member", "display_name": "Greer, Julia R." }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/S7JS-A837", "abstract": "This thesis presents a concept for ultra-lightweight deformable mirrors based on a thin substrate of optical surface quality coated with continuous active piezopolymer layers that provide modes of actuation and shape correction. This concept eliminates any kind of stiff backing structure for the mirror surface and exploits micro-fabrication technologies to provide a tight integration of the active materials into the mirror structure, to avoid actuator print-through effects. Proof-of-concept, 10-cm-diameter mirrors with a low areal density of about 0.5 kg/m² have been designed, built and tested to measure their shape-correction performance and verify the models used for design. The low cost manufacturing scheme uses replication techniques, and strives for minimizing residual stresses that deviate the optical figure from the master mandrel. It does not require precision tolerancing, is lightweight, and is therefore potentially scalable to larger diameters for use in large, modular space telescopes. Other potential applications for such a laminate could include ground-based mirrors for solar energy collection, adaptive optics for atmospheric turbulence, laser communications, and other shape control applications.
\r\n\r\nThe immediate application for these mirrors is for the Autonomous Assembly and Reconfiguration of a Space Telescope (AAReST) mission, which is a university mission under development by Caltech, the University of Surrey, and JPL. The design concept, fabrication methodology, material behaviors and measurements, mirror modeling, mounting and control electronics design, shape control experiments, predictive performance analysis, and remaining challenges are presented herein. The experiments have validated numerical models of the mirror, and the mirror models have been used within a model of the telescope in order to predict the optical performance. A demonstration of this mirror concept, along with other new telescope technologies, is planned to take place during the AAReST mission.
\r\n\r\n" }, { "name": "Rabinovitch, Jason", "degree": "PhD", "year": "2014", "title": "Advancing EDL Technologies for Future Space Missions: From Ground Testing Facilities to Ablative Heatshields", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:05302014-140011538", "creators": [ { "name": { "family": "Rabinovitch", "given": "Jason" }, "id": "Rabinovitch-Jason", "orcid": "0000-0002-1914-7964", "display_name": "Rabinovitch, Jason" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XKM7-7368", "abstract": "Motivated by recent MSL results where the ablation rate of the PICA heatshield was over-predicted, and staying true to the objectives outlined in the NASA Space Technology Roadmaps and Priorities report, this work focuses on advancing EDL technologies for future space missions.
\r\n\r\nDue to the difficulties in performing flight tests in the hypervelocity regime, a new ground testing facility called the vertical expansion tunnel is proposed. The adverse effects from secondary diaphragm rupture in an expansion tunnel may be reduced or eliminated by orienting the tunnel vertically, matching the test gas pressure and the accelerator gas pressure, and initially separating the test gas from the accelerator gas by density stratification. If some sacrifice of the reservoir conditions can be made, the VET can be utilized in hypervelocity ground testing, without the problems associated with secondary diaphragm rupture.
\r\n\r\nThe performance of different constraints for the Rate-Controlled Constrained-Equilibrium (RCCE) method is investigated in the context of modeling reacting flows characteristic to ground testing facilities, and re-entry conditions. The effectiveness of different constraints are isolated, and new constraints previously unmentioned in the literature are introduced. Three main benefits from the RCCE method were determined: 1) the reduction in number of equations that need to be solved to model a reacting flow; 2) the reduction in stiffness of the system of equations needed to be solved; and 3) the ability to tabulate chemical properties as a function of a constraint once, prior to running a simulation, along with the ability to use the same table for multiple simulations.
\r\n\r\nFinally, published physical properties of PICA are compiled, and the composition of the pyrolysis gases that form at high temperatures internal to a heatshield is investigated. A necessary link between the composition of the solid resin, and the composition of the pyrolysis gases created is provided. This link, combined with a detailed investigation into a reacting pyrolysis gas mixture, allows a much needed consistent, and thorough description of many of the physical phenomena occurring in a PICA heatshield, and their implications, to be presented.
\r\n\r\nThrough the use of computational fluid mechanics and computational chemistry methods, significant contributions have been made to advancing ground testing facilities, computational methods for reacting flows, and ablation modeling.
" }, { "name": "Richmond, Victoria Stolyar", "degree": "PhD", "year": "2014", "title": "Techniques for Strength Measurement at High Pressures and Strain-Rates using Transverse Waves", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:01152014-115401299", "creators": [ { "name": { "family": "Richmond", "given": "Victoria Stolyar" }, "id": "Richmond-Victoria-Stolyar", "display_name": "Richmond, Victoria Stolyar" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "orcid": "0000-0002-9112-6615", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "space" ], "doi": "10.7907/SH60-5659", "abstract": "The study of the strength of a material is relevant to a variety of applications including automobile collisions, armor penetration and inertial confinement fusion. Although dynamic behavior of materials at high pressures and strain-rates has been studied extensively using plate impact experiments, the results provide measurements in one direction only. Material behavior that is dependent on strength is unaccounted for. The research in this study proposes two novel configurations to mitigate this problem.
\r\n\r\nThe first configuration introduced is the oblique wedge experiment, which is comprised of a driver material, an angled target of interest and a backing material used to measure in-situ velocities. Upon impact, a shock wave is generated in the driver material. As the shock encounters the angled target, it is reflected back into the driver and transmitted into the target. Due to the angle of obliquity of the incident wave, a transverse wave is generated that allows the target to be subjected to shear while being compressed by the initial longitudinal shock such that the material does not slip. Using numerical simulations, this study shows that a variety of oblique wedge configurations can be used to study the shear response of materials and this can be extended to strength measurement as well. Experiments were performed on an oblique wedge setup with a copper impactor, polymethylmethacrylate driver, aluminum 6061-t6 target, and a lithium fluoride window. Particle velocities were measured using laser interferometry and results agree well with the simulations.
\r\n\r\nThe second novel configuration is the y-cut quartz sandwich design, which uses the anisotropic properties of y-cut quartz to generate a shear wave that is transmitted into a thin sample. By using an anvil material to back the thin sample, particle velocities measured at the rear surface of the backing plate can be implemented to calculate the shear stress in the material and subsequently the strength. Numerical simulations were conducted to show that this configuration has the ability to measure the strength for a variety of materials.
\r\n" }, { "name": "Saito, Namiko", "degree": "PhD", "year": "2014", "title": "Large-Eddy Simulations of Fully Developed Turbulent Channel and Pipe Flows with Smooth and Rough Walls", "advisor": "Pullin, Dale I.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02142014-112419793", "creators": [ { "name": { "family": "Saito", "given": "Namiko" }, "id": "Saito-Namiko", "display_name": "Saito, Namiko" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale I." }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale I." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WKNJ-ET18", "abstract": "Studies in turbulence often focus on two flow conditions, both of which occur frequently in real-world flows and are sought-after for their value in advancing turbulence theory. These are the high Reynolds number regime and the effect of wall surface roughness. In this dissertation, a Large-Eddy Simulation (LES) recreates both conditions over a wide range of Reynolds numbers Reτ = O(102)-O(108) and accounts for roughness by locally modeling the statistical effects of near-wall anisotropic fine scales in a thin layer immediately above the rough surface. A subgrid, roughness-corrected wall model is introduced to dynamically transmit this modeled information from the wall to the outer LES, which uses a stretched-vortex subgrid-scale model operating in the bulk of the flow. Of primary interest is the Reynolds number and roughness dependence of these flows in terms of first and second order statistics. The LES is first applied to a fully turbulent uniformly-smooth/rough channel flow to capture the flow dynamics over smooth, transitionally rough and fully rough regimes. Results include a Moody-like diagram for the wall averaged friction factor, believed to be the first of its kind obtained from LES. Confirmation is found for experimentally observed logarithmic behavior in the normalized stream-wise turbulent intensities. Tight logarithmic collapse, scaled on the wall friction velocity, is found for smooth-wall flows when Reτ ≥ O(106) and in fully rough cases. Since the wall model operates locally and dynamically, the framework is used to investigate non-uniform roughness distribution cases in a channel, where the flow adjustments to sudden surface changes are investigated. Recovery of mean quantities and turbulent statistics after transitions are discussed qualitatively and quantitatively at various roughness and Reynolds number levels. The internal boundary layer, which is defined as the border between the flow affected by the new surface condition and the unaffected part, is computed, and a collapse of the profiles on a length scale containing the logarithm of friction Reynolds number is presented. Finally, we turn to the possibility of expanding the present framework to accommodate more general geometries. As a first step, the whole LES framework is modified for use in the curvilinear geometry of a fully-developed turbulent pipe flow, with implementation carried out in a spectral element solver capable of handling complex wall profiles. The friction factors have shown favorable agreement with the superpipe data, and the LES estimates of the Karman constant and additive constant of the log-law closely match values obtained from experiment." }, { "name": "Verma, Siddhartha", "degree": "PhD", "year": "2014", "title": "Velocity Resolved - Scalar Modeled Simulations of High Schmidt Number Turbulent Transport", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:06042014-163735743", "creators": [ { "name": { "family": "Verma", "given": "Siddhartha" }, "id": "Verma-Siddhartha", "display_name": "Verma, Siddhartha" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PTD9-W004", "abstract": "The objective of this thesis is to develop a framework to conduct velocity resolved - scalar modeled (VR-SM) simulations, which will enable accurate simulations at higher Reynolds and Schmidt (Sc) numbers than are currently feasible. The framework established will serve as a first step to enable future simulation studies for practical applications. To achieve this goal, in-depth analyses of the physical, numerical, and modeling aspects related to Sc>>1 are presented, specifically when modeling in the viscous-convective subrange. Transport characteristics are scrutinized by examining scalar-velocity Fourier mode interactions in Direct Numerical Simulation (DNS) datasets and suggest that scalar modes in the viscous-convective subrange do not directly affect large-scale transport for high Sc. Further observations confirm that discretization errors inherent in numerical schemes can be sufficiently large to wipe out any meaningful contribution from subfilter models. This provides strong incentive to develop more effective numerical schemes to support high Sc simulations. To lower numerical dissipation while maintaining physically and mathematically appropriate scalar bounds during the convection step, a novel method of enforcing bounds is formulated, specifically for use with cubic Hermite polynomials. Boundedness of the scalar being transported is effected by applying derivative limiting techniques, and physically plausible single sub-cell extrema are allowed to exist to help minimize numerical dissipation. The proposed bounding algorithm results in significant performance gain in DNS of turbulent mixing layers and of homogeneous isotropic turbulence. Next, the combined physical/mathematical behavior of the subfilter scalar-flux vector is analyzed in homogeneous isotropic turbulence, by examining vector orientation in the strain-rate eigenframe. The results indicate no discernible dependence on the modeled scalar field, and lead to the identification of the tensor-diffusivity model as a good representation of the subfilter flux. Velocity resolved - scalar modeled simulations of homogeneous isotropic turbulence are conducted to confirm the behavior theorized in these a priori analyses, and suggest that the tensor-diffusivity model is ideal for use in the viscous-convective subrange. Simulations of a turbulent mixing layer are also discussed, with the partial objective of analyzing Schmidt number dependence of a variety of scalar statistics. Large-scale statistics are confirmed to be relatively independent of the Schmidt number for Sc>>1, which is explained by the dominance of subfilter dissipation over resolved molecular dissipation in the simulations. Overall, the VR-SM framework presented is quite effective in predicting large-scale transport characteristics of high Schmidt number scalars, however, it is determined that prediction of subfilter quantities would entail additional modeling intended specifically for this purpose. The VR-SM simulations presented in this thesis provide us with the opportunity to overlap with experimental studies, while at the same time creating an assortment of baseline datasets for future validation of LES models, thereby satisfying the objectives outlined for this work." }, { "name": "Xuan, Yuan", "degree": "PhD", "year": "2014", "title": "Progress in Numerical Modeling of Non-Premixed Combustion", "advisor": "Blanquart, Guillaume", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292014-112456783", "creators": [ { "name": { "family": "Xuan", "given": "Yuan" }, "id": "Xuan-Yuan", "orcid": "0000-0001-9326-2197", "display_name": "Xuan, Yuan" } ], "advisors": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "advisor", "display_name": "Blanquart, Guillaume" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZED4-A872", "abstract": "Progress is made on the numerical modeling of both laminar and turbulent non-premixed flames. Instead of solving the transport equations for the numerous species involved in the combustion process, the present study proposes reduced-order combustion models based on local flame structures.
\r\n\r\nFor laminar non-premixed flames, curvature and multi-dimensional diffusion effects are found critical for the accurate prediction of sooting tendencies. A new numerical model based on modified flamelet equations is proposed. Sooting tendencies are calculated numerically using the proposed model for a wide range of species. These first numerically-computed sooting tendencies are in good agreement with experimental data. To further quantify curvature and multi-dimensional effects, a general flamelet formulation is derived mathematically. A budget analysis of the general flamelet equations is performed on an axisymmetric laminar diffusion flame. A new chemistry tabulation method based on the general flamelet formulation is proposed. This new tabulation method is applied to the same flame and demonstrates significant improvement compared to previous techniques.
\r\n\r\nFor turbulent non-premixed flames, a new model to account for chemistry-turbulence interactions is proposed. %It is found that these interactions are not important for radicals and small species, but substantial for aromatic species. The validity of various existing flamelet-based chemistry tabulation methods is examined, and a new linear relaxation model is proposed for aromatic species. The proposed relaxation model is validated against full chemistry calculations. To further quantify the importance of aromatic chemistry-turbulence interactions, Large-Eddy Simulations (LES) have been performed on a turbulent sooting jet flame. %The aforementioned relaxation model is used to provide closure for the chemical source terms of transported aromatic species. The effects of turbulent unsteadiness on soot are highlighted by comparing the LES results with a separate LES using fully-tabulated chemistry. It is shown that turbulent unsteady effects are of critical importance for the accurate prediction of not only the inception locations, but also the magnitude and fluctuations of soot.
" }, { "name": "Aria, Adrianus Indrat", "degree": "PhD", "year": "2013", "title": "Control of Wettability of Carbon Nanotube Array by Reversible Dry Oxidation for Superhydrophobic Coating and Supercapacitor Applications", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06012013-192515668", "creators": [ { "name": { "family": "Aria", "given": "Adrianus Indrat" }, "id": "Aria-Adrianus-Indrat", "display_name": "Aria, Adrianus Indrat" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "chair", "display_name": "Dabiri, John O." }, { "name": { "family": "Heath", "given": "James R." }, "id": "Heath-J-R", "orcid": "0000-0001-5356-4385", "role": "member", "display_name": "Heath, James R." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/B6MF-FX89", "abstract": "In this thesis, dry chemical modification methods involving UV/ozone, oxygen plasma, and vacuum annealing treatments are explored to precisely control the wettability of CNT arrays. By varying the exposure time of these treatments the surface concentration of oxygenated groups adsorbed on the CNT arrays can be controlled. CNT arrays with very low amount of oxygenated groups exhibit a superhydrophobic behavior. In addition to their extremely high static contact angle, they cannot be dispersed in DI water and their impedance in aqueous electrolytes is extremely high. These arrays have an extreme water repellency capability such that a water droplet will bounce off of their surface upon impact and a thin film of air is formed on their surface as they are immersed in a deep pool of water. In contrast, CNT arrays with very high surface concentration of oxygenated functional groups exhibit an extreme hydrophilic behavior. In addition to their extremely low static contact angle, they can be dispersed easily in DI water and their impedance in aqueous electrolytes is tremendously low. Since the bulk structure of the CNT arrays are preserved during the UV/ozone, oxygen plasma, and vacuum annealing treatments, all CNT arrays can be repeatedly switched between superhydrophilic and superhydrophobic, as long as their O/C ratio is kept below 18%.
\r\n\r\nThe effect of oxidation using UV/ozone and oxygen plasma treatments is highly reversible as long as the O/C ratio of the CNT arrays is kept below 18%. At O/C ratios higher than 18%, the effect of oxidation is no longer reversible. This irreversible oxidation is caused by irreversible changes to the CNT atomic structure during the oxidation process. During the oxidation process, CNT arrays undergo three different processes. For CNT arrays with O/C ratios lower than 40%, the oxidation process results in the functionalization of CNT outer walls by oxygenated groups. Although this functionalization process introduces defects, vacancies and micropores opening, the graphitic structure of the CNT is still largely intact. For CNT arrays with O/C ratios between 40% and 45%, the oxidation process results in the etching of CNT outer walls. This etching process introduces large scale defects and holes that can be obviously seen under TEM at high magnification. Most of these holes are found to be several layers deep and, in some cases, a large portion of the CNT side walls are cut open. For CNT arrays with O/C ratios higher than 45%, the oxidation process results in the exfoliation of the CNT walls and amorphization of the remaining CNT structure. This amorphization process can be implied from the disappearance of C-C sp2 peak in the XPS spectra associated with the pi-bond network.
\r\n\r\nThe impact behavior of water droplet impinging on superhydrophobic CNT arrays in a low viscosity regime is investigated for the first time. Here, the experimental data are presented in the form of several important impact behavior characteristics including critical Weber number, volume ratio, restitution coefficient, and maximum spreading diameter. As observed experimentally, three different impact regimes are identified while another impact regime is proposed. These regimes are partitioned by three critical Weber numbers, two of which are experimentally observed. The volume ratio between the primary and the secondary droplets is found to decrease with the increase of Weber number in all impact regimes other than the first one. In the first impact regime, this is found to be independent of Weber number since the droplet remains intact during and subsequent to the impingement. Experimental data show that the coefficient of restitution decreases with the increase of Weber number in all impact regimes. The rate of decrease of the coefficient of restitution in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Experimental data also show that the maximum spreading factor increases with the increase of Weber number in all impact regimes. The rate of increase of the maximum spreading factor in the high Weber number regime is found to be higher than that in the low and moderate Weber number. Phenomenological approximations and interpretations of the experimental data, as well as brief comparisons to the previously proposed scaling laws, are shown here.
\r\n\r\nDry oxidation methods are used for the first time to characterize the influence of oxidation on the capacitive behavior of CNT array EDLCs. The capacitive behavior of CNT array EDLCs can be tailored by varying their oxygen content, represented by their O/C ratio. The specific capacitance of these CNT arrays increases with the increase of their oxygen content in both KOH and Et4NBF4/PC electrolytes. As a result, their gravimetric energy density increases with the increase of their oxygen content. However, their gravimetric power density decreases with the increase of their oxygen content. The optimally oxidized CNT arrays are able to withstand more than 35,000 charge/discharge cycles in Et4NBF4/PC at a current density of 5 A/g while only losing 10% of their original capacitance.
" }, { "name": "Bourguignon, Jean-Loup", "degree": "PhD", "year": "2013", "title": "Models of Turbulent Pipe Flow", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:11272012-130849053", "creators": [ { "name": { "family": "Bourguignon", "given": "Jean-Loup" }, "id": "Bourguignon-Jean-Loup", "display_name": "Bourguignon, Jean-Loup" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "orcid": "0000-0002-1828-2486", "role": "member", "display_name": "Doyle, John Comstock" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BPEZ-VM28", "abstract": "The physics of turbulent pipe flow was investigated via the use of two models based on simplified versions of the Navier-Stokes equations. The first model was a streamwise-constant projection of these equations, and was used to study the change in mean flow that occurs during transition to turbulence. The second model was based on the analysis of the turbulent pipe flow resolvent, and provided a radial basis for the modal decomposition of turbulent pipe flow. The two models were tested numerically and validated against experimental and numerical data.
\r\n\r\nAnalysis of the streamwise-constant model showed that both non-normal and nonlinear effects are required to capture the blunting of the velocity profile, which occurs during pipe flow transition. The model generated flow fields characterized by the presence of high- and low-speed streaks, whose distribution over the cross-section of the pipe was remarkably similar to the one observed in the velocity field near the trailing edge of the puff structures present in pipe flow transition.
\r\n\r\nA modal decomposition of turbulent pipe flow, in the three spatial directions and in time, was performed, and made possible by the significant reduction in data requirements achieved via the use of compressive sampling and model-based radial basis functions. The application and efficiency of compressive sampling in wall-bounded turbulence was demonstrated.
\r\n\r\nApproximately sparse representations of turbulent pipe flow by propagating waves with model-based radial basis functions, were derived. The basis functions, obtained by singular value decomposition of the resolvent, captured the wall-normal coherence of the flow; and provided a link between the propagating waves and the governing equations, allowing for the identification of the dominant mechanims sustaining the waves, as a function of their streamwise wavenumber.
\r\n\r\nAnalysis of the resolvent showed that the long streamwise waves are amplified mainly via non-normality effects, and are also constrained to be tall in the wall-normal direction, which decreases the influence of viscous dissipation. The short streamwise waves were shown to be localized near the critical-layer (defined as the wall-normal location where the convection velocity of the wave equals the local mean velocity), and thus exhibit amplification with a large contribution from criticality. The work in this thesis allows the reconciliation of the well-known results concerning optimal disturbance amplification due to non-normal effects with recent resolvent analyses, which highlighted the importance of criticality effects.
\r\n" }, { "name": "Damazo, Jason Scott", "degree": "PhD", "year": "2013", "title": "Planar Reflection of Gaseous Detonation", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06112013-153305610", "creators": [ { "name": { "family": "Damazo", "given": "Jason Scott" }, "id": "Damazo-Jason-Scott", "orcid": "0000-0002-4155-7177", "display_name": "Damazo, Jason Scott" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4QW7-TK55", "abstract": "Pipes containing flammable gaseous mixtures may be subjected to internal detonation. When the detonation normally impinges on a closed end, a reflected shock wave is created to bring the flow back to rest. This study built on the work of Karnesky (2010) and examined deformation of thin-walled stainless steel tubes subjected to internal reflected gaseous detonations. A ripple pattern was observed in the tube wall for certain fill pressures, and a criterion was developed that predicted when the ripple pattern would form. A two-dimensional finite element analysis was performed using Johnson-Cook material properties; the pressure loading created by reflected gaseous detonations was accounted for with a previously developed pressure model. The residual plastic strain between experiments and computations was in good agreement.
\r\n\r\nDuring the examination of detonation-driven deformation, discrepancies were discovered in our understanding of reflected gaseous detonation behavior. Previous models did not accurately describe the nature of the reflected shock wave, which motivated further experiments in a detonation tube with optical access. Pressure sensors and schlieren images were used to examine reflected shock behavior, and it was determined that the discrepancies were related to the reaction zone thickness extant behind the detonation front. During these experiments reflected shock bifurcation did not appear to occur, but the unfocused visualization system made certainty impossible. This prompted construction of a focused schlieren system that investigated possible shock wave-boundary layer interaction, and heat-flux gauges analyzed the boundary layer behind the detonation front. Using these data with an analytical boundary layer solution, it was determined that the strong thermal boundary layer present behind the detonation front inhibits the development of reflected shock wave bifurcation.
" }, { "name": "Gdoutos, Eleftherios E.", "degree": "PhD", "year": "2013", "title": "Thin Metastructures with Engineered Thermal Expansion", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292013-162505920", "creators": [ { "name": { "family": "Gdoutos", "given": "Eleftherios E." }, "id": "Gdoutos-Eleftherios-E", "display_name": "Gdoutos, Eleftherios E." } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "chair", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Shapiro", "given": "Andrew A." }, "id": "Shapiro-A-A", "role": "member", "display_name": "Shapiro, Andrew A." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M69R-3A76", "abstract": "The geometry and constituent materials of metastructures can be used to engineer the thermal expansion coefficient. In this thesis, we design, fabricate, and test thin thermally stable metastructures consisting of bi-metallic unit cells and show how the coefficient of thermal expansion (CTE) of these metastructures can be finely and coarsely tuned by varying the CTE of the constituent materials and the unit cell geometry. Planar and three-dimensional finite element method modeling is used to drive the design and inform experiments, and predict the response of these metastructures. We demonstrate computationally the significance of out-of-plane effects in the metastructure response. We develop an experimental setup using digital image correlation and an infrared camera to experimentally measure full displacement and temperature fields during testing and accurately measure the metastructures\u2019 CTE. We experimentally demonstrate high aspect ratio metastructures of Ti/Al and Kovar/Al which exhibit near-zero and negative CTE, respectively. We demonstrate robust fabrication procedures for thermally stable samples with high aspect ratios in thin foil and thin film scales. We investigate the lattice structure and mechanical properties of thin films comprising a near-zero CTE metastructure. The mechanics developed in this work can be used to engineer metastructures of arbitrary CTE and can be extended to three dimensions." }, { "name": "Jacobi, Ian", "degree": "PhD", "year": "2013", "title": "Structure of the Turbulent Boundary Layer under Static and Dynamic Impulsive Roughness Perturbation", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:07102012-152431583", "creators": [ { "name": { "family": "Jacobi", "given": "Ian" }, "id": "Jacobi-Ian", "display_name": "Jacobi, Ian" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Hussain", "given": "Fazle" }, "id": "Hussain-F", "orcid": "0000-0002-2209-9270", "role": "member", "display_name": "Hussain, Fazle" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/H5WJ-RK31", "abstract": "The zero-pressure gradient turbulent boundary layer at Reynolds numbers (based on momentum thickness) ranging from 2700--4100 was perturbed using an impulsively short patch of two-dimensional, spanwise roughness elements. A spatial perturbation was considered in which the roughness patch was held statically on the flat-plate, and the flow downstream of the perturbation was measured by hotwire and particle-image velocimetry. A dynamic perturbation, in which the roughness patch was actuated periodically in time, was also studied, and additional measurements were taken by phase-locking to the dynamic actuation itself.
\r\n\r\nThe static perturbation distorted the boundary layer through the generation of a `stress bore' which modified the mean streamwise velocity gradient. The effect of this stress bore was observed in a modification of statistical and spectral measures of the turbulence, as well as a redistribution of coherent structures in the boundary layer. The characterization of the statically perturbed boundary layer provided a base flow from which to consider the dynamically perturbed flow. The dynamically perturbed flow manifested both effects analogous to the static perturbation, as well as a coherent, periodic, large-scale velocity fluctuation. The extent to which these two features could be treated as linearly independent was studied by a variety of statistical and spectral means. Moreover, the very large scale motion synthesized by the dynamic perturbation was isolated by phase-locked measurement, and its behavior was predicted with reasonable success by employing a resolvent operator approach to a forced version of the Orr-Sommerfeld equation.
\r\n\r\nThe relationship between large-scale motions and an envelope of small-scale motions in the turbulent boundary layer was studied in both the unperturbed and perturbed flows. A variety of correlation techniques were used to interpret the interaction between the different scale motions in the context of a phase-relationship between large and small scales. This phase relationship was shown to provide a physically-grounded perspective on the relationship between the synthetic very large scale motion produced by the dynamic perturbation and the smaller scales in the flow, and was able to provide a foundation for thinking about new approaches to controlling turbulence through large-scale forcing.
" }, { "name": "Johnson, Gwendolyn Brook", "degree": "PhD", "year": "2013", "title": "Modeling, Simulation, and Design of Self-Assembling Space Systems: Accurate Collision Detection, Robust Time Integration, and Optimal Control", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:09132012-125328533", "creators": [ { "name": { "family": "Johnson", "given": "Gwendolyn Brook" }, "id": "Johnson-G-B", "display_name": "Johnson, Gwendolyn Brook" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Leyendecker", "given": "Sigrid" }, "id": "Leyendecker-S", "role": "member", "display_name": "Leyendecker, Sigrid" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/73S0-Y593", "abstract": "Motivated by issues inherent in modeling and designing self-assembling systems (e.g. multiple collisions, collisions between non-smooth bodies, clumping and jamming behaviors, etc.), the goal of this thesis is to develop robust numerical tools that enable ecient and accurate direct simulation of self assembling systems and the application of optimal control methods to this type of system. The systems will be alternately modeled using linear nite elements, rigid bodies, or chains of rigid bodies. To this end, this work begins with development of a linear programming based collision detection algorithm for general convex polyhedral bodies. The resulting linear program has several features which render it extremely useful in determining the force system at the time of contact in numerical collision integrators. With robust collision detection in hand, three related numerical integration methods for dynamics with collisions are treated; a direct potential-based approach, and exact collision integrator in a discrete variational setting, and a decomposition-based algorithm, again in the discrete variational setting. Finally, several control problems are treated in the Discrete Mechanics and Optimal Control{Constrained (DMOCC) framework in which collisions between non-smooth bodies either need to be avoided or explicitly included in the optimal control problem. A globally stable feedback controller and a family of trajectories for spacecraft docking are also developed and tested with an accurate representation of an optimized CubeSat docking system." }, { "name": "Kwok, Kawai", "degree": "PhD", "year": "2013", "title": "Mechanics of Viscoelastic Thin-Walled Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06122012-184825377", "creators": [ { "name": { "family": "Kwok", "given": "Kawai" }, "id": "Kwok-Kawai", "orcid": "0000-0001-6876-4267", "display_name": "Kwok, Kawai" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Andrade", "given": "Jose E." }, "id": "Andrade-J-E", "role": "member", "display_name": "Andrade, Jose E." }, { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "member", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RSSF-1C35", "abstract": "Thin-walled structures made of polymers and reinforced polymer composites are prominent candidates for constructing large lightweight structures. A major challenge in designing polymer-based thin-walled structures is their time and temperature dependent behavior originating from material viscoelasticity and its interaction with the highly geometrically nonlinear response due to thinness of the walls. Although polymer viscoelasticity and geometric nonlinearity have been extensively studied, the mechanics of structures exhibiting both phenomena are not well understood.
\r\n\r\nThis thesis presents a combination of experimental, numerical, and analytical investigations of the behavior of viscoelastic thin-walled structures. The first goal of this research is to establish general methods of analysis for two types of structural components, namely composite shells and polymer membranes, that will serve as the basis for full-scale structural analysis. The second goal is to demonstrate the capability of the developed methods by analyzing time and temperature dependent behavior of deployable structures and balloon structures.
\r\n\r\nIn the study of deployable structures, the deployment and shape recovery processes after stowage are investigated. Fundamental features of viscoelastic deployable structures are studied first with homogeneous polymer beams and shells. A simple closed-form solution describing the shape evolution of a beam after stowage is proposed. The effects of rate and temperature on the bending instability of shells are revealed. Building on the understanding gained from the analysis of homogeneous structures, modeling techniques are developed for polymer composite structures. A micromechanical viscoelastic model for carbon fiber reinforced polymer thin shells is established through finite element homogenization and applied to evaluate the effects of long-term stowage in a representative composite deployable structure.
\r\n\r\nIn the study of balloon structures, a membrane model is developed to study polymer balloon films with stress concentrations due to thickness variation. A nonlinear viscoelastic constitutive model is first formulated for the film material. The wrinkling instability behavior is incorporated into the model through correction of stress and strain states in the presence of wrinkling. Stress concentration factors in balloon films are predicted and measured with the membrane model and full-field displacement measurement techniques, respectively.
" }, { "name": "Lopez Ortega, Alejandro", "degree": "PhD", "year": "2013", "title": "Simulation of Richtmyer-Meshkov Flows for Elastic-Plastic Solids in Planar and Converging Geometries Using an Eulerian Framework", "advisor": "Pullin, Dale Ian; Meiron, Daniel I.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02202013-185004693", "creators": [ { "name": { "family": "Lopez Ortega", "given": "Alejandro" }, "id": "Lopez-Ortega-Alejandro", "display_name": "Lopez Ortega, Alejandro" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "co-advisor", "display_name": "Meiron, Daniel I." } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "role": "member", "display_name": "Bruno, Oscar P." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4WJ6-D795", "abstract": "This thesis presents a numerical and analytical study of two problems of interest involving shock waves propagating through elastic-plastic media: the motion of converging (imploding) shocks and the Richtmyer-Meshkov (RM) instability. Since the stress conditions encountered in these cases normally produce large deformations in the materials, an Eulerian description, in which the spatial coordinates are fixed, is employed. This formulation enables a direct comparison of similarities and differences between the present study of phenomena driven by shock-loading in elastic-plastic solids, and in fluids, where they have been studied extensively. In the first application, Whitham's shock dynamics (WSD) theory is employed for obtaining an approximate description of the motion of an elastic-plastic material processed by a cylindrically/spherically converging shock. Comparison with numerical simulations of the full set of equations of motion reveal that WSD is an accurate tool for characterizing the evolution of converging shocks at all stages. The study of the Richtmyer-Meshkov flow (i.e., interaction between the interface separating two materials of different density and a shock wave incoming at an angle) in solids is performed by means of analytical models for purely elastic solids and numerical simulations when plasticity is included in the material model. To this effect, an updated version of a previously developed multi-material, level-set-based, Eulerian framework for solid mechanics is employed. The revised code includes the use of a multi-material HLLD Riemann problem for imposing material boundary conditions, and a new formulation of the equations of motion that makes use of the stretch tensor while avoiding the degeneracy of the stress tensor under rotation. Results reveal that the interface separating two elastic solids always behaves in a stable oscillatory or decaying oscillatory manner due to the existence of shear waves, which are able to transport the initial vorticity away from the interface. In the case of elastic-plastic materials, the interface behaves at first in an unstable manner similar to a fluid. Ejecta formation is appreciated under certain initial conditions while in other conditions, after an initial period of growth, the interface displays a quasi-stationary long-term behavior due to stress relaxation. The effect of secondary shock-interface interactions (re-shocks) in converging geometries is also studied. A turbulent mixing zone, similar to what is observed in gas--gas interfaces, is created, especially when materials with low strength driven by moderate to strong shocks are considered." }, { "name": "Mihaly, Jonathan Michael", "degree": "PhD", "year": "2013", "title": "Investigation of Hypervelocity Impact Phenomena Using Real-time Concurrent Diagnostics", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072013-143355354", "creators": [ { "name": { "family": "Mihaly", "given": "Jonathan Michael" }, "id": "Mihaly-Jonathan-Michael", "display_name": "Mihaly, Jonathan Michael" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/V3A7-7686", "abstract": "Hypervelocity impact of meteoroids and orbital debris poses a serious and growing threat to spacecraft. To study hypervelocity impact phenomena, a comprehensive ensemble of real-time concurrently operated diagnostics has been developed and implemented in the Small Particle Hypervelocity Impact Range (SPHIR) facility. This suite of simultaneously operated instrumentation provides multiple complementary measurements that facilitate the characterization of many impact phenomena in a single experiment. The investigation of hypervelocity impact phenomena described in this work focuses on normal impacts of 1.8 mm nylon 6/6 cylinder projectiles and variable thickness aluminum targets. The SPHIR facility two-stage light-gas gun is capable of routinely launching 5.5 mg nylon impactors to speeds of 5 to 7 km/s. Refinement of legacy SPHIR operation procedures and the investigation of first-stage pressure have improved the velocity performance of the facility, resulting in an increase in average impact velocity of at least 0.57 km/s. Results for the perforation area indicate the considered range of target thicknesses represent multiple regimes describing the non-monotonic scaling of target perforation with decreasing target thickness. The laser side-lighting (LSL) system has been developed to provide ultra-high-speed shadowgraph images of the impact event. This novel optical technique is demonstrated to characterize the propagation velocity and two-dimensional optical density of impact-generated debris clouds. Additionally, a debris capture system is located behind the target during every experiment to provide complementary information regarding the trajectory distribution and penetration depth of individual debris particles. The utilization of a coherent, collimated illumination source in the LSL system facilitates the simultaneous measurement of impact phenomena with near-IR and UV-vis spectrograph systems. Comparison of LSL images to concurrent IR results indicates two distinctly different phenomena. A high-speed, pressure-dependent IR-emitting cloud is observed in experiments to expand at velocities much higher than the debris and ejecta phenomena observed using the LSL system. In double-plate target configurations, this phenomena is observed to interact with the rear-wall several micro-seconds before the subsequent arrival of the debris cloud. Additionally, dimensional analysis presented by Whitham for blast waves is shown to describe the pressure-dependent radial expansion of the observed IR-emitting phenomena. Although this work focuses on a single hypervelocity impact configuration, the diagnostic capabilities and techniques described can be used with a wide variety of impactors, materials, and geometries to investigate any number of engineering and scientific problems." }, { "name": "O'Farrell, Clara", "degree": "PhD", "year": "2013", "title": "A Dynamical Systems Analysis of Vortex Pinch-Off", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05032013-161632237", "creators": [ { "name": { "family": "O'Farrell", "given": "Clara" }, "id": "O'Farrell-Clara", "display_name": "O'Farrell, Clara" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "chair", "display_name": "Dabiri, John O." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "orcid": "0000-0002-1828-2486", "role": "member", "display_name": "Doyle, John Comstock" } ], "option_major": [ "cds" ], "doi": "10.7907/AFAA-KF43", "abstract": "Vortex rings constitute the main structure in the wakes of a wide class of swimming and flying animals, as well as in cardiac flows and in the jets generated by some moss and fungi. However, there is a physical limit, determined by an energy maximization principle called the Kelvin-Benjamin principle, to the size that axisymmetric vortex rings can achieve. The existence of this limit is known to lead to the separation of a growing vortex ring from the shear layer feeding it, a process known as `vortex pinch-off', and characterized by the dimensionless vortex formation number. The goal of this thesis is to improve our understanding of vortex pinch-off as it relates to biological propulsion, and to provide future researchers with tools to assist in identifying and predicting pinch-off in biological flows.
\r\n\r\nTo this end, we introduce a method for identifying pinch-off in starting jets using the Lagrangian coherent structures in the flow, and apply this criterion to an experimentally generated starting jet. Since most naturally occurring vortex rings are not circular, we extend the definition of the vortex formation number to include non-axisymmetric vortex rings, and find that the formation number for moderately non-axisymmetric vortices is similar to that of circular vortex rings. This suggests that naturally occurring vortex rings may be modeled as axisymmetric vortex rings. Therefore, we consider the perturbation response of the Norbury family of axisymmetric vortex rings. This family is chosen to model vortex rings of increasing thickness and circulation, and their response to prolate shape perturbations is simulated using contour dynamics. Finally, the response of more realistic models for vortex rings, constructed from experimental data using nested contours, to perturbations which resemble those encountered by forming vortices more closely, is simulated using contour dynamics. In both families of models, a change in response analogous to pinch-off is found as members of the family with progressively thicker cores are considered. We posit that this analogy may be exploited to understand and predict pinch-off in complex biological flows, where current methods are not applicable in practice, and criteria based on the properties of vortex rings alone are necessary.
" }, { "name": "Parziale, Nicholaus J.", "degree": "PhD", "year": "2013", "title": "Slender-Body Hypervelocity Boundary-Layer Instability", "advisor": "Shepherd, Joseph E.; Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312013-164534236", "creators": [ { "name": { "family": "Parziale", "given": "Nicholaus J." }, "id": "Parziale-Nicholaus-J", "orcid": "0000-0001-9880-1727", "display_name": "Parziale, Nicholaus J." } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "co-advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "chair", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KZJ1-Y009", "abstract": "With novel application of optical techniques, the slender-body hypervelocity boundary-layer instability is characterized in the previously unexplored regime where thermo-chemical effects are important. Narrowband disturbances (500-3000 kHz) are measured in boundary layers with edge velocities of up to 5~km/s at two points along the generator of a 5 degree half angle cone. Experimental amplification factor spectra are presented. Linear stability and PSE analysis is performed, with fair prediction of the frequency content of the disturbances; however, the analysis over-predicts the amplification of disturbances. The results of this work have two key implications: 1) the acoustic instability is present and may be studied in a large-scale hypervelocity reflected-shock tunnel, and 2) the new data set provides a new basis on which the instability can be studied.
" }, { "name": "Szelengowicz, Ivan Michel Nicolas", "degree": "PhD", "year": "2013", "title": "Analysis and Optimization of Stress Wave Propagation in Two-Dimensional Granular Crystals with Defects", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:05082013-161911202", "creators": [ { "name": { "family": "Szelengowicz", "given": "Ivan Michel Nicolas" }, "id": "Szelengowicz-Ivan-Michel-Nicolas", "display_name": "Szelengowicz, Ivan Michel Nicolas" } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Kochmann", "given": "Dennis M." }, "id": "Kochmann-D-M", "role": "chair", "display_name": "Kochmann, Dennis M." }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "role": "member", "display_name": "Bruno, Oscar P." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TE86-1A15", "abstract": "Granular crystals are compact periodic assemblies of elastic particles in Hertzian contact whose dynamic response can be tuned from strongly nonlinear to linear by the addition of a static precompression force. This unique feature allows for a wide range of studies that include the investigation of new fundamental nonlinear phenomena in discrete systems such as solitary waves, shock waves, discrete breathers and other defect modes. In the absence of precompression, a particularly interesting property of these systems is their ability to support the formation and propagation of spatially localized soliton-like waves with highly tunable properties. The wealth of parameters one can modify (particle size, geometry and material properties, periodicity of the crystal, presence of a static force, type of excitation, etc.) makes them ideal candidates for the design of new materials for practical applications. This thesis describes several ways to optimally control and tailor the propagation of stress waves in granular crystals through the use of heterogeneities (interstitial defect particles and material heterogeneities) in otherwise perfectly ordered systems. We focus on uncompressed two-dimensional granular crystals with interstitial spherical intruders and composite hexagonal packings and study their dynamic response using a combination of experimental, numerical and analytical techniques. We first investigate the interaction of defect particles with a solitary wave and utilize this fundamental knowledge in the optimal design of novel composite wave guides, shock or vibration absorbers obtained using gradient-based optimization methods." }, { "name": "Whittlesey, Robert Wells", "degree": "PhD", "year": "2013", "title": "Dynamics and Scaling of Self-Excited Passive Vortex Generators for Underwater Propulsion", "advisor": "Dabiri, John O.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282013-114822808", "creators": [ { "name": { "family": "Whittlesey", "given": "Robert Wells" }, "id": "Whittlesey-Robert-Wells", "display_name": "Whittlesey, Robert Wells" } ], "advisors": [ { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "advisor", "display_name": "Dabiri, John O." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SC4M-8896", "abstract": "A series of experiments was conducted on the use of a device to passively generate vortex rings, henceforth a passive vortex generator (PVG). The device is intended as a means of propulsion for underwater vehicles, as the use of vortex rings has been shown to decrease the fuel consumption of a vehicle by up to 40% Ruiz (2010).
\r\n\r\nThe PVG was constructed out of a collapsible tube encased in a rigid, airtight box. By adjusting the pressure within the airtight box while fluid was flowing through the tube, it was possible to create a pulsed jet with vortex rings via self-excited oscillations of the collapsible tube.
\r\n\r\nA study of PVG integration into an existing autonomous underwater vehicle (AUV) system was conducted. A small AUV was used to retrofit a PVG with limited alterations to the original vehicle. The PVG-integrated AUV was used for self-propelled testing to measure the hydrodynamic (Froude) efficiency of the system. The results show that the PVG-integrated AUV had a 22% increase in the Froude efficiency using a pulsed jet over a steady jet. The maximum increase in the Froude efficiency was realized when the formation time of the pulsed jet, a nondimensional time to characterize vortex ring formation, was coincident with vortex ring pinch-off. This is consistent with previous studies that indicate that the maximization of efficiency for a pulsed jet vehicle is realized when the formation of vortex rings maximizes the vortex ring energy and size.
\r\n\r\nThe other study was a parameter study of the physical dimensions of a PVG. This study was conducted to determine the effect of the tube diameter and length on the oscillation characteristics such as the frequency. By changing the tube diameter and length by factors of 3, the frequency of self-excited oscillations was found to scale as f~D_0^{-1/2} L_0^0, where D_0 is the tube diameter and L_0 the tube length. The mechanism of operation is suggested to rely on traveling waves between the tube throat and the end of the tube. A model based on this mechanism yields oscillation frequencies that are within the range observed by the experiment.
" }, { "name": "Boettcher, Philipp Andreas", "degree": "PhD", "year": "2012", "title": "Thermal Ignition", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05162012-131336010", "creators": [ { "name": { "family": "Boettcher", "given": "Philipp Andreas" }, "id": "Boettcher-Philipp-Andreas", "display_name": "Boettcher, Philipp Andreas" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "orcid": "0000-0002-5074-9728", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/H2W9-ZK95", "abstract": "Accidental ignition of flammable gases is a critical safety concern in many industrial applications. Particularly in the aviation industry, the main areas of concern on an aircraft are the fuel tank and adjoining regions, where spilled fuel has a high likelihood of creating a flammable mixture. To this end, a fundamental understanding of the ignition phenomenon is necessary in order to develop more accurate test methods and standards as a means of designing safer air vehicles. The focus of this work is thermal ignition, particularly auto-ignition with emphasis on the effect of heating rate, hot surface ignition and flame propagation, and puffing flames.
\r\n\r\nCombustion of hydrocarbon fuels is traditionally separated into slow reaction, cool flame, and ignition regimes based on pressure and temperature. Standard tests, such as the ASTM E659, are used to determine the lowest temperature required to ignite a specific fuel mixed with air at atmospheric pressure. It is expected that the initial pressure and the rate at which the mixture is heated also influences the limiting temperature and the type of combustion. This study investigates the effect of heating rate, between 4 and 15 K/min, and initial pressure, in the range of 25 to 100 kPa, on ignition of n-hexane air mixtures. Mixtures with equivalence ratio ranging from 0.6 to = 1.2 were investigated. The problem is also modeled computationally using an extension of Semenov's classical auto-ignition theory with a detailed chemical mechanism. Experiments and simulations both show that in the same reactor either a slow reaction or an ignition event can take place depending on the heating rate. Analysis of the detailed chemistry demonstrates that a mixture which approaches the ignition region slowly undergoes a significant modification of its composition. This change in composition induces a progressive shift of the explosion limit until the mixture is no longer flammable. A mixture that approaches the ignition region sufficiently rapidly undergoes only a moderate amount of thermal decomposition and explodes quite violently. This behavior can also be captured and analyzed using a one-step reaction model, where the heat release is in competition with the depletion of reactants.
\r\n\r\nHot surface ignition is examined using a glow plug or heated nickel element in a series of premixed n-hexane air mixtures. High-speed schlieren photography, a thermocouple, and a fast response pressure transducer are used to record flame characteristics such as ignition temperature, flame speed, pressure rises, and combustion mode. The ignition event is captured by considering the dominant balance of diffusion and chemical reaction that occurs near a hot surface. Experiments and models show a dependence of ignition temperature on mixture composition, initial pressure, and hot surface size. The mixtures exhibit the known lower flammability limit where the maximum temperature of the hot surface was insufficient at igniting the mixture. Away from the lower flammability limit, the ignition temperature drops to an almost constant value over a wide range of equivalence ratios (0.7 to 2.8) with large variations as the upper flammability limit is approached. Variations in the initial pressure and equivalence ratio also give rise to different modes of combustion: single flame, re-ignition, and puffing flames. These results are successfully compared to computational results obtained using a flamelet model and a detailed chemical mechanism for n-heptane. These different regimes can be delineated by considering the competition between inertia, i.e., flame propagation, and buoyancy, which can be expressed in the Richardson number.
\r\n\r\nIn experiments of hot surface ignition and subsequent flame propagation a 10 Hz puffing flame instability is visible in mixtures that are stagnant and premixed prior to the ignition sequence. By varying the size of the hot surface, power input, and combustion vessel volume, we determined that the instability is a function of the interaction of the flame with the fluid flow induced by the combustion products rather than the initial plume established by the hot surface. The phenomenon is accurately reproduced in numerical simulations and a detailed flow field analysis revealed a competition between the inflow velocity at the base of the flame and the flame propagation speed. The increasing inflow velocity, which exceeds the flame propagation speed, is ultimately responsible for creating a puff. The puff is then accelerated upward, allowing for the creation of the subsequent instabilities. The frequency of the puffing is proportional to the gravitational acceleration and inversely proportional to the flame speed. We propose a relation describing the dependence of the frequency on gravitational acceleration, hot surface diameter, and flame speed. This relation shows good agreement for lean and rich n-hexane-air as well as lean hydrogen-air flames.
" }, { "name": "Capece, Angela Maria", "degree": "PhD", "year": "2012", "title": "Plasma-Surface Interactions in Hollow Cathode Discharges for Electric Propulsion\r ", "advisor": "Shepherd, Joseph E.; Polk, James E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05312012-113856351", "creators": [ { "name": { "family": "Capece", "given": "Angela Maria" }, "id": "Capece-Angela-Maria", "orcid": "0000-0003-4147-7174", "display_name": "Capece, Angela Maria" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "co-advisor", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "co-advisor", "display_name": "Polk, James E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Bellan", "given": "Paul Murray" }, "id": "Bellan-P-M", "role": "member", "display_name": "Bellan, Paul Murray" }, { "name": { "family": "Giapis", "given": "Konstantinos P." }, "id": "Giapis-K-P", "role": "member", "display_name": "Giapis, Konstantinos P." }, { "name": { "family": "Goebel", "given": "Dan M." }, "id": "Goebel-D-M", "role": "member", "display_name": "Goebel, Dan M." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "member", "display_name": "Polk, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7TDQ-DR81", "abstract": "Electric thrusters generate high exhaust velocities and can achieve specific impulses in excess of 1000 s. The low thrust generation and high specific impulse make electric propulsion ideal for interplanetary missions, spacecraft station keeping, and orbit raising maneuvers. Consequently, these devices have been used on a variety of space missions including Deep Space 1, Dawn, and hundreds of commercial spacecraft in Earth orbit. In order to provide the required total impulses, thruster burn time can often exceed 10,000 hours, making thruster lifetime essential.
\r\n\r\nOne of the main life-limiting components on ion engines is the hollow cathode, which serves as the electron source for ionization of the xenon propellant gas. Reactive contaminants such as oxygen can modify the cathode surface morphology and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase evaporation of the emission materials and reduce cathode life. A deeper understanding of the mechanisms initiating cathode failure will improve thruster operation, increase lifetime, and ultimately reduce cost.
\r\n \r\nA significant amount of work has been done previously to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity, and its role during cathode poisoning is not completely understood. The work presented here represents the first attempt at understanding how oxygen impurities in the xenon discharge plasma alter the emitter surface and affect operation of a 4:1:1 BaO-CaO-Al2O3 hollow cathode.
\r\n \r\nA combination of experimentation and modeling was used to investigate how oxygen impurities in the discharge plasma alter the emitter surface and reduce the electron emission capability. The experimental effort involved operating a 4:1:1 hollow cathode at various conditions with oxygen impurities in the xenon flow. Since direct measurements of the emitter surface state cannot be obtained because of the cathode geometry and high particles fluxes, measurements of the emitter temperature using a two-color pyrometer were used to determine the oxygen surface coverage and characterize the rate processes that occur during poisoning.
\r\n \r\nA model describing the material transport in the plasma discharge was developed and is used to predict the barium and oxygen fluxes to the emitter surface during cathode operation by solving the species continuity and momentum equations. The dominant ionization process for molecular oxygen in the plasma gas is resonant charge exchange with xenon ions. Barium is effectively recycled in the plasma; however, BaO and O2 are not. The model shows that the oxygen flux to the surface is not diffusion limited.
\r\n\r\nExperimental results indicate that the oxygen poisoning rate is slow and that the oxygen poisoning coverage on the emitter surface is less than 3%. A time-dependent model of the reaction kinetics of oxygen and barium at the tungsten surface was developed using the experimental results.
\r\n \r\nThe experiments and kinetics model indicate that the dominant processes at the emitter surface are dissociative adsorption of O2, sputtering of the O2 precursor, and desorption of O. Ion sputtering of the weakly bound O2 precursor state limits the poisoning rate and yields low oxygen coverage. Removal of chemisorbed atomic oxygen is dominated by thermal processes. Based on the low oxygen coverage and long poisoning transients, plasma cathodes appear to be able to withstand higher oxygen concentrations than vacuum cathodes.
" }, { "name": "Deng, Xiaowei", "degree": "PhD", "year": "2012", "title": "Clefted Equilibrium Shapes of Superpressure Balloon Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06062012-202646378", "creators": [ { "name": { "family": "Deng", "given": "Xiaowei" }, "id": "Deng-Xiaowei", "display_name": "Deng, Xiaowei" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YYTP-2005", "abstract": "This thesis presents a numerical and analytical study of the clefted equilibrium shape of superpressure balloon structures. Lobed superpressure balloons have shown a tendency to deploy into unexpected asymmetric shapes, hence their design has to strike a balance between the lower stresses achieved by increasing lobing and the risk of incomplete deployment. Extensive clefting is a regular feature of balloons that are incompletely inflated, and is regularly seen during launch and ascent. Our particular interest in the research is in clefts that remain once a balloon has reached its float altitude and is fully pressurized.
\r\n\r\nA simplified simulation technique for orthotropic viscoelastic membranes is presented in the thesis. Wrinkling is detected by a combined stress-strain criterion and an iterative scheme searches for the wrinkle angle using a pseudoelastic material stiffness matrix based on a nonlinear viscoelastic constitutive model. This simplified model has been implemented in ABAQUS/Explicit and is able to compute the behavior of a membrane structure by superposition of a small number of response increments. The model has been tested against a published solution for a time-independent isotropic membrane under simple shear and also against experimental results on StratoFilm 420 under simple shear.
\r\n\r\n\r\nA fully three-dimensional finite element model of balloon structures incorporating wrinkling and frictionless contact, able to simulate the shapes taken up by lobed superpressure balloons during the final stages of their ascent has been established. Two different methods have been considered to predict the clefts: (i) deflation and\r\ninflation method and (ii) constraint shift method. In method (i), the starting configuration is obtained by deflating an initially symmetric balloon subject to uniform pressure. The deflation simulation is continued until the differential pressure at the bottom of the balloon has become negative, at which point the balloon is extensively clefted. The balloon is then inflated by increasing the bottom pressure while maintaining a uniform vertical ressure gradient, and the evolution of the shape and stress distribution of the balloon is studied. Two different designs of uperpressure balloons are investigated: a flat facet balloon and a ighly lobed balloon. It is found that the flat facet balloon follows essentially the same path during deflation and inflation, and hence will deploy into a unique, symmetric shape. For the lobed balloon it is found that it follows different paths during deflation and inflation, and deploys into an alternate, clefted equilibrium shape.
\r\n\r\nCompared to method (i), method (ii) is computationally a more efficient clefting test. The test consists in setting up the balloon in its symmetrically inflated configuration, then breaking the symmetry of this shape by artificially introducing a clefting imperfection, and finally determining the equilibrium shape of the balloon. The clefting imperfection is computed by shifting the constraint at the bottom of the balloon and removing the pressure in the bottom region, below the shifted constraint. The clefting test is applied successfully to three 27~m diameter superpressure balloons that have been tested indoors by NASA, of which one had remained clefted when it was inflated and the other two had deployed completely.
\r\n\r\nIn addition to numerical simulations, formulation of a new cleft factor, employed as an indicator of tendency to S-cleft for superpressure balloons based on constant-stress design has been established through dimensional analysis. The cleft factor, defined as the ratio of clefted volume to cyclically symmetrical volume, is expressed in the form of power law relation of the dimensionless groups. An example illustrates how to calculate the coefficients of the analytical formula and analyze sensitivity of design parameters to clefting.
" }, { "name": "Inoue, Michio", "degree": "PhD", "year": "2012", "title": "Large-Eddy Simulation of the Flat-Plate Turbulent Boundary Layer at High Reynolds Numbers", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:05222012-183141047", "creators": [ { "name": { "family": "Inoue", "given": "Michio" }, "id": "Inoue-Michio", "display_name": "Inoue, Michio" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PXTM-W616", "abstract": "The near-wall, subgrid-scale (SGS) model [Chung and Pullin, \"Large-eddy simulation and wall-modeling of turbulent channel flow\", J. Fluid Mech. 631, 281--309 (2009)] is used to perform large-eddy simulations (LES) of the incompressible developing, smooth-wall, flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. This is then used to study several aspects of zero- and adverse-pressure gradient turbulent boundary layers.
\r\n\r\nFirst, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Re\u03b8 based on the free-stream velocity and the momentum thickness in the range Re\u03b8 = 103 - 1012. Results include the inverse skin friction coefficient, \u221a2/Cf, velocity profiles, the shape factor H, the Karman \"constant\", and the Coles wake factor as functions of Re\u03b8. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Re\u03b8 = O(106. At extremely large Re\u03b8, the empirical Coles-Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot of itself probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that the LES is suggestive of the asymptotic, infinite Reynolds-number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum Re\u03b8 of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger Re\u03b8 could be achieved with quad- or higher-precision arithmetic.
\r\n\r\nSecond, the time series velocity signals obtained from LES within the logarithmic region of the zero-pressure gradient turbulent boundary layer are used in combination with an empirical, predictive inner--outer wall model [Marusic et al., \"Predictive model for wall-bounded turbulent flow\", Science 329, 193 (2010)] to calculate the statistics of the fluctuating streamwise velocity in the inner region of the zero-pressure gradient turbulent boundary layer. Results, including spectra and moments up to fourth order, are compared with equivalent predictions using experimental time series, as well as with direct experimental measurements at Reynolds numbers Re\u03c4 based on the friction velocity and the boundary layer thickness, Re\u03c4 =7,300, 13,600 and 19,000. LES combined with the wall model are then used to extend the inner-layer predictions to Reynolds numbers Re\u03c4 =62,000, 100,000 and 200,000 that lie within a gap in log(Re\u03c4) space between laboratory measurements and surface-layer, atmospheric experiments. The present results support a log-like increase in the near-wall peak of the streamwise turbulence intensities with Re\u03c4 and also provide a means of extending LES results at large Reynolds numbers to the near-wall region of wall-bounded turbulent flows.
\r\n\r\nFinally, we apply the wall model to LES of a turbulent boundary layer subject to an adverse pressure gradient. Computed statistics are found to be consistent with recent experiments and some Reynolds number similarity is observed over a range of two orders of magnitude.
" }, { "name": "Khatri, Devvrath", "degree": "PhD", "year": "2012", "title": "Non-Destructive Evaluation of Material System Using Highly Nonlinear Acoustic Waves", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:05102012-091402754", "creators": [ { "name": { "family": "Khatri", "given": "Devvrath" }, "id": "Khatri-Devvrath", "display_name": "Khatri, Devvrath" } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Yang", "given": "Jinkyu" }, "id": "Yang-Jinkyu", "role": "member", "display_name": "Yang, Jinkyu" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/P3VR-Q582", "abstract": "A chain of granular particles is one of the most studied examples of highly nonlinear systems deriving its response from the nonlinear Hertzian contact interaction between particles. Interest in these systems derives from their tunable dynamic response, encompassing linear, weakly nonlinear, and strongly nonlinear regimes, controlled by varying the static and dynamic load applied. In chains with a very weak (or zero) static precompression, the system supports the formation and propagation of highly nonlinear solitary waves (HNSWs). The dual-nonlinear interaction between particles (i.e., a power-law type contact potential in compression, and zero strength in tension) combined with discreteness of the system, makes the granular system highly tunable. The propagation properties of these waves, such as traveling pulse width, wave speed, number of separated pulses (single or train of pulses), etc., can be controlled by modifying one or many of the parameters, like the particle's dimension, material properties, static and dynamic force amplitude, the type and duration of the initial excitation applied to the system, and/or the periodicity of the chain. The ability to control the wave properties in such chains has been proposed for several different practical engineering applications.
\r\n\r\nThe dynamic properties of these granular chains have been conventionally studied using discrete particle models (DPMs) which consider the particles in the chains as point masses connected by nonlinear Hertzian springs with the neighboring particles. Although, this is a good approximation under proper circumstances, it does not capture many features of the three dimensional elastic particles such as the elastic wave propagation within the particles, the local deformation of the particles in the vicinity of the contact point, the corresponding changes in the contact area, and the collective vibrations of the particles among others. This thesis focuses on the development of a nite element model (FEM)using the commercially available software Abaqus, which takes into account many of these characteristic features. The nite element model discretizes particles by considering them as three-dimensional deformable bodies of revolution and describes the nonlinear dynamic response of one-dimensional granular chains composed of particles with various geometries and orientations. We showed that particles' geometries and orientations provide additional design parameters for controlling the dynamic response of the system, compared to chains composed of spherical particles. We also showed that the tunable and compact nature of these waves can be used to tailor the properties of HNSWs for specfic application, such as information carriers for actuation and sensing of mechanical properties and boundary effects of adjoining media in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM). Using experiments and numerics, we characterized interface dynamics between granular media and adjoining linear elastic media, and found that the coupling produced temporary localization of the incident waves at the boundaries between the two media and their decomposition into reflected waves. We monitored the formation of reflected solitary waves propagating back from the interface and found that their properties are sensitive to the geometric and material properties of the adjoining media. The work done in this research enhances our understanding of the basic physics and tunability of nonlinear granular media, and further establishes a theoretical and numerical foundation\r\nin the applications of HNSWs as information carriers.
" }, { "name": "LeHew, Jeffrey Allen", "degree": "PhD", "year": "2012", "title": "Spatio-Temporal Analysis of the Turbulent Boundary Layer and An Investigation of the Effects of Periodic Disturbances", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05232012-142127799", "creators": [ { "name": { "family": "LeHew", "given": "Jeffrey Allen" }, "id": "LeHew-Jeffrey-Allen", "display_name": "LeHew, Jeffrey Allen" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/20CM-EV70", "abstract": "The purpose of this study was to investigate the turbulent boundary layer to learn more about the dynamics of the flow and how it might be controlled through the input of spatially and/or temporally periodic disturbances. The first part of this work studies the structure of a zero-pressure-gradient turbulent boundary layer using time-resolved particle image velocimetry in both wall-normal and wall-parallel planes. Using data from wall-parallel measurements, a 3D spectrum over streamwise, spanwise, and temporal wavelengths was constructed for the first time, a major focus of this work. Among several uses, this spectrum allows the calculation of a scale-based convection velocity, that is, a convection velocity for each streamwise-spanwise scale pair present in the flow. This data set also provided a method for investigating the temporal evolution of coherent structures in the flow, of which, swirling coherent structures (SCS), indicative of vortices, and low-momentum regions were investigated thoroughly. The convection velocity and lifetime of the SCS were measured; using histograms of the SCS convection velocity in multiple wall-parallel planes, it was possible to statistically infer different SCS structures that could be categorized as ``attached'' or ``detached'' from the wall.
\r\n\t\r\nA study was also performed on the response of the turbulent boundary layer to a stationary periodic roughness inspired by the scale pattern on the sailfish. The roughness was relatively sparse with element spacing on the order of the boundary layer thickness allowing the measurement of turbulent statistics at different points along the roughness as well as below the crests of the roughness elements, a region not commonly accessible in rough-wall boundary layer studies. The streamwise turbulent statistics were studied using hotwire anemometry from which it was found that while the outer part of the flow remained similar, the near-wall region was perturbed by structures of size similar to the roughness spacing.
" }, { "name": "Mello, Michael", "degree": "PhD", "year": "2012", "title": "Identifying the Unique Ground Motion Signatures of Supershear Earthquakes: Theory and Experiments", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072012-032023169", "creators": [ { "name": { "family": "Mello", "given": "Michael" }, "id": "Mello-Michael", "orcid": "0000-0003-2129-9235", "display_name": "Mello, Michael" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "role": "member", "display_name": "Kanamori, Hiroo" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DJDD-2487", "abstract": "The near-field ground motion signatures associated with sub-Rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by Rosakis and co-workers (Xia et al., 2004, 2005a; Lu et al., 2007; Rosakis et al., 2007). Heterodyne laser interferometers enable continuous, high-bandwidth measurements of fault-normal (FN), fault-parallel (FP), and vertical (V) particle velocity ``ground motion\" records at discrete locations on the surface of a Homalite-100 test specimen as a sub-Rayleigh or a supershear rupture sweeps along the frictional fault. Photoelastic interference fringes, acquired using high-speed digital photography, provide a synchronized, spatially resolved, whole field view of the advancing rupture tip and surrounding maximum shear stress field.
\r\n\r\nThe first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures. Experimental results demonstrate that a shear Mach front produced by a stable supershear rupture is characterized by a dominant FP velocity component. The situation is shown to reverse in the sub-Rayleigh rupture speed regime whereby the FN particle velocity component dominates the ground motion record. Additional distinguishing particle velocity signatures, consistent with theoretical and numerical predictions, and repeatedly observed in experimental records are, (1) a pronounced peak in the FP velocity record induced by the leading dilatational field, which sweeps the measurement station in advance of the shear Mach front, and (2) a pronounced velocity swing in the FN record associated with the arrival of a trailing Rayleigh sub-Rayleigh (secondary) rupture, which follows the arrival of the shear Mach front. Analysis of the particle velocity records also confirms 2D steady-state theoretical predictions pertaining to the separation, attenuation, and radiation partitioning of the shear and dilatational portions of the rupture velocity field components.
\r\n\r\nThe second phase of our experimental investigations re-examine the 2002, Mw7.9, Denali fault earthquake and the remarkable set of near-source ground motion records obtained at (PS10), located approximately 85 km east of the epicenter and just 3 km north of the fault along the Alaska pipeline. Motivated by the analysis and interpretation of these records by (Ellsworth et al., 2004; Dunham and Archuleta, 2004, 2005), we attempt to mimic the Denali strike-slip rupture scenario and replicate the PS10 ground motion signatures using a laboratory earthquake experiment. The experiments feature a left-to-right (west-to-east) propagating right lateral rupture within a Homalite-100 test specimen with particle velocity data collected at a near-field station situated just above (north of) the fault. Both sub-Rayleigh and supershear laboratory earthquake experiments are conducted using the Denali PS10 configuration in order to compare and contrast the resulting particle velocity signatures. Supershear laboratory records capture all of the prominent features displayed within the PS10 ground motion records. Noted velocity signatures are correlated to the location of the rupture fronts and their noted arrival times in the synchronized photoelastic image sequence. Scaling relationships are also presented which transform the laboratory records through six orders of magnitude in time, to match the scale of the PS10 ground motion records. The strong correlation between the scaled experimental records and the actual PS10 ground motion records support the hypothesis that the Denali strike-slip fault exhibited a supershear burst.
\r\n\r\nFinally, we present a 2D steady state, stress-velocity formulation that relates the FP and FN particle velocity records measured close to the fault, to the evolution of the stress tensor at the same location. A locally steady-state condition is assumed within a restricted time interval in order to invoke these relationships and estimate the dynamic stresses, \u03c3xx(t) and \u03c4(t), at the near-fault station. Dynamic stress measurements enable a new class of friction investigations using the laboratory earthquake configuration. Experimental findings are presented, which capture the temporal and spatial distributions of \u03c3xx and \u03c4, evolution of the dynamic friction coefficient, and velocity weakening behavior of a supershear slip-pulse.
" }, { "name": "Ziegler, John Lewis (Jack)", "degree": "PhD", "year": "2012", "title": "Simulations of Compressible, Diffusive, Reactive Flows with Detailed Chemistry Using a High-Order Hybrid WENO-CD Scheme", "advisor": "Pullin, Dale Ian; Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12302011-185742249", "creators": [ { "name": { "family": "Ziegler", "given": "John Lewis (Jack)" }, "id": "Ziegler-John-Lewis-Jack", "orcid": "0000-0001-6127-5567", "display_name": "Ziegler, John Lewis (Jack)" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "co-advisor", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "co-advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZKW8-ES97", "abstract": "A hybrid weighted essentially non-oscillatory (WENO)/centered-difference (CD) numerical method, with low numerical dissipation, high-order shock-capturing, and structured adaptive mesh refinement (SAMR), has been developed for the direct numerical simulation (DNS) of the multicomponent, compressive, reactive Navier-Stokes equations. The method enables accurate resolution of diffusive processes within reaction zones. This numerical method is verified with a series of one- and two-dimensional test problems, including a convergence test of a two-dimensional unsteady reactive double Mach reflection problem. Validation of the method is conducted with experimental comparisons of three applications all of which model multi-dimensional, unsteady reactive flow: an irregular propane detonation, shock and detonation bifurcations, and spark ignition deflagrations.
\r\n\r\nThe numerical approach combines time-split reactive source terms with a high-order, shock-capturing scheme specifically designed for diffusive flows. A description of the order-optimized, symmetric, finite difference, flux-based, hybrid WENO / centered-difference scheme is given, along with its implementation in a high-order SAMR framework. The implementation of new techniques for discontinuity flagging, scheme-switching, and high-order prolongation and restriction is described. In particular, the refined methodology does not require upwinded WENO at grid refinement interfaces for stability, allowing high-order prolongation and thereby eliminating a significant source of numerical diffusion within the overall code performance.
\r\n\r\nA minimally reduced irregular detonation mixture mechanism (22 species and 53 reversible reactions) is developed and combined with the WENO-CD numerical method to accurately model two-dimensional hydrocarbon (propane) detonations with detailed chemistry and transport. First of its kind, resolved double Mach reflection (DMR) detonation simulations with a large hyrdocarbon mixture are presented. Detailed discussions and comparisons of the influence of grid resolution, lower-order numerical methods, and inviscid approximations are made in addition to the detailed presentation of fluid dynamics found in an unsteady, highly unstable, reactive DMR simulation. Also conducted are direct experimental comparisons to soot foils and schlieren images with an unresolved large-scale propane detonation channel simulation.
\r\n\r\nThe numerical method is also applied to the DNS of two other problems, detonation/shock bifurcations and spark ignited deflagrations. Through the resolution of viscous/diffusive scales, new insights into how a bifurcated foot develops after a detonation end wall reflection, and how geometry can influence the development of a flame kernel after spark ignition are found.
\r\n" }, { "name": "Boechler, Nicholas Sebastian", "degree": "PhD", "year": "2011", "title": "Granular Crystals: Controlling Mechanical Energy with Nonlinearity and Discreteness", "advisor": "Daraio, Chiara", "url": "https://resolver.caltech.edu/CaltechTHESIS:05162011-131929134", "creators": [ { "name": { "family": "Boechler", "given": "Nicholas Sebastian" }, "id": "Boechler-Nicholas-Sebastian", "orcid": "0000-0001-9639-1533", "display_name": "Boechler, Nicholas Sebastian" } ], "advisors": [ { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "advisor", "display_name": "Daraio, Chiara" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Cross", "given": "Michael Clifford" }, "id": "Cross-M-C", "role": "member", "display_name": "Cross, Michael Clifford" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Painter", "given": "Oskar J." }, "id": "Painter-O", "role": "member", "display_name": "Painter, Oskar J." }, { "name": { "family": "Davis", "given": "Gregory L." }, "id": "Davis-G-L", "role": "member", "display_name": "Davis, Gregory L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BAHK-WD44", "abstract": "The presence of structural discreteness and periodicity can affect the propagation of phonons, sound, and other mechanical waves. A fundamental property of many of the periodic structures and materials designed for this purpose is the presence of complete band gaps in their dispersion relation. Waves with frequencies in the band gap cannot propagate and are reflected by the material. Like the concept of a band gap, the functionality of these periodic structures has historically been based on concepts from linear dynamics. Nonlinear systems can offer increased flexibility over linear systems including new ways to localize energy, convert energy between frequencies, and tune the response of the system. Granular crystals are arrays of elastic particles that interact nonlinearly via Hertzian contact, and are a type of nonlinear periodic structure whose response to dynamic excitations can be tuned to encompass linear, weakly nonlinear, and strongly nonlinear regimes. Drawing on ideas from condensed matter physics and nonlinear science, this thesis focuses on how the nonlinearity and structural discreteness of granular crystals can be used to control mechanical energy. The dynamic response of one-dimensional granular crystals composed of compressed elastic spheres (or cylinders) is studied using a combination of experimental, numerical, and analytical techniques. The discovery of fundamental physical phenomena occurring in the linear and weakly nonlinear regimes is described, along with how such phenomena can be used to create new ways to control the propagation of mechanical wave energy. The specific mechanisms investigated include tunable frequency band gaps, discrete breathers, nonlinear localized defect modes, and bifurcations. These mechanisms are utilized to create novel devices for tunable vibration filtering, energy harvesting and conversion, and tunable acoustic rectification." }, { "name": "Gonzalez, Marcial", "degree": "PhD", "year": "2011", "title": "Energy and Force Stepping Integrators in Lagrangian Mechanics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:10052010-230939247", "creators": [ { "name": { "family": "Gonzalez", "given": "Marcial" }, "id": "Gonzalez-Marcial", "display_name": "Gonzalez, Marcial" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SP10-A207", "abstract": "The overarching goal of this thesis is to develop new numerical time integration schemes for Lagrangian mechanics that better cope with the challenges of understanding the dynamic behavior of materials. We specifically address the formulation of convergent time integration schemes that exhibit good long-term behavior---such as conferred by symplecticity and exact conservation properties---and that have the ability to automatically and asynchronously modulate the time step in different regions of the domain. We achieve these properties in a progression of three developments: (i) energy-stepping, (ii) force-stepping, and (iii) asynchronous energy-stepping integrators. These developments are based on a new method of approximation for Lagrangian mechanics, proposed in this thesis, that consists of replacing the Lagrangian of the system by a sequence of approximate Lagrangians that can be solved exactly. Then, energy-stepping integrators result from replacing the potential energy by a piecewise constant approximation, force-stepping integrators result from replacing the potential energy by a piecewise affine approximation, and asynchronous energy-stepping integrators result from replacing localized potential energies by piecewise constant approximations. Throughout the dissertation, the properties of these time integrators are theoretically predicted and born out by a number of selected examples of application. Furthermore, we address the challenges of understanding the propagation of solitary waves in granular crystals at low impact velocity conditions by investigating the role of energy-trapping effects with the numerical time integration schemes developed in this work.\r\n" }, { "name": "Lopez Jimenez, Francisco", "degree": "PhD", "year": "2011", "title": "Mechanics of Thin Carbon Fiber Composites with a Silicone Matrix", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:03152011-154253229", "creators": [ { "name": { "family": "Lopez Jimenez", "given": "Francisco" }, "id": "Lopez-Jimenez-Francisco", "display_name": "Lopez Jimenez, Francisco" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/A773-KF92", "abstract": "This thesis presents an experimental, numerical and analytical study of the behavior of thin fiber composites with a silicone matrix. The main difference with respect to traditional composites with epoxy matrix is the fact that the soft matrix allows the fibers to microbuckle without breaking. This process acts as a stress relief mechanism during folding, and allows the material to reach very high curvatures, which makes them particularly interesting as components of space deployable structures. The goal of this study is to characterize the behavior and understand the mechanics of this type of composite.
\r\n\r\nExperimental testing of the bending behavior of unidirectional composites with a silicone matrix shows a highly non-linear moment vs. curvature relationship, as well as strain softening under cyclic loading. These effects are not usually observed in composites with an epoxy matrix. In the case of tension in the direction transverse to the fibers, the behavior shows again non-linearity and strain softening, as well as an initial stiffness much higher than what would be expected based on the traditional estimates for fiber composites.
\r\n\r\nThe micro mechanics of the material have been studied with a finite element model. It uses solid elements and a random fiber arrangement produced with a reconstruction process based on micrographs of the material cross section. The simulations capture the macroscopic non-linear response, as well as the fiber microbuckling, and show how microbuckling reduces the strain in the fibers. The model shows good agreement for the bending stiffness of specimens with low fiber volume fraction, but it overestimates the effect of the matrix for more densely packed fibers. This is due to the high matrix strain that derives from the assumption of perfect bonding between fiber and matrix. In the case of tension transverse to the fibers, the model shows a much better agreement with experiments than traditional composite theory, and shows that the reason for the observed high stiffness is the incompressibility of the matrix. In order to capture the strain softening due to fiber debonding, cohesive elements have been introduced between the fibers and the matrix. This allows the model to capture quantitatively the non-linear behavior in the case of loading transverse to the fibers, and the damage due to cyclic loading. A single set of parameters for the cohesive elements produce good agreement with the experimental results for very different values of the fiber volume fraction, and could also be used in the analysis of more complicated loading cases, such as bending or biaxial tension.
\r\n\r\nIn addition to the simulations, a homogenized analytical model has also been created. It extends previous analysis of composites with a soft matrix to the case of very thin composites. It provides a good qualitative description of the material behavior, and it helps understand the mechanics that take place within the material, such as the equilibrium of energy terms leading to a finite wave length, as opposed to microbuckling under compression.
" }, { "name": "Reina Romo, Celia", "degree": "PhD", "year": "2011", "title": "Multiscale Modeling and Simulation of Damage by Void Nucleation and Growth", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:11022010-080434454", "creators": [ { "name": { "family": "Reina Romo", "given": "Celia" }, "id": "Reina-Romo-Celua", "display_name": "Reina Romo, Celia" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Marian", "given": "Jaime" }, "id": "Marian-J", "role": "member", "display_name": "Marian, Jaime" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WFYW-AS22", "abstract": "Voids are observed to be generated under sufficient loading in many materials, ranging from polymers and metals to biological tissues. The presence of these voids can have drastic implications at the macroscopic level including strong material softening and more incipient fracture. Developing tools to appropriately account for these effects is therefore very desirable.
\r\n\r\nThis thesis is concerned with both, the appearance of voids (nucleation process) and the modeling and simulation of materials in the presence of voids. A particular nucleation mechanism based on vacancy aggregation in high purity metallic single crystals is analyzed. A multiscale model is developed in order to obtain an approximate value of the time required for vacancies to form sufficiently large clusters for further growth by plastic deformation. It is based on quantum mechanical results, kinetic Monte Carlo methods and continuum mechanics estimates calibrated with quasi-continuum results. The ultimate goal of these simulations is to determine the feasibility of this nucleation mechanism under shock loading conditions, where the temperature and tensions are high and vacancy diffusion is promoted.
\r\n\r\nOn the other hand, the effective behavior of materials with pre-existent voids is analyzed within the general framework of continuum mechanics and is therefore applicable to any material. The overall properties of the heterogeneous material are obtained through a two-level characterization: a representative volume element consisting of a hollow sphere is used to describe the \"microscopic\" fields, and an equivalent homogeneous material is used for the \"macroscopic\" behavior. A variational formulation of this two-scale model is presented. It provides a consistent definition of the macro-variables under general loading conditions, extending the well-known static averaging results so as to include microdynamic effects under finite deformations. This variational framework also provides a suitable starting point for time discretization and consistent definitions within discrete time. The spatial boundary value problem resulting from this multiscale model is solved with a particular spherical shell element specially developed for this problem. The approximation space is based on spherical harmonics, which respects the symmetries of the porous material and allows the representation of the fields on the sphere with very few degrees of freedom. Numerical tools, such as the exact representation of the boundary conditions and an exact quadrature rule, are also provided. The resulting numerical model is verified extensively, demonstrating good convergence results, and its applicability is shown through several material point calculations and a full two-scale finite element implementation.
" }, { "name": "Silva, Michael Lee", "degree": "PhD", "year": "2011", "title": "Damage Evolution in Composite Materials and Sandwich Structures Under Impulse Loading", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:05122011-154526450", "creators": [ { "name": { "family": "Silva", "given": "Michael Lee" }, "id": "Silva-Michael-Lee", "display_name": "Silva, Michael Lee" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Shukla", "given": "Arun" }, "id": "Shukla-A", "role": "member", "display_name": "Shukla, Arun" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CRX1-7D43", "abstract": "Damage evolution in composite materials is a rather complex phenomenon. There are numerous failure modes in composite materials stemming from the interaction of the various constituent materials and the particular loading conditions. This thesis is concerned with investigating damage evolution in sandwich structures under repeated transient loading conditions associated with impulse loading due to hull slamming of high-speed marine craft. To fully understand the complex stress interactions, a full field technique to reveal stress or strain is required. Several full field techniques exist but are limited to materials with particular optical properties. A full field technique applicable to most materials is known as thermoelastic stress analysis (TSA) and reveals the variation in sum of principal stresses of a cyclically loaded sample by correlating the stresses to a small temperature change occurring at the loading frequency. Digital image correlation (DIC) is another noncontact full field technique that reveals the deformation field by tracking the motion of subsets of a random speckle pattern during the loading cycles.
\r\n\r\nA novel experimental technique to aid in the study of damage progression that combines TSA and DIC simultaneously utilizing a single infrared camera is presented in this thesis. A technique to reliably perform DIC with an infrared (IR) camera is developed utilizing variable emissivity paint. The thermal data can then be corrected for rigid-body motion and deformation such that each pixel represents the same material point in all frames. TSA is then performed on this corrected data, reducing motion blur and increasing accuracy. This combined method with a single infrared camera has several advantages, including a straightforward experimental setup without the need to correct for geometric effects of two spatially separate cameras. Additionally, there is no need for external lighting in TSA as the measured electromagnetic radiation is emitted by the sample\u2019s thermal fields.
\r\n\r\nThe particular stress resolution of TSA will depend on properties of the material of interest but the noise floor for the temperature variation is universal to the camera utilized. For the camera system in this thesis, the noise floor was found to be fairly frequency independent with a magnitude of 0.01 oC, giving the minimum measurable stress for 2024 aluminum alloy of 3.6 MPa and for Nylon of 0.84 MPa. The average displacement range found during a static DIC test with IR images was 0.1 pixels. The maximum displacement variation at 1 Hz was 0.018 pixels. The average variation in strain at 1 Hz was 25 microstrain comparable to traditional DIC measurements in the visible optical regime.
\r\n\r\nThe combined TSA-DIC method in IR was validated with several benchmark example problems including plate structures with holes, cracks, and bimaterials. The validated technique was applied to foam-core sandwich composite beams under repeated simulated wave slamming loading. There are numerous failure modes in sandwich composite materials and the full field stress and strain from TSA and DIC, respectively, allow for improved failure analysis and prediction. Understanding damage in sandwich structures under impulse loading is a complex open area of research and the combined TSA-DIC method provides further insight into the failure process.
\r\n" }, { "name": "Stohlman, Olive Remington", "degree": "PhD", "year": "2011", "title": "Repeatability of Joint-Dominated Deployable Masts", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:05242011-022845109", "creators": [ { "name": { "family": "Stohlman", "given": "Olive Remington" }, "id": "Stohlman-Olive-Remington", "display_name": "Stohlman, Olive Remington" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Kim", "given": "Yunjin" }, "id": "Kim-Yunjin", "role": "member", "display_name": "Kim, Yunjin" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/D3AR-G573", "abstract": "Deployable masts are a class of structure that can be stowed in a small volume and expanded into long, slender, and stable booms. Their greatest benefit as space structures is their packing ratio: masts can typically be packed to a fraction of their deployed length at a diameter only modestly wider than their deployed width. This thesis is concerned with precision deployable masts, which can be stowed and deployed with repeatability of the tip position of better than 1 mm over 60 m. The methods of investigation are experimental measurements of a sample mast and numerical modeling of the mast with specially attention to hysteretic joints.
\r\n\r\nA test article of an ADAM mast was used for the experimental work. Two categories of experi- ment were pursued: measurements of mast components as inputs to the model, and measurements of full bays as validation cases for the model. Measurements of the longeron ball end joint friction, cable preload, and latch behavior are of particular note, and were evaluated for their variability. Further measurements were made of a bay in torsion and a short two-bay mast in shear, showing that there is residual displacement in this mast after shear loading is applied and released.
\r\n\r\nThe modeling approach is described in detail, with attention to the treatment of the mast latches, which lock the structure in its deployed configuration. A user element subroutine was used within the framework of the Abaqus finite element analysis solver to model the behavior of the latches with high fidelity.
\r\n\r\nValidation cases for the model are presented in comparison with experimental observations of a two-bay mast. These cases show that the model captures a number of important and complex nonlinear effects of the hysteretic mast components. Parametric studies of the impacts of component behaviors and modeling practices are explored, emphasizing the impacts of part variability and the idealization of the mast latching mechanisms.
" }, { "name": "Suryanarayana, Phanish", "degree": "PhD", "year": "2011", "title": "Coarse-Graining Kohn-Sham Density Functional Theory", "advisor": "Bhattacharya, Kaushik; Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:05292011-200916324", "creators": [ { "name": { "family": "Suryanarayana", "given": "Phanish" }, "id": "Suryanarayana-Phanish", "display_name": "Suryanarayana, Phanish" } ], "advisors": [ { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "advisor", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "co-advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GCKH-EX20", "abstract": "Defects, though present in relatively minute concentrations, play a significant role in determining macroscopic properties. Even vacancies, the simplest and most common type of defect, are fundamental to phenomena like creep, spall and radiation ageing. This necessitates an accurate characterization of defects at physically relevant concentrations, which is typically in parts per million. This represents a unique challenge since both the electronic structure of the defect core as well as the long range elastic field need to be resolved simultaneously. Unfortunately, accurate ab-initio electronic structure calculations are limited to a few hundred atoms, which is orders of magnitude smaller than that necessary for a complete description. Thus, defects represent a truly challenging multiscale problem.
\r\n\r\nDensity functional theory developed by Hohenberg, Kohn and Sham (DFT) is a widely accepted, reliable ab-initio method for computing a wide range of material properties. We present a real-space, non-periodic, finite-element and max-ent formulation for DFT. We transform the original variational problem into a local saddle-point problem, and show its well-posedness by proving the existence of minimizers. Further, we prove the convergence of finite-element approximations including numerical quadratures. Based on domain decomposition, we develop parallel finite-element and max-ent implementations of this formulation capable of performing both all-electron and pseudopotential calculations. We assess the accuracy of the formulation through selected test cases and demonstrate good agreement with the literature.
\r\n\r\nTraditional implementations of DFT solve for the wavefunctions, a procedure which has cubic-scaling with respect to the number of atoms. This places serious limitations on the size of the system which can be studied. Further, they are not amenable to coarse-graining since the wavefunctions need to be orthonormal, a global constraint. To overcome this, we develop a linear-scaling method for DFT where the key idea is to directly evaluate the electron density without solving for the individual wavefunctions. Based on this linear-scaling method, we develop a numerical scheme to coarse-grain DFT derived solely based on approximation theory, without the introduction of any new equations and resultant spurious physics. This allows us to study defects at a fraction of the original computational cost, without any significant loss of accuracy. We demonstrate the efficiency and efficacy of the proposed methods through examples. This work enables the study of defects like vacancies, dislocations, interfaces and crack tips using DFT to be computationally viable.
" }, { "name": "Tchieu, Andrew Allen", "degree": "PhD", "year": "2011", "title": "The Development of Low-Order Models for the Study of Fluid-Structure Interactions", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-133354529", "creators": [ { "name": { "family": "Tchieu", "given": "Andrew Allen" }, "id": "Tchieu-Andrew-Allen", "display_name": "Tchieu, Andrew Allen" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SYHX-8A77", "abstract": "In this work, several low-order models are derived to describe and simulate fluid-structure interaction problems with rigid bodies at a modest computational cost. The models are based on the inviscid flow assumption such that potential theory can be used with, in some cases, point vortices in the flow. Three general areas of application are considered. First, a thin airfoil undergoing small-scale unsteady motions in the presence of a freestream flow is investigated. The low-order model that is developed has only one ordinary differential equation for the fluid dynamic variables. This model is used to briefly investigate vortex-induced flutter in the attached-flow regime and control of a free-flying airfoil using synthetic jet actuators. Second, the vortex-induced vibrations of an arbitrary bluff body in the presence of vortices, with or without a freestream flow, are considered. Several examples of the canonical mass-spring-damper system for a circular cylinder and a flat plate are given to demonstrate the use of the vortex-based model for these applications. Finally, the two-body problem in a potential flow is addressed. A relatively simple solution specific to the doubly connected domain is determined and its resulting force and moment are coupled to the rigid bodies to investigate the mutual interactions between the two bodies. Aspects of drafting behind a forced body, the role of the fluid in elastic collision, and flapping flight are discussed in this context. Although a few specific examples and applications are given for each chapter, the main purpose of the thesis is to present low-order potential flow methods that are applicable to a variety of situations.\r\n" }, { "name": "Venturini, Gabriela Natalia", "degree": "PhD", "year": "2011", "title": "Topics in Multiscale Modeling of Metals and Metallic Alloys", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:11222010-114324484", "creators": [ { "name": { "family": "Venturini", "given": "Gabriela Natalia" }, "id": "Venturini-Gabriela-Natalia", "display_name": "Venturini, Gabriela Natalia" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "orcid": "0000-0001-6558-0323", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" }, { "name": { "family": "Marian", "given": "Jaime" }, "id": "Marian-J", "orcid": "0000-0001-9000-3405", "role": "member", "display_name": "Marian, Jaime" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/D6YS-B365", "abstract": "In a number of areas of application, the behavior of systems depends sensitively on properties that pertain to the atomistic scale, i. e., the angstrom and femtosecond scales. However, generally the behaviors of interest are macroscopic and are characterized by slow evolution on the scale of meters and years. This broad disparity of length and time scales places extraordinary challenges in computational material science.
\r\n\r\nThe overarching objective of this dissertation is to address the problem of multiple space and time scales in atomistic systems undergoing slow macroscopic evolution while retaining full atomistic detail. Our approach may be summarized as follows:
\r\n\r\n(1) The issue of accounting for finite temperature in coarse grained systems has not been solved entirely. For finite temperature systems at equilibrium, constructing an effective free energy in terms of a reduced set of atomic degrees of freedom is still an open area of research. In particular, the thermal vibrations of the missing degrees of freedom need to be accounted for. This is specially important if the aim of the simulation is to determine the dynamic properties of a system, or to allow the transmission of dynamic information between regions of different spatial discretization. To this end, we introduce a framework to simulate (spatially) coarse dynamic systems using the Quasicontinuum method (QC). The equations of motion are strictly derived from dissipative Lagrangian mechanics, which provides a classical Langevin implementation where the characteristic time is governed by the vibrations of the finest length scale in the computational cell. In order to assess the framework's ability to transmit information across scales, we study the phonon impoverish spectra in coarse regions and the resulting underestimation of thermal equilibrium properties.
\r\n\r\n(2) Atomistic simulations have been employed for the past thirty years to determine structural and thermodynamic (equilibrium) properties of solids and their defects over a wide range of temperatures and pressures. The traditional Monte Carlo (MC) and Molecular Dynamics (MD) methods, while ideally suited to these calculations, require appreciable computational resources in order to calculate the long-time averages from which properties are obtained. In order to permit a reasonably quick, but accurate determination of the equilibrium properties of interest, we present an extension of the \u201cmaximum entropy\u201d method to build effective alloy potentials while avoiding the treatment of all the system's atomic degrees of freedom. We assess the validity of the model by testing its ability to reproduce experimental measurements.
\r\n\r\n(3) Based upon these effective potentials, we present a numerical framework capable of following the time evolution of atomistic systems over time windows currently beyond the scope of traditional atomistic methods such as Molecular Dynamics (MD) or Monte Carlo (MC). This is accomplished while retaining the underlying atomistic description of the material. We formulate a discrete variational setting in which the simulation of time-dependent phenomena is reduced to a sequence of incremental problems, each characterized by a variational principle. In this fashion we are able to study the interplay between deformation and diffusion using time steps or strain rates that are orders of magnitude larger or smaller than their MD|MC counterparts.
\r\n\r\n(4) We formulate a new class of \u201cReplica Time Integrators\u201d (RTIs) that allows for the two-way transmission of thermal phonons across mesh interfaces. This two-way transmission is accomplished by representing the state of the coarse region by a collection of identical copies or \u201creplicas\u201d of itself. Each replica runs at its own slow time step and is out-of-phase with respect to the others by one fast time step. Then, each replica is capable of absorbing from the fine region the elementary signal that is in phase with the replica. Conversely, each replica is capable of supporting --and transmitting to the fine region-- an elementary signal of a certain phase. Since fine and coarse regions evolve asynchronously in time, RTIs permit both spatial and temporal coarse graining of the system of interest. Using a combination of phase-error analysis and numerical testing we find that RTIs are convergent, and allow step waves and thermal phonons to cross mesh interfaces in both directions losslessly.
\r\n" }, { "name": "Ward, Geoffrey M.", "degree": "PhD", "year": "2011", "title": "The Simulation of Shock- and Impact-Driven Flows with Mie-Gr\u00fcneisen Equations of State", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:12162010-115725941", "creators": [ { "name": { "family": "Ward", "given": "Geoffrey M." }, "id": "Ward-Geoffrey-M", "display_name": "Ward, Geoffrey M." } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8Q2Q-GT29", "abstract": "An investigation of shock- and impact-driven flows with Mie-Gr\u00fcneisen equation of state derived from a linear shock-particle speed Hugoniot relationship is presented. Cartesian mesh methods using structured adaptive refinement are applied to simulate several flows of interest in an Eulerian frame of reference. The flows central to the investigation include planar Richtmyer-Meshkov instability, the impact of a sphere with a plate, and an impact-driven Mach stem.
\r\n\r\nFirst, for multicomponent shock-driven flows, a dimensionally unsplit, spatially high-order, hybrid, center-difference, limiter methodology is developed. Effective switching between center-difference and upwinding schemes is achieved by a set of robust tolerance and Lax-entropy-based criteria [49]. Oscillations that result from such a mixed stencil scheme are minimized by requiring that the upwinding method approaches the center-difference method in smooth regions. To attain this property a blending limiter is introduced based on the norm of the deviation of WENO reconstruction weights from ideal. The scheme is first demonstrated successfully for the linear advection equation in spatially fourth- and sixth-order forms. A spatially fourth-order version of the method that combines a skew-symmetric kinetic-energy preserving center-difference scheme with a Roe-Riemann solver is then developed and implemented in Caltech's Adaptive Mesh Refinement, Object-oriented C++ (AMROC) [16,17] framework for Euler flows.
\r\n\r\nThe solver is then applied to investigate planar Richtmyer-Meshkov instability in the context of an equation of state comparison. Comparisons of simulations with materials modeled by isotropic stress Mie-Gr\u00fcneisen equations of state derived from a linear shock-particle speed Hugoniot relationship [36,52] to those of perfect gases are made with the intention of exposing the role of the equation of state. First, results for single- and triple-mode planar Richtmyer-Meshkov instability between mid-ocean ridge basalt (MORB) and molybdenum modeled by Mie-Gr\u00fcneisen equations of state are presented for the case of a reflected shock. The single-mode case is explored for incident shock Mach numbers of 1.5 and 2.5. For the planar triple-mode case a single incident Mach number of 2.5 is examined with the initial corrugation wave numbers related by k\u2081=k\u2082+k\u2083. A comparison is drawn to Richtmyer-Meshkov instability in fluids with perfect gas equations of state utilizing matching of a nondimensional pressure jump across the incident shock, the post-shock Atwood ratio, post-shock amplitude-to-wavelength ratio, and time nondimensionalized by the Rcithmyer linear-growth rate time constant prediction. Result comparison demonstrates difference in start-up time and growth rate oscillations. Growth rate oscillation frequency is seen to correlate directly to the expected oscillation frequency of the transmitted and reflected shocks. For the single-mode cases, further comparison is given for vorticity distribution and corrugation centerline shortly after shock interaction that demonstrates only minor differences.
\r\n\r\nAdditionally, examined is single-mode Richtmyer-Meshkov instability when a reflected expansion wave is present for incident Mach numbers of 1.5 and 2.5. Comparison to perfect gas solutions in such cases yields a higher degree of similarity in start-up time and growth rate oscillations. Vorticity distribution and corrugation centerline shortly after shock interaction is also examined. The formation of incipient weak shock waves in the heavy fluid driven by waves emanating from the perturbed transmitted shock is observed when an expansion wave is reflected.
\r\n\r\nNext, the ghost fluid method [83] is explored for application to impact-driven flows with Mie-Gr\u00fcneisen equations of state in a vacuum. Free surfaces are defined utilizing a level-set approach. The level-set is reinitialized to the signed distance function periodically by solution to a Hamilton-Jacobi differential equation in artificial time. Flux reconstruction along each Cartesian direction of the domain is performed by subdividing in a way that allows for robust treatment of grid-scale sized voids. Ghost cells in voided regions near the material-vacuum interface are determined from surface-normal Riemann problem solution. The method is then applied to several impact problems of interest. First, a one-dimensional impact problem is examined in Mie-Gr\u00fcneisen aluminum with simple point erosion used to model separation by spallation under high tension. A similar three-dimensional axisymmetric simulation of two rods impacting is then performed without a model for spallation. Further results for three-dimensional axisymmetric simulation of a sphere hitting a plate are then presented.
\r\n\r\nFinally, a brief investigation of the assumptions utilized in modeling solids as isotropic fluids is undertaken. An Eulerian solver approach to handling elastic and elastic-plastic solids is utilized for comparison to the simple fluid model assumption. First, in one dimension an impact problem is examined for elastic, elastic-plastic, and fluid equations of state for aluminum. The results demonstrate that in one dimension the fluid models the plastic shock structure of the flow well. Further investigation is made using a three-dimensional axisymmetric simulation of an impact problem involving a copper cylinder surrounded by aluminum. An aluminum slab impact drives a faster shock in the outer aluminum region yielding a Mach reflection in the copper. The results demonstrate similar plastic shock structures. Several differences are also notable that include a lack of roll-up instability at the material interface and slip-line emanating from the Mach stem's triple point.
" }, { "name": "Yang, Yue", "degree": "PhD", "year": "2011", "title": "Lagrangian and Vortex-Surface Fields in Turbulence", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechTHESIS:02212011-233246689", "creators": [ { "name": { "family": "Yang", "given": "Yue" }, "id": "Yang-Yue", "display_name": "Yang, Yue" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hou", "given": "Thomas Y." }, "id": "Hou-T-Y", "role": "member", "display_name": "Hou, Thomas Y." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DF3E-G629", "abstract": "In this thesis, we focus on Lagrangian investigations of isotropic turbulence, wall-bounded turbulence and vortex dynamics. In particular, the evolutionary multi-scale geometry of Lagrangian structures is quantified and analyzed. Additionally, we also study the dynamics of vortex-surface fields for some simple viscous flows with both Taylor--Green and Kida--Pelz initial conditions.
\r\n\r\nFirst, we study the non-local geometry of finite-sized Lagrangian structures in both stationary, evolving homogenous isotropic turbulence and also with a frozen turbulent velocity field. The multi-scale geometric analysis is applied on the evolution of Lagrangian fields, obtained by a particle-backward-tracking method, to extract Lagrangian structures at different length scales and to characterize their non-local geometry in a space of reduced geometrical parameters. Next, we report a geometric study of both evolving Lagrangian, and also instantaneous Eulerian structures in turbulent channel flow at low and moderate Reynolds numbers. A multi-scale and multi-directional analysis, based on the mirror-extended curvelet transform, is developed to quantify flow structure geometry including the averaged inclination and sweep angles of both classes of turbulent structures at multiple scales ranging from the half-height of the channel to several viscous length scales. Results for turbulent channel flow include the geometry of candidate quasi-streamwise vortices in the near-wall region, the structural evolution of near-wall vortices, and evidence for the existence and geometry of structure packets based on statistical inter-scale correlations.
\r\n\r\nIn order to explore the connection and corresponding representations between Lagrangian kinematics and vortex dynamics, we develop a theoretical formulation and numerical methods for computation of the evolution of a vortex-surface field. Iso-surfaces of the vortex-surface field define vortex surfaces. A systematic methodology is developed for constructing smooth vortex-surface fields for initial Taylor--Green and Kida--Pelz velocity fields by using an optimization approach. Equations describing the evolution of vortex-surface fields are then obtained for both inviscid and viscous incompressible flows. Numerical results on the evolution of vortex-surface fields clarify the continuous vortex dynamics in viscous Taylor--Green and Kida--Pelz flows including the vortex reconnection, rolling-up of vortex tubes, vorticity intensification between anti-parallel vortex tubes, and vortex stretching and twisting. This suggests a possible scenario for explaining the transition from a smooth laminar flow to turbulent flow in terms of topology and geometry of vortex surfaces.
\r\n" }, { "name": "Bane, Sally Page Moffett", "degree": "PhD", "year": "2010", "title": "Spark Ignition: Experimental and Numerical Investigation With Application to Aviation Safety", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-173243262", "creators": [ { "name": { "family": "Bane", "given": "Sally Page Moffett" }, "id": "Bane-Sally-Page-Moffett", "orcid": "0000-0002-4764-3228", "display_name": "Bane, Sally Page Moffett" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "member", "display_name": "Blanquart, Guillaume" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/W1NB-5W06", "abstract": "Determining the risk of accidental ignition of flammable mixtures is a topic of tremendous importance in industry and aviation safety. The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, in recent years, particularly in the aviation safety industry, the viewpoint has changed to one where ignition is statistical in nature. Approaching ignition as statistical rather than a threshold phenomenon appears to be more consistent with the inherent variability in the engineering test data.
\r\n\r\nIgnition tests were performed in lean hydrogen-based aviation test mixtures and in two hexane-air mixtures using low-energy capacitive spark ignition systems. Tests were carried out using both short, fixed sparks (1 to 2 mm) and variable length sparks up to 10 mm. The results were analyzed using statistical tools to obtain probability distributions for ignition versus spark energy and spark energy density (energy per unit spark length). Results show that a single threshold MIE value does not exist, and that the energy per unit length may be a more appropriate parameter for quantifying the risk of ignition than only the energy. The probability of ignition versus spark charge was also investigated, and the statistical results for the spark charge and spark energy density were compared. It was found that the test results were less variable with respect to the spark charge than the energy density. However, variability was still present due to phenomena such as plasma instabilities and cathode effects that are caused by the electrodynamics.
\r\n\r\nWork was also done to develop a two-dimensional numerical model of spark ignition that accurately simulates all physical scales of the fluid mechanics and chemistry. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. One-step chemistry models were used to allow for highly resolved simulations, and methods for calculating effective one-step parameters were developed using constant pressure explosion theory. The one-step model was tuned to accurately simulate the flame speed, temperature, and straining behavior using one-dimensional flame computations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process.
\r\n" }, { "name": "Karnesky, James Alan", "degree": "PhD", "year": "2010", "title": "Detonation Induced Strain in Tubes", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05142010-174001426", "creators": [ { "name": { "family": "Karnesky", "given": "James Alan" }, "id": "Karnesky-James-Alan", "display_name": "Karnesky, James Alan" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Blanquart", "given": "Guillaume" }, "id": "Blanquart-G", "role": "member", "display_name": "Blanquart, Guillaume" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GTKC-FY91", "abstract": "When a detonation wave propagates through a piping system, it acts as a traveling pressure load to the pipe wall. The detonation wave must be followed by an expansion wave in order to bring the combustion products to zero velocity at the ignition end. When it reaches a closed end-wall, a reflected shock is formed which propagates back into the tube with a decaying pressure. The present study aims to develop predictive models for the stresses and strains produced in such a situation. To this end, two series of experiments are discussed. The first series used strain gauges and a laser vibrometer to measure the elastic response of the tube to the incident detonation in thin aluminum tubes. The second series used strain gauges and high speed video to measure the plastic response of steel tubes to incident detonations and reflected shocks. In these experiments a novel mode of plastic deformation was discovered in which the residual plastic deformation in the tube wall had a periodic sinusoidal pattern.
\r\n\r\nA semi-empirical model of the pressure history was developed for use as a boundary condition in models of the mechanical response of the tube. This model was tested against experiment, and it was found that the pressure and arrival time could not be simultaneously predicted from the simple model. This and the general form of the pressure traces in the experiment seem to suggest an interaction between the reflected shock and the boundary layer behind the detonation resulting in a possible bifurcation in the reflected shock wave.
\r\n\r\nWith these considerations in mind, the model was applied to single degree of freedom and finite element models of the tube wall. The ripples observed in the experiment were present in the 1-D single degree of freedom models, indicating that they are a result of the interaction of the reflected shock wave with the elastic oscillations set in motion by the detonation wave. Strain-rate hardening was found to be an important consideration under detonation loading conditions. With proper consideration of rate hardening, a single material model may be used to arrive at reasonable predictions the plastic strains resulting from detonations and reflections at initial pressures of 2 and 3 bar initial pressures.
" }, { "name": "Kim, Daegyoum", "degree": "PhD", "year": "2010", "title": "Characteristics of Three-dimensional Vortex Formation and Propulsive Performance in Flapping Locomotion", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072010-114858790", "creators": [ { "name": { "family": "Kim", "given": "Daegyoum" }, "id": "Kim-Daegyoum", "display_name": "Kim, Daegyoum" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8VZJ-7Z78", "abstract": "Three-dimensional vortex formation and propulsive performance were studied experimentally to identify some of the main characteristic mechanisms of flapping locomotion. Mechanical models with thin plates were used to simulate flapping and translating motions of animal propulsors. Three-dimensional flow fields were mapped quantitatively using defocusing digital particle image velocimetry.
\r\n\r\nFirst, vortex structures made by impulsively translating low aspect-ratio plates were studied. The investigation of translating plates with a 90 degree angle of attack is important since it is a fundamental model for a better understanding of drag-based propulsion systems. Rectangular flat-rigid, flexible, and curved-rigid thin plastic plates with the same aspect ratio were used to compare their vortex structures and hydrodynamic forces. The interaction of the tip flow and the nearby vortex is a critical flow phenomenon to distinguish vortex patterns among these three cases. In the flexible plate case, slow development of the vortex structure causes a small initial peak in hydrodynamic force during the acceleration phase. However, after the initial peak, the flexible plate generates large force magnitude comparable to that of the flat-rigid plate case.
\r\n \r\nDrag-based paddling propulsion was also studied to explain some of the fundamental differences in vortex formation of lift-based and drag-based propulsions. While the temporal change of the inner area enclosed by the vortex loop is an important factor in thrust generation of lift-based propulsion, the temporal change of the vortex strength becomes more important in drag-based propulsion. Spanwise flow behind the paddling plate plays an important role in tip vortex motion and thrust generation. The distribution of spanwise flow depends on the propulsor shape and the Reynolds number. A delta-shaped propulsor generates strong spanwise flow compared to a rectangular propulsor. For the low Reynolds number case, the spanwise flow is not as strong as that of the high Reynolds number case. The flexible propulsor can smooth out force peaks during impulsive motions without sacrificing total impulse, which is advantageous in avoiding structural failures and stabilizing body motion. The role of the stopping vortex was addressed in optimizing a stroke angle of paddling animals.
\r\n\r\nIn addition, vortex formation of clapping propulsion was investigated by varying aspect ratio and stroke angle. A low aspect-ratio propulsor produces larger total impulse than a high aspect-ratio propulsor. As the aspect ratio increases, circulation of the vortex is strengthened, and the inner area enclosed by the vortex structure tends to enlarge. Moreover, in terms of thrust, the advantage of a single plate over double clapping plates is larger for the lower aspect-ratio case. These results offer information to better understand the benefit of low aspect-ratio wings in force generation under specific locomotion modes. When a pair of plates claps, a vortex loop forms from two counter-rotating tip vortices by a reconnection process. The dynamics of wake structures are dependent on the aspect ratio and the stroke angle.
\r\n\r\nVortex formation and vorticity transport processes of translating and rotating plates with a 45 degree angle of attack were investigated as well. In both translating and rotating cases, the spanwise flow over the plate and the vorticity tilting process inside the leading-edge vortex were observed. The distribution of spanwise flow is a prominent distinction between the vortex structures of these two cases. While spanwise flow is confined inside the leading-edge vortex for the translating case, it is widely present over the plate and the wake region of the rotating case. As the Reynolds number decreases, due to the increase in viscosity, leading-edge and tip vortices tend to spread inside the area swept by the rotating plate, which results in lower lift force generation.
\r\n\r\nLastly, for translating motion, the dynamics of the vortex in corner regions was compared among three different corner shapes. For a large corner angle, the forward movement of the vortex tends to be uniform along the plate edges. However, for a small corner angle, the vortex close to the corner moves forward following the plate while the vortex away from the corner retards its forward movement.
" }, { "name": "Lamberson, Leslie Elise", "degree": "PhD", "year": "2010", "title": "Dynamic Optical Investigations of Hypervelocity Impact Damage", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05282010-183132978", "creators": [ { "name": { "family": "Lamberson", "given": "Leslie Elise" }, "id": "Lamberson-Leslie-Elise", "orcid": "0000-0002-1340-4667", "display_name": "Lamberson, Leslie Elise" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Greer", "given": "Julia R." }, "id": "Greer-J-R", "role": "member", "display_name": "Greer, Julia R." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AQJH-3D60", "abstract": "One of the prominent threats in the endeavor to develop next-generation space assets is the risk of space debris impact in earth\u2019s orbit and micrometeoroid impact damage in near-earth orbit and deep space. To date, there is no study available which concentrates on the analysis of dynamic crack growth from hypervelocity impacts on such structures, resulting in their eventual catastrophic degradation. Experiments conducted using a unique two-stage light-gas gun facility have examined the in situ dynamic fracture of brittle polymers subjected to this high-energy-density event. Optical techniques of caustics and photoelasticity, combined with high-speed photography up to 100 million frames per second, analyze crack growth behavior of Mylar and Homalite 100 thin plates after impact by a 1.8 mm diameter nylon 6-6 right cylindrical slug at velocities ranging from 3 to 7 km/s (7000\u201315500 mph). Crack speeds in both polymers averaged between 0.2 and 0.47 cR, the Rayleigh wave speed (450\u20131000 mph). Shadow spots and surrounding caustics reveal time histories of the dynamic stress intensity factor, as well as the energy release rate ahead of the mode-I, or opening, crack tips. Results indicate that even under extreme impact conditions of out of-plane loading, highly localized heating, and energetic impact phenomena involving plasma formation and ejecta, the dynamic fracture process occurs during a deformation regime dominated by in-plane loading. These findings imply that the reliability of impacted, thin-walled, plate and shell space structures, idealized by the experimental configuration investigated, can be predicted by the well defined principles of classical dynamic fracture mechanics." }, { "name": "Norman, Adam Keith", "degree": "PhD", "year": "2010", "title": "Effect of Surface Morphological Changes on Flow Over a Sphere", "advisor": "McKeon, Beverley J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:05212010-124044645", "creators": [ { "name": { "family": "Norman", "given": "Adam Keith" }, "id": "Norman-Adam-Keith", "display_name": "Norman, Adam Keith" } ], "advisors": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "advisor", "display_name": "McKeon, Beverley J." } ], "committee": [ { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "chair", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Hussain", "given": "Fazle" }, "id": "Hussain-F", "role": "member", "display_name": "Hussain, Fazle" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SED1-5250", "abstract": "An experimental investigation was undertaken to examine the effect of a morphing surface on the flow over a sphere in the Reynolds number range of 5x10\u2074 to 5x10\u2075. Here, a morphing surface is defined as a continuous surface that undergoes small amplitude changes in order to excite flow instabilities, rather than utilizing large mechanical changes to the overall shape as with traditional aerodynamic control surfaces. The sphere was chosen as an ideal geometry for testing morphing surfaces, because of the well-known sensitivity of the flow to small asymmetries on the surface. In this study, an approximation of a morphing surface was made by dynamically moving a small isolated roughness element along the sphere, thus producing small amplitude time-dependent changes to the surface shape. An experimental apparatus was designed that produced the actuation with an internal motor, which moved the roughness element via magnetic interaction. A three-component piezoelectric force sensor placed inside the sphere allowed for accurate, instantaneous measurements of the global effect of the actuator on the flow. It was found that the moving roughness could produce an instantaneous lateral force as large as the drag. Simultaneous force and particle image velocimetry measurements in the subcritical regime were used to show that there is a relatively long timescale associated with the instability growth, entrainment of fluid, and local change of the position of separation. This allowed the roughness to trip an extended region of the flow at once. It is shown that the three-dimensionality of the disturbance leads to the production of two helical counter-rotating vortices in the wake. In addition, it is demonstrated that a mean side force can be obtained by oscillating the roughness element about a point, producing a lateral force an order of magnitude larger than the force caused by a stationary roughness element. Finally, the results from the dynamic roughness were used to help interpret the underlying physical mechanisms that govern the forcing on a smooth sphere." }, { "name": "Sullivan, Regina Mariko", "degree": "PhD", "year": "2010", "title": "The Physics of High-Velocity Ions in the Hall Thruster Near-Field", "advisor": "Shepherd, Joseph E.; Johnson, Lee K.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04022010-134100215", "creators": [ { "name": { "family": "Sullivan", "given": "Regina Mariko" }, "id": "Sullivan-Regina-Mariko", "display_name": "Sullivan, Regina Mariko" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Johnson", "given": "Lee K." }, "id": "Johnson-L-K", "role": "co-advisor", "display_name": "Johnson, Lee K." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Bellan", "given": "Paul Murray" }, "id": "Bellan-P-M", "role": "member", "display_name": "Bellan, Paul Murray" }, { "name": { "family": "Johnson", "given": "Lee K." }, "id": "Johnson-L-K", "role": "member", "display_name": "Johnson, Lee K." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8F2Y-NM32", "abstract": "A study of the physics underlying high velocity ion trajectories within the near-field region of a Hall thruster plume is presented. In this context, \"high velocity\" ions are ions that have been accelerated through the full potential drop of the thruster (sometimes referred to as \"primary energy\" or \"primary beam energy\" ions). Results from an experimental survey of an SPT-70 thruster plume are shown, along with simulated data from a Hall thruster code and from a plasma sheath model. Two main features are examined: the central jet along the Hall thruster centerline, and the population of high velocity ions at high angles.
\r\n\r\nIn the experimental portion of the investigation, three diagnostic instruments were employed: (1) a Faraday probe for measuring ion current density, (2) an ExB velocity filter for mapping ions with the primary beam energy, and (3) a Retarding Potential Analyzer (RPA) for determining ion energy distributions. In the numerical portion, two codes were employed: (1) a hybrid-PIC Hall thruster code known as HPHall, and (2) a model of the plasma sheath near the exit plane of the thruster, which was developed by the author.
\r\n\r\nA comparison between the measured and simulated data sets is made, to analyze the degree to which different mechanisms are responsible for the evolution of the thruster\r\nplume in the near-field region. This analysis shows that the central jet is both a function of symmetric expansion of the ion beam as well as asymmetry in the internal potential field of the thruster. Additionally, it is suggested that high energy, high angle ions could be generated given a specific internal electric field configuration, while oscillations are ruled out as the cause of these ions. The results from the sheath model show that while the sheath can change trajectory angles by 10 to 20 degrees, it can not fully explain the presence of\r\nhigh angle ions with high energies.
" }, { "name": "Chung, Daniel", "degree": "PhD", "year": "2009", "title": "Numerical Simulation and Subgrid-Scale Modeling of Mixing and Wall-Bounded Turbulent Flows", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05292009-123828", "creators": [ { "name": { "family": "Chung", "given": "Daniel" }, "id": "Chung-Daniel", "orcid": "0000-0003-3732-364X", "display_name": "Chung, Daniel" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "orcid": "0000-0003-4220-1583", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "orcid": "0000-0001-8369-3014", "role": "member", "display_name": "Bruno, Oscar P." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NE1Y-M812", "abstract": "We extend the idea of multiscale large-eddy simulation (LES), the underresolved fluid dynamical simulation that is augmented with a physical description of subgrid-scale (SGS) dynamics. Using a vortex-based SGS model, we consider two areas of specialization: active (buoyant) scalar mixing and wall-bounded turbulence.
\r\n\r\nFirst, we develop a novel method to perform direct numerical simulation (DNS) of statistically stationary buoyancy-driven turbulence by using the fringe-region technique within a triply periodic domain, in which a mixing region is sandwiched between two fringes that supply the flow with unmixed fluids---heavy on top of light. Spectra exhibit small-scale universality, as evidenced by collapse in inner scales. A comparison with high-resolution DNS spectra from Rayleigh--Taylor turbulence reveals some similarities.
\r\n\r\nWe perform LES of this flow to show that a passive scalar SGS model can also be used in an unstably stratified environment. LES spectra, including subgrid extensions, show good agreement with DNS data. For stably stratified flows, we develop an active scalar SGS model by performing a perturbation expansion in small Richardson numbers of the passive scalar SGS model to obtain an expression for the SGS scalar flux that contains buoyancy corrections.
\r\n\r\nWe then develop a wall model for LES in which the near-wall region is unresolved. A special near-wall SGS model is constructed by averaging the streamwise momentum equation together with an assumption of local--inner scaling, giving an ordinary differential equation for the local wall shear stress that is coupled with the LES. An extended form of the stretched-vortex SGS model, which incorporates the production of near-wall Reynolds shear stresses due to the winding of streamwise momentum by near-wall attached SGS vortices, then provides a log relation for the off-wall LES boundary conditions. A Karman-like constant is calculated dynamically as part of the LES. With this closure we perform LES of turbulent channel flow for friction-velocity Reynolds numbers $Rey_\tau=2,\textrm{k}$--$20,\textrm{M}$. Results, including SGS-extended spectra, compare favorably with DNS at Rey_\tau=2,\textrm{k}$, and maintain an $O(1)$ grid dependence on $Rey_\tau$.
\r\n\r\nFinally, we apply the wall model to LES of long channels to capture effects of large-scale structures. Computed correlations are found to be consistent with recent experiments.
" }, { "name": "Kramer, Richard Michael Jack", "degree": "PhD", "year": "2009", "title": "Stable High-Order Finite-Difference Interface Schemes with Application to the Richtmyer-Meshkov Instability", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-03132009-095507", "creators": [ { "name": { "family": "Kramer", "given": "Richard Michael Jack" }, "id": "Kramer-Richard-Michael-Jack", "display_name": "Kramer, Richard Michael Jack" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HXGM-DC92", "abstract": "High-order adaptive mesh refinement offers the potential for accurate and efficient resolution of problems in fluid dynamics and other fields where a wide range of length scales is present. A critical requirement for the interface closures used with these methods is stability in the context of hyperbolic systems of partial differential equations.
\r\n\r\nIn this study, a class of energy-stable high-order finite-difference interface closures is presented for grids with step resolution changes in one dimension. Asymptotic stability in time for these schemes is achieved by imposing a summation-by-parts condition on the interface closure, which is thus also nondissipative. Interface closures compatible with interior fourth- and sixth-order explicit, and fourth-order implicit centered schemes are presented. Validation tests include linear and nonlinear problems in one and in two dimensions with tensor-product grid refinement.
\r\n\r\nA second class of stable high-order interface closures is presented for two-dimensional cell-centered grids with patch-refinement and step-changes in resolution. For these grids, coarse and fine nodes are not aligned at the mesh interfaces, resulting in hanging nodes. Stability is achieved by again imposing a summation-by-parts condition, resulting in nondissipative closures, at the cost of accuracy at corner interfaces. Interface stencils for an explicit fourth-order finite-difference scheme are presented for each geometry. Validation tests confirm the stability and accuracy of these closures for linear and nonlinear problems.
\r\n\r\nThe Richtmyer-Meshkov instability is investigated using a novel first-order perturbation of the two-dimensional Navier-Stokes equations about a shock-resolved base flow. The computational domain is efficiently resolved using the one-dimensional fourth-order interface scheme. Results are compared to analytic models of the instability, showing agreement with predicted asymptotic growth rates in the inviscid range, while significant discrepancies are noted in the transient growth phase. Viscous effects are found to be poorly predicted by existing models.
\r\n" }, { "name": "Kramer, Sharlotte Lorraine Bolyard", "degree": "PhD", "year": "2009", "title": "Phase-Shifting Full-Field Interferometric Methods for In-Plane Tensorial Stress Determination for Fracture Studies", "advisor": "Ravichandran, Guruswami; Bhattacharya, Kaushik", "url": "https://resolver.caltech.edu/CaltechETD:etd-05272009-094456", "creators": [ { "name": { "family": "Kramer", "given": "Sharlotte Lorraine Bolyard" }, "id": "Kramer-Sharlotte-Lorraine-Bolyard", "orcid": "0000-0001-6015-8385", "display_name": "Kramer, Sharlotte Lorraine Bolyard" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "advisor", "display_name": "Bhattacharya, Kaushik" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "role": "member", "display_name": "Pellegrino, Sergio" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M9NV-T722", "abstract": "Anisotropic fracture criteria can be established with understanding of full-field stresses near a crack. The anisotropy of the stresses implies that the full in-plane tensorial stress is required, but current experimental optical techniques only give the sum or difference of principal stresses, motivating development of experimental methods that combines two experimental techniques to determine all of the stress components, such as the proposed hybrid experimental method of phase-shifting photoelasticity and transmission Coherent Gradient Sensing (CGS). This thesis establishes this method for stress determination around cracks in photoelastic materials.
\r\n\r\nThis experimental method first requires a new theory for the use of CGS, a wavefront shearing interferometry technique, for photoelastic materials. The first analysis of transmission wavefront shearing interferometry for photoelastic materials is experimentally demonstrated using CGS in full field for a compressed polycarbonate plate with a side V-shaped notch with good agreement with theoretical data. For the hybrid experimental method, a six-step phase-shifting photoelasticity method determines principal stress directions and the difference of principal stresses, and the transmission CGS method utilizes a standard four-step phase-shifting method to measure the x and y first derivatives of the sum of principal stresses, which are numerically integrated for the sum of principal stresses. The full-field principal stresses may then be separated, followed by the Cartesian and polar coordinate stresses using the principal stress directions and the polar angle. The method is first demonstrated for in-plane tensorial stress determination for a compressed polycarbonate plate with a side V-shaped notch with good comparison to theoretical stress fields. The CGS-photoelasticity experimental method is then applied to determine stresses around Mode I-dominant cracks in Homalite-100. The experimental stress fields have excellent agreement with the full-field 2D asymptotic crack solution using the Mode I and Mode II stress intensity factor values calculated from the experimental data. With this foundation of stress determination around cracks in photoelastic materials and with some future analysis, this experimental method can be extended to determine stresses in anisotropic crystals for fracture studies.
\r\n" }, { "name": "Li, Bo", "degree": "PhD", "year": "2009", "title": "The Optimal Transportation Method in Solid Mechanics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05212009-173044", "creators": [ { "name": { "family": "Li", "given": "Bo" }, "id": "Li-Bo", "orcid": "0000-0002-0127-8210", "display_name": "Li, Bo" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FAT3-0247", "abstract": "This dissertation is concerned with the development of a robust and efficient meshless method, the Optimal Transportation Method (OTM), for general solid flows involving extremely large deformation, fast, transient loading and hydrodynamic phenomena. This method is a Lagrangian particle method through an integration of optimal transportation theory with meshless interpolation and material point integrations. The theoretical framework developed in this thesis generalized the Benamou-Brenier differential formulation of optimal transportation problems and leads to a multi-field variational characterization of solid flows, including elasticity, inelasticity, equation of state, and general geometries and boundary conditions. To this end, the accuracy, robustness and versatility of OTM is assessed and demonstrated with convergence and stability test, Taylor anvil test and a series of full three-dimensional simulations of high/hyper-velocity impact examples with the aid of a novel meshless dynamic contact algorithm presented in this thesis.\r\n" }, { "name": "Lu, Xiao", "degree": "PhD", "year": "2009", "title": "Combined Experimental and Numerical Study of Spontaneous Dynamic Rupture on Frictional Interfaces", "advisor": "Lapusta, Nadia; Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10242008-070701", "creators": [ { "name": { "family": "Lu", "given": "Xiao" }, "id": "Lu-Xiao", "display_name": "Lu, Xiao" } ], "advisors": [ { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "advisor", "display_name": "Lapusta, Nadia" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Avouac", "given": "Jean-Philippe" }, "id": "Avouac-J-P", "role": "member", "display_name": "Avouac, Jean-Philippe" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BGGT-MC04", "abstract": "The process of spontaneous dynamic frictional sliding along the interface of two elastic solids is of great interest to a number of disciplines in engineering and sciences. Applications include frictional rupture processes in earthquakes, delamination of layered composite materials, and sliding between soft membranes in biological systems. The transient nature of rupture dynamics presents an array of fascinating yet challenging questions, including the nucleation process, the mechanism of interface failure, and the speed and mode of rupture propagation.
\r\n\r\nThis thesis presents such a combined experimental and theoretical study aimed at understanding the conditions for selecting pulse-like vs. crack-like rupture modes and subshear vs. supershear rupture speeds. There are two major contributions in this work. The first one is high-resolution experimental study of the rupture modes on a frictional interface. The study presents first experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes, reveals how different rupture modes are selected based on the level of fault prestress, demonstrates that both rupture modes can transition to supershear speeds, and advocates, based on comparison with theoretical studies, importance of velocity-weakening friction for earthquake dynamics. The second major contribution is the numerical modeling of the rupture experiments that reveal the importance of the rupture nucleation mechanism and friction formulations. The modeling of sub-Rayleigh to supershear transition has demonstrated the influence of rupture nucleation mechanism on supershear transition distance, as well as on the mechanism of supershear transition. The modeling of pulse-like to crack-like rupture mode transition has confirmed the necessity of velocity weakening friction for producing pulse-like rupture to match the experimental observations.
\r\n" }, { "name": "Poon, Poh Chieh Benny", "degree": "PhD", "year": "2009", "title": "A Critical Appraisal of Nanoindentation with Application to Elastic-Plastic Solids and Soft Materials", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-09162008-023546", "creators": [ { "name": { "family": "Poon", "given": "Poh Chieh Benny" }, "id": "Poon-Poh-Chieh-Benny", "display_name": "Poon, Poh Chieh Benny" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Rittel", "given": "Daniel" }, "id": "Rittel-D", "role": "member", "display_name": "Rittel, Daniel" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/J1WM-BW36", "abstract": "This study examines the accuracy of the extracted elastic properties using nanoindentation. Since the conventional method to extract these properties utilizes Sneddon\u2019s elastic solution, this study first considers indentations of linearly elastic solids for direct comparison. The study proposes a criterion for a converged specimen\u2019s geometry and modifies Sneddon\u2019s equation to account for the finite tip radius and specimen compressibility effects. A composite correction factor is derived to account for the violations of the underlying assumptions behind Sneddon\u2019s derivation. This factor is a function of indentation depth, and a critical depth is derived beyond which the finite tip radius effect will be insignificant. Techniques to identify the radius of curvature of the indenter and to decouple the elastic constants for linear elastic materials are proposed. Experimental results on nanoindentation of natural latex are reported and discussed in light of the proposed modified relation and techniques.
\r\n\r\nThe second part of the study examines the accuracy of the extracted material properties in elastic-plastic nanoindentations. The study establishes that the accurate determination of the projected area of contact, A, is crucial. However, the conventional method to determine A is largely limited to elastic materials, hence a new electrical resistance method is proposed to measure A for elastic-plastic materials. With an accurate A, the error associated with the extracted elastic material properties is reduced by more than 50% in some cases. This error remains to be a function of the material\u2019s Poisson\u2019s ratio, which is identified to influence the amount of residual stresses at the plastic imprint.
\r\n\r\nFinally, this study examines the accuracy of the extracted material properties in the nanoindentation of soft materials using an Atomic Force Microscope (AFM). The effects of cantilever stiffness, preload, and surface interaction forces are observed to influence the measurements. Three set of experiments were performed to decouple these effects. The effect of a preload resembles a shift of nanoindentation load-displacement curve, while the cantilever stiffness is observed to have significant influence on the measurement of the surface forces. Lastly, a novel technique to account for these effects is proposed, in order to accurately extract the material properties of interest.
" }, { "name": "Rimoli, Julian Jose", "degree": "PhD", "year": "2009", "title": "A Computational Model for Intergranular Stress Corrosion Cracking", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05142009-135909", "creators": [ { "name": { "family": "Rimoli", "given": "Julian Jose" }, "id": "Rimoli-Julian-Jose", "orcid": "0000-0002-8707-2968", "display_name": "Rimoli, Julian Jose" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Krishnan", "given": "Swaminathan" }, "id": "Krishnan-S", "role": "member", "display_name": "Krishnan, Swaminathan" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K1HJ-DZ56", "abstract": "Stress corrosion cracking (SCC) is a very common failure mechanism characterized by a slow, environmentally induced crack propagation in structural components. Time-to-failure tests and crack-growth-rate tests are widespread practices for studying the response of various materials undergoing SCC. However, due to the large amount of factors affecting the phenomenon and the scattered data, they do not provide enough information for quantifying the effects of main SCC mechanisms. This thesis is concerned with the development of a novel 3-dimensional, multiphysics model for understanding the intergranular SCC of polycrystalline materials under the effect of impurity-enhanced decohesion. This new model is based upon: (i) a robust algorithm capable of generating the geometry of polycrystals for objects of arbitrary shape; (ii) a continuum finite element model of the crystals including crystal plasticity; (iii) a grain boundary diffusion model informed with first-principles computations of diffusion coefficients; and (iv) an intergranular cohesive model described by concentration-dependent constitutive relations also derived from first-principles. Results are validated and compared against crack-growth-rate and initiation time tests.\r\n" }, { "name": "Bermejo-Moreno, Ivan", "degree": "PhD", "year": "2008", "title": "On the Non-Local Geometry of Turbulence", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05092008-173614", "creators": [ { "name": { "family": "Bermejo-Moreno", "given": "Ivan" }, "id": "Bermejo-Moreno-Ivan", "display_name": "Bermejo-Moreno, Ivan" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DH9W-Y894", "abstract": "A multi-scale methodology for the study of the non-local geometry of eddy structures in turbulence is developed. Starting from a given three-dimensional field, this consists of three main steps: extraction, characterization and classification of structures. The extraction step is done in two stages. First, a multi-scale decomposition based on the curvelet transform is applied to the full three-dimensional field, resulting in a finite set of component fields, one per scale. Second, by iso-contouring each component field at one or more iso-contour levels, a set of closed iso-surfaces is obtained that represents the structures at that scale. For periodic domains, those structures intersecting boundaries are reconnected with their continuation in the opposite boundaries. The characterization stage is based on the joint probability density function (jpdf), in terms of area coverage on each individual iso-surface, of two differential-geometry properties, the shape index and curvedness, plus the stretching parameter, a dimensionless global invariant of the surface. Taken together, this defines the geometrical signature of the iso-surface. The classification step is based on the construction of a finite set of parameters, obtained from algebraic functions of moments of the jpdf of each structure, that specify its location as a point in a multi-dimensional 'feature space'. At each scale the set of points in feature space represents all structures at that scale, for the specified iso-contour value. This allows the application, to the set, of clustering techniques that search for groups of structures with a common geometry.
\r\n\r\nResults are presented of a first application of this technique to a passive scalar field obtained from 512\u00b3 direct numerical simulation of scalar mixing by forced, isotropic turbulence (Re\u03bb=265). These show transition, with decreasing scale, from blob-like structures in the larger scales to blob- and tube-like structures with small or moderate stretching in the inertial range of scales, and then toward tube and, predominantly, sheet-like structures with high level of stretching in the dissipation range of scales. Implications of these results for the dynamical behavior of passive scalar stirring and mixing by turbulence are discussed.
\r\n\r\nWe apply the same methodology to the enstrophy and kinetic energy dissipation rate instantaneous fields of a second numerical database of incompressible homogeneous isotropic turbulence decaying in time obtained by DNS in a periodic box. Three different resolutions are considered: 256\u00b3, 512\u00b3 and 1024\u00b3 grid points, with kmax\u03b7\u0305 approximately 1, 2, and 4, respectively, the same initial conditions and Re\u03bb \u2248 77. This allows a comparison of the geometry of the structures obtained for different resolutions. For the highest resolution, structures of enstrophy and dissipation evolve in a continuous distribution from blob-like and moderately stretched tube-like shapes at the large scales to highly stretched sheet-like structures at the small scales. The intermediate scales show a predominance of tube-like structures for both fields, much more pronounced for the enstrophy field. The dissipation field shows a tendency toward structures with lower curvedness than those of the enstrophy, for intermediate and small scales. The 256\u00b3 grid resolution case (kmax\u03b7\u0305 \u2248 1) was unable to detect the predominance of highly stretched sheet-like structures at the smaller scales.
\r\n\r\nThe same methodology, but without the multi-scale decomposition, is then applied to two scalar fields used by existing local criteria for the eduction of tube- and sheet-like structures in turbulence, Q and [Aij]+ respectively, obtained from invariants of the velocity gradient tensor and alike in the 1024\u00b3 case. This adds the non-local geometrical characterization and classification to those local criteria, assessing their validity in educing particular geometries.
\r\n\r\nFinally we introduce a new methodology for the study of proximity issues among different sets of structures, based also on geometrical and non-local analyses. We apply it to four of the fields previously studied. Tube-like structures of Q are mainly surrounded by sheets of [Aij]+, which appear at close distances. For the enstrophy, tube-like structures at an intermediate scale are primarily surrounded by sheets of smaller scales of the enstrophy and structures of dissipation at the same and smaller scales. A secondary contribution results from tubes of enstrophy at smaller scales appearing at farther distances. Different configurations of composite structures are presented.
\r\n" }, { "name": "Franck, Christian", "degree": "PhD", "year": "2008", "title": "Quantitative Characterization of 3D Deformations of Cell Interactions with Soft Biomaterials", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-05292008-163638", "creators": [ { "name": { "family": "Franck", "given": "Christian" }, "id": "Franck-Christian", "display_name": "Franck, Christian" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Tirrell", "given": "David A." }, "id": "Tirrell-D-A", "role": "member", "display_name": "Tirrell, David A." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VMN5-SP86", "abstract": "In recent years, the importance of mechanical forces in directing cellular function has been recognized as a significant factor in biological and physiological processes. In fact, these physical forces are now viewed equally as important as biochemical stimuli in controlling cellular response. Not only do these cellular forces, or cell tractions, play an important role in cell migration, they are also significant to many other physiological and pathological processes, both at the tissue and organ level, including wound healing, inflammation, angiogenesis, and embryogenesis. A complete quantification of cell tractions during cell-material interactions can lead to a deeper understanding of the fundamental role these forces play in cell biology. Thus, understanding the function and role of a cell from a mechanical framework can have important implications towards the development of new implant materials and drug treatments.
\r\n\r\nPrevious research has contributed significant descriptions of cell-tissue interactions by quantifying cell tractions in two-dimensional environments; however, most physiological processes are three-dimensional in nature. Recent studies have shown morphological differences in cells cultured on two-dimensional substrates versus three-dimensional matrices, and that the intrinsic extracellular matrix interactions and migration behavior are different in three dimensions versus two dimensions. Hence, measurement techniques are needed to investigate cellular behavior in all three dimensions.
\r\n\r\nThis thesis presents a full-field imaging technique capable of quantitatively measuring cell traction forces in all three spatial dimensions, and hence addresses the need of a three-dimensional quantitative imaging technique to gain insight into the fundamental role of physical forces in biological processes. The technique combines laser scanning confocal microscopy (LSCM) with digital volume correlation (DVC) to track the motion of fluorescent particles during cell-induced or externally applied deformations. This method is validated by comparing experimentally measured non-uniform deformation fields near hard and soft spherical inclusions under uniaxial compression with the corresponding analytical solution. Utilization of a newly developed computationally efficient stretch-correlation and deconvolution algorithm is shown to improve the overall measurement accuracy, in particular under large deformations.
\r\n\r\nUsing this technique, the full three-dimensional substrate displacement fields are experimentally determined during the migration of individual fibroblast cells on polyacrylamide gels. This is the first study to show the highly three-dimensional structure of cell-induced displacement and traction fields. These new findings suggest a three-dimensional push-pull cell motility, which differs from the traditional theories based on two-dimensional data. These results provide new insight into the dynamic cell-matrix force exchange or mechanotransduction of migrating cells, and will aid in the development of new three-dimensional cell motility and adhesion models.
\r\n\r\nAs this study reveals, the mechanical interactions of cells and their extracellular matrix appear to be highly three-dimensional. It also shows that the LSCM-DVC technique is well suited for investigating the mechanics of cell-matrix interactions while providing a platform to access detailed information of the intricate biomechanical coupling for many cellular responses. Thus, this method has the capability to provide direct quantitative experimental data showing how cells interact with their surroundings in three dimensions and might stimulate new avenues of scientific thought in understanding the fundamental role physical forces play in regulating cell behavior.
" }, { "name": "Lombardini, Manuel", "degree": "PhD", "year": "2008", "title": "Richtmyer-Meshkov Instability in Converging Geometries", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05302008-140331", "creators": [ { "name": { "family": "Lombardini", "given": "Manuel" }, "id": "Lombardini-Manuel", "display_name": "Lombardini, Manuel" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5SNE-4003", "abstract": "We investigate the Richtmyer-Meshkov instability (RMI) in converging geometries analytically and computationally. The linear, or small amplitude, regime is first covered as it is the onset to subsequent non-linear stages of the perturbation growth. While the plane interaction of a shock with a slightly perturbed density interface is classically viewed as a single interface evolving as baroclinic vorticity have been initially deposited on it, we propose a simple but more complete model characterizing the early interaction between the interface and the receding waves produced by the shock-interface interaction, in the case of a reflected shock. A universal time scale representing the time needed by the RMI to reach its asymptotic growth rate is found analytically and confirmed by ideal gas computations for various incident shock Mach numbers MI and Atwood ratios A, and could be useful especially for experimentalists in non-dimensionalizing their data.
\r\n\r\nConsidering again linear perturbations, we then obtain a general analytical model for the asymptotic growth rate reached by the instability during the concentric interaction of an imploding/exploding cylindrical shock with a cylindrical interface containing three-dimensional orthogonal perturbations, in the azimuthal and axial directions. Stable perturbations, typical of the converging geometry, are discovered. Comparisons are made with simulations where the effects of compressibility, wave reverberations, and flow convergence are isolated. Azimuthal and axial perturbation evolution are compared with results obtained for the plane RMI at comparable initial wavelengths.
\r\n\r\nA second interaction occurs when the transmitted shock, produced by the incident converging shock impacting the interface, converges to the axis and reflects to reshock the initially accelerated interface. This leads to highly non-linear perturbation growth. To isolate the complex wave interaction process, the interface is considered initially unperturbed so that the flow is radially symmetric. An accurate visualization procedure is performed to characterize the underlying physics behind the reshock event. We study extensively the cylindrical and spherical geometry, for various MI and for the air \u2192 SF6 (A=0.67) and SF6 \u2192 (A=-0.67) interactions, and draw important differences with the equivalent plane configuration.
\r\n\r\nA hybrid, low-numerical dissipation/shock-capturing method, embedded into an adaptive mesh refinement framework is optimized in order to achieve large-eddy simulations of the self-similar cylindrical converging shock-driven RMI and the turbulent mixing generated by the reshock. Computations are produced for MI=1.3 and 2.0, and for air -> SF6 SF6 -> air interfaces. We develop statistics tools to study extensively the growth of the turbulent mixing zone using cylindrical averages as well as various measures such as probability density functions of the mixing and turbulent power spectra, with the objectives of understanding the turbulent mixing in this particular geometry.
" }, { "name": "Matheou, Georgios", "degree": "PhD", "year": "2008", "title": "Large-Eddy Simulations of Molecular Mixing in a Recirculating Shear Flow", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05262008-152803", "creators": [ { "name": { "family": "Matheou", "given": "Georgios" }, "id": "Matheou-Georgios", "orcid": "0000-0003-4024-4571", "display_name": "Matheou, Georgios" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VFKF-SC30", "abstract": "The flow field and mixing in an expansion-ramp geometry is studied using large-eddy simulation (LES) with subgrid scale (SGS) modeling based on the stretched-vortex model. The expansionramp geometry was developed to provide enhanced mixing and flameholding characteristics while maintaining low total-pressure losses, elements that are important in the design and performance of combustors for hypersonic air-breathing propulsion applications. The mixing was studied by tracking a passive scalar without taking into account the effects of chemical reactions and heat release.
\r\n\r\nIn order to verify the solver and the boundary closure implementation, a method utilizing results from linear stability analysis (LSA) theory is developed. LSA can be used to compute unstable perturbations to a flow, subject to certain approximations. The perturbations computed from LSA are used as an inflow condition to the flow computed by the solver been assessed. A projection based metric is constructed that only assumes the shape of the solution and not the growth rate of the perturbations, thus also allowing the latter to be determined as part of the verification. The growth rate of the perturbations for an unbounded (effectively) incompressible shear layer and a confined compressible shear layer is found to be in agreement with the prediction of the LSA.
\r\n\r\nThe flow and mixing predictions of the LES are in good agreement with experimental measurements. Total (resolved and subgrid) probability density functions (PDFs) of the passive scalar are estimated using an assumed beta-distribution model for the subgrid scalar field. The improved mixing characteristics of the expansion-ramp geometry compared to free shear layers are illustrated by the shapes of the PDFs. Moreover, the temperature rise and the probability of mixed fluid profiles are in good agreement with the experimental measurements, indicating that the mixing on a molecular scale is correctly predicted by the LES\u2013SGS model. Finally, the predictions of the LES are shown to be resolution-independent. The mean fields and passive scalar PDFs have essentially converged at the two finer grid-resolutions used.
" }, { "name": "Taira, Kunihiko (Sam)", "degree": "PhD", "year": "2008", "title": "The Immersed Boundary Projection Method and Its Application to Simulation and Control of Flows Around Low-Aspect-Ratio Wings", "advisor": "Colonius, Tim", "url": "https://resolver.caltech.edu/CaltechETD:etd-05232008-124342", "creators": [ { "name": { "family": "Taira", "given": "Kunihiko (Sam)" }, "id": "Taira-Kunihiko-Sam", "orcid": "0000-0002-3762-8075", "display_name": "Taira, Kunihiko (Sam)" } ], "advisors": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "advisor", "display_name": "Colonius, Tim" } ], "committee": [ { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "chair", "display_name": "Colonius, Tim" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "member", "display_name": "Hunt, Melany L." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "mecheng" ], "doi": "10.7907/VSDD-P465", "abstract": "First, we present a new formulation of the immersed boundary method that is algebraically identical to the traditional fractional step algorithm. This method, called the immersed boundary projection method, allows for the simulations of incompressible flows over arbitrarily shaped bodies under motion and/or deformation in both two and three dimensions. The no-slip condition along the immersed boundary is enforced simultaneously with the incompressibility constraint through a single projection. The boundary force is determined implicitly without any constitutive relations for the rigid body formulation, which in turn allows the use of high CFL numbers in our simulations compared to past methods.
\r\n\r\nNext, the above immersed boundary projection method is used to analyze three-dimensional separated flows around low-aspect-ratio flat-plate wings. A number of simulations highlighting the unsteady nature of the separated flows are performed for Re = 300 and 500 with various aspect ratios, angles of attack, and planform geometries. The aspect ratio and angle of attack are found to have a large influence on the stability of the wake profile and the force experienced by the low-aspect-ratio wing. At early times, following an impulsive start, topologies of the wake vortices are found to be the same across different aspect ratios and angles of attack. Behind low-aspect-ratio rectangular plates, leading-edge vortices form and eventually separate as hairpin vortices following the start-up. This phenomenon is found to be similar to dynamic stall observed behind pitching plates. The detached structure would then interact with the tip vortices, reducing the downward velocity induced by the tip vortices acting upon the leading-edge vortex. At large time, depending on the aspect ratio and angles of attack, the wakes reach one of the three states: (i) a steady state, (ii) a periodic unsteady state, or (iii) an aperiodic unsteady state. We have observed that the tip effects in three-dimensional flows can stabilize the flow and also exhibit nonlinear interaction with the shedding vortices.
\r\n\r\nAt last, we apply steady blowing to separated flows behind the low-aspect-ratio rectangular wings. The objective of the flow control is to enhance lift at post-stall angles of attack by changing the dynamics of the wake vortices. This controller strengthens the tip vortices by engulfing the trailing-edge vortex sheet to increase the downward thrust and the downward induced velocity onto the leading-edge vortices. The tip vortices that are traditionally considered as an aerodynamic nuisance, have been used favorably to increase lift in post-stall flows for the considered low-aspect-ratio wings.
" }, { "name": "Graff, Emilio Casta\u00f1o", "degree": "PhD", "year": "2007", "title": "On the Development of Defocusing Digital Particle Image Velocimetry with Full Characterization", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-05252007-140239", "creators": [ { "name": { "family": "Graff", "given": "Emilio Casta\u00f1o" }, "id": "Graff-Emilio-Casta\u00f1o", "display_name": "Graff, Emilio Casta\u00f1o" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Yang", "given": "Changhuei" }, "id": "Yang-Changhuei", "orcid": "0000-0001-8791-0354", "role": "member", "display_name": "Yang, Changhuei" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z9HM56F8", "abstract": "Defocusing Digital Particle Image Velocimetry is the first volumetric, three-dimensional PIV method ever put into practice. This manuscript contains the details of its development, a detailed analysis of its performance (both through simulation and real measurements), and a series of experimental demonstrations of the capability of the technique. The system is capable of resolving upwards of 7,000 vectors per pair with an absolute error on the order of 0.03% of the volume size." }, { "name": "Kidd, Theresa Hiromi", "degree": "PhD", "year": "2007", "title": "Mechanical Characterization of Damage and Failure in Polymeric Foams and Glass/Epoxy Composites", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-11102006-182329", "creators": [ { "name": { "family": "Kidd", "given": "Theresa Hiromi" }, "id": "Kidd-Theresa-Hiromi", "display_name": "Kidd, Theresa Hiromi" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/G25Y-KE07", "abstract": "The mechanical characterization including evolution of damage and failure of foams and composites are becoming increasingly important, as they form the basic components of sandwich structures. Sandwich structures consist of two faceplates that surround a core material. In many modern applications, faceplates and cores are typically comprised of composite materials and polymeric foam, respectively. Knowledge of the failure behavior of these individual components is necessary for understanding the failure behavior and design of sandwich structures. A systematic investigation of the damage evolution and failure behavior of foams and composites was conducted using a variety of experimental techniques.
\r\n\r\nIn-situ ultrasonic measurements were used to track the damage behavior in PVC polymeric foams with densities ranging from 130 to 250 kg/m\u00b3. The wave speeds were measured by two quartz piezoelectric shear transducers with a resonant frequency of 5 MHz in the transmission mode. A fixture was developed and constructed to protect the transducers during compression, while allowing them to take sound speed measurements of the sample along the axis of the load train. This fixture was placed in a servo-hydraulic MTS (Materials Testing System) machine, where the load-displacement response of the foam was recorded. A digital image correlation (DIC) method was used to capture the progression of failure under compression. Two dominant failure modes, elastic buckling and plastic collapse, were identified \u2013 and their onsets corresponded to the change in elastic wave speeds in the material, measured by the in-situ ultrasonic technique.
\r\n\r\nThe transverse response of S-Glass/Epoxy unidirectional composites was investigated under varying degrees of confinement and strain rates. The experimental setup utilizes a fixture that allowed for independent measurement of the three principal stresses in a confined specimen. A servo-hydraulic materials testing system and a Kolsky (split Hopkinson) pressure bar generated strain rates between 10\u207b\u00b3 to 10\u2074 s\u207b\u00b9. Post-test scanning electron microscopy (SEM) observations suggest that under transverse loading at low-strain rates, confinement contributes to localized band formation. In addition, micrographs indicated that macroscopic transverse failure is dominated by shear stress, and occurs within these localized bands. These shear dominated failure bands were found inclined in a direction approximately 35\u00b0 to the direction of loading. Implications of this orientation deviation of failure bands from maximum shear trajectories at 45\u00b0 are discussed in reference to the state of confinement.
" }, { "name": "Klamo, Joseph Thomas", "degree": "PhD", "year": "2007", "title": "Effects of Damping and Reynolds Number on Vortex-Induced Vibrations", "advisor": "Leonard, Anthony; Gharib, Morteza; Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-11292006-120631", "creators": [ { "name": { "family": "Klamo", "given": "Joseph Thomas" }, "id": "Klamo-Joseph-Thomas", "orcid": "0000-0003-1458-0379", "display_name": "Klamo, Joseph Thomas" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "co-advisor", "display_name": "Gharib, Morteza" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "co-advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "member", "display_name": "Brennen, Christopher E." }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" } ], "option_major": [ "mecheng" ], "doi": "10.7907/S27Z-J533", "abstract": "Vortex-induced vibrations have been studied experimentally with emphasis on damping and Reynolds number effects. Our system was an elastically-mounted rigid circular cylinder, free to oscillate only transverse to the flow direction, with very low inherent damping. We were able to prescribe the mass, damping, and elasticity of the system over a wide range of values, with the damping controlled by a custom-made variable magnetic eddy-current damping system.
\r\n\r\nSpecial emphasis is put on a nontraditional parameter formulation. The advantages of this formulation are explained, and an important new parameter, effective stiffness, is introduced. Using this new formulation, the amplitude and frequency responses are only a function of damping, Reynolds number, and effective stiffness. We show the effects that damping and Reynolds number each have on the amplitude and frequency response profiles and make the interesting observation that changes in damping or Reynolds number have similar effects.
\r\n\r\nThe maximum amplitudes of our systems are studied in detail. We theoretically show that they should be functions of both damping and Reynolds number. This allows us to create constant-Reynolds-number curves of maximum amplitude over a large range of damping values, which we call a \"generalized\" Griffin plot. We also define maximum amplitudes in the case of zero damping as limiting amplitudes, and show that they are only a function of Reynolds number. We experimentally determine our limiting amplitude dependence on Reynolds number over the range 200 < Reynolds number < 5050.
\r\n\r\nDiscontinuities in the amplitude response profile are also investigated. The discontinuity between the initial branch and the large-amplitude, upper branch is studied in two ways. First, the time-averaged behavior is examined to understand what controls the discontinuity and look for damping and Reynolds number effects. Second, we track the cycle-by-cycle transient response through this discontinuous amplitude change, induced either by changes in the tunnel velocity or system damping. Finally, we also find a new discontinuity hysteresis region between the lower branch and the desynchronized region, which appears to be a low Reynolds number effect and is only seen in systems with Reynolds number < 1000.
" }, { "name": "Latini, Marco", "degree": "PhD", "year": "2007", "title": "Simulations and Analysis of Two- and Three-Dimensional Single-Mode Richtmyer-Meshkov Instability using Weighted Essentially Non-Oscillatory and Vortex Methods", "advisor": "Meiron, Daniel I.; Schilling, Oleg", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082006-124547", "creators": [ { "name": { "family": "Latini", "given": "Marco" }, "id": "Latini-Marco", "display_name": "Latini, Marco" } ], "advisors": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "advisor", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Schilling", "given": "Oleg" }, "id": "Schilling-O", "role": "advisor", "display_name": "Schilling, Oleg" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Pierce", "given": "Niles A." }, "id": "Pierce-N-A", "role": "member", "display_name": "Pierce, Niles A." }, { "name": { "family": "Schilling", "given": "Oleg" }, "id": "Schilling-O", "role": "member", "display_name": "Schilling, Oleg" }, { "name": { "family": "Hou", "given": "Thomas Y." }, "id": "Hou-T-Y", "role": "member", "display_name": "Hou, Thomas Y." } ], "option_major": [ "appliedmath" ], "doi": "10.7907/1397-GZ04", "abstract": "An incompressible vorticity-streamfunction (VS) method is developed to investigate the single-mode Richtmyer-Meshkov instability in two and three dimensions. The initial vortex sheet (representing the initial shocked interface) is thickened to regularize the limit of classical Lagrangian vortex methods. In the limit of smaller thickness, the initial velocity converges to the velocity of a vortex sheet. The vorticity on the Cartesian grid follows the vorticity evolution equation augmented by the baroclinic vorticity production term (to capture the effects of the instability on the layer) and a viscous dissipation term. The equations are discretized using a fourth-order in space and third-order in time semi-implicit Adams-Bashforth backward differentiation scheme. The convergence properties of the method with respect to varying the diffuse interface thickness and viscosity are investigated. It is shown that the small-scale structures within the roll-up are more sensitive to the diffuse interface thickness than to the viscosity. By contrast, the large-scale quantities, including the perturbation, bubble, and spike amplitudes are less sensitive. Fourth-order point-wise convergence is achieved, provided that a sufficiently fine grid is used.
\r\n\r\nIn two dimensions, the VS method is applied to investigate late-time nonlinear effects of the single-mode Mach 1.3 air(acetone)/SF_6 shock tube experiment of Jacobs and Krivets. The results are also compared to those from compressible ninth-order weighted essentially non-oscillatory (WENO) simulations. The density fields from the WENO and VS methods agree with the experimental PLIF images in the large-scale structures but differ in the small-scale structures. The WENO method exhibits small-scale disordered structure similar to that in the experiment, while the VS method does not capture such structure, but shows a strong rotating core. The perturbation amplitudes from the two methods are in good agreement and match the experimental data points well. The WENO bubble amplitude is smaller than the VS amplitude and vice versa for the spike amplitude. Comparing amplitudes from simulations with varying Mach number shows that as the Mach number increases, the differences in the bubble and spike amplitudes increase due to intensifying pressure perturbations not present in the incompressible VS method. The perturbation amplitude from the WENO and VS methods is also compared to the predictions of nonlinear amplitude growth models in which the growth rate was reduced to account for the diffuse initial interface. In general, the model predictions agree with the simulation amplitudes at early-to-intermediate times and underpredict at later times, corresponding to the late nonlinear regime.
\r\n\r\nThe WENO simulation is used to investigate reshock, which occurs when the transmitted shock reflects from the end wall of the test section and interacts with the evolving layer. The post-reshock mixing layer width agrees well with the predictions of reshock models for short times until the interaction of the reflected rarefaction with the layer.
\r\n\r\nThe VS simulation was also compared to classical Lagrangian and vortex-in-cell simulations as the Atwood number was varied. For low Atwood numbers, all three simulations agree. As the Atwood number increases, the VS simulation shows differences in the bubble and spike amplitudes compared to the Lagrangian and VIC simulations, as the baroclinic vorticity production for a diffuse layer is different from that of a thin layer. The simulation amplitudes agree with the predictions of nonlinear amplitude growth models at early times. The growth models underpredict the amplitudes at later times.
\r\n\r\nThe investigation is extended to three dimensions, where the initial perturbation is a product of sinusoids and the initial vorticity deposition is given by linear instability analysis. The instability evolution and dynamics of vorticity are visualized using the mass fraction and enstrophy isosurface, respectively. For the WENO and VS methods, two roll-ups corresponding to the bubble and spike regions form, and the vorticity shows the formation of a ring-like structure. The perturbation amplitudes from the WENO and VS methods are in excellent agreement. The bubble and spike amplitude are in good agreement at early times. At later times, the WENO bubble amplitude is smaller than the VS amplitude and vice versa for the spike. The nonlinear three-dimensional Zhang-Sohn model agrees with the simulation amplitudes at early times, and underpredicts later. In three dimensions, the enstrophy iso-surface after reshock shows significant fragmentation and the formation of small, short, tubular structures. Simulations with different initial amplitudes show that the mixing layer width after reshock does not depend on the pre-shock amplitude. Finally, the effects of Atwood number are investigated using the VS method and the amplitudes are compared to the predictions of the Zhang-Sohn model. The simulation and the models are in agreement at early times, while the models underpredict later.
\r\n\r\nThe VS method constitutes a useful numerical approach to investigate the Richtmyer-Meshkov instability in two and three dimensions. The VS method and, more generally, vortex methods are valid tools for predicting the large-scale instability features, including the perturbation amplitudes, into the late nonlinear regime.
" }, { "name": "Loumes, Laurence", "degree": "PhD", "year": "2007", "title": "Multilayer Impedance Pump: A Bio-Inspired Valveless Pump with Medical Applications", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-01082007-103832", "creators": [ { "name": { "family": "Loumes", "given": "Laurence" }, "id": "Loumes-Laurence", "display_name": "Loumes, Laurence" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Daraio", "given": "Chiara" }, "id": "Daraio-C", "orcid": "0000-0001-5296-4440", "role": "member", "display_name": "Daraio, Chiara" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Dabiri", "given": "John O." }, "id": "Dabiri-J-O", "orcid": "0000-0002-6722-9008", "role": "member", "display_name": "Dabiri, John O." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MT2J-AR81", "abstract": "This thesis introduces the concept of multilayer impedance pump, a novel pumping mechanism inspired from the embryonic heart structure.
\r\n\r\nThe multilayer impedance pump is a composite two-layer fluid-filled elastic tube featuring a thick, gelatin-like internal layer similar in nature to the embryonic cardiac jelly, and that is used to amplify longitudinal elastic waves. Pumping is based on the impedance pumping mechanism. Elastic waves are generated upon small external periodic compressions of the elastic tube. They propagate along the tube\u2019s walls, reflect at the tube\u2019s extremities and drive the flow in a preferential direction. This fully coupled fluid-structure interaction problem is solved for the flow and the structure using the finite element method over a relevant range of frequencies of excitation. Results show that the two-layer configuration can be an efficient wave propagation combination, and that it allows the pump to produce significant flow for small excitations. The multilayer impedance pump is a complex system in which flow and structure exhibit a resonant behavior. At resonance, a constructive elastic wave interaction coupled with a most efficient energy transmission between the elastic walls and the fluid is responsible for the maximum exit flow. The pump efficiency reaches its highest at resonance, highlighting furthermore the concept of resonance pumping.
\r\n\r\nUsing the proposed multilayer impedance pump model, we are able to bring an additional proof on the impedance nature of the embryonic heart by comparing a peristaltic and an impedance multilayer pump both excited in similar fashion to the one observed in the embryonic heart.
\r\n\r\nThe gelatin layer that models the embryonic cardiac jelly occupies most of the tube walls and is essential to the propagation of elastic waves. A comparison between the exact same impedance pump with and without the additional gelatin layer sheds light on the dynamic role of the cardiac jelly in the embryonic heart and on nature's optimized design.
\r\n\r\nFinally, several biomedical applications of multilayer impedance pumping are presented. A physiologically correct model of aorta is proposed to test the pump as an implantable cardiovascular assist device.
\r\n" }, { "name": "Morris, Bradford S.", "degree": "PhD", "year": "2007", "title": "Charge-Exchange Collision Dynamics and Ion Engine Grid Geometry Optimization", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02282007-154751", "creators": [ { "name": { "family": "Morris", "given": "Bradford S." }, "id": "Morris-Bradford-S", "display_name": "Morris, Bradford S." } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Johnson", "given": "Lee K." }, "id": "Johnson-L-K", "role": "member", "display_name": "Johnson, Lee K." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/W996-M220", "abstract": "The development of a new three-dimensional model for determining the absolute energy distribution of ions at points corresponding to spacecraft surfaces to the side of an ion engine is presented. The ions resulting from elastic collisions, both charge-exchange (CEX) and direct, between energetic primary ions and thermal neutral xenon atoms are accounted for. Highly resolved energy distributions of CEX ions are found by integration over contributions from all points in space within the main beam formed by the primary ions.
\r\n\r\nThe sputtering rate due to impingement of these ions on a surface is calculated. The CEX ions that obtain significant energy (10 eV or more) in the collision are responsible for the majority of the sputtering, though this can depend on the specific material being sputtered. In the case of a molybdenum surface located 60 cm to the side of a 30 cm diameter grid, nearly 90% of the sputtering is due to the 5% of ions with the highest collision exit energies. Previous models that do not model collision energetics cannot predict this. The present results agree with other models and predict that the majority of the ion density is due to collisions where little to no energy is transferred.
\r\n\r\nThe sputtering model is combined with a grid-structure model in an optimization procedure where the sputtering rate at specified locations is minimized by adjustment of parameters defining the physical shape of the engine grids. Constraints are imposed that require that the deflection of the grid under a specified load does not exceed a maximum value, in order to ensure survivability of the grids during launch. To faciliate faster execution of the calculations, simplifications based on the predicted behavior of the CEX ions are implemented. For diametrically opposed sputtering locations, a rounded barrel-vault shape reduces the expected sputtering rate by up to 30% in comparison to an NSTAR-shaped grid.
" }, { "name": "Mouton, Christopher Andre", "degree": "PhD", "year": "2007", "title": "Transition Between Regular Reflection and Mach Reflection in the Dual-Solution Domain", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01052007-125557", "creators": [ { "name": { "family": "Mouton", "given": "Christopher Andre" }, "id": "Mouton-Christopher-Andre", "display_name": "Mouton, Christopher Andre" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "McKeon", "given": "Beverley J." }, "id": "McKeon-B-J", "role": "member", "display_name": "McKeon, Beverley J." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TEA0-Q468", "abstract": "A study of the shock-reflection domain for steady flow is presented. Conditions defining boundaries between different possible shock-reflection solutions are given, and where possible, simple analytic expressions for these conditions are presented. A new, more accurate estimate of the steady-state Mach stem height is derived based on geometric considerations of the flow. In particular, the location of the sonic throat through which the subsonic convergent flow behind the Mach stem is accelerated to divergent supersonic flow is considered. Comparisons with previous computational and experimental work show that the theory presented in this thesis more accurately predicts the Mach stem height than previous theories. The Mach stem height theory is generalized to allow for a moving triple point. Based on this moving triple point theory, a Mach stem growth rate theory is developed. This theory agrees well with computational and experimental results. Numerical computations of the effects of water vapor disturbances are also presented. These disturbances are shown to be sufficient to cause transition from regular reflection to Mach reflection in the dual-solution domain. These disturbances are also modeled as a simple energy deposition on one of the wedges, and an estimate for the minimum energy required to cause transition is derived.
\r\n\r\nExperimental results using an asymmetric wedge configuration in the Ludwieg tube facility at the California institute of Technology are presented. A Mach 4.0 nozzle was designed and built for the Ludwieg tube facility. This Mach number is sufficient to provide a large dual-solution domain, while being small enough not to require preheating of the test gas. The test time of the facility is 100ms, which requires the use of high-speed cinematography and a fast motor to rotate one of the two wedges. Hysteresis in the transition between regular to Mach reflection was successfully demonstrated in the Ludwieg tube facility. The experiments show that regular reflection could be maintained up to a shock angle approximately halfway between the von Neumann condition and the detachment condition.
\r\n\r\nEnergy deposition studies were performed using an Nd:YAG laser. Triggering transition in this manner is found to depend on the location of the energy deposition. This finding is consistent with the numerical work presented in this thesis. Experiments were also performed to measure the Mach stem height and its growth rate. These results are compared with the theoretical estimates presented in this thesis. Excellent agreement between the steady-state Mach stem height and the theoretical estimates is seen. Comparisons of Mach stem growth rate with theoretical estimates show significant differences, but do show good agreement regarding the time required to reach the steady-state height.
" }, { "name": "Rubel, Michael Thomas", "degree": "PhD", "year": "2007", "title": "A Theory of Stationarity and Asymptotic Approach in Dissipative Systems", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-01122007-114557", "creators": [ { "name": { "family": "Rubel", "given": "Michael Thomas" }, "id": "Rubel-Michael-Thomas", "display_name": "Rubel, Michael Thomas" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "member", "display_name": "Marsden, Jerrold E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VWDE-GB16", "abstract": "The approximate dynamics of many physical phenomena, including turbulence, can be represented by dissipative systems of ordinary differential equations. One often turns to numerical integration to solve them. There is an incompatibility, however, between the answers it can produce (i.e., specific solution trajectories) and the questions one might wish to ask (e.g., what behavior would be typical in the laboratory?) To determine its outcome, numerical integration requires more detailed initial conditions than a laboratory could normally provide. In place of initial conditions, experiments stipulate how tests should be carried out: only under statistically stationary conditions, for example, or only during asymptotic approach to a final state. Stipulations such as these, rather than initial conditions, are what determine outcomes in the laboratory.
\r\n\r\nThis theoretical study examines whether the points of view can be reconciled: What is the relationship between one's statistical stipulations for how an experiment should be carried out--stationarity or asymptotic approach--and the expected results? How might those results be determined without invoking initial conditions explicitly?
\r\n\r\nTo answer these questions, stationarity and asymptotic approach conditions are analyzed in detail. Each condition is treated as a statistical constraint on the system--a restriction on the probability density of states that might be occupied when measurements take place. For stationarity, this reasoning leads to a singular, invariant probability density which is already familiar from dynamical systems theory. For asymptotic approach, it leads to a new, more regular probability density field. A conjecture regarding what appears to be a limit relationship between the two densities is presented.
\r\n\r\nBy making use of the new probability densities, one can derive output statistics directly, avoiding the need to create or manipulate initial data, and thereby avoiding the conceptual incompatibility mentioned above. This approach also provides a clean way to derive reduced-order models, complete with local and global error estimates, as well as a way to compare existing reduced-order models objectively.
\r\n\r\nThe new approach is explored in the context of five separate test problems: a trivial one-dimensional linear system, a damped unforced linear oscillator in two dimensions, the isothermal Rayleigh-Plesset equation, Lorenz's equations, and the Stokes limit of Burgers' equation in one space dimension. In each case, various output statistics are deduced without recourse to initial conditions. Further, reduced-order models are constructed for asymptotic approach of the damped unforced linear oscillator, the isothermal Rayleigh-Plesset system, and Lorenz's equations, and for stationarity of Lorenz's equations.
" }, { "name": "Sone, Kazuo", "degree": "PhD", "year": "2007", "title": "Modeling and Simulation of Axisymmetric Stagnation Flames", "advisor": "Dimotakis, Paul E.; Meiron, Daniel I.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04252007-170838", "creators": [ { "name": { "family": "Sone", "given": "Kazuo" }, "id": "Sone-Kazuo", "display_name": "Sone, Kazuo" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "advisor", "display_name": "Meiron, Daniel I." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "co-chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DYGA-YJ20", "abstract": "Laminar flame modeling is an important element in turbulent combustion research. The accuracy of a turbulent combustion model is highly dependent upon our understanding of laminar flames and their behavior in many situations. How much we understand combustion can only be measured by how well the model describes and predicts combustion phenomena. One of the most commonly used methane combustion models is GRI-Mech 3.0. However, how well the model describes the reacting flow phenomena is still uncertain even after many attempts to validate the model or quantify uncertainties.
\r\n\r\nIn the present study, the behavior of laminar flames under different aerodynamic and thermodynamic conditions is studied numerically in a stagnation-flow configuration. In order to make such a numerical study possible, the spectral element method is reformulated to accommodate the large density variations in methane reacting flows. In addition, a new axisymmetric basis function set for the spectral element method that satisfies the correct behavior near the axis is developed, and efficient integration techniques are developed to accurately model axisymmetric reacting flow within a reasonable amount of computational time. The numerical method is implemented using an object-oriented programming technique, and the resulting computer program is verified with several different verification methods.
\r\n\r\nThe present study then shows variances with the commonly used GRI-Mech 3.0 chemical kinetics model through a direct simulation of laboratory flames that allows direct comparison to experimental data. It is shown that the methane combustion model based on GRI-Mech 3.0 works well for methane-air mixtures near stoichiometry. However, GRI-Mech 3.0 leads to an overprediction of laminar flame speed for lean mixtures and an underprediction for rich mixtures. This result is slightly different from conclusion drawn in previous work, in which experimental data are compared with a one-dimensional numerical solutions. Detailed analysis reveals that flame speed is sensitive to even slight flame front curvature as well as its finite extension in the radial direction. Neither of these can be incorporated in one-dimensional flow modeling.
" }, { "name": "Laurence, Stuart Jon", "degree": "PhD", "year": "2006", "title": "Proximal Bodies in Hypersonic Flow", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04242006-172719", "creators": [ { "name": { "family": "Laurence", "given": "Stuart Jon" }, "id": "Laurence-Stuart-Jon", "orcid": "0000-0001-8760-8366", "display_name": "Laurence, Stuart Jon" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Stevenson", "given": "David John" }, "id": "Stevenson-D-J", "orcid": "0000-0001-9432-7159", "role": "member", "display_name": "Stevenson, David John" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VZJV-KJ48", "abstract": "The problem of proximal bodies in hypersonic flow is encountered in several important situations, both natural and man-made. The present work seeks to investigate one aspect of this problem by exploring the forces experienced by a secondary body when some part of it is within the shocked region created by a primary body travelling at hypersonic speeds.
\r\n\r\nAn analytical methodology based on the blast wave analogy is developed and used to predict the secondary force coefficients for simple geometries in both two and three dimensions. When the secondary body is entirely inside the primary shocked region, the nature of the lateral coefficient is found to depend strongly on the relative size of the two bodies. For two spheres, the methodology predicts that the secondary body will experience an exclusively attractive lateral force if the secondary diameter is larger then one-sixth the primary diameter. The analytical results are compared with numerical simulations carried out using the AMROC software and good agreement is obtained if an appropriate normalization for the lateral displacement is used.
\r\n\r\nResults from a series of experiments in the T5 hypervelocity shock tunnel are also presented and compared with perfect-gas numerical simulations, again with good agreement. In order to model this situation experimentally, a new force-measurement technique for short-duration hypersonic facilities has been developed, and results from the validation experiments are included.
\r\n\r\nFinally, the analytical methodology is used to model two physical situations. First, the entry of a binary asteroid system into the Earth's atmosphere is simulated. Second, a model for a fragmenting meteoroid in a planetary atmosphere is developed, and simulations are carried out to determine whether the secondary scatter patterns in the Sikhote-Alin crater field may be attributed to aerodynamic interactions between fragments rather than to secondary fragmentation. It is found that while aerodynamic interactions lead to increased secondary crater grouping, these groups do not exhibit the typically elliptical shape that we would expect secondary fragmentation to produce.
" }, { "name": "Lieberman, Daniel Howard", "degree": "PhD", "year": "2006", "title": "Detonation Interaction with Sharp and Diffuse Interfaces", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11172005-092205", "creators": [ { "name": { "family": "Lieberman", "given": "Daniel Howard" }, "id": "Lieberman-Daniel-Howard", "display_name": "Lieberman, Daniel Howard" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "role": "member", "display_name": "Bruno, Oscar P." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9JZE-X524", "abstract": "Detonation interaction with an interface was investigated, where the interface separated a combustible from an oxidizing mixture. The ethylene-oxygen combustible mixture had a fuel-rich composition to promote secondary combustion with the oxidizer in the turbulent mixing zone that resulted from the interaction. Both sharp and diffuse interfaces were studied.
\r\n\r\nDiffuse interfaces were created by the formation of a gravity current using a sliding valve that initially separated the test gas and combustible mixture. Opening the valve allowed a gravity current to develop before the detonation was initiated. By varying the delay between opening the valve and initiating the detonation it was possible to achieve a wide range of interface conditions. Sharp interfaces were created by using a nitro-cellulose membrane to separate the two mixtures. The membrane was destroyed by the detonation wave.
\r\n\r\nThe interface orientation and thickness with respect to the detonation wave have a profound effect on the outcome of the interaction. Diffuse interfaces result in curved detonation waves with a transmitted shock and following turbulent mixing zone. Sharp interfaces result in an interaction occurring at a node point similar to regular shock refraction (Henderson, 1989). The impulse was measured to quantify the degree of secondary combustion accounting for 5-6% of the total impulse. A model was developed that estimated the volume expansion of a fluid element due to combustion in the turbulent mixing zone (Dimotakis, 1991) to predict the impulse in the limit of infinite Damkohler number.
" }, { "name": "Parkin, Kevin L.G.", "degree": "PhD", "year": "2006", "title": "The Microwave Thermal Thruster and Its Application to the Launch Problem", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06022006-160023", "creators": [ { "name": { "family": "Parkin", "given": "Kevin L.G." }, "id": "Parkin-Kevin-L-G", "orcid": "0000-0003-4521-8559", "display_name": "Parkin, Kevin L.G." } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Barmatz", "given": "Martin B." }, "id": "Barmatz-M-B", "role": "member", "display_name": "Barmatz, Martin B." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "role": "member", "display_name": "Hunt, Melany L." }, { "name": { "family": "Worden", "given": "Simon P." }, "id": "Worden-S-P", "role": "member", "display_name": "Worden, Simon P." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T337-T709", "abstract": "Nuclear thermal thrusters long ago bypassed the 50-year-old specific impulse (Isp) limitation of conventional thrusters, using nuclear powered heat exchangers in place of conventional combustion to heat a hydrogen propellant. These heat exchanger thrusters experimentally achieved an Isp of 825 seconds, but with a thrust-to-weight ratio (T/W) of less than ten they have thus far been too heavy to propel rockets into orbit.
\r\n\r\nThis thesis proposes a new idea to achieve both high Isp and high T/W: The Microwave Thermal Thruster. This thruster covers the underside of a rocket aeroshell with a lightweight microwave absorbent heat exchange layer that may double as a re-entry heat shield. By illuminating the layer with microwaves directed from a ground-based phased array, an Isp of 700\u2013900 seconds and T/W of 50\u2013150 is possible using a hydrogen propellant. The single propellant simplifies vehicle design, and the high Isp increases payload fraction and structural margins. These factors combined could have a profound effect on the economics of building and reusing rockets.
\r\n\r\nA laboratory-scale microwave thermal heat exchanger is constructed using a single channel in a cylindrical microwave resonant cavity, and new type of coupled electromagnetic-conduction-convection model is developed to simulate it. The resonant cavity approach to small-scale testing reveals several drawbacks, including an unexpected oscillatory behavior. Stable operation of the laboratory-scale thruster is nevertheless successful, and the simulations are consistent with the experimental results.
\r\n\r\nIn addition to proposing a new type of propulsion and demonstrating it, this thesis provides three other principal contributions: The first is a new perspective on the launch problem, placing it in a wider economic context. The second is a new type of ascent trajectory that significantly reduces the diameter, and hence cost, of the ground-based phased array. The third is an eclectic collection of data, techniques, and ideas that constitute a Microwave Thermal Rocket as it is presently conceived, in turn selecting and motivating the particular experimental and computational analyses undertaken.
" }, { "name": "Zhang, Rongjing", "degree": "PhD", "year": "2006", "title": "Mechanical Characterization of Thin Films with Application to Ferroelectrics", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-01312006-170959", "creators": [ { "name": { "family": "Zhang", "given": "Rongjing" }, "id": "Zhang-Rongjing", "display_name": "Zhang, Rongjing" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Molinari", "given": "Alain" }, "id": "Molinari-A", "role": "member", "display_name": "Molinari, Alain" }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Haile", "given": "Sossina M." }, "id": "Haile-S-M", "role": "member", "display_name": "Haile, Sossina M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CJR5-DK94", "abstract": "One important part of the motivation for this research work comes from the microelectromechanical systems (MEMS) technology. Its basic concept of high volume production and low unit cost can only be achieved when the devices made by microelectronics technique are reliable. The success in this area largely depends on the understanding of materials. However, the mechanical characterization is lagged behind the theoretical work and designing software development. The standard characterization method is still not established. For MEMS actuators, especially for active materials, the desired characterization system for obtaining mechanical properties requires load control feature and the capability of doing dynamic tests. However, there is no such method among the currently available tools for mechanical characterization.
\r\n\r\nThe other part of the motivation comes from the comprehensive research work of Caltech ferroelectric group. This group, which consists of nine faculty members, is aiming to develop new devices, especially new actuators, by the aid of multi-scale theory tools and selected experimental methods. The work presented in this dissertation is an important and key step of this ambitious project: the electromechanical characterization of devices. This will provide validation for the multi-scale materials modeling framework and help to increase the reliability of the actuators and devices.
\r\n\r\nIn this work, two techniques were developed for mechanical characterization, which satisfy the challenging requirements for thin film structures and devices: being able to do dynamic study on fragile ceramic thin film samples with load control feature. The first technique is a new method to characterize mechanical properties of released thin films under concentrated load. This technique can be used to apply load in the ?N?mN range with displacement measured with high accuracy of 0.1 ?m. The successful characterization of Si3N4 free-standing membranes demonstrated the capability and reliability of this new technique. The elastic modulus and residual stress of Si3N4 free-standing thin film were measured to be around 250 GPa and 450 MPa, respectively. These values were in close agreement with values obtained using a different technique as well as those found in the literature. This technique has the potential application on elastic-plastic characterization and characterization of other functional thin film materials such as shape memory alloys.
\r\n\r\nPressure bulge test technique, which is another type of load control method suitable for dynamic test, was also developed. The apparatus was designed to be compact to fit into the x-ray diffractometer for in-situ XRD study and had additional compatibility for polarized light microscopy study. Characterization of free standing thin film of single layer amorphous silicon nitride (Si3N4) and multi-layered PBT/Si3N4, and thick film of single crystal barium titanate (BaTiO3) showed the capability and reliability of this technique. Excellent agreement of the Si3N4 Young\u2019s modulus between these two developed methods gave the confidence for using these techniques to understand new materials.
\r\n\r\nIn situ x-ray diffraction study was carried out on the single crystal thick films which were loaded with distributed mechanical loading by pressure bulge setup. Direct evidence of 90o domain switching was obtained from the in situ XRD results with the intensity changing in both (002) and (200) orientations. Obvious changes in domain patterns were observed by using the polarized light microscope. The Young\u2019s modulus of this barium titanate single crystal thick film with thickness of 100 ?m was characterized before the XRD exam. Using this information, in-plane stress can be analyzed, and the relation between the driving force (the stress) and the microstructural change (volume fraction change in a-domain or c-domain) can be determined.
" }, { "name": "Zielonka, Matias Gabriel", "degree": "PhD", "year": "2006", "title": "Configurational Forces and Variational Mesh Adaption in Solid Dynamics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05112006-162905", "creators": [ { "name": { "family": "Zielonka", "given": "Matias Gabriel" }, "id": "Zielonka-Matias-Gabriel", "display_name": "Zielonka, Matias Gabriel" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "member", "display_name": "Marsden, Jerrold E." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/V6RB-FR94", "abstract": "This thesis is concerned with the exploration and development of a variational finite element mesh adaption framework for non-linear solid dynamics and its conceptual links with the theory of dynamic configurational forces. The distinctive attribute of this methodology is that the underlying variational principle of the problem under study is used to supply both the discretized fields and the mesh on which the discretization is supported. To this end a mixed-multifield version of Hamilton's principle of stationary action and Lagrange-d'Alembert principle is proposed, a fresh perspective on the theory of dynamic configurational forces is presented, and a unifying variational formulation that generalizes the framework to systems with general dissipative behavior is developed. A mixed finite element formulation with independent spatial interpolations for deformations and velocities and a mixed variational integrator with independent time interpolations for the resulting nodal parameters is constructed. This discretization is supported on a continuously deforming mesh that is not prescribed at the outset but computed as part of the solution. The resulting space-time discretization satisfies exact discrete configurational force balance and exhibits excellent long term global energy stability behavior. The robustness of the mesh adaption framework is assessed and demonstrated with a set of examples and convergence tests." }, { "name": "Bergthorson, Jeffrey Myles", "degree": "PhD", "year": "2005", "title": "Experiments and Modeling of Impinging Jets and Premixed Hydrocarbon Stagnation Flames", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05242005-165713", "creators": [ { "name": { "family": "Bergthorson", "given": "Jeffrey Myles" }, "id": "Bergthorson-Jeffrey-Myles", "orcid": "0000-0003-2924-7317", "display_name": "Bergthorson, Jeffrey Myles" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Kuppermann", "given": "Aron" }, "id": "Kuppermann-A", "role": "member", "display_name": "Kuppermann, Aron" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7FQZ-EY88", "abstract": "To model the combustion of long-chain hydrocarbon fuels, an accurate kinetics mechanism must first be developed for the oxidation of small hydrocarbons, such as methane, ethane, and ethylene. Even for methane, a generally accepted mechanism is still elusive due to a lack of kinetically independent experimental data. In this work, a combined experimental and modeling technique is developed to validate and further optimize these mechanisms. This technique relies on detailed measurements of strained flames in a jet-wall stagnation flow using simultaneous Particle Streak Velocimetry (PSV) and CH Planar Laser Induced Fluorescence (PLIF). Stagnation flames are simulated using an axisymmetric, one-dimensional model with accurate specification of the requisite boundary conditions. Direct comparisons between experiment and simulation allow for an assessment of the various models employed, with an emphasis on the chemistry model performance.
\r\n\r\nThe flow field for a cold impinging laminar jet is found to be independent of the nozzle-to-plate separation distance if velocities are scaled by the Bernoulli velocity. The one-dimensional formulation is found to accurately model the stagnation flow if the velocity boundary conditions are appropriately specified. The boundary-layer-displacement-thickness corrected diameter is found to be an appropriate scale for axial distances and allows the identification of an empirical, analytical expression for the flow field of the impinging laminar jet.
\r\n\r\nStrained methane-air flame experiments confirm that the reacting flow is also independent of the nozzle-to-plate separation distance. Methane, ethane, and ethylene flames are studied as functions of the applied strain rate, mixture dilution, and mixture fraction. Mechanism performance is found to be relatively insensitive to both the mixture dilution and the imposed strain rate, while exhibiting a stronger dependence on the fuel type and flame stoichiometry. The approach and diagnostics presented here permit an assessment of the predictions of strained-hydrocarbon flames for several combustion chemistry mechanisms. The data presented in this thesis are made available to kineticists looking for optimization targets, with the goal of developing a predictive kinetics model for hydrocarbon fuels. The methodology described here can allow new optimization targets to be rapidly measured, reducing the experimental burden required to fully constrain the chemistry models.
" }, { "name": "Chatelain, Philippe", "degree": "PhD", "year": "2005", "title": "Contributions to the Three-Dimensional Vortex Element Method and Spinning Bluff Body Flows", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-02012005-061553", "creators": [ { "name": { "family": "Chatelain", "given": "Philippe" }, "id": "Chatelain-Philippe", "orcid": "0000-0001-9891-5265", "display_name": "Chatelain, Philippe" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YEDS-0V89", "abstract": "Several contributions to the three-dimensional vortex element method for incompressible flows are presented. We introduce redistribution schemes based on the hexagonal lattice in two dimensions, and the face-centered cubic lattice in three dimensions. Interpolation properties are studied in the frequency domain and are used to build high-order schemes that are more compact and isotropic than equivalent cubic schemes. We investigate the reconnection of vortex rings at small Reynolds numbers for a variety of configurations. In particular, we trace their dissipative nature to the formation of secondary structures.
\r\n\r\nA method for flows with moving boundaries is implemented. The contributions of rotating or deforming boundaries to the Biot-Savart law are derived in terms of surface integrals. They are implemented for rigid boundaries in a fast multipole algorithm. Near-wall vorticity is discretized with attached panels. The shape function and Biot-Savart contributions of these elements account for the presence of the boundary and its curvature. A conservative strength exchange scheme was designed to compute the viscous flux from these panels to free elements.
\r\n\r\nThe flow past a spinning sphere is studied for a Reynolds number of 300 and a wall velocity that is equal to half the free-stream velocity. Three directions of the angular velocity are considered. Good agreement with previous numerical and experimental measurements of the force coefficients is observed. Topological features such as the separation and critical points are investigated and compared amongst the configurations.
\r\n\r\nFinally, preliminary results for flapping motions are presented. Simple rigid geometries are used to model a fish swimming in a free-stream and a flapping plate.
" }, { "name": "Dabiri, John Oluseun", "degree": "PhD", "year": "2005", "title": "Unsteady Fluid Mechanics of Starting-Flow Vortex Generators with Time-Dependent Boundary Conditions", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-04112005-151435", "creators": [ { "name": { "family": "Dabiri", "given": "John Oluseun" }, "id": "Dabiri-John-Oluseun", "orcid": "0000-0002-6722-9008", "display_name": "Dabiri, John Oluseun" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Brady", "given": "John F." }, "id": "Brady-J-F", "orcid": "0000-0001-5817-9128", "role": "member", "display_name": "Brady, John F." }, { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "member", "display_name": "Dickinson, Michael H." } ], "option_major": [ "bioeng" ], "doi": "10.7907/QV8Y-YZ12", "abstract": "Nature has repeatedly converged on the use of starting flows for mass, momentum, and energy transport. The vortex loops that form during flow initiation have been reproduced in the laboratory and have been shown to make a proportionally larger contribution to fluid transport than an equivalent steady jet. However, physical processes limit growth of the vortex loops, suggesting that these flows may be amenable to optimization. Although it has been speculated that optimal vortex formation might occur naturally in biological systems, previous efforts to quantify the biological mechanisms of vortex formation have been inconclusive. In addition, the unsteady fluid dynamical effects associated with starting flow vortex generators are poorly understood.
\r\n\r\nThis thesis describes a combination of new experimental techniques and in vivo animal measurements that determine the effects of fluid-structure interactions on vortex formation by starting flow propulsors. Results indicate that vortex formation across various biological systems is manipulated by these kinematics in order to maximize thrust and/or propulsive efficiency. An emphasis on observed vortex dynamics and transient boundary conditions facilitates quantitative comparisons across fluid transport schemes, irrespective of their individual biological functions and physical scales.
\r\n\r\nThe primary contributions of this thesis are the achievement of quantitative measures of unsteady vortex dynamics via fluid entrainment and added-mass effects, and the development of a robust framework to facilitate the discovery of general design principles for effective fluid transport in engineering technologies and biological therapies. The utility of this new research paradigm is demonstrated through a variety of examples.
" }, { "name": "Dooley, Bradley Scott", "degree": "PhD", "year": "2005", "title": "Stereo Digital Particle Image Velocimetry Investigation of a Free Surface Mixing Layer", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-06022005-180557", "creators": [ { "name": { "family": "Dooley", "given": "Bradley Scott" }, "id": "Dooley-Bradley-Scott", "display_name": "Dooley, Bradley Scott" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EH41-N436", "abstract": "Shear flows in the vicinity of a free surface are a problem with numerous applications, perhaps the most obvious being the wakes of seagoing surface vessels. The flow behind a full-scale ship is extremely complex \u2013 so much so that it is frequently more instructive to consider simpler cases highlighting particular elements of the larger problem. To that end, an experimental investigation has been conducted to study the behavior of a turbulent plane mixing layer intersecting a free surface at low Froude number. The local Reynolds number, based on the velocity differential across the layer and the momentum thickness, was approximately 10,000.
\r\n\r\nThe technique of Stereoscopic Digital Particle Image Velocimetry (SDPIV) was implemented to obtain instantaneous three-component velocity measurements within planar slices of the steady-state, spatially developing mixing layer flow. Guided by previous studies of the same flow conditions, specific depths were chosen at a single downstream station for investigation \u2013 specifically those in and around counter-rotating streamwise vortices known to exist in the mean flow very near the free surface. 3,000 consecutive SDPIV image pairs were recorded at a rate of 15 per second at each location, giving ample data for Reynolds decomposition and spectral analysis of the velocity fields.
\r\n\r\nThe present study has found that the anisotropy known to exist in some other free surface flows, such as surface-parallel submerged jets, is also present in the case of the mixing layer. Power spectra of all three velocity components are shown to capture part of the inertial subrange; the isotropic energy cascade seen to be present away from the free surface is also seen to disappear near the surface, as surface-normal velocity fluctuations are severely attenuated.
\r\n\r\nAdditionally, a low-frequency spanwise oscillation is deduced from the velocity power spectra and cospectra in the immediate vicinity of the mean streamwise vortices. Not present at all at significant depth, the motions at this frequency are also observed to markedly decrease \u2013 in all components \u2013 at locations closer to the surface. These observations appear to have both parallels and key differences compared to previously observed meandering of model boat wakes, and the possibility that the oscillation stems from the vortex-pair instability is discussed.
" }, { "name": "Faddy, James Malcolm", "degree": "PhD", "year": "2005", "title": "Flow Structure in a Model of Aircraft Trailing Vortices", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-163801", "creators": [ { "name": { "family": "Faddy", "given": "James Malcolm" }, "id": "Faddy-James-Malcolm", "display_name": "Faddy, James Malcolm" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Schneider", "given": "Tapio" }, "id": "Schneider-T", "orcid": "0000-0001-5687-2287", "role": "member", "display_name": "Schneider, Tapio" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RC42-JY40", "abstract": "We consider a model of incompressible trailing vortices consisting of an array of counter-rotating structures in a doubly periodic domain, infinite in the vertical direction. The two-dimensional vortex array of Mallier and Maslowe is combined with an axial velocity profile chosen proportional to the initial axial vorticity to provide an initial condition for the vortex wake. This base flow is a weak solution of the three component steady Euler equations in two dimensions thus allowing its linear stability properties to be investigated. These are used to interpret several stages in the development of vortex structure observed in fully three-dimensional DNS at Reynolds numbers Gamma/(2 pi nu)=O(1000). For sufficiently high axial velocity, itseffect can be seen, in that each vortex in the linear array first develops helical structures before undergoing a period of relaminarization. At later times the more slowly growing co-operative elliptical instabilities become apparent; however, the helical structure persists and the observed vortical structures remain coherent for longer periods than in the absence of axial velocity. Using the stretched vortex subgrid model, large-eddy simulation runs are performed at higher Reynolds numbers and a mixing transition identified at about Re = 1-2 x 10\u2074. Similar phenomena are observed in these simulations as are seen in the DNS. Next the spatial nature of the true aircraft wake is compared to the temporal approximation commonly employed to simplify the computational complexity of the problem. A model is formulated to acount for the average axial pressure gradients that develops in the spatial wake but is absent from the temporal simulation. The model enables jet- and wake-like axial flows to be distinguished and the subtle differences in the ensuing turbulent states investigated. Finally, the model is used to investigate co-rotating vortex merger, the new thrust term providing a mechanism to enhance the axial flow further destabilizing the base flow." }, { "name": "Fung, Jimmy", "degree": "PhD", "year": "2005", "title": "Coarse Analysis of Multiscale Systems: Diffuser Flows, Charged Particle Motion, and Connections to Averaging Theory", "advisor": "Murray, Richard M.; Marsden, Jerrold E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-165938", "creators": [ { "name": { "family": "Fung", "given": "Jimmy" }, "id": "Fung-Jimmy", "orcid": "0000-0002-6612-2209", "display_name": "Fung, Jimmy" } ], "advisors": [ { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "advisor", "display_name": "Murray, Richard M." }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "advisor", "display_name": "Marsden, Jerrold E." } ], "committee": [ { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "chair", "display_name": "Murray, Richard M." }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "member", "display_name": "Marsden, Jerrold E." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/wn0z-gn57", "abstract": "We describe a technique for the efficient computation of the dominant-scale dynamics of a fluid system when only a high-fidelity simulation is available. Such a technique is desirable when governing equations for the dominant scales are unavailable, when model reduction is impractical, or when the original high-fidelity computation is expensive. We adopt the coarse analysis framework proposed by I. G. Kevrekidis (Comm. Math. Sci. 2003), where a computational superstructure is designed to use short-time, high-fidelity simulations to extract the dominant features for a multiscale system. We apply this technique to compute the dominant features of the compressible flow through a planar diffuser. We apply the proper orthogonal decomposition to classify the dominant and subdominant scales of diffuser flows. We derive a suitable coarse projective Adams-Bashforth time integration routine and apply it to compute averaged diffuser flows. The results include accurate tracking of the dominant-scale dynamics for a range of parameter values for the computational superstructure. These results demonstrate that coarse analysis methods are useful for solving fluid flow problems of a multiscale nature.
\r\n\r\nIn order to elucidate the behavior of coarse analysis techniques, we make comparisons to averaging theory. To this end, we derive governing equations for the average motion of charged particles in a magnetic field in a number of different settings. First, we apply a novel procedure, inspired by WKB theory and Whitham averaging, to average the variational principle. The resulting equations are equivalent to the guiding center equations for charged particle motion; this marks an instance where averaging and variational principles commute. Secondly, we apply Lagrangian averaging techniques, previously applied in fluid mechanics, to derive averaged equations. Making comparisons to the WKB/Whitham-style derivation allows for the necessary closure of the Lagrangian averaging formulation. We also discuss the Hamiltonian setting and show that averaged Hamiltonian systems may be derivable using concepts from coarse analysis. Finally, we apply a prototypical coarse analysis procedure to the system of charged particles and generate trajectories that resemble guiding center trajectories. We make connections to perturbation theory to derive guidelines for the design of coarse analysis techniques and comment on the prototypical coarse analysis application.
" }, { "name": "Jackson, Scott Irving", "degree": "PhD", "year": "2005", "title": "Gaseous Detonation Initiation Via Wave Implosion", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05242005-151253", "creators": [ { "name": { "family": "Jackson", "given": "Scott Irving" }, "id": "Jackson-Scott-Irving", "orcid": "0000-0002-6814-3468", "display_name": "Jackson, Scott Irving" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "co-chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "co-chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Heaton", "given": "Thomas H." }, "id": "Heaton-T-H", "role": "member", "display_name": "Heaton, Thomas H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MKP3-VC84", "abstract": "Efficient detonation initiation is a topic of intense interest to designers of pulse detonation engines. This experimental work is the first to detonate propane-air mixtures with an imploding detonation wave and to detonate a gas mixture with a non-reflected, imploding shock. In order to do this, a unique device has been developed that is capable of generating an imploding toroidal detonation wave inside of a tube from a single ignition point without any obstruction to the tube flow path. As part of this study, an initiator that creates a large-aspect-ratio planar detonation wave in gas-phase explosive from a single ignition point has also been developed.
\r\n\r\nThe effectiveness of our initiation devices has been evaluated. The minimum energy required by the imploding shock for initiation was determined to scale linearly with the induction zone length, indicating the presence of a planar initiation mode. The imploding toroidal detonation initiator was found to be more effective at detonation initiation than the imploding shock initiator, using a comparable energy input to that of current initiator tubes.
" }, { "name": "Kowalewsky, Olga", "degree": "PhD", "year": "2005", "title": "Theory of Complex Lattice Quasicontinuum and Its Application to Ferroelectrics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-12202004-182638", "creators": [ { "name": { "family": "Kowalewsky", "given": "Olga" }, "id": "Kowalewsky-Olga", "display_name": "Kowalewsky, Olga" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "chair", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/rb0c-9534", "abstract": "Complex lattice Quasicontinuum theory is developed and applied to the description of ferroelectric phenomena. Quasicontinuum theory is a multiscale theory that provides a unified description of materials by combining atomistic and continuum approaches. It provides a seamless transition between atomistics and continuum, but the description of the material is derived directly from the underlying atomic structure, using the computationally expensive atomistics only where needed, at the location of phenomena of atomistic origin.
\r\n\r\nComplex Lattice Quasicontinuum theory can be applied to complex lattice crystals consisting of many kinds of atoms. One highlight of it is treatment of each component lattice as separately and independently as possible. The component Quasicontinua are coupled through the microscopic forces within nodal clusters, making the complex atomistics of the heterogeneous lattice the basis of the description.
\r\n\r\nFerroelectrics are especially suited to the application of Quasicontinuum theory. The nature of defects in ferroelectric materials is atomistic, but their influence over the material is long ranged due to induced elastic fields. Many different ferroelectric phenomena involving the perovskite ferroelectrics Barium Titanate and Lead Titanate are investigated and simulated. For Barium Titanate: the 180 degree domain wall structure and quasistatic crack under load. For Lead Titanate: the 180 degree domain wall structure and a domain wall step.
\r\n\r\nThe results for the domain walls show that the domain wall thickness is atomistically small, of the order of few lattice constants, which is in agreement with recent ab initio molecular dynamics simulations, but we also observe long range effects resulting from the presence of the wall. During crack loading in the sample of Barium Titanate we observe polarization changes around the crack tip which are consistent with experimental observations of an increase of fracture toughness. The quasicontinuum study of a domain wall step gives an atomistical view into the equilibrium structure of the step.
\r\n\r\nQuasicontinuum is able to model these phenomena with atomistic precision around the defects and non-homogeneities, and also capture the influence of long-ranging effects in the samples. These studies could also give valuable modeling input for larger scale continuum approaches.
" }, { "name": "Papachristodoulou, Antonis", "degree": "PhD", "year": "2005", "title": "Scalable Analysis of Nonlinear Systems Using Convex Optimization", "advisor": "Doyle, John Comstock", "url": "https://resolver.caltech.edu/CaltechETD:etd-05082005-100243", "creators": [ { "name": { "family": "Papachristodoulou", "given": "Antonis" }, "id": "Papachristodoulou-Antonis", "orcid": "0000-0002-3565-8967", "display_name": "Papachristodoulou, Antonis" } ], "advisors": [ { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "role": "advisor", "display_name": "Doyle, John Comstock" } ], "committee": [ { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "role": "chair", "display_name": "Doyle, John Comstock" }, { "name": { "family": "Rantzer", "given": "Anders" }, "id": "Rantzer-A", "role": "member", "display_name": "Rantzer, Anders" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "role": "member", "display_name": "Murray, Richard M." }, { "name": { "family": "Low", "given": "Steven H." }, "id": "Low-S-H", "role": "member", "display_name": "Low, Steven H." } ], "option_major": [ "cds" ], "doi": "10.7907/5YG6-JG32", "abstract": "In this thesis, we investigate how convex optimization can be used to analyze different classes of nonlinear systems at various scales algorithmically. The methodology is based on the construction of appropriate Lyapunov-type certificates using sum of squares techniques.
\r\n\r\nAfter a brief introduction on the mathematical tools that we will be using, we turn our attention to robust stability and performance analysis of systems described by Ordinary Differential Equations. A general framework for constrained systems analysis is developed, under which stability of systems with polynomial, non polynomial vector fields and switching systems, as well as estimating the region of attraction and the L2 gain can be treated in a unified manner. Examples from biology and aerospace illustrate our methodology.
\r\n\r\nWe then consider systems described by Functional Differential Equations (FDEs), i.e., time-delay systems. Their main characteristic is that they are infinite dimensional, which complicates their analysis. We first show how the complete Lyapunov-Krasovskii functional can be constructed algorithmically for linear time delay systems. Then, we concentrate on delay-independent and delay-dependent stability analysis of nonlinear FDEs using sum of squares techniques. An example from ecology is given.
\r\n\r\nThe scalable stability analysis of congestion control algorithms for the Internet is investigated next. The models we use result in an arbitrary interconnection of FDE subsystems, for which we require that stability holds for arbitrary delays, network topologies and link capacities. Through a constructive proof, we develop a Lyapunov functional for FAST - a recently developed network congestion control scheme - so that the Lyapunov stability properties scale with the system size. We also show how other network congestion control schemes can be analyzed in the same way.
\r\n\r\nFinally, we concentrate on systems described by Partial Differential Equations. We show that axially constant perturbations of the Navier-Stokes equations for Hagen-Poiseuille flow are globally stable, even though the background noise is amplified as R3 where R is the Reynolds number, giving a 'robust yet fragile' interpretation. We also propose a sum of squares methodology for the analysis of systems described by parabolic PDEs.
\r\n\r\nWe conclude this work with an account for future research.
" }, { "name": "Ponchaut, Nicolas Frederic", "degree": "PhD", "year": "2005", "title": "Part I: 3DPTV: Advances and Error Analysis. Part II: Extension of Guderley's Solution for Converging Shock Waves", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05312005-111401", "creators": [ { "name": { "family": "Ponchaut", "given": "Nicolas Frederic" }, "id": "Ponchaut-Nicolas-Frederic", "display_name": "Ponchaut, Nicolas Frederic" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/09ZH-9M66", "abstract": "This work is divided into two unrelated parts. In the first part, a full three-dimensional particle tracking system was developed and tested. Three images, from three separate CCDs placed at the vertices of an equilateral triangle, permit the three-dimensional location of particles to be determined by triangulation. Particle locations measured at two different times can then be used to create a three-component, three-dimensional velocity field. Key developments are the ability to accurately process overlapping particle images, offset CCDs to significantly improve effective resolution, treatment of dim particle images, and a hybrid particle tracking technique ideal for three-dimensional flows when only two sets of images exist. An in-depth theoretical error analysis was performed, which gives the important sources of error and their effect on the overall system. This error analysis was verified through a series of experiments, and a vortex flow measurement was performed.\r\n\r\nIn the second part, the problem of a cylindrically or spherically imploding and reflecting shock wave in a flow initially at rest was examined. Guderley's strong shock solution around the origin was improved by adding two more terms in the series expansion solution for both the incoming and the reflected shock waves. A series expansion was also constructed for the case where the shock is still very far from the origin. In addition, a program based on the characteristics method was written. Thanks to an appropriate change of variables, the shock motion could be computed from virtually infinity to very close to the reflection point. Comparisons were made between the series expansions, the characteristics program, and the results obtained using an Euler solver. These comparisons showed that the addition of two terms to the Guderley solution significantly increases the accuracy of the series expansion." }, { "name": "Thunnissen, Daniel Pierre", "degree": "PhD", "year": "2005", "title": "Propagating and Mitigating Uncertainty in the Design of Complex Multidisciplinary Systems", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01072005-162147", "creators": [ { "name": { "family": "Thunnissen", "given": "Daniel Pierre" }, "id": "Thunnissen-Daniel-Pierre", "display_name": "Thunnissen, Daniel Pierre" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Antonsson", "given": "Erik K." }, "id": "Antonsson-E-K", "role": "member", "display_name": "Antonsson, Erik K." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Ledyard", "given": "John O." }, "id": "Ledyard-J-O", "role": "member", "display_name": "Ledyard, John O." } ], "option_major": [ "mecheng" ], "doi": "10.7907/0FX2-AM50", "abstract": "As humanity has developed increasingly ingenious and complicated systems, it has not been able to accurately predict the performance, development time, reliability, or cost of such systems. This inability to accurately predict parameters of interest in the design of complex multidisciplinary systems such as automobiles, aircraft, or spacecraft is due in great part to uncertainty. Uncertainty in complex multidisciplinary system design is currently mitigated through the use of heuristic margins. The use of these heuristic margins can result in a system being overdesigned during development or failing during operation.
\r\n\r\nThis thesis proposes a formal method to propagate and mitigate uncertainty in the design of complex multidisciplinary systems. Specifically, applying the proposed method produces a rigorous foundation for determining design margins. The method comprises five distinct steps: identifying tradable parameters; generating analysis models; classifying and addressing uncertainties; quantifying interaction uncertainty; and determining margins, analyzing the design, and trading parameters. The five steps of the proposed method are defined in detail. Margins are now a function of risk tolerance and are measured relative to mean expected system performance, not variations in design parameters measured relative to heuristic values.
\r\n\r\nAs an example, the proposed method is applied to the preliminary design of a spacecraft attitude determination and control system. In particular, the design of the attitude control system on the Mars Exploration Rover spacecraft cruise stage is used. Use of the proposed method for the example presented yields significant differences between the calculated design margins and the values assumed by the Mars Exploration Rover project.
\r\n\r\nIn addition to providing a formal and rigorous method for determining design margins, this thesis provides three other principal contributions. The first is an uncertainty taxonomy for use in the design of complex multidisciplinary systems with detailed definitions for each uncertainty type. The second is the modification of two simulation techniques, the mean value method and subset simulation, that can significantly reduce the computational burden in applying the proposed method. The third is a set of diverse application examples and various simulation techniques that demonstrate the generality and benefit of the proposed method.
" }, { "name": "Wheatley, Vincent", "degree": "PhD", "year": "2005", "title": "On the Richtmyer-Meshkov Instability in Magnetohydrodynamics", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-145538", "creators": [ { "name": { "family": "Wheatley", "given": "Vincent" }, "id": "Wheatley-Vincent", "orcid": "0000-0002-7287-7659", "display_name": "Wheatley, Vincent" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Schneider", "given": "Tapio" }, "id": "Schneider-T", "orcid": "0000-0001-5687-2287", "role": "member", "display_name": "Schneider, Tapio" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/N407-2B54", "abstract": "The Richtmyer-Meshkov instability is important in a wide variety of applications including inertial confinement fusion and astrophysical phenomena. In some of these applications, the fluids involved may be plasmas and hence be affected by magnetic fields. For one configuration, it has been numerically demonstrated that the growth of the instability in magnetohydrodynamics is suppressed in the presence of a magnetic field. Here, the nature of this suppression is theoretically and numerically investigated.
\r\n\r\nIn the framework of ideal incompressible magnetohydrodynamics, we examine the stability of an impulsively accelerated, sinusoidally perturbed density interface in the presence of a magnetic field that is parallel to the acceleration. This is accomplished by analytically solving the linearized initial value problem, which is a model for the Richtmyer-Meshkov instability. We find that the initial growth rate of the interface is unaffected by the presence of a magnetic field, but for a finite magnetic field the interface amplitude asymptotes to a constant value. Thus the instability of the interface is suppressed. The interface behavior from the analytical solution is compared to the results of both linearized and non-linear compressible numerical simulations for a wide variety of conditions.
\r\n\r\nWe then consider the problem of the regular refraction of a shock at an oblique, planar contact discontinuity separating conducting fluids of different densities in the presence of a magnetic field aligned with the incident shock velocity. Planar ideal MHD simulations indicate that the presence of a magnetic field inhibits the deposition of vorticity on the shocked contact, which leads to the suppression of the Richtmyer-Meshkov instability. We show that the shock refraction process produces a system of five to seven plane waves that may include fast, intermediate, and slow MHD shocks, slow compound waves, 180\u00b0 rotational discontinuities, and slow-mode expansion fans that intersect at a point. In all solutions, the shocked contact is vorticity free and hence stable. These solutions are not unique, but differ in the type of waves that participate. The set of equations governing the structure of these multiple-wave solutions is obtained in which fluid property variation is allowed only in the azimuthal direction about the wave-intersection point. Corresponding solutions are referred to as either quintuple-points, sextuple-points, or septuple-points, depending on the number of participating waves. A numerical method of solution is described and examples are compared to the results of numerical simulations for moderate magnetic field strengths. The limit of vanishing magnetic field at fixed permeability and pressure is studied for two solution types. The relevant solutions correspond to the hydrodynamic triple-point with the shocked contact replaced by a singular structure consisting of a wedge, whose angle scales with the applied field magnitude, bounded by either two slow compound waves or two 180\u00b0 rotational discontinuities, each followed by a slow-mode expansion fan. These bracket the MHD contact which itself cannot support a tangential velocity jump in the presence of a non-parallel magnetic field. The magnetic field within the singular wedge is finite and the shock-induced change in tangential velocity across the wedge is supported by the expansion fans that form part of the compound waves or follow the rotational discontinuities. To verify these findings, an approximate leading order asymptotic solution appropriate for both flow structures was computed. The full and asymptotic solutions are compared quantitatively and there is shown to be excellent agreement between the two.
" }, { "name": "Wirz, Richard Edward", "degree": "PhD", "year": "2005", "title": "Discharge Plasma Processes of Ring-Cusp Ion Thrusters", "advisor": "Culick, Fred E. C.; Polk, James E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05232005-162628", "creators": [ { "name": { "family": "Wirz", "given": "Richard Edward" }, "id": "Wirz-Richard-Edward", "orcid": "0000-0001-5309-3659", "display_name": "Wirz, Richard Edward" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "co-advisor", "display_name": "Polk, James E." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "member", "display_name": "Polk, James E." }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "role": "member", "display_name": "Bruno, Oscar P." }, { "name": { "family": "Bellan", "given": "Paul Murray" }, "id": "Bellan-P-M", "role": "member", "display_name": "Bellan, Paul Murray" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VKKE-PC20", "abstract": "This study has increased the viability of miniature ion thruster technology, advanced state-of-the-art discharge modeling, and revealed important aspects of discharge plasma processes. These extensions of existing ion thruster technology and understanding are necessary to fulfill the needs of future space missions. Experimental comparisons of the discharge performance of an array of miniature (3cm diameter) ion thruster discharge configurations were conducted and showed that a 3-ring configuration with length-to-diameter of 1.0 exhibited the best performance. A compact and lightweight version of this configuration, using small accelerator grid holes, exhibited discharge losses of 250-550eV/ion and propellant efficiency of as much as 87%. This performance represents a significant advancement in miniature (less than 5cm diameter) ion thruster technology and demonstrates that a miniature ion thruster of low magnet and thruster weight can yield desirable performance.
\r\n\r\nA multi-component hybrid 2-D computational Discharge Model was developed to help identify important ion thruster discharge processes and investigate miniaturization issues. Combining experimental and computational results reveals that magnetic field optimization for a miniature ion thruster is bracketed by considerations of primary electron utilization and discharge stability. Discharge Model analysis of the larger (30cm diameter) NSTAR thruster revealed that the peak observed in the NSTAR beam profile is due to double ions that are created by over-confinement of primary electrons on the thruster axis. Design sensitivity results show that, at the NSTAR thruster scale, efficient confinement of primary electrons is relatively easy to achieve; therefore, efforts to improve thruster performance should focus on effectively utilizing the primary electrons to minimize double ion production and maximize the number of single ions extracted to the beam.
\r\n\r\nThe observations from this study have furthered the understanding of discharge processes and should improve future ion thruster design and modeling efforts. The Discharge Model advances state-of-the-art ion thruster modeling and provides a framework for a complete thruster model that can be used for long-life performance assessment and life validation.
" }, { "name": "Barba, Lorena A.", "degree": "PhD", "year": "2004", "title": "Vortex Method for Computing High-Reynolds Number Flows: Increased Accuracy with a Fully Mesh-Less Formulation", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-030854", "creators": [ { "name": { "family": "Barba", "given": "Lorena A." }, "id": "Barba-Lorena-A", "orcid": "0000-0001-5812-2711", "display_name": "Barba, Lorena A." } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TSR5-DE67", "abstract": "For the applications of high Reynolds number flows, the vortex method presents the advantage of being free from numerically dissipative truncation error. In practice, however, many vortex methods introduce some numerical dissipation in mesh-based spatial adaption stages, or schemes such as vortex particle splitting. The need for spatial adaption in vortex methods arises from the Lagrangian framework, which results in an increase of discretization error over time. Presently, a vortex method is devised that incorporates radial basis function (RBF) interpolation to provide spatial adaption in a fully mesh-less formulation. Numerical experiments show that there is a potential for higher accuracy in comparison with the standard remeshing techniques. The rate of convergence of the new spatial adaption method is exponential, however convection error limits the vortex method to second order convergence. Avenues for future research involve decreasing convection error, for example by means of deformable basis functions. Nevertheless, the RBF-based spatial adaption scheme has various advantages, in addition to a demonstrated higher accuracy and the obvious benefit of not requiring a regular arrangement of particles or mesh. One presently demonstrated advantage is automatic core size control for the core spreading viscous method, without the need for vortex particle splitting.
\r\n\r\nThree applications have been successfully treated with the presently developed vortex method. The relaxation of monopoles under non-linear perturbations has been computed, resulting in noticeable improvements compared to previously published results. The existence of a quasi-steady state consisting of a rotating tripole has been corroborated, for the case of large amplitude perturbations. The second application consists of the early adaptation of two co-rotating vortices at high Reynolds number, characterized by elliptical deformation of the cores, as well as small scale deformation in the weak areas of vorticity. This is considered to pose a severe test on the present method, or indeed any method. Comparison with results using spectral methods demonstrate in practice the potential for high accuracy computations using a mesh-less method, and in addition show that the naturally adaptive vortex method can result in considerably reduced problem sizes. Finally, for the calculation of non-symmetric Burgers vortices, a correction to the core spreading method for out-of-plane strain was developed. The results establish the capability of the vortex method for the computation of vortices under three-dimensional strain.
" }, { "name": "Bobba, Kumar Manoj", "degree": "PhD", "year": "2004", "title": "Robust Flow Stability: Theory, Computations and Experiments in Near Wall Turbulence", "advisor": "Doyle, John Comstock; Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-143324", "creators": [ { "name": { "family": "Bobba", "given": "Kumar Manoj" }, "id": "Bobba-Kumar-Manoj", "display_name": "Bobba, Kumar Manoj" } ], "advisors": [ { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "orcid": "0000-0002-1828-2486", "role": "advisor", "display_name": "Doyle, John Comstock" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Doyle", "given": "John Comstock" }, "id": "Doyle-J-C", "orcid": "0000-0002-1828-2486", "role": "co-chair", "display_name": "Doyle, John Comstock" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hou", "given": "Thomas Y." }, "id": "Hou-T-Y", "orcid": "0000-0001-6287-1133", "role": "member", "display_name": "Hou, Thomas Y." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0J1D-1B18", "abstract": "Helmholtz established the field of hydrodynamic stability with his pioneering work in 1868. From then on, hydrodynamic stability became an important tool in understanding various fundamental fluid flow phenomena in engineering (mechanical, aeronautics, chemical, materials, civil, etc.) and science (astrophysics, geophysics, biophysics, etc.), and turbulence in particular. However, there are many discrepancies between classical hydrodynamic stability theory and experiments. In this thesis, the limitations of traditional hydrodynamic stability theory are shown and a framework for robust flow stability theory is formulated. A host of new techniques like gramians, singular values, operator norms, etc. are introduced to understand the role of various kinds of uncertainty. An interesting feature of this framework is the close interplay between theory and computations. It is shown that a subset of Navier-Stokes equations are globally, non-nonlinearly stable for all Reynolds number. Yet, invoking this new theory, it is shown that these equations produce structures (vortices and streaks) as seen in the experiments. The experiments are done in zero pressure gradient transiting boundary layer on a flat plate in free surface tunnel. Digital particle image velocimetry, and MEMS based laser Doppler velocimeter and shear stress sensors have been used to make quantitative measurements of the flow. Various theoretical and computational predictions are in excellent agreement with the experimental data. A closely related topic of modeling, simulation and complexity reduction of large mechanics problems with multiple spatial and temporal scales is also studied. A nice method that rigorously quantifies the important scales and automatically gives models of the problem to various levels of accuracy is introduced. Computations done using spectral methods are presented.\r\n" }, { "name": "Chao, Tong Wa", "degree": "PhD", "year": "2004", "title": "Gaseous Detonation-Driven Fracture of Tubes", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04062004-165940", "creators": [ { "name": { "family": "Chao", "given": "Tong Wa" }, "id": "Chao-Tong-Wa", "display_name": "Chao, Tong Wa" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TEZP-YC46", "abstract": "An experimental investigation of fracture response of aluminum 6061-T6 tubes under internal gaseous detonation loading has been carried out. The pressure load, with speeds exceeding 2 km/s, can be characterized as a pressure peak (ranging from 2 to 6 MPa) followed by an expansion wave. The unique combination of this particular traveling load and tube geometry produced fracture data not available before in the open literature. Experimental data of this type are useful for studying the fluid-structure-fracture interaction and various crack curving and branching phenomena, and also for validation for multi-physics and multi-scale modeling.
\r\n\r\nAxial surface flaws were introduced to control the crack initiation site. Fracture threshold models were developed by combining a static fracture model and an extensively studied dynamic amplification factor for tubes under internal traveling loads. Experiments were also performed on hydrostatically loaded preflawed aluminum 6061-T6 tubes for comparison. Significantly different fracture behavior was observed and the difference was explained by fluid dynamics and energy considerations. The experiments yielded comparison on crack speeds, strain, and pressure histories.
\r\n\r\nIn other experiments, the specimens were also pre-torqued to control the propagation direction of the cracks. Measurements were made on the detonation velocity, strain history, blast pressure from the crack opening, and crack speeds. The curved crack paths were digitized. The Chapman-Jouguet pressure, initial axial flaw length, and torsion level were varied to obtain different crack patterns. The incipient crack kinking angle was found to be consistent with fracture under mixed-mode loading. High-speed movies of the fracture events and blast wave were taken and these were used in interpreting the quantitative data.
\r\n\r\nNumerical simulations were performed using the commercial explicit finite-element software LS-Dyna. The detonation wave was modeled as a traveling boundary load. Both non-fracturing linear elastic simulations and elastoplastic simulations with fracture were conducted on three-dimensional models. The simulated fracture was compared directly with an experiment with the same conditions. The overall qualitative fracture behavior was captured by the simulation. The forward and backward cracks were observed to branch in both the experiment and simulation.
" }, { "name": "Fago, Matthew Justin", "degree": "PhD", "year": "2004", "title": "Constrained Sequential Lamination: Nonconvex Optimization and Material Microstructure", "advisor": "Ortiz, Michael; Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-05142004-144712", "creators": [ { "name": { "family": "Fago", "given": "Matthew Justin" }, "id": "Fago-Matthew-Justin", "display_name": "Fago, Matthew Justin" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "co-advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Lapusta", "given": "Nadia" }, "id": "Lapusta-N", "role": "member", "display_name": "Lapusta, Nadia" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/P1PK-E179", "abstract": "A practical algorithm has been developed to construct, through sequential lamination, the partial relaxation of multiwell energy densities such as those characteristic of shape memory alloys. The resulting microstructures are in static and configurational equilibrium, and admit arbitrary deformations. The laminate topology evolves during deformation through branching and pruning operations, while a continuity constraint provides a simple model of metastability and hysteresis. In cases with strict separation of length scales, the method may be integrated into a finite element calculation at the subgrid level. This capability is demonstrated with a calculation of the indentation of a Cu-Al-Ni shape memory alloy by a spherical indenter.
\r\n\r\nIn verification tests the algorithm attained the analytic solution in the computation of three benchmark problems. In the fourth case, the four-well problem (of, e.g., Tartar), results indicate that the method for microstructural evolution imposes an energy barrier for branching, hindering microstructural development in some cases. Although this effect is undesirable for purely mathematical problems, it is reflective of the activation energies and metastabilities present in applications involving natural processes.
\r\n\r\nThe method was further used to model Shield's tension test experiment, with initial calculations generating reasonable transformation strains and microstructures that compared well with the sequential laminates obtained experimentally.
" }, { "name": "Malhotra, Sanjeev", "degree": "PhD", "year": "2004", "title": "On Combustion Instability in Solid Rocket Motors", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05062004-130913", "creators": [ { "name": { "family": "Malhotra", "given": "Sanjeev" }, "id": "Malhotra-Sanjeev", "display_name": "Malhotra, Sanjeev" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Flandro", "given": "G. A." }, "id": "Flandro-G-A", "role": "member", "display_name": "Flandro, G. A." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/V1WS-S833", "abstract": "An investigation of combustion instability in solid rocket motors was conducted using perturbation techniques, with particular emphasis placed upon understanding the fluid dynamics of the chamber environment. It was shown that although the phenomena generally manifests itself as oscillations of pressure, with the frequencies measured in tests well predicted by classical acoustic formulas, important aspects of the behavior cannot be explained without due recognition of the two basic processes of fluid dynamics\u2014i.e., the compressing/expanding process and the shearing process.
\r\n\r\nThus, a new framework for studying these instabilities that accommodated both linear and nonlinear behavior was developed. The approach differed from previous work in its use of linear stability eigenfunctions\u2014that satisfy the no-slip boundary condition\u2014as a basis for the expansion, with adjoints used to effect a spatial averaging. Among other things, this allowed for the self-consistent inclusion of vortical flow effects.
\r\n\r\nWith respect to the linear behavior, two dominant vorticity-related pathways were shown to exist: one because of sound creating vorticity, and the other, because of that vorticity, in turn, creating more sound. These effects cancel however and thus to leading order no net contribution exists. Though this finding had been reported in an earlier study, restrictive assumptions were introduced. In contrast, we establish that the result is independent of grain geometry and holds for any fluid motion, turbulent or otherwise.
\r\n\r\nA nonlinear coupling to the flame zone owing to vorticity creation was also identified. The term was left unevaluated however, since no satisfactory model of the flame response presently exists. To help circumvent this difficulty, i.e., that much remains to be done on modeling nonlinear processes, the amplitude equations were studied in a general way using perturbation techniques based on ideas of resonance. The advantage of such an approach is that the nonlinear coefficients need not be specified a priori\u2014only conditions on the linear behavior of the system need to be placed. Closed form results were derived for the limiting periodic behavior when the first mode is unstable and compared against results from numerical integration. Striking agreement was shown.
" }, { "name": "O'Gorman, Paul Ambrose", "degree": "PhD", "year": "2004", "title": "Theory and Simulation of Passive Scalar Mixing in the Presence of a Mean Scalar Gradient", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05272004-150652", "creators": [ { "name": { "family": "O'Gorman", "given": "Paul Ambrose" }, "id": "O'Gorman-Paul-Ambrose", "display_name": "O'Gorman, Paul Ambrose" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Bruno", "given": "Oscar P." }, "id": "Bruno-O-P", "role": "member", "display_name": "Bruno, Oscar P." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K02H-HK48", "abstract": "The turbulent mixing of a passive scalar in the presence of a mean scalar gradient was investigated using theory and simulation. The velocity-scalar cospectrum measures the distribution of the mean scalar flux across scales. An inequality is shown to bound the magnitude of the cospectrum in terms of the shell-summed energy and scalar spectra. At high Schmidt number this bound limits the possible contribution of the sub-Kolmogorov scales to the scalar flux. At low Schmidt number we use an argument of Batchelor, Howells, and Townsend (1959) to derive a new asymptotic result for the cospectrum in the inertial-diffusive range, with a -11/3 power law wavenumber dependence. A comparison is made with results from large-eddy simulation at low Schmidt number.
\r\n\r\nThe sparse direct-interaction perturbation (SDIP) was used to calculate the cospectrum for a range of Schmidt numbers. The Kolmogorov type scaling result is recovered in the inertial-convective range, and the constant of proportionality was calculated. At high Schmidt numbers, the cospectrum is found to decay exponentially in the viscous-convective range, and at low Schmidt numbers the -11/3 power law is observed in the inertial-diffusive range. The stretched-spiral vortex model was used to calculate the cospectrum, and asymptotic expressions were found for the contribution to the cospectrum from the axial velocity in the vortex structures. Results are reported for the cospectrum from a direct numerical simulation at a Taylor Reynolds number of 265, and a comparison is made of results for the cospectrum from the SDIP, the stretched-spiral vortex model, simulation, and experiment.
\r\n\r\nThe stretched-spiral vortex model was also used to derive expressions for the modal time correlation functions of the velocity and scalar fields. These expressions were evaluated numerically and asymptotically. Winding by the vortex core is shown to lead to an inertial timescale, and movement of the vortex structures by the large scale flow leads to a sweeping timescale. The velocity and scalar modal time correlation functions were calculated in the direct numerical simulation. They coincide for large enough wavenumber, and are found to collapse to universal forms when a sweeping timescale is used.
" }, { "name": "O'Reilly, Gerard Kieran", "degree": "PhD", "year": "2004", "title": "Compressible Vortices and Shock-Vortex Interactions", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-05262004-145030", "creators": [ { "name": { "family": "O'Reilly", "given": "Gerard Kieran" }, "id": "O'Reilly-Gerard-Kieran", "display_name": "O'Reilly, Gerard Kieran" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FGJD-0Z31", "abstract": "Secondary instabilities on the organized, spanwise, vortical structures in incompressible shear layers, play an important role in generating the onset of three-dimensional turbulence in such flows. The effect of increasing compressibility on these instabilities is examined by using the compressible Stuart vortex as a model for a compressible shear layer. It is found that both two- and three-dimensional subharmonic instabilities cease to promote pairing events even at moderate free stream Mach numbers. The fundamental mode becomes dominant as the free stream Mach number is increased, and a new instability corresponding to an instability on a parallel shear layer is observed. The interaction of a shock with a compressible vortex may be viewed as a simplified model of the general interaction of a shock with the coherent structures in a turbulent flow field. An approximate theory for computing shock-compressible-vortex interactions is developed, based on Ribner (1954). The problem of convection of a frozen patern of vorticity, dilatation, temperature and entropy through a planar shock wave is considered. The refraction and modification of the upstream disturbances into the three basis modes permitted by the linear Euler equations is derived, as well as the perturbation to the shock wave. This theory is used to compute approximate post-shock states corresponding to shock-CSV interactions, a model for shock shear layer interactions. The method is verified by comparing its approximate post-shock fields with those computed explicitly using AMROC, a finite difference, AMR-WENO code. Finally, numerical solutions corresponding to a compressible analogue of the Mallier and Maslowe vortex (a periodic array of counter-rotating vortices) are presented. These solutions admit the existence of large regions of smooth supersonic flow, and could potentially be used to model the counter-rotating vortices arising from the single- and multi-mode Richtmyer-Meshkov instability." }, { "name": "Ringuette, Matthew James", "degree": "PhD", "year": "2004", "title": "Vortex Formation and Drag on Low Aspect Ratio, Normal Flat Plates", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-05292004-183807", "creators": [ { "name": { "family": "Ringuette", "given": "Matthew James" }, "id": "Ringuette-Matthew-James", "orcid": "0000-0002-8998-9489", "display_name": "Ringuette, Matthew James" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Burdick", "given": "Joel Wakeman" }, "id": "Burdick-J-W", "orcid": "0000-0002-3091-540X", "role": "member", "display_name": "Burdick, Joel Wakeman" }, { "name": { "family": "Dickinson", "given": "Michael H." }, "id": "Dickinson-M-H", "orcid": "0000-0002-8587-9936", "role": "member", "display_name": "Dickinson, Michael H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/907K-2F28", "abstract": "Experiments were done to investigate the role of vortex formation in the drag force generation of low aspect ratio, normal flat plates starting from rest. This very simplified case is a first, fundamental step toward understanding the more complicated flow of hovering flight, which relies primarily on drag for propulsion. The relative importance of the plate's free end, or tip, with varying aspect ratio was also studied.
\r\n\r\nIdentifying the relationship among aspect ratio, vortex formation, and drag force can provide insight into the wing aspect ratios and kinematics found nature, with the eventual goal of designing man-made flapping wing micro air vehicles.
\r\n\r\nThe experiments were carried out using flat plate models in a towing tank at a moderate Reynolds number of 3000. Two aspect ratios, 6 and 2, were considered, the latter in order to have a highly tip-dominated case. A force balance measured the time-varying drag, and multiple, perpendicular sections of the flow velocity were measured quantitatively using digital particle image velocimetry. Vorticity fields were calculated from the velocity data, and features in the drag force for different aspect ratios were related to the vortex dynamics. Finally, since the flow is highly three-dimensional, dye flow visualization was done to characterize its structure and to augment the two-dimensional digital particle image velocimetry data.
" }, { "name": "Wintenberger, Eric", "degree": "PhD", "year": "2004", "title": "Application of Steady and Unsteady Detonation Waves to Propulsion", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04222004-121013", "creators": [ { "name": { "family": "Wintenberger", "given": "Eric" }, "id": "Wintenberger-Eric", "display_name": "Wintenberger, Eric" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "member", "display_name": "Polk, James E." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "role": "member", "display_name": "Hunt, Melany L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2NXT-SE76", "abstract": "The present work investigates the applications of steady and unsteady detonation waves to air-breathing propulsion systems. The efficiency of ideal detonation-based propulsion systems is first investigated based on thermodynamics. We reformulate the Hugoniot analysis of steady combustion waves for a fixed initial stagnation state to conclude that steady detonation waves are less desirable than deflagrations for propulsion. However, a thermostatic approach shows that unsteady detonations have the potential for generating more work than constant-pressure combustion. The subsequent work focuses on specific engine concepts. A flow path analysis of ideal steady detonation engines is conducted and shows that their performance is limited and poorer than that of the ideal ramjet or turbojet engines. The limitations associated with the use of a steady detonation in the combustor are drastic and such engines do not appear to be practical. This leads us to focus on unsteady detonation engines, i.e., pulse detonation engnes. The unsteady generation of thrust in the simple configuration of a detonation tube is first analyzed using gas dynamics. We develop one of the first models to quickly and reliably estimate the impulse of a pulse detonation tube. The impulse is found to scale directly with the mass of explosive in the tube and the square root of the energy release per unit mass of the mixture. Impulse values for typical fuel-oxidizer mixtures are found to be on the order of 160 s for hydrocarbon-oxygen mixtures and 120 s for fuel-air mixtures at standard conditions. These results are then used as a basis to develop the first complete system-level performance analysis of a supersonic, single-tube, air-breathing pulse detonation engine. We show that hydrogen- and JP10-fueled pulse detonation engines generate thrust up to a Mach number of 4, and that the specific impulse decreases quasi-linearly with increasing flight Mach number. Finally, we find that the performance of our pulse detonation engine exceeds that of the ramjet below a Mach number of 1.35." }, { "name": "Yang, Qiang", "degree": "PhD", "year": "2004", "title": "Thermomechanical Variational Principles for Dissipative Materials with Application to Strain Localization in Bulk Metallic Glasses", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-152537", "creators": [ { "name": { "family": "Yang", "given": "Qiang" }, "id": "Yang-Qiang", "display_name": "Yang, Qiang" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Johnson", "given": "William Lewis" }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William Lewis" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6FV2-KV63", "abstract": "This thesis is concerned with variational principles for general coupled thermomechanical problems in dissipative materials including finite elastic and plastic deformation, non-Newtonian viscosity, rate sensitivity, arbitrary flow and hardening rule, as well as heat conduction. It is shown that there exists a potential function such that both the conservation of energy and balance of linear momentum are the Euler-Lagrange equations of its first variation. Inspired from the time-discretized version of the variational formulation, we present a procedure for variational thermomechanical update, which generalizes the isothermal approach under a variational thermodynamic framework. This variational formulation then serves as a basis for temperature change as well as constitutive updates.
\r\n\r\nAn important application of the variational formulation is to optimize the shear band thickness in strain localization processes. We show that this optimization takes the form of a configurational-force equilibrium and results in a well-defined band thickness. We further implement displacement discontinuities into a class of strain-localization finite elements. These elements consist of two surfaces, attached to the abutting volume elements, which can separate and slip relative to each other, and thus enable the accurate and efficient simulation of the dynamical formation of stain localization.
\r\n\r\nThe variational formulation also leads to a finite-deformation continuum modeling of bulk metallic glasses. It is shown that the strain softening of bulk metallic glasses is due to the increase of free volume (and thus the decrease of viscosity), while temperature rise accelerates the localization of the deformation. The model reproduces the constitutive behavior of Vitreloy 1 bulk metallic glass at various strain rates and temperatures.
" }, { "name": "Arienti, Marco", "degree": "PhD", "year": "2003", "title": "A Numerical and Analytical Study of Detonation Diffraction", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02122003-152525", "creators": [ { "name": { "family": "Arienti", "given": "Marco" }, "id": "Arienti-Marco", "orcid": "0000-0001-8166-0016", "display_name": "Arienti, Marco" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MAGN-R628", "abstract": "An investigation of detonation diffraction through an abrupt area change has been carried out via two-dimensional, parallel simulations. The existence of critical conditions for successful diffraction is closely related to the occurrence of localized re-initiation mechanisms, and is relevant to propulsion and safety concepts concerning detonation transmission. Our analysis is specialized to a reactive mixture with perfect gas equation of state and a single-step reaction in the Arrhenius form. The concept of shock decoupling from the reaction zone is the simplest idea used to explain the behavior of a diffracting detonation front. Lagrangian particles are injected into the flow in order to identify the dominant terms in the equation that describes the temperature rate of change of a fluid element, expressed in a shock-based reference system. Conveniently simplified, this equation provides an insight into the competition between the energy release rate and the expansion rate behind the diffracting front. We also examine the mechanism of spontaneous generation of transverse waves along the front. This mechanism is related to the sensitivity of the reaction rate to temperature, and it is investigated in the form of a parametric study for the activation energy. We study in detail three highly resolved cases of detonation diffraction that illustrate different types of behavior, super-, sub-, and near-critical diffraction. We review the applicability of existing shock dynamics models to the corner-turning problem. Numerical results from the parametric study are compared with predictions from these theories in the attempt to find a formula for shock decay in a quenching detonation. This estimate is then used in the simplified temperature rate of change equation to provide a relation between critical channel width and activation energy. We conclude this study by examining the spontaneous formation of transverse waves along the wavefront of a successfully transmitted detonation. The problem is simplified to a planar CJ detonation moving in a channel over a small obstacle to investigate how acoustic waves propagate within the reaction zone. Depending on the reaction kinetics, we show that such waves may be amplified due to feedback between the chemical reaction and fluid motion. The amplification can lead to shock steepening and formation of transverse detonation waves.\r\n" }, { "name": "Austin, Joanna Maria Karol", "degree": "PhD", "year": "2003", "title": "The Role of Instability in Gaseous Detonation", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05292003-150534", "creators": [ { "name": { "family": "Austin", "given": "Joanna Maria Karol" }, "id": "Austin-Joanna-Maria-Karol", "orcid": "0000-0003-3129-5035", "display_name": "Austin, Joanna Maria Karol" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/X7YH-T687", "abstract": "In detonation, the coupling between fluid dynamics and chemical energy release is critical. The reaction rate behind the shock front is extremely sensitive to temperature perturbations and, as a result, detonation waves in gases are always unstable. A broad spectrum of behavior has been reported for which no comprehensive theory has been developed. The problem is extremely challenging due to the nonlinearity of the chemistry-fluid mechanics coupling and extraordinary range of length and time scales exhibited in these flows. Past work has shown that the strength of the leading shock front oscillates and secondary shock waves propagate transversely to the main front. A key unresolved issue has emerged from the past 50 years of research on this problem: What is the precise nature of the flow within the reaction zone and how do the instabilities of the shock front influence the combustion mechanism?
\r\n\r\nThis issue has been examined through dynamic experimentation in two facilities. Key diagnostic tools include unique visualizations of superimposed shock and reaction fronts, as well as short but informative high-speed movies. We study a range of fuel-oxidizer systems, including hydrocarbons, and broadly categorize these mixtures by considering the hydrodynamic stability of the reaction zone. From these observations and calculations, we show that transverse shock waves do not essentially alter the classic detonation structure of Zeldovich-von Neumann-Doring (ZND) in weakly unstable detonations, there is one length scale in the instability, and the combustion mechanism is simply shock-induced chemical-thermal explosion behind a piecewise-smooth leading shock front. In contrast, we observe that highly unstable detonations have substantially different behavior involving large excursions in the lead shock strength, a rough leading shock front, and localized explosions within the reaction zone. The critical decay rate model of Eckett et al. (JFM 2000) is combined with experimental observations to show that one essential difference in highly unstable waves is that the shock and reaction front may decouple locally. It is not clear how the ZND model can be effectively applied in highly unstable waves. There is a spectrum of length scales and it may be possible that a type of \"turbulent\" combustion occurs. We consider how the coupling between chemistry and fluid dynamics can produce a large range of length scales and how possible combustion regimes within the front may be bounded.
" }, { "name": "Hung, Patrick Hin Fun", "degree": "PhD", "year": "2003", "title": "Algorithms for Reaction Mechanism Reduction and Numerical Simulation of Detonations Initiated by Projectiles", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05302003-142744", "creators": [ { "name": { "family": "Hung", "given": "Patrick Hin Fun" }, "id": "Hung-Patrick-Hin-Fun", "display_name": "Hung, Patrick Hin Fun" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "role": "member", "display_name": "Hunt, Melany L." } ], "option_major": [ "mecheng" ], "doi": "10.7907/H5GV-PV33", "abstract": "The evolution of a homogeneous, chemically reactive system with n species forms a dynamical system in chemical state-space. Under suitable constraints, unique and stable equilibrium exists and can be interpreted as zeroth-dimensional (point like) attractors in this n-dimensional space. At these equilibrium compositions, the rates of all reversible reactions vanish and can, in fact, be determined from thermodynamics independent of chemical kinetics.
\r\n\r\nGeneralizing this concept, an m-dimensional Intrinsic Low Dimensional Manifold (ILDM) represents an m-dimensional subspace in chemical state-space where all but the m-slowest aggregate reactions are in equilibrium, and these aggregate reactions are determined by eigenvalue considerations of the chemical kinetics. In this context, a certain composition is said to be m-dimensional if it is on an m-, but not an (m-1)-, dimensional ILDM.
\r\n\r\nTwo new algorithms are proposed that allow the dimensionality of chemical compositions be determined simply. The first method is based on recasting the Maas and Pope algorithm. The second, and more efficient, method is inspired by the mathematical structure of the Maas and Pope algorithm and makes use of the technique known as arc-length reparameterization. In addition, a new algorithm for the construction of ILDM, and the application of these ideas to detonation simulations, is discussed.
\r\n\r\nIn the second part of the thesis, numerical simulations of detonation waves initiated by hypervelocity projectiles are presented. Using detailed kinetics, only the shock-induced combustion regime is realized as simulating the conditions required for a stabilized detonation is beyond the reach of our current computational resources. Resorting to a one-step irreversible reaction model, the transition from shock-induced combustion to stabilized oblique detonation is observed, and an analysis of this transition based on the critical decay-rate model of Kaneshige (1999) is presented.
" }, { "name": "Koslowski, Marisol", "degree": "PhD", "year": "2003", "title": "A Phase-Field Model of Dislocations in Ductile Single Crystals", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05302003-094155", "creators": [ { "name": { "family": "Koslowski", "given": "Marisol" }, "id": "Koslowski-Marisol", "orcid": "0000-0001-9650-2168", "display_name": "Koslowski, Marisol" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Molinari", "given": "Alain" }, "id": "Molinari-A", "role": "member", "display_name": "Molinari, Alain" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SFMJ-1B50", "abstract": "A phase-field theory of dislocations, strain hardening and hysteresis in ductile single crystals is developed. The theory accounts for an arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations; the interaction between the dislocations and an applied resolved shear stress field; and the irreversible interactions with short-range obstacles, resulting in hardening, path dependency and hysteresis.
\r\n\r\nWe introduce a variational formulation for the statistical mechanics of dissipative systems. The influence of finite temperature as well as the mechanics in the phase-field theory are modeled with a Metropolis Monte Carlo algorithm and a mean field approximation.
\r\n\r\nA chief advantage of the present theory is that at zero temperature it is analytically tractable, in the sense that the complexity of the calculations may be reduced, with the aid of closed form analytical solutions, to the determination of the value of the phase field at point-obstacle sites. The theory predicts a range of behaviors which are in qualitative agreement with observation, including hardening and dislocation multiplication in single slip under monotonic loading; the Bauschinger effect under reverse loading; the fading memory effect; the evolution of the dislocation density under cycling loading; temperature softening; strain rate dependence; and others.
\r\n\r\nThe model also reproduces the formation of dislocation networks observed in grain boundaries for different crystal structures and orientations. Simultaneously with the stable configurations the theory naturally predicts the equilibrium dislocation density independently of initial values or sources.
" }, { "name": "Kumar, Sanjay", "degree": "PhD", "year": "2003", "title": "An Experimental Investigation of Richtmyer-Meshkov Instability", "advisor": "Hornung, Hans G.; Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-02212003-140109", "creators": [ { "name": { "family": "Kumar", "given": "Sanjay" }, "id": "Kumar-Sanjay", "display_name": "Kumar, Sanjay" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NKDR-TT39", "abstract": "In this study, the interaction of a shock wave with an interface between two gases is studied experimentally. The basic mechanism for the initial growth of perturbations on the interface is the baroclinic generation of vorticity which results from the misalignment of the pressure gradient in the shock and the density gradient at the interface. The growth of perturbations soon enters into a nonlinear regime with the appearance of bubbles of light fluid rising into heavy fluid and spikes of heavy fluid falling into light fluid. In the nonlinear regime, interaction between various scales and the appearance of other instabilities, such as Kelvin-Helmholtz instability, along the boundaries of the spikes occur, which results in the breakup of the interface. These processes lead to a turbulent mixing zone (TMZ) which grows with time. The main focus of this study is to understand the growth of TMZ with time in a cylindrical geometry with square cross section and for the the first time study the effect of area convergence in a conical geometry on its growth rate. The present set of experiments is done in the GALCIT 17 in. shock tube with air and sulfur hexafluoride as light and heavy gases. The growth of the TMZ is studied in a straight test section for single-mode initial perturbation consisting of two different wavelength and amplitude combinations at incident shock Mach number of 1.55. The multimode initial perturbation growth at late times is studied in a conical geometry to study the effect of area convergence at incident Mach numbers of 1.55 and 1.39. The results are compared with the experiments of Vetter which were done in the same shock tube with a straight test section with no area convergence and at the same Mach number.
\r\n\r\nIn the study of the Richtmyer-Meshkov (RM) instability of single-scale perturbations on air/sulfur-hexafluoride interface in a straight test section, the initially sinusoidal interface is formed by a polymeric membrane of thickness 1.5 micron and the flow visualization is done using schlieren imaging technique. The interface thickness is measured visually from the photographs. It is found that the growth rate decreases rapidly with time with a small dependence on the initial wavelength persisting until late times.
\r\n\r\nIn the case of the RM instability, growth of multimode initial perturbations in a conical geometry, it is found from the schlieren flow visualization images that the interface thickness grows about 40-50 % more rapidly than in Vetter's experiments. Experimental results for laser-induced scattering at late times are presented for air/He gas combinations at the interface. In situations when the rear of the interface is not clearly demarcated, the thickness is determined by an image processing technique. This technique is also used to determine the possible dominant eddy/blob size in the TMZ from the schlieren images. Some inviscid computational studies, with a planar or spherical shock interacting with a planar or spherical initial interface in light-heavy (air/sulfur-hexafluoride) and heavy-light (air/He) configurations, are also presented. In the conical geometry there is a reflected shock originating from the triple point. This reflection is a consequence of the transition from the cylindrical shock tube to the converging cone. Due to the vorticity created by the interaction of reflected shock from the cone wall with the interface in initial stage, it is found that the interface curves toward or away from the apex of the cone, depending on the sign of density gradient. This curving of the interface could have a role to play in the diffuse rear boundary of the interface in schlieren flow visualization images but the laser-induced scattering image shows that the mixing zone indeed does not have a well-defined rear boundary. Rather, small blobs of fluids on the right are scattered in the mixing zone. An inviscid computational study is also done on cylindrical and conical test section geometries to study the effect of transverse reflected waves on the growth of small sinusoidal initial perturbations. It is found by comparison with cylindrical geometry (where reflected waves do not exist) that the transverse reflected waves do not affect the growth of perturbations on the interface.
" }, { "name": "Lew, Adri\u00e1n Jos\u00e9", "degree": "PhD", "year": "2003", "title": "Variational Time Integrators in Computational Solid Mechanics", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05262003-200254", "creators": [ { "name": { "family": "Lew", "given": "Adri\u00e1n Jos\u00e9" }, "id": "Lew-Adri\u00e1n-Jos\u00e9", "display_name": "Lew, Adri\u00e1n Jos\u00e9" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Sulsky", "given": "Deborah" }, "id": "Sulsky-D", "role": "member", "display_name": "Sulsky, Deborah" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "member", "display_name": "Marsden, Jerrold E." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6C74-GC16", "abstract": "This thesis develops the theory and implementation of variational integrators for computational solid mechanics problems, and to some extent, for fluid mechanics problems as well. Variational integrators for finite dimensional mechanical systems are succinctly reviewed, and used as the foundations for the extension to continuum systems. The latter is accomplished by way of a space-time formulation for Lagrangian continuum mechanics that unifies the derivation of the balance of linear momentum, energy and configurational forces, all of them as Euler-Lagrange equations of an extended Hamilton's principle. In this formulation, energy conservation and the path independence of the J- and L-integrals are conserved quantities emanating from Noether's theorem. Variational integrators for continuum mechanics are constructed by mimicking this variational structure, and a discrete Noether's theorem for rather general space-time discretizations is presented. Additionally, the algorithms are automatically (multi)symplectic, and the (multi)symplectic form is uniquely defined by the theory. For instance, in nonlinear elastodynamics the algorithms exactly preserve linear and angular momenta, whenever the continuous system does.
\r\n\r\nA class of variational algorithms is constructed, termed asynchronous variational integrators (AVI), which permit the selection of independent time steps in each element of a finite element mesh, and the local time steps need not bear an integral relation to each other. The conservation properties of both synchronous and asynchronous variational integrators are discussed in detail. In particular, AVI are found to nearly conserve energy both locally and globally, a distinguishing feature of variational integrators. The possibility of adapting the elemental time step to exactly satisfy the local energy balance equation, obtained from the extended variational principle, is analyzed. The AVI are also extended to include dissipative systems. The excellent accuracy, conservation and convergence characteristics of AVI are demonstrated via selected numerical examples, both for conservative and dissipative systems. In these tests AVI are found to result in substantial speedups, at equal accuracy, relative to explicit Newmark.
\r\n\r\nIn elastostatics, the variational structure leads to the formulation of discrete path-independent integrals and a characterization of the configurational forces acting in discrete systems. A notable example is a discrete, path-independent J-integral at the tip of a crack in a finite element mesh.
" }, { "name": "Pottebaum, Tait Sherman", "degree": "PhD", "year": "2003", "title": "The Relationship Between Near-Wake Structure and Heat Transfer for an Oscillating Circular Cylinder in Cross-Flow", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-05202003-145011", "creators": [ { "name": { "family": "Pottebaum", "given": "Tait Sherman" }, "id": "Pottebaum-Tait-Sherman", "display_name": "Pottebaum, Tait Sherman" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Richardson", "given": "Mark I." }, "id": "Richardson-M-I", "role": "member", "display_name": "Richardson, Mark I." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "member", "display_name": "Hunt, Melany L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/C9NQ-N832", "abstract": "A series of experiments were carried out in order to understand the relationship between wake structure and heat transfer for a transversely oscillating circular cylinder in cross-flow and to explore the dynamics of the vortex formation process in the wake. The cylinder's heat transfer coefficient was determined over a range of oscillation amplitudes up to 1.5 cylinder diameters and oscillation frequencies up to 5 times the stationary cylinder natural shedding frequency. The results were compared to established relationships between oscillation conditions and wake structure. Digital particle image thermometry/velocimetry (DPIT/V) was used to measure the temperature and velocity fields in the near-wake for a set of cases chosen to be representative of the variety of wake structures that exist for this type of flow. The experiments were carried out in a water tunnel at a Reynolds number of 690.
\r\n\r\nIt was found that wake structure and heat transfer both significantly affect one another. The wake mode, a label indicating the number and type of vortices shed in each oscillation period, is directly related to the observed heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, explain this relationship. The streamwise spacing between shed vortices was also shown to affect heat transfer coefficient for the 2S mode, which consists of two single vortices shed per cycle. The streamwise spacing is believed to influence entrainment of freestream temperature fluid by the forming vortices, thereby affecting the temperature gradient at the cylinder base. This effect may exist for other wake modes, as well.
\r\n\r\nThe cylinder's transverse velocity was shown to influence the heat transfer by affecting the circulation of the wake vortices. For a fixed wake structure, the effectiveness of the wake vortices at enhancing heat transfer depends on their circulation. Also, the cylinder's transverse velocity continually changes the orientation of the wake with respect to the freestream flow, thereby spreading the main source of heat transfer enhancement--the vortices near the cylinder base--over a larger portion of the cylinder surface.
\r\n\r\nPreviously observed heat transfer enhancement associated with oscillations at frequencies near the natural shedding frequency and its harmonics were shown to be limited to amplitudes of less than about 0.5 cylinder diameters.
\r\n\r\nA new phenomenon was discovered in which the wake structure switches back and forth between distinct wake modes. Temperature induced variations in the fluid viscosity are believed to be the cause of this mode-switching. It is hypothesized that the viscosity variations change the vorticity and kinetic energy fluxes into the wake, thereby changing the wake mode and the heat transfer coefficient. This discovery underscores the role of viscosity and shear layer fluxes in determining wake mode, potentially leading to improved understanding of wake vortex formation and pinch-off processes in general.
\r\n\r\nAspect ratio appears to play a role in determining the heat transfer coefficient mainly for non-oscillating cylinders. The heat transfer is also affected by aspect ratio for oscillation conditions characterized by weak synchronization of the wake to the oscillation frequency.
" }, { "name": "Thoutireddy, Pururav", "degree": "PhD", "year": "2003", "title": "Variational Arbitrary Lagrangian-Eulerian Method", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-05292003-113845", "creators": [ { "name": { "family": "Thoutireddy", "given": "Pururav" }, "id": "Thoutireddy-Pururav", "display_name": "Thoutireddy, Pururav" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "chair", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Marsden", "given": "Jerrold E." }, "id": "Marsden-J-E", "role": "member", "display_name": "Marsden, Jerrold E." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DQT0-5104", "abstract": "This thesis is concerned with the development of Variational Arbitrary Lagrangian-Eulerian method (VALE) method. VALE is essentially finite element method generalized to account for horizontal variations, in particular, variations in nodal coordinates. The distinguishing characteristic of the method is that the variational principle simultaneously supplies the solution, the optimal mesh and, in case problems of shape optimization, optimal shape. This is accomplished by rendering the functional associated with the variational principle stationary with respect to nodal field values as well as with respect to the nodal positions of triangulation of the domain of analysis. Stationarity with respect to the nodal positions has the effect of the equilibriating the energetic or configurational forces acting in the nodes. Further, configurational force equilibrium provides precise criterion for mesh optimality. The solution so obtained corresponds to minimum of energy functional (minimum principle) in static case and to the stationarity of action sum (discrete Hamilton's stationarity principle) in dynamic case, with respect to both nodal variables and nodal positions. Further, the resulting mesh adaption scheme is devoid of error estimates and mesh-to-mesh transfer interpolation errors. We illustrate the versatility and convergence characteristics of the method by way of selected numerical tests and applications, including the problem of semi-infinite crack, the shape optimization of elastic inclusions and free vibration of 1-d rod." }, { "name": "Chasiotis, Ioannis", "degree": "PhD", "year": "2002", "title": "The Strength of Polycrystalline Silicon at the Micro- and Nano-Scales with Applications to MEMS", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-09142005-105805", "creators": [ { "name": { "family": "Chasiotis", "given": "Ioannis" }, "id": "Chasiotis-Ioannis", "display_name": "Chasiotis, Ioannis" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "chair", "display_name": "Goodwin, David G." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Ustundag", "given": "Ersan" }, "id": "Ustundag-E", "orcid": "0000-0002-0812-7028", "role": "member", "display_name": "Ustundag, Ersan" }, { "name": { "family": "Tai", "given": "Yu-Chong" }, "id": "Tai-Yu-Chong", "orcid": "0000-0001-8529-106X", "role": "member", "display_name": "Tai, Yu-Chong" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JPRZ-P277", "abstract": "Three aspects concerning the reliability of MicroElectroMechanical Systems (MEMS) are discussed in this work. These aspects are: (1) the development of a new tensile testing technique for measuring the elastic modulus and rupture strengths of thin films, (2) an assessment of stress concentration and specimen size effects in failure of micron-sized specimens and (3) the consequences of Hydrofluoric Acid (HF) chemical treatment on the microstructural integrity and the tensile strength of polycrystalline silicon.
\r\n\r\nA new method for tensile testing of thin films by means of an improved apparatus has been developed to measure the elastic properties (Young's modulus, tensile strength) of surface micromachined polycrystalline silicon specimens. The newly designed tensile tester makes use of an Ultraviolet (UV) light curable adhesive to clamp micron-sized specimens. It permits for the first time the testing of thin film materials possessing high failure strength. The properties determination utilizes surface topologies of deforming specimens, acquired with an Atomic Force Microscope (AFM), for determining strain fields by means of Digital Image Correlation (DIC). This full-field, direct and local measurements technique provides the capability of testing any type of thin film materials with nanometer resolution. The gage section of the specimens tested in this study varied between 200 and 1000 \u00b5m in length, 6 and 50 \u00b5m in width, all for a nominal thickness of 2 \u00b5m.
\r\n\r\nThe dependence of fracture strength on micron and sub-micron sized geometries was studied by means of specimens containing various degrees of stress concentrations. A systematic study of small-scale size effects was thus performed by tensioning elliptically perforated specimens (minimum radius of curvature of 1 gm) so as to: (a) vary the stress concentration with constant radius of curvature, (b) increasing radius of curvature of micronotches relative to the grain size. The results demonstrate a strong influence of the size of the highly strained domain (decreasing notch radii) on the failure strength of MEMS scale specimens, while the effect of varying the stress concentration factor is rather insignificant. In addition, tests performed on unnotched tensile specimens of varying dimensions revealed a specimen size effect by which the values of strength scaled with the specimen length. The Young's modulus, however, is found to be rather insensitive to the specimen dimensions at the scale of microns.
\r\n\r\nIn an effort to assess the unexplained and puzzling large variation in properties reported for very small polysilicon specimens a study was conducted to search for a cause. Contrary to the common belief that 49% HF wet release represents a safe post-process for manufacturing polycrystalline silicon, this study has clearly identified the release process as a key item in determining thin film failure properties. It is found that surface roughness as characterized by groove formation at the grain boundaries depends distinctly on the HF release time. In addition, while the actual failure mechanism in polysilicon follows a transgranular fracture, moderate exposure in HF results in partial intergranular fracture at the film surface that is responsible for complete failure. Long exposures yield films of low mechanical strength that demonstrate clear intergranular failure.
" }, { "name": "Gonz\u00e1lez Li\u00f1ero, Luis", "degree": "PhD", "year": "2002", "title": "Global Fracture Analysis of Laminated Composite Materials for Aerospace Structures", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechTHESIS:05032011-085654224", "creators": [ { "name": { "family": "Gonz\u00e1lez Li\u00f1ero", "given": "Luis" }, "id": "Gonz\u00e1lez-Li\u00f1ero-Luis", "display_name": "Gonz\u00e1lez Li\u00f1ero, Luis" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Jennings", "given": "Paul C." }, "id": "Jennings-P-C", "role": "member", "display_name": "Jennings, Paul C." }, { "name": { "family": "Phillips", "given": "Robert B." }, "id": "Phillips-R", "orcid": "0000-0003-3082-2809", "role": "member", "display_name": "Phillips, Robert B." }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FCJ8-EW63", "abstract": "The failure process of laminated composite materials originating from precut sharp cracks, as well as their propagation, is studied from a \"global\" perspective,\r\nappropriate for structural analysis. The size effect in the damage development is explored and the question of \"scaling\" of the results is addressed.\r\n\r\nTwo globally orthotropic sets of panels with the notches aligned along the axes of orthrotopy are studied. The internally evolving damage in the crack tip region is\r\nexamined through enhanced x-ray radiographic inspection and surface strain fields are measured by means of the Digital Image Correlation method (the applicability and limitations of which are analyzed and discussed). The results obtained from these two experimental techniques are joined to assess the feasibility of identifying internal damage solely from surface measurements.\r\n\r\nThe shape of the region of influence of the crack is described and its extension measured. A simplified model for damage progression analysis is proposed.\r\n\r\nThe process of initiation of the damage propagation is described in detail and the different responses for the two different layups are discussed. The maximum stress/strain and the Tsai-Hill failure criteria are compared with the experimental results on the laminates, and their reliability and limitations are addressed.\r\n\r\nThe effective properties of the two sets of laminates are measured at three different loading rates and compared to theory, and the relevance of the time dependence of the material is studied.\r\n" }, { "name": "Nguyen, Olivier Thanh", "degree": "PhD", "year": "2002", "title": "Cohesive Models of Fatigue Crack Growth and Stress-Corrosion Cracking", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-12032004-161201", "creators": [ { "name": { "family": "Nguyen", "given": "Olivier Thanh" }, "id": "Nguyen-Olivier-Thanh", "display_name": "Nguyen, Olivier Thanh" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Pandolfi", "given": "Anna" }, "id": "Pandolfi-A", "role": "member", "display_name": "Pandolfi, Anna" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/C3KP-4M44", "abstract": "The aim of this dissertation was to develop models of fatigue crack growth and stress-corrosion cracking by investigating cohesive theories of fracture. These models were integrated in a finite-element framework embedding a contact algorithm and techniques of remeshing and adaptive meshing.\r\n\r\nFor the fatigue model, we developed a phenomenological cohesive law which exhibits unloading-reloading hysteresis. This model qualitatively predicts fatigue crack growth rates in metals under constant amplitude regime for short and long cracks, as well as growth retardation due to overload. Quantitative predictions were obtained in the case of long cracks.\r\n\r\nWe developed a chemistry-dependent cohesive law which serves as a basis for the stress-corrosion cracking model. In order to determine this cohesive law, two approaches, based on energy relaxation and the renormalization group, were used for coarse-graining interplanar potentials. We analyzed the cohesive behavior of a large--but finite--number of interatomic planes and found that the macroscopic cohesive law adopts a universal asymptotic form. The resulting stress-corrosion crack growth rates agreed well with those observed experimentally in 'static' fatigue tests given in the literature.\r\n" }, { "name": "Xu, Luoyu Roy", "degree": "PhD", "year": "2002", "title": "Dynamic Failure Characteristics in Layered Materials and Structures", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04252011-111825843", "creators": [ { "name": { "family": "Xu", "given": "Luoyu Roy" }, "id": "Xu-Luoyu-Roy", "display_name": "Xu, Luoyu Roy" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Johnson", "given": "William Lewis" }, "id": "Johnson-W-L", "role": "member", "display_name": "Johnson, William Lewis" }, { "name": { "family": "Heaton", "given": "Thomas H." }, "id": "Heaton-T-H", "orcid": "0000-0003-3363-2197", "role": "member", "display_name": "Heaton, Thomas H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/wver-8342", "abstract": "Systematic investigations were carried out to understand the general nature of dynamic failure mechanisms in layered materials and structures such as composite and\r\nsandwich structures, thin films, layered armors and layered rock. A series of impact experiments on model-layered specimens were conducted using high-speed photography\r\nand dynamic photoelasticity.\r\n\r\nFor the first time, the sequence and interaction of two major dynamic failure modes in layered materials-inter-layer cracking and intra-layer cracking were revealed\r\nin real time. For heterogeneous three-layer systems, shear-dominated inter-layer cracking was always the first failure event for specimens subjected to low-speed impact. Interlayer cracking generally nucleated from interfacial locations where the inter-layer shear stress acquired a local maximum. Depending on impact speed and bond strength\r\ncharacteristics, inter-layer cracks were very transient and often became intersonic even under moderate impact speeds. Intra-layer cracking always initiated after the development of inter-layer cracks as a result of inter-layer crack kinking into the adjacent layer. The resulting intra-layer mode I cracks often accelerated and branched as they\r\nattained high speeds, causing core layer fragmentation. For homogenous-layered systems composed of bonded layers of Homalite, intra-layer cracks appeared in the form of cracks\r\nradiating from the impact site. As soon as these cracks approached an interface, interlayer cracks were often induced depending on the angle between the crack path and the interface. Direct experimental evidence of the dynamic equivalent of \"Cook-Gordon mechanism\" was recorded, i.e., two intersonic interfacial cracks nucleated and propagated along the interface before a fan of mode I incident cracks was ever able to reach the interface. Also, significant dependence of the failure characteristics on impact speeds and interfacial strengths was found. For the heterogeneous three-layer system subjected to a high impact speed, two clear shear shock waves associated with the intersonic inter-layer cracks were observed at the specimen center. Shock waves were also observed along the interface in heterogeneous three-layer systems featuring weak and ductile bonds. The impact momentum and loading duration were identified as two important parameters in damage spreading for a given impact energy.\r\n\r\nMotivated by the experimental observations of crack deflection/penetration at an interface, a novel wedge-loaded impact specimen was designed to explore the basic\r\nmechanics nature of this phenomenon. The deflection/penetration behavior of an incoming dynamic crack at an interface was found to depend on the interfacial angle and the interfacial fracture toughness. A dynamic fracture model, together with an energy criterion, were proposed and were found to agree reasonably well with the experimental observations." }, { "name": "Zhuang, Shiming", "degree": "PhD", "year": "2002", "title": "Shock Wave Propagation in Periodically Layered Composites", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechTHESIS:05102011-141326530", "creators": [ { "name": { "family": "Zhuang", "given": "Shiming" }, "id": "Zhuang-Shiming", "display_name": "Zhuang, Shiming" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Grady", "given": "Dennis E." }, "id": "Grady-D-E", "role": "member", "display_name": "Grady, Dennis E." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/988X-1V27", "abstract": "Mathematically, a shock wave is treated as a discontinuity in a medium. In reality, however, a shock wave is always structured, i.e., its front takes a finite time to rise from an initial material state to the final shocked state. The structuring of a shock front is due to the competition between the nonlinearity of material behavior and the dissipation processes occurring during the wave propagation. There are many mechanisms which may be responsible for the dissipation and/or dispersion of shock wave energy. In homogeneous media, such as metals, one common interpretation for the structuring of a shock wave is that the viscoplasticity processes (dislocation, twinning, etc.) are responsible for the dissipation of energy. While in heterogeneous composites, besides the viscous dissipative processes existing in each of its constituents, due to the existence of internal interfaces, the scattering induced by the interface during shock compression could be another important mechanism.\r\n\r\nIn this study, the interface scattering effects on shock wave propagation in heterogeneous media were investigated by subjecting periodically layered composites to planar impact loading with a flyer plate. The flyer plate was accelerated to a desired velocity using a powder gun loading system. In order to measure shock particle velocity time history at an internal or the free surface of the specimen, the so-called VISAR (Velocity Interferometry System for Any Reflector) diagnostic system was constructed and used during shock compression experiments. Manganin stress gages were embedded inside the specimen at selected internal interfaces to measure shock stress time history. To study the scattering mechanisms of the interface to waves, two-component composite specimens with different interface mechanical properties and heterogeneity were prepared and tested. Different types of composites were prepared with differing mechanical impedance. Specimens with different heterogeneity were obtained by changing the geometrical configuration (length scale) of the layered stack. Two-dimensional numerical simulations were also carried out to understand the process of shock wave evolution in the layered composites.\r\n\r\nExperimental and numerical studies show that periodically layered composites support steady structured shock waves. The influence of internal interfaces on the shock wave propagation is through the scattering mechanism, i.e., multiple reflection of waves in the layers and their interaction with the shock wave. The interface scattering affects both the bulk and the deviatoric response of the composite to shock compression. The influence of scattering on the bulk behavior is to slow down the velocity of the shock wave in the composites, while its influence on the deviatoric response is to structure the shock wave profile. If all the dissipative and dispersive effects are collectively termed as viscosity, which causes the shock front structuring, i.e., the shock front rise-time increasing, then the effective shock viscosity increases with the increase of interface impedance mismatch and decreases with the increase of interface density (interface area per unit volume) and shock loading strength. The existing mixture model for constructing the constitutive relation for composites based on the known properties of its component materials can only, at best, reasonably predict the response of the composites under strong shock loading conditions. In order to fully describe the response of a heterogeneous composite to shock compression loading, accurate physics-based constitutive relations need to be formulated to take into account the scattering effects induced by the heterogeneous microstructure.\r\n" }, { "name": "Burcsu, Eric Noboru", "degree": "PhD", "year": "2001", "title": "Investigation of Large Strain Actuation in Barium Titanate", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-10232001-192042", "creators": [ { "name": { "family": "Burcsu", "given": "Eric Noboru" }, "id": "Burcsu-Eric-Noboru", "display_name": "Burcsu, Eric Noboru" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "co-chair", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "orcid": "0000-0001-5877-4824", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Haile", "given": "Sossina M." }, "id": "Haile-S-M", "orcid": "0000-0002-5293-6252", "role": "member", "display_name": "Haile, Sossina M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XT3Y-Z860", "abstract": "Sensors and actuators based on ferroelectric materials have become indispensable in the fields of aerospace, high technology, and medical instruments. Most devices rely on the linear piezoelectric behavior of formulations of PZT which offer high bandwidth, linear actuation but very low strains of around 0.1%. The nonlinear electromechanical behavior of these materials is largely governed by the motion of domains and is highly affected by stress as well as electric field. The recent theories of Shu and Bhattacharya have sought to address some of the issues related to the structure and behavior of these materials at the mesoscale. One result of the theories is the prediction of another mode of actuation in ferroelectric crystals based on a combined electrical and mechanical loading that could result in strains of up to 6%.
\r\n\r\nDescriptions of the phenomenological theories of ferroelectrics are presented including the classical Landau-Ginsburg-Devonshire theory and the more recent theory of Shu and Bhattacharya. Predictions are made, based on the theory, of the electromechanical behavior of ferroelectric crystals that are addressed by the experiments. An experimental setup has been designed to investigate large strain actuation in single crystal ferroelectrics based on combined electrical and mechanical loading. An investigation of the stress dependence of the electrostrictive response has been carried out with in situ observations of the domain patterns under constant compressive stress and variable electric field. Experiments have been performed on initially single domain crystals of barium titanate with (100) and (001) orientation at compressive stresses between 0 and 5 MPa. Global strain and polarization histories have been recorded. The electrostrictive response is shown to be highly dependent on the level of applied stress with a maximum strain of 0.9% measured at a compressive stress of about 2 MPa. An unusual secondary hysteresis has been observed in the polarization signal at high levels of stress that indicates an intermediate structural configuration, possibly the orthorhombic state. Polarized light microscopy has been used to observe the evolution of the domain pattern simultaneously with the strain and polarization measurement. These results are discussed and suggestions for future work are proposed.
" }, { "name": "Ching, Weng Ki", "degree": "PhD", "year": "2001", "title": "Disinfection by Pulsed Power Discharges", "advisor": "Hoffmann, Michael R.", "url": "https://resolver.caltech.edu/CaltechETD:etd-07252005-142630", "creators": [ { "name": { "family": "Ching", "given": "Weng Ki" }, "id": "Ching-Weng-Ki", "display_name": "Ching, Weng Ki" } ], "advisors": [ { "name": { "family": "Hoffmann", "given": "Michael R." }, "id": "Hoffmann-M-R", "orcid": "0000-0001-6495-1946", "role": "advisor", "display_name": "Hoffmann, Michael R." } ], "committee": [ { "name": { "family": "Hoffmann", "given": "Michael R." }, "id": "Hoffmann-M-R", "orcid": "0000-0001-6495-1946", "role": "chair", "display_name": "Hoffmann, Michael R." }, { "name": { "family": "Nealson", "given": "Kenneth H." }, "id": "Nealson-K-H", "orcid": "0000-0001-5189-3732", "role": "member", "display_name": "Nealson, Kenneth H." }, { "name": { "family": "Newman", "given": "Dianne K." }, "id": "Newman-D-K", "orcid": "0000-0003-1647-1918", "role": "member", "display_name": "Newman, Dianne K." }, { "name": { "family": "Leadbetter", "given": "Jared R." }, "id": "Leadbetter-J-R", "orcid": "0000-0002-7033-0844", "role": "member", "display_name": "Leadbetter, Jared R." } ], "option_major": [ "envreng" ], "doi": "10.7907/RV3M-0849", "abstract": "We study the disinfection of Escherichia coli in a pulsed power discharge reactor. The pulsed power discharge process is an electrohydraulic phenomenon characterized by a rapid release of electrical energy across a submerged electrode pair.
\r\n\t\r\nThe survival kinetics of pure strains of E. coli suspensions exposed to 5.5 kV, 90 kA electrohydraulic discharges (EHD) is investigated. The probability of survival Pn of a 2 x 10\u2077 E. coli CFU mL\u207b\u00b9 population after 50 consecutive EHD's follows a logit distribution that corresponds to lethal doses of LD\u2085\u2080 = 2.2 and L:D\u2089\u2080 = 10.8 EHDs. Variation of the initial cell concentration produced nearly constant values of LD\u2085\u2080 = 0.9\u00b10.1 in the range 2 x 10\u00b3 \u2264 E. coli/CFU mL\u207b\u00b9 \u2264 2 x 10\u2076. Beyond 10\u2076 CFU mL\u207b\u00b9, the LD\u2085\u2080 values increase exponentially due to nonlinear light absorbance with increasing E. coli concentrations. Qualitatively similar initial cell concentration dependence is observed for survival under low intensity 254 nm irradiation, in contrast with lower values of LD\u2085\u2080 obtained in denser colonies to 20 kHz power ultrasound exposure.
\r\n\r\nThe high intensity (3.3 x 10\u00b9\u2070 W m\u207b\u00b2) ultraviolet radiation emitted by the electrohydraulic discharge is completely suppressed in the presence of less than 100 mg L\u207b\u00b9 2,2'-dihydroxy-4,4'-dimethoxybenzophenone-5,5'-disulfonic acid (BP9), a well-known sunscreen agent. Concentrations of BP9, in the range (0 - 100) mg L\u207b\u00b9, are varied to measure the sterilization kinetics of ~3 x 10\u2077 CFU mL\u207b\u00b9 suspensions to varying degrees of high intensity UV exposure. The slope of the logit plots of E. coli as function of BP9 concentration is consistent with the screening of radiation as the sole lethal agent. Computed values of biologically available light fluences are 5.0 x 10\u00b9\u00b9 photons CFU\u207b\u00b9 for high intensity, high power, pulsed EHD experiments and 6.6 x 10\u2078 photons CFU\u207b\u00b9 for separate low power continuous UV experiments. The net availability of 3 orders of magnitude more photons during high intensity UV exposure suggests the possibility of a multiphotonic disinfection mechanism at play in the EHD process relative to low intensity case.
\r\n\r\nThe overall resistance to long term exposure to EHD is also investigated. Selective pressure experiments with E. coli exposed to 11 cycles of 50 consecutive EHDs each show a weak kinetic change in the dose-response curves reflected in the nearly constant values of LD\u2085\u2080 - 0.24 \u00b1 0.03. A greater than 98% metabolic similarity in carbon source consumption between initial and final E. coli populations are enzymatically related. In addition, the results indicate that no bacterial contaminants are propagated throughout the experiment.
" }, { "name": "Chow, Benjamin Bin", "degree": "PhD", "year": "2001", "title": "Application of dynamic fracture mechanics to the investigation of catastrophic failure in aircraft structures", "advisor": "Rosakis, Ares J.; Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-08112005-103246", "creators": [ { "name": { "family": "Chow", "given": "Benjamin Bin" }, "id": "Chow-B-B", "display_name": "Chow, Benjamin Bin" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/745f-mb29", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA dynamic fracture mechanics approach to the estimation of the residual strength of aircraft structures is presented. The dependence of the dynamic crack initiation toughness of aluminum 2024-T3 on loading rate is first studied experimentally. A drop of up to 40% in the value of dynamic initiation toughness, [...], is discovered for loading rates in the range of [...]. This range of loading rate corresponds to the typical rates found in an aircraft fuselage experiencing explosive loading conditions. A dramatic increase in the value of dynamic crack initiation toughness is also found for loading rates above [...]. Based on these results and on established dynamic fracture mechanic concepts, a fracture mechanics based failure model is established and is used to estimate the residual strength of aircraft structures.\r\n\r\nA methodology to determine residual strength of dynamically loaded structures based on global structural analysis coupled with local finite element analysis is introduced. Local finite element calculations were performed for different loading rates, [...], ranging from [...] to [...], to simulate the conditions encountered in an explosively loaded aircraft fuselage. Simulations were conducted at a number of loading rates for the following cases of relevance to aircraft fuselage: (i) center cracked panels, (ii) rivet holes with wing cracks, (iii) biaxially loaded panels and (iv) panels prestressed to simulate pressurization. The results from the analyses were then used in conjunction with the experimental results for the dynamic fracture toughness of a 2024-T3 aluminum alloy as a function of loading rate, [...], to determine the time to failure, [...], for a given loading rate. A failure envelope, [...], based on the failure model and finite element analysis, is presented for the different cases and the implications for the residual strength of aircraft structures is discussed.\r\n\r\nMixed mode dynamic crack initiation in aluminum 2024-T3 alloy is investigated by combining experiments with numerical simulations. Pre-fatigued single edge notched specimens and three point bend specimens are subjected to dynamic symmetric and asymmetric loading to generate a range of mode mixity at the cracktip. The optical technique of coherent gradient sensing (CGS) and a strain gage method are employed to study the evolution of the mixed mode stress intensity factors. The dynamic mixed mode failure envelope is obtained using the crack initiation data from the experiments at a nominal loading rate of [...] and is compared with the static counterpart for 2024-T3 aluminum alloy. The fracture surfaces near the crack initiation site are investigated using a scanning electron microscope and reveal ductile void growth and coalescence. Numerical simulations of the experiments are conducted to both help in designing the experiments and to validate the results of the experiments. The numerical simulations show good correlation with the experimental results." }, { "name": "Coker, Demirkan", "degree": "PhD", "year": "2001", "title": "Dynamic Initiation and Propagation of Cracks in Unidirectional Composite Plates", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:10112010-152127073", "creators": [ { "name": { "family": "Coker", "given": "Demirkan" }, "id": "Coker-Demirkan", "orcid": "0000-0001-7385-7089", "display_name": "Coker, Demirkan" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Heaton", "given": "Thomas H." }, "id": "Heaton-T-H", "role": "member", "display_name": "Heaton, Thomas H." }, { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "role": "member", "display_name": "Kanamori, Hiroo" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/yrm2-4b88", "abstract": "Dynamic crack growth along weak planes is a significant mode of failure in composites and other layered/sandwiched structures and is also the principal mechanism of shallow crustal earthquakes. In order to shed light on this phenomenon dynamic crack initiation and propagation characteristics of a model fiber-reinforced unidirectional graphite/epoxy composite plate was investigated experimentally. Dynamic fracture experiments were conducted by subjecting the composite plates to in-plane, symmetric and asymmetric, impact loading. The lateral shearing interferometric technique of coherent gradient sensing (CGS) in conjunction with high-speed photography was used to visualize the failure process in real time. It was found that mode-I cracks propagated subsonically with crack speeds increasing to the neighborhood of the Rayleigh wave speed of the composite. Also in mode-I, the dependence of the dynamic initiation fracture toughness on the loading rate was determined and was found to be constant for low loading rates and to increase rapidly above K̇dI > 10\u2075. The dynamic crack propagation toughness, KID, was observed to decrease with crack tip speed up to the Rayleigh wave speed of the composite.
\r\n\r\nFor asymmetric, mode-II, types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibers. These cracks propagated with unprecedented speeds reaching 7400 m/s which is the dilatational wave speed of the composite along the fibers. For intersonic crack growth, the interferograms featured a shock wave structure typical of disturbances traveling with speeds higher than one of the characteristic wave speeds in the solid. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating non-uniform crack face frictional contact. In addition, shear dominated dynamic crack growth is investigated along composite/Homalite interfaces subjected to impact loading. The crack growth phenomenon was observed usivvvvng dynamic photoelasticity in conjunction with high-speed photography. Three quantized intersonic and supersonic crack tip speed regimes were identified. First conclusive evidence of crack growth at supersonic speeds with respect to lower speed material and sonic speeds with respect to the unidirectional composite was obtained. Furthermore, this investigation documents the first experimental observation of a mother/daughter crack mechanism allowing a subsonic crack to evolve into an intersonic crack.
\r\n\r\n" }, { "name": "Duchemin, Olivier Bernard", "degree": "PhD", "year": "2001", "title": "An investigation of ion engine erosion by low energy sputtering", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02242002-122344", "creators": [ { "name": { "family": "Duchemin", "given": "Olivier Bernard" }, "id": "Duchemin-O-B", "display_name": "Duchemin, Olivier Bernard" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "co-chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Polk", "given": "James E." }, "id": "Polk-J-E", "role": "member", "display_name": "Polk, James E." }, { "name": { "family": "Brophy", "given": "John R." }, "id": "Brophy-J-R", "role": "member", "display_name": "Brophy, John R." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K408-J123", "abstract": "Unlike chemical propulsion systems, which are fundamentally limited in performance by propellant energy density, electric propulsion devices, such as ion engines, are limited in iotal deliverable impulse by maximum propellant throughput due to engine wear.\r\n\r\nIn order to perform realistic modeling of engine lifetime, the erosion mechanisms involved must be understood. In particular, the damage---or sputtering---caused by slow ions on solid surfaces is extremely difficult to quantify. We first review the engine failure modes in which sputtering of molybdenum by slow xenon ions plays a critical role. We then present the relevant physical mechanisms, and describe a model for estimating the minimum kinetic energy necessary to dislodge a surface atom.\r\n\r\nOver seventeen analytical approaches to the energy dependence of sputtering have been published in the literature. We implement the four that are most relevant to ion engine erosion processes. In addition, we use the Monte-Carlo simulation program TRIM to calculate sputtering yields. We find, in particular, that the relative sensitivity of sputtering yield to surface binding energy increases dramatically near the sputtering threshold energy. Although the surface binding energy is a (weak) function of temperature, we show that the sputtering yield should not increase significantly at temperatures typical of ion engine operation.\r\n\r\nAn experimental approach to the measurement of low energy sputtering yields is implemented and validated. Based on the Quartz Crystal Microbalance (QCM) technique, this method takes advantage of the differential mass sensitivity exhibited by the piezoelectric quartz resonator used in this study. Because of the importance of surface contamination in low energy sputtering, a surface kinetics model is presented to describe a surface under the simultaneous cleaning effect of ion bombardment, and background gas flow contamination.\r\n\r\nA special case of simultaneous surface contamination and erosion occurs during engine ground testing, where carbon is backsputtered on the accelerator grid from the facility. We describe experiments to measure ion-induced desorption cross-sections for carbon on molybdenum, before concluding that the protective effect of the carbon contamination is unlikely to significantly affect engine erosion, so that ground testing results are applicable to space operations" }, { "name": "Eckett, Christopher Adam", "degree": "PhD", "year": "2001", "title": "Numerical and Analytical Studies of the Dynamics of Gaseous Detonations", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-143255", "creators": [ { "name": { "family": "Eckett", "given": "Christopher Adam" }, "id": "Eckett-Christopher-Adam", "display_name": "Eckett, Christopher Adam" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/83ME-8076", "abstract": "This thesis examines two dynamic parameters of gaseous detonations, critical energy and cell size. The first part is concerned with the direct initiation of gaseous detonations by a blast wave and the associated critical energy. Numerical simulations of the spherically symmetric direct initiation event with a simple chemical reaction model are presented. Local analysis of the computed unsteady reaction zone structure identities a competition between heat release rate, front curvature and unsteadiness. The primary failure mechanism is found to be unsteadiness in the induction zone arising from the deceleration of the shock front. On this basis, simplifying assumptions are applied to the governing equations, permitting solution of an analytical model for the critical shock decay rate. The local analysis is validated by integration of reaction zone structure equations with detailed chemical kinetics and prescribed unsteadiness. The model is then applied to the global initiation problem to produce an analytical equation for the critical energy. Unlike previous phenomenological models, this equation is not dependent on other experimentally determined parameters. For different fuel-oxidizer mixtures, it is found to give agreement with experimental data to within an order of magnitude. The second part of the thesis is concerned with the development of improved reaction models for accurate quantitative simulations of detonation cell size and cellular structure. The mechanism reduction method of Intrinsic Low-Dimensional Manifolds, originally developed for flame calculations, is shown to be a viable option for detonation simulations when coupled with a separate model in the induction zone. The agreement with detailed chemistry calculations of constant volume reactions and one-dimensional steady detonations is almost perfect, a substantial improvement on previous models. The method is applied to a two-dimensional simulation of a cellular detonation in hydrogen-oxygen-argon. The results agree well with an earlier detailed chemistry calculation and experimental data. The computational time is reduced by a factor of 15 compared with a detailed chemistry simulation." }, { "name": "Guduru, Pradeep Reddy", "degree": "PhD", "year": "2001", "title": "An investigation of dynamic failure events in steels using full field high-speed infrared thermography and high-speed photography", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11052003-085740", "creators": [ { "name": { "family": "Guduru", "given": "Pradeep Reddy" }, "id": "Guduru-P-R", "display_name": "Guduru, Pradeep Reddy" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3w5m-qb27", "abstract": "An infrared (IR) imaging system has been developed recently at Caltech for measuring the temperature increase during the dynamic deformation of materials. The system consists of an 8x8 HgCdTe focal plane array, with 64 parallel preamplifiers. Outputs from the 64 detector/preamplifiers are digitized using a row-parallel scheme. In this approach, all 64 signals are simultaneously acquired and held using a bank of track and hold amplifiers. An array of eight 8:1 multiplexers then routes the signals to eight 10MHz digitizers, acquiring data from each row of detectors in parallel. The maximum rate is one million frames per second.\r\n\r\nCrack tip temperature rise during dynamic deformation is known to alter the fracture mechanisms and consequently the fracture toughness of a material. However, no direct experimental measurements have ever been made to determine the same because of limited diagnostic tools. Further, the temperature rise in the vicinity of the crack tip could potentially be used as a direct measure of loading and could serve as a diagnostic tool in order to extract appropriate fracture parameters. By transcending the existing experimental limitations, this investigation presents detailed, real time evolution of the transient crack tip temperature fields in two different steels (C300 and HY100 steels), using the 2-D high speed IR camera. The crack tip temperature rise at initiation in C300 steel was found to be about 55K. In case of HY100, the crack tip temperature rise was above 90K and was seen to be a strong function of loading rate. HRR elastic-plastic singular field has been used to extract J integral evolution from the measured temperature field. Critical value of J integral at initiation was seen to increase with loading rate.\r\n\r\nAn experimental investigation has been conducted to study the initiation and propagation characteristics of dynamic shear bands in C300 maraging steel. Pre-fatigued single edge notched specimens were impacted on the edge under the notch to produce shear dominated mixed mode stress fields. The optical technique of coherent gradient sensing (CGS) was employed to study the evolution of the mixed mode stress intensity factors. Simultaneously, the newly developed 2-D high speed infrared (IR) camera was employed to obtain the temperature field evolution during the initiation and propagation of the shear bands. A criterion for shear band initiation is proposed in terms of a critical mode II stress intensity factor. The IR images, for the first time, revealed the transition of crack tip plastic zone into a shear band and also captured the structure of the tip of a propagating shear band. These thermographs support the notion of a diffuse shear band tip and reveal \"hot spots\" distributed along the length of a well developed shear band." }, { "name": "Huang, Ying", "degree": "PhD", "year": "2001", "title": "Scanning Tunneling Microscopy and Digital Image Correlation in Nanomechanics Investigations", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechTHESIS:12212010-112400412", "creators": [ { "name": { "family": "Huang", "given": "Ying" }, "id": "Huang-Ying", "display_name": "Huang, Ying" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/exy7-5z58", "abstract": "Probe microscopy (scanning tunneling microscopy and atomic force microscopy) and digital image correlation together serve as a potentially powerful tool for experimentally\r\ninvestigating the mechanical behaviors of materials at the sub-micron and nanometer scales. Based on the tunneling effect in quantum physics, the scanning tunneling\r\nmicroscope (STM) records surface topography quantitatively and can achieve angstrom resolution. The digital image correlation (DIC) extracts the displacements and gradients\r\nfrom the undeformed and deformed topographical images.\r\nIn this work, a calibration has been performed on the existing STM built \"in-house\" and the coefficients used in the STM system were confirmed. Major improvements on several\r\ncomponents of the system have been made, including constructing a new actuator probe to decouple its in-plane and out-of-plane movements, designing and implementing a new\r\nfirst-stage amplifier to reduce the noise output by a factor of 10 and modeling of the controller in the STM feedback loop.\r\nFurther, systematic study of the digital image correlation has been conducted. In the simple case of one-dimensional correlation, key parameters involved are the subset size,\r\nvariables in the displacement representation, frequency content of the signal and noise. The one-dimensional study was then extended to two dimensions. In addition to those\r\nkey parameters identified in the one-dimensional study, the sampling rate poses substantial influence on the correlation accuracy. Low amplitude, high frequency noise\r\nstill increases the correlation error significantly." }, { "name": "Isella, Giorgio Carlo", "degree": "PhD", "year": "2001", "title": "Modeling and Simulation of Combustion Chamber and Propellant Dynamics and Issues in Active Control of Combustion Instabilities", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03012006-093758", "creators": [ { "name": { "family": "Isella", "given": "Giorgio Carlo" }, "id": "Isella-Giorgio-Carlo", "display_name": "Isella, Giorgio Carlo" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/k1rf-a525", "abstract": "A method for a comprehensive approach to analysis of the dynamics of an actively controlled combustion chamber, with detailed analysis of the combustion models for the case of a solid rocket propellant, is presented here. The objective is to model the system as interconnected blocks describing the dynamics of the chamber, combustion and control (including sensors and actuators).
\r\n\r\nThe analytical framework for the analysis of the dynamics of a combustion chamber is based on spatial averaging, as introduced by Culick. This method results in the determination of a set of coupled oscillator equations that are then integrated with the appropriate forcing terms deriving from combustion and control.
\r\n\r\nCombustion dynamics are analyzed for the case of a solid propellant. Considerable data exists suggesting that the response functions for many solid propellants tend to have higher values, in some ranges of frequencies, than predicted by the conventional quasi-steady theory. Hence, quasi-steady theory is extended to include the dynamics of the gas-phase and also of a surface layer interposed between the gaseous flame zone and the heated solid phase of the propellant. The models are constructed so that they produce a combustion response function for the solid propellant that can be immediately introduced in the our analytical framework. The principal objective of this analysis is to determine which characteristics of the solid propellant are responsible for the large sensitivity, observed experimentally, of propellant burning response to small variations in the conditions. We show that velocity coupling, and not pressure coupling, has the potential to be the mechanism responsible for that high sensitivity. Some issues related to the modeling of solid propellant are also discussed, namely the importance of particulate modeling and its effect on the global dynamics of the chamber and a revisited interpretation of the intrinsic stability limit for burning of solid propellants.
\r\n\r\nActive control is also considered in the analysis. A critical discussion about the most commonly used control strategies used in combustion allows us to define which are the most promising algorithms to use on future experiments. Particular attention is devoted to the effect of time delay (between sensing and actuation) on the control strategy; several methods to compensate for it are presented and discussed, with numerical examples based on the approximate analysis produced by our framework.
\r\n\r\nExperimental results are presented for the case of a Dump Combustor. The combustor exhibits an unstable burning mode, defined through the measurement of the pressure trace and shadowgraph imaging. The transition between stable and unstable modes of operation is characterized by the presence of hysteresis, also observed in other experimental works, and hence not a special characteristic of this combustor. Control is introduced in the form of pulsed secondary fuel. We show the capability of forcing the transition from unstable to stable burning, hence extending the stable operating regime of the combustor. The transition, characterized by the use of a shadowgraph movie sequence, is attributed to a combined fluid-mechanic and combustion mechanism.
" }, { "name": "Krueger, Paul Samuel", "degree": "PhD", "year": "2001", "title": "The Significance of Vortex Ring Formation and Nozzle Exit Over-Pressure to Pulsatile Jet Propulsion", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-09142005-111030", "creators": [ { "name": { "family": "Krueger", "given": "Paul Samuel" }, "id": "Krueger-Paul-Samuel", "display_name": "Krueger, Paul Samuel" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3QWF-8G05", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nPulsatile jet propulsion can be accomplished using a fully-pulsed jet (i.e., a periodic series of starting jets or pulses), the unsteady nature of which engenders vortex ring formation. The significance of vortex ring formation for this type of propulsion is studied experimentally using a piston-cylinder mechanism to generate starting and fully-pulsed, round jets of water into water at a maximum jet Reynolds number of 13,000. Starting jets are considered separately since they are the limiting case of a fully-pulsed jet at zero pulsing frequency. Direct measurements of the total impulse per pulse (starting jets) and time-averaged thrust (fully-pulsed jets) are made using a force balance. Hotfilm anemometry is used to measure the jet velocity and Digital Particle Image Velocimetry (DPIV) is used to measure vortex ring position, vorticity, energy, circulation, and impulse.\r\n\r\nThe pulses for both types of jets are generated using piston stroke to diameter ratios (L/D) in the range 2 to 8 for piston velocity programs in a generally positive-sloping (PS) or negative-sloping (NS) family. The range of L/D considered brackets the transition between the case where an individual vortex ring is produced with each pulse (small L/D) and the case where the vortex ring stops growing and pinches off from its generating jet, producing a trailing jet (large L/D). This transition occurs at a higher L/D for the PS ramps, allowing the effects of vortex ring formation and pinch off to be illuminated by comparison of the results for the NS and PS ramps.\r\n\r\nThe significance of vortex ring formation is first analyzed for starting jets. Measurements of the total impulse per pulse as a function of L/D show that a leading vortex ring adds more impulse per unit L/D than a trailing jet. This leads to a maximum in the average thrust during a pulse at the L/Ds just before vortex ring pinch off is observed for both the PS and NS ramps. The propulsive benefit provided by a leading vortex ring over a trailing jet is connected to over-pressure at the nozzle exit plane during vortex ring formation. DPIV measurements demonstrate that nozzle exit over-pressure also makes an important contribution to energy and circulation. It is shown that this over-pressure can be related to the momentum that must be supplied by the forming vortex ring to ambient fluid in the form of added and entrained mass. A model is proposed for nozzle exit over-pressure near the initiation of an impulsive velocity program where entrainment can be ignored. The model readily accounts for the pressure contribution to circulation in the NS ramps, but modeling of entrainment is required to properly determine impulse and energy.\r\n\r\nFor the fully-pulsed jet experiments, a normalized thrust, [...], is introduced to characterize the pressure effects associated with vortex ring formation. The pulsing frequency is expressed in dimensionless form as [...], which is between 0 and 1 for all fully-pulsed jets. A propulsive benefit from pressure ([...]) is observed for all L/D and [...] considered. At low [...], the results are similar to those for the starting jets. At higher [...], [...] decreases with L/D as with the starting jets, which is related to the existence of vortex ring pinch off for all observed [...]. At a fixed L/D, two dominant decreasing trends in [...] with [...] appear and seem to be related to the effects of previously ejected pulses on forming vortex rings. No dramatic increase in [...] with [...] (associated with the increased convective velocity of multiple coaxial vortex rings over that of individual vortex rings) is observed since (a) the ring separation is never reduced low enough to see an increase in the ring velocity (even for [...]), and (b) the vortex rings don't remain coaxial or coherent as [...]." }, { "name": "Lokhandwalla, Murtuza", "degree": "PhD", "year": "2001", "title": "Damage Mechanisms in Shock Wave Lithotripsy (SWL)", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-03162005-130412", "creators": [ { "name": { "family": "Lokhandwalla", "given": "Murtuza" }, "id": "Lokhandwalla-Murtuza", "display_name": "Lokhandwalla, Murtuza" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VZWS-7Z85", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nShock wave lithotripsy is a 'non-invasive' therapy for treating kidney stones. Focused shock waves fragment stones to a size that can be passed naturally. There is, however, considerable tissue injury associated with this treatment, and the mechanisms of stone fragmentation and tissue injury are not well understood. This work investigates potential tissue damage mechanisms, with an aim towards modifying the wave-field parameters, so as to enhance stone fragmentation and minimize tissue damage.\r\n\r\nLysis of red blood cells (RBC's) due to in vitro exposure to shock waves was considered as a model of cellular level damage. Fluid flow-fields induced by a non-uniform shock wave, as well as radial expansion/implosion of a bubble was hypothesized to cause lysis of cells. Both the above flow-fields constitute an unsteady, extensional flow exerting inertial as well as viscous forces on the RBC membrane. The resultant membrane tension and the membrane areal strain ([Delta]A/A) due to the above flow-fields were estimated. Both were found to exert a significantly higher inertial force (50 - 100 mN/m) than the critical membrane tension (10 mN/m). Bubble-induced flow-field was estimated to last for a longer duration ([...]) compared to the shock-induced flow ([...]) and hence, was predicted to be lytically more effective, in typical in vitro experimental conditions. However, in vivo conditions severely constrain bubble growth, and cell lysis due to shock-induced shear could be dominant.\r\n\r\nHemolysis due to shock-induced shear, in absence of cavitation, was experimentally investigated. The lithotripter-generated shock wave was refocused by a parabolic reflector. This refocused wave-field had a tighter focus (smaller beam-width and a higher amplitude) than the lithotripter wave-field. Cavitation was eliminated by applying overpressure to the fluid. A novel passive cavitation detector (HP-PCD) operating at high overpressure (upto 7 MPa) was used to measure acoustic emission due to bubble activity. Aluminum foils were also used to differentiate cavitational from non-cavitational mode of damage. RBC's suspended in phosphate-buffered saline PBS) were exposed to the reflected wave-field from the parabolic reflector and also from a flat reflector, the latter serving as a control experiment. Exposure to the wave-field from the parabolic reflector increased hemolysis four-fold compared to untreated controls and was twice that of cell lysis with the flat reflector. This result corroborated the hypothesis of shock-induced shear as a cell damage mechanism in the absence of cavitation.\r\n" }, { "name": "Molinari, Jean-Francois Roland", "degree": "PhD", "year": "2001", "title": "Three dimensional finite element analysis of impact damage and erosion of metallic targets", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-102656", "creators": [ { "name": { "family": "Molinari", "given": "Jean-Francois Roland" }, "id": "Molinari-J-R", "display_name": "Molinari, Jean-Francois Roland" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6ZJV-B162", "abstract": "This dissertation is concerned with the development of a robust three-dimensional finite-element framework for the simulation of complex problems in mechanics and physics of solids. This approach is intended to shine light on impact and erosion mechanisms among other multiscale, multiphysics problems. The components of the computational framework are a contact algorithm including friction, wear, finite deformation plasticity, heat generation, heat transfer, and adaptive meshing coupled with error estimation. The adaptive meshing is a key development that enhances the efficiency and robustness of the method. We demonstrate the ability of the methodology to simulate diverse problems such as shear banding, impact, and wear.\r\n" }, { "name": "Ol, Michael Volf", "degree": "PhD", "year": "2001", "title": "The Passage Toward Stall of Nonslender Delta Wings at Low Reynolds Number", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:11192010-085724406", "creators": [ { "name": { "family": "Ol", "given": "Michael Volf" }, "id": "Ol-Michael-Volf", "display_name": "Ol, Michael Volf" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/016w-2332", "abstract": "Separated flow over the leeside of relatively nonslender delta wings was studied experimentally. Such flowfields are more complex than those of the slender delta wing\r\nof very low aspect ratio. A version of Stereo Digital Particle Image Velocimetry was applied to measurements in a low speed water tunnel, at Reynolds numbers below 20,000, for delta wing models of 50\u00b0 and 65\u00b0 leading edge sweep angles and 30\u00b0 windward-side leading edge bevels. Since the objective was to draw comparisons to the stall of classical high aspect ratio wings, low angles of attack were emphasized, with most data points taken in the 5\u00b0-20\u00b0 angle of attack range. Measurements were taken over the starboard portion of the wing planform in crossflow planar slices near the apex region, yielding all three components of the velocity field, albeit restricted to planar cuts.\r\nVorticity and circulation were calculated from these measurements. All three components of vorticity were obtained in select cases, by central-differencing velocity\r\ndata across triplets of adjacent interrogation planes. In addition, flow visualization by dye injection into the windward apex stagnation region was used to confirm the presence of primary and secondary leading edge vortices, to qualitatively verify the locations of vortex breakdown, and to verify the stereo digital particle image velocimetry results.\r\nBoth delta wings exhibit stable, coherent leading edge vortices at very low angles of attack, down to 2.5\u00b0. Results for the 65\u00b0 wing were in accordance with the literature. The 50\u00b0 wing, however, exhibited flow characteristics akin to both slender delta wings, and wings of high aspect ratio, and generally exhibited stronger and more robust leading edge vortices than usually observed. For the 50\u00b0 wing, the primary leading edge vortices were stable below 10\u00b0 angle of attack, with gradual and steady upstream progression of the vortex breakdown region with increasing angles of attack, from aft of the trailing\r\nedge to approximately the midchord. Secondary leading edge vortices were found to decay more abruptly, and at lower angle of attack than the primaries, all but disappearing\r\nby 10\u00b0 angle of attack. This fact has the potential of serving as the basis for a predictive criterion for breakdown of the primary vortices.\r\nThe entire vortex system undergoes large-scale instabilities in the 12\u00b0-20\u00b0 angle of attack range. While the flow visualization was inconclusive, particle image\r\nvelocimetry confirmed that breakdown sweeps over the entire forward third of the wing planform in going from 12.5\u00b0 to 15\u00b0 angle of attack. This change is characterized by a\r\nsharp drop in axial velocity in the primary leading edge vortex core region, along with a loss of coherent vortical structure normally associated with this region. The leading edge shear layer, however, remains in an organized rolled-up state. By 20\u00b0, the flow over the leeward side of the wing is at the threshold of complete separation, with flow along the wing centerline stalling as the left and right separated regions grow and merge.\r\nBoth wings exhibited a largely stagnant region outboard of the primary LEV and inboard of the leading edge shear layer, especially at angles of attack beyond 10\u00b0. This\r\nphenomenon is consistent with some prior observations at Reynolds numbers on the order of 20,000 and below, and differs sharply from that at higher Reynolds numbers. Further experiments are necessary to elucidate the cause and extent of Reynolds number influence on separation near the leeward surface. Also, the 50\u00b0 wing is probably of too\r\nhigh sweep to be a true limiting case for the existence of coherent leading edge vortices, for the conditions of the present experiment. But the abruptness of its stall and the close relationship between the leading edge vortex flow and the leeward surface boundary layer are qualitatively indicative of such a transitional case from slender delta wing separation to classical airfoil stall." }, { "name": "Rasheed, Adam", "degree": "PhD", "year": "2001", "title": "Passive Hypervelocity Boundary Layer Control Using an Ultrasonically Absorptive Surface", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08192001-143746", "creators": [ { "name": { "family": "Rasheed", "given": "Adam" }, "id": "Rasheed-Adam", "display_name": "Rasheed, Adam" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "chair", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "busecon" ], "doi": "10.7907/EFZZ-X764", "abstract": "A series of exploratory boundary layer transition experiments was performed on a sharp 5.06 degree half-angle round cone at zero angle-of-attack in the T5 Hypervelocity Shock Tunnel in order to test a novel hypersonic boundary layer control scheme. Recently performed linear stability analyses suggested that transition could be delayed in hypersonic boundary layers by using an ultrasonically absorptive surface that would damp the second mode (Mack mode). The cone used in the experiments was constructed with a smooth surface on half the cone (to serve as a control) and an acoustically absorptive porous surface on the other half. It was instrumented with flush-mounted thermocouples to detect the transition location. Test gases investigated included nitrogen and carbon dioxide at M = 5 with specific reservoir enthalpy ranging from 1.3 MJ/kg to 13.0 MJ/kg and reservoir pressure ranging from 9.0 MPa to 50.0 MPa. Detailed comparisons were performed to insure that previous results obtained in similar boundary layer transition experiments (on a regular smooth surface) were reproduced and the results were extended to examine the effects of the porous surface. These experiments indicated that the porous surface was highly effective in delaying transition provided that the hole size was significantly smaller than the viscous length scale." }, { "name": "Samudrala, Omprakash", "degree": "PhD", "year": "2001", "title": "Subsonic and intersonic crack growth along weak planes and bimaterial interfaces", "advisor": "Knauss, Wolfgang Gustav; Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechTHESIS:10122010-134248309", "creators": [ { "name": { "family": "Samudrala", "given": "Omprakash" }, "id": "Samudrala-Omprakash", "display_name": "Samudrala, Omprakash" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/51b0-1c87", "abstract": "A combined experimental and analytical study has been conducted to investigate the phenomena of intersonic crack propagation along weak planes in homogeneous solids and dissimilar material interfaces. A single edge notch/crack oriented along a weak plane in a brittle polymer or along a polymer/metal interface was loaded in shear by impacting the specimen with a high velocity projectile fired from a gas gun. Homalite-100 or PMMA was chosen for the polymer and 6061 Aluminum or 4340 steel was chosen for the metal. The stress field information around the propagating crack tip was recorded in real time by two different optical techniques which provide complimentary information - photoelasticity and coherent gradient sensing (CGS), in conjunction with high speed photography. Along weak planes in Homalite-100, dynamic shear cracks were observed to initiate and propagate at speeds exceeding the shear wave speed (c_s) of the polymer. The isochromatic fringe patterns reveal two distinct lines of strong stress field discontinuity (Mach waves) emanating from the crack tip. Intersonic cracks were observed to initially accelerate up to the longitudinal wave speed (c_l) of Homalite and thereafter slow down to propagate at a near constant velocity slightly above \u221a2c_s . A series of short secondary opening cracks parallel to each other and at a steep angle to the weak plane (\u2248 80\u00b0) were also observed to initiate behind the main intersonic crack tip. Motivated by the experimental observations, an asymptotic analysis was performed to obtain the near tip fields for an intersonically propagating steady state mode II crack with a finite sized shear cohesive zone in front of it. The cohesive shear stress was chosen to be either a constant or to depend linearly on the magnitude of the local slip rate. Decohesion was chosen to occur when the relative slip between the two cohesive surfaces reaches a material/interface specific critical value. Unlike the case of a point sized dissipative region, it is shown that with a finite cohesive zone, the dynamic energy release rate is finite through out the intersonic regime. The influence of crack plane shear strength and of the rate parameter on the crack propagation behavior is investigated. Isochromatic fringe patterns were constructed using the cohesive crack tip fields, which compare favorably with the experimentally observed fringe patterns, and an attempt is made to extract the relevant analytical parameters. Unlike for a mode-I crack, a cohesive stress distribution that decreases with the local slip rate is found to match the experimental observations. The rate parameter was extracted by fitting the secondary crack angle observed in the experiments to that predicted by the analytical solution based on a maximum principal stress fracture criterion. Edge notches/cracks on polymer/metal interfaces were loaded under different impact configurations and the conditions governing the attainment of intersonic crack growth along a bimaterial interface were investigated. High resolution isochromatic fringe patterns were obtained to study the nature of the crack tip fields during subsonic/intersonic transition. Careful observations of the transition of an interface crack into the intersonic regime showed the formation of crack face contact at speeds beyond c_R of the polymer. Subsequently, the contact zone is observed to expand in size, detach from the intersonic crack tip and finally vanish. The recorded isochromatic fringe patterns showed multiple Mach wave formation associated with such a scenario. Along PMMA/ Al and PMMA/steel bimaterial interfaces, dynamic cracks initiating from edge notches were observed to accelerate to speeds higher than c_l of PMMA (supersonic), almost reaching c_R of aluminum. The resulting crack growth was observed to be highly transient and the gradients of in-plane normal stress components were recorded using CGS interferometry. Motivated by the aforementioned experimental observations, an asymptotic analysis was performed to obtain the stress and deformation fields around a steadily propagating intersonic crack on an elastic-rigid interface with a finite zone of crack face frictional sliding contact located a finite distance behind the tip. A linear frictional contact model is adopted, wherein the shear stress is proportional to the normal stress through a constant, the coefficient of dynamic friction. Isochromatic fringe patterns predicted by the near-tip fields exhibit the essential features observed during the experiments. Frictional sliding contact is shown to be possible only for velocities between c_s and \u221a2c_s of the polymer. The relevant analytical parameters were predicted by comparing the model to the experimental isochromatic fringe patterns and comments are made on the merits of the model presented." }, { "name": "Sane, Sandeep Bhalchandra", "degree": "PhD", "year": "2001", "title": "Time-dependent compressibility of poly (methyl methacrylate) (PMMA) : an experimental and molecular dynamics investigation", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechTHESIS:04262011-100757709", "creators": [ { "name": { "family": "Sane", "given": "Sandeep Bhalchandra" }, "id": "Sane-S-B", "display_name": "Sane, Sandeep Bhalchandra" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/saw5-7p32", "abstract": "This thesis contains three chapters, which describe different aspects of an investigation of the bulk response of Poly(Methyl Methacrylate) (PMMA). The first chapter describes the physical measurements by means of a Belcher/McKinney-type apparatus. Used earlier for the measurement of the bulk response of Poly(Vinyl Acetate), it was now adapted for making measurements at higher temperatures commensurate with the glass transition\r\ntemperature of PMMA. The dynamic bulk compliance of PMMA was measured at atmospheric pressure over a wide range of temperatures and frequencies, from which the master curves for the bulk compliance were generated by means of the time-temperature superposition principle. It was found that the extent of the transition ranges for the bulk and shear response were comparable. Comparison of the shift factors for bulk and shear responses supports the idea that different molecular mechanisms contribute to shear and\r\nbulk deformations.\r\n\r\nThe second chapter delineates molecular dynamics computations for the bulk response for a range of pressures and temperatures. The model(s) consisted of 2256 atoms\r\nformed into three polymer chains with fifty monomer units per chain per unit cell. The time scales accessed were limited to tens of pico seconds. It was found that, in addition to the typical energy minimization and temperature annealing cycles for establishing equilibrium models, it is advantageous to subject the model samples to a cycle of\r\nrelatively large pressures (GPa-range) for improving the equilibrium state. On comparing the computations with the experimentally determined \"glassy\" behavior, one finds that,\r\nalthough the computations were limited to small samples in a physical sense, the primary limitation rests in the very short times (pico seconds). The molecular dynamics computations do not model the physically observed temperature sensitivity of PMMA, even if one employs a hypothetical time-temperature shift to account for the large\r\ndifference in time scales between experiment and computation. The values computed by the molecular dynamics method do agree with the values measured at the coldest\r\ntemperature and at the highest frequency of one kiloHertz.\r\n\r\nThe third chapter draws on measurements of uniaxial, shear and Poisson response conducted previously in our laboratory. With the availability of four time or frequency-dependent material functions for the same material, the process of interconversion between different material functions was investigated. Computed material functions were\r\nevaluated against the direct experimental measurements and the limitations imposed on successful interconversion due to the experimental errors in the underlying physical data\r\nwere explored. Differences were observed that are larger than the experimental errors would suggest.\r\n" }, { "name": "Seywert, Claude", "degree": "PhD", "year": "2001", "title": "Combustion Instabilities: Issues in Modeling and Control", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01252007-135242", "creators": [ { "name": { "family": "Seywert", "given": "Claude" }, "id": "Seywert-Claude", "display_name": "Seywert, Claude" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Murray", "given": "Richard M." }, "id": "Murray-R-M", "orcid": "0000-0002-5785-7481", "role": "member", "display_name": "Murray, Richard M." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JKFD-7W43", "abstract": "This study deals with various aspects in the development of active control of combustion instabilities.\r\n\r\nA low-order model is developed, reconciling along the way two different approaches taken by researchers to attack the description of combustion instabilities. The model is demonstrated with application to a Rijke tube and compared to experiments. The Rijke burner experiments suggest two major discrepancies with the model: the presence of a hysteresis loop is unaccounted for and the model does not describe the seemingly random fluctuations in the amplitude of the pressure oscillations in the 'unstable' regime. So far no explanation for the hysteresis can be given; however, this phenomenon is successfully exploited by using a novel nonlinear control technique to expand the stable operating range of the burner. The origin of the 'noise' in the pressure trace is explained by considering entropy and vorticity waves in the combustor. Their presence leads to a slight modification of the original model, introducing stochastic source terms into the oscillator equations. The consequences of the presence of these terms is analyzed by means of simulations. One interesting result is that they allow for the identification of model parameters from a single experimental run of a stable combustion system.\r\n\r\nFinally, a unified approach to controlling combustion instabilities is presented. The formulation and analysis account for truncation to a few modes; uncertainties in the description of the system (including uncertain sensing and actuating); external disturbances; and intrinsic noise sources. An explicit expression is derived against which any controller can be checked for stability." }, { "name": "Shan, Wei-Jen Jerry", "degree": "PhD", "year": "2001", "title": "Mixing and Isosurface Geometry in Turbulent Transverse Jets", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01082002-152648", "creators": [ { "name": { "family": "Shan", "given": "Wei-Jen Jerry" }, "id": "Shan-Wei-Jen-Jerry", "display_name": "Shan, Wei-Jen Jerry" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0R6S-DY34", "abstract": "Mixing and the geometry of jet-fluid-concentration level sets in turbulent transverse jets were experimentally studied. Jet-fluid concentration fields were measured with laser-induced fluorescence and digital imaging techniques for Reynolds numbers between 1000 and 20000. The scalar field is assessed in terms of classical measures, such as two-dimensional power spectra, as well as probability-density functions (PDFs). Enhanced scalar mixing with increasing Reynolds number is found in the evolution of PDFs of jet-fluid concentration. In the far field of the transverse jet, the scalar PDF is seen to evolve from a monotonically-decreasing function to a strongly-peaked distribution with increasing Reynolds number. Turbulent mixing is found to be flow dependent, based on differences between PDFs of scalar fields in transverse jets and axisymmetric, turbulent jets. The distribution of scalar increments is also studied for separations of varying distance and direction. A novel technique for whole-field measurement of scalar increments is introduced. Probability-density functions of scalar increments are found to trend toward exponential-tailed distributions with decreasing separation distances. The scalar field is anisotropic with decreasing scale, as seen in the two-dimensional power spectra, directional scalar microscales, and in directional PDFs of scalar increments.\r\n\r\nThe geometric complexity of level-sets (iso-concentration contours) in turbulent mixing is assessed within the framework of fractal geometry.\r\nGeneralized coverage statistics are introduced for anisotropic, non-self-similar geometries. This generalized coverage counting involves covering with parallelepipeds of varying size and aspect ratio. A scale-dependent measure of the anisotropy of a set is also introduced. It is shown that this scale-dependent measure transforms the generalized coverage count to isotropy through a scale-dependent normalization of the coordinates. Level sets of jet-fluid concentration in the transverse jet are found to be anisotropic at both large and small scales. The small-scale anisotropy is explained by vertically-oriented extensional strain caused by a counter-rotating vortex pair, while the large-scale anisotropy is associated with the horizontally-elongated shape of the cross-section of the transverse jet. For the special case of isotropic box-counting, the scale-dependent coverage dimension is found to vary from unity, at the smallest length scales, to 2, at the largest length scales, indicating that the isosurfaces produced by turbulent mixing are more complex than can be described by power-law fractals" }, { "name": "Yu, Chengxiang Rena", "degree": "PhD", "year": "2001", "title": "Three-Dimensional Cohesive Modeling of Impact Damage of Composites", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechTHESIS:10112010-130530819", "creators": [ { "name": { "family": "Yu", "given": "Chengxiang Rena" }, "id": "Yu-Chengxiang-Rena", "orcid": "0000-0003-4176-0324", "display_name": "Yu, Chengxiang Rena" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Hou", "given": "Thomas Y." }, "id": "Hou-T-Y", "role": "member", "display_name": "Hou, Thomas Y." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Pandolfi", "given": "Anna" }, "id": "Pandolfi-A", "role": "member", "display_name": "Pandolfi, Anna" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/nd8e-tc84", "abstract": "The objective of this work is to establish the applicability of cohesive theories of fracture in situations involving material interface, material heterogeneity (e.g., layered composites), material anisotropy(e.g., fiber-reinforced composites), shear cracks, intersonic dynamic crack growth and dynamic crack branching. The widely used cohesive model is extended to orthotropic range. The so-developed computational tool, completed by a self-adaptive fracture procedure and a frictional contact algorithm, is capable of following the evolution of three-dimensional damage processes, modeling the progressive decohesion of interfaces and anisotropic materials. The material parameters required by cohesive laws are directly obtained from static experiments. The ability of the methodology to simulate diverse problems such as delamination between fibers of graphite/epoxy composites, as well as sandwich structures and branching within brittle bulk materials has been demonstrated.
" }, { "name": "Zuhal, Lavi Rizki", "degree": "PhD", "year": "2001", "title": "Formation and Near-Field Dynamics of a Wing Tip Vortex", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:02072013-122723580", "creators": [ { "name": { "family": "Zuhal", "given": "Lavi Rizki" }, "id": "Zuhal-Lavi-Rizki", "display_name": "Zuhal, Lavi Rizki" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VNJW-6592", "abstract": "The search for a more efficient method to destroy aircraft trailing vortices requires\r\na good understanding of the early development of the vortices. For that purpose, an\r\nexperimental investigation has been conducted to study the formation and near-field\r\ndynamics of a wing tip vortex.
\r\n\r\nTwo versions of the Digital Particle Image Velocimetry (DPIV) technique were\r\nused in the studies. Planar DPIV was used to obtain velocity fields adjacent to the wing\r\nsurface. Stereoscopic DPIV, which allows instantaneous measurements of all three\r\ncomponents of velocity within a planar slice, was used to measure velocity fields behind\r\nthe wing. The trailing vortex was produced by a rectangular half-wing model with an\r\nNACA 0012 profile. All measurements were made at Reynolds number, based on chord\r\nlength, of 9040.
\r\n\r\nThe present study has found that the wing sheds multiple vortices. A structure\r\nthat closely resembles a wing tip vortex is first observed on the suction side of the wing\r\nnear the tip at the mid-chord section of the wing. At the trailing edge of the wing, a\r\nsmaller vortex with an opposite sense of rotation is observed next to the tip vortex. In\r\naddition to the two vortices, two vortex layers with opposite sense of rotation, one on the\r\npressure side and one on the suction side, are apparent at the trailing edge. Farther\r\ndownstream, most of the vorticity in the vortex layer, with the same sense of rotation as\r\nthe tip vortex, rolls up into the wing tip vortex. The vortices, with opposite sense of\r\nrotation, break up into smaller vortices which orbit around the tip vortex. At least one\r\nrelatively strong satellite vortex appears in some of the instantaneous fields. The studies\r\nfound that the interaction of the tip vortex and satellite vortices give rise to the unsteady\r\nmotion of the wing tip vortex. In addition, the studies also examined the effects of the\r\nboundary layer and the tip geometry to the strength and motion of the trailing vortex.
" }, { "name": "Brady, Mark A.", "degree": "PhD", "year": "2000", "title": "Regularized vortex sheet evolution in three dimensions", "advisor": "Pozrikidis, Constantine; Pullin, Dale Ian; Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechTHESIS:10012010-145525634", "creators": [ { "name": { "family": "Brady", "given": "Mark A." }, "id": "Brady-M-A", "display_name": "Brady, Mark A." } ], "advisors": [ { "name": { "family": "Pozrikidis", "given": "Constantine" }, "id": "Pozrikidis-C", "role": "advisor", "display_name": "Pozrikidis, Constantine" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/pbyj-k986", "abstract": "A computational method is presented to follow the evolution of regularized three-dimensional (3D) vortex sheets through an otherwise irrotational, inviscid, constant-density fluid. The sheet surface is represented by a triangulated mesh with interpolating functions locally defined inside each triangle. C^1 continuity is maintained between triangles via combinations of cubic B\u00e9zier triangular interpolants. The self-induced sheet motion generally results in a highly deformed surface which is adaptively refined as needed to capture regions of increasing curvature and to avoid severe Lagrangian deformation. Automatic mesh refinement is implemented with an advancing front technique. Sheet motion is regularized by adding a length scale cut-off to the BiotSavart kernel. Velocity evaluation takes less time than the standard O(N^2) scaling, due to utilization of multi-pole expansions of the kernel. Zero, singly, and doubly periodic vortex sheets are simulated, modeling vortex rings, vortex/jet combinations and standard shear layers. Comparisons with previous two-dimensional (2D) results are favorable and 3D simulations are presented. The perturbed 3D planar shear layer is simulated and compared with a similar experiment revealing qualitatively similar results and agreement on the mechanism by which streamwise vorticity is created. We find the ratio of spanwise to streamwise vorticity to vary between 7 and 9 during early stages of roll-up.\r\n\r\nA new technique for estimating the curvature singularity time of true vortex sheets (i.e., non-regularized motion) is presented. The motion and singularity time of a planar, doubly periodic sheet, evolving from a 3D normal mode perturbation, is found to reduce to that of a well known singly periodic (and only two-dimensional) problem, an unexpected extension of Moore's [38] non-linear analysis for 2D vortex sheets." }, { "name": "Jeon, David S.", "degree": "PhD", "year": "2000", "title": "On Cylinders Undergoing One- and Two-Degree of Freedom Forced Vibrations in a Steady Flow", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechTHESIS:09302010-151500998", "creators": [ { "name": { "family": "Jeon", "given": "David S." }, "id": "Jeon-David-S", "display_name": "Jeon, David S." } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ky58-en52", "abstract": " Formation of vortices in the near wake of circular cylinders is discussed. Two different cases are compared: starting flow around an initially stationary cylinder and flow around an oscillating cylinder in a steady freestream. The effects of formation time on the morphology of the vortices are shown, as well as some consequences thereof.\r\n\r\n For starting flows, the critical formation time defines the point where the wake transitions from the initially symmetrical state to the intermediate asymmetrical state. The asymmetrical state breaks down into the periodic shedding state normally associated with cylinder flows. It appears that the wake reaches a critical level of vorticity annihilation at the critical time. This triggers an exponential growth of asymmetry in the near wake. Evidence of this process can also be seen in the early time force data.\r\n\r\n For oscillating flows, the critical time defines the transition from vortex to vortex-and-tail morphology. First, phase averaged vorticity fields are presented showing the changes in the wake with forcing frequency and streamwise motion. These changes are related to the formation time, and related to similar effects seen in other flows. In addition, prolonged formation is related to the observed switch in the phase of the vortex shedding. The effects of streamwise motion are also shown, including the increased phase coherence of the shedding via coordination of the shedding process and the ability to adjust formation time via streamwise acceleration. The latter was used to demonstrate a plausible explanation for the vortex pair formation process observed by some researchers by showing how the formation process affects the number of vortices generated per cycle." }, { "name": "Moeleker, Piet", "degree": "PhD", "year": "2000", "title": "The filtered advection-diffusion equation : Lagrangian methods and modeling", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechTHESIS:10062010-114741410", "creators": [ { "name": { "family": "Moeleker", "given": "Piet" }, "id": "Moeleker-P", "display_name": "Moeleker, Piet" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2q17-4g27", "abstract": "This research focuses on the incompressible scalar advection-diffusion equation. After applying a Gaussian filter, an infinite series expansion is found for the advection term to obtain a closed equation. Only the first two terms in this expansion are retained yielding the tensor-diffusivity subgrid model. This model can be interpreted as a tensor-diffusivity term which is proportional to the rate-of-strain tensor of the large-scale\r\nfiltered velocity field. Due to the negative diffusion in the stretching directions, care needs to be taken in the choice of a numerical method.\r\n\r\nThe scalar field is decomposed in a collection of anisotropic or axisymmetric Gaussian particles. Equations of motion for the location and the shape/size of the particles are derived using an expansion in Hermite polynomials. A novel, accurate remeshing scheme was found resulting in explicit expressions for the amplitudes of the new set of particles.\r\n\r\nA stagnation flow was used for illustrative purposes and validation. Using a 2D time-dependent velocity field yielding chaotic advection, both axisymmetric and\r\nanisotropic particles yield good agreement with filtered direct numerical simulations and compare favorably with the Smagorinsky subgrid model. Computational efficiency\r\nmakes axisymmetric particles the preferred choice.\r\n\r\nA literature study using a 3D stationary one-parameter chaotic velocity field was used to validate model and particle-method in 3D. For highly chaotic fields good\r\nagreement was obtained with this study. Computations have been performed for 3D forced isotropic periodic turbulence to study scalar mixing. Comparisons with literature are made.\r\n\r\nIt was shown that when the unfiltered velocity field is known, the most accurate results are obtained by moving particles using this field. It was concluded that a good\r\nsubgrid model modifies the equation of motion to get a good approximation to the unfiltered velocity field.\r\n" }, { "name": "Schlamp, Stefan", "degree": "PhD", "year": "2000", "title": "Laser-induced thermal acoustic velocimetry", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechTHESIS:10012010-130705165", "creators": [ { "name": { "family": "Schlamp", "given": "Stefan" }, "id": "Schlamp-S", "display_name": "Schlamp, Stefan" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/skm0-tk82", "abstract": "Laser-Induced Thermal Acoustics (LITA) is a non-intrusive, remote, four-wave mixing laser diagnostic technique for measurements of the speed of sound and of the thermal diffusivity in gases. If the gas composition is known, then its temperature and density can be inferred. Beam misalignments and bulk fluid velocities can influence the time history and intensity of LITA signals. A closed-form analytic expression for LITA signals incorporating these effects is derived. The magnitude of beam misalignment and the flow velocity can be inferred from the signal shape using a least-squares fit of this model to the experimental data. High-speed velocimetry using homo dyne detection is demonstrated with NO_2-seeded air in a supersonic blow-down nozzle. The measured speed of sound deviates less than 2% from the theoretical value assuming isentropic quasi-1D flow. Boundary layer effects degrade the velocity measurements to errors of 20%. Heterodyne detection is used for low-speed velocimetry up to Mach number M = 0.1. The uncertainty of the velocity measurements was ~ 0.2 m/s. The sound speed measurements were repeatable to 0.5%. The agreement between theory and experiments is very good. A one-hidden-layer feed-forward neural network is trained using back-propagation learning and a steepest descent learning rule to extract the speed of sound and flow velocity from a heterodyne LITA signal. The effect of the network size on the performance is demonstrated. The accuracy is determined with a second set of LITA signals that were not used during the training phase. The accuracy is found to be better than that of a conventional frequency decomposition technique while being computationally as efficient. This data analysis method is robust with respect to noise, numerically stable, and fast enough for real-time data analysis. The accuracy and uncertainty of non-resonant LITA measurements is investigated. The error in measurements of the speed of sound and of the thermal diffusivity initially decreases with increasing signal intensity (excitation beam pulse energy) and increases again after passing a minimum. The location of the minimum error for the speed of sound and for the thermal diffusivity coincide. The errors at the minimum are 0.03% and 1%, respectively. The uncertainties for the speed of sound and the thermal diffusivity decrease monotonically to 0.25% and 5%, respectively. The increased error for high excitation beam pulse energies results from finite-strength waves that cannot be treated using the linearized equations of motion." }, { "name": "Schultz, Eric", "degree": "PhD", "year": "2000", "title": "Detonation Diffraction Through an Abrupt Area Expansion", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-180459", "creators": [ { "name": { "family": "Schultz", "given": "Eric" }, "id": "Schultz-Eric", "display_name": "Schultz, Eric" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/96F1-QR61", "abstract": "The problem of a self-sustaining detonation wave diffracting from confinement into an unconfined space through an abrupt area change is characterized by the geometric scale of the confinement and the reaction scale of the detonation. Previous investigations have shown that this expansion associated with a detonation transitioning from planar to spherical geometry can result in two possible outcomes depending upon the combustible mixture composition, initial thermodynamic state, and confining geometry. Competition between the energy release rate and expansion rate behind the diffracting wave is crucial. The sub-critical case is characterized by the rate of expansion exceeding the energy release rate. As the chemical reactions are quenched, the shock wave decouples from the reaction zone and rapidly decays. The energy release rate dominates the expansion rate in the super-critical case, maintaining the coupling between the shock and reaction zone which permits successful transition across the area change. A critical diffraction model has been developed in the present research effort from which the initial conditions separating the sub-critical and super-critical cases can be analytically determined. Chemical equilibrium calculations and detonation simulations with validated detailed reaction mechanisms provide the model input parameters. Experiments over a wide range of initial conditions with single- and multi-sequence shadowgraphy and digital chemiluminescence imaging support the model derivation and numerical calculations. Good agreement has been obtained between the critical diffraction model and experimental results." }, { "name": "Swenson, Grant Douglas", "degree": "PhD", "year": "2000", "title": "Numerical simulations of combustion instabilities in gas turbine combustors, with applications", "advisor": "Culick, Fred E. C.; Zukoski, Edward E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01162008-140810", "creators": [ { "name": { "family": "Swenson", "given": "Grant Douglas" }, "id": "Swenson-G-D", "display_name": "Swenson, Grant Douglas" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "co-advisor", "display_name": "Zukoski, Edward E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3QG6-JN34", "abstract": "Recent advances in technology have opened up a potential market for small gas turbine power systems in the 50-100 MW range. In an effort to improve their systems, the gas-turbine industry is interested in understanding and controlling combustion instabilities as well as reducing pollutant production. To understand the dynamics inherent in a combustion system, information about the flow field behavior is required. Because of a scarcity of available experimental or numerical results for full-scale gas-turbine combustors, we decided to use numerical simulations to provide the required information about the flow field dynamics. The ability of the numerical simulations to reproduce unstable behavior in combustion environments will be presented. The investigation of the flow field dynamics has been conducted for three test cases; a planar heat source in a tube, premixed flow in a dump combustor, and premixed and diffusion flames in a full-scale gas turbine combustor. The numerically determined unsteady acoustic modes will be shown to compare well with theory and experiments. An investigation of the local heat release response to an unsteady flow field is conducted for incorporation into an approximate analysis method. The results of including a Helmholtz resonator in a dump combustor as a passive control mechanism will be presented. The production of NOx and CO will be compared between stable and unstable flow configurations. The pollutant results indicate that for the planar flame in a tube and the dump combustor, the NOx levels at the exit plane are reduced when the system is unstable.\n" }, { "name": "Voelkl, Tobias", "degree": "PhD", "year": "2000", "title": "A physical-space version of the stretched-vortex subgrid-stress model for large-eddy simulation of incompressible flow", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-07092007-154618", "creators": [ { "name": { "family": "Voelkl", "given": "Tobias" }, "id": "Voelkl-T", "display_name": "Voelkl, Tobias" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/d900-mk74", "abstract": "In large-eddy simulations of turbulence, the large scales of the flow are resolved by a numerical solution of the equations of motion for these scales, but the contribution of the fine-scale turbulence must be modeled. The stretched-vortex model estimates the influence of these unresolved subgrid-scale turbulence fluctuations on the resolved-scale velocities by using kinematic results for homogeneous, anisotropic turbulence consisting of locally straight, unidirectional vortex structures [D. I. Pullin and P. G. Saffman, Phys. Fluids 6 (5), 1994]. A new method is presented to dynamically determine the value of model constants related to the subgrid kinetic energy. For this purpose, a relation between the resolved-scale velocity structure function of second order and the energy spectrum is derived based on the kinematics of the model vortex structures, and therefore without the assumption of isotropy. Implementation of this relation using a local, circular average allows application of the model to wallbounded turbulent flows without special modifications. The resulting algebraic model is completely localized, i.e., no global flow quantities like the resolved-scale spectrum are required. This facilitates the application of the model in physical-space numerical methods using, for example, finite differences or Lagrangian-interpolation polynomials. The model includes an estimate of the subgrid kinetic energy, which is used to compute subgrid contributions to low-order turbulence statistics of the full flowfield. Results will be shown for the decay of kinetic energy and energy spectra of decaying, isotropic turbulence, for mean velocities, root-mean-square velocity fluctuations and turbulence-kinetic-energy budgets of channel flow up to a Reynolds number of approximately 23 000 (based on channel halfwidth and centerline velocity), and for mean velocities and turbulence kinetic energy of channel flow under spanwise rotation. The results are compared to unfiltered data from direct numerical simulations and experiment." }, { "name": "Bond, Christopher Llewellyn", "degree": "PhD", "year": "1999", "title": "Reynolds number effects on mixing in the turbulent shear layer", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03242005-162912", "creators": [ { "name": { "family": "Bond", "given": "Christopher Llewellyn" }, "id": "Bond-C-L", "display_name": "Bond, Christopher Llewellyn" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3KKM-HB66", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nExperiments have been performed in turbulent shear layers to assess the effects of Reynolds number on mixing. The experiments extend the upper range of incompressible, chemically-reacting flow data from [...] to [...] and, in a subsequent collaboration with M. Slessor and others, to 7 x 10[...]. The experiments address an ambiguity in recent measurements of mixing in compressible shear layers, whose differences from measurements in incompressible shear layers could have been due either to compressibility or Reynolds number effects. The current results, collectively with previous results, show that the decrease of mixing with increasing Reynolds number first seen by Mungal et al. (1985) continues to the Reynolds numbers studied, but do not show a further decrease in mixing. The conclusions must be tempered by the presence of changes in the initial boundary layers and acoustic environment, and by the compressibility ([...] = 0.25) of the highest Reynolds number (Re = 7 x 10[...]) case.\r\n" }, { "name": "Davis, Jean-Paul", "degree": "PhD", "year": "1999", "title": "High-enthalpy shock/boundary-layer interaction on a double wedge", "advisor": "Sturtevant, Bradford; Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02272008-125333", "creators": [ { "name": { "family": "Davis", "given": "Jean-Paul" }, "id": "Davis-J", "display_name": "Davis, Jean-Paul" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Goodwin", "given": "David G." }, "id": "Goodwin-D-G", "role": "member", "display_name": "Goodwin, David G." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4C98-MN23", "abstract": "Interaction between a shock wave and a boundary layer at a compression corner can produce a region of separated flow. The length of separation is important in determining aerodynamic forces, and the heat transfer at reattachment is important for the design of thermal protection systems. The effects of high-enthalpy flow on these phenomenon, particularly separation length, are not well known. Experiments to measure separation length and reattachment heating are performed in the T5 Hypervelocity Shock Tunnel using nitrogen test gas and a double-wedge geometry which allows greater control over local flow conditions at separation and, at high incidence angle, may produce real-gas effects due to dissociation behind the leading shock. Local external flow conditions were found by computational reconstruction of the inviscid nonequilibrium flow field.\n\nApplication of results from asymptotic theory to a simple model for separation leads to a new scaling parameter which approximately accounts for wall temperature effects on separation length for a laminar nonreacting boundary layer and extends previous results to arbitrary viscosity law. A. classification is introduced which divides mechanisms for real-gas effects into those acting internal and external to viscous regions of the flow, with internal mechanisms further subdivided into those arising upstream and downstream of separation. Application of the ideal dissociating gas model to a scaling law based on local external flow parameters and a nonreacting boundary layer shows that external mechanisms due to dissociation decrease separation length at low incidence but depend on the free-stream dissociation at high incidence, and have only a small effect on peak heating. A limited numerical study of reacting boundary layers shows that internal mechanisms due to recombination in the upstream boundary layer cause a slight decrease in separation length and a large increase in heat flux relative to a nonreacting boundary layer with the same external conditions.\n\nCorrelations are presented of experimentally measured separation length using local external flow parameters computed for reacting flow, which scales out external mechanisms but not internal mechanisms. These show the importance of the new scaling parameter in high-enthalpy flows, a linear relationship between separation length and reattachment pressure ratio as found previously for supersonic interactions, and a Reynolds-number effect for transitional interactions. A significant increase in scaled separation length is observed for high-enthalpy data in the laminar regime, and this is attributed to an internal recombination mechanism occurring in the separated shear layer. Experimental data for reattachment heat flux are found to agree roughly with existing correlations and to exhibit an increase due to an internal recombination mechanism, but cannot provide further insight due to large scatter." }, { "name": "Gharib, Mohammad Reza", "degree": "PhD", "year": "1999", "title": "Vortex-induced vibration, absence of lock-in and fluid force deduction", "advisor": "", "url": "https://resolver.caltech.edu/CaltechETD:etd-07132005-133506", "creators": [ { "name": { "family": "Gharib", "given": "Mohammad Reza" }, "id": "Gharib-M-R", "display_name": "Gharib, Mohammad Reza" } ], "advisors": [], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CF0Y-5A59", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nVortex-induced vibration (VIV) of elastically supported cylinders in cross-flow is studied experimentally for cylinder mass ratios (average cylinder density / fluid density), [...]. For small mass ratios below 10, a new VIV mode is discovered which does not involve a lock-in behavior. The oscillation and the shedding frequencies coalesce and deviate slightly from the nominal Strouhal frequency of St=0.2 to smaller values with increasing free stream velocity U. With increasing mass ratio above 10 (m*>10), the frequency growth with free stream velocity U appears to approach the lock-in limit while the amplitude and the frequency range of oscillations diminish. Additionally, a novel technique is employed to deduce the unsteady lift coefficient on the body using VIV time traces of the cylinder displacement and their numerical derivatives.\n\nAn analytical study of the dynamical equation shows that the oscillation amplitude (A/D) is inversely proportional to effective stiffness, [...], where U represents the non-dimensional flow speed and [...], the ratio of the oscillation to natural frequencies. It is hence maintained that at high mass ratio cases studied previously (m*>100), lock-in behavior ([...] for U ~ 1) is a prerequisite for nominal vibration amplitudes. At low values of mass ratio, however, [...] is minimized naturally without a need for lock-in.\n\nThrough a detailed study of a large number of cases with low to medium mass ratios in different experimental settings, it is additionally argued that lock-in is a sporadic phenomenon that appears at various mass ratios. The few occurrences of lock-in at low mass ratios with nominal damping and the unexpected absence of any oscillations at medium mass ratios (m*~30), except for a few cases exhibiting lock-in tendencies indicate that lock-in is not as common as classically believed.\n" }, { "name": "Leyva, Ivett A.", "degree": "PhD", "year": "1999", "title": "Shock detachment process on cones in hypervelocity flows", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02082008-162753", "creators": [ { "name": { "family": "Leyva", "given": "Ivett A." }, "id": "Leyva-I-A", "display_name": "Leyva, Ivett A." } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/62TN-GA26", "abstract": "The shock detachment process on cones in hypervelocity flows is one of the most sensitive flows to relaxation effects. The critical angle for shock detachment under frozen conditions can be very different from the critical angle under chemical and thermal equilibrium. The rate of increase of the detachment distance with cone angle is also affected by the relaxation rate.
\r\n\r\nThe purpose of this study is to explain the effects of nonequilibrium on the shock detachment distance and its growth rate on cones in hypervelocity flows. The study consists of an experimental and a computational program. The experimental part has been carried out at Caltech's hypervelocity reflected shock tunnel (T5). Six different free-stream conditions have been chosen, four using N2 as the test gas and two using CO2. About 170 shots were performed on 24 cones. The cones range in diameter from 2 cm to 16 cm with half-angles varying from 55\u00b0 to 75\u00b0. The experimental data obtained are holographic interferograms of every shot, and surface temperature and pressure measurements for the bigger cones. Extensive numerical simulations were made for the N2 flows and some were also made for the CO2 flows. The code employed is a Navier-Stokes solver that can account for thermal and chemical nonequilibrium in axisymmetric flows.
\r\n\r\nThe experimental and computational data obtained for the shock detachment distance confirms a previous theoretical model that predicts the detachment distance will grow more slowly for relaxing flows than for frozen or equilibrium flows. This difference is explained in terms of the behavior of the sonic line inside the shock layer. Different growth rates result when the detachment distance is controlled by the diameter of the cone (frozen and equilibrium cases) than when it is controlled by the extent of the relaxation zone inside the shock layer (nonequilibrium flows). The experimental data are also complemented with computational data to observe the behavior of the detachment distance from the frozen to equilibrium limits for a given cone half-angle and free-stream condition. As deduced by a previous simple scaling argument, the ratio of the detachment distance to the diameter of the cone is constant in the two extremes and rapidly switches from one value to the other for cone diameters of about 2 cm to 16 cm. The experimental interferograms are also compared with numerical ones in terms of the detachment distance, the number of fringes in the shock layer, and the shape of the fringes.
\r\n\r\nThe heat flux traces obtained from the temperature measurements show different behaviors for the attached and detached cases, but these effects can be related to the conditions at the edge of and inside the boundary layer and to the Reynolds number of the flow rather than to nonequilibrium effects. The pressure measurements were insensitive to the degree of nonequilibrium.
\r\n" }, { "name": "Maheo, Patrice Michel", "degree": "PhD", "year": "1999", "title": "Free-Surface Turbulent Shear Flows", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-02212008-104541", "creators": [ { "name": { "family": "Maheo", "given": "Patrice Michel" }, "id": "Maheo-Patrice-Michel", "display_name": "Maheo, Patrice Michel" } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Raichlen", "given": "Fredric" }, "id": "Raichlen-F", "role": "member", "display_name": "Raichlen, Fredric" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Wu-T-Y-T", "role": "member", "display_name": "Wu, Theodore Yao-tsu" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TCTV-BM26", "abstract": "The structure and dynamics of turbulent wakes and shear layers in the presence of a clean free surface have been investigated experimentally using digital particle image velocimetry (DPIV). The purpose of this study was to determine the extent and characteristics of the influence, if any, of the free surface on these underlying turbulent shear flows.
\r\n\r\nThe free surface was found to affect the dynamics of turbulence within a surface layer on the order of one half-width of the submerged wake and one half of the local vorticity thickness of the submerged shear layer. Within this layer, the vertical velocity fluctuations are inhibited and the turbulence kinetic energy is redistributed to the horizontal components. The self-induced motion of surface-parallel vortical structures under the influence of their images was shown to lead to large-scale mean streamwise secondary flows and associated outward surface currents-symmetric for the wake and asymmetric for the shear layer. This motion was the origin of the significantly higher lateral spreading rates of these surface shear flows compared to the spreading rates of their fully-submerged counterparts \u2014 20% and 25% for the wake and shear layer respectively. In addition, the evolution of the streamwise and surface-normal enstrophy components within the surface layer was consistent with the normal connection of vortical structures required at a free surface.
\r\n\r\nThe influence of the secondary flows was tracked back to the splitter plate's turbulent boundary layers where they were hence deduced to originate. A simple analysis of the mixed-boundary corner flows of the splitter plate made using the mean streamwise vorticity equation coupled with the evolution of the values of the transverse velocity confirmed the latter. In this picture of the mean flow, the secondary flows present in the near-surface edges of these shear flows were related to the pair of outer secondary vortices generated thereby. Furthermore, using a simplified equation for the surface-normal Reynolds stress, it was shown that the mutual interaction of the surface-parallel vortical structures with their images yielded a decrease in vertical velocity fluctuations as the free surface was approached. This equation shed further light on the redistribution of the vertical kinetic energy of turbulence into the other two Reynolds normal stresses. The resulting free-surface Reynolds-stress anisotropy in turn gave birth to the two streamwise secondary flows.
\r\n\r\n\r\n" }, { "name": "Shariff, Shaun", "degree": "PhD", "year": "1999", "title": "Numerical simulation of viscous reacting hypersonic flow past cones", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-02212008-155520", "creators": [ { "name": { "family": "Shariff", "given": "Shaun" }, "id": "Shariff-S", "display_name": "Shariff, Shaun" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/D6TR-5Z18", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\n\r\nThe stepback and partial grid techniques were used in the numerical simulation of laminar viscous reacting flows past cones at incidence. The stepback technique is a method for computing exactly flows which are truly conical, and can be an effective approximate method for nearly conical flows such as viscous flow past cones. The partial grid technique uses a stepback solution as the upstream boundary condition for a time-marching calculation, and is more accurate than the stepback method. Both frozen and chemically active equilibrium flow were considered, using the Ideal Dissociating Gas model. Computations were performed for frozen hypersonic flow past a 10[...] half-angle cone inclined at 24[...] incidence, and for hypervelocity flow past a 15[...] half-angle cone at 30[...] incidence with active dissociation and recombination chemistry. These computations were compared with experiments, and the effects of equilibrium and non-equilibrium chemistry were observed. These calculations also show that the effects of chemistry on heat transfer and separation location are small and of the same order as the errors associated with the stepback method. Therefore, for high accuracy in computing reacting flows, the partial grid method should be used.\r\n" }, { "name": "Lee, Sangwook", "degree": "PhD", "year": "1998", "title": "Failure of laminated composites at thickness discontinuities under complex loading and elevated temperatures", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-01222008-140642", "creators": [ { "name": { "family": "Lee", "given": "Sangwook" }, "id": "Lee-Sang", "display_name": "Lee, Sangwook" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9p36-ht49", "abstract": "Failure initiation of laminated composites with discontinuous thickness is examined in terms of typical structural load description (tension, shear force and bending moment) rather than in terms of micromechanics considerations. Because transverse shear produced relatively small effects in failure initiation, results are presented as tension-bending interactions. Two loading frames were designed to apply moments and tension simultaneously. Four types of specimens of different stacking sequence were examined to determine failure initiation, and analyzed subsequently via a finite element analysis (ABAQUS). Depending on the stacking sequence across the interface of the step, two different failure modes are identified: For uni-directional fiber orientation across the interface in the tension direction, failure occurs through cracking and delamination which is governed by a fracture mechanics criterion. While the initiation strength for this failure mode is higher than for the cross-ply configurations, the residual strength after initiation is only marginally higher, providing virtually no margin of safety (10%). For cases involving cross-plies on either side of the interface, failure initiation occurs by matrix cracking, with a critical strain across the fibers providing a universal failure criterion. In these cases the residual load bearing capability was 30 to 45% higher than the failure initiation loads. The interaction between moment and tension at failure initiation is linear, an observation that does not hold for the delamination failure driven by crack propagation. It is found that all failures can be described in terms of a common fracture principle; the stress or strain criteria are interchangeable with the fracture energy computations, provided one allows for a range of values of associated fracture energies. Assuming that time dependent aspects of the failure process are not dominant, elevated temperatures did not change the general results of how bending and tension loads interact, provided one accounts for residual thermal stresses; however, the stress magnitude at which the failure initiation occurs decreases with increasing temperature.\r\n" }, { "name": "Park, Han G.", "degree": "PhD", "year": "1998", "title": "A Study of Heat Transport Processes in the Wake of a Stationary and Oscillating Circular Cylinder Using Digital Particle Image Velocimetry/Thermometry", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-04132004-150955", "creators": [ { "name": { "family": "Park", "given": "Han G." }, "id": "Park-Han-G", "display_name": "Park, Han G." } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "orcid": "0000-0001-5592-2334", "role": "member", "display_name": "Hunt, Melany L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/C9KN-RQ12", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\t\t\t\t\t\r\nAn experimental investigation is carried out on the processes of heat transfer associated with a heated circular cylinder in crossflow. Two studies are made. First, a study of the transport of heat in the near wake (x/D<5) of a stationary and transversely oscillated cylinder is made at Reynolds number of 610. Second, a study is made of the surface heat transfer from a cylinder which is undergoing forced oscillations in the transverse direction.\r\n\r\nThe studies are made using the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) which allows simultaneous measurements of both the velocity and temperature fields of the flow. The temperature is measured by seeding the flow with thermochromic liquid crystal (TLC) particles which change their reflected wavelength as function of temperature. By calibrating reflected wavelength versus temperature using a color multi-CCD camera, the local temperature of the flow may be deduced. The velocity is measured by using the same particles as Lagrangian flow tracers, and local velocity or displacement of the flow may be measured by cross-correlating two sequential images. A limitation of DPIV/T, which is the low level of precision (5% - 20% of the temperature span of TLC particles), may be overcome by a process in which the temperature at a given location is computed by averaging the temperatures of the particles within a specified sampling window. This process increases the precision to 2% - 10%.\r\n\r\nIn the study of the heat transport in the near wake, the velocity and temperature measurements obtained from DPIV/T are decomposed into their mean, coherent, and incoherent components using the triple decomposition. It is found that the heat from the cylinder is transported down the wake mostly by the mean heat flux and is laterally transported out of the wake by the coherent and the incoherent heat fluxes. In examining the direction of the turbulent heat flux vectors, the vectors are found not to be co-linear with the gradient of mean temperature. This misalignment implies that the gradient transport models are inappropriate for modeling the turbulent heat transport in the near wake of a circular cylinder. In examining the production of turbulence, it is found that that kinetic energy fluctuations are produced in the saddle regions (regions where the fluid is being stretched in one direction and compressed in another) while the temperature fluctuations are produced at the edges of center regions (regions where the fluid is rotating), i.e., the edges of the vortex cores.\r\n\r\nFrom the study of the heat convection from a cylinder as function of forced oscillation frequency [...] and amplitudes (A/D=0.1, 0.2), it is found that besides the previously known increase near the natural vortex shedding frequency, there also exists a large increase in the heat transfer at approximately three times this frequency for A/D=0.1. For A/D=0.2, there exist large increases at roughly two and three times the natural vortex shedding frequency. From a DPIV/T study, it is found that the wake pattern becomes synchronized with the mechanical oscillation of the cylinder at these frequencies where the heat transfer increases significantly. At the frequencies corresponding to roughly two and three times the unforced vortex shedding frequency, the wake pattern may become synchronized by processes of period doubling and tripling with respect to the cylinder oscillation period, respectively. The increase in the heat transfer rate is found to correlate with the distance at which vortices roll-up behind the cylinder. The distance is observed to decrease sharply at the frequencies corresponding to a sharp increase in the heat transfer. Therefore, the near wake is found to play a critical role in the heat transfer from the surface of a circular cylinder, and the cause of the increase in heat transfer is believed to the removal of the stagnant and low heat convecting fluid at the base of the cylinder during the roll-up of the vortices." }, { "name": "Radovitzky, Raul A", "degree": "PhD", "year": "1998", "title": "Error estimation and adaptive meshing in strongly nonlinear dynamic problems", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-113427", "creators": [ { "name": { "family": "Radovitzky", "given": "Raul A" }, "id": "Radovitzky-R-A", "display_name": "Radovitzky, Raul A" } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MF7F-YK03", "abstract": "This dissertation is concerned with the development of a general computational framework for mesh adaption such as is required in the three-dimensional lagrangian finite element simulation of strongly nonlinear, possibly dynamic, problems. It is shown that, for a very general constitutive framework, the solutions of the incremental boundary value problem obey a minimum principle, provided that the constitutive updates are formulated appropriately. This minimum principle is taken as a basis for asymptotic error estimation. In particular, we chose to monitor the error of a lower-order projection of the finite element solution. The optimal mesh size distribution then follows from a posteriori error indicators which are purely local, i. e., can be computed element-by-element.\n\nA sine qua non condition for the successful accomplishment of the kind of analysis envisioned in this work is the possibility to mesh the deforming domains of analysis. In the first section of this thesis a method is presented for mesh generation in complex geometries and general--possibly non-manifold--topologies.\n\nThe robustness and versatility of the computational framework is demonstrated with the aid of convergence studies and selected examples of application and the results contrasted with previous approaches" }, { "name": "Repetto, Eduardo A.", "degree": "PhD", "year": "1998", "title": "On the fatigue behavior of ductile F.C.C. metals", "advisor": "Ortiz, Michael", "url": "https://resolver.caltech.edu/CaltechETD:etd-01242008-133649", "creators": [ { "name": { "family": "Repetto", "given": "Eduardo A." }, "id": "Repetto-E-A", "display_name": "Repetto, Eduardo A." } ], "advisors": [ { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "advisor", "display_name": "Ortiz, Michael" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/zkfb-sr87", "abstract": "A micromechanically based model for fatigue crack nucleation in ductile F.C.C. metals is developed. The theory includes a model of F.C.C. crystal-plasticity in finite deformations that takes into account the Bauschinger effect, dipole annihilation in the persistent slip bands (PSBs), with vacancy generation and PSB elongation as a byproduct, as well as coupled vacancy diffusion and the attendant surface motion due to the flux of vacancies out of the body.\r\n\r\nFinite element simulations are performed in order to establish the predictive capability of the theory. Detailed modelling of the intersection of the PSB with a free surface, enhanced by the use of remeshing and surface evolution techniques, enable the prediction of nucleation sites, life expectancy, surface profile, alternate slip between the sides of the PSB and strain localization at the grooves.\r\n\r\nIn an attempt to resolve the dislocation structures experimentally observed during cyclic loading, a theory based on the non-convexity of a pseudo-energy density is developed. Non-homogeneous minimizers are found containing variants oriented in coincidence with the dislocation walls observed experimentally. Due to the latent hardening and geometrical softening, the minimizing structures are found to consist of regions of single slip which is in accordance with the observed \"patchy slip.\"" }, { "name": "Shiels, Douglas G.", "degree": "PhD", "year": "1998", "title": "Simulation of Controlled Bluff Body Flow with a Viscous Vortex Method", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-03162004-133652", "creators": [ { "name": { "family": "Shiels", "given": "Douglas G." }, "id": "Shiels-Dpouglas-G", "display_name": "Shiels, Douglas G." } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/49A0-VA12", "abstract": "Bluff body flows controlled in various manners are simulated with a high-resolution, gridless vortex method. Two-dimensional, unsteady, viscous simulations are utilized to illuminate the physical phenomenon underpinning certain flows of this class. Flows past a rotationally oscillating circular cylinder and flows past an elastically mounted circular cylinder are studied, providing a variety of new insights about these systems. A computational method facilitating longtime, high-resolution vortex simulations is developed whose grid-free nature enables future extension to complex geometries.\r\n\r\nThe significant fluid forces experienced by bluff bodies are of much practical concern and are induced by flowfields that are often complex. The studies in this thesis aim to contribute to the understanding of the relation between wake development and forces and how to exploit this relationship to achieve flow control. A circular cylinder undergoing rotational oscillation is known to experience a significant deviation in forces from unforced flow. Computations from Re=150-15000 verify past experimental observation of significant drag reduction for certain forcing parameters. These simulations also illuminate the mechanism which renders this control effective - a forced boundary layer instability triggering premature shedding of multipole vortex structures.\r\n\r\nNew insights were also provided by studies of flow over a model of an elastically mounted cylinder. A two-dimensional cylinder modeled as a damped oscillator can serve as an approximation to three-dimensional situations such as a cable under tension. Simulations clarified the behavior of such a two-dimensional system and, contrary to a line of classical thinking, revealed an unexpected adaptivity in wake evolution. New scaling is also suggested which better classifies these systems under certain conditions.\r\n\r\nVortex methods are well-suited for incompressible bluff body flow in many ways. However, the handling of viscous diffusion causes complications for such simulations. A relatively unexplored approach, the core expansion method, is studied, extended, and implemented in this work in order to balance accuracy with preservation of the gridless foundation of vortex methods. This viscous technique is found to enable long-time calculations that are prohibitive with other techniques while preserving a high level of accuracy." }, { "name": "Slessor, Michael David", "degree": "PhD", "year": "1998", "title": "Aspects of turbulent-shear-layer dynamics and mixing", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03292005-085835", "creators": [ { "name": { "family": "Slessor", "given": "Michael David" }, "id": "Slessor-M-D", "display_name": "Slessor, Michael David" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K55F-8589", "abstract": "Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility to investigate some aspects of high-Reynolds-number, turbulent, shearlayer flows in both incompressible- and compressible-flow regimes. Experiments designed to address several issues were performed; effects of inflow boundary conditions, freestream conditions (supersonic/subsonic flow), and compressibility, on both large-scale dynamics and small-scale mixing, are described.\r\n\r\nChemically-reacting and non-reacting flows were investigated, the former relying on the (H2 + NO/F2) chemical system, in the fast-kinetic regime, to infer the structure and amount of molecular-scale mixing through use of \"flip\" experiments. A variety of experimental techniques, including a color-schlieren visualization system developed as part of this work, were used to study the flows.\r\n\r\nBoth inflow conditions and compressibility are found to have significant effects on the flow. In particular, inflow conditions are \"remembered\" for long distances downstream, a sensitivity similar to that observed in low-dimensionality, non-linear (chaotic) systems. The global flowfields (freestreams coupled by the shear layer) of transonic flows exhibit a sensitivity to imposed boundary conditions, i. e., local area ratios. A previously-proposed mode-selection rule for turbulent-structure convection speeds, based on the presence of a lab-frame subsonic freestream, was experimentally demonstrated to be incorrect. Compressibility, when decoupled from all other parameters, e.g., Reynolds number, velocity and density ratios, etc., reduces laxge-scale entrainment and turbulent growth, but slightly enhances smallscale mixing, with an associated change in the structure of the molecularly-mixed fluid. This reduction in shear-layer growth rate is examined and a new parameter that interprets compressibility as an energy-exchange mechanism is proposed. The parameter reconciles and collapses experimentally-observed growth rates.\r\n" }, { "name": "Adam, Philippe H.", "degree": "PhD", "year": "1997", "title": "Enthalpy Effects on Hypervelocity Boundary Layers", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01072008-111636", "creators": [ { "name": { "family": "Adam", "given": "Philippe H." }, "id": "Adam-Philippe-H", "display_name": "Adam, Philippe H." } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YF0K-HK91", "abstract": "More than 50 shots with air and 35 shots with carbon dioxide were carried out in the T5 shock tunnel at GALCIT to study enthalpy effects on hypervelocity boundary layers. The model tested was a 5\u00b0 half-angle cone measuring approximately 1 meter in length. It was instrumented with 51 chromel-constantan coaxial thermocouples and the surface heat transfer rate was computed to deduce the state of the boundary layer and, when applicable, the transition location.\r\n\r\nTransitional boundary layers obtained confirm the stabilizing effect of enthalpy. As the reservoir enthalpy is increased, the transition Reynolds number evaluated at the reference conditions increases as well. The stabilizing effect is more rapid in gases with lower dissociation energy and it seems to level off when no further dissociation can be achieved. These effects do not appear when the transition location is normalized with the edge conditions. Further normalizing the reservoir enthalpy with the edge enthalpy appears to collapse the data for all gases onto a single curve. A similar collapse is obtained when normalizing both the transition location and the reservoir enthalpy with maximum temperature conditions obtained with BLIMPK, a nonequilibrium boundary layer code.\r\n\r\nThe observation that the reference conditions seem more appropriate to normalize high enthalpy transition data was taken a step further by comparing the tunnel data with results from a reentry experiment. When the edge conditions are used, the tunnel data are around an order of magnitude below the flight data. This is commonly attributed to the fact that disturbance levels in tunnels are high, causing the boundary layer to transition prematurely. However, when the conditions at the reference temperature are used instead, the data come within striking distance of one another although the trend with enthalpy seems to be a destabilizing one for the flight data. This difference could be due to the cone bending and blunting observed during the reentry.\r\n\r\nExperimental laminar heat transfer levels were compared to numerical results obtained with BLIMPK. Results for air indicate that the reactions are probably in nonequilibrium and that the wall is catalytic. The catalycity is seen to yield higher surface heat transfer rates than the noncatalytic and frozen chemistry models. The results for carbon dioxide, however, are inconclusive. This is, perhaps, because of inadequate modeling of the actual reactions. Experimentally, an anomalous yet repeatable, rise in the laminar heat transfer level can be seen at medium enthalpies in carbon dioxide boundary layers.\r\n" }, { "name": "Hodowany, Jon", "degree": "PhD", "year": "1997", "title": "On the conversion of plastic work into heat", "advisor": "Ravichandran, Guruswami; Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01102008-074409", "creators": [ { "name": { "family": "Hodowany", "given": "Jon" }, "id": "Hodowany-J", "display_name": "Hodowany, Jon" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FENH-ZK36", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe present study investigated heat evolution in metal plasticity. Specifically, experiments were designed to measure the partition of plastic work into heat and stored energy during dynamic deformations. The fraction of plastic work converted into heat has implications in a wide range of thermomechanical phenomena, including shear bands, dynamic fracture, ballistic penetration and high speed machining.\r\n\r\nKolsky bars, in compression and torsion, were used to determine mechanical properties at strain rates between [...] and [...]. For dynamic loading, in-situ temperature changes were measured using a high-speed HgCdTe photoconductive detector. Specially designed infrared optics, configured in tandem with the HgCdTe detector and the Kolsky bar constituted a novel experimental configuration for determining the fraction of plastic work converted into heat, and thus, the amount of energy stored in metals. The temperature detection system was ideally suited for small temperature excursions from ambient conditions, and was sensitive to temperature changes as little as 0.5 \u00b0C. The emissivity of metals was found to increase above certain high levels of plastic strain due to changes in surface roughness, which can affect the validity of temperature calibration. A technique of sample recovery, rough surface layer removal, and reloading was employed to obtain large plastic strains in the Kolsky bar. A Materials Testing System (MTS) servo-hydraulic load frame was used to measure mechanical properties at lower strain rates, [...] to [...] When temperature measurement was needed within this range of strain rates, a fast E-type thin wire thermocouple, with a time response of 1 ms, was employed.\r\n\r\nThe fraction of plastic work converted into heat, [beta], was treated as a constitutive function of strain and strain rate in the heat conduction equation. 2024 aluminum alloy and commercially pure [alpha]-titanium were the metal systems used in the current study to determine the functional dependence of [beta] on strain and strain rate. The T351, T4 and T6 tempers of 2024 aluminum did not exhibit strain rate dependence in flow stress over the entire range of strain rates tested. At low levels of plastic strain, all tempers of 2024 aluminum stored more than 50% of the input plastic work. At some level of plastic strain, depending on temper, 2024 aluminum could no longer store plastic work. After this point, [beta] increased to a value near 1.0 and remained nearly constant during subsequent plastic deformation. When averaged over all strains, [beta] was 0.85-0.95 depending on the particular heat treatment. The fraction of plastic work dissipated as heat was not found to be sensitive to strain rate over a wide range of strain rates. In contrast, the flow stress of [alpha]-titanium was strongly dependent on strain rate. The initial flow stress increased by more than 15% between strain rates of [...] and [...]. In addition, the strain hardening was also observed to be rate dependent. For fixed plastic strain, the tangent modulus increased as strain rate increased. Titanium dissipated a greater proportion of energy as heat at low strains than all tempers of 2024 aluminum. The ability to store energy in titanium decreased with increasing plastic strain. For plastic strains above 0.3, titanium dissipated nearly all input plastic work as heat. The proportion of energy dissipated as heat at fixed strain increased as strain rate increased.\r\n" }, { "name": "Krok, James Christopher", "degree": "PhD", "year": "1997", "title": "Jet Initiation of Deflagration and Detonation", "advisor": "Shepherd, Joseph E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-181337", "creators": [ { "name": { "family": "Krok", "given": "James Christopher" }, "id": "Krok-James-Christopher", "display_name": "Krok, James Christopher" } ], "advisors": [ { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "advisor", "display_name": "Shepherd, Joseph E." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "orcid": "0000-0002-2912-0001", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Kirschvink", "given": "Joseph L." }, "id": "Kirschvink-J-L", "orcid": "0000-0001-9486-6689", "role": "member", "display_name": "Kirschvink, Joseph L." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PSTR-T717", "abstract": "We have constructed a facility for the study of jet-initiated deflagration and detonation in hydrogen-air-steam mixtures. The facility is built around two pressure vessels. Mixtures of hydrogen, oxygen and nitrogen are spark-ignited in the driver vessel, generating a hot mixture of combustion products. The pressure rise ruptures a diaphragm, venting the products into the receiver vessel through nozzles of 12.7-92 mm diameter. The receiver is filled with hydrogen-air and hydrogen-oxygen mixtures diluted with either nitrogen or steam.
\r\n\r\nThe deflagration tests studied the lean and maximum-dilution limits of hydrogen-air mixtures ignited by a hydrogen-steam jet. The lean limit of 6% hydrogen was comparable to other studies. The maximum dilution limit for steam was 60%. This is higher than the limit found in spark/glow plug ignition experiments. Shock oscillations in the receiver increased with nozzle size.
\r\n\r\nFurther tests studied the initiation of detonation in both hydrogen-air and stoichiometric hydrogen-oxygen-diluent mixtures. In terms of jet diameter, D, and receiver detonation cell size, \u03bb, we found initiation limits of 2 < D/\u03bb < 7, where other experiments required a D/\u03bb of 11 or more. We propose that the D/\u03bb model does not adequately characterize jet initiation, as it does not reflect the conditions in the driver.
\r\n\r\nThe tests indicated that shock focusing plays an important role, promoting strong secondary explosions with or without prompt initiation of detonation. Mixtures with steam dilution were prone to DDT near the detonation limit, as the slower flame speed allows shock reflection and pressurization to occur before the reactants are consumed. Tests with nitrogen dilution had no DDT regime. Because of DDT and shock focusing, peak pressures were highest in mixtures that were slightly less sensitive than the detonation threshold. Schlieren movies confirmed the formation of a detonation near the nozzle exit.
" }, { "name": "Lu, Hongbing", "degree": "PhD", "year": "1997", "title": "Nonlinear thermo-mechanical behavior of polymers under multiaxial loading", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11072003-103136", "creators": [ { "name": { "family": "Lu", "given": "Hongbing" }, "id": "Lu-H", "display_name": "Lu, Hongbing" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SH42-W153", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [?]. Abstract is included in .pdf document.\n\nIn order to better understand phenomena related to \"yield-like\" behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22[degrees] and 100[degrees]C. A non-contact measurement technique based on digital image correlation method is developed to determine the surface deformations on curved cylindrical specimens at various temperatures, including temperatures close to the glass transition when the specimens become soft. The study indicates that in contrast to tile mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong coupling between all deformation or stress invariants as assessed in creep experiments for strains in excess of' 0.4%. While shear stresses alone elicit nonlinear response in creep (rates) as \"intrinsically\" nonlinear shear response, the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in quantitative agreement with free volume arguments. Passing below the glass transition still produces greater creep acceleration from positive dilatation than from a decrease in specific volume, but the dominance of the intrinsic shear nonlinearity vis-a-vis the dilatational ilifluence increases below [...] as the temperature drops. A constitutive model based oil free volume consideration is modified from the models proposed by Losi and Knauss (1992). Comparison of experimental results with model prediction indicates a good qualitative agreement, and reasonably good quantitative agreement." }, { "name": "Noca, Flavio", "degree": "PhD", "year": "1997", "title": "On the Evaluation of Time-Dependent Fluid-Dynamic Forces on Bluff Bodies", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-08102005-132659", "creators": [ { "name": { "family": "Noca", "given": "Flavio" }, "id": "Noca-Flavio", "display_name": "Noca, Flavio" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "member", "display_name": "Brennen, Christopher E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K2Z0-9016", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nWe present some exact expressions for the evaluation of time-dependent forces on a body in an incompressible and viscous cross-flow which only require the knowledge of the velocity field (and its derivatives) in a finite and arbitrarily chosen region enclosing the body.\r\n\r\nGiven a control volume V with external surface S which encloses an arbitrary body, the fluid-dynamic force F on the body can be evaluated from one of the following three expressions (in abbreviated form):\r\n\r\nF = [...] + body motion terms,\r\n\r\nF = [...] + body motion terms,\r\n\r\nF = no volume integral terms + [...] + body motion terms,\r\n \r\nwhere N is the space dimension, u is the flow velocity, [...] is the vorticity, x is the position vector, and the tensors [...], [...], [...] depend only on the velocity field u and its (spatial and temporal) derivatives.\r\n\r\nThe first equation is already known for either simply connected domains or inviscid flows. We re-derive it here for viscous flows in doubly connected domains (i.e. domains which include a body). We then obtain the second and third equation through a simple algebraic manipulation of the first equation.\r\n\r\nThese expressions are particularly useful for experimental techniques like Digital particle Image Velocimetry (DPIV) which provide time sequences of 2D velocity fields but not pressure fields.\r\n\r\nThey are tested experimentally with DPIV on two-dimensional, low Reynolds number circular cylinder flows. Both steady and unsteady motions are studied." }, { "name": "Papalexandris, Miltiadis Vassilios", "degree": "PhD", "year": "1997", "title": "Unsplit Numerical Schemes for Hyperbolic Systems of Conservation Laws with Source Terms", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-06032005-161139", "creators": [ { "name": { "family": "Papalexandris", "given": "Miltiadis Vassilios" }, "id": "Papalexandris-Miltiadis-Vassilios", "display_name": "Papalexandris, Miltiadis Vassilios" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Colonius", "given": "Tim" }, "id": "Colonius-T", "orcid": "0000-0003-0326-3909", "role": "member", "display_name": "Colonius, Tim" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HW7S-AR36", "abstract": "In this thesis, a new method for the design of unsplit numerical schemes for hyperbolic systems of conservation laws with source terms is developed. Appropriate curves in space-time are introduced, along which the conservation equations decouple to the characteristic equations of the corresponding one-dimensional homogeneous system. The local geometry of these curves depends on the source terms and the spatial derivatives of the solution vector. Numerical integration of the characteristic equations is performed on these curves.\r\n\r\nIn the first chapter, a scalar conservation law with a stiff, nonlinear source term is studied using the proposed unsplit scheme. Various tests are made, and the results are compared with the ones obtained by conventional schemes. The effect of the stiffness of the source term is also examined.\r\n\r\nIn the second chapter, the scheme is extended to the one-dimensional, unsteady Euler equations for compressible, chemically-reacting flows. A numerical study of unstable detonations is performed. Detonations in the regime of low overdrive factors are also studied. The numerical simulations verify that the dynamics of the flow-field exhibit chaotic behavior in this regime.\r\n\r\nThe third chapter deals with the development and implementation of the unsplit scheme, for the two-dimensional, reactive Euler equations. In systems with more than two independent variables there are one-parameter families of curves, forming manifolds in space-time, along which the one-dimensional characteristic equations hold. The local geometry of these manifolds and their position relative to the classical characteristic rays are studied. These manifolds might be space-like or time-like, depending on the local flow gradients and the source terms.\r\n\r\nIn the fourth chapter a numerical study of two-dimensional detonations in performed. These flows are intrinsically unstable and produce very complicated patterns, such as cellular structures and vortex sheets. The proposed scheme appears to be capable of capturing many of the the important details of the flow-fields. Unlike traditional schemes, no explicit artificial-viscosity mechanisms need to be used with the proposed scheme." }, { "name": "Prochazka, Aurelius", "degree": "PhD", "year": "1997", "title": "Stability and structure of stretched vortices", "advisor": "Pullin, Dale Ian", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142008-091416", "creators": [ { "name": { "family": "Prochazka", "given": "Aurelius" }, "id": "Prochazka-A", "display_name": "Prochazka, Aurelius" } ], "advisors": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "advisor", "display_name": "Pullin, Dale Ian" } ], "committee": [ { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "chair", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Grether", "given": "David M." }, "id": "Grether-D-M", "role": "member", "display_name": "Grether, David M." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/sc0x-1g40", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nWe investigate, numerically and analytically, the structure and stability of steady and quasi-steady solutions of the Navier-Stokes equations corresponding to steady stretched vortices embedded in a uniform nonsymmetric straining field, [...], [...], one principal axis of extensional strain of which is aligned with the vorticity. These are known as nonsymmetric Burgers vortices studied first by Robinson and Saffman (1984). We consider vortex Reynolds numbers [...] where [...] is the vortex circulation and [...] the kinematic vorticity, in the range [...], and a broad range of strain ratios [...] including [...], and in some cases [...]. A pseudo-spectral method is used to obtain numerical solutions corresponding to steady vortex states over our whole ([...] parameter space including [...], where arguments proposed by Moffatt, Kida, and Ohkitani (1994) suggest the nonexistence of steady solutions. When [...] and [...], we find an accurate asymptotic form for the vorticity in a region [...], giving, in some cases, near machine-precision agreement with our numerical solutions. This suggests the existence of an extended region where the exponentially small vorticity is confined to a near cat's-eye shaped region of the almost two-dimensional flow, and takes a constant value nearly equal to [...] on bounding streamlines. This allows an estimate of the leakage rate of circulation to infinity as [...] = [...] with corresponding exponentially slow decay of the vortex when [...]. This leakage rate differs substantially from that estimated by Moffatt, Kida, and Ohkitani. The normal-mode linear stability of the axisymmetric Burgers vortex [...] to two-dimensional disturbances is calculated in detail and the vortex is found to be stable at all Reynolds numbers. An iterative technique based on the Power method is used to estimate the largest eigenvalues for the nonsymmetric case [...]. Stability is found for [...], and a neutrally convective mode of instability is found and analyzed analytically for [...]. Our general conclusion is that the generalized nonsymmetric Burgers vortex is unconditionally stable to two-dimensional disturbances for all [...], and that the vortex will tend to move with the background strain when [...], but maintain its structure which will change only through exponentially slow leakage of vorticity, indicating extreme robustness in this case.\n" }, { "name": "Warncke, Amy E.", "degree": "PhD", "year": "1997", "title": "The Effects of Surfactants on Free-Surface Flows", "advisor": "Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-01172008-092233", "creators": [ { "name": { "family": "Warncke", "given": "Amy E." }, "id": "Warncke-Amy-E", "display_name": "Warncke, Amy E." } ], "advisors": [ { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1xg2-yy62", "abstract": "This experimental investigation into the nature of free surface flows is to study the effects of surfactants on the boundary condition at the free surface and the resulting flow field. In particular, the flow field associated with a stationary Reynolds ridge was investigated as well as the wake behind a surface-piercing cylinder, where experimental techniques such as Digital Particle Image Velocimetry and a new surface slope measurement technique were utilized.\r\n\r\nResults show a large change in the flow field in the free-surface vicinity depending on the presence of surface tension gradients and thus shear stresses at the free surface. In particular, the boundary layer beneath a Reynolds ridge was measured and it is shown that the primary source of vorticity at the free surface can be attributed to the free surface deceleration at the ridge. Also, in the wake of the cylinder, depending on the surface condition, the connection of the shedding vortex filaments was found to be greatly altered with the propensity of surface tension gradients to redirect the vorticity near the free surface to that of the surface-parallel component. Thus it is shown that surfactants can dramatically alter the flow field due to the change in the free-surface boundary condition and resulting vorticity generation and conversion in the vicinity of the free surface." }, { "name": "Burnley, Victor Scott", "degree": "PhD", "year": "1996", "title": "Nonlinear Combustion Instabilities and Stochastic Sources", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04012005-140017", "creators": [ { "name": { "family": "Burnley", "given": "Victor Scott" }, "id": "Burnley-Victor-Scott", "display_name": "Burnley, Victor Scott" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "chair", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Levine", "given": "Jay" }, "id": "Levine-J", "role": "member", "display_name": "Levine, Jay" }, { "name": { "family": "Franklin", "given": "Joel N." }, "id": "Franklin-J-N", "role": "member", "display_name": "Franklin, Joel N." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/013z-q287", "abstract": "An investigation of combustion instabilities was conducted using an approximate analysis which allows any relevant physical processes to be included. The resulting system of coupled nonlinear oscillator equations was studied using the methods of dynamical systems theory. Previous investigations have further simplified the system using the method of time-averaging and truncation to a small number of modes. We have investigated the consequences of using these additional approximations, a case which had not been addressed completely in the literature. It was determined that application of the method of time-averaging introduces a stability boundary which limits the range in which the averaged equations are valid.
\r\n\r\nTransverse oscillations in a cylindrical chamber were also treated. It was established that in addition to its role in energy transfer between modes, nonlinear gasdynamics also provides a means of shifting the frequencies of oscillations to integral multiples of the fundamental. This additional role can reduce the efficiency of energy transfer, thus increasing the acoustic amplitudes. An example of a low amplitude transverse oscillation was produced suggesting a means by which the amplitudes of transverse modes, as well as nonintegral longitudinal modes, may be reduced.
\r\n\r\nThe coupling between combustion processes and acoustic oscillations was studied as a possible explanation of the phenomenon known as triggering. Using several ad hoc models, the effects of nonlinear pressure coupling and velocity coupling on the behavior of the system were investigated. Substantial regions of possible triggering were produced when using a model of velocity coupling with a threshold, but only if nonlinear gas dynamics was also included.
\r\n\r\nThe interaction between combustion noise and acoustic instabilities has received relatively little attention. The sources of noise in a combustion chamber are associated with vorticity and entropy waves. By including these contributions in the approximate analysis, the general forms of the stochastic excitations were obtained. Subsequently, the efects of these excitations on the amplitudes of acoustic modes were studied. When only nonlinear gasdynamics was included, no cases of bimodal probability density functions, characteristic of triggering, were found. However, when the model of velocity coupling with a threshold is added, bimodal probability densities can occur.
" }, { "name": "Cates, Joseph Eugene", "degree": "PhD", "year": "1996", "title": "Studies of shock wave focusing using geometrical shock dynamics", "advisor": "", "url": "https://resolver.caltech.edu/CaltechETD:etd-10042006-142406", "creators": [ { "name": { "family": "Cates", "given": "Joseph Eugene" }, "id": "Cates-J-E", "display_name": "Cates, Joseph Eugene" } ], "advisors": [], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/bfhf-3w47", "abstract": "A finite-difference numerical method for geometrical shock dynamics has been developed, based on the analogy between the equations and the supersonic potential equation. The method has proven to be a valuable tool for analyzing the complex nonlinear processes that occur in shock focusing. The approximate shock dynamics theory is able to capture the effects of initial Mach number and aperture angle on the focal region. The numerical results duplicate the strong, moderate, and weak shock behaviors observed in experiments, with good agreement for focal pressure and triple-point path. The primary error arises due to the inability of shock dynamics to allow regular reflection along the centerline. Adequate resolution of the focal region proves to be particularly important to properly judge the accuracy of the shock dynamics solution. The appropriate shock dynamics equations are developed for the case of shock propagation into a nonuniform media for a general equation of state with nonuniform freestream velocity. The modification of the numerical method to this more general problem is straightforward. The complete shock dynamics equations are derived for both perfect gas and water using the modified Tait equation. The results for propagation of a planar shock over cylindrical gas inhomogeneities shows excellent agreement with experimental results.\n\nThe propagation of sonic booms through the atmosphere provides examples of all major types of weak shock behavior. The extensive seismic network in Southern California, consisting of over two hundred sites covering over 50,000 square kilometers, is used to map primary and secondary sonic boom carpets. The results show sonic boom ground exposure under the real atmosphere is much larger than previously expected.\n" }, { "name": "Catrakis, Haris J.", "degree": "PhD", "year": "1996", "title": "Mixing and the Geometry of Isosurfaces in Turbulent Jets", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03312005-152819", "creators": [ { "name": { "family": "Catrakis", "given": "Haris J." }, "id": "Catrakis-Haris-J", "display_name": "Catrakis, Haris J." } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GPS6-DW08", "abstract": "Experiments have been conducted to investigate mixing and the geometry of scalar isosurfaces in turbulent jets. Specifically, images of the jet-fluid concentration in the far-field of round, liquid-phase, turbulent jets have been recorded at high resolution and signal-to-noise ratio using laser-induced-fluorescence digital-imaging techniques, in the Reynolds number range 4.5 x 10\u00b3 \u2264 Re \u2264 18 x 10\u00b3. Analysis of these data indicates that this Reynolds-number range spans a mixing transition in the far field of turbulent jets. This is manifested in the probability-density function of the scalar field, as well as in other scalar-field and scalar-isosurface measures. Classical as well as fractal measures of the isosurfaces have been computed, from small to large spatial scales, and are found to be functions of both scalar threshold and Reynolds number. The coverage of level sets of jet-fluid concentration in the two-dimensional images is found to possess a scale-dependent-fractal dimension that increases continuously with increasing scale, from near unity, at the smallest scales, to 2, at the largest scales. The geometry of the scalar isosurfaces is, therefore, more complex than power-law fractal, exhibiting an increasing complexity with increasing scale. This behavior necessitates a scale-dependent generalization of power-law-fractal geometry. A connection between scale-dependent-fractal geometry and the distribution of scales is established and used to compute the distribution of spatial scales in the flow. A lognormal model of scales is proposed. The data also indicate a lognormal distribution of size of the isoscalar islands and lakes, and a powerlaw distribution of shape complexity, with values of the latter that increase with increasing size.
" }, { "name": "Howard, Danny Dwayne", "degree": "PhD", "year": "1996", "title": "Part I. Mechanisms of injury associated with extracorporeal shock wave lithotripsy; Part II. Exsolution of volatiles", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-10242005-083544", "creators": [ { "name": { "family": "Howard", "given": "Danny Dwayne" }, "id": "Howard-D-D", "display_name": "Howard, Danny Dwayne" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "member", "display_name": "Brennen, Christopher E." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "role": "member", "display_name": "Shepherd, Joseph E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/995X-8517", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nPart I - Shock waves are focused in extracorporeal shock wave lithotripsy (ESWL) machines to strengths sufficient to fracture kidney stones. Substantial side effects-most of them acute-have resulted from this procedure, including injury to soft tissue. The focusing of shock waves through various layers of tissue is a complex process which stimulates many bio-mechano-chemical responses.\n\nThis thesis presents results of an in vitro study of the initial mechanical stimulus. Planar nitrocellulose membranes of order 10 um thick were used as models of thin tissue structures. Two modes of failure were recorded: Failure due to cavitation collapsing on or near the membranes, and failure induced by altering the structure of shock waves. Tests were done in water at and around F2 to characterize the extent of cavitation damage, and was found to be confined within the focal region, 1.2 cm along the axis of focus.\n\nScattering media were used to simulate the effects of acoustic nonuniformity of tissue and to alter the structure of focusing shock waves. 40 um diameter (average) hollow glass spheres were added to ethylene glycol, glycerine and castor oil to vary the properties of the scattering media. Multiple layer samples of various types of phantom tissue were tested in degassed castor oil to gauge the validity of the scattering media. The scattering media and tissue samples increased the rise time decreased strain rate in a similar fashion. Membranes were damaged by the decreased strain rate and accumulated effects of the altered structure: After about 20 or so shocks immersed in the scattering media and after about 100 shocks behind the tissue samples. The mode of failure was tearing with multiple tears in some cases from about .1 cm to about 3 cm depending of the number of shocks and membrane thickness.\n\nPart II - This work examines the exsolution of volatiles-carbon dioxide from water-in a cylindrical test cell under different pressure conditions. Water was supersaturated with carbon dioxide under various pressures (620 to 1062 kPa), and depressurized rapidly to investigate how carbon dioxide is undissolved, exsolution, and its effects on the surrounding environment. Cavities grow as a result of convective diffusion: They move before depleting carbon dioxide in a given region. The radius of a cavity in this environment grows at a faster rate [...] than that of a cavity at rest [...]. Bubble growth rates were inferred by measuring the bulk liquid using high speed motion pictures. Water in the test-cell is accelerated as a result of buoyancy induced by cavity growth. Cavities are elliptical in shape and grow until mutual interaction causes them to fragment. Accelerations range from 10 to 100 g were measured with velocities ranging from 7 to 13 m/s.\n" }, { "name": "Kuzo, David Matthew", "degree": "PhD", "year": "1996", "title": "An experimental study of the turbulent transverse jet", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-02232005-160358", "creators": [ { "name": { "family": "Kuzo", "given": "David Matthew" }, "id": "Kuzo-D-M", "display_name": "Kuzo, David Matthew" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/xa4v-qb95", "abstract": "An experimental study that examines the mean flow properties of the turbulent transverse jet using Digital Particle Image Velocimetry (DPIV) is reported.
\r\n\r\nThis study examines various jet flows over a range of Reynolds numbers based upon jet velocity and diameter. Within this range it is documented that mean flow fields, identified as alternate mean flow states, other than the symmetric vortex pair exist. These alternate flow states are characterized by unsteadiness, asymmetry, and multiple streamwise vortices. Flow visualization shows that these alternate flow states are formed during initial jet roll-up, and persist far downstream without developing into the (expected) vortex pair. A map of the domain of existence, in velocity ratio - Reynolds number space, of these alternate states is provided, and a discussion of the underlying physics is included.
\r\n\r\nWith this new understanding, selected symmetric jet flows at velocity ratios of 5, 10, and 20 are generated and studied using DPIV. The mean flow is quantified through measurements of vortex circulation, size, and impulse, and the relationship of these properties to the (initial) jet momentum flux is examined.
" }, { "name": "Moore, Cyrille Dennis", "degree": "PhD", "year": "1996", "title": "Experiments in axisymmetric supersonic jets", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-10172005-151911", "creators": [ { "name": { "family": "Moore", "given": "Cyrille Dennis" }, "id": "Moore-C-D", "display_name": "Moore, Cyrille Dennis" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1g15-nz55", "abstract": "An experimental study of the effects of exit Mach number and density ratio on the development of axisymmetric jets is described in this thesis. Jet exit Mach numbers of 1.41, 2.0, and 3.0, were studied for jets of helium, argon, and nitrogen. The jets exit into a gas at rest (velocity ratio = 0), in order to better isolate the effects of compressibility and density ratio. Density ratios vary from 0.23 to 5.5.\n\nIn order to generate shock free-jets, unique nozzles were designed and constructed for each gas and Mach number combination. A plating method for the construction of the nozzles was developed to ensure high-accuracy and a good surface finish at a cost significantly less than direct-machining techniques.\n\nThe spreading rate of the jet for several downstream locations is measured with a pitot probe. Centerline data are used to characterise the length of the potential core of the jet, which correlates well with the relative spreading rates. Limited frequency data is obtained through the use of piezo-resistive pressure probes. This method is promising for flows that are not conducive to hot-wire probes.\n\nSpark shadography is used to visualize both the mean and instantaneous flow, with the minimum spark time being 20 nanoseconds. The convection velocity of large-scale disturbances is estimated from the visible Mach-type acoustic waves emanating from the jet.\n\nFor a wide range of jet Mach and Reynolds numbers, the convection velocity of the large scale disturbances in the potential core region of the jet is approximately 0.8 times the jet velocity, the approximate velocity of the first helical instability mode of the jet.\n\nThe main objectives of the present work were to investigate the effects of compressibility and density on the initial development of the axisymmetric jet. Although the data are not sufficient to determine if the convective Mach number concept used in 2-d shear layer research will work in the case of an axisymmetric jet, it is clear that the axisymmetric data do not collapse onto the 2-d curve. However, the density ratio scaling used for the 2-d shear layer appears to work well for the axisymmetric jet, based on the available data.\n\nThe data appear to indicate that the initial development of the jet is dominated by instability modes of the jet as a whole, rather than the shear layers.\n\nOne anomaly noted was that there were long period variations in the centerline total pressure, with times on the order of 3000 jet time scales. The fluctuations did not appear to be experimental artifacts." }, { "name": "Moore, Kevin C.", "degree": "PhD", "year": "1996", "title": "Experiments on the interaction of a coflowing light gas jet with a weak oblique shock wave", "advisor": "Zukoski, Edward E.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02072013-095344022", "creators": [ { "name": { "family": "Moore", "given": "Kevin C." }, "id": "Moore-K-C", "display_name": "Moore, Kevin C." } ], "advisors": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "advisor", "display_name": "Zukoski, Edward E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/qgz4-en52", "abstract": "Results of experiments performed in the GALCIT Supersonic Wind Tunnel involving\r\na coflowing helium jet in a Mach 2.4 air freestream are presented. Measurements\r\nof the undisturbed jet arc compared with those made after the jet interacts with\r\ndifferent weak oblique shock waves. Data collected using stagnation temperature\r\nand pitot pressure probes is used to form time averaged cross sectional views of the\r\nflow for different downstream stations. These views show that the baroclinic vorticity\r\ngenerated when the jet passes through the shock wave causes the jet to develop\r\ninto a pair of counter-rotating vortices whose axes are aligned with the flow. Previously\r\nproposed models for the circulation deposited on the jet-freestream interface\r\nand characteristic time of an analogous unsteady two-dimensional flow are adapted\r\nto this flow. Comparison of the jet cross sections for corresponding characteristic\r\ntimes shows agreement with experiments and analysis previously performed on the\r\nuustcady two-dimensional analogy. Shear between the jet and the freestream is not\r\nseen to materially affect the development of the flow, but the development of the\r\njet is seen to strongly depend on the strength of the oblique shock. Scattering of\r\na laser light sheet off of ice crystals present in the air but absent from the helium\r\nprovides time-resolved flow visualization pictures which show large departures from\r\nthe mean as the flow moves downstream. This demonstrates the need to consider\r\nturbulence effects which were not considered in previous two-dimensional studies.\r\nThey also show that the presence of the shock wave increases the apparent randomness\r\nof the flow as well as the spreading rate of the jet, likely increasing mixing.\r\nA jet cross section extracted from the average of several Rayleigh scattered images\r\nagrees well with the general size and shape of the jet cross section taken from the\r\ncorresponding time averaged temperature data. The design and construction of the\r\nactive feedback control loop in a gas blending system used to independently control\r\nthe mass flowrate and temperature of the jet is described.\r\n" }, { "name": "Valluri, Siddhartha", "degree": "PhD", "year": "1996", "title": "Bluff Body Flows in the Presence of a Free Surface", "advisor": "Roshko, Anatol; Gharib, Morteza", "url": "https://resolver.caltech.edu/CaltechETD:etd-06082007-075443", "creators": [ { "name": { "family": "Valluri", "given": "Siddhartha" }, "id": "Valluri-Siddhartha", "display_name": "Valluri, Siddhartha" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "advisor", "display_name": "Gharib, Morteza" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "co-chair", "display_name": "Gharib, Morteza" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Wu-T-Y-T", "role": "member", "display_name": "Wu, Theodore Yao-tsu" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2wbk-9k81", "abstract": "An experimental study is performed in a water tunnel (Re = 40,000 to Re = 60,000) to study the interaction between the wake of a circular disk and the free surface. The deformation of the free surface is correlated with the behavior of the wake by utilizing surface pictures, wake flow visualization, drag measurement and Digital Particle Image Velocimetry techniques. It is observed that the wake can exist in two modes with different stabilities. The flow can switch between these two modes and the switching process exhibits hysteresis. The topological differences between these modes and their relation to the observed surface patterns are discussed. The changes in the wake are reflected by an increase in Cd which reaches a maximum value when the upper edge of the disk is 0.125 diameters from the surface. Comparison is also made with a disk approaching a solid boundary." }, { "name": "Chen, Weinong", "degree": "PhD", "year": "1995", "title": "Dynamic failure behavior of ceramics under multiaxial compression", "advisor": "Ravichandran, Guruswami", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-101839", "creators": [ { "name": { "family": "Chen", "given": "Weinong" }, "id": "Chen-Weinong", "display_name": "Chen, Weinong" } ], "advisors": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "advisor", "display_name": "Ravichandran, Guruswami" } ], "committee": [ { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "chair", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Ortiz", "given": "Michael" }, "id": "Ortiz-M", "role": "member", "display_name": "Ortiz, Michael" }, { "name": { "family": "Ahrens", "given": "Thomas J." }, "id": "Ahrens-T-J", "role": "member", "display_name": "Ahrens, Thomas J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0NNE-JD20", "abstract": "An experimental technique has been developed that is capable of (1) dynamically loading the specimen in multiaxial compression; (2) controlling the stress state in the specimen in the range from uniaxial stress to uniaxial strain; and (3) allowing the recovery of the sample after loaded by a single, well defined pulse for the characterization of the failure mode. In this technique, cylindrical ceramic specimens were loaded in the axial direction using a split Hopkinson pressure bar modified to apply a single loading pulse, and were confined laterally either by shrink fit sleeves, or by eletro-magnetic force.\r\n\r\nQuasi-static and dynamic multiaxial compression experiments have been performed on a machinable glass ceramic, Macor, and a monolithic engineering ceramic, sintered aluminum nitride (A1N). The cylindrical ceramic specimens were confned laterally by shrink fit sleeves: the amount of confining pressure (0-230 MPa) was varied by using different sleeve materials. The quasi-static axial load was applied by a hydraulic driven Material Test System (MTS), whereas the dynamic axial load was provided by a modified split Hopkinson (Kolsky) pressure bar (SHPB). Under both quasi-static and dynamic loading conditions, the experimental results for both materials showed that the failure mode changed from fragmentation by axial splitting under conditions of uniaxial stress (without lateral confinement) to localized deformation on faults under moderate lateral confinement. The fault initiation process was studied experimentally in detail. Based on the experimental results, a compressive brittle failure process was summarized. A transition from brittle to ductile behavior was observed in Macor under high confinement pressure which was achieved using a second sleeve around the inner sleeve. The compressive failure strengths of both materials increased with increasing confinement pressure under both quasi-static and dynamic loading conditions. The highest dynamic compressive strengths of Macor and A1N measured in the experiments were 1.35 GPa and 5.40 GPa, respectively, whereas their quasi-static compressive strength were measured to be 0.43 GPa and 2.5 GPa, respectively.\r\n\r\nBased on the experimental results on A1N together with available data in the literature, a failure/flow criterion was developed for ceramic materials under multiaxial loading. A Mohr-Coulomb criterion and an improved Johnson-Holmquist model were found to fit the experimental data for brittle failure, whereas the materials exhibited pressure insensitive plastic flow at high pressures. Observations made in other types of dynamic experiments (e.g., shock wave loading) were rationalized based on the postulated failure mechanisms and the possibility of plastic flow beyond the Hugoniot elastic limit (HEL). The effect of various material properties on the failure behavior was investigated using the proposed failure criterion. The applicability of the present model to a range of ceramics was also explored and the limitations of the model were outlined.\r\n" }, { "name": "Cummings, Eric Bryant", "degree": "PhD", "year": "1995", "title": "Laser-Induced Thermal Ccoustics", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09182007-085047", "creators": [ { "name": { "family": "Cummings", "given": "Eric Bryant" }, "id": "Cummings-Eric-Bryant", "display_name": "Cummings, Eric Bryant" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Shepherd", "given": "Joseph E." }, "id": "Shepherd-J-E", "orcid": "0000-0003-3181-9310", "role": "member", "display_name": "Shepherd, Joseph E." }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/p7mb-d967", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nLaser-induced thermal acoustics (LITA) is a new technique for remote nonintrusive measurement of thermophysical gas properties. LITA involves forming, via opto-acoustic effects, grating-shaped perturbations of gas properties using intersecting beams from a short-pulse laser. A third beam scatters coherently into a signal beam off the perturbation grating via acousto-optical effects. The evolution of the gas perturbations modulates the scattered signal beam. Accurate values of the sound speed, transport properties, and composition of the gas can be extracted by analyzing the signal beam.\r\n\r\nAn analytical expression for the spectrum, absolute magnitude, and time history of the LITA signal is derived. The optoacoustic effects of thermalization and electrostriction are treated. Finite beam-diameter, beam-duration, and thermalization-rate effects are included in the analysis. The expression accurately models experimental signals over a wide range of gas conditions.\r\n\r\nExperimental tests using LITA have been conducted on pure and [...]-seeded air and helium at pressures ranging from ~0.1 kPa-14 MPa. Carbon dioxide has been explored near its liquid-vapor critical point. Accuracies of 0.1% in sound speed measurements have been achieved in these tests. Accuracies of ~1% have been achieved in measurements of thermal diffusivity, although beam misalignment effects have typically degraded this accuracy by a factor of ~10-20. Using LITA, susceptibility spectra have been taken of approximately a femtogram of [...]. The effects of fluid motion and turbulence have been explored. LITA velocimetry has been demonstrated, in which the Doppler shift of light scattered from a flowing fluid is measured. LITA velocimetry requires no particle seeding, has a coherent signal beam, and can be applied to pulsed flows. LITA has also been applied to measure single-shot [...] or \"Rayleigh scattering\" spectra of a gas using a technique of wavelength-division multiplexing, called multiplex LITA. The LITA apparatus used in these tests costs about one-tenth that of many conventional laser diagnostics. Narrowband LITA measurements of the sound speed and transport properties and multiplex LITA measurements of the spectral properties of gases may be taken in a single laser shot.\r\n" }, { "name": "Min, Inki A.", "degree": "PhD", "year": "1995", "title": "Transport, stirring and mixing in two-dimensional vortex flows", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-10182007-132306", "creators": [ { "name": { "family": "Min", "given": "Inki A." }, "id": "Min-I-A", "display_name": "Min, Inki A." } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Wiggins", "given": "Stephen R." }, "id": "Wiggins-S-R", "role": "member", "display_name": "Wiggins, Stephen R." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8y2c-8203", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nTo understand the basic contribution of vortex motion in the transport and mixing of passive fluid, we study a system of N discrete vortices. With variation of N and [...] (a vorticity distribution parameter), we are able to experiment with a range of vortex dynamics sufficient to capture many of the features of two-dimensional turbulence in their elementary form - such as vortex merging (inverse cascade of energy), filamentation (enstrophy cascade), etc. With this model the mixing of the fluid is numerically studied via stretch statistics and the spatial distribution of a non-diffusive scalar interface. The spectrum of spatial distribution of scalars as a result of the stirring motion of the N vortices is particularly important in view of the recent (as well as historical) interest in the characterization of the scalar distribution in turbulence. We also examine the velocity field statistics and the Lagrangian motion of fluid particles. It is also instructive to look at the kinematic causes behind the types of statistics that are obtained for the velocity structure functions. A 'building block' approach to understanding these effects in turbulence may lie in building up from a collection of discrete vortices, as done in this thesis, to adding vortices of different scales and the three-dimensional effects. It is in the context of these wider issues that we study the N-vortex problem.\n\nIn the final part of this thesis we investigate the two-dimensional mixing produced by large scale vortical structures during the evolution of a spatially developing mixing layer. Although the advent of three-dimensionality and fully developed turbulence are essential features of mixing layers, it is still dominated by the large scale two-dimensional structures and its effect on the mixing is illustrated here.\n" }, { "name": "Sanderson, Simon Ralph", "degree": "PhD", "year": "1995", "title": "Shock wave interaction in hypervelocity flow", "advisor": "Sturtevant, Bradford; Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11092004-094744", "creators": [ { "name": { "family": "Sanderson", "given": "Simon Ralph" }, "id": "Sanderson-S-R", "display_name": "Sanderson, Simon Ralph" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3GPK-5G29", "abstract": "The interaction of a weak oblique shock with the strong bow shock ahead of a blunt body in supersonic flow produces extreme heat transfer rates and surface pressures. Although the problem has been studied extensively in low enthalpy flows, the influences of high enthalpy real gas effects are poorly understood. Existing perfect gas models predict greatly increased heating with increasing Mach number and decreasing ratio of specific heats.\r\n\r\nExperiments are conducted in a free piston shock tunnel to determine the effects of thermochemistry on the problem at high enthalpy. The flow topology is simplified by studying the nominally two-dimensional flow about a cylinder with a coplanar impinging shock wave. High resolution holographic interferometry is used to investigate changes in the flow structure as the location of the impinging shock wave is varied. Fast response heat transfer gauges provide time resolved measurements of the model surface temperature. The data that are obtained do not support the existing predictions of greatly increased heat transfer at high enthalpy.\r\n\r\nA model is developed to study the thermochemical processes occurring in the interaction region. The phenomenon arises because the stagnation streamline is forced to pass through a system of oblique shock waves that produce less entropy than the undisturbed bow shock. Peak heating is shown to result from a balancing of the strengths of the oblique shock waves. This condition is demonstrated to simultaneously minimize the influence of thermochemistry on the flow. Real gas effects are shown to become important at lower Mach numbers (< 7.5) and for shock angles weaker or stronger than that which produces maximum heating. The model accurately reproduces the experimental observations.\r\n\r\nA nonequilibrium approximation is introduced that applies when the oblique waves are weak with respect to the undisturbed bow shock. Within the scope of the approximation, non-monotonic behavior with the reaction rate is predicted. The reaction rate is not varied as an independent parameter in the current experiments.\r\n" }, { "name": "Al Juhany, Khalid Ahmed Bin Talal", "degree": "PhD", "year": "1994", "title": "Supersonic Film Cooling Including the Effect of Shock Wave Interaction", "advisor": "Hunt, Melany L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12112007-084103", "creators": [ { "name": { "family": "Al Juhany", "given": "Khalid Ahmed Bin Talal" }, "id": "Al-Juhany-Khalid-Ahmed-Bin-Talal", "display_name": "Al Juhany, Khalid Ahmed Bin Talal" } ], "advisors": [ { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "role": "advisor", "display_name": "Hunt, Melany L." } ], "committee": [ { "name": { "family": "Hunt", "given": "Melany L." }, "id": "Hunt-M-L", "role": "chair", "display_name": "Hunt, Melany L." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Sabersky", "given": "Rolf H." }, "id": "Sabersky-R-H", "role": "member", "display_name": "Sabersky, Rolf H." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7ry6-fy87", "abstract": "The current work is an investigation of supersonic film cooling effectiveness including interactions with a two-dimensional shock wave. Air and helium, which are either heated or cooled, are injected at Mach numbers between 1.2 and 2.2 into a Mach 2.4 air freestream. The adiabatic wall temperature is measured directly. The injection velocity and mass flux are varied by changing the total temperature and Mach number while maintaining matched pressure conditions.
\r\n\r\nHeated injection, with the injectant to freestream velocity ratios greater than 1, exhibit a rise in wall temperature downstream of the slot yielding effectiveness values greater than one. The temperature rise, which also occurs for cooled injection, is attributed to the merging of the injectant boundary layer and the lip-wake. As a result comparisons between heated and cooled injection may not be valid. With the exception of heated helium runs, larger injection Mach numbers slightly increase the effective cooling length per mass injection rate. The results for helium injection indicate an increase in effectiveness as compared to that for air injection. The experimental results are compared with studies in the literature.
\r\n\r\nFlow profiles at several axial locations, up to 90 slot heights, indicate that for the same Mach number the helium injections induce a larger wake and a thicker boundary layer than air injection.
\r\n\r\nThe influence of the shock impingement on the recovery temperature is not large if the flow remains attached. Once separation occurs the temperature changes drastically with downstream distance. The shock strength for incipient separation is smaller when helium is injected than when no film coolant is present. However, the converse is true with air injection even though, for the same Mach number, the momentum flux for the air injection is less than that for the helium injection. The induced separation in the case of helium is attributed to the reduced fullness of its momentum flux profile prior to interaction. This research demonstrates how the performance of supersonic film cooling for thermal control is undermined by the susceptibility to shock induced separation, and raises concerns about hydrogen film cooling for N.A.S.P.
" }, { "name": "Germain, Patrick", "degree": "PhD", "year": "1994", "title": "The boundary layer on a sharp cone in high-enthalpy flow", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10182005-112714", "creators": [ { "name": { "family": "Germain", "given": "Patrick" }, "id": "Germain-P", "display_name": "Germain, Patrick" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/hfje-z896", "abstract": "An exploratory study of the laminar, transitional and turbulent boundary layer on a 5 deg. half-angle cone in hypervelocity flow was conducted in the high-enthalpy shock tunnel T5 by measurement of the heat flux distribution and by qualitative flow visualization. A novel flow visualization technique using sodium seeding to increase the sensitivity of conventional interferometric techniques by resonant enhancement of the refractivity of the medium was developed to study the boundary layer structure. The experiments were designed to cover a large range of specific reservoir enthalpy, ranging from the perfect-gas regime to the range where significant oxygen and some nitrogen dissociation and recombination effects may be expected in the boundary layer. The presence of atomic species is due to the combined effect of nozzle freezing and frictional heating in the boundary layer. In the laminar regime and in the latter range, the following effects were found to be present: At the same nominal conditions, heat flux levels are higher in air than in nitrogen because of a larger heat release from oxygen recombination at the wall. By varying the reservoir specific enthalpy in air and nitrogen, and from measurements in carbon dioxide, it was found that real-gas effects stabilize the boundary layer. If the transition Reynolds number is renormalized by evaluating it at the reference temperature, the data for a given gas becomes correlated in a plot against reservoir enthalpy. Increasing enthalpy stabilizes the flow. The stabilizing effect is stronger with gases whose lowest activation energy is low. This behavior is opposite to the prediction made by the linear stability theory regarding the second linear mode of instability. The linear stability theory predicts, however, that real-gas effects stabilize the Tollmien-Schlichting mode. Flow visualization results suggest that the dominant instability mode in the present experiments was the Tollmien-Schlichting mode. Finally, the flow visualization pictures show structures that are not qualitatively different from those of an incompressible turbulent boundary layer, but they do not indicate if real-gas effects change significantly the structure of the turbulent boundary layer. The heat transfer measurements compare well with semi-empirical predictions." }, { "name": "Lambros, John", "degree": "PhD", "year": "1994", "title": "Dynamic decohesion of bimaterial interfaces", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042007-075432", "creators": [ { "name": { "family": "Lambros", "given": "John" }, "id": "Lambros-J", "display_name": "Lambros, John" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/36hw-c185", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nIn the present work, findings of an experimental study of dynamic decohesion of bimaterial systems composed of constituents with a large material property mismatch are presented. PMMA/steel or PMMA/aluminum bimaterial fracture specimens are used. Dynamic one point bend loading is accomplished with a drop weight tower device (for low and intermediate loading rates) or a high speed gas gun (for high loading rates). High speed interferometric measurements are made using the lateral shearing interferometer of Coherent Gradient Sensing in conjunction with high speed photography. Very high crack propagation speeds (terminal crack tip speeds up to [...], where [...] is the shear wave speed of PMMA) and high accelerations ([...], where g is the acceleration of gravity) are observed and reported. Issues regarding data analysis of the high speed interferograms are discussed. The effects of near tip three dimensionality are also analyzed. In crack propagation regions governed by large crack tip accelerations it is found that for accurate analysis of the optical data use of a transient elastodynamic crack tip field is necessary. Otherwise use of a Kd-dominant analysis is sufficient. Using the dynamic complex stress factor histories obtained by fitting the experimental data, a dynamic crack growth criterion is proposed. In the subsonic regime of crack growth it is seen that the opening and shearing displacements behind the propagating crack tip remain constant, i.e., the crack retains a self-similar profile during crack growth at any speed. This forms the basis of the proposed dynamic interfacial fracture criterion. It is also found that the process of dynamic interfacial fracture is highly unstable. This is corroborated by both the very large measured values of crack tip speed and acceleration and by the observation that the energy release rate at the propagating crack tip decreases with increasing crack tip speed. A mechanism of energy transfer from the metal to the PMMA side of the specimen is believed to be responsible for the high transient and transonic effects. An analysis and discussion of this phenomenon is also presented in this work.\n" }, { "name": "Vendroux, Guillaume", "degree": "PhD", "year": "1994", "title": "Scanning tunneling microscopy in micromechanics investigations", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-06162005-104436", "creators": [ { "name": { "family": "Vendroux", "given": "Guillaume" }, "id": "Vendroux-G", "display_name": "Vendroux, Guillaume" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KVSX-Q863", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nA new experimental method is proposed for studying deformations of micromechanical material systems at the submicron scale. To that end, a Scanning Tunneling Microscope (STM) was designed and built to allow placement on a mechanically deforming specimen. Operating in constant current mode, this digitally controlled STM records detailed topographies of specimen surfaces with a resolution of 10.15 nm in-plane and [...] out-of-plane, over a [...] area.\n\nA pattern recognition type algorithm was written to extract the 3-D displacement field from topographies of a given specimen area but under different loading conditions. This Digital Image Correlation (DIC) scheme was found to have very robust convergence characteristics and a higher resolution than that of the images it compares. The accuracy of the DIC code on STM scans was assessed by measuring displacement fields resulting from a translation of the specimen under the microscope. Two major causes of noise were identified, namely drifting of the specimen during scan acquisition and hysteresis distortion of the scan grid. An experimental procedure was devised to limit the occurrence of such perturbations and under these guidelines the resolution of the DIC scheme was found to be 4.8 nm for in-plane displacement measurements and 1.5 nm for out-of plane's.\n\nA micromechanical study of the deformation mechanism of PolyVinylChloride (PVC) was undertaken. Analysis of STM scans revealed that, upon first loading the surface of PVC specimens is deformed irreversibly even at low strain levels. The size of the strain induced topographic changes suggests that, at the scale of [...], a continuum type constitutive modeling of PVC may not be appropriate. This investigation also uncovered the present limitations of the STM-DIC scheme in measuring displacement fields consistently at the nanometer scale." }, { "name": "Wen, Chihyung", "degree": "PhD", "year": "1994", "title": "Hypervelocity flow over spheres", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12212004-092435", "creators": [ { "name": { "family": "Wen", "given": "Chihyung" }, "id": "Wen-C", "display_name": "Wen, Chihyung" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5DDV-GW34", "abstract": "The nature of the nonequilibrium flow of dissociating gases over spheres was investigated experimentally, numerically and theoretically. A series of experiments with three different gases, nitrogen, air and carbon dioxide, was performed in the shock tunnel T5 at GALCIT. Five spheres of different radii equipped with thermocouples for surface heat flux measurements were used. The state-of-the-art numerical method by Candler (1988) was used to conduct a parallel study which strongly complemented the experimental and theoretical efforts.\n\nExperimental heat flux measurements are presented. Good agreement was observed among the measured stagnation point heat transfer rates, computational results and Fay and Riddell\u2019s theoretical predictions. For nitrogen and air, the measured heat flux distributions were also in good agreement with numerical computation results and Lees\u2019 theory. For carbon dioxide, large deviations were observed. Early transition tripped by surface roughness is a possible cause for the deviation of heat flux distribution from the theory. The experimental differential interferograms were compared with the images constructed from computational flowfields. Good agreement of fringe pattern and shock shape was observed.\n\nAn analytical solution is obtained for inviscid hypervelocity dissociating flow over spheres. The solution explains the correlation between the dimensionless stand-off distance and the dimensionless reaction rate parameter previously observed by Hornung (1972) for nitrogen. The physics of the correlation can be shown as the binary scaling. Based on the solution, a new dimensionless reaction rate parameter is defined to generalize Hornung\u2019s correlation for more complex gases than nitrogen. Experimental and numerical results confirm the new correlation.\n\nThe effect of nonequilibrium recombination downstream of a curved two-dimensional shock was also addressed. An analytical solution for an ideal dissociating gas was obtained, giving an expression for dissociation fraction as a function of temperature on a streamline. The solution agrees well with the numerical result and provides a rule of thumb to check the validity of binary scaling for the experimental conditions. The effects upon the binary scaling of the large difference in freestream temperature between flight and free-piston shock tunnel conditions are discussed." }, { "name": "Belanger, Jacques", "degree": "PhD", "year": "1993", "title": "Studies of Mixing and Combustion in Hypervelocity Flows with Hot hydrogen injection", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08222007-092852", "creators": [ { "name": { "family": "Belanger", "given": "Jacques" }, "id": "Belanger-Jacques", "display_name": "Belanger, Jacques" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/012K-GE91", "abstract": "The ability to build an air-breathing single-stage-to-orbit propulsion system requires examination of key elements such as turbulent mixing rates, especially at the \"zero shear\" fuel-air mixing condition, and combustion efficiency. The required data can only be obtained in experiments which simultaneously match the flight total pressure and total enthalpy as well as the fuel conditions. GALCIT, with its new free piston shock tunnel T5, has the capability to do some of these combustion experiments. But prior to these tests, it was felt that there was a need to simulate the gas dynamical processes in the free piston shock tunnel and also in a new combustion driven shock tunnel built for these experiments so that both systems could be used as efficiently as possible. The numerical code helped explain the piston motion in the free piston shock tunnel. The code was also very useful for the design of the combustion driven shock tunnel.
\r\n\r\nBecause hydrogen has to be injected into the combustion chamber of the propulsion system after being used as a cooling fluid, a combustion driven shock tunnel was built to reproduce this \"hot\" hydrogen fuel. The system has been used successfully to supply hydrogen at up to 1500 K for the experiments. To reduce the complexity of the problem, a very basic configuration for the hydrogen injection system was tested. This was first done with an injection system mounted flush with the surface of a flat plate in the test section of T5. Different test conditions as well as Mach 2 and 5 nozzle injectors at angles of 15\u00b0 or 30\u00b0 were tested to determine criteria for significant combustion. Lower limits in pressure and enthalpy were found where hydrogen combustion becomes very limited using this \"hot\" hydrogen fuel. The second set of experiments still used an injection system mounted flush with the surface but involved a small combustor model previously tested in the hypervelocity HYPULSE facility. Low pressure experiments were performed to reproduce some of the HYPULSE tests and excellent agreement was found. Experiments at high pressure were also performed to better match the real flight total pressure and some hydrogen combustion was detected in these tests.
" }, { "name": "Bridges, David Henry", "degree": "PhD", "year": "1993", "title": "Tip effects on the vortex wake of an axisymmetric body at angle of attack", "advisor": "Hornung, Hans G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08232007-081713", "creators": [ { "name": { "family": "Bridges", "given": "David Henry" }, "id": "Bridges-D-H", "display_name": "Bridges, David Henry" } ], "advisors": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "advisor", "display_name": "Hornung, Hans G." } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Peck", "given": "Charles W." }, "id": "Peck-C-W", "role": "member", "display_name": "Peck, Charles W." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DZ4H-M010", "abstract": "Experiments on the effects of conditions at the tip of an axisymmetric body on the vortex wake of the body at large angle of attack in incompressible flow have been performed. The separation patterns that occur at the nose of a cone with a hemispherical tip have been documented for ranges of Reynolds number based on tip radius and of relative angle of attack. Tips with cross sections that vary smoothly from elliptic to circular have been designed and tested on a cone. The effectiveness of the tips in varying the cone side force coefficient as the tip is rotated has been demonstrated. The relation between tip roll angle and side force coefficient has been shown to be essentially independent of Reynolds number, for the range of Reynolds number tested, as well as cone base conditions and roll direction. The relation between tip roll angle and side force coefficient has been shown to be a strong function of angle of attack. The reasons for nonsmooth variation of side force coefficient with tip roll angle at higher angles of attack have been determined by examinations of the vortex wake geometry. Peaks in the magnitude of the side force coefficient after zero crossings have been shown to be caused by a rapid shift in the wake vortices away from a symmetric configuration as the tip rotates away from a symmetry condition. Reductions in magnitude of the side force coefficient between magnitude peaks have been demonstrated to be a result of the near vortex crossing over the cone centerline. This vortex crossover has been shown to occur near and after the breakaway from the cone of the far vortex. The ability of one of the elliptic cross section tips to produce smooth variations of side force coefficient with roll angle ahead of vortex breakaway has been demonstrated, suggesting that the tip might be an effective yaw control device for aircraft at high angle of attack." }, { "name": "Cardell, Gregory Scott", "degree": "PhD", "year": "1993", "title": "Flow past a circular cylinder with a permeable wake splitter plate", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-04012005-092116", "creators": [ { "name": { "family": "Cardell", "given": "Gregory Scott" }, "id": "Cardell-G-S", "display_name": "Cardell, Gregory Scott" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/25C5-1150", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nMeasurements in the near wake region of a circular cylinder in a uniform flow in the Reynolds number range [...] with permeable splitter plates spanning the wake center plane are presented. Permeability is defined by the pressure drop across the plates, and the relationship between permeability and plate solidity is determined for a set of plates constructed from woven wire mesh, permitting unambiguous characterization of the splitter plates by the solidity. The effects of different solidities on the flow in the near wake are investigated using smoke wire flow visualization, hot-wire anemometry, and measurements of the mean pressure at the cylinder surface, and the results are related to cylinder flow without a splitter plate.\n\nFlow visualization results demonstrate that the introduction of low solidity splitter plates does not change the basic near wake structure, and that sufficiently high solidity uncouples the large-scale wake instability from the body, with the primary vortex formation occurring downstream of the separation bubble due to instability of the wake profile. Hotwire and surface pressure measurements confirm and quantify the flow visualization results, showing that the permeable splitter plates reduce the drag and modify the primary wake frequency. When the solidity is high enough that the wake is convectively unstable, the base pressure is independent of the Reynolds number and solidity. For a wide range of solidities, the same asymptotic value of the Strouhal number is reached at high Reynolds numbers. The relationship between the Strouhal number and the base pressure is discussed.\n\nDetailed measurements in the separating shear layers with splitter plates moderating the primary vortex formation show that in the mean the development of the separating shear layers is similar to that of the plane mixing layer. The presence of the splitter plates enhances shear layer development, and it is found that for all solidities the instability amplifies a broad frequency band without discrete spectral features. Turbulent transition in the shear layer and its role in the pronounced Reynolds number dependency of the flow in this Reynolds number range is discussed. Acoustic excitation of the separated shear layers confirms the broad band frequency response of the shear layer instability, and the effect of the driven shear layers on the near wake is investigated." }, { "name": "Geubelle, Philippe H.", "degree": "PhD", "year": "1993", "title": "Nonlinear effects in interfacial fracture", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-08272007-104817", "creators": [ { "name": { "family": "Geubelle", "given": "Philippe H." }, "id": "Geubelle-P-H", "display_name": "Geubelle, Philippe H." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/rkt0-7k90", "abstract": "The issue of the non-coplanar quasi-static propagation of a crack in homogeneous and bimaterial sheets is investigated. Through a preliminary linear analysis, it is shown that the interface crack kinking problem is confronted, in most practical cases, with difficulties which do not arise in the homogeneous situation: the crack path as predicted by the maximum energy release rate criterion cannot be determined uniquely and an additional length parameter, absent in the homogeneous case, needs to be specified to assure uniqueness. Following that development, the assumption of small deformations is relinquished and it is shown how the size of the nonlinear zone imparts possibly the physical significance of the additional length parameter. The analysis is performed numerically in the homogeneous and bimaterial cases within the framework of the nonlinearly elastic theory of plane stress and using a \"boundary-layer\" approach. Material and geometrical nonlinearities are combined through the use of the Generalized Neo-Hookean (GNH) model. As the length of the crack extension becomes comparable to the size of the nonlinear zone, a transition is observed between the value of the energetically most favorable kink angle predicted by the linear theory and a unique \"nonlinear\" value which is found to be independent of the crack extension length and the far-field loading conditions.\n\nThe results of the crack propagation analysis are related to those of a detailed asymptotic analysis of the structure of the near-tip stress and deformation fields for the GNU class of hyperelastic materials. The investigation addresses a) the symmetric (mode I) and non-symmetric (mixed-mode) homogeneous situations, b) the rigid substrate case and c) the general bimaterial problem which allows for an arbitrary choice, on both sides of the interface, of the three material parameters characterizing the GNH model. The asymptotic analysis allows to quantify the effect of the \"hardening\" characteristics on the blunting of the crack and the associated stress and strain singularities, and shows that the near-tip fields corresponding to a general nonsymmetric loading are, in the homogeneous situation, related to those of the symmetric (mode I) case through a rotation which depends on the material characteristics and the far-field loading conditions. A somewhat similar property is obtained in the bimaterial problem, where the existence of a non-oscillatory and \"contact-free\" solution is confirmed for all material combinations." }, { "name": "Hammer, Jay A.", "degree": "PhD", "year": "1993", "title": "Lifted turbulent jet flames", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-08272007-131353", "creators": [ { "name": { "family": "Hammer", "given": "Jay A." }, "id": "Hammer-J-A", "display_name": "Hammer, Jay A." } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/fn1k-za85", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nExperiments were conducted on lifted, turbulent jet diffusion flames. An automated technique using a linear photodiode array was implemented to measure the temporal history of the liftoff height h. The measurements enabled accurate determination of the mean liftoff height [...] under a wide range of flow conditions, including several fuels, nozzle diameters, and exit velocities [...]. The results showed an approximately linear relationship between [...] and [...], with a slight dependence on Reynolds number. A strain-rate model for liftoff, based on far-field scaling of turbulent jets, provides an explanation for the linear dependence of [...] on [...]. Measurements were also made in which the nozzle fluid contained varying amounts of air, where it was found that the slope of the [...] vs. [...] line increases faster than predicted by far-field scaling of turbulent jets. The discrepancy is attributed to near-field effects.\n\nThe amplitudes of the fluctuations in h were found to be of the order of the local large scale of the jet. There is a slight increase in normalized fluctuation level [...] with [...], and there is some variation of [...] with fuel type. The time scales of the fluctuations of h were found to be considerably longer than the local large-scale time of the turbulence [...]. By using fuels of different chemical times to vary [...], the measured correlation time [...] normalized by [...] was found to collapse with Richardson number [...]. Experiments in which the nozzles were oriented horizontally showed no change in [...], however. Additional experiments were conducted to investigate alternative explanations for the variation of [...] with [...]. These experiments included measuring the flame length L simultaneously with h, and measuring the visible radiation I simultaneously with h. L(t) was found to be nearly uncorrelated with h(t), dismissing the possibility that a feedback mechanism from L to h controls the fluctuations of h. Although I(t) is highly correlated with h(t) for the most sooting fuel, acetylene, it is not deemed responsible for the longer correlation times of that fuel. This was deduced from experiments using mixtures of hydrogen with other fuels, which produce very little radiation, but which have values of [...] comparable to those of acetylene flames.\n\nAnother experiment was conducted in which two-dimensional images of fuel concentration (CH4) and reaction zones (indicated by CH) were obtained. The images showed a wide variety of structure types, indicating that there is no universal description of the flow field at the flame base. The flame stabilization position showed large fluctuations in both the axial and radial directions. The shot to shot variation in methane number density at the flame base was also large." }, { "name": "Koumoutsakos, Petros D.", "degree": "PhD", "year": "1993", "title": "Direct numerical simulations of unsteady separated flows using vortex methods", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-082957", "creators": [ { "name": { "family": "Koumoutsakos", "given": "Petros D." }, "id": "Koumoutsakos-P-D", "display_name": "Koumoutsakos, Petros D." } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TCQ9-9C86", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nNumerical simulations are presented for viscous incompressible flows with and without solid wall boundaries. Our numerical method is based on vortex methods. The classical inviscid scheme is enhanced to account for viscous effects via the method of particle strength exchange. The method is extended to account for the enforcement of the no-slip boundary condition as well by appropriately modifying the strength of the particles. Computations are possible for extended times by periodically remeshing the vorticity field.\n\nThe particles are advanced using the Blot-Savart law for the evaluation of the velocity. Computations are made using up to [...](10[superscript 6]) vortex particles by efficiently implementing the method of multipole expansions for vector computer architectures to obtain an [...](N) algorithm.\n\nThe method is used to simulate the inviscid evolution of an elliptical vortex in an unbounded fluid as well as unsteady separated flows around circular cylinders for a wide range of Reynolds numbers (40 - 9500). Direct comparisons are made of the results of the present method with those from a variety of theoretical, computational and experimental studies. The results exhibit the robustness and validity of the present method and allow to gain physical insight as to vorticity formation and its relation to the forces experienced by the body." }, { "name": "Lappas, Tasso", "degree": "PhD", "year": "1993", "title": "An adaptive Lagrangian method for computing 1-D reacting flows, and, The theory of Riemann invariant manifolds for the compressible Euler equations", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-09192007-130342", "creators": [ { "name": { "family": "Lappas", "given": "Tasso" }, "id": "Lappas-T", "display_name": "Lappas, Tasso" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/bw0e-2789", "abstract": "In the first part of this thesis, a method for computing one-dimensional, unsteady compressible flows, with and without chemical reactions, is presented. This work has focused on accurate computation of the discontinuous waves that arise in such flows. The main feature of the method is the use of an adaptive Lagrangian grid. This allows the computation of discontinuous waves and their interactions with the accuracy of front-tracking algorithms. This is done without the use of additional grid points representing shocks, in contrast to conventional, front-tracking schemes. The Lagrangian character of the present scheme also allows contact discontinuities to be captured easily. The algorithm avoids interpolation across discontinuities in a natural and efficient way. The method has been used on a variety of reacting and non-reacting flows in order to test its ability to compute complicated wave interactions accurately and in a robust way.\r\n\r\nIn the second part of this thesis, a new approach is presented for computing multidimensional flows of an inviscid gas. The goal is to use the knowledge of the one-dimensional, characteristic problem for gas dynamics to compute genuinely multidimensional flows in a mathematically consistent way. A family of spacetime manifolds is found on which an equivalent 1-D problem holds. These manifolds are referred to as Riemann Invariant Manifolds. Their geometry depends on the local, spatial gradients of the flow, and they provide locally a convenient system of coordinate surfaces for spacetime. In the case of zero entropy gradients, functions analogous to the Riemann invariants of 1-D gas dynamics can be introduced. These generalized Riemann Invariants are constant on the Riemann Invariant Manifolds. The equations of motion are integrable on these manifolds, and the problem of computing the solution becomes that of determining the geometry of these manifolds locally in spacetime.\r\n\r\nThe geometry of these manifolds is examined, and in particular, their relation to the characteristic surfaces. It turns out that they can be space-like or time-like, depending on the flow gradients. An important parameter is introduced, which plays the role of a Mach number for the wave fronts that these manifolds represent. Finally, the issue of determining the solution at points in spacetime, using information that propagates along space-like surfaces is discussed. The question of whether it is possible to use information outside the domain of dependence of a point in spacetime to determine the solution is discussed in relation to the existence and uniqueness theorems, which introduce the concept of domain of dependence.\r\n\r\nThis theory can be viewed as an extension of the method of characteristics to multidimensional, unsteady flows. There are many ways of using the theory to develop practical, numerical schemes. It is shown how it is possible to correct a conventional, second-order Godunov scheme for multidimensional effects, using this theory. A family of second-order, conservative Godunov schemes is derived, using the theory of Riemann Invariant Manifolds, for the case of two-dimensional flow. The extension to three dimensions is straightforward. One of these schemes is used to compute two standard test cases and a two-dimensional, inviscid, shear layer.\r\n" }, { "name": "Lisoski, Derek Lee Ashton", "degree": "PhD", "year": "1993", "title": "Nominally 2-Dimensional Flow About a Normal Flat Plate", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-04042005-105646", "creators": [ { "name": { "family": "Lisoski", "given": "Derek Lee Ashton" }, "id": "Lisoski-Derek-Lee-Ashton", "display_name": "Lisoski, Derek Lee Ashton" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "List", "given": "E. John" }, "id": "List-E-J", "role": "member", "display_name": "List, E. John" }, { "name": { "family": "Gharib", "given": "Morteza" }, "id": "Gharib-M", "orcid": "0000-0003-0754-4193", "role": "member", "display_name": "Gharib, Morteza" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AZEG-2T16", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nTowing tank and water channel experiments and a two-dimensional vortex element numerical model were used to study the forces experienced by a bluff flat plate set normal to a nominally two-dimensional flow. Intrinsic (small scale) and extrinsic (large scale) three-dimensional motions in the experimental flow were isolated and their separate and combined effects on forces and overall wake development were studied. Transient flow development starting from rest, as well as steady flow conditions, were investigated.\r\n\r\nA force balance was used to measure the unsteady lift and drag of vertically oriented models projecting through a free surface with various lower end conditions; simultaneous LIF flow visualizations imaged the structure of the vortices in the wake. Plate aspect ratio, lower end condition and angle of attack were varied to effect changes in large scale three-dimensional motions, while changes in Reynolds number and Richardson number (flow stratification) modified the small scale three dimensionality intrinsic to the flow.\r\n\r\nTowing tank experiments indicated that normal plates required sixty to one hundred chord lengths of travel to establish steady vortex shedding. An initial drag peak during acceleration was followed by a drag minimum of [...] reached while the wake was confined to a symmetric vortex bubble. Subsequent to the breakdown of this bubble, a region of symmetric flow with [...] and no vortex shedding was apparent for twenty to thirty chord lengths, followed by the final onset of vortex shedding which occurred exponentially. During this onset forces overshot their final steady-state values [...]. Flows with less large scale extrinsic three dimensionality (higher aspect ratio, \"more two-dimensional\" end conditions, and stratified flow) had longer development times and higher subsequent overshoot levels.\r\n\r\nSmall geometric asymmetries (angle of attack variations) increased the minimum drag level seen after the acceleration and resulted in an earlier breakdown of the closed wake, followed by an immediate transition to steady shedding. The breakdown of the initial bubble in this case was more coherent spanwise and did not result in a long-lasting symmetric nonshedding flow.\r\n\r\nDuring \"steady-state\" shedding, modulation in the vortex shedding amplitude at a time scale of five to ten Strouhal periods resulted in a twenty percent fluctuation in mean drag level, with a corresponding increase in rms lift. This modulation accompanied a slow oscillation in the formation distance of the shed vorticity, the period of which was Reynolds number independent but decreased with increasing aspect ratio, reaching a minimum value of six Strouhal periods for aspect ratios greater than ten.\r\n\r\nAgreement between three-dimensional experimental and two-dimensional numerical-model results was good at early times, indicating the experiments were two-dimensional until the breakdown of the closed wake bubble following the initial acceleration. Prior to this breakdown the numerical model of a normal plate gave a drag coefficient [...], similar to that given by the Kirchhoff-Rayleigh free-streamline prediction but lower than experiments. Small asymmetries of the 2d model resulted in an increase in the minimum drag level to [...] . Subsequent to the closed wake breakdown, drag levels of [...] are 65% higher than steady-state experimental values.\r\n\r\nAlthough no region corresponding to the post-acceleration non-vortex-shedding seen experimentally was found in the basic numerical results, the addition of circulation decay to the numerical-model resulted in a region which appeared qualitatively similar. This circulation decay also decreased mean drag levels [...] and gave an exponential shedding onset with subsequent long period shedding modulation.\r\n\r\nStabilizing spanwise stratification of the experimental flow had little effect for Richardson numbers [...] (based on chord). For [...] and [...] a longer lasting post-acceleration closed wake was followed by strong initial shedding and a large drag overshoot, with a subsequent decrease in shedding amplitude and increase in formation distance to the levels seen in the unstratified [...] case, which exhibited considerable Reynolds number dependence. For plates at [...] angle of attack the symmetric nonvortex shedding region was reduced in duration and subsequent \"steady-state\" drag levels were increased ten to fifteen percent [...] from the unstratified case." }, { "name": "Pulos, Guillermo C.", "degree": "PhD", "year": "1993", "title": "Nonsteady crack propagation and craze behavior in PMMA", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-08312007-104217", "creators": [ { "name": { "family": "Pulos", "given": "Guillermo C." }, "id": "Pulos-G-C", "display_name": "Pulos, Guillermo C." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4kqe-ec90", "abstract": "This work is devoted to the study of nonsteady crack propagation under cyclic loading in polymers, specifically PMMA. The first part deals with the delineation of a precision loading facility allowing ultra-precise load or displacement control commensurate with the high resolution measurements of crack tip material response.\r\n\r\nA method of determining the advance of crack tip through combined microscope and computer-analyzed observation is presented. In particular, the experimental set up and software development is described by which these measurements are achieved. It is shown that automated crack tip location is possible with a precision of one to two microns, which is amply sufficient for present purposes to make definitive statements about the smoothness or discontinuity of crack propagation.\r\n\r\nThe craze and crack opening displacements are measured near the free surface of the specimen both under quasi-static step loading and cyclic loading. Eleven craze opening profiles for equal load increments are acquired during a single cycle under fatigue loading. A multi-linear craze stress model is used to match the opening displacements to the measurements. While the primordial thickness can be defined from the Lorentz-Lorenz equation and from the assumption of a constant index of refraction for the quasi-static loading, the effect of load history may prevent such determinations for cyclic loading. The damage accumulated through cyclic deformation reduces the strength of the fibrils in the middle of the craze and produces a drop in the middle of the stress distribution. The craze and crack opening displacement are monitored in connection with a jump-like crack/craze advance constituting 50% of the craze length. The newly-drawn craze fibrils after the jump show mechanical behavior that is different from their behavior before the jump and exhibit no deterioration in their stress-displacement relation." }, { "name": "Bonazza, Riccardo", "degree": "PhD", "year": "1992", "title": "X-ray measurements of shock-induced mixing at an air/xenon interface", "advisor": "Sturtevant, Bradford; Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06232005-111112", "creators": [ { "name": { "family": "Bonazza", "given": "Riccardo" }, "id": "Bonazza-R", "display_name": "Bonazza, Riccardo" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "co-advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3h5j-yb71", "abstract": "A new experimental technique to measure the density of a high atomic number gas at an interface has been developed and demonstrated. It is based on the absorption of X-rays by the high atomic number gas, and it was implemented in a vertical square shock tube for the study of shock-accelerated air/xenon interfaces. These were prepared by retracting a metal plate initially separating the two gases, prior to the release of the shock wave. Thus the interfaces were all of initial finite thickness. Interfaces of two types, quasi-sinusoidal and nominally flat, were examined. Object of study were the amplitude of large wavelength (25 - 100 mm) perturbations on the interface, and the thickness of the interface. An integral definition for the interface mean line (proposed in a previous numerical work at GALCIT) was adopted; a new integral definition for the interface thickness was proposed, making it feasible to study for the first time the thickness of quasi sinusoidal interfaces. Experiments were performed to image interfaces having interacted with the incident shock, the incident and the reflected shock, or a series of weak waves reverberating between the interface and the shock tube end wall. The results for the growth rates of the amplitudes were compared against a model based on the linear theory: The measured values are larger than the predicted ones in the case when the interface only interacts with the incident and the first reflected shocks. They are smaller than the theoretical ones in the case of multiple reverberations. The interface thickness exhibits essentially no growth upon interaction with the incident shock. The interaction of the reflected shock with the turbulent boundary layer behind the incident one generates random acoustical disturbances which reach the interface and cause the subsequent thickness growth. The thickness growth rates of nominally flat interfaces are larger than those previously found at GALCIT in a schlieren visualization experiment, in the case of two shock interactions. They are smaller in the case of multiple interactions. In the case of two shock interactions, wall vortices generated by the interaction of the reflected shock with the boundary layer behind the incident one severely affected the measurements. A correction was proposed to account for this effect in the measurements of the interface thickness. The need remains for a different experimental technique, capable of eliminating these adverse effects altogether." }, { "name": "Budzinski, John Michael", "degree": "PhD", "year": "1992", "title": "Planar Rayleigh Scattering Measurements of Shock Enhanced Mixing", "advisor": "Zukoski, Edward E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-07202007-130628", "creators": [ { "name": { "family": "Budzinski", "given": "John Michael" }, "id": "Budzinski-John-Michael", "display_name": "Budzinski, John Michael" } ], "advisors": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "advisor", "display_name": "Zukoski, Edward E." } ], "committee": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "chair", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/pwj0-h036", "abstract": "This investigation was concerned with the mixing which occurs after the unsteady interaction of a shock wave with a laminar jet of helium. The jet of helium was injected normal to the direction of the propagation of the shock. The vorticity created at the boundaries of the jet during the shock interaction generates a stream of air which divides the initial circular cross section of the jet into two lobes. Each lobe is further divided into a tail region and a vortex core. The vortex cores from each lobe form a vortex pair that pulls ahead of the tail regions. In the present investigation the primary diagnostic, planar Rayleigh scattering, had sufficient spatial and temporal resolution to resolve the smallest diffusion scales present and allowed helium mole fractions to be measured in two-dimensional planes normal to the original jet flow direction. The amount of molecular mixing was evaluated with a mass distribution function at increasing times after the shock interaction. The total masses of helium contained in regions where the molar concentration of helium was at least 30% and 50% were also calculated. The shock Mach number was varied, and the effect of a reflected shock was studied. The velocity and spacing of the vortex pairs was measured. It was found that shock interactions can significantly increase the mixing between the air and helium. As the Mach number increases, better mixing occurs as the stream of air divides the jet. However, less mixing occurs at the later times when the vortex pairs are moving ahead of the tails. A rough collapse of the mixing data occurs when time is normalized with the change in velocity of the air behind the shock. The measured velocities and estimated values of the circulation agree very well with previous computational results. An increase in the enhancement of mixing occurred after the interaction with the reflected shock." }, { "name": "Frieler, Clifford Eugene", "degree": "PhD", "year": "1992", "title": "Mixing and reaction in the subsonic 2-D turbulent free shear layer", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03292005-135259", "creators": [ { "name": { "family": "Frieler", "given": "Clifford Eugene" }, "id": "Frieler-C-E", "display_name": "Frieler, Clifford Eugene" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/fsen-at31", "abstract": "Several aspects of mixing and reaction in a turbulent two-dimensional shear layer have been studied. Experiments have been performed with reacting H2, F2, and NO in inert diluent gases. Sensing the heat release by these reactions, several aspects of the mixing process can be examined without the usual resolution limitations. For example, in contrast with direct measurements of composition, the amount of mixed fluid can be conservatively estimated with the results of the \"flip\" experiments. These have been performed over a range of density ratios, Reynolds numbers and heat release.\r\n\r\nThe effects of initial conditions are of primary importance when comparisons to other studies are undertaken. Aspects as fundamental as growth rate of the turbulent region, or as obscure as the mixed fluid flux ratio depend strongly on the boundary conditions of this flow. These effects are examined in conjunction with those of Reynolds number and density ratio. For most cases studied here, tripping of the high speed boundary layer led to growth rate decreases. An exception was found for the case of high density ratio where the opposite effect was observed. This anomalous result occurred at conditions under which a new mode of instability has been shown to exist. Parallels exist between this unusual result and those of Batt in the uniform density case.\r\n\r\nAn extensive study of the effects of density ratio on the mixing and reaction in the 2-D shear layer has been performed. Results indicate that several aspects of the mixing process are remarkably similar. Profiles of mixed fluid change little as the density ratio varies by a factor of 30. The integral amount of mixed fluid varies less than 6% for all density ratios examined. This insensitivity contrasts with that of the profiles of mixed fluid composition. While having very similar shapes the profiles are offset by an amount which depends very strongly upon the density ratio. The entrainment into the mixing layer has also been examined. Power spectral densities of the temperature time series were calculated and found to collapse upon normalization with the adiabatic flame temperature and large structure passage frequency. Least squares fits of the probability density functions were also examined.\r\n\r\nThe initial work of Mungal and Frieler (1988) on the effects of chemical kinetics on the formation of product in the 2-D mixing layer have been greatly expanded. Measurements have been extended to include a wider range of NO concentrations and have been performed for two other stoichiometries. Results indicate that the simple model envisioned in Mungal and Frieler may only be suited for cases with extreme stoichiometry (very high or very low). Further investigations have turned up a serious discrepancy reflecting both on the experimental technique and on theory and modeling of this reacting flow. Experiments run under otherwise identical conditions demonstrate that more product is formed when F2 is the rich reactant than when H2 is the rich reactant. This dependence upon molecular character is counter intuitive and stems from a coupling of the effects of differing diffusivity and chemical kinetics. Numerical calculations based on simplified flow models are reported which demonstrate this coupling. These results indicate that even subtle diffusion effects can measurably effect reacting flows and imply that assumptions common among current modeling efforts must be re-examined.\r\n\r\nThe effects of Reynolds number on mixing and reaction in the 2-D turbulent mixing layer have been examined. Evidence of the remnants of the initial roll up and mixing transition are seen for Reynolds numbers as large as 30,000. Indications of a resonance with the acoustic mode of the apparatus exist which affect results for Reynolds numbers up to 60,000. Natural transition of the high and low speed boundary layer on the splitter plate complicate comparisons of the high Reynolds number data with the remainder. In spite of all of these qualifications, the amount of mixed fluid is nearly constant. Over the range of Reynolds numbers 10,000 to 200,000, it varies by less than 12%. No evidence of an asymptotic decline in the amount of mixed fluid is observed.\r\n" }, { "name": "Lu, Lingyun", "degree": "PhD", "year": "1992", "title": "On the development and application of a modified boundary element method for the analysis of three-dimensional elastostatic problems in thick plates", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08072007-131209", "creators": [ { "name": { "family": "Lu", "given": "Lingyun" }, "id": "Lu-L", "display_name": "Lu, Lingyun" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/tkxp-f839", "abstract": "A modified three-dimensional Boundary Element Method (BEM) is developed. This method is specially tailored towards applications in three-dimensional elasticity, involving regions containing two parallel planar surfaces. Typical structures are the three-dimensional plate structures. The formulation makes use of the three-dimensional fundamental solution of a concentrated load applied in an infinite three-dimensional plate of uniform finite thickness (obtained by Benitez and Rosakis in 1985). The most attractive feature of this modified BEM is that, for the class of problems involving structures described above subjected to traction-free boundary conditions on the planar surfaces, discretization is only required on the lateral surfaces of the plate and the surfaces of the cavities, holes, and cracks in the plate. No discretization is needed on the planar surfaces of the plate. In this initial study, three problems involving a pressurized hole in an infinite three-dimensional plate are analyzed. The shapes of the holes include a circular hole and two elliptical holes with the aspect ratios of 4 and 10, respectively. In all the cases, the accuracy of the modified BEM is established by direct comparison of its results with those of finite element calculations. The results of the modified BEM are also compared with the plane-stress and plane-strain approximations of the problems under consideration. This comparison make it possible to access the important three-dimensional effects near the surface of the elliptical hole." }, { "name": "Minahen, Timothy M.", "degree": "PhD", "year": "1992", "title": "Structural instabilities involving time dependent materials : theory and experiment", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-08082007-100442", "creators": [ { "name": { "family": "Minahen", "given": "Timothy M." }, "id": "Minahen-T-M", "display_name": "Minahen, Timothy M." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "chair", "display_name": "Knowles, James K." }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" }, { "name": { "family": "Christman", "given": "Tom" }, "id": "Christman-T", "role": "member", "display_name": "Christman, Tom" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7h70-2p51", "abstract": "The creep buckling of viscoelastic structures is studied analytically and experimentally to investigate structural stability in the presence of time dependent materials. The theory of linear viscoelasticity is used to model polymeric column specimens subjected to constant compressive end loads. A strength of materials approach (Euler-Bernoulli beam theory) is employed to model the moment-curvature relation for the column. The growth of initial imperfections is calculated using the hereditary integral formulation. Solution techniques are developed for small displacements and then generalized to include the effects of large displacements and rotations. A failure criterion based on maximum deformation allows the column life to be estimated directly from the material relaxation modulus. A discussion generalizing the results to include plates and shells is presented.\n\nRectangular cross-section polymethylmethacrylate (PMMA) specimens with hinged boundary conditions are used to study viscoelastic buckling experimentally. Constant compressive end loads are applied using a servo-controlled load frame while the specimens are kept in a temperature cabinet at elevated temperatures (accelerating the creep behavior). Specimen shortening and out-of-plane deflections are monitored during the tests. The relaxation modulus of PMMA is approximated by a Prony-Dirichlet series and the model is used to simulate the laboratory experiments. Model and experimental results show good agreement during the \"glassy\" and slow growth phases of the column response. As the growth rate increases some deviations between theory and experiment are seen. It is shown that the deviations are not a result of geometric nonlinearities, but may, in part, be explained by material nonlinearities not accounted for in the model.\n" }, { "name": "Nadiga, Balasubramanya T.", "degree": "PhD", "year": "1992", "title": "A study of multi-speed discrete-velocity gases", "advisor": "Sturtevant, Bradford; Broadwell, James E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08092007-090234", "creators": [ { "name": { "family": "Nadiga", "given": "Balasubramanya T." }, "id": "Nadiga-B-T", "display_name": "Nadiga, Balasubramanya T." } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "advisor", "display_name": "Broadwell, James E." } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Pullin", "given": "Dale Ian" }, "id": "Pullin-D-I", "role": "member", "display_name": "Pullin, Dale Ian" }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/tz85-x511", "abstract": "The applicability of multi-speed discrete-velocity gases to compressible flow situations is considered. First, the equation of state, the anisotropies and the advection velocities for any multi-speed model on the square and triangular lattices are derived. The dependence on the model of any of these to leading order in the flow velocity is shown to be only through a fourth moment of the stationary equilibrium speed distribution. Next, a computation scheme is introduced, wherein adjacent cells in a cell network interact through an exchange of particles, commensurate with the equilibrium fluxes of mass, momentum, and energy. This corresponds to the infinite collision rate limit of the model gas, resulting in very low viscosities. Finally, a simple multi-speed model, the nine-velocity model is studied in detail: Solving the shock tube flow with the model yields almost all phenomenology associated with a perfect gas. An exact shock profile is computed for the model and is compared to a Navier-Stokes shock profile. An adiabatic channel flow is simulated with the model and the results compared to an integral solution of the Navier-Stokes equation. The comparisons in both the cases are excellent. It is also shown that the nine-velocity gas does not permit steady supersonic flow." }, { "name": "Sullivan, Gregory Daniel", "degree": "PhD", "year": "1992", "title": "An investigation of mixing and transport at a sheared density interface", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-08152007-082818", "creators": [ { "name": { "family": "Sullivan", "given": "Gregory Daniel" }, "id": "Sullivan-G-D", "display_name": "Sullivan, Gregory Daniel" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/931s-r394", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nScalar transport is investigated at a density interface imbedded in a turbulent shear flow. This problem is investigated first experimentally, and based on the experimental findings, a physical model for entrainment is developed.\n\nSteady-state interfacial shear flows are generated in a laboratory water channel for layer Richardson numbers, Ri, between about 1 and 10. The flow field is made optically homogeneous, enabling the combined use of laser-Doppler velocimetry and laser-induced fluorescence with photodiode array imaging to measure the velocity and concentration fields at high resolution.\n\nFalse-color images of the concentration field provide valuable insight into mixing and transport at the interface. The dominant interfacial mixing mechanism is found to depend on the local mean shear Richardson number, [...] for [...] less than about 0.40 to 0.45, local mixing is dominated by Kelvin-Helmholtz (K-H) instabilities; for somewhat larger values of [...], mixing is dominated by interfacial wave breaking. In both cases, vertical transport of mixed fluid into adjacent turbulent layers is accomplished by large-scale turbulent eddies which impinge on the interface and scour fluid from its outer edges.\n\nBased on the experimental results, a model for interfacial mixing and entrainment is developed. A local equilibrium is assumed in which the rate of loss of interfacial fluid by eddy scouring is balanced by the rate of production (local mixing) by interfacial instabilities and molecular diffusion. In the case of one-sided entrainment, the model results are as follows: when interfacial mixing is diffusion dominated, [...] and [...], where [...] is the interface thickness, h is the boundary layer thickness, Pe is the Peclet number, and E is the normalized entrainment velocity; when mixing is wave breaking dominated, [...] and [...]; and when mixing is K-H dominated, [...] and [...]. In all cases the maximum concentration anomaly is [...]. The model for single-sided entrainment is simply extended to the case in which both layers are entraining. In the latter case it is found that entrainment depends on combinations of parameters from both layers.\n\nThe proposed entrainment model is supported by experimental results from this and previous studies. The data from this study are in agreement with [...] and are consistent with model results for [...]; results from previous studies support model predictions for E and [...]." }, { "name": "Wissler, John B.", "degree": "PhD", "year": "1992", "title": "Transmission of thin light beams through turbulent mixing layers", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-08162007-133929", "creators": [ { "name": { "family": "Wissler", "given": "John B." }, "id": "Wissler-J-B", "display_name": "Wissler, John B." } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/pw33-qc68", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nLight transmission through incompressible gaseous turbulent mixing layers is investigated with the objective of understanding the effects of large-scale coherent structures and mixing transition on the optical quality of the mixing layer. Experiments are done in a vertically flowing mixing layer which is enclosed inside a pressure tank and fed by two banks of high-pressure gas bottles. The study considers both the unequal density (high-speed [...] and low-speed [...]) and equal density (high-speed [...] and low-speed [...]) cases; the mixing of dissimilar gases is the source of the optical aberrations. Large-scale Reynolds numbers range between 3500 and 80000 over pressures from 2 to 6 bar. Light transmission characteristics are first studied qualitatively using a network of thin sheets of short-exposure ([...]) white light which are aberrated by the mixing layer and then image directly onto photographic film. Light transmission characteristics are then studied quantitatively using a lateral effect detector to dynamically track a thin He-Ne laser beam as it wanders under the influence of the passing coherent structures.\n\nThe study finds that the spanwise coherent structures generate systematic deflections of the light beam in the streamwise direction; the greatest deflections occur near the trailing edges of the structures at a formation called the cusp, where the high-speed fluid and low-speed fluid are entrained into the vortex core. The streamwise coherent structures, which form later in the mixing layer's development than the spanwise structures, generate substantial beam deflections in the span-wise direction which are closely associated with the streamwise streaks in plan-view shadowgraphs. The rms fluctuations of the streamwise and spanwise deflection angles increase rapidly during mixing transition, peaking at 380 high-speed-side momentum thicknesses downstream from the splitter plate, then decrease far down-stream to asymptotic values of 0.6 to 0.8 as scaled by the static pressure and the Gladstone-Dale constant shift across the mixing layer. The data suggest that a possible mechanism for the deflections is the interaction of the beam with the thin interfaces which bound relatively uniform bodies of fluid inside the structures." }, { "name": "Gilbrech, Richard Joseph", "degree": "PhD", "year": "1991", "title": "An experimental investigation of chemically-reacting, gas-phase turbulent jets", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06272007-091419", "creators": [ { "name": { "family": "Gilbrech", "given": "Richard Joseph" }, "id": "Gilbrech-R-J", "display_name": "Gilbrech, Richard Joseph" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/p80s-h321", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\n\nA new high pressure combustion facility was built to investigate mixing in axisymmetric, turbulent jets exiting into quiescent reservoirs. The facility uses fluorine and nitric oxide, diluted with nitrogen, for chemical product formation that is accompanied by heat release. The average temperature was measured by a set of long, thin, resistance wire thermometers stretched across the jet centerline at 16 downstream locations from x/d[subscript 0] = 30 to 240. Runs at several stoichiometric mixture ratios [phi], for Reynolds numbers ranging from 10,000 [...] Re [...] 150,000, were performed to determine any dependence of flame length on Reynolds number. The Reynolds number was varied through density, i.e., pressure, while the jet exit velocity and exit diameter were held constant. The time-averaged line integral of temperature, measured along the transverse axis of the jet by the wires, displays a logarithmic dependence on x/d* within the flame zone, and asymptotes to a constant value beyond the flame tip, as predicted from scaling and similarity arguments for a momentum-dominated, turbulent jet. The main result of the work is that the flame length, as estimated from the temperature measurements, varies with changes in Reynolds number, suggesting that the mixing process is not Reynolds number independent up to Re = 150,000. Specifically, the normalized flame length Lf/d* displays a linear dependence on [phi], with a slope that decreases from Re = 10,000 to 20,000, and then remains constant for Re > 20,000. Additionally, the measurements revealed a \"mixing virtual origin,\" defined as the far-field flame length extrapolated to [phi] = 0, that increases with increasing Re for Re [...]20,000 and then decreases with increasing Re for Re > 20, 000. A separate set of experiments indicated that the runs described above were momentum dominated to the farthest measuring station and that the kinetics of the chemical reactions were fast compared to the characteristic mixing time. The transition of the jet flow from a momentum- dominated to a buoyancy-dominated regime was identified in another set of experiments." }, { "name": "Hall, Jeffery Lawrence", "degree": "PhD", "year": "1991", "title": "An experimental investigation of structure, mixing and combustion in compressible turbulent shear layers", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09232005-141544", "creators": [ { "name": { "family": "Hall", "given": "Jeffery Lawrence" }, "id": "Hall-J-L", "display_name": "Hall, Jeffery Lawrence" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T0MN-J472", "abstract": "Two-dimensional, compressible, turbulent shear layers are studied in a new wind tunnel facility. Both reacting and non-reacting flows are investigated, with one free stream velocity supersonic and the other subsonic. The combustion experiments are based on the use of low concentrations of hydrogen, nitric oxide and fluorine gases. Side-view Schlieren photographs of these reacting and non-reacting flows appear devoid of the two-dimensional, large-scale structures seen in incompressible flows. Comparison with all-subsonic flows produced in the same facility suggests that this lack of two-dimensional structure is due to the presence of the supersonic high-speed free stream velocity. Travelling shock and expansion waves are observed in the high compressibility flows, evidently created by turbulent structures convecting at supersonic velocities. Such waves are seen only in the low-speed fluid, with apparent convection velocities much higher than those predicted on the basis of isentropic pressure-matching arguments. The measured shear layer growth rates agree with previous results by other experimenters, except for a few cases at low compressibility and low density ratio. The fast chemistry regime is attained in some of the high compressibility flows tested. \"Flip\" experiments conducted in this regime indicate that the volume fraction of mixed fluid in the layer is substantially reduced as compared to previous incompressible results. These same flip experiments also reveal that compressibility significantly alters the entrainment ratio. Finally, it is observed that the shear layer growth rate is relatively insensitive to incident shock /expansion waves and significant heat release inside the shear layer." }, { "name": "Hill, Larry Glenn", "degree": "PhD", "year": "1991", "title": "An experimental study of evaporation waves in a superheated liquid", "advisor": "", "url": "https://resolver.caltech.edu/CaltechETD:etd-10242005-103224", "creators": [ { "name": { "family": "Hill", "given": "Larry Glenn" }, "id": "Hill-L-G", "display_name": "Hill, Larry Glenn" } ], "advisors": [], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NF49-ME48", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nEvaporation waves in superheated liquids are studied using a rapid-depressurization facility consisting of a vertical glass test cell situated beneath a large, low-pressure reservoir. The objective of this study is to learn more about the physical mechanisms of explosive boiling (of which an evaporation wave is a specific example), as well as properties of the flow it produces.\r\n\r\nThe test cell is initially sealed from the reservoir by a foil diaphragm, and is partially filled with a volatile liquid (Refrigerant 12 or 114). An experiment is initiated by rupturing the diaphragm via a pneumatically driven cutter. The instrumentation consists of fast-response pressure measurements, high-speed motion pictures, and spark-illuminated still photographs. The liquid temperature is typically 20\u00b0C; the liquid superheat is controlled by setting the reservoir pressure to values between vacuum and 1 atm. The pressures subsequent to depressurization are very much less than the critical pressure, and the initial temperatures are sufficiently low that, although the test liquid is highly superheated, the superheat limit is not approached. Evaporation waves in which bubble nucleation within the liquid column is suppressed entirely are considered almost exclusively.\r\n\r\nWhen the diaphragm is ruptured, the liquid pressure drops to virtually the reservoir value within a few milliseconds. Provided that the liquid superheat so obtained is sufficiently high, the free surface then erupts in a process known as explosive boiling, which is characterized by violent, fine-scale fragmentation of the superheated liquid and extremely rapid evaporation. The explosive boiling process proceeds as a \"wavefront\" into the liquid column, producing a highspeed, two-phase flow that travels upward into the low-pressure reservoir, emptying the test cell in a few hundred milliseconds. The speed of the wavefront varies between 0.2 and 0.6 m/s, depending on run conditions; the corresponding two-phase flow varies between about 5 and 35 m/s.\r\n\r\nIn the highest superheat case for the more volatile liquid (Refrigerant 12), explosive boiling usually initiates by the rapid formation of nucleation sites at random spots on the liquid free surface and at the glass/free-surface contact line. Boiling spreads to the remaining surface within 160 [...]. In the highest superheat case for the less volatile liquid (Refrigerant 114), nucleation begins only at the glass/free-surface contact line. Boiling then spreads radially inward toward the center. In the lower superheated cases for both liquids, nucleation begins at one or more sites on the glass/free-surface contact line, and propagates across the free surface.\r\n\r\nAt the higher superheats, explosive boiling initiates within a few milliseconds from diaphragm burst, the same time scale as that of liquid depressurization. No distinction is made between the onset of nucleation and that of explosive boiling. However, if the reservoir pressure is raised above a certain approximate value, the onset of explosive boiling is delayed. During the delay period, relatively slow bubbling (initiated at one or more nucleation sites at the glass/free-surface contact line) occurs, and a cluster of bubbles forms in the vicinity of the initial site. The bubble cluster then \"explodes,\" marking the transition to explosive boiling. The delay period increases significantly as the reservoir pressure is raised slightly further. Reservoir pressures corresponding to a delay period of order 100 ms define an approximate self-start threshold pressure, above which the transition to explosive boiling does not occur. \r\n\r\nWithin about 10 ms of initiation, the wave reaches a quasi-steady condition in which the average wave speed, two-phase flow speed, and base and exit pressures are constant. However, the instantaneous propagation rate and the mechanisms that generate the mean flow are observed to be highly nonsteady. The wavefront appears to propagate by heterogeneous bubble nucleation at its leading edge, and any given region of the wavefront tends to propagate in surges associated with new nucleation and/or very fine-scale surface perturbations. Measurements of the instantaneous position of the upstream tip of the wavefront indicate that local velocity fluctuations are the same order as the mean velocity. The leading-edge bubble lifetimes and diameters are statistically distributed; mean values are of order 1 ms and 1 mm, respectively. The leading-edge bubbles are fragmented in violent \"bursts\" of aerosol. Bursts have a tendency to sweep over the leading-edge bubble layer in a wavelike manner: They are \"large-scale structures\" associated with the fragmentation of many bubbles.\r\n\r\nFragmentation, rapid evaporation, flow acceleration, and pressure drop occur primarily within about 1 cm of the leading edge. Downstream of this region, the average speed and appearance of the flow are virtually constant. This developed flow is a highly nonuniform, two-phase spray containing streaklike structures. Its liquid phase is composed of drops (with a maximum diameter of about 100 [...]), as well as clusters and chains of bubbles (with a diameter of a few hundred microns). A thin liquid layer begins climbing the wall upon wave initiation. Its speed is a few m/s-significantly slower than that of the two-phase flow through the center. Exit pressure measurements indicate that the flow chokes for sufficiently low reservoir pressure; at higher reservoir pressures the flow is unchoked.\r\n\r\nThe self-start threshold is not a propagation threshold, as waves are observed to propagate at somewhat lower superheats if started artificially. This is accomplished in Refrigerant 114 by \"jump-starting\" the wave, using the more volatile Refrigerant 12. For sufficiently high reservoir pressures, an \"absolute\" threshold is reached at which the quasi-steady rapid evaporation processes break down.\r\n\r\nPossible mechanisms for explosive boiling are discussed in light of the present results. While neither of the two previous schools of thought (interfacial instability hypotheses and the secondary nucleation hypothesis) are alone adequate to explain the observed behavior, there is evidence that both may play a role. It is here proposed that the bursting phenomenon and bubble nucleation at the wavefront leading edge are mutually interactive processes-bursting occurring as the violent breakup of interstitial bubble liquid, and nucleation (and fine-scale perturbations) being caused by burst-generated aerosol striking the leading-edge surface. It is not understood what role interfacial instability may play in the bursting process.\r\n\r\nAn evaporation wave is analogous to a premixed flame in that both are classified as \"weak deflagration\" waves in gasdynamic theory. It is shown that using several approximations that are valid for the type of evaporation waves studied, the conservation equations (jump conditions) can be reduced to a single, simple expression in terms of readily measured and inferred properties.\r\n" }, { "name": "Pfaff, Richard D.", "degree": "PhD", "year": "1991", "title": "Three-dimensional effects in nonlinear fracture explored with interferometry", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-07102007-104514", "creators": [ { "name": { "family": "Pfaff", "given": "Richard D." }, "id": "Pfaff-R-D", "display_name": "Pfaff, Richard D." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Ravichandran", "given": "Guruswami" }, "id": "Ravichandran-G", "role": "member", "display_name": "Ravichandran, Guruswami" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6jnw-kh62", "abstract": "The prospects for understanding fracture mechanics in terms of a general material constitutive description are explored. The effort consists of three distinct components.\n\nFirst, optical interferometry, in its various forms (Twyman-Green, diffraction moire, etc.), can potentially be used under a wide range of conditions to very accurately measure the displacement and strain fields associated with the deformation surrounding a cracktip. To broaden the range of fracture problems to which interferometry may be applied, certain of the necessary experimental improvements have been developed:\n\n1. High speed camera designs capable of extremely high (> 10(9) frames/second) framing rates with large array sizes, (> 4000 x 4000 pixels per frame) so that the application of optical techniques to solid mechanics may be considered without limitation on the rate of deformation.\n\n2. An accurate and adaptable device for dynamic loading of fracture specimens to high load levels utilizing electromagnetic (Lorentz force) loading with ultrahigh (> 2,000,000 Amp/cm(2)) current flux densities.\n\n3. Implementation of high sensitivity (2 nm), large range (2 nm x 3,200,000) interferometry achieved with wide field array sizes of 50,000 x 50,000 and 8 bit gray scale (error restricted to 1 bit) for surface deformation measurements on fracture specimens.\n\nSecond, functional descriptions for certain aspects of the displacement fields associated with fracture specimens are developed. It is found that the fully three-dimensional crack tip field surrounding a through-thickness crack in a plate of elastic-plastic material shows a hierarchical structure of organization and that the primary aspects of the deformation field would seem to have a relatively simple form of expression if the deformation is viewed in a properly normalized form.\n\nThird, a comparison is made between interferometrically measured surface displacements for a notched 3-point-bend speciemn of a ductile heat treatment of 4340 steel and a numerical simulation of the specimen based on a material constitutive description determined from uniaxial tests performed on the same material. The small but finite notch tip radius (0.15 mm) fabricated by a wire-cutting electrical discharge machine allows one to explore the limits of applicability of standard continuum plasticity theories without involving a process zone model for the very near tip region extent in a cracked specimen geometry.\n" }, { "name": "Schultheisz, Carl R.", "degree": "PhD", "year": "1991", "title": "Comparison of experimental and computational crack-tip deformations using Moire interferometry and finite elements", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-07122007-132245", "creators": [ { "name": { "family": "Schultheisz", "given": "Carl R." }, "id": "Schultheisz-C-R", "display_name": "Schultheisz, Carl R." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/etqv-z784", "abstract": "The large plastic deformations at the tip of a crack in a ductile heat treatment of 4340 steel are studied experimentally and numerically to investigate the details of the deformation in a tough material. The specimen is loaded in a three-point-bend arrangement. The finite-element model of the experiment uses a small-strain, incremental plasticity law, with a power-law hardening behavior. Both the in-plane and out-of-plane deformations were measured on the same specimen at the same time.\n\nThe experimental technique of moire interferometry is used to measure the in-plane displacements. This technique is described in detail, including an analysis of the effect of out-of-plane rotations on the use of the technique. A four-beam interferometer for measuring orthogonal displacement components is described, and its performance analyzed.\n\nThe three-dimensional, finite-element model has 11913 degrees of freedom, and provides data for comparison with the experiment between 4000 N (linear behavior) up to 73.5 kN (continuous fracture of the steel specimen). The model material properties are determined from a uniaxial test on specimens taken from the same bar as the fracture specimens and with identical heat treatment. This model characterizes the crack as a rounded notch to match the notch in the steel fracture specimen. The effects of tunneling of the crack are introduced through the release of nodes along the crack plane corresponding to measured crack profiles.\n\nResults indicate that the numerical model matches the experiment quite well up to a load of 52.3 kN; mismatch at higher loads may be caused by a lack of finite-strain formulation in the code. The finite notch tip negates the singularity in either the stress or strain fields; the HRR field seems to have no region of dominance. However, the function of the J-integral appropriate to the HRR field does normalize the stresses and strains well, indicating that the J-integral is still a good fracture criterion. The effects of the added tunnel indicate that failure of the material depends on both the plastic strain and the hydrostatic stress." }, { "name": "Sugioka, Ichiro", "degree": "PhD", "year": "1991", "title": "Particle Transport by Rapid Vaporization of Superheated Liquid", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-07172007-082412", "creators": [ { "name": { "family": "Sugioka", "given": "Ichiro" }, "id": "Sugioka-Ichiro", "display_name": "Sugioka, Ichiro" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "co-chair", "display_name": "Hornung, Hans G." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "co-chair", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Sieh", "given": "Kerry E." }, "id": "Sieh-K-E", "role": "member", "display_name": "Sieh, Kerry E." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZS1H-VE29", "abstract": "Superheated liquid vaporizing explosively in a particle bed inside a cylindrical test cell has been studied using a rapid depressurization apparatus. The experiments provide insights into the explosive vaporization phenomenon and the multiphase flow which is generated by the rapid production of vapor.
\r\n\r\nInside the sealed test cell, spherical glass particles are immersed in a volatile liquid, Refrigerant 12 or 114 at 300K. When the diaphragm at the upper end of the test cell is ruptured, the liquid pressure is reduced to a predetermined pressure within milliseconds. Since the liquid temperature is higher than the boiling temperature at reduced pressure, the liquid achieves a superheated state and nucleate boiling begins among the particles. The particle-liquid-vapor flow produced by the rapid release of vapor has been found to differ depending on whether the pressure is reduced below a critical level, which is 55% of the vapor pressure in the experiments conducted. When the final pressure is greater than critical, vapor pockets continue to grow throughout the particle bed and displace a liquid-particles mixture out from the test cell. When the final pressure is below critical, the particles are dispersed by a wave-like phenomenon (disruption front) where explosive vaporization appears to be localized in a narrow region. A disruption front in R12 travels at about 380 cm/s, and at about 200 cm/s in R114.
\r\n\r\nExperiments have been performed at various conditions to study the vaporization and transport process. High-speed cinematography and fast response pressure gauges have provided data on the particle acceleration process. The inertial effect on particle acceleration has been studied by conducting similar experiments in a centrifuge. Using this data, the transport process associated with the disruption front has been examined in detail. An empirical relationship between the particle weight and viscous drag is presented for this particular case. This study concludes with discussions based on analytical models of the disruption front to approximate flows properties which are intractable experimentally. It is suggested that a disruption front is an expansion process which maximizes vaporization and entropy.
" }, { "name": "Tokumaru, Phillip Takeo", "degree": "PhD", "year": "1991", "title": "Active control of the flow past a cylinder executing rotary motions", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04132004-141454", "creators": [ { "name": { "family": "Tokumaru", "given": "Phillip Takeo" }, "id": "Tokumaru-P-T", "display_name": "Tokumaru, Phillip Takeo" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/v7p7-d977", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\t\t\t\t\t\r\nExploratory experiments have been performed on circular cylinders executing forced rotary motions in a steady uniform flow. These motions include harmonic oscillations, steady rotation, and combinations of the two. Flow visualization and laser-Doppler velocimetry measurements were used to characterize the wake structure, and to estimate the convection speed, spacing, and strength of the vortical structures. Laser-Doppler velocimetry measurements were also made to estimate the cylinder drag coefficient and wake displacement thickness. In addition, the periodic flow close the cylinder and in the near wake region was mapped for a particular forced case. The data show that a considerable amount of control can be exerted over the flow by such means. In particular, a large increase, or decrease, in the resulting displacement thickness, estimated cylinder drag, and associated mixing with the free stream can be achieved, depending on the frequency and amplitude of oscillation. In order to assess the effects of oscillatory forcing on a cylinder with a net (mean) rotation rate, a novel method for estimating the steady lift forces was employed. Using this method, it was also found that the addition of forced rotary oscillations to the steady rotation of the cylinder helped to increase [...] in the cases where the wake would normally be separated in the steadily rotating case, and decrease it otherwise. Results obtained for a steadily rotating cylinder (no forced oscillations) compare favorably with similar data published in the literature." }, { "name": "Waitz, Ian Anton", "degree": "PhD", "year": "1991", "title": "An Investigation of Contoured Wall Injectors for Hypervelocity Mixing Augmentation", "advisor": "Zukoski, Edward E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-07172007-083103", "creators": [ { "name": { "family": "Waitz", "given": "Ian Anton" }, "id": "Waitz-Ian-Anton", "display_name": "Waitz, Ian Anton" } ], "advisors": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "advisor", "display_name": "Zukoski, Edward E." } ], "committee": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "chair", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MPJS-1Q71", "abstract": "A parametric study of a class of contoured wall fuel injectors is presented. The injectors were aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Short combustor residence time, a requirement for fuel injection parallel to the freestream, and strong sensitivity of overall vehicle performance to combustion efficiency motivated the investigation. Several salient parametric dependencies were investigated. Injector performance was evaluated in terms of mixing, losses, jet penetration and heating considerations.
\r\n\r\nA large portion of the research involved a series of tests conducted at the NASA Langley High - Reynolds Number Mach 6 Wind-Tunnel. Helium was used as an injectant gas to simulate hydrogen fuel. The parameters investigated include injector spacing, boundary layer height, and injectant to freestream pressure and velocity ratios. Conclusions concerning injector performance and parameter dependencies are supported by extensive three-dimensional flow field surveys as well as data from a variety of flow visualization techniques including Rayleigh scattering, Schlieren, spark-shadowgraph, and surface oil flow.
\r\n\r\nAs an adjunct to these experiments, a three-dimensional Navier-Stokes solver was used to conduct a parametric study which closely tracked the experimental effort. The results of these investigations strongly complemented the experimental work. Use of the code also allowed research beyond the fairly rigid bounds of the experimental test matrix. These studies included both basic investigations of shock-enhanced mixing on generic injectors, and applied efforts such as combining film-cooling with the contoured wall injectors.
\r\n\r\nLocation of an oblique shock at the base of the injection plane was found to be a loss-effective method for enhancing hypervelocity mixing through baroclinic generation of vorticity and subsequent convection and diffusion. Injector performance was strongly dependent on the displacement effect of the hypersonic boundary layer which acted to modify the effective wall geometry. Strong dependence on injectant to freestream pressure ratio was also displayed. Mixing enhancement related to interaction of the unsteady component of the boundary layer with both steady and unsteady components of the flow field was found to be secondary, as were effects due to variation in mean shear between the injectant and the freestream in the exit plane.
" }, { "name": "Chua, Kiat", "degree": "PhD", "year": "1990", "title": "Vortex Simulation of Separated Flows in Two and Three Dimensions", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-08092005-142847", "creators": [ { "name": { "family": "Chua", "given": "Kiat" }, "id": "Chua-Kiat", "display_name": "Chua, Kiat" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9ENS-EP36", "abstract": "This thesis is concerned with the applications of vortex methods to the problem of unsteady, separated flows in two and three dimensions, and can be divided into three parts. In the first part, an improved method for satisfying the boundary conditions on a flat plate is developed and applied to the two-dimensional separated flow problem. In this method, boundary layers on both side of the plate are represented by stacks of multiple vortex panels, the strength of which are determined by enforcing both the no-through flow and no-slip boundary conditions at the plate. Vortex shedding at the sharp edge of the plate is represented as the separation of the boundary vortex elements. Both forced and unforced flows are studied and comparisons to experiments are carried out. For the case without forcing, large discrepancy between calculations and experiments, which is also reported by other workers using a different vortex method or Navier-Stokes calculations, is observed. In the case with forcing, the discrepancy is reduced with lateral forcing at low amplitude; and eliminated, regardless of amplitude, with streamwise forcing (acceleration). In the second part, an improved three-dimensional vortex particle method is developed. In this method, vortex elements of vorticity that move with the local velocity and are stretched and rotated according to the local strain field, are used. To mimic the effects of vorticity cancellations, close pairs of opposite sign vortex elements are replaced by high order dipoles. The method is designed to handle complex high Reynolds number vortical flows and a non-linear viscosity model is included to treat small-scale effects in such flows. Applications to two problems involving strong interactions of vortex tubes are carried out and core deformation with complex internal strucures and induced axial flow within vortex tubes are observed. Qualitative comparison to experiments are encouraging. In the third part, the two-dimensional method developed in the first part is modified and extended to three dimensions. Here, solenoidal condition for vorticity is considered and closed vortex loops are used to represent the boundary layer vorticity and the vorticity at shedding. For the evolution of the vortex wake, the vortex particle method developed in the second part is used. Applications to the flow past a normal square plate is carried out and the early stages of the flow are studied.
" }, { "name": "Fric, Thomas Frank", "degree": "PhD", "year": "1990", "title": "Structure in the Near Field of the Transverse Jet", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-02232007-075829", "creators": [ { "name": { "family": "Fric", "given": "Thomas Frank" }, "id": "Fric-Thomas-Frank", "display_name": "Fric, Thomas Frank" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JVHG-E582", "abstract": "Photographs of an axisymmetric turbulent jet issuing from a wall into a crossflow display the four types of vortical structures which exist in the near field: the jet shear layer vortices, the nascent far field vortex pair, the near wall horseshoe vortices, and a system of vortices in the wake of the jet.
\r\n\r\nAdditionally, results of hot-wire measurements in the wake of the transverse jet are presented. Among these results are characteristic wake Strouhal frequencies, which vary with the jet to crossflow velocity ratio, and wake velocity profiles.
\r\n\r\nIt is found that the wake vorticity is not \"shed\" from the jet but is formed from vorticity which originated in the wall boundary layer. Therefore, analogies between the wakes of transverse jets and the wakes of solid cylinders are incorrect. Since the jet is not a solid obstacle to the crossflow, as a cylinder is, new vorticity is not generated at the interface between the jet and the crossflow. Instead, the boundary layer on the wall from which the jet issues separates near the downstream side of the jet because it cannot negotiate the adverse pressure gradient imposed on it by the flow around the jet, which is not \"separated\" as it is for a cylinder. The wake vortices subsequently formed are found to be most coherent near a jet to crossflow velocity ratio of four.
\r\n\r\nThe near field development of the counterrotating vortex pair, which is the dominant structure of the far field jet, is also addressed. It is argued that the source of vorticity for the vortex pair is the vorticity from the boundary layer within the jet nozzle. Estimates for the strength of these vortices are obtained by considering the flux of vorticity emanating from the nozzle.
\r\n\r\nPossible implications for mixing are briefly discussed.
" }, { "name": "Goldstein, David Benjamin", "degree": "PhD", "year": "1990", "title": "Investigations of a Discrete Velocity Gas", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-05152007-093505", "creators": [ { "name": { "family": "Goldstein", "given": "David Benjamin" }, "id": "Goldstein-David-Benjamin", "display_name": "Goldstein, David Benjamin" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Seitz", "given": "Charles L." }, "id": "Seitz-C-L", "role": "member", "display_name": "Seitz, Charles L." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DK4P-AK28", "abstract": "A new model of molecular gasdynamics with discrete molecular velocity components has been implemented for parallel computation. When the suitably normalized velocity components can take only integer values and time is discretized for digital computation, the particles travel between a regular array of points in physical and velocity space, and the gas is called a \"lattice gas.\" Calculations of molecular motions are thereby simplified. The outcome of binary collisions between particles is determined by reflections about axes of symmetry in the center-of-mass frame of reference. The procedure speeds calculations of collisions. Of interest is the insight the discrete model provides into complex physical behavior and the effect that physically realistic simplifications have on the accuracy and speed of parallel calculations of a flow.
\r\n\r\nThe equilibrium state of a discrete-velocity gas and the influence of limited velocity resolution are explained. It is found that the equilibrium velocity distribution functions of the present model agree with those of the discrete Boltzmann equation at very low velocity resolution and the continuous-velocity Boltzmann equation at higher velocity resolution. The time development of non-equilibrium velocity distribution functions is presented. The model is applied to unsteady flows involving strong shock waves, heat transfer between solid surfaces, and unsteady shear layer development.
\r\n\r\nWhen the model is applied to gas mixtures, numerical experiments show that the required number of values of each component of molecular velocity depends strongly upon the mass ratios of the particle species involved. However, fewer than ten values of each velocity component are necessary to produce results of satisfactory accuracy in calculations of a shock wave in a single species gas. A unique, self-adaptive mesh for parallel computation, used either for the present lattice gas model or earlier direct simulation Monte Carlo (Bird, 1976) models, is described. The mesh balances the load between the processors of the multicomputer and maintains the cell size at approximately a fixed number of local mean free paths throughout the flow field.
" }, { "name": "Ide, Kayo", "degree": "PhD", "year": "1990", "title": "Regular and chaotic motion of uniform elliptical vortices in external linear time dependent velocity fields", "advisor": "", "url": "https://resolver.caltech.edu/CaltechETD:etd-04132004-092941", "creators": [ { "name": { "family": "Ide", "given": "Kayo" }, "id": "Ide-K", "display_name": "Ide, Kayo" } ], "advisors": [], "committee": [ { "name": { "family": "Wiggins", "given": "Stephen R." }, "id": "Wiggins-S-R", "role": "chair", "display_name": "Wiggins, Stephen R." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/sc2s-3749", "abstract": "The motion of a uniform vorticity of an elliptical shape (UEV) induced by an external linear flow field and a self induced effect in a two dimensional incompressible inviscid flow is investigated. The fluid particle motion around the UEV is also investigated. The techniques from dynamical systems are intensively used. In particular, we develop some modified versions of the Melnikov techniques which show how the perturbations influence the dynamics and allow us to predict the perturbed dynamics by knowing the unperturbed dynamics.\n\nThe UEV motions in steady external linear flow fields are regular, however there are quite a few possibilities for types of motion depending on the UEV initial configuration and the external linear flow field parameters. When time periodic perturbation is imposed, the UEV motion drastically changes and various new types of motion become possible. This includes transition dynamics (i.e., the UEV changes its type of motion), chaotic motion, irregular oscillation and rotation, and quasiperiodic motion. The Melnikov techniques tell that the excess kinetic energy play significant roles in determining the UEV dynamics.\n\t\nA periodic motion of the UEV in a steady external linear flow field results in the generation of chaotic fluid particle mixing regions around the UEV where the fluid particles from distinct flow regions are chaotically transported. Our study emphasizes on the finite core size effect of the UEV in comparison to flow fields induced by a point vortex in unsteady external linear flow fields. We show that the size of the UEV core and the UEV initial configuration are important factors in determining the size of the mixing region.\n" }, { "name": "Jahnke, Craig C.", "degree": "PhD", "year": "1990", "title": "Application of Dynamical Systems Theory to Nonlinear Aircraft Dynamics", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05092007-134504", "creators": [ { "name": { "family": "Jahnke", "given": "Craig C." }, "id": "Jahnke-Craig-C", "display_name": "Jahnke, Craig C." } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Keller", "given": "Herbert Bishop" }, "id": "Keller-H-B", "role": "member", "display_name": "Keller, Herbert Bishop" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "orcid": "0000-0002-4903-8419", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9KZS-XC46", "abstract": "A continuation method has been used to determine the steady states of three nonlinear aircraft models: a general aviation aircraft with a canard configuration, a generic jet fighter, and the F-14. The continuation method calculated the steady states of the aircraft as functions of the control surface deflections. Bifurcations of these steady states were determined and shown to cause instabilities which resulted in qualitative changes in the state of the aircraft. A longitudinal instability which resulted in a deep stall was determined for the general aviation aircraft. Roll-coupling and high angle of attack instabilities were determined for the generic jet fighter, and wing rock, directional divergence and high angle of attack instabilities were determined for the F-14.
\r\n\r\nKnowledge of the control surface deflections at which bifurcations occurred was used to either put limits on the control surface deflections or to program the control surface deflections such that a combination of control surface deflections at which bifurcations occur could not be attained. Simple control systems were included in the aircraft models to determine the effects of control systems on the instabilities of each aircraft. Steady spin modes were determined for each aircraft. A successful recovery technique was determined for the general aviation aircraft, but no successful recovery technique could be found for the F-14.
" }, { "name": "Losi, Giancarlo Umberto Maria", "degree": "PhD", "year": "1990", "title": "Nonlinear thermoviscoelastic behavior of polymers", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-103818", "creators": [ { "name": { "family": "Losi", "given": "Giancarlo Umberto Maria" }, "id": "Losi-G-U-M", "display_name": "Losi, Giancarlo Umberto Maria" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2dk7-z575", "abstract": "The rheological behavior of polymers in the neighborhood of the glass transition has been investigated in the framework of the free volume theory of nonlinear viscoelastic behavior. Free volume theory as normally applied above the glass transition was modified to account for the effect of the residual volume of vacancies below the glass transition; this modification was accomplished by modelling the changes in the state of the polymer as the sum of viscoelastic changes and a random disturbance deriving from the thermal collisions between molecules. The changes in mechanical properties going across the glass transition follow from the freezing-in of relaxation mechanisms and of free volume. The pressure dependence of the glass transition was found to be in qualitative agreement with measurements on PVAc, while the ratio of the glassy and rubbery heat capacities was found to coincide with the ratio of the equilibrium bulk compliances in the glassy and rubbery domains. The predictions of the model for the problem of transient and residual thermal stresses were compared with those of two simpler models.\n\nThe second part of the thesis studies the consequences of the nonlinear viscoelastic behavior on the decohesion zone in front of a crack propagating through an adhesive layer. The softening of the material response in the cohesive zone is taken to be effected by free volume induced change in relaxation times of the cohesive material and by void growth; the latter is assumed to depend on a critical value of strain at the beginning of the cohesive zone. The stress intensity factor for steady crack propagation is obtained by imposing the finiteness of strains at the crack tip. For the case where the properties of the adherends are the same as the linearized properties of the adhesive, the predictions show three regimes of crack propagation: a low speed regime where the adherends behave elastically with the rubbery properties, an intermediate range where their response becomes increasingly stiffer, and a high speed regime characterized by glassy behavior of the adherends and control of the crack growth process exclusively by the nonlinearly viscoelastic behavior of the failing material." }, { "name": "Pepin, Francois Michel", "degree": "PhD", "year": "1990", "title": "Simulation of the flow past an impulsively started cylinder using a discrete vortex method", "advisor": "", "url": "https://resolver.caltech.edu/CaltechETD:etd-08162005-104131", "creators": [ { "name": { "family": "Pepin", "given": "Francois Michel" }, "id": "Pepin-F-M", "display_name": "Pepin, Francois Michel" } ], "advisors": [], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7svd-7213", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nVortex methods are a powerful tool for the simulation of incompressible flows at high Reynolds number. They rely on a discrete Lagrangian representation of the vorticity field to approximately satisfy the Kelvin and Helmholtz theorems which govern the dynamics of vorticity for inviscid flows.\r\n\r\nA time splitting technique can be used to include viscous effects. The diffusion equation is considered separately after convecting the particles with an inviscid vortex method. In this thesis, the viscous effects are represented by the so-called deterministic method. The approach was extended to problems where a flux of vorticity is used to enforce the no-slip boundary condition. The ability of such a scheme to create the right amount of vorticity at the wall and to adequately redistribute it within the fluid is demonstrated by simulating the viscous flow induced by an oscillating cylinder.\r\n\r\nIn order to accurately compute the viscous transport of vorticity, gradients need to be well resolved. As the Reynolds number is increased, these gradients get steeper and more particles are required to achieve the requisite resolution. In practice, the computing cost associated with the convection step dictates the number of vortex particles and puts an upper bound on the Reynolds number that can be simulated with confidence.\r\n\r\nThat threshold can be increased by reducing the asymptotic time complexity of the convection step from [...] to [...]. The near-field of every vortex particle is identified. Within that region, the velocity is computed by considering the pairwise interaction of vortices. The speed-up is achieved by approximating the influence of the rest of the domain, the far-field. In that context, the interaction of two vortex particles is treated differently depending on their spatial relation. The resulting computer code does not lend itself to vectorization but has been successfully implemented on concurrent computers.\r\n\r\nThe combination of a fully viscous vortex method with a fast parallel algorithm is used to simulate the flow past an impulsively started cylinder. Experiments have shown that this flow is characterized by the presence of a secondary eddy within the main recirculating region. The secondary structures over a wide range of Reynolds number (Re=550 to 9500). It was observed that the secondary phenomenon can lead to a major flow reorganization by drastically altering the transport of vorticity. The separating boundary layer acts as a source of vorticity and, at Re=550, the resulting vortex sheet smoothly rolls up into the primary vortex. For Re=3000 and 9500, however, the secondary eddy interferes with that process and the flux of vorticity is redirected toward the cylinder where it accumulates into a new vortical structure.\r\n\r\nThe impulsive start is followed by a [...] singularity in the drag coefficients. The numerical simulations captured this behavior and the computed drag history for short times is in close agreement with the one predicted by a matched asymptotics analysis." }, { "name": "Smedley, Gregory Todd", "degree": "PhD", "year": "1990", "title": "A study of immiscible liquids, liquid behavior at zero gravity, and dynamic contact lines and angles", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-08182004-154208", "creators": [ { "name": { "family": "Smedley", "given": "Gregory Todd" }, "id": "Smedley-G-T", "display_name": "Smedley, Gregory Todd" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AQR1-4B73", "abstract": "The work reported here covers three topics that were investigated as part of preparation for a space based experiment. The intention of the proposed experiment was to study the relationship between container geometry and interface geometry for a free liquid surface at zero gravity.\n\nA mathematical theory proposed by Paul Concus and Robert Finn in 1974 and recently developed by Finn yields explicit geometrical criteria for the position of the free surface of a liquid at zero gravity in a cylindrical container of specified cross section. It is possible to find geometrical criteria that promise a particular liquid location. A unified graphical presentation of four geometries is given that can be used directly for the design of containments for liquids at zero gravity. As one application of these design curves, a container was built and tested in a series of 2 second droptower experiments at NASA Lewis Research Center.\n\nIt was apparent that the space based experiment would require use of sophisticated optical instrumentation that would be most effective if a pair of immiscible liquids were used rather than a single liquid under its vapor. This work identifies 121 transparent immiscible liquid pairs that have properties compatible with optical instrumentation based on laser-induced fluorescence. Physical data such as specific gravity, index of refraction, viscosity, flash point, and toxicity were found in the literature. Compatibility with plexiglas (PMMA), contact angles of the internal meniscus on glass and PMMA, meniscus formation times, and clearing times were measured. A useful noninvasive technique for determining interfacial tensions is explained and used.\n\nThe contact angle is a critical parameter in the consideration of liquid behavior at zero gravity, therefore, a technique, based on laser light refraction, was developed to objectively measure it. Dynamic contact line experiments were conducted at various velocities, both advancing and receding, using one of the 121 immiscible liquid pairs (nonane/formamide) in contact with glass.\n" }, { "name": "Washabaugh, Peter D.", "degree": "PhD", "year": "1990", "title": "An experimental investigation of mode-I crack tip deformation", "advisor": "Knauss, Wolfgang Gustav; Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11092007-101356", "creators": [ { "name": { "family": "Washabaugh", "given": "Peter D." }, "id": "Washabaugh-P-D", "display_name": "Washabaugh, Peter D." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/wv0e-g535", "abstract": "The out-of-plane displacement of amorphous polymethylmethacrylate plates rupturing at slow (0.1 mm/s), and fast (0.5 to 0.9 mm/\u00b5s) rates are measured using a Twymann-Green interferometer. The measured surface shapes within one plate thickness of the crack-tip do not compare well with the two-dimensional planar asymptotic approximation, but compare favorably with the published slopes for three-dimensional finite element solutions when normalized with the static material properties. Discrepancies, on the order of ten percent, between the magnitude of the three-dimensional finite element solutions suggest that the stress intensity factor does not fully characterize the near tip deformations.\n\nA dynamically propagating crack is found to move in a non-steady, periodic, submicrosecond fashion. This result is supported both by the surface measurements and the fracture morphology. The material toughening, as measured by the surface roughness, correlates well with the stress intensity factor and not with the crack velocity. The details of the sub-microsecond propagation and toughening was not resolvable with the microsecond temporal resolution of the experiment.\n\nInhibiting the material toughening at the crack tip by artificially introducing a weak material plane augments the crack motion to velocities close to the material's shear wave speed. The crack propagates more steadily along the weak plane than through a virgin solid, while maintaining the character of the out-of-plane displacement of a crack propagating in an unsullied material.\n" }, { "name": "Zhuang, Mei", "degree": "PhD", "year": "1990", "title": "An investigation of the inviscid spatial instability of compressible mixing layers", "advisor": "Dimotakis, Paul E.; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-11132007-094001", "creators": [ { "name": { "family": "Zhuang", "given": "Mei" }, "id": "Zhuang-M", "display_name": "Zhuang, Mei" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "co-advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/650s-9t96", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe behavior of both unbounded and bounded compressible plane mixing layers with respect to two- and three-dimensional, spatially growing wave disturbances is investigated using linear stability analysis. The mixing layer is formed by two parallel streams with different gases and the flow is assumed to be inviscid and non-reacting.\r\n\r\nFor unbounded mixing layers, the effects of the free stream Mach number, velocity ratio, temperature ratio, gas constant (molecular weight) ratio and the ratios of specific heats on the linear spatial instability characteristics of a mixing layer are determined. A nearly universal dependence of the normalized maximum amplification rate on the convective Mach number is found for two-dimensional spatially growing disturbances. The effects of the mean flow profiles on the instability behavior of the mixing layers are also studied. It is shown that decreasing the thickness of the total temperature profile relative to the mean velocity profile, or adding a wake component in the mean velocity profile can make the normalized amplification rate decrease slower as the convective Mach number increases for both subsonic and supersonic convective Mach numbers.\r\n\r\nFor an unbounded mixing layer with subsonic convective Mach numbers, there is only one unstable mode propagating with a phase velocity [...] approximately equal to the isentropically estimated convective velocity of the large scale structures [...]. As the convective Mach number approaches or exceeds unity, there are always two unstable spatial modes. One is with a phase velocity [...] (slow mode and the other is with a phase velocity [...] (fast mode). For the low supersonic convective Mach numbers, the fast mode is more unstable than the slow mode when the heavy gas is on the low speed side and the slow mode is dominant when the heavy gas is on the high speed side.\r\n\r\nThe effect of parallel flow guide walls on a spatially growing mixing layer is also investigated. It is shown that, in this case, if the convective Mach number exceeds a critical value of approximately unity, there are many supersonic unstable modes. The maximum amplification rates of mixing layers approach an asymptotic value and this maximum amplification rate increases to a maximum value and decreases again as the distance between the walls decreases. For a mixing layer inside parallel flow guide walls, the growth rate of three-dimensional modes is larger than the corresponding two-dimensional mode at high convective Mach numbers. But the growth rate of two-dimensional supersonic instability waves has a larger value than their three-dimensional counterparts for a mixing layer inside a rectangular duct (Tam & Hu [1988], [1989]). Contour plots of the pressure perturbation fields for both unbounded and bounded mixing layers indicate that there are waves propagating outward from the mixing layer along the Mach angle, and that the walls provide a feedback mechanism between the growing mixing layer and this compression/expansion wave system. The bounded mixing layers are more unstable than the corresponding free mixing layers for supersonic convective Mach numbers. The streaklines of the flow confirm that the spreading rate of the mixing layer is unusually small for supersonic disturbances.\r\n" }, { "name": "Anilkumar, A. V.", "degree": "PhD", "year": "1989", "title": "Experimental Studies of High-Speed Dense Dusty Gases", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-08312006-130447", "creators": [ { "name": { "family": "Anilkumar", "given": "A. V." }, "id": "Anilkumar-A-V", "display_name": "Anilkumar, A. V." } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "mecheng" ], "doi": "10.7907/5B1B-AY62", "abstract": "An experimental study of the flow of high-speed dense dusty gases has been conducted in a novel shock tube facility. The flow is generated through rapid depressurization and subsequent fluidization of a stationary packed bed of particles loaded under pressure in the vertical driver section of the shock tube. The flow was studied with high-speed photography and fast-response pressure transducers.
\r\n\r\nThe studies have been exploratory in nature. The entire process of lofting and disassembly of packed particle beds has been documented. A wide spectrum of dusty flows with particle loadings ranging from that of a fully packed plug to that of a dilute disperse particle flow was observed in this facility. Only extreme flow fields like packed plug flows and very dilute disperse particle flows were found to be uniform. All other flow fields, with intermediate particle loadings, were characterized by the simultaneous presence of dense filamentary structures and dilute dispersions of particles. Typically, while operating with 0.5 mm glass beads, flows reached speeds of 60 meters per second in a period of 25 milliseconds.
\r\n\r\nTwo lofting configurations of the packed beds were set up. In the first configuration, the rapid depressurization of the interstitial bed fluid and the consequent initiation of bed expansion was examined. Bed expansion starts along horizontal fractures that partition the bed into slabs. While the bed is accelerating, particles rain down from the bottom surfaces of the slabs partitioning the fractures into bubbles with a characteristic honeycomb pattern. The bubbles eventually compete and the dominant ones prevail. The observed instability of the bottom surfaces of the slabs is analogous to the Rayleigh-Taylor instability observed in continuous media. The flow development in this configuration was not influenced by any wall effects.
\r\n\r\nThe second lofting configuration is a high-speed fluidization configuration. Here, the role of the fluid entering from below the bed, in continuing the bed expansion initiated by the rapid depressurization of the interstitial bed fluid, was examined. The bed expansion occurs along expanding and elongating bubbles and the bubble walls are stretched into dense filamentary structures. Beds initially stacked with a gradient in particle size or density or both showed drastic differences in response to fluidization. The morphology of the expanded flow field in all cases was essentially the same: nonuniform, interspersed with dense filamentary structures and dilute dispersions of particles.
\r\n\r\nIn the second lofting configuration, only the late stages of flow development were influenced by wall effects. Wall effects manifest as faster moving fluid along the walls and denser accumulation of flow structures towards the center of the channel. The bottom of the dusty flow is characterized by the presence of a tail; a concentric dense particle column formed by the accumulation of particles, initially present in the bottom regions of the flow. The tail terminates in a bulbous and streamlined bottom from which particles are slowly eroded by the coflowing fluid.
\r\n\r\nA multi-transducer probe was installed in the dusty gas flow for making dynamic pressure measurements and for correlating observations with those made through extensive flow visualization.
" }, { "name": "Beran, Philip Stewart", "degree": "PhD", "year": "1989", "title": "An Investigation of the Bursting of Trailing Vortices Using Numerical Simulation", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02012007-105641", "creators": [ { "name": { "family": "Beran", "given": "Philip Stewart" }, "id": "Beran-Philip-Stewart", "display_name": "Beran, Philip Stewart" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Keller", "given": "Herbert Bishop" }, "id": "Keller-H-B", "role": "member", "display_name": "Keller, Herbert Bishop" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/chyb-nk54", "abstract": "Solutions of the Navier-Stokes equations are obtained for the flow of an isolated, trailing vortex, and for the swirling flow through a frictionless pipe. In both cases, the flow is assumed to be steady, incompressible and rotationally symmetric. Solutions are computed using Newton's method and Gaussian elimination for a wide range of values of two parameters: Reynolds number, Re, and vortex strength, V. Pseudo-arclength continuation is employed to facilitate the computation of solution points in the parameter space. The numerical procedure is validated through comparison of solutions with solutions obtained in previous investigations for the case of a trailing vortex. Solutions are also compared with results reported by Brown and Lopez (1988) for the case of flow through a pipe.
\r\n\r\nSolutions of the quasi-cylindrical equations are obtained for the flow of a trailing vortex. Solutions are computed using an explicit, space-marching scheme, and are compared with solutions of the Navier-Stokes equations.
\r\n\r\nProvided that Re is about 200, or larger, four vortex states are observed.
\r\n\r\n1. When V is sufficiently small, the flow is entirely supercritical.
\r\n\r\n2. As V is increased, the flow at an axial station becomes critical and a transition point forms. At the point, the flow departs from an upstream state that is supercritical to a downstream state that is marked by large-amplitude, spatial oscillations of core radius. When Re is large, the downstream state is nearly periodic. The general features of transition are well described by the conjugate-flow theory of Benjamin 1967). Failure of the quasi-cylindrical equations is found to be a necessary and sufficient condition for the existence of a transition point. As V is further increased, the transition point moves upstream. Reversed flow is not observed.
\r\n\r\n3. Over a narrow range of vortex strengths, a small bubble of reversed flow is observed downstream of the transition point.
\r\n\r\n4. When V is large, the entire flow is marked by large-amplitude, spatial oscillations of core radius. A transition point is not evident within the computational domain. Typically, large regions of reversed flow are observed.
" }, { "name": "Brouillette, Martin", "degree": "PhD", "year": "1989", "title": "On the Interaction of Shock Waves with Contact Surfaces Between Gases of Different Densities", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-102505", "creators": [ { "name": { "family": "Brouillette", "given": "Martin" }, "id": "Brouillette-Martin", "display_name": "Brouillette, Martin" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Elachi", "given": "Charles" }, "id": "Elachi-C", "role": "member", "display_name": "Elachi, Charles" }, { "name": { "family": "Hornung", "given": "Hans G." }, "id": "Hornung-H-G", "role": "member", "display_name": "Hornung, Hans G." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9JGS-ZX78", "abstract": "The interaction of shock waves with a contact surface between gases of different densities has been studied experimentally and theoretically. The basic mechanism for the instability of perturbations at the interface is baroclinic vorticity generation resulting from the misalignment of the pressure gradient of the shock and the density gradient of the interface. In the present study, the effects of interface density contrast and initial thickness, and incident wave strength on the development of the instability at the interface are investigated. The experiments were performed in a new vertical shock tube facility where the interaction of a shock wave with either a discontinuous interface, formed by a thin (0.5 \u00b5m) plastic membrane, or a continuous interface, created by retracting a metal plate initially separating the two gases, was studied. Air was used on one side of the interface and either helium, carbon dioxide, refrigerant-22 or sulphur hexafluoride was used on the other side as the test gas.
\r\n\r\nExperiments to study the time evolution of quasi-sinusoidal perturbations on a continuous interface have shown that the growth rates are reduced as the interface thickness is increased. It has been observed that growth rates of perturbations of wavelength \u03bb ~ 25 mm on interfaces of thickness \u03b4 ~ 10 mm are about three times smaller than those predicted by the linear theory for the impulsive acceleration of discontinuous interfaces. A new model that accounts for the growth rate reduction caused by the presence of a finite density gradient on the interface has been proposed, and good agreement was obtained with the present experimental results.
\r\n\r\nExperiments were also performed to observe the schlieren visual thickness of plane discontinuous or continuous interfaces with random small-scale perturbations after interaction with the incident shock wave and its reverberations. The interface was initially located near the end wall of the shock tube to permit the observation of the development of the interface phenomena after the arrival of the incident shock and its reverberations. It is found that the interaction of a shock wave with a discontinuous interface causes the appearance of a turbulent mixing zone between the two gases, whose growth rate slows down as time increases, owing to a decrease in turbulence intensity and the action of viscosity. Because of the large uncertainty associated with the measurements a short time after the interaction with the incident shock, the accurate determination of a possible universal power law governing the thickening of the interface is not feasible. Results for the interaction of the first reverberation of the primary wave with the already turbulent interface have demonstrated that this growth is sensitive to the initial pre-growth state of the interface. It also appears that the thickening of the turbulent mixing zone is accomplished by the merging of large structures within the interface. However, since the energy available for the turbulent motions at the impulsively accelerated interface remains constant after the interaction with the shock and also depends on the wavelength of the initial perturbation, it is not certain whether the development of mixing at the interface achieves an asymptotic stage of self-similar turbulence independent of initial conditions, as has been observed for the gravity-driven interfaces. Also, it has been found that the growth rates measured in the present experiments with discontinuous interfaces are nearly an order of magnitude lower than those reported by previous investigators. The continuous interfaces formed by the retracting plate are smoothed by molecular diffusion, and thus the combination of low density gradient and small initial perturbations is such that they exhibit growth only after being perturbed by acoustic noise introduced by the reverberation of waves between the interface, the side walls and the end of the shock tube.
\r\n\r\nThe development of viscous boundary layers on the side walls of the test section can cause the bifurcation of waves reflected from the end wall of the shock tube, and, thereafter, the formation of wall bubbles and interface contaminating jets. Moreover, the generation of vortical structures by the baroclinic instability excited by the interaction of reflected waves with the distorted interface within the boundary layer has been demonstrated. Significant contamination of the test gas can by achieved by these structures, even if reflected-wave bifurcation is absent. Moreover, the strain induced by the vorticity in these wall structures tends to thin the interface; the magnitude of this effect on the growth rates in the present plane interface experiments is estimated to be of order 10% for discontinuous interfaces and 50% for continuous interfaces.
\r\n" }, { "name": "Krishnaswamy, Sridhar", "degree": "PhD", "year": "1989", "title": "On the Domain of Dominance of the Asymptotic Elastodynamic Crack-Tip Fields", "advisor": "Rosakis, Ares J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-134326", "creators": [ { "name": { "family": "Krishnaswamy", "given": "Sridhar" }, "id": "Krishnaswamy-Sridhar", "display_name": "Krishnaswamy, Sridhar" } ], "advisors": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "advisor", "display_name": "Rosakis, Ares J." } ], "committee": [ { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "chair", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Wood", "given": "David Shotwell" }, "id": "Wood-D-S", "role": "member", "display_name": "Wood, David Shotwell" }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/cfz9-nk80", "abstract": "A substantial part of the experimental data in dynamic fracture mechanics has been obtained under the assumption that the two-dimensional asymptotic elastodynamic stress-intensity factor field (the KdI-field) is dominant over at least the region around the crack-tip over which the experimental measurements are made. The validity of this assumption is investigated in this thesis both experimentally and through finite-element simulations of the experiments.
\r\n\r\nThe experiments reported in this work were on 4340 steel, three-point bend specimens loaded dynamically using a drop-weight tower. The two cases of dynamically loaded stationary cracks and dynamically propagating cracks were considered. An optical configuration is proposed that leads to a bifocal high-speed camera capable of focusing on two different planes simultaneously. This was used in conjunction with the method of caustics to measure the apparent stress-intensity factor simultaneously from two different regions (initial-curves) around the crack-tip. If the initial-curves lie within the domain of dominance of the asymptotic field, the measured values of the dynamic stress-intensity factor must agree to within experimental error. By suitably adjusting the optical set-up, a range of initial-curves was scanned in an attempt to map the domain of dominance of the KdI-field.
\r\n\r\nThe impact hammer and supports of the drop-weight loading device were instrumented in order to monitor the time dependent loads acting on the specimen. These loads were subsequently used as boundary tractions in dynamic two- and three-dimensional finite-element simulations of the experiments. The simulations were carried only up to the point of crack initiation. Comparison of the numerical simulations with the experimental results help in identifying the role of three-dimensionality and transient conditions on the measured stress-intensity factor values.
\r\n\r\nOn the basis of both the experimental results as well as the numerical simulations, no sizeable annulus of dominance for the asymptotic elastodynamic field was found for the laboratory situation studied. It appears that the assumption of an underlying KdI-dominant (or two-dimensional) field might not hold to a level of accuracy that would warrant many of the conclusions made in the literature regarding the crack-initiation toughness values as well as the uniqueness of the dynamic fracture toughness - crack velocity relation or its specimen and acceleration dependence.
" }, { "name": "Schneider, Steven Philip", "degree": "PhD", "year": "1989", "title": "Effects of Controlled Three-Dimensional Perturbations on Boundary Layer Transition", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-02142007-094214", "creators": [ { "name": { "family": "Schneider", "given": "Steven Philip" }, "id": "Schneider-Steven-Philip", "display_name": "Schneider, Steven Philip" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZJ0M-GG83", "abstract": "The laminar-turbulent transition in a flat plate boundary layer was studied experimentally using a spanwise array of computer-controlled surface heating elements. The elements were used to introduce disturbances at a point just downstream of the critical Reynolds number. When sinusoidal heating at an unstable frequency is carried out, instability waves develop and grow as they travel downstream. Measurements were made using flush-mounted hot-film wall shear sensors, and the later stages of transition were visualized using dye injection. Oblique Tollmien-Schlichting waves were successfully introduced, and their downstream development into the turbulent regime was studied. Exploratory studies of other types of 3D forcing are also reported.
\r\n\r\nMeasurements of oblique waves in the linear region yielded phase speeds and wave angles that were consistent with the linear theory. Subharmonics of the oblique-wave wall shear were seen downstream, in the nonlinear region. Surprisingly, the amplitude of these subharmonic waves decreased abruptly with increasing oblique-wave angle, so that an oblique wave of about 10 degrees had a subharmonic amplitude which was an order of magnitude below that for a 2D wave. Waves of larger oblique angles did not produce detectable subharmonics. A simple explanation of this behavior is given, in terms of the wave-interaction theory.
\r\n\r\nThe intermittency, defined as the fraction of time in which the wall shear is turbulent, was measured to determine the relative location of transition. These measurements, carried out further downstream, show that the introduction of a 2D wave is most effective in moving the transition point upstream, for a given power input. This upstream movement of transition slowly decreases as the oblique wave angle is increased. The fact that there is no abrupt movement of transition corresponding to the abrupt disappearance of the subharmonic nonlinear breakdown mechanism suggests there should be a simpler explanation for the nonlinear breakdown.
" }, { "name": "Winckelmans, Gr\u00e9goire St\u00e9phane", "degree": "PhD", "year": "1989", "title": "Topics in Vortex Methods for the Computation of Three- and Two-Dimensional Incompressible Unsteady Flows", "advisor": "Leonard, Anthony", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-112216", "creators": [ { "name": { "family": "Winckelmans", "given": "Gr\u00e9goire St\u00e9phane" }, "id": "Winckelmans-Gr\u00e9goire-St\u00e9phane", "display_name": "Winckelmans, Gr\u00e9goire St\u00e9phane" } ], "advisors": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "advisor", "display_name": "Leonard, Anthony" } ], "committee": [ { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "chair", "display_name": "Leonard, Anthony" }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/19HD-DF80", "abstract": "Contributions to vortex methods for the computation of incompressible unsteady flows are presented. Three methods are investigated, both theoretically and numerically.
\r\n\r\nThe first method to be considered is the inviscid method of vortex filaments in three dimensions, and the following topics are presented: (a) review of the method of regularized vortex filaments and of convergence results for multiple-filament computations, (b) modeling of a vortex tube by a single filament convected with the regularized Biot-Savart velocity applied on the centerline: velocity of the thin filament vortex ring and dispersion relation of the rectilinear filament, and (c) development of a new regularization of the Biot-Savart law that reproduces the lowest mode dispersion relation of the rectilinear vortex tube in the range of large to medium wavelengths.
\r\n\r\nNext the method of vortex particles in three dimensions is investigated, and the following contributions are discussed: (a) review of the method of singular vortex particles: investigation of different evolution equations for the particle strength vector and weak solutions of the vorticity equation, (b) review of the method of regularized vortex particles and of convergence results, and introduction of a new algebraic smoothing with convergence properties as good as those of Gaussian smoothing, (c) development of a new viscous method in which viscous diffusion is taken into account by a scheme that redistributes the particle strength vectors, and application of the method to the computation of the fusion of two vortex rings at Re = 400, and (d) investigation of the particle method with respect to the conservation laws and derivation of new expressions for the evaluation of the quadratic diagnostics: energy, helicity and enstrophy.
\r\n\r\nThe third method considered is the method of contour dynamics in two dimensions. The particular efforts presented are (a) review of the classical inviscid method and development of a new regularized version of the method, (b) development of a new vector particle version of the method, both singular and regularized: the method of particles of vorticity gradient, (c) development of a viscous version of the method of regularized particles and application of the method to computation of the reconnection of two vortex patches of same sign vorticity, and (d) investigation of the particle method with respect to the conservation laws and derivation of new expressions for the evaluation of linear and quadratic diagnostics.
" }, { "name": "Dowling, David Russell", "degree": "PhD", "year": "1988", "title": "Mixing in Gas Phase Turbulent Jets", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06132005-160404", "creators": [ { "name": { "family": "Dowling", "given": "David Russell" }, "id": "Dowling-David-Russell", "display_name": "Dowling, David Russell" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9233-5476", "abstract": "This work is an experimental investigation of the mixing of the nozzle fluid of a round, turbulent jet with the entrained reservoir fluid, using laser-Rayleigh scattering methods. The measurements, at Reynolds numbers of 5,000 and 16,000, cover the axial range from 20 to 90 jet exit diameters and resolve the full range of temporal and spatial concentration scales. The measured mean and rms values of the concentration, and the mean scalar dissipation rate, when estimated from the time derivative of concentration, are consistent with jet similarity laws. Concentration fluctuation power spectra are found to be self-similar along rays emanating from the virtual origin of the jet, and are consistent with the universal form of scalar spectra proposed by Gibson (1968 II). The probability density functions for the concentration, the time derivative of concentration, and the square of the time derivative of concentration, are compiled and are also found to be self-similar along rays. Features of the measured distributions and spectra are consistent with the existence of large-scale structures within the flow that span the local diameter of the jet's turbulent cone. On the centerline of the jet, the scaled probability density function of jet gas concentration is found to be almost independent of the Reynolds number while the local mixing rate in the inner part of jet is not. The usual assumptions concerning isotropy and correlation of derivatives are found to lead to erroneous results for the probablility density function of the scalar dissipation rate.
" }, { "name": "Giezen, Jurgen Johannes", "degree": "PhD", "year": "1988", "title": "Plastic Buckling of Cylinders Under Biaxial Loading", "advisor": "Babcock, Charles D.; Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11212007-083506", "creators": [ { "name": { "family": "Giezen", "given": "Jurgen Johannes" }, "id": "Giezen-Jurgen-Johannes", "display_name": "Giezen, Jurgen Johannes" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "co-advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Beck", "given": "James L." }, "id": "Beck-J-L", "role": "member", "display_name": "Beck, James L." }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Singer", "given": "Josef" }, "id": "Singer-J", "role": "member", "display_name": "Singer, Josef" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ycv3-kp17", "abstract": "An experimental investigation is carried out to study the effects of nonproportional loading in the plastic range on a buckling load. The discrepancy between experimental and theoretical results points to some principal shortcoming in the analysis. The problem has been simplified by applying axial tensile load and external press to simple cylindrical shell specimen and observing the buckling load for various nonproportional load-paths. Results are compared to numerical predictions (BOSOR5) using classical type plasticity models such as J\u2082 deformation and J\u2082 incremental theory. Significant discrepancy was found an attributed to inadequate modeling of the nonlinear material behavior. The effects of geometrical imperfections and large deflections were found to insignificant, thereby leading to an idea how much of the discrepancy between test and theory is due to a use of inadequate plastic model. The introduction of the Southwell plot into elastic shell buckling problem reduced the already minor effects of geometric imperfections.
\r\n\r\nThe Christoffersen-Hutchinson corner theory model was introduced into BOSAR5 in its simplest form as presented by Poh-Sang Lam. Results obtained with this model, which allows corners to form on an initially smooth yield surface, displayed better agreement with experimental data. However, increased computational time and problems related to abrupt changes in load-path at the corner are a major concern at this present time.
\r\n" }, { "name": "Pugh, Jeffrey David", "degree": "PhD", "year": "1988", "title": "Finite Amplitude Waves in Plane Poiseuille Flow", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11072007-104253", "creators": [ { "name": { "family": "Pugh", "given": "Jeffrey David" }, "id": "Pugh-Jeffrey-David", "display_name": "Pugh, Jeffrey David" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "chair", "display_name": "Saffman, Philip G." }, { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "role": "member", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lorenz", "given": "Jens" }, "id": "Lorenz-Jens", "role": "member", "display_name": "Lorenz, Jens" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XHFQ-MJ23", "abstract": "Nonlinear behavior in plane Poiseuille flow has attracted theoretical interest over the last decade, both because of its tractability and because it is believed that some of the results may be applicable to phenomena occurring in the boundary layer. We have investigated the existence of three-dimensional finite amplitude waves in plane Poiseuille flow, in the hope of finding candidates for a class of simple flows which might provide insight into the nature of turbulence. These so-called vortical states would exist as attractors for the turbulent flow and mimic many of its properties.
\r\n\r\nOne of the requisite properties of these simple flows is existence at the low Reynolds numbers observed in experimental studies of transition to turbulence in plane Poiseuille flow. Although no such three-dimensional solutions were found in our study, a number of new insights have been made into the structure and stability of two- and three-dimensional steady wave solutions in plane Poiseuille flow. These in turn suggest new areas of investigation for finding vortical states.
" }, { "name": "Waas, Anthony Marius", "degree": "PhD", "year": "1988", "title": "Compression Failure of Fibrous Laminated Composites in the Presence of Stress Gradients: Experiment and Analysis", "advisor": "Babcock, Charles D.; Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11062003-092741", "creators": [ { "name": { "family": "Waas", "given": "Anthony Marius" }, "id": "Waas-Anthony-Marius", "orcid": "0000-0002-5258-2749", "display_name": "Waas, Anthony Marius" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "co-chair", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "co-chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "role": "member", "display_name": "Rosakis, Ares J." }, { "name": { "family": "Hall", "given": "John F." }, "id": "Hall-J-F", "role": "member", "display_name": "Hall, John F." }, { "name": { "family": "Lorenz", "given": "Jens" }, "id": "Lorenz-Jens", "role": "member", "display_name": "Lorenz, Jens" }, { "name": { "family": "Singer", "given": "Josef" }, "id": "Singer-J", "role": "member", "display_name": "Singer, Josef" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PP86-PE50", "abstract": "A series of experiments were performed to determine the mechanism of failure in compressively loaded laminated plates in the presence of stress gradients generated by a circular cutout. Real time holographic interferometry and in-situ photomicrography of the hole surface, were used to observe the progression of failure.
\r\n\r\nThe test specimens are multi-layered composite flat plates, which are loaded in compression. The plates are made of two material systems, T300/BP907 and IM7/8551-7. Two different lay-ups of T300/BP907 and four different lay-ups of IM7/8551-7 are investigated.
\r\n\r\nThe load on the specimen is slowly increased and a series of interferograms are produced during the load cycle. These interferograms are video-recorded. The results obtained from the interferograms and photo-micrographs are substantiated by sectioning studies and ultrasonic C-scanning of some specimens which are unloaded prior to catastrophic failure, but beyond failure initiation. This is made possible by the servo-controlled loading mechanism that regulates the load application and offers the flexibility of unloading a specimen at any given instance in the loadtime history.
\r\n\r\nAn underlying objective of the present investigation is the identification of the physics of the failure initiation process. This required testing specimens with different stacking sequences, for a fixed hole diameter, so that consistent trends in the failure process could be identified.
\r\n\r\nIt is revealed that the failure is initiated as a localized instability in the 0\u00b0 plies at the hole surface, approximately at right angles to the loading direction. This instability emanating at the hole edge and propagating into the interior of the specimen within the 0\u00b0 plies is found to be fiber microbuckling. The microbuckling is found to occur at a local strain level of \u2243 8600 \u00b5strain at the hole edge for the IM material system. This initial failure renders a narrow zone of fibers within the 0\u00b0 plies to loose structural integrity. Subsequent to the 0\u00b0-ply failure, extensive delamination cracking is observed with increasing load. The through thickness location of these delaminations is found to depend on the position of the 0\u00b0 plies.
\r\n\r\nThe delaminated portions spread to the undamaged areas of the laminate by a combination of delamination buckling and growth, the buckling further enhancing the growth. When the delaminated area reaches a critical size, about 75-100% of the hole radius in extent, an accelerated growth rate of the delaminated portions is observed. The culmination of this last event is the complete loss of flexural stiffness of each of the delaminated portions leading to catastrophic failure of the plate. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.
\r\n\r\nA simple mechanical model is presented for the microbuckling problem. This model addresses the buckling instability of a semi-infinte layered half-plane alternatingly stacked with fibers and matrix, loaded parallel to the surface of the half-plane. The fibers are modelled using Bernoulli-Navier beam theory, and the matrix is assumed to be a linearly elastic foundation. The predicted buckling strains are found to overestimate the experimental result. However, the dependence of the buckling strain on parameters such as the fiber volume fraction, ratio of Youngs moduli of the constituents and Poisson's ratio of the matrix are obtained from the analysis. It is seen that a high fiber volume fraction, increased matrix stiffness, and perfect bonding between fiber and matrix are desirable properties for increasing the compressive strength.
" }, { "name": "Arakeri, Jaywant H.", "degree": "PhD", "year": "1987", "title": "Structure Measurements in a Synthetic Turbulent Boundary Layer", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-03192008-085206", "creators": [ { "name": { "family": "Arakeri", "given": "Jaywant H." }, "id": "Arakeri-Jaywant-H", "display_name": "Arakeri, Jaywant H." } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Hall", "given": "John F." }, "id": "Hall-J-F", "role": "member", "display_name": "Hall, John F." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FABW-YZ77", "abstract": "Extensive hot-wire measurements have been made to determine the structure of the large eddy in a synthejc turbulent boundary layer on a flat-plate model. The experiments were carried out in a wind tunnel at a nominal free-stream velocity of 12 m/s. The synthetic turbulent boundary layer had a hexagonal pattern of eddies and a ratio of streamwise scale to spanwise scale of 3.2:1. The measured celerity of the large eddy was 84.2 percent of the free-stream velocity. There was some loss of coherence, but very little distortion, as the eddies moved downstream. Several mean properties of the synthetic boundary layer were found to agree quite well with the mean properties of a natural turbulent boundary layer at the same Reynolds number.
\r\n\r\nThe large eddy is composed of a pair of primary counter-rotating vortices about five \u03b4 long in the streamwise direction and about one \u03b4 apart in the spanwise direction, where \u03b4 is the mean boundary-layer thickness. The sense of the primary pair is such as to pump fluid away from the wall in the region between the vortices. A secondary pair of counter-rotating streamwise vortices, having a sense opposite to that of the primary pair, is observed outside of and slightly downstream from the primary vortices. Both pairs of vortices extend across the full thickness of the boundary layer and are inclined at a shallow angle to the surface of the flat plate. The data show that the mean vorticity vectors are not tangential to the large-eddy vortices. In fact, the streamwise and normal vorticity components that signal the presence of the eddy are of the same order of magnitude. Definite signatures are obtained in terms of the mean skin-friction coefficient and the mean wake parameter averaged at constant phase. Velocities induced by the vortices are partly responsible for entrainment of irrotational fluid, for transport of momentum, for generation of Reynolds stresses, and for maintenance of streamwise and normal vorticity in the outer flow. A stretching mechanism is important in matching spanwise vorticity close to the wall to variations in turbulent shearing stress. Regions where the stretching term is large coincide with regions of large wall shearing stress and large turbulence production.
" }, { "name": "Lacerda, Nehemias Lima", "degree": "PhD", "year": "1987", "title": "On the Start Up of Supersonic Underexpanded Jets", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-03042008-081340", "creators": [ { "name": { "family": "Lacerda", "given": "Nehemias Lima" }, "id": "Lacerda-Nehemias-Lima", "display_name": "Lacerda, Nehemias Lima" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Hall", "given": "John F." }, "id": "Hall-J-F", "role": "member", "display_name": "Hall, John F." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/W3CX-2Z48", "abstract": "An impulsively started jet can be formed by a gas confined in a high pressure reservoir that escapes suddenly through an exit orifice, into a controlled atmosphere. Supersonic gas jets of this type are unsteady and differ from the steady jet that develops later by the presence of a bow shock, a jet head and a nonstationary Mach disk. The effects of the pressure ratio between the high pressure gas inside the reservoir and the lower pressure atmospheric gas, as well as the gas combination used, are studied experimentally. The gases used for the jet and the atmosphere were selected from helium, nitrogen and sulfur hexafluoride.
\r\n\r\nThe data acquisition consisted of: high resolution flash photography to obtain detail from the pictures; high-speed movie pictures to obtain the time development of selected features; and fast-response pressure transducers located at the reservoir end plate, the tank end plate and the jet exit. The initial development of the jet is highly time dependent. During this phase, the shape that the jet assumes varies with pressure ratio and with the choice of gas. In particular an extremely light gas exhausting into a heavy atmosphere, exhibits an uncommon shape. It develops as a bubble wrapped by the bow shock, that increases its volume with flow time and pressure ratio. As the pressure ratio increases, it becomes more tightly wrapped by the bow shock. At later times the jet assumes conventional linear growth.
\r\n\r\nAfter the jet starts, a Mach disk is observed close to the jet exit which moves downstream as the exit pressure builds up. The monotonic increase in exit pressure is caused by the slow breaking of the diaphragm. The position of the Mach disk is furthest from the jet exit when the exit pressure is a maximum. After that it oscillates around the location predicted by the steady theory of Ashkenas and Sherman (1966) at a frequency close to one of the resonant frequencies of the reservoir. The features observed for the inner structure of the jet were verified to agree with those obtained for impulsive flow generated by a muzzle blast.
\r\n\r\nThe frontal part of the jet forms the jet head, whose shape changes with the flow conditions. The initial evolution of the jet head is linear but after propagating a distance of around ten exit diameters, it reaches asymptotic behavior with an evolution that is approximately proportional to square root of time. The head creates a bow shock ahead of it that propagates downstream and increases the pressure of the atmospheric gas. This bow shock was found to be less attenuated than in spherically symmetric explosions. The asymptotic behavior of the bow shock was reached after about eight exit diameters.
" }, { "name": "Lin, Jane Ming-Chin", "degree": "PhD", "year": "1987", "title": "Transient Gas Jets into Liquid", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-04262004-113723", "creators": [ { "name": { "family": "Lin", "given": "Jane Ming-Chin" }, "id": "Lin-Jane-Ming-Chin", "display_name": "Lin, Jane Ming-Chin" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4qp0-4x37", "abstract": "An experimental investigation of the development of high velocity, impulsively initiated gas jets into liquid was conducted in an effort to understand some of the physical processes that occur for a jet of very light fluid into a dense ambient atmosphere. Four gases, refrigerants 12 and 22, nitrogen, and helium were injected into water at nozzle exit Mach numbers from 1.0 to 2.2.
\r\n\r\nThe study showed that a gas jet into water develops in at least three stages: startup, transition, and global steady state. The startup is characterized by bubble growth; the growth rate is well predicted by classical bubble-growth theory. Jet transition is marked by axially directed flow, which penetrates through the startup bubble and which forms a cylindrical protrusion along the axis of symmetry. A combination of strong recirculating flow and liquid entrainment causes the startup bubble to deflate and to lift off and move downstream. In the steady state, instantaneous photographs show small-scale fluctuations of the jet boundary, but time-averaged photographs show the expected conical spreading of the steady jet; the measured spreading angles range from 18-25 degrees.
\r\n\r\nHowever, the most significant finding of this study is that under some conditions, the gas jet into liquid never reaches the global steady state. Instead, the jet boundary exhibits chugging: large nonlinear oscillations which lead to irregular collapses of the gas column followed by explosive outward bursts of gas. The unsteadiness observed is much more violent than the familiar fluctuations typical of constant-density jets. The length scale of the motion is generally on the order of several jet diameters; the time scale is on the order of the period for bubble collapse.
\r\n\r\nIt was found that the amplitude and frequency of chugging are strongly dependent on the ratio of the liquid density to the gas density, the jet Mach number, and the operating pressure ratio. The conditions under which unsteadiness occurs were determined experimentally. In particular, a quantitative measure of jet susceptibility to unsteadiness has been established. Steady jets can be achieved in two ways: by being discharged from deLaval nozzles (increasing the exit Mach number) or by being overpressured.
\r\n\r\nThe unsteady behavior is modeled as the collapse of a bubble in liquid; comparisons of collapse times show good agreement. A mechanism for the unsteadiness is discussed. It is proposed that the chugging is the response of the jet boundary to a pressure difference between the jet and surrounding liquid, which arises as the result of the rapid expansion of a light fluid into a dense ambient atmosphere. The flow is shown to be similar to the discharge of a gas from a nozzle into a channel of larger cross section. An upper limit to the pressure difference is determined based on estimates of the minimum base pressure for such channel flows; a lower limit is established for the collapse time. All experimental values are within the bounds. The derived values indicate that the pressure differences between the jet and liquid may be more than 90 percent of the ambient pressure.
" }, { "name": "Papamoschou, Dimitri", "degree": "PhD", "year": "1987", "title": "Experimental Investigation of Heterogeneous Compressible Shear Layers", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-12192007-085328", "creators": [ { "name": { "family": "Papamoschou", "given": "Dimitri" }, "id": "Papamoschou-Dimitri", "display_name": "Papamoschou, Dimitri" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Blandford", "given": "Roger D." }, "id": "Blandford-R-D", "role": "member", "display_name": "Blandford, Roger D." }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "member", "display_name": "Lees, Lester" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M7PQ-Y494", "abstract": "The compressible, two-dimensional shear layer is investigated experimentally in a novel facility. In this facility, it is possible to flow similar or, dissimilar gases of different densities and to select different Mach numbers for each stream over a wide range of Reynolds numbers. In the current experiments, ten combinations of gases and Mach numbers are studied in which the freestream Mach numbers range from 0.2 to 4, the density ratio varies from 0.2 to 9.2, and the velocity ratio varies from 0.13 to 1. The growth of the turbulent region of the layer is measured by means of pitot pressure profiles obtained at several streamwise locations. The resulting growth rate is estimated to be about 80% of the visual growth rate. The transition from laminar to turbulent flow, as well as the structure of the turbulent layer, are observed with Schlieren photographs of 20 nanosecond duration. Streamwise pressure distribution and total pressures are measured by means of a Scanivalve-pressure transducer system.
\r\n\r\nAn underlying objective of this investigation was the definition of a compressibility-effect parameter that correlates and consolidates the experimental results, especially the turbulent growth rates. A brief analytical investigation of the vortex sheet suggests that such a parameter is the Mach number in a frame of reference moving with the phase speed of the disturbance, called here the convective Mach number. In a similar manner, the convective Mach number of a turbulent shear layer is defined as the one seen by an observer moving with the convective velocity of the dominant waves and structures. It happens to have about the same value for each stream. In the current experiments, it ranges from 0 to 1.9.
\r\n\r\nThe correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases gradually with increasing convective Mach number, reaching an asymptotic value of about 0.25 for supersonic convective Mach numbers. The above behavior is in qualitative agreement with results of linear stability theory as well as with those of previous, one-stream experiments.
\r\n\r\nLarge-scale structures, resembling those observed in subsonic shear layers, are evident in the Schlieren photographs. It is estimated that the mean structure spacing, normalized by the local thickness, is reduced to about half its incompressible value as the convective Mach number becomes supersonic.
\r\n\r\nAn estimate of the transition Reynolds number has been obtained from the photographs of two shear layers having quite different convective Mach numbers, one low subsonic and the other sonic. In both cases, it is about 2 x 105, based on distance to transition and properties of the high unit Reynolds number stream, thus suggesting that, in this experiment, transition is dominated by instabilities of the wake, rather than of the shear layer.
\r\n" }, { "name": "Schatzle, Paul Russell", "degree": "PhD", "year": "1987", "title": "An Experimental Study of Fusion of Vortex Rings", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-04232004-092555", "creators": [ { "name": { "family": "Schatzle", "given": "Paul Russell" }, "id": "Schatzle-Paul-Russell", "display_name": "Schatzle, Paul Russell" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "role": "member", "display_name": "Caughey, Thomas Kirk" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KK00-ZJ41", "abstract": "A two-component laser-Doppler velocimeter with frequency bias is used to measure the velocity field produced by the oblique collision of two laminar vortex rings. The Reynolds number, defined here as the ratio of ring circulation to kinematic viscosity, is about 1800. The rings are generated underwater and approach each other along intersecting paths. The plane defined by the two paths is a plane of symmetry, and is the plane in which the measurements were done. The out-of-plane components of vorticity and strain rate are computed from finite differences of the velocity data. The state of the rings prior to the collision is determined in order to provide a set of starting conditions for future numerical simulations. In addition, the circulation of each vortex core is computed as a function of time during the collision.
\r\n\r\nThe principal result of this work is that the circulation of each vortex core is observed to decrease during the interaction; furthermore, the fluid viscosity is shown to be responsible for this decrease. The exact mechanism by which the reduction in circulation is effected is unclear, but it appears to be related to the out-of-plane stretching. A time scale is proposed for this cancellation process which combines the effects of viscosity and stretching.
\r\n\r\nThe measurements also show that the structure of the vorticity field changes during the collision. In particular, each vortex core is observed to split into two vortices under the action of the local in-plane strain field.
" }, { "name": "Papantoniou, Dimitris Antony", "degree": "PhD", "year": "1986", "title": "Observations in Turbulent Buoyant Jets by Use of Laser-Induced Fluorescence", "advisor": "List, E. John", "url": "https://resolver.caltech.edu/CaltechETD:etd-03142008-142149", "creators": [ { "name": { "family": "Papantoniou", "given": "Dimitris Antony" }, "id": "Papantoniou-Dimitris-Antony", "display_name": "Papantoniou, Dimitris Antony" } ], "advisors": [ { "name": { "family": "List", "given": "E. John" }, "id": "List-E-J", "role": "advisor", "display_name": "List, E. John" } ], "committee": [ { "name": { "family": "List", "given": "E. John" }, "id": "List-E-J", "role": "chair", "display_name": "List, E. John" }, { "name": { "family": "Sabersky", "given": "Rolf H." }, "id": "Sabersky-R-H", "role": "member", "display_name": "Sabersky, Rolf H." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Psaltis", "given": "Demetri" }, "id": "Psaltis-D", "orcid": "0000-0003-4684-8800", "role": "member", "display_name": "Psaltis, Demetri" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." } ], "option_major": [ "eng" ], "doi": "10.7907/SX4D-3555", "abstract": "The entrainment mechanism and mixing process are investigated in the far field of a liquid phase buoyant jet issuing into an unconfined, quiescent medium, by an experimental technique based on laser-induced fluorescence (LIF). Visualization experiments show the existence of a large scale organization in the far field, with structures spanning the radial extent of the conical flow region. Quantitative, high-resolution measurements of scalar concentration were performed along the radial direction in the far field region. For each data set, a large number of successive instantaneous concentration profiles were obtained by combining LIF techniques with linear photodiode array imaging and high speed data acquisition. The measurements revealed that the instantaneous profile bears no resemblance to the time-averaged profile. The flow interior is characterized by large spatial gradients of concentration, associated with interfaces between mixed jet fluid and fresh, entrained ambient fluid transported to regions deep into the flow. This is inconsistent with the description of transport by gradient diffusion concepts. The probability of finding unmixed ambient fluid and the concentration variance are greatly increased under the action of buoyancy. At any axial location, the arrival of a structure front is marked by a spatially coherent (along the radial direction) increase in the local concentration level. It is found that, within the structure, values of the concentration are generally decreasing in the upstream direction; substantial uniformity within the mixed fluid portion is observed along the radial direction. In the conical flow field of the momentum jet, a central region (in fixed spatial coordinates) may be identified within which the local mixed fluid composition is relatively uniform. This is not the case for the buoyancy driven plume, due to a greater variance in the position of the large structure and the high value of the intermittency. It is suggested that fluid is entrained by vortical motions mainly from the back and side regions of the large structure. Flow visualization reveals vorticity in the axial direction which enhances the mixing process; this vorticity appears stronger in the buoyancy driven flow. The results of these experiments are interpreted through a simple conceptual model of entrainment and mixing that encompasses the observed large scale organization of the buoyant jet flow.
" }, { "name": "Robey, Harry Francis, III", "degree": "PhD", "year": "1986", "title": "The Nature of Oblique Instability Waves in Boundary Layer Transition", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-05242007-150746", "creators": [ { "name": { "family": "Robey", "given": "Harry Francis, III" }, "id": "Robey-Harry-Francis-III", "display_name": "Robey, Harry Francis, III" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "chair", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7VG8-Y513", "abstract": "An experimental study of both the weakly non-linear as well as the three-dimensional nature of boundary layer transition is conducted using the active surface heating technique of Liepmann et al. In the present study, this technique is extended to provide a means for controllably and repeatably introducing three-dimensional disturbances into a laminar boundary layer. A review of the surface heating technique is presented along with a discussion of some peculiarities encountered in extending this technique to three-dimensional geometries. A thorough description of the design and operation of a programmable 32-element heater array and the supporting instrumentation are given as well.
\r\n\r\nThe heater array is first used to study the effect of weak nonlinearity on boundary layer transition. By keeping the forced disturbances as two-dimensional as possible, it is shown that the effects of weak non-linearity are relatively benign. The growth rates are seen to follow the linear theory up to perturbation amplitudes (\u03c4'w\u03c4̅w) of nearly twelve percent. The only deviation from the linear theory arises in the form of non-linearly generated harmonics phase-locked to the fundamental. It is concluded that although these non-linearly generated harmonics do alter the wave behavior to some extent, they are by themselves not sufficient to explain the transition from small linear oscillations to the large amplitude, broad-band, three-dimensional oscillations characteristic of a fully turbulent boundary layer.
\r\n\r\nThe effect of three-dimensionality on boundary layer transition is then investigated through an analytical and experimental study of single oblique instability waves. This subject has remained largely unexplored, as such disturbances were generally thought to be more stable and therefore less dangerous than their two-dimensional counterparts. Through a series of experiments, however, it is shown that certain conditions exist for which oblique waves are observed to be more unstable than any two-dimensional wave. It is shown that oblique waves exhibit a non-stationary period-doubling behavior that is not seen in two-dimensional disturbances. A vortex pairing mechanism is proposed to explain this behavior, and is shown to occur in a manner consistent with the Biot-Savart law for the induced velocity field.
" }, { "name": "Sigurdson, Lorenz Willard", "degree": "PhD", "year": "1986", "title": "The Structure and Control of a Turbulent Reattaching Flow", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-03102008-104655", "creators": [ { "name": { "family": "Sigurdson", "given": "Lorenz Willard" }, "id": "Sigurdson-Lorenz-Willard", "display_name": "Sigurdson, Lorenz Willard" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Leonard", "given": "Anthony" }, "id": "Leonard-A", "role": "member", "display_name": "Leonard, Anthony" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/prfm-x404", "abstract": "An unsteady and three-dimensional large-scale structure is proposed for the reattachment region of a separation bubble, based on a visualization study of the flow over a plate with a square leading edge and its axisymmetric counterpart, a flat-faced circular cylinder aligned coaxially with the free-stream. The initial free shear layer structures are primarily two-dimensional but evolve into boundary layer type structures as they near reattachment and interact with the wall. Some segments form \"loops\" which convect away from the wall and downstream, while spanwise adjacent parts convect toward the wall and upstream. The loops are sometimes clearly arranged in a staggered pattern. Their legs form a series of counter-rotating streamwise vortex pairs which bridge the reattachment zone. These observations reconcile apparently contradictory propositions concerning the fate of the structures as they encounter reattachment. The interaction between successive vortices at alternating spanwise locations is fundamental to several flows. The structure of turbulent wakes is also discussed.
\r\n\r\nAn experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp- edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge.
\r\n\r\nIn both forced and unforced cases it was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the \"shedding\" type instability of the entire bubble. The latter results from an interaction with the image vortices due to the presence of the wall. A method of frequency scaling is proposed that correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.
\r\n\r\nNew methods for approximating axisymmetric flows are presented. Transition of shear-layers and separation bubbles is also discussed.
" }, { "name": "Taylor, Stephen", "degree": "PhD", "year": "1986", "title": "The Effects of Large-Eddy Manipulator Devices on the Turbulent Spot and the Turbulent Boundary Layer", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03102008-083411", "creators": [ { "name": { "family": "Taylor", "given": "Stephen" }, "id": "Taylor-Stephen", "orcid": "0000-0003-0264-1453", "display_name": "Taylor, Stephen" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7520-qj30", "abstract": "Recent experimental studies indicate that net drag reductions can be achieved in a turbulent boundary layer by placing a tandem configuration of large-eddy manipulator blades in the outer region of the boundary layer. However, the mechanisms responsible for the observed wall-shear reductions are not well understood. Furthermore, discrepancies exist among independent experimental studies regarding the magnitude of the attainable net drag reduction.
\r\n\r\nA fundamental argument is made regarding the source of the observed wall-shear reductions. It is shown that the tandem manipulator is not a low-drag device. The implication is that the momentum deficit in the wake of the manipulator is a prominent contributor to the observed wall-shear reductions, not necessarily that the ability of the large eddies to transport momentum is hampered. The behavior of the wall shear downstream of the device, obtained using hot-film wall-shear sensors, is consistent with the entrainment, mixing, and consequent deceleration of low-momentum fluid from the wake of the manipulator.
\r\n\r\nWith the aid of direct measurements of wall shear, an upper bound is placed on the attainable net drag reduction by establishing a lower bound for the device drag. It is concluded that small net reductions (~ 5 percent) may be attained at large downstream distances (\u2273 100 boundary-layer thicknesses). This conclusion is consistent with most net drag assessments made independently by others in which the momentum-balance technique was employed. However, the result is not consistent with reports of large net reductions (~ 20 percent) over shorter distances (~ 50 boundary-layer thicknesses).
\r\n\r\nEfforts are also made to explain the observed effects in terms of turbulent structure. The turbulent spot is employed as a prototype structure for the large-scale, organized motions in the turbulent boundary layer. Dramatic wall-shear reductions occur in the region of the spot occupied by the large vortex structure. Such reductions are also evident when the spot is propagating in transitional and fully turbulent ambient boundary layers. Although the transport properties of the vortex structure may be affected by the manipulator, it is proposed that an important source of the wall-shear reductions is the transport by the large vortex of low-momentum fluid in the wake of the manipulator.
\r\n\r\nSome effects of a three-dimensional manipulator are also explored. The design of the device is based on a crude model of the three-dimensional structure of the turbulent spot. Although it appears that the device inhibits the spanwise growth of the spot, its overall effect on the wall shear of the spot and the turbulent boundary layer does not represent an improvement over the effectiveness of a comparable tandem configuration.
" }, { "name": "Aaron, Kim Maynard", "degree": "PhD", "year": "1985", "title": "Edgetones and Acoustic Resonances in a Duct", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01222007-092606", "creators": [ { "name": { "family": "Aaron", "given": "Kim Maynard" }, "id": "Aaron-Kim-Maynard", "display_name": "Aaron, Kim Maynard" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "chair", "display_name": "Kubota, Toshi" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Acosta", "given": "Allan J." }, "id": "Acosta-A-J", "role": "member", "display_name": "Acosta, Allan J." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/bfy8-1995", "abstract": "Undesirable sound generation in the combustion chamber of segmented solid propellant rocket motors has been attributed to vortex shedding from obstructions that are uncovered as the propellant burns back. This phenomenon has been investigated experimentally and the mechanism explained.
\r\n\r\nA pair of aluminum baffles within a lucite duct through which air is drawn models the important aspects which enable the sound generation mechanism to operate. The baffles form an edgetone system which interacts with the longitudinal acoustic modes of the chamber. Acoustic tones occur spontaneously, at frequencies determined by the acoustic resonances, when the spacing between the baffles satisfies certain criteria.
\r\n\r\nFlow visualization using smoke and a strobe light triggered by the pressure oscillations indicate that vortex shedding occurs at the first baffle in phase with the acoustic velocity oscillations there. The interaction of these vortices with the downstream baffle drives the acoustic resonance which, in turn, triggers the formation of new vortices at the upstream separation point.
\r\n\r\nThe phase relations for this feedback to operate require that there be close to an integral number of wavelengths, or vortices, from the separation point to the impingement point.
\r\n\r\nA model has been developed which predicts the experimentally observed behaviour well. Pressure amplitudes are predicted within an order of magnitude. Mean flow rates and baffle spacings yielding maximum response are determined correctly by the model.
" }, { "name": "Dahm, Werner Johann Anton", "degree": "PhD", "year": "1985", "title": "Experiments on Entrainment, Mixing and Chemical Reactions in Turbulent Jets at Large Schmidt Number", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12062006-104125", "creators": [ { "name": { "family": "Dahm", "given": "Werner Johann Anton" }, "id": "Dahm-Werner-Johann-Anton", "orcid": "0000-0002-0779-3280", "display_name": "Dahm, Werner Johann Anton" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/k93g-jh12", "abstract": "Entrainment, mixing and chemical reactions are investigated in the far field of steady, axisymmetric, momentum-driven, turbulent jets issuing into an unconfined, quiescent medium in the large Schmidt number (liquid-phase) regime. Visualization experiments using both passive and chemically sensitive planar laser induced fluorescence (LIF) techniques show the importance of large scale transport in the jet far field, and suggest that entrainment, mixing and chemical reactions in the far field are dominated by a large scale organization of the flow. Successive instantaneous profiles of the jet fluid concentration along the axial and radial directions in the jet far field are measured by combining these LIF techniques with direct, high-resolution, linear photodiode array imaging and high-speed digital data acquisition. These imaging measurements have revealed an axial similarity concentration variable for which probability density functions (PDFs) in the jet far field are self-similar along rays. A chemical reaction method is presented which allows the self-similar form of these PDFs to be measured with full resolution at all scales of transport and mixing. Furthermore, these imaging measurements have shown that instantaneous radial profiles of the jet fluid concentration do not resemble the mean concentration profile. Specifically, unmixed ambient fluid is found deep within the jet and the composition of molecularly mixed fluid within large regions in the jet is approximately uniform. The results from these experiments are interpreted in the context of a simple conceptual model for large scale organization of entrainment, mixing and chemical reactions in the far field of turbulent jets.
" }, { "name": "Elyada, Dov", "degree": "PhD", "year": "1985", "title": "Structural Analysis of Imperfect Three-Legged Truss Columns for Large Space Structures Applications", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11182004-161353", "creators": [ { "name": { "family": "Elyada", "given": "Dov" }, "id": "Elyada-Dov", "display_name": "Elyada, Dov" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "chair", "display_name": "Babcock, Charles D." }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Rosakis", "given": "Ares J." }, "id": "Rosakis-A-J", "orcid": "0000-0003-0559-0794", "role": "member", "display_name": "Rosakis, Ares J." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YXNE-YE65", "abstract": "Three-legged truss columns are basic structural components of many envisioned large outer-space structures. They constitute three longerons ('legs') forming, in the column cross-section, the vertices of an equiliateral triangle. Their longerons are held together by uniformly spaced battens while a shear web, usually made of diagonals, restrains shear deformation.
\r\n\r\nThis work deals with configurations characterized by having relatively stiff battens, longerons which are pinned to the battens and prestressed string diagonals. Considered are only simply-supported slender columns having slender longeron segments and relatively thin and lightly preloaded diagonals. The columns are allowed to have global (overall) as well as local (longeron segment) geometrical imperfections - not necessarily small ones.
\r\n\r\nInvestigated is the static structural behavior of such columns when loaded by purely axial compressive concentrated forces acting at the supports. Addressed are the topics of global and local buckling, post-buckling, imperfection sensitivity, global-local mode interaction, complete non-linear response, limit loads and diagonals slackening and post-slackening.
\r\n\r\nThe approach is a theoretical one; a system of non-linear, ordinary differential equations is set up which represents the column, and results, mostly in closed form, are obtained by solving that system for a variety of cases of varying generality.
\r\n\r\nFirst, a highly idealized case is studied in detail, in which the diagonals are removed and infinite shear rigidity is postulated instead. The results exhibit most of the essential features of the more complicated cases. Next, the case of the undeflected or only-slightly deflected column is considered. Results include the prebuckling behavior, slackening and local buckling loads, global buckling load, initial post-buckling behavior and imperfection sensitivities. Diagonals slackening in a deflecting column is studied next. This is done by means of slackening loci constructed in the load-deflection plane. Solutions are obtained for some special cases of a deflecting column. These include a complete analysis of the locally-perfect case and the cases of small load and high defection. Also obtained is an engineering-oriented load-deflection working relation valid for the most general case but based on generalization rather than on rigorous solution. A torsion-compression mode, dominant in post-slackening, is also analyzed. The work is concluded by investigating the error committed in treating continuous longerons as if they were discontinuous-pinned.
\r\n" }, { "name": "Frost, David Lawrence", "degree": "PhD", "year": "1985", "title": "Effects of Ambient Pressure on the Instability of a Liquid Boiling Explosively at the Superheat Limit", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-04102008-081638", "creators": [ { "name": { "family": "Frost", "given": "David Lawrence" }, "id": "Frost-David-Lawrence", "orcid": "0000-0002-3579-6091", "display_name": "Frost, David Lawrence" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "member", "display_name": "Brennen, Christopher E." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8J3K-PS54", "abstract": "The effect of ambient pressure on the dynamical behaviour of a single droplet (1-2 mm diameter) of volatile liquid boiling explosively at the limit of superheat is studied experimentally and theoretically. In a series of experiments it is shown that the evaporative instability, observed earlier by Shepherd & Sturtevant (1982) during the rapid vapourization of butane droplets at atmospheric pressure, is suppressed at high pressure. Three other fluids (pentane, isopentane, and ether) are tested to establish the generality of the instability and other transient processes previously observed. Direct evidence is obtained showing that during violently unstable boiling small liquid particles are torn from the liquid-vapour interface. This ejection of fine droplets from the evaporating surface produces a mass flux orders of magnitude greater than that characteristic of ordinary boiling.
\r\n\r\nRaising the ambient pressure lowers the superheat attained at the superheat limit, which decreases the vapourization rate. At high pressure boiling consists of normal slow vapourization from a smooth interface. Observed bubble growth rates show reasonable agreement with theory. At intermediate pressures a transitional regime of stability occurs in which a drop initially vapourizes stably for several milliseconds while incipient instability waves develop on the evaporating interface. When only a small amount of liquid remains in the drop in the shape of a thin cap, heat transfer from the surrounding hot host fluid initiates violent boiling at the edge of the liquid cap. The subsequent rapid vapourization generates a radiated pressure field two orders of magnitude larger than during stable boiling, and sets the bubble into violent oscillation. The bubble is subject to the Rayleigh-Taylor instability and rapidly disintegrates into a cloud of small bubbles.
\r\n\r\nLowering the ambient pressure decreases the time delay between nucleation and onset of unstable boiling. For example, in ether at atmospheric pressure the instability is triggered less than 8 \u00b5sec after nucleation, shortly after the smooth vapour bubble contacts the droplet surface. Heterogeneous nucleation spreads out along the surface of the drop while disturbances (with a length scale of 100 \u00b5m) distort the unstably evaporating interface within the drop, substantially enhancing the vapourization rate. At early times, droplets torn from the evaporating surface evaporate before the instability-driven jet impinges upon the surrounding fluid, bulging the bubble surface. The last portion of liquid in a drop boils particularly violently and droplets ejected from the evaporating interface at this time remain intact to splatter the bubble surface. At subatmospheric pressures the most rapid vapourization occurs and temperature gradients within a drop produce spatial variations in vapourization rate.
\r\n\r\nThe Landau mechanism for the instability of laminar flames is adapted to the case of evaporation to investigate the effects of variable ambient pressure. A spherical version of the theory, applicable before the vapour bubble contacts the droplet surface, predicts absolute stability at atmospheric pressure. At later times the spherical constraint is inappropriate and planar theory yields results in general agreement with observation. Differences in fluid properties make some fluids more prone to instability than others. The product of the maximum growth rate with the time interval the interface is predicted to be linearly unstable measures the susceptibility to instability. For practical estimates it is suggested that a value of 3 of this parameter be taken as the lower limit for instability. The sensitivity of the instability to temperature suggests that small temperature nonuniformities may be responsible for quantitative departures of the behaviour from predictions.
\r\n" }, { "name": "Hermanson, James Carl", "degree": "PhD", "year": "1985", "title": "Heat Release Effects in a Turbulent, Reacting Shear Layer", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06132007-075717", "creators": [ { "name": { "family": "Hermanson", "given": "James Carl" }, "id": "Hermanson-James-Carl", "display_name": "Hermanson, James Carl" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "chair", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/y722-za34", "abstract": "The effects of heat release were studied in a planar, gaseous reacting mixing layer formed between free streams containing hydrogen and fluorine in inert diluents. Sufficiently high concentrations of reactants were employed to produce adiabatic flame temperature rises of up to 940 K (1240 K absolute). The Reynolds number at the measuring station, based on velocity difference, 1% temperature thickness and cold kinematic viscosity was approximately 6x104. The temperature field was measured with cold wire resistance thermometers and thermocouples. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from mean pitot probe dynamic pressure measurements.
\r\n\r\nThough the displacement thickness of the layer, for zero streamwise pressure gradient, increased with increasing heat release, the actual growth rate of the layer did not increase, but instead decreased slightly. The overall entrainment into the layer was seen to be substantially reduced as a consequence of heat release. Calculations showed that the decrease in layer growth rate can be accounted for by a corresponding reduction in turbulent shear stress.
\r\n\r\nThe mean temperature rise profiles, normalized by the adiabatic flame temperature rise, were not greatly changed in shape by heat release. A small decrease in normalized mean temperature rise with heat release was observed. Large scale coherent structures were observed to persist at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease exceeded the rate of layer growth rate reduction, and suggests that the mechanisms of vortex amalgamation were, to some extent, inhibited by heat release.
\r\n\r\nImposition of a favorable pressure gradient resulted in additional thinning of the layer, and caused a slight increase in the mixing and amount of chemical product formation. The change in layer growth rate can be shown to be related to a change in free stream velocity ratio induced by pressure gradient.
" }, { "name": "Roberts, Fredrick Allen", "degree": "PhD", "year": "1985", "title": "Effects of a Periodic Disturbance on Structure and Mixing in Turbulent Shear Layers and Wakes", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-03252008-144801", "creators": [ { "name": { "family": "Roberts", "given": "Fredrick Allen" }, "id": "Roberts-Fredrick-Allen", "display_name": "Roberts, Fredrick Allen" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "chair", "display_name": "Broadwell, James E." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/syy5-a334", "abstract": "Large scale structure and mixing processes are investigated in chemically reacting wakes and shear layers to which a periodic disturbance is applied. The experiments employ a diffusion-limited acid-base reaction to directly measure the extent of mixing. Optical diagnostics used include laser absorption and laser induced fluorescence. Absorption of laser light by reacted product provides a measure of cross-stream average product. Fluorescence was measured by a self-scanning linear photodiode array using high speed computer data acquisition to obtain the product distribution across the layer.
\r\n\r\nPrevious results showing that forcing alters the structure and growth rate of shear layers are confirmed. Forcing artificially extends the lifetime of vortices whose size is consistent with the disturbance wavelength. Amalgamation of smaller vortices is enhanced over that in the natural layer until the frequency locked scale is achieved. At high Reynolds number product measurements show reduction of product with forcing. At moderate Reynolds numbers, on the other hand, there is an increase in product when forced. In one case a five fold increase in product was observed. The differences are related to the different effects of forcing on entrainment, composition ratio and secondary structure.
\r\n\r\nA dramatic, order of magnitude increase in mixing was discovered for certain forced wake flows. This effect is strongly associated with an interaction between the spanwise organized wake vortices and the test-section side walls.
\r\n" }, { "name": "R\u00f6sgen, Thomas", "degree": "PhD", "year": "1985", "title": "Second Sound Scattering in Superfluid Helium", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-161340", "creators": [ { "name": { "family": "R\u00f6sgen", "given": "Thomas" }, "id": "R\u00f6sgen-Thomas", "display_name": "R\u00f6sgen, Thomas" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/r8ge-j070", "abstract": "Focusing cavities are used to study the scattering of second sound in liquid helium II. The special geometries reduce wall interference effects and allow measurements in very small test volumes.
\r\n\r\nIn a first experiment, a double elliptical cavity is used to focus a second sound wave onto a small wire target. A thin film bolometer measures the side scattered wave component. The agreement with a theoretical estimate is reasonable, although some problems arise from the small measurement volume and associated alignment requirements.
\r\n\r\nA second cavity is based on confocal parabolas, thus enabling the use of large planar sensors. A cylindrical heater produces again a focused second sound wave. Three sensors monitor the transmitted wave component as well as the side scatter in two different directions. The side looking sensors have very high sensitivities due to their large size and resistance. Specially developed cryogenic amplifiers are used to match them to the signal cables.
\r\n\r\nIn one case, a second auxiliary heater is used to set up a strong counterflow in the focal region. The second sound wave then scatters from the induced fluid disturbances.
\r\n\r\nAttempts to observe scattering from quantized vortex lines in the rotating parabolic cavity ultimately did not succeed, although a theoretical estimate seems to indicate a basic feasibility.
" }, { "name": "Cimbala, John Michael", "degree": "PhD", "year": "1984", "title": "Large Structure in the Far Wakes of Two-Dimensional Bluff Bodies", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-09132005-131650", "creators": [ { "name": { "family": "Cimbala", "given": "John Michael" }, "id": "Cimbala-John-Michael", "orcid": "0000-0003-3210-6347", "display_name": "Cimbala, John Michael" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." }, { "name": { "family": "Raichlen", "given": "Fredric" }, "id": "Raichlen-F", "role": "member", "display_name": "Raichlen, Fredric" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KZ05-YC02", "abstract": "Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Karman vortex street frequency is observed. Beyond this region of decay, larger-scale (lower wave-number) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the center of the band shifting to lower frequencies as downstream distance is increased.
\r\n\r\nThe far-wake structure does not depend directly on the scale or frequency of the original Karman vortices; the growth of this structure is due to hydro-dynamic instability of the developing mean wake profile; it is not caused by amalgamation of the Karman vortices. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re \u2248 6000), which do not initially shed Karman-type vortices into the wake.
\r\n\r\nHot-wire measurements show that two-dimensional locally-parallel inviscid linear stability theory is adequate to explain the growth of downstream structure. Namely, measured prominent frequencies in the cylinder wake are in close agreement with those predicted by the theory, when streamwise growth of wake width is taken into account.
\r\n\r\nFinally, three-dimensionality in the far wake of a circular cylinder is briefly discussed.
" }, { "name": "Haas, Jean-Fran\u00e7ois Luc", "degree": "PhD", "year": "1984", "title": "Interaction of Weak Shock Waves and Discrete Gas Inhomogeneities", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-06232005-110318", "creators": [ { "name": { "family": "Haas", "given": "Jean-Fran\u00e7ois Luc" }, "id": "Haas-Jean-Fran\u00e7ois-Luc", "orcid": "0000-0002-7730-5107", "display_name": "Haas, Jean-Fran\u00e7ois Luc" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "chair", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T37C-X215", "abstract": "An experimental investigation of the interaction of shock waves with discrete gas inhomogeneities is conducted in the GALCIT 15 cm diameter shock tube. The gas volumes are cylindrical refraction cells of 5 cm diameter with a 0.5 \u00b5m thick membrane separating the test gas (helium or Freon 22) from the ambient air and large spherical soap bubbles containing the same gases. The incident wave Mach numbers are nominally 1.09 and 1.22. The wave pattern and the deformation of the gas volumes are documented by shadowgraphs. The transmitted and diffracted wave pressure profiles are recorded by pressure transducers at various distances behind the cylinders. The basic phenomena of acoustic wave refraction, reflection and diffraction by cylindrical acoustic lenses, with indices of refraction appropriate to the gases used in the experiments, are illustrated with computer-generated ray and wave-front diagrams.
\r\n\r\nIn the case of a Freon 22-filled cylinder, the wave diffracted externally around the body precedes the wave transmitted from the interior which goes through a focus just behind the cylinder, while in the case of the helium-filled cylinder the expanding transmitted wave runs ahead of the diffracted wave. Both sets of waves merge a few cylinder diameters downstream. The wave patterns inside the cylinder, showing initially the refracted waves and later the same waves reflected internally, present some interesting phenomena.
\r\n\r\nThe mechanisms by which the gas volumes are transformed into vertical structures by the shock motion are observed. The unique effect of shock acceleration and Rayleigh-Taylor instability on the spherical volume of helium leads to the formation of a strong vortex ring which rapidly separates from the main volume of helium. Measurements of the wave and gas-interface velocities are compared to values calculated for one-dimensional interactions and for a simple model of shock-induced Taylor instability. The behavior of thin liquid membranes accelerated by shocks under varying conditions is documented by high speed photography.
\r\n\r\nIn a related experiment, shock waves of Mach number between 1.005 and 1.36 interact with a dense random array of 2 mm diameter helium filled soap bubbles. Experimental results (based on shadowgraphs and pressure measurements) show that very weak shock waves (Ms \u2264 1.01) are strongly scattered by the array, which is left undisturbed by the shock, and that stronger shock waves, only locally disturbed by each bubble, maintain undisturbed pressure profiles because of nonlinear effects, while the array undergoes shock-induced mixing. A simple criterion for multiple scattering shows that the combined effect of many bubbles is necessary in order to produce important modifications on the shock wave pressure profile.
" }, { "name": "Koochesfahani, Manoochehr Mohseni", "degree": "PhD", "year": "1984", "title": "Experiments on Turbulent Mixing and Chemical Reactions in a Liquid Mixing Layer", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12132006-131143", "creators": [ { "name": { "family": "Koochesfahani", "given": "Manoochehr Mohseni" }, "id": "Koochesfahani-Manoochehr-Mohseni", "orcid": "0000-0002-7001-8455", "display_name": "Koochesfahani, Manoochehr Mohseni" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Y7BR-C556", "abstract": "The processes of entrainment and mixing are investigated in reacting and non-reacting, uniform density, liquid mixing layers over a wide range of Reynolds numbers. In non-reacting cases, a passive scalar technique is used to measure the probability density function (pdf) of the composition field. Chemically reacting experiments employ a diffusion-limited acid-base reaction to directly measure the extent of mixing. The diagnostics are based entirely on the laser induced fluorescence technique. The fluorescence signal is measured by self-scanning linear photodiode arrays using high speed, real-time computer data acquisition. The system is capable of yielding species concentration data with a spatial resolution of 100 \u00b5m and a temporal resolution of 0.8 msec.
\r\n\r\nResults show that the vertical structures in the mixing layer initially roll up with a large excess of high speed fluid in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but the composition also changes. It is found that the pdf of the mixed fluid, above the mixing transition, is quite uniform across the entire transverse extent of the layer. Furthermore, it is asymmetric and biased toward the high speed fluid. Experimental evidence indicates that the turbulent transport, in the cases studied, is dominated by large scale structures and is not adequately described by standard gradient-diffusion models. The fluid composition in the mixing layer, suggested by the present results, is in qualitative agreement with many aspects of the recent theoretical model of Broadwell and Breidenthal. The amount of product formed in the layer is compared to Mungal's measurements in gas, and, it is observed that the liquid layer has about 50% less product. The mean concentration of the mixed fluid, for a mixing layer at a velocity ratio of 0.38, becomes constant at 0.57 above the mixing transition. This corresponds to an entrainment ratio of 1.32, in agreement with the gaseous result of Konrad at the same velocity ratio.
" }, { "name": "Wang, Chiun", "degree": "PhD", "year": "1984", "title": "The Effects of Curvature on Turbulent Mixing Layers", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-01222007-142548", "creators": [ { "name": { "family": "Wang", "given": "Chiun" }, "id": "Wang-Chiun", "display_name": "Wang, Chiun" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BC48-BE98", "abstract": "Experimental studies have been conducted in a curved mixing layer in which both the velocity ratio and the density ratio were variable. Flow visualization studies and profile measurements covered a wide range of experimental conditions. The structures observed experimentally were examined in the light of three different instability mechanisms which can exist in the same mean flow.
\r\n\r\nFor the case of mixing layers with uniform density, it was found that the normal large spanwise vortex structures can be weakened or inhibited by Taylor-Gortler instability if the inner stream is faster than the outer stream. For the case of mixing layers with different densities, three-dimensionality is greatly enhanced by Rayleigh-Taylor instability if the inner stream is heavier than the outer stream, and especially if the inner stream is also faster. In the former case the growth rate of the mixing layer was found to be insensitive to changes in the velocity ratio.
\r\n\r\nThe effects of curvature on the structure of the curved turbulent mixing layer were explored in terms of length scales and celerity for the large spanwise structures where these structures could be observed. Other things being equal, the celerity of the large structures was found to depend on density ratio and velocity ratio but not on the sense of the mean streamline curvature.
" }, { "name": "Gharib, Morteza (Mory)", "degree": "PhD", "year": "1983", "title": "The Effect of Flow Oscillations on Cavity Drag, and a Technique for their Control", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-133001", "creators": [ { "name": { "family": "Gharib", "given": "Morteza (Mory)" }, "id": "Gharib-Morteza", "orcid": "0000-0003-0754-4193", "display_name": "Gharib, Morteza (Mory)" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Corngold", "given": "Noel Robert" }, "id": "Corngold-N-R", "role": "member", "display_name": "Corngold, Noel Robert" }, { "name": { "family": "Blandford", "given": "Roger D." }, "id": "Blandford-R-D", "role": "member", "display_name": "Blandford, Roger D." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JJYA-CK31", "abstract": "The phenomenon of cavity flow oscillation is investigated to determine the conditions for onset of periodic oscillations and to understand the relationship between the state of the shear layer and the cavity drag. Experiments have been performed in a water tunnel using a 4\" axisymmetric cavity model instrumented with a strip heater on the nose cone and pressure taps in and around the cavity. A complete set of measurements of oscillation phase, amplitude amplification along the flow direction, distribution of shear stress and other momentum flux is obtained by means of a laser Doppler velocimeter. Drag measurements were made by integrating the mean pressure over the solid surfaces of the cavity. Results indicated exponential cavity drag dependence on the length of the cavity. A jump in the cavity drag coefrcient is observed as the cavity flow shows a bluff body wake type behavior. An independent estimate of the drag, which is obtained by integration of shear and mean momentum transfer terms over the peripheral area of the cavity, confirms the exponential dependence of drag on the length of the cavity. Results, also reveal that the drag of the cavity in the non-oscillating mode is less than the case if the cavity were replaced by a solid surface. Natural and forced oscillations of the cavity shear layer spanning the gap are studied. The forced oscillations are introduced by a sinusoidally heated thin-film strip which excites the Tollmein-Schlichting waves in the boundary layer upstream of the gap, For a sufficiently large gap, self-sustained periodic oscillations are observed while for smaller gaps, which do not oscillate naturally, periodic oscillations can be obtained by external forcing through the strip-heater. In the latter case resonance is observed whenever the forcing frequency satisfies the phase criterion \u03c6/(2\u03c0) = N, and its amplitude exceeds certain threshold levels, but the phenomenon is non-self-supporting. The drag of the cavity can be increased by one order of magnitude in the non-oscillating case through external forcing. For naturally occurring oscillations, it is possible for two waves to co-exist in the shear layer (natural and forced). Also, it is possible to completely eliminate mode switching by applying external forcing. For the first time a test is performed to cancel or dampen the amplitude of Kelvin-Helmholtz wave in the cavity shear layer. This is done through introducing an external perturbation with the same frequency of the natural component but having a different phase. Reduction by a factor of 2 is obtained in the amplitude of the oscillation.
" }, { "name": "Heymans, Luc J.", "degree": "PhD", "year": "1983", "title": "An Engineering Analysis of Polymer Film Adhesion to Rigid Substrates", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-05152007-111322", "creators": [ { "name": { "family": "Heymans", "given": "Luc J." }, "id": "Heymans-Luc-J", "display_name": "Heymans, Luc J." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Tschoegl", "given": "Nicholas W." }, "id": "Tschoegl-N-W", "role": "member", "display_name": "Tschoegl, Nicholas W." }, { "name": { "family": "Fornberg", "given": "Bengt" }, "id": "Fornberg-Bengt", "role": "member", "display_name": "Fornberg, Bengt" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3GQW-YP87", "abstract": "An important source of interface fracture contributing to adhesive failure in a bimaterial sandwich, consisting of a rigid substrate and a viscoelastic encapsulant material, arises from residual stresses. The encapsulant is often deposited on the substrate above its glass transition temperature region but used below this temperature range. In order to determine the magnitude of the residual stresses a viscoelastic stress analysis of a bimaterial sandwich is carried out, taking into account the time-dependent material properties of the polymeric layer and the environmental \"loading\" conditions. The theoretical analysis is paralleled by an experimental examination of the time-dependent out-of-plane deformation of thin, circular sandwiches.
\r\n\r\nPolyvinyl acetate was chosen as a model material exhibiting significant viscoelastic effects under room test conditions. Therefore the pertinent physical and mechanical properties of PYAc are determined; these include the thermal coefficient of expansion, the shear creep compliance and the relaxation modulus. In the experimental work BK-7 glass is taken as the \"rigid\" substrate. The measurements connected to the stress analysis are monitored with laser interferometry (Newton's rings). A comparison between theory and experiment completes the viscoelastic stress analysis.
\r\n\r\nIn the second part of this study time dependent adhesive failure of rubbery materials is investigated. Polymeric materials are being used increasingly for a wide variety of applications. Some of these materials are applied as protective layers to isolate their substrates from a hostile environment. Others achieve remarkable structural bond strengths thereby displacing the traditional mechanical fasteners like bolts and rivets. If one wants to investigate the long time integrity of a layer assembly the time dependence of the material properties of the adhesives needs to be carefully analyzed. This time dependence is also reflected in the energy required to create new surfaces as interfacial debonding proceeds the adhesive fracture energy is one of the dominant parameters in time dependent adhesive failure. In our investigation it is characterized through peel testing.
\r\n\r\nWith the knowledge of the pertinent material properties as well as of the adhesive fracture energy, we then proceed to formulate a criterion for continuing interfacial crack propagation. The analysis is carried out for elastic solids, with the effect of viscoelastic behavior incorporated later on. Debond tests provide a way to check how well the theoretical predictions correspond to experimental debond results.
" }, { "name": "Mungal, Mark Godfrey", "degree": "PhD", "year": "1983", "title": "Experiments on Mixing and Combustion with Low Heat Release in a Turbulent Shear Flow", "advisor": "Dimotakis, Paul E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09142006-144655", "creators": [ { "name": { "family": "Mungal", "given": "Mark Godfrey" }, "id": "Mungal-Mark-Godfrey", "display_name": "Mungal, Mark Godfrey" } ], "advisors": [ { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "advisor", "display_name": "Dimotakis, Paul E." } ], "committee": [ { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "chair", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Broadwell", "given": "James E." }, "id": "Broadwell-J-E", "role": "member", "display_name": "Broadwell, James E." }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QZ4F-V692", "abstract": "A new blowdown facility to study mixing and combustion in a turbulent shear layer has been built. The system is capable of 100 m/s for three seconds in a 5 x 20 cm exit area on the high speed side, and 50 m/s in a 10 x 20 cm exit area on the low speed side. Dilute concentrations of hydrogen and fluorine, carried in an inert gas, react when both fluid streams meet at the tip of a splitter plate. The reaction is spontaneous, rapid, and highly exothermic. The resulting temperature field has been studied using a rake of eight fast response thermometers placed across the width of the layer. Runs have been performed for low heat release over a wide range of equivalence (concentration) ratios, at a Reynolds number of 30,800 based on velocity difference and vorticity thickness. The heat release is sufficiently low so that the overall properties of the mixing layer are not significantly changed from the cold case.
\r\n\r\nThe results show the presence of large, hot structures within the flow together with cool, irrotational tongues of freestream fluid that penetrate deep into the layer. Thus, it is possible for the entire width of the layer to be quite hot, owing to the passage of a large structure, or for the layer to be quite cool, owing to the presence of the cool fluid tongues. The mean temperature results from a duty cycle whereby a given point in the flow sees alternating hot and cool fluid which averages into the local mean. The mean temperature profiles do not achieve the adiabatic flame temperature at any location across the layer, with the maximum mean temperature, depending upon the equivalence ratio, varying from 54% to 67% of the adiabatic flame temperature. The location of the maximum mean temperature shifts by about 25% of the visual thickness of the layer for a change of equivalence ratio by a factor of 64. The amount of product formed in the layer is compared to earlier measurements in water, and, it is found that at a speed ratio of 0.40, there exists 20 to 25% more product in gaseous flows, implying that molecular diffusion, or in nondimensional form the Schmidt number, plays a role in mixing at large Reynolds number. The present results compare favorably with the recent theoretical model of Broadwell and Breidenthal for mixing and chemical reaction in a turbulent shear layer. With this model it is possible to bring the results for gases and liquids into quantitative agreement.
" }, { "name": "Sargent, William Stapf", "degree": "PhD", "year": "1983", "title": "Natural Convection Flows and Associated Heat Transfer Processes in Room Fires", "advisor": "Zukoski, Edward E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02052007-132409", "creators": [ { "name": { "family": "Sargent", "given": "William Stapf" }, "id": "Sargent-William-Stapf", "display_name": "Sargent, William Stapf" } ], "advisors": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "advisor", "display_name": "Zukoski, Edward E." } ], "committee": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "chair", "display_name": "Kubota, Toshi" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sabersky", "given": "Rolf H." }, "id": "Sabersky-R-H", "role": "member", "display_name": "Sabersky, Rolf H." }, { "name": { "family": "List", "given": "E. John" }, "id": "List-E-J", "role": "member", "display_name": "List, E. John" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/n7kr-k165", "abstract": "This report presents the results of experimental investigations of natural convection flows and associated heat transfer processes produced by small fires in rooms with a single door or window opening. Calculation procedures have been developed to model the major aspects of these flows.
\r\n\r\nTwo distinct sets of experiments were undertaken.
\r\n\r\nFirst, in a roughly 1/4 scale facility, a slightly dense solution of brine was allowed to flow into a tank of fresh water. The resulting density difference produced a flow which simulated a very small fire in a room with adiabatic walls. Second, in an approximately 1/2 scale test room, a nearly stoichioinetric mixture of air and natural gas was burned at floor level to model moderate strength fires. In this latter facility, we directly measured the heat conducted through the walls, in addition to determining the gas temperature and composition throughout the room.
\r\n\r\nThese two facilities complemented each other. The former offered good flow visualization and allowed us to observe the basic flow phenomena in the absence of heat transfer effects. On the other hand, the latter, which involved relatively larger fires, was a more realistic simulation of an actual room fire, and allowed us to calculate the convective heat transfer to the ceiling and walls. In addition, the stronger sources present in these 1/2 scale tests produced significant secondary flows. These secondary flows along with heat transfer effects act to modify the gas temperature or density profiles within the room from those observed in the 1/4 scale experiments.
\r\n\r\nSeveral calculation procedures have been developed, based on the far field properties of plumes when the density differences are small (the Boussinesq approximation). The simple point source plume solution is used along with hydraulic analysis of flow through an orifice to estimate the temperatures of the hot ceiling layer gas and of the cooler floor zone fluid, as well as the height of the interface between them. A finite source plume model is combined with conservation equations across the interface to compute the evolution of the plume above the interface. This calculation then provides the starting point for an integral analysis of the flow and heat transfer in the turbulent ceiling jet.
\r\n\r\nThe computed results both for the average floor and ceiling zone gas temperatures, and for the connective heat transfer in the ceiling jet agreed reasonably well with our experimental data. This agreement suggests that our computational procedures can be applied to answer practical questions, such as whether the connective heat flux from a given fire in a real room would be sufficient to trigger sprinklers or other detection systems in a given amount of time.
" }, { "name": "Bauer, Thomas Patrick", "degree": "PhD", "year": "1982", "title": "Low-Thrust Perturbation Guidance", "advisor": "Caughey, Thomas Kirk", "url": "https://resolver.caltech.edu/CaltechETD:etd-08122005-133355", "creators": [ { "name": { "family": "Bauer", "given": "Thomas Patrick" }, "id": "Bauer-Thomas-Patrick", "display_name": "Bauer, Thomas Patrick" } ], "advisors": [ { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "role": "advisor", "display_name": "Caughey, Thomas Kirk" } ], "committee": [ { "name": { "family": "Wood", "given": "Lincoln" }, "id": "Wood-L", "role": "chair", "display_name": "Wood, Lincoln" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Muhleman", "given": "Duane Owen" }, "id": "Muhleman-D-O", "role": "member", "display_name": "Muhleman, Duane Owen" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "display_name": "Caughey, Thomas Kirk" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TVQ3-FV70", "abstract": "Low-thrust perturbation guidance, as applied to the minimum time problem of an Earth to Mars rendezvous, has been reexamined and shown to perform orders of magnitude better, as measured by the terminal state error, than previous studies indicated. The orbits of Earth and Mars were assumed to be inclined and elliptical. The only forces considered were the Sun's gravity and that of the constant thrust rocket engine.
\r\n\r\nFirst order necessary conditions of the calculus of variations were developed for the nominal trajectory. The resulting nonlinear two-point boundary value problem was solved with the Backward Sweep Method. Feedback gain related and trajectory information is stored on a file during the optimization of the nominal trajectory to be retrieved later in the guidance programs by a high order interpolator.
\r\n\r\nTwo guidance schemes, Time-To-Go Guidance and Minimum Distance Guidance, were investigated for several initial perturbations in velocity and position. The performance of the two schemes was found to be clearly acceptable although not quite as good as reoptimization. The two schemes are equivalent in performance. Moreover, a simplified version of the schemes, Current Time Guidance, was found to be comparable in performance to the more elaborate guidance schemes.
\r\n\r\nA comparison of the current results with those of previous studies was made showing that terminal state errors can be reduced 100 to 10,000 times more than found earlier. This apparent improvement may possibly be explained by the use of a high fidelity integrator and other enhancements implemented in the software, although algorithm and programming mistakes in the earlier studies are suspected.
\r\n\r\nA similar minimum time problem, that of a two-dimensional Earth to Mars orbit transfer using a solar sail, was also reexamined. The optimized trajectory was found to be very similar to those obtained by most earlier studies. A recent report which prompted the study was found to have an error in a transversality condition causing anomalous results.
" }, { "name": "Catherasoo, Christopher Jeyaparan", "degree": "PhD", "year": "1982", "title": "Shock Dynamics in Non-Uniform Media", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-09132006-143337", "creators": [ { "name": { "family": "Catherasoo", "given": "Christopher Jeyaparan" }, "id": "Catherasoo-Christopher-Jeyaparan", "display_name": "Catherasoo, Christopher Jeyaparan" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Housner", "given": "George W." }, "id": "Housner-G-W", "role": "member", "display_name": "Housner, George W." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7knb-5s50", "abstract": "The theory of shock dynamics in two dimensions is reformulated to treat shock propagation in a non-uniform medium. The analysis yields a system of hyperbolic equations with source terms representing the generation of disturbances on the shock wave as it propagates into the fluid non-uniformities. The theory is applied to problems involving the refraction of a plane shock wave at a free plane gaseous interface. The \"slow-fast\" interface is investigated in detail, while the \"fast-slow\" interface is treated only briefly. Intrinsic to the theory is a relationship analogous to Snell's law of refraction at an interface. The theory predicts both regular and irregular (Mach) refraction, and a criterion is developed for the transition from one to the other. Quantitative results for several different shock strengths, angles of incidence and sound speed ratios are presented. An analogy between shock refraction and the motion of a force field in unsteady one-dimensional gasdynamics is pointed out. Also discussed is the limiting case for a shock front to be continuous at the interface. Comparison of results is made with existing experimental data, with transition calculations based on three-shock theory, and with the simple case of normal interaction.
" }, { "name": "Chai, Herzl", "degree": "PhD", "year": "1982", "title": "The Growth of Impact Damage in Compressively Loaded Laminates", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04132004-084629", "creators": [ { "name": { "family": "Chai", "given": "Herzl" }, "id": "Chai-Herzl", "display_name": "Chai, Herzl" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "chair", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Miklowitz", "given": "Julius" }, "id": "Miklowitz-J", "role": "member", "display_name": "Miklowitz, Julius" }, { "name": { "family": "Housner", "given": "George W." }, "id": "Housner-G-W", "role": "member", "display_name": "Housner, George W." }, { "name": { "family": "Corcoran", "given": "William Harrison" }, "id": "Corcoran-W-H", "role": "member", "display_name": "Corcoran, William Harrison" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/58D3-ZZ41", "abstract": "The thesis is divided into two chapters. The first chapter describes an experimental program carried out to determine the phenomenological aspects of composite panel failure (graphite/epoxy laminate) under simultaneous in-plane compression and low velocity transverse impact (0-250 ft/sec). High-speed photography and the shadow moire technique were used to record a full-field surface deformation history of the impacted plate. The information gained from these records, supplemented by plate sectioning and observation for interior damage, has shown that the predominant failure mechanism is the coupled effect of delamination and buckling.
\r\n\r\nIn chapter 2, this process of failure is modelled by delamination buckling wherein the local delamination stability, growth or arrest are governed by an energy release rate criterion. Various degrees of problem simplifications are employed, starting with a one-dimensional, delaminated-beam model. In the most sophisticated treatment, it is assumed that the damaged area has an elliptical shape. It was found that this model is capable of describing the growth conditions and the growth behavior of the damaged area. It was also found that the predominant parameter controlling delamination growth or arrest is the fracture energy associated with delamination.
\r\n\r\nIn the appendix at the end of this work, the fracture energy of a composite laminate is considered. A modified cleavage analysis is developed, and is applied to evaluate this quantity for two composite materials of current interest. The test results are then examined with reference to impact damage tolerance data available for these materials. A viscoelastic characterization of the two resins used to fabricate the two composites mentioned above is also provided. The results of these measurements are then examined with reference to long-term applicability of the matrix material.
" }, { "name": "Nosenchuck, Daniel Mark", "degree": "PhD", "year": "1982", "title": "Passive and Active Control of Boundary Layer Transition", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-09182006-135720", "creators": [ { "name": { "family": "Nosenchuck", "given": "Daniel Mark" }, "id": "Nosenchuck-Daniel-Mark", "display_name": "Nosenchuck, Daniel Mark" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "chair", "display_name": "Knowles, James K." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CKFA-E875", "abstract": "It is well known that laminar-turbulent boundary layer transition is initiated by the formation of Tollmien-Schlichting laminar instability waves. The amplification rates of these waves are strongly dependent on the shape of the boundary layer velocity profile. Consequently, the transition process can be controlled by modifying the velocity profile. This can be accomplished by controlling the pressure gradient (dp/dx), using boundary layer suction, installing surface roughness elements, or by surface heating or cooling. Methods used to modify the transition process through changes in the mean velocity profile are called \"passive\" in this paper. There exists a large set of experiments and theory on the application of passive methods for boundary layer control. In the present work only surface heating will be addressed.
\r\n\r\nTransition measurements were made on a heated flat plate in water. Results are presented for several plate wall temperature distributions. An increase by a factor of 2.5 in transition Reynolds number was observed for a 5\u00b0C isothermal wall overheat. Buoyancy effects on transition were minimal due to the small Richardson and Grashof numbers encountered in the experiments.
\r\n\r\nThe amplification of laminar instability waves is comparatively to process, taking place over many boundary layer thicknesses. After the slow amplification of the laminar instability waves, transition occurs by a strong three dimensional dynamic instability. It appears possible to attenuate (or reinforce) the instability waves by introducing amplitude-and phase-controlled perturbations into the laminar boundary layer using feedback control system. This method is called \"active\" control and forms the larger part of the research reported in this thesis.
\r\n\r\nA combination of sensors, activators and feedback control electronics is required for active control. The sensors used in the experiments are flush-mounted hot film wall shear robes. A new type of activator was developed using thin, flush-mounted surface heating elements to excite instability waves in the laminar boundary layer by periodic (active) heating.
\r\n\r\nExperimental evidence is presented illustrating the effects of periodically heated flush mounted strips in perturbing a flat plate boundary layer in water. The results of superposition of forced laminar instability waves are also given. Finally, an active feedback-control system using a single hot film probe and strip heater was developed to control natural laminar instability waves in real time. It is shown that when the natural waves were attenuated, the transition length was increased by 25%, requiring only 10 watts of strip heater power. To accomplish the same transition delay using passive heating, the internal heating pads had to supply 1900 watts of power.
" }, { "name": "Ravi-Chandar, K.", "degree": "PhD", "year": "1982", "title": "An Experimental Investigation into the Mechanics of Dynamic Fracture", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-104624", "creators": [ { "name": { "family": "Ravi-Chandar", "given": "K." }, "id": "Ravi-Chandar-K", "display_name": "Ravi-Chandar, K." } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "chair", "display_name": "Babcock, Charles D." }, { "name": { "family": "Housner", "given": "George W." }, "id": "Housner-G-W", "role": "member", "display_name": "Housner, George W." }, { "name": { "family": "Kanamori", "given": "Hiroo" }, "id": "Kanamori-H", "orcid": "0000-0001-8219-9428", "role": "member", "display_name": "Kanamori, Hiroo" }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZSX5-Q128", "abstract": "Current theories of dynamic fracture are based on elastodynamic analyses of mathematically sharp plane cracks and as such do not explain the observed terminal velocities or the phenomenon of crack branching satisfactorily. The present investigation addresses the above problems by using both microscopic and macroscopic interpretations. The experimental scheme that is used in this investigation is the configuration of a pressure loaded semi-infinite crack in an infinite medium. The loading is achieved through an electromagnetic device which provides highly repeatable loading. The method of caustics is used in conjunction with a high speed camera to obtain the time histories of the crack tip stress intensity factor and the crack position.
\r\n\r\nThe problems of crack initiation and crack arrest are explored. The stress intensity factor at initiation is found to be independent of the rate of applied loading when the latter is below about 104MPA/sec, but the initiation stress intensity factor increases considerably when the loading rate is increased further. Crack arrest is obtained in large specimen by using very low energy loading pulses. It was found that the stress intensity factor at crack arrest was constant and also that, within the time resolution of the high speed camera (5 \u03bcsec), the crack comes to a stop abruptly.
\r\n\r\nThe crack propagation and branching aspects were investigated first using post-mortem analysis of the fracture surfaces and high speed photomicrography to get an idea of the microscopic processes that occur in the fracure process. From this investigation, it was found that crack propagation involving high stress intensity factor and high velocity situations takes place by the growth and interaction of microcracks, due to the voids present in the material. A surprising result of this investigation was that cracks propagated at a constant velocity, although the stress intensity factor varied. Current theories of dynamic fracture cannot explain such behaviour. The crack branching process was found to be a continuous process arising out of propagation along a straight line. High speed photomicrographs of the branching process indicated the presence of a number of part-through attempted branches that interact with one another and finally the successful emergence of a few full fledged branches.
\r\n\r\nThe microscopic observations on the crack propagation and branching process leads to a new interpretation of dynamic fracture that attempts to qualitatively explain the constancy of the velocity of propagation, the terminal velocity and crack branching. The crack branching mechanism is a logical continuation of the mechanism for crack propagation.
\r\n" }, { "name": "Shieh, An Huh", "degree": "PhD", "year": "1982", "title": "Non-Stationary Lattice Theory", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-09142006-112813", "creators": [ { "name": { "family": "Shieh", "given": "An Huh" }, "id": "Shieh-An-Huh", "display_name": "Shieh, An Huh" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/xknf-r113", "abstract": "No abstract submitted." }, { "name": "Trebitz, Bernd Otto", "degree": "PhD", "year": "1982", "title": "Acoustic Transmission Imaging for Flow Diagnostics", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-145444", "creators": [ { "name": { "family": "Trebitz", "given": "Bernd Otto" }, "id": "Trebitz-Bernd-Otto", "display_name": "Trebitz, Bernd Otto" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "chair", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Cohen", "given": "Donald S." }, "id": "Cohen-D-S", "role": "member", "display_name": "Cohen, Donald S." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/pnsp-rk30", "abstract": "Wave propagation through a given flow field can be utilized to obtain information about the flow. Acoustic waves in particular offer the possibility of measuring velocity fields, because sound waves are convected with the flow, and temperature fields, because the acoustic index of refraction is a strong function of temperature. This work concentrates on situations where the first effect is dominating compared to the latter one.
\r\n\r\nA sound system capable of \"illuminating\" a flow field with ultrasonic waves and measuring the amplitude and phase distribution of the transmitted wave as a function of time was constructed. A large area transmitter and a linear receiving array were used as transducers. Parallel signal processing and interleaved data conversion and acquisition result in a maximum frame rate of 10 kHz.
\r\n\r\nThe feasibility of measuring velocity disturbances with ultrasound was demonstrated by transmitting sound through a vortex, which was generated in a duct by an airfoil swirl generator. Assuming an exponential fit for the tangential velocity component, inner core radius and circulation can be determined directly from the phase change of the transmitted wave due to the vortex. A more accurate representation of the radial velocity profile can be found by digital reconstruction via the Abel inversion formula, which allows reconstruction of rotationally symmetric objects from line projections. Even though the flow field under investigation was steady, this is neither a restriction of the technique, nor of the apparatus. However, the repetition rate for consecutive data frames depends on the operating procedure.
" }, { "name": "Bernal, Luis Paulino", "degree": "PhD", "year": "1981", "title": "The Coherent Structure of Turbulent Mixing Layers. I. Similarity of the Primary Vortex Structure. II. Secondary Streamwise Vortex Structure", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-08262005-154143", "creators": [ { "name": { "family": "Bernal", "given": "Luis Paulino" }, "id": "Bernal-Luis-Paulino", "display_name": "Bernal, Luis Paulino" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "chair", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." }, { "name": { "family": "Ingersoll", "given": "Andrew P." }, "id": "Ingersoll-A-P", "orcid": "0000-0002-2035-9198", "role": "member", "display_name": "Ingersoll, Andrew P." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z52D-SA34", "abstract": "The primary spanwise organized vortex structure and the secondary streamwise vortex structure of turbulent mixing layers have been investigated. Flow visualization motion pictures of a constant density mixing layer were used to measure the properties of the large scale vortices. It was found that after an initial transition region mean properties of the large scale vortices reach the expected linear growth with downstream distance required by similarity. In the self-similar region, the vortex core area and visual thickness increase continuously during its life-span.
\r\n\r\nA theoretical model of probability distribution function for the large-scale vortex circulation was developed. This distribution is found to be lognormal and to have a standard deviation, normalized with the mean of 0.28. From this model the mean life-span of the vortices could also be obtained and was found to be 0.67 times the mean life-span position.
\r\n\r\nThe streamwise streak pattern observed by Konrad (1976) and Breidenthal (1978) in plan-view pictures of the mixing layer was investigated, using flow visualization and spanwise concentration measurements. It was confirmed that this pattern is the result of a secondary vortex structure dominated by streamwise, counterrotating vortices. A detailed description of its spatial relation to the primary, spanwise vortex structure is presented. From time average flow pictures, the onset position and initial scale of the secondary structures were determined. From concentration measurements, spanwise variations in mean properties, resulting from the secondary structure, were found. This also showed an increase of the spanwise scale with downstream distance and the existence of the streamwise vortices in the fully developed turbulent region. In this region the mean spacing is found approximately equal to the vorticity thickness.
" }, { "name": "Glezer, Ari", "degree": "PhD", "year": "1981", "title": "An Experimental Study of a Turbulent Vortex Ring", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-10092006-080016", "creators": [ { "name": { "family": "Glezer", "given": "Ari" }, "id": "Glezer-Ari", "display_name": "Glezer, Ari" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "chair", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "member", "display_name": "Knauss, Wolfgang Gustav" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "member", "display_name": "Kubota, Toshi" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RC4P-DA56", "abstract": "An apparatus has been constructed to study the formation and dynamics of vortex rings. The rings are formed in water by a momentary jet discharge downward into a glass tank from an orifice in a submerged plate. The necessary impulse is provided by a pressurized reservoir and is controlled by a fast programmable solenoid valve.
\r\n\r\nA particular turbulent vortex ring with a relatively thin core was chosen for detailed study. The velocity field of this ring was measured at numerous locations in the tank, using a two-channel laser-Doppler velocimeter with digital tracking electronics. Beamsplitting and frequency-shifting were accomplished by two partially-overlapping radial phase gratings driven by hysteresis-synchronous motors.
\r\n\r\nThe main aim of the research was to verify the similarity properties of the turbulent vortex ring and to determine mean particle paths and mean vorticity contours in the appropriate nonsteady similarity coordinates x/t\u00bc, r/t\u00bc. Some novel pattern-recognition techniques were developed to overcome the problem of dispersion in the trajectories of individual vortices.
" }, { "name": "Ruth, Edward Kenneth", "degree": "PhD", "year": "1981", "title": "Experiments with Unconventional Cross Flow Heat Exchangers", "advisor": "Rannie, W. Duncan", "url": "https://resolver.caltech.edu/CaltechETD:etd-10042006-110003", "creators": [ { "name": { "family": "Ruth", "given": "Edward Kenneth" }, "id": "Ruth-Edward-Kenneth", "display_name": "Ruth, Edward Kenneth" } ], "advisors": [ { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/P6TP-GR40", "abstract": "These experiments consisted of measurements of pressure losses and heat transfer rates for two unconventional crossflow heat exchanger configurations.\r\n\r\nThe first type consisted of an arrangement of circular tubes in a sawtooth pattern. Performance of this type did not appear to be an improvement over more conventional heat exchangers.\r\n\r\nThe second type used exchanger tubes of a special lenticular cross section so spaced as to keep the velocity of the fluid flowing outside the tubes nearly constant in magnitude reducing separation and drag. The lenticular tubes' performance was superior to the conventional types, especially at higher Reynolds numbers." }, { "name": "Shih, Choon-Foo", "degree": "PhD", "year": "1981", "title": "Failure of Liquid Storage Tanks Due to Earthquake Excitation", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechTHESIS:04132018-090928397", "creators": [ { "name": { "family": "Shih", "given": "Choon-Foo" }, "id": "Shih-Choon-Foo", "display_name": "Shih, Choon-Foo" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "chair", "display_name": "Babcock, Charles D." }, { "name": { "family": "Housner", "given": "George W." }, "id": "Housner-G-W", "role": "member", "display_name": "Housner, George W." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Miklowitz", "given": "Julius" }, "id": "Miklowitz-J", "role": "member", "display_name": "Miklowitz, Julius" }, { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Wu-T-Y-T", "role": "member", "display_name": "Wu, Theodore Yao-tsu" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/m0v8-hs31", "abstract": "Above ground liquid storage tanks have suffered serious damage during earthquakes. The damage of tanks can vary from local yielding or buckling of the tank wall, to loss of contents, or to collapse which leads to an unrepairable tank. Considerable work has been carried out on this problem with varying degree of success. However, the results are largely directed toward response rather than failure prediction. The information on failure mechanisms is very limited. The present work consists of scale model testing, correlation with existing analysis and failure prediction with laboratory verification. The scale model testing incorporates dynamic similarity of the fluid/structure interaction problem. The model study shows that small plastic models can be useful in studying the dynamics and buckling of liquid-filled tanks under ground excitation even though the model does not display complete similitude. The buckling criterion proposed in this study is based upon static considerations and the complex stress field in the shell wall is supplanted by a simple field for which analytical/experimental results are available. Harmonic buckling tests demonstrate that the static buckling criterion is satisfactory even though the prebuckling stress field is time dependent. The harmonic buckling tests, when correlated with the stresses from a response analysis, also indicate that the buckling is largely dependent upon the n = 1 response, Transient buckling tests are also carried out and the results show that the linear analysis together with the static buckling criterion gives a good prediction of the failure of a full fluid-filled tank. The test parameters in these buckling tests include water depth, title angle, thickness of tank wall, top end condition, ground excitation pattern, etc. In addition, buckling tests of unanchored tanks are conducted to study the influence of changing the anchorage of the tank base. An analytical model is suggested to predict the response of an unanchored tank due to overturning moment. The current design criterion of an un\u00adanchored tank is also assessed in this study. The results of this investigation, in addition to those carried out previously, provide a better understanding of the forced vibration problem, failure criterion and appropriate design procedure for a liquid storage tank.
\r\n" }, { "name": "Kyriakides, Stelios", "degree": "PhD", "year": "1980", "title": "On the Propagating Buckle and Its Arrest", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-134840", "creators": [ { "name": { "family": "Kyriakides", "given": "Stelios" }, "id": "Kyriakides-Stelios", "display_name": "Kyriakides, Stelios" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/c81x-nk02", "abstract": "If an offshore pipeline locally buckles in the presence of sufficiently large external pressure, a propagating buckle is initiated. The buckle propagates along the pipeline until it encounters a region of adverse conditions -- low pressure or an arresting device. The lowest pressure at which a buckle propagates is defined as the Propagation Pressure. An experimental study of this quantity is presented and a semiempirical expression derived by examining various models of the phenomenon. The dynamics of the Propagating Buckle are also examined and a parametric study of the steady state velocity of Propagation is carried out. A systematic way of empirically deriving the parametric dependence of arresting devices is presented and experimental results of two such arrestors are discussed. During this study a unique \"flip-flop\" mode of propagation was discovered and studied. An explanation of the phenomenon is also attempted." }, { "name": "Liechti, Kenneth Macdougall", "degree": "PhD", "year": "1980", "title": "The Application of Optical Interferometry to Time Dependent Unbonding: Parts 1, 2, and 3", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-10292003-135400", "creators": [ { "name": { "family": "Liechti", "given": "Kenneth Macdougall" }, "id": "Liechti-Kenneth-Macdougall", "display_name": "Liechti, Kenneth Macdougall" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9yk1-v212", "abstract": "Improved methods of adhesive joining for constructing structural elements have led to an increase in the use of bonding. Bonding is already widely used in the aerospace industry. It allows potential for greater weight and manufacturing cost savings when compared to mechanical fastening as well as providing better fatigue resistant capabilities. More accurate design or life prediction based on a better understanding of adhesive joint failure will allow far more extensive and safer use of adhesively bonded structures. The possibility of time dependent failure is introduced in bonding,by the fact that many adhesives are polymers and measurably viscoelastic under many circumstances. Failure can occur by unbonding at the adherend-adhesive interface or totally within the adhesive. Here the time dependent failure by unbonding at the interface is addressed as one initial investigation of the whole problem.\r\n\r\nIn many engineering problems the modelling of failure has been based on linear fracture mechanics. The extent to which the use of the linear theory can be justified is often questionable in view of the nonlinearity of the local crack front deformation field. One aspect of the current investigation is therefore the measurement of the local crack front deformation to examine the limits of the validity for the linear theory and to provide a basis for future modelling, Within this framework, particular attention is paid to the aspects of local failure mode interaction and the time dependence of the unbonding process.\r\n\r\nThe small displacements in the crack front region require a resolution on the order of a wavelength of light. The experimental techniques used are therefore based on optical interferometry. The displacements applied to the adherends are controlled to the same order by a novel, thermally actuated servoloading device.\r\n\r\nThe thesis is divided into three distinct parts. The first part deals with the development of the experimental techniques used to measure the unbound profiles as well as to measure and control the displacements applied to the adherends. For stationary cracks, the second part describes experiments which examine the extent of the nonlinearity in the crack front deformation field while fracture criteria are developed and compared for unbonding rates in steadily propagating cracks. Finally, proposals for improvement of the experimental techniques and further experiments are discussed in the third part." }, { "name": "Turner, Timothy Neal", "degree": "PhD", "year": "1980", "title": "Second Sound Shock Waves and Critical Velocities in Liquid Helium II", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10132006-075044", "creators": [ { "name": { "family": "Turner", "given": "Timothy Neal" }, "id": "Turner-Timothy-Neal", "display_name": "Turner, Timothy Neal" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/cg05-6436", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nLarge amplitude second-sound shock waves have been generated and the experimental results compared to the theory of nonlinear second-sound. The structure and thickness of second-sound shock fronts is calculated and compared to experimental data. Theoretically it is shown that at T = 1.88\u00b0K, where the nonlinear wave steepening vanishes, the thickness of a very weak shock must diverge. In a region near this temperature, a finite-amplitude shock pulse will evolve into an unusual double-shock configuration consisting of a front steepened, temperature raising shock followed by a temperature lowering shock. Double-shocks are experimentally verified. The theoretical dependence of the shock induce temperature jump on the Mach number is successfully verified for large amplitudes ([...]) after the response of a thin-film superconducting temperature sensor is analyzed.\r\n\r\nThe ability of second-sound shock waves to simultaneously produce and measure very large relative velocities in regions away from the disruptive influence of walls makes them an invaluable tool in the study of critical velocities intrinsic to the fluid. It was experimentally discovered that very large second-sound shock waves initiate a breakdown in the superfluidity of helium II, which is dramatically displayed as a limit to the maximum attainable shock strength. Although the observed breakdown could not be definitely attributed to a critical velocity, the value of the maximum shock-induced relative velocity represents a significant lower bound to the intrinsic critical velocity of helium II. The observed limits within which superfluidity was still maintained (w=3.67 m/sec at T = 1.45\u00b0K, and w = 3.20 m/sec at T = 1.85\u00b0K) are the largest counterflow velocities ever obtained outside of restricted geometries." }, { "name": "Breidenthal, Robert Edward", "degree": "PhD", "year": "1979", "title": "A Chemically Reacting, Turbulent Shear Layer", "advisor": "Roshko, Anatol; Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10272006-111937", "creators": [ { "name": { "family": "Breidenthal", "given": "Robert Edward" }, "id": "Breidenthal-Robert-Edward", "display_name": "Breidenthal, Robert Edward" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "chair", "display_name": "Saffman, Philip G." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Dimotakis", "given": "Paul E." }, "id": "Dimotakis-P-E", "role": "member", "display_name": "Dimotakis, Paul E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/hj67-9h91", "abstract": "A chemically reacting turbulent shear layer was investigated in a new, blow-down water tunnel. In a diffusion-limited reaction, a pH indicator, phenolphthalein, in one stream mixed and reacted with a base, sodium hydroxide, in the other stream to form a visible reaction product. Using optical densitometry techniques, the amount of product was measured as a function of Reynolds number, at a relatively high Schmidt number of approximately 600. The results were compared with both the previous mixing measurements of Konrad in a gaseous shear layer (Sc = 0.7) and the simple mixing model of Broadwell.\r\n\r\nThe product was found to be distributed, as expected, in concentrated lumps associated with the large, spanwise-coherent structures of the turbulence. The time averaged amount of product in the layer exhibited a rapid transition at a large-structure Reynolds number of about 5 x 10(3) for a velocity ratio of 0.38. Above the transition, the amount of product within the layer was independent of Reynolds number.\r\n\r\nThis transition is related to the introduction of small scale, three-dimensional motions into the layer. In the initial region, where the flow was already unsteady and contained large structures but was strictly two-dimensional, very little mixing was observed. Downstream the flow became unstable to three-dimensional perturbations and small scale, three-dimensional motions were introduced into the layer. Across this transition, the aqueous mixing increased by an order of magnitude, indicating the sensitivity of mixing to small scales of the turbulence in a high Schmidt number fluid. At high Reynolds numbers, changing the Schmidt number by three orders of magnitude only altered the molecular mixing by about a factor of two or less. The mixing model of Broadwell, which addresses the effect of Schmidt number, is in satisfactory qualitative agreement with the observations.\r\n\r\nThe unique flow visualization of the visible reaction product in water permitted a study of the three-dimensional instability and evolution of small scale motions in the layer. Streamwise streaks which had been previously observed in the Brown-Roshko gas apparatus were found to originate from a spanwise-sinuous wiggle which appeared at a large-structure Reynolds number which varied with velocity ratio, indicating an influence of initial conditions on the instability." }, { "name": "Oey, Nicolaus Kiam Thian", "degree": "PhD", "year": "1979", "title": "Leading Edge Flutter of a Supercavitating Hydrofoil", "advisor": "Babcock, Charles D.; Brennen, Christopher E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032006-091936", "creators": [ { "name": { "family": "Oey", "given": "Nicolaus Kiam Thian" }, "id": "Oey-Nicolaus-Kiam-Thian", "display_name": "Oey, Nicolaus Kiam Thian" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "advisor", "display_name": "Brennen, Christopher E." } ], "committee": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "chair", "display_name": "Babcock, Charles D." }, { "name": { "family": "Brennen", "given": "Christopher E." }, "id": "Brennen-C-E", "role": "member", "display_name": "Brennen, Christopher E." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Davis", "given": "Leverett" }, "id": "Davis-Leverett", "role": "member", "display_name": "Davis, Leverett" }, { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Wu-T-Y-T", "role": "member", "display_name": "Wu, Theodore Yao-tsu" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9DGB-DW93", "abstract": "Leading edge flutter is a problem that is unique to a super-cavitating hydrofoil. At high speed, the leading edge portion has been observed to oscillate while the trailing edge remains motionless.
\r\n\r\nIn this study, several flat plate hydrofoils were tested. The experimental results indicate that the phenomenon is a complex function of speed, angle of attack, cavitation number and mass ratio. Leading edge flutter was also observed to cause cavity pinching. A theoretical study was also conducted. Two mathematical models are presented here. The first one models the flexible chord foil as a rigid chord foil hinged at the trailing edge; the second model treats the fluid-structure interaction problem of a flexible chord foil cantilevered at the trailing edge. Both models resemble leading edge flutter near zero cavitation number in some respects. At short and moderate cavity lengths, leading edge flutter phenomenon is influenced by the cavity closure condition.
\r\n" }, { "name": "Sava\u015f, \u00d6mer", "degree": "PhD", "year": "1979", "title": "Some Measurements in Synthetic Turbulent Boundary Layers", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-11012006-083249", "creators": [ { "name": { "family": "Sava\u015f", "given": "\u00d6mer" }, "id": "Sava\u015f-\u00d6mer", "display_name": "Sava\u015f, \u00d6mer" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XZ05-2555", "abstract": "Synthetic turbulent boundary layers were constructed on a flat plate by generating systematic moving patterns of turbulent spots in a laminar flow. The experiments were carried out in a wind tunnel at a Reynolds number based on plate length of 1.7 x 106. Spots were generated periodically in space and time near the leading edge to form a regular hexagonal pattern. The disturbance mechanism was a camshaft which displaced small pins momentarily into the laminar flow at frequencies up to 80 Hz. The main instrumentation was a rake of 24 hot wires placed across the flow in a line parallel to the surface. The main measured variable was local intermittency; i.e., the probability of observing turbulent flow at a particular point in space and time. The results are reported in numerous x-t diagrams showing the evolution of various synthetic flows along the plate. The celerity or phase velocity of the large eddies was found to be 0.88, independent of eddy scale. All patterns with sufficiently small scales eventually showed loss of coherence as they moved downstream. A novel phenomenon called eddy transposition was observed in several flows which contained appreciable laminar regions. The large eddies shifted in formation to new positions, intermediate to their original ones, while preserving their hexagonal pattern. The present results, together with some empirical properties of a turbulent spot, were used to estimate the best choice of scales for constructing a synthetic boundary layer suitable for detailed study. The values recommended are: spanwise period/thickness \u22483.2, streamwise period/thickness \u224811." }, { "name": "Vinkler, Aharon P.", "degree": "PhD", "year": "1979", "title": "Optimal Controller Design Methods for linear Systems with Uncertain Parameters: Development, Evaluation and Comparison", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-10312006-104019", "creators": [ { "name": { "family": "Vinkler", "given": "Aharon P." }, "id": "Vinkler-Aharon-P", "display_name": "Vinkler, Aharon P." } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9fwb-er39", "abstract": "In many physical systems, an accurate knowledge of certain parameters is very difficult or very expensive to obtain. The designer of a remotely piloted vehicle flight control system, for example, frequently has available little data regarding aerodynamic coefficients, due to a lack of wind tunnel tests. Commonly used controller design methods, based on nominal values of plant parameters, often fail to achieve a satisfactory design in the face of parameter uncertainty.\r\n\r\nIn this work two methods have been developed for the design of linear, constant gain feedback controllers for systems with uncertain parameters:\r\n\r\n1) The multistep guaranteed cost control method is based on the concept of minimizing an upper bound of a cost functional in the face of parameter uncertainty. An algorithm has been developed to analyze the effect of parameter uncertainties on closed-loop system stability. An extension of this algorithm results in a technique for choosing constant feedback gains which guarantee a stable closed-loop system that possesses some of the desirable features of optimally designed control systems.\r\n\r\n2) The minimum discrete expected cost method is based on the concept of minimizing the expected value of a cost functional over a finite number of points in the range of parameter uncertainty. The design process makes use of statistical information about the uncertain parameters and incorporates in its cost functional whatever effects accompany a large departure in the plant parameters from their nominal values.\r\n\r\nAn extensive comparison of these two methods, together with the guaranteed cost control method, the minimax method, and the uncertainty weighting method, has been done in the context of the design of a fifth-order lateral autopilot for an RPV with uncertain aerodynamic coefficients. All five methods were evaluated on the bases of performance and design effort required. Both new methods were found to avoid some of the drawbacks associated with other techniques. The two newly developed methods are easy to implement and offer the designer tools for use in real control system design." }, { "name": "Wise, Jack LeRoy, III", "degree": "PhD", "year": "1979", "title": "Experimental Investigation of First- and Second-Sound Shock Waves in liquid Helium II", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032006-095703", "creators": [ { "name": { "family": "Wise", "given": "Jack LeRoy, III" }, "id": "Wise-Jack-LeRoy-III", "display_name": "Wise, Jack LeRoy, III" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/c49w-bh82", "abstract": "The cryogenic shock tube is used to generate a gasdynamic shock which propagates through saturated helium vapor and subsequently reflects from the upper surface of a column of LHeII. Superconducting thin-film detectors, produced by evaporation of aluminum in an oxygen atmosphere, yield highly repeatable arrival time data for the incident gasdynamic shock and the resultant first- and second-sound shocks in the liquid. Accurate x-t diagrams of the shock trajectories have been constructed for initial liquid temperatures of T(0) = 1.522, 1.665, 1.751, 1.832, 1.989, 2.031, and 2.095\u00b0K. Consistent discrepancies are observed between experimental and theoretical wave trajectories.\r\n\r\nThe detector signals qualitatively verify theoretical predictions that the temperature decreases through the pressure shock and increases through the temperature shock. Amplitude measurements based on static detector calibrations indicate that the magnitude of the temperature jump across the pressure shock agrees approximately with the theoretical calculation. Temperature jump measurements for the coupled second-sound shock imply shock-induced relative velocities, w =v(n)-v(s), on the order of 2.5 m/sec.\r\n\r\nFor initial conditions close to the [lambda]-transition (e.g., T(0) = 2.095\u00b0K), the pressure jump across the first-sound shock is sufficient to cause a change in phase from LHeII to LHeI. This change is experimentally evidenced by detector outputs indicating the absence of the temperature shock in the wake of a sufficiently strong pressure shock." }, { "name": "Koenig, Keith", "degree": "PhD", "year": "1978", "title": "Interference Effects on the Drag of Bluff Bodies in Tandem", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042006-140552", "creators": [ { "name": { "family": "Koenig", "given": "Keith" }, "id": "Koenig-Keith", "display_name": "Koenig, Keith" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EXEV-R625", "abstract": "The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations and from various add-on devices designed to reduce their drag.\r\n\r\nThe object of the present investigation is an axisymmetric configuration which seems to have first been studied by Saunders (1966). A disc of diameter d1 is coaxially placed in front of a flat faced cylinder of diameter d2. For a given ratio d1/d2, there is a value of gap ratio, g*/d2 for which the drag of the system is a minimum. In the most optimum configuration, d1/d2 = 0.75, g*/d2 = 0.375, and the corresponding drag coefficient is 0.01, a remarkable reduction from the value of 0.72 for the cylinder alone. For each value of d1/d2, the minimum drag configuration g*/d2 appears to correspond to a condition in which the separation streamsurface just matches (joins tangentially onto) the rearbody. Support for this idea is furnished by comparison with results derived from free-streamline theory and from flow visualization experiments. However, when g*/d2 exceeds a critical value of about 0.5, the value of CD, while still optimum, is almost an order of magnitude higher than for subcritical optimum gap ratios. The increase seems to be connected with the onset of cavity oscillations.\r\n\r\nMeasurements of the velocity field in the vicinity of the forebody have been made using a frequency-shifted laser-Doppler velocimeter. These measurements indicate an order of magnitude difference in the shear stress along the separation surface between optimum subcritical and supercritical geometries.\r\n\r\nThe drag characteristics of the axisymmetric forebody system are altered by modifying the shape of the component bodies. Modifications that change the conditions at separation from the frontbody, interfere with the cavity flow or effect the flow on the rearbody face can produce significant changes in the forebody drag.\r\n\r\nFor non-axisymmetric geometry (square cross-sections) the separation surface cannot exactly match the rear body and the subcritical minimum values of drag are higher than for circular cross-sections." }, { "name": "Magiawala, Kiran Ramanlal", "degree": "PhD", "year": "1978", "title": "Measurements of Energy Exchange Between Acoustic Fields and Non-Uniform Steady Flow Fields", "advisor": "Culick, Fred E. C.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302006-154249", "creators": [ { "name": { "family": "Magiawala", "given": "Kiran Ramanlal" }, "id": "Magiawala-Kiran-Ramanlal", "display_name": "Magiawala, Kiran Ramanlal" } ], "advisors": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "advisor", "display_name": "Culick, Fred E. C." } ], "committee": [ { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "chair", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Sabersky", "given": "Rolf H." }, "id": "Sabersky-R-H", "role": "member", "display_name": "Sabersky, Rolf H." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "member", "display_name": "Sturtevant, Bradford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TR93-1793", "abstract": "Study of the unsteady burning of solid propellants can be best carried out under widely varying conditions and at relatively inexpensive cost in a simple test device known as the T-burner. This simple configuration is used to observe the spontaneous growth and decay of oscillations. Knowing the losses involved in the system, one can infer the frequency response of the burning surface within the approximations of linearity.\r\n\r\nA significant undertainty in the interpretation of data taken with T-burners arises because very little has been known about some of the acoustics, in particular the influence of the exhaust vent. The present investigation is a study of the influence of a subsonic exhaust vent. The primary apparatus is a resonance tube operated at room temperature with different resonance frequencies of the first longitudinal mode of oscillation. Experiments have been done over ranges of the average Mach number of the flow in the resonance tube, and with vent having different sizes and shapes.\r\n\r\nAccording to the one-dimensional linear stability analysis, the attenuation constant associated with the influence of the exhaust vent is given by the product of four times the resonance frequency of oscillation times the average Mach number of the flow in the resonance tube. The following major conclusions were predicted and verified:\r\n\r\n (i) the vent produces a gain of acoustic energy proportional to the average Mach number of the flow in main resonance tube\r\n\r\n (ii) the gain is proportional to the frequency of the fundamental longitudinal mode\r\n\r\n (iii) the gain is independent of the shape and size of the vent. \r\n\r\nThe influence of the exhaust vent, hence, cannot be neglected in the interpretation of T-burner data." }, { "name": "Wadcock, Alan James", "degree": "PhD", "year": "1978", "title": "Flying-Hot-Wire Study of Two-Dimensional Turbulent Separation on an NACA 4412 Airfoil at Maximum Lift", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-10312006-095330", "creators": [ { "name": { "family": "Wadcock", "given": "Alan James" }, "id": "Wadcock-Alan-James", "display_name": "Wadcock, Alan James" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5TB8-1G75", "abstract": "Hot-wire measurements have been made in the boundary layer, the separated region, and the near wake for flow past an NACA 4412 airfoil at maximum lift. The Reynolds number based on chord was about 1,500,000. Special care was taken to achieve a two-dimensional mean flow. The main instrumentation was a flying hot wire; that is, a hot-wire probe mounted on the end of a rotating arm. The probe velocity was sufficiently high to avoid the usual rectification problem by keeping the relative flow direction always within a range of \u00b130 degrees to the probe axis. A digital computer was used to control synchronized sampling and storage of hot-wire data at closely spaced points along the probe arc. Data were obtained at several thousand locations in the flow field. These data include intermittency, two components of mean velocity, and mean values for three double, four triple, and five quadruple products of two velocity fluctuations. No information was obtained about the third (spanwise) velocity component. The data are available on punched cards in raw form and also in processed form, after use of smoothing and interpolation routines to obtain values on a fine rectangular mesh aligned with the airfoil chord. The data are displayed as contour plots of the fifteen variables." }, { "name": "Berg, Dale Evan", "degree": "PhD", "year": "1977", "title": "Surface Roughness Effects on the Hypersonic Turbulent Boundary Layer", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-11142006-154140", "creators": [ { "name": { "family": "Berg", "given": "Dale Evan" }, "id": "Berg-Dale-Evan", "display_name": "Berg, Dale Evan" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "chair", "display_name": "Kubota, Toshi" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "member", "display_name": "Zukoski, Edward E." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2S44-JV38", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental investigation of the response of a hypersonic turbulent boundary layer to a step change in surface roughness has been performed. The boundary layer on a flat nozzle wall of a Mach 6 wind tunnel was subjected to abrupt changes in surface roughness and its adjustment to the new surface conditions was examined. Both mean and fluctuating flow properties were acquired for smooth-to-rough and rough-to-smooth surface configurations.\r\n\r\nThe boundary layer was found to respond gradually and to attain new equilibrium profiles, for both the mean and the fluctuating properties, some 10 to 25 [...] downstream of the step change. Mean flow self-similarity was the first to establish itself, followed by the mass flux fluctuations, followed in turn by the total temperature fluctuations.\r\n\r\nUse of a modified Van Driest transformation resulted in good correlation of smooth and rough wall data in the form of the incompressible law of the wall. This is true even in the nonequilibrium vicinity of the step for small roughness heights.\r\n\r\nThe present data are found to correlate well with previously published roughness effect data from low and high speed flows when the roughnesses are characterized by an equivalent sand grain roughness height.\r\n\r\nExisting correlations based on low speed data were found to be unsuccessful in predicting the effect of this roughness on the skin friction and velocity profile. The indiscriminate use of low speed roughness effects correlations to predict the effects of roughness on supersonic and hypersonic flows must therefore be regarded as a procedure subject to gross errors.\r\n\r\nSignificant pressure and temperature history effects were observed throughout the boundary layer. The existence of these effects was found to create a nozzle wall boundary layer whose properties were far different than those in a boundary layer on a flat plate in the freestream, raising questions about the validity of simulating the flat plate boundary layer with the nozzle wall boundary layer." }, { "name": "Craig, James Eldon", "degree": "PhD", "year": "1977", "title": "Weak Shocks in Open-Ended Ducts with Complex Geometry", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-11302006-133224", "creators": [ { "name": { "family": "Craig", "given": "James Eldon" }, "id": "Craig-James-Eldon", "display_name": "Craig, James Eldon" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Culick", "given": "Fred E. C." }, "id": "Culick-F-E-C", "role": "member", "display_name": "Culick, Fred E. C." }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" }, { "name": { "family": "Jennings", "given": "Paul C." }, "id": "Jennings-P-C", "role": "member", "display_name": "Jennings, Paul C." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QAFR-PS29", "abstract": "The dynamics of weak shocks in ducts of complex geometry and the sound radiation produced by the reflection of a weak shock from the open end of a duct have been investigated. Duct geometries include expansion chambers with and without inlet or outlet tubes extended and enclosed perforated tubes. Internal and external pressure histories of the interaction of weak shocks with simple muffler elements have been recorded using a standard one-shot shock tube and a resonating shock tube. The excitation shock Mach number ranged from 1.05 to 1.55. Analytical investigations, including a synthesis of existing works on internal weak-shock interactions of an acoustic treatment of the sound radiation produced by weak shock waves, are presented. Combining the above analyses, models for the reduction in radiated sound per unit of incident shock amplitude, as a result of inserting a muffler between the source and the tailpipe exit, are developed.\r\n\r\nFor expansion chambers with and without extensions, the dependence of the transmitted and reflected waves and of the radiated sound on area ratio is compared with predictions. In particular, measured transmission coefficients for expansion chambers agree reasonably well with the predictions for all shock strengths; however, for large area ratios, the predicted sound attenuation is not observed, as waves diffracted at the upstream junction cause more sound to be radiated. For expansion chambers with internal extensions, sound attenuation is increased for low incident shock strengths; while for increasing incident shock strength, the internal transmission characteristics deteriorate, the reducing the sound attenuation.\r\n\r\nFor enclosed perforated tubes, the dependence of the transmitted and reflected waves and of the radiated sound on the perforated area ratio and incident shock strength is compared with predictions. For perforated tubes with infinite enclosure, the transmission and reflection coefficients depend on both incident shock strength and perforated area ratio, as predicted. However, agreement with data is obtained only after inserting a perforated discharge coefficient with the perforated area ratio in the theory. The reduction of sound radiation with perforated area ratio is measured for one incident shock strength and then compared with predictions. For small area ratios, there is agreement but for large area ratios the measurements show that less sound is radiated than predicted. For large area ratios, gradual compressions with smooth fronts (not shock fronts) are transmitted, resulting in less radiated sound. Enclosures have no effect on the sound attenuation for small perforate area ratios; however, as the perforate area ratio increases, the enclosure eventually inhibits further increase in sound attenuation." }, { "name": "Higuchi, Hiroshi", "degree": "PhD", "year": "1977", "title": "Experimental Investigation on Axisymmetric Turbulent Wakes with Zero Momentum Defect", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-11282006-111227", "creators": [ { "name": { "family": "Higuchi", "given": "Hiroshi" }, "id": "Higuchi-Hiroshi", "display_name": "Higuchi, Hiroshi" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/121B-VN50", "abstract": "An experimental investigation of a turbulent axisymmetric wake with zero momentum defect was carried out. The experiment was conducted in a low speed wind tunnel with a circular tube mounted parallel to the stream. A controlled amount of air was injected into the stream at the end of the model to cancel the drag produced by the turbulent boundary layer on the model. The measurements on the mean flow and the fluctuation quantities were carried up to 90 diameters downstream. By adjusting the strength of the injections the behavior of the pure wake, the co-flowing jet and the matched injection were examined, and the self similar properties both in the mean velocity and the turbulent intensity were found to exist in these cases. Rapid decays off the centerline velocity and the maximum turbulent intensity were observed in the matched injection case. The effect of the initial condition was studied by artificially thickening the boundary layer on the model, and it was observed that the wake relaxes into the final decay law sooner. The unmatched cases, both strong and weak injections, were also investigated; the strong injection case was found to relax into the self-similar weak jet profile and the weaker injection case was observed to approach to the similar wake profile.\r\n\r\nFlow visualization was conducted on the plane of symmetry and the entrainment process by the engulfing large eddies was visualized in the pure wake and in the co-flowing jet.\tA lack of large rotations was observed in the matched injection case and the lack of production of turbulent energy was attributed to the faster decay of the flow properties." }, { "name": "Konrad, John Harrison", "degree": "PhD", "year": "1977", "title": "An Experimental Investigation of Mixing in Two-Dimensional Turbulent Shear Flows with Applications to Diffusion-Limited Chemical Reactions", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-10132005-105700", "creators": [ { "name": { "family": "Konrad", "given": "John Harrison" }, "id": "Konrad-John-Harrison", "display_name": "Konrad, John Harrison" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZE81-ZM79", "abstract": "The extent of molecular mixing in several two-dimensional free turbulent shear flows was measured using a concentration probe with a frequency response of 100 kHz and a spatial resolution of 0.1 mm. The flows investigated were (i) a shear layer in which the gases on either side of the layer are of unequal density, (ii) a shear layer in which the gases on either side of the layer are of equal density, and (iii) a wake in which the gases on either side of the wake are of unequal densities. The extent of mixing was measured as a function of Reynolds number for the first case.
\r\n\r\nIt was found that at a critical Reynolds number the extent of molecular mixing sharply increased (25%). Power spectral density curves of the concentration time histories also indicated a marked increase in the high frequency fluctuations above this Reynolds number. A shadowgraph investigation of this phenomenon revealed that three-dimensional Taylor-type vortices whose axes of rotation are basically in the flow direction exist in the flow in addition to the two-dimensional large structures previously observed. These Taylor vortices were found to be unstable above the critical Reynolds number and were producing the increase in molecular mixing. The growth and development of the two-dimensional large structures were found to be basically unaffected by this instability. It is proposed that the fully developed turbulence of shear flows is maintained by a combination of the development of the large structures and of the coupling between the large structures and these unstable Taylor vortices.
\r\n\r\nThese data were also used to predict results for shear flows in which diffusion-limited chemical reactions have been incorporated.
\r\n" }, { "name": "Shah, Piyush Chimanlal", "degree": "PhD", "year": "1977", "title": "Estimation of Properties in Petroleum Reservoirs", "advisor": "Gavalas, George R.; Seinfeld, John H.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042006-081055", "creators": [ { "name": { "family": "Shah", "given": "Piyush Chimanlal" }, "id": "Shah-Piyush-Chimanlal", "display_name": "Shah, Piyush Chimanlal" } ], "advisors": [ { "name": { "family": "Gavalas", "given": "George R." }, "id": "Gavalas-G-R", "role": "advisor", "display_name": "Gavalas, George R." }, { "name": { "family": "Seinfeld", "given": "John H." }, "id": "Seinfeld-J-H", "role": "advisor", "display_name": "Seinfeld, John H." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7DC3-EX92", "abstract": "The determination of parameters in a dynamical system, on the basis of noisy observations of its state is variously known as parameter estimation, identification or the inverse problem. In this work, the determination of porous rock property distribution in a petroleum reservoir using the production rate records and observed pressures (the history matching problem) is considered.\r\n\r\nThe history matching problem is inherently underdetermined because of the large number of unknown parameters relative to the available data. The number of unknowns can be reduced by representing the distributions by a small number of parameters (parameterization). The commonly used zonation approach involves a parameterization, but introduces a considerable modeling error. In chapter 1, Bayesian estimation theory is extended to history matching as an alternative to zonation; it is sought to alleviate the underdeterminacy through specification of a priori statistical information about the unknown parameters. Application of Bayesian estimation and zonation to the problem of porosity and permeability estimation in a one-dimensional single-phase reservoir indicates that the former yields superior estimates; this holds true even when the prior statistics involve large errors. The application of the conjugate gradient and the Gauss-Newton (or Marquardt's) algorithms for history matching is investigated, and the numerical effort for zonation and Bayesian estimation in one- and two-dimensional reservoirs is estimated in detail.\r\n\r\nIn chapter 2, analytic expressions are derived for the sensitivities of an observed oil pressure to small, arbitrary changes in the porosity and permeability distributions in a one-dimensional reservoir. The results indicate that highly oscillatory components of either have very small influence on the pressure and thus cannot be determined by history matching. Further, the dependence of all the observed pressures on the unknown parameters is linearized, for small deviation, about two reference property distributions. The linear relation is analyzed to yield quantitative information concerning the statistical properties of the problem. Iterative corrections in the history matching algorithms are identified with various pseudo-inverses of the linear relation, thus explaining the properties of the resulting estimates. The nature of the linear relation is found to be not strongly dependent on the reference property distributions used for linearization; thus such analysis can be performed prior to estimation. It is discussed how the linearized analysis can be used to determine the determinacy of any given parameterization.\r\n\r\nThe information derived from the linearized analysis and that in the a priori statistics is synthesized in chapter 3 to predict covariances for the zonation and Bayesian estimates. Since the results of the linearized analysis depend only weakly on the reference distribution, the predicted covariances are valid for a class of reservoirs having \"true\" property distributions with identical prior statistics. A good agreement is found when the predicted variances are compared with actual mean square estimate errors in simulations with four distributions with given prior statistics. The sensitivity of the estimates and their covariance to changes and errors in the specification of the prior statistics are investigated in considerable detail. The determination of zonation with smallest trace of estimate covariance for a given problem is considered. The design of Marquardt's algorithm to yield the smallest expected total estimate error for a given zonation is discussed." }, { "name": "Wojcik, Gregory Lynn", "degree": "PhD", "year": "1977", "title": "Self-Similar Elastodynamic Solutions for the Plane Wedge", "advisor": "Sechler, Ernest Edwin; Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11132006-080224", "creators": [ { "name": { "family": "Wojcik", "given": "Gregory Lynn" }, "id": "Wojcik-Gregory-Lynn", "display_name": "Wojcik, Gregory Lynn" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "chair", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "member", "display_name": "Knowles, James K." }, { "name": { "family": "Miklowitz", "given": "Julius" }, "id": "Miklowitz-J", "role": "member", "display_name": "Miklowitz, Julius" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E1D3-0T11", "abstract": "Wave propagation in a two-dimensional elastic wedge is fundamental to a large class of problems in elastodynamic theory, however until now analytical solutions to all but certain degenerate cases were unknown. In this thesis a general elastodynamic solution is derived for the wedge in a state of plane strain. Surface tractions are, restricted to uniform normal and shear loads spreading from the wedge vertex at constant velocity. The geometry and loading then allow self-similar solutions of the governing differential equations and boundary conditions in hyperbolic and elliptic domains. Hyperbolic solutions are found in terms of the elliptic solutions by the method of characteristics, while elliptic solutions are reduced using analytic function theory to two independent Fredholm integral equations of the second kind in one dimension. Although numerical solutions are beyond the scope of the investigation, the integral equations are solvable by standard techniques. Such solutions can be used to solve a number of plane elastodynamic problems involving an edge." }, { "name": "Bofah, Kwasi Kete", "degree": "PhD", "year": "1976", "title": "A Study of the Trailing Vortices Behind a Ring Wing", "advisor": "Liepmann, Hans Wolfgang; Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11092006-132109", "creators": [ { "name": { "family": "Bofah", "given": "Kwasi Kete" }, "id": "Bofah-Kwasi-Kete", "display_name": "Bofah, Kwasi Kete" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/050Z-4228", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe flow field of a laminar vortex wake behind a ring wing was investigated. Experiments were conducted in the GALCIT Low Speed Water Channel, using laser Doppler velocimetry techniques to measure vertical and axial velocity components in the trailing vortex wake. A thin cylindrical ring wing model was tested at various axial angles of attack and free stream velocities. Velocity profiles were measured at several downstream stations from the trailing edge to 45 wing diameters downstream.\r\n\r\nThe inviscid roll-up of the trailing vortex sheet shed by a ring wing was numerically examined. A line vortex representation was used to calculate the evolution of the initially cylindrical vortex sheet. The vortex sheet was found to distort in shape and then smoothly roll up into a pair of doubly connected spirals whose centers originate from approximately the center of gravity of vorticity in the upper quadrants of the ring wing's circular trailing edge. (This origin is at an angle of 38\u00b0 measured from the horizontal wing diameter.)\r\n\r\nThe experimental and flow visualization results are consistent with the numerical data and show that a pair of counter-rotating vortices do develop from the rolling up of the vortex sheet shed by a ring wing in a nonaxial flow. The vortices trail, downstream of the wing, with their vorticity centroids spaced by [...]/4 wing diameters.\r\n\r\nSaffman and Moore's theory of axial flow in laminar trailing vortices was adapted and found to be in reasonable agreement with the experimental results. The flow field near the trailing edge was found to be in fair agreement with Weissinger's inviscid calculations." }, { "name": "Cantwell, Brian Joseph", "degree": "PhD", "year": "1976", "title": "A Flying Hot Wire Study of the Turbulent Near Wake of a Circular Cylinder at a Reynolds Number of 140,000", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-085806", "creators": [ { "name": { "family": "Cantwell", "given": "Brian Joseph" }, "id": "Cantwell-Brian-Joseph", "display_name": "Cantwell, Brian Joseph" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QKGW-CA84", "abstract": "An experiment was performed in the GALCIT 10-foot wind tunnel to study the flow in the near wake of a circular cylinder at a Reynolds number of 140,000.\r\n\r\nThe main objective of this investigation was to study the phenomenology of the processes of vortex formation and transport in the near wake, at a Reynolds number sufficiently high to insure a fully turbulent wake, but low enough to insure a laminar separation. The latter requirement anticipates the eventual use of the results as a test case for advanced calculation codes.\r\n\r\nMuch current experimental work on turbulent flows is concerned with large, coherent, organized vortex structures which have a relatively long lifetime and which account for much of the transport of mass, momentum and heat in turbulent shear flows. High Reynolds number flow past a cylinder is one case where such structures dominate.\r\n\r\nThe apparatus developed for measuring this flow consists of x-array hot wire probes mounted on the ends of a pair of whirling arms. In such a flow, where large changes in flow direction occur, a fixed hot wire would rectify the velocity signal and give ambiguous results. However, by applying a large enough bias velocity to the wires, the relative velocity vector can be maintained within the [+/-]30 degree range of sensitivity of the x-array. One useful property of this technique is that a rotation of the arms in a uniform flow applies a wide range of relative flow angles to the x-arrays, making them inherently self-calibrating in pitch.\r\n\r\nThe most important element of the instrumentation concomitant to the flying hot wire is a computer controlled data acquisition system which is slaved to the position of the rotating arm and which manages, monitors, edits and records the vast profusion of data which is continuously poured out by the device. A fast sensor responding to model surface pressure was used to generate a signal synchronized with the vortex-shedding process. This signal was recorded along with the hot wire data and used later to sort the data into populations having the same phase. Ensemble averages conditioned this way yield an average picture of the instantaneous flow field in which the vortices are frozen as they would be in a photograph.\r\n\r\nIn addition to the conventional velocity, pressure and stress data, results are presented which show the instantaneous (in the sense of an average at constant phase) velocity, intermittency, vorticity and stress fields as a function of phase for the first six diameters of the near wake.\r\n\r\nIn the present study, the Reynolds stresses are broken up into the contribution from large scale periodic motions and that from background or random turbulence, and, when dissected in this way, permit an enlightening look at the anatomy of this turbulent flow. Laid against the background of the instantaneous velocity, vorticity and intermittency, the stresses in the near wake emerge as a concatenation of peaks and valleys, some the result of strong induced motions in the outer flow which cause free stream fluid to move rapidly inward toward the center of the wake, others the result of the random motions of the background turbulence." }, { "name": "Cosner, Raymond Robert", "degree": "PhD", "year": "1976", "title": "Experiments on Thin Airfoils Spanning a Transonic Shear Flow", "advisor": "Zukoski, Edward E.; Rannie, W. Duncan", "url": "https://resolver.caltech.edu/CaltechETD:etd-11292006-134453", "creators": [ { "name": { "family": "Cosner", "given": "Raymond Robert" }, "id": "Cosner-Raymond-Robert", "display_name": "Cosner, Raymond Robert" } ], "advisors": [ { "name": { "family": "Zukoski", "given": "Edward E." }, "id": "Zukoski-E-E", "role": "advisor", "display_name": "Zukoski, Edward E." }, { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YJNQ-ZP43", "abstract": "A wind tunnel was built at the California Institute of Technology to provide two uniform coflowing streams at Mach numbers of 0.6 and l.4 with a plane mixing layer in between. Preliminary studies were made of this free shear layer, indicating a region of self-similar behavior and general agreement in growth rate with previous studies.\r\n\r\nA program of experimental and theoretical work was completed in which wedges were installed in the supersonic stream to create shock waves incident on the shear layer. Similar studies were performed with wedges in the subsonic stream. Good agreement was found between the analytic first-order theory and experiment in showing that the upstream propagation of pressure disturbances ahead of their source in the uniform subsonic stream is limited to about 0.6 subsonic layer thicknesses.\r\n\r\nThe problem of an airfoil in spanwise-varying transonic shear was studied experimentally with the goal of understanding the interactions in the shear region between the supersonic and subsonic streams, especially in terms of deviations from quasi-two-dimensional behavior. The effect of modest angle of attack was also examined." }, { "name": "Shea, John Richard, III", "degree": "PhD", "year": "1976", "title": "A Chemical Reaction in a Turbulent Jet", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11282006-152216", "creators": [ { "name": { "family": "Shea", "given": "John Richard, III" }, "id": "Shea-John-Richard-III", "display_name": "Shea, John Richard, III" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "chair", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Shair", "given": "Fredrick H." }, "id": "Shair-F-H", "role": "member", "display_name": "Shair, Fredrick H." }, { "name": { "family": "Wulf", "given": "Oliver Reynolds" }, "id": "Wulf-O-R", "role": "member", "display_name": "Wulf, Oliver Reynolds" }, { "name": { "family": "Setchell", "given": "Robert E." }, "id": "Setchell-R-E", "role": "member", "display_name": "Setchell, Robert E." }, { "name": { "family": "Kaufmann", "given": "Aaron" }, "id": "Kaufmann-A", "role": "member", "display_name": "Kaufmann, Aaron" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BJQT-9A50", "abstract": "The turbulent mixing and subsequent chemical reaction of gases is an essential part of many technological processes ranging from gas furnaces to chemical lasers. Surprisingly, there is very little information, either theoretical or experimental, about the actual rate of the chemical reaction in such processes. Generally the chemical kinetics are well understood, but the process of turbulent mixing is not. Many measurements of mixing in turbulent jets have been made in the past, but they have generally failed to distinguish essentially unmixed gas in the turbulent mixing zone from gas which is mixed on a molecular scale. Knowledge of where turbulent fluid is mixed on a molecular scale is critical for predicting chemical reaction rates in the flow.
\r\n\r\nIn this experiment the rate of a chemical reaction in an axisymmetric turbulent jet is studied, and the results are used to determine the rate of molecular mixing in the jet. A turbulent jet containing dilute ozone in an inert mixture of nitrogen and oxygen flows into a stagnant tank of nitric oxide and nitrogen. When the gases mix on a molecular scale, the ozone and nitric oxide rapidly react to produce oxygen and nitrogen dioxide. The rate at which the mixing and chemical reaction proceeds is determined by using an ultraviolet light absorption technique to measure the time averaged ozone concentration at points throughout the jets mixing zone.
\r\n\r\nThe experiment establishes a criterion for determining when a reaction of known chemical kinetics is sufficiently rapid that chemical nonequilibrium has a negligible effect on the mean reactant profile. When a reacting jet satisfies this criterion for equilibrium chemistry, the reactant profiles are found to be independent of jet Reynolds numbers from 4,000 to 32,000 based on the nozzle diameter.
\r\n\r\nIn addition, a mixing fraction, η, is defined to measure the extent of local molecular scale mixing independently of a chemical reaction occurring in the jet. The fraction assumes values of unity in the unmixed primary jet, zero in unmixed ambient fluid, and intermediate fractions for mixtures of all proportions. Points on nonreacting jet profiles are related to time averages of η. A limiting highly reacting ozone profile, found when a large excess of nitric oxide is present in the ambient fluid, is related to the time average of an intermittency function, J(η), defined equal to unity when η is within a specified neighborhood of one and zero elsewhere. Thus the experimental measurements of ozone profiles are directly related to the statistics of molecular scale mixing in the jet.
" }, { "name": "Chang, Liang-Chou", "degree": "PhD", "year": "1975", "title": "Theoretical Investigations of Turbulent Boundary Layer Over a Wavy Surface", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechTHESIS:11122015-115400945", "creators": [ { "name": { "family": "Chang", "given": "Liang-Chou" }, "id": "Chang-Liang-Chou", "display_name": "Chang, Liang-Chou" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ANTA-K262", "abstract": "The important features of the two-dimensional incompressible\r\nturbulent flow over a wavy surface of wavelength comparable with the boundary\r\nlayer thickness are analyzed.
\r\n\r\nA turbulent field method using model equation for turbulent\r\nshear stress similar to the scheme of Bradshaw, Ferriss and Atwell\r\n(1967) is employed with suitable modification to cover the viscous\r\nsublayer. The governing differential equations are linearized based\r\non the small but finite amplitude to wavelength ratio. An orthogonal\r\nwavy coordinate system, accurate to the second order in the amplitude\r\nratio, is adopted to avoid the severe restriction to the validity of\r\nlinearization due to the large mean velocity gradient near the wall.\r\nAnalytic solution up to the second order is obtained by using the method\r\nof matched-asymptotic-expansion based on the large Reynolds number\r\nand hence the small skin friction coefficient.
\r\n\r\nIn the outer part of the layer, the perturbed flow is practically\r\n\"inviscid.\" Solutions for the velocity, Reynolds stress and also the\r\nwall pressure distributions agree well with the experimental measurement. \r\nIn the wall region where the perturbed Reynolds stress plays an\r\nimportant role in the process of momentum transport, only a qualitative\r\nagreement is obtained. The results also show that the nonlinear\r\nsecond-order effect is negligible for amplitude ratio of 0.03.\r\nThe discrepancies in the detailed structure of the velocity, shear\r\nstress, and skin friction distributions near the wall suggest modifications\r\nto the model are required to describe the present problem.
" }, { "name": "Kulkarny, Vijay Anand", "degree": "PhD", "year": "1975", "title": "An Experimental Investigation on Focussing of Weak Shock Waves in Air", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-09042007-082903", "creators": [ { "name": { "family": "Kulkarny", "given": "Vijay Anand" }, "id": "Kulkarny-Vijay-Anand", "display_name": "Kulkarny, Vijay Anand" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KK7V-6827", "abstract": "The behavior of focussing weak shock waves is experimentally investigated with a view to observe and understand the processes occurring near the focus, especially the processes that control the maximum amplitude. Concave reflectors are used against the endwall of a large 17\" diameter shock tube, to focus the plane incident shock. Reflectors producing line and point foci, and cusped and smooth caustics are examined for incident shock Mach numbers ranging between 1.005 to 1.5. The flowfield is observed with spark shadowgraphs to visualize the motion of various wavefronts. Pressure histories measured at various points in the flow with miniature piezoelectric gauges provide additional information about the various processes occurring near the focus.\r\n\r\nShadowgraphs show that for weak shocks, the observed foci are predominantly nonlinear, even though away from the focus, the shockfronts appear to be almost acoustic. Thus a weak shockfront, after the focus, crosses itself and forms a loop, which is an essential feature of acoustic wavefronts. Nonetheless, at the focus, distortion in the geometry of the fronts due to nonlinear effects is very prominent. Inherently nonlinear phenomena, such as formation of three-shock intersections, lead to foci of finite size, in which, as the pressure measurements show, the amplitudes are finite.\r\n\r\nThe amplitude dependence of these phenomena confirms that they are basically nonlinear. The geometrical distortion and the focus are larger for stronger shock waves, and the maximum amplification is smaller. Further, when the distortion becomes significant compared to the size of the initial shockfront, a transition occurs in the geometry of the focussed shockfront. In this case, the focussed front does not cross and remains \"unlooped\", which is consistent with the nonlinear behavior predicted by shock dynamics.\r\n\r\nThe transition in the geometry of the wavefronts is related to the behavior of the three-shock intersections formed near the focus. In fact, it is shown that the occurrence of crossed or uncrossed shockfronts is very parallel to the occurrence of regular or Mach reflection, respectively, in the case of a shock diffracted by a wedge. (The reflecting wedge surface corresponds to the axis of symmetry in a focussing process.) The dependence on the steepness of the approaching waves is also similar in the two cases; rapid convergence of waves suppresses nonlinear effects, whereas in a slow convergence, nonlinear effects gain prominence.\r\n\r\nThe pressure histories at various locations, when correlated with the waves occurring there, show that nonlinear diffraction processes are very important. In fact, it is shown that the formation of the three-shock intersection occurs due to nonlinear distortion and breaking of a compressive diffraction, and that, in the focus, the limiting and reduction of the peak amplitude occurs by a diffracted expansion overtaking the shock due to nonlinear effects." }, { "name": "Moronval, Marc Jules", "degree": "PhD", "year": "1975", "title": "Optimization of Arch and Shell Structures", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechTHESIS:09272010-093909310", "creators": [ { "name": { "family": "Moronval", "given": "Marc Jules" }, "id": "Moronval-Marc-Jules", "display_name": "Moronval, Marc Jules" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EMCP-7017", "abstract": "Structural optimization of structures with respect to their shape and thickness distribution is studied using a variational approach. The behavioral constraint is either the state of stress or the stiffness.\r\nThe boundary value problems, derived using Optimal Control theory, are solved with the parallel shooting technique. For statically determinant arches subjected to a uniform pressure, the contribution of the shear force is included in the behavioral constraints to prevent the problems from being singular. For the case of membrane shells of revolution supporting a combined pressure and end traction loading case, solutions were obtained up to a critical value of the load coefficient. A physical interpretation of the singularity is obtained by including the possibility\r\nof discrete rings in the formulation. A set of optimality conditions for shells of revolution described by the bending theory, satisfying a stiffness constraint, is derived. The problem is found to be ill posed when the shear contribution is not included in the structural operator.\r\n" }, { "name": "Sarohia, Virendra", "degree": "PhD", "year": "1975", "title": "Experimental and Analytical Investigation of Oscillations in Flows over Cavities", "advisor": "Roshko, Anatol; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-05032007-131245", "creators": [ { "name": { "family": "Sarohia", "given": "Virendra" }, "id": "Sarohia-Virendra", "display_name": "Sarohia, Virendra" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WJC1-X942", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nIn this study, an analytical and experimental approach has been used to investigate the phenomenon of flow induced oscillations in cavities. Laminar axisymmetric flows over shallow cavities at low subsonic speeds were experimentally investigated using constant temperature hot-wire anemometry. This study comprised the following: study of the effect of the freestream and cavity configuration on onset of cavity oscillations; measurements of cavity shear layer under a wide range of cavity and flow configurations, and the distribution of the phase of the propagating disturbances during both first and second mode of cavity oscillation for a fixed Reynolds number at the upstream corner. Both motion and instant pictures of cavity shear flow, visualized by smoke injection, were obtained. Experiments were also done to investigate the effect of artificial excitation and of mass injection on the onset of cavity oscillations.\r\n\r\nThe present study indicates that the cavity depth has little effect on oscillations in shallow cavities, except when the depth is of the order of the thickness of the cavity shear flow. For such cavity configurations, measurements indicate a strong stabilizing effect of depth on laminar cavity shear layer. Results of motion pictures and hot-wire surveys of the cavity shear layer show that, close to the downstream cavity corner, large lateral motion of the shear layer occurs, which results in a periodic shedding of vortices at a frequency of cavity oscillations. Mean velocity measurements show growth rates as high as [...] 0.022 where [...] is the shear layer momentum thickness and x is the streamwise coordinate. These are attributed to strong imposed velocity fluctuations on the flow, by the oscillating cavity system.\r\n\r\nPhase measurements indicate that the disturbances propagate at a constant phase speed through the cavity shear layer. The wave length of the propagating disturbance bears an approximate integral relation to cavity width, in each mode of cavity oscillation given by [...] where b is the cavity width, [...] the wave length of the propagating disturbance and N is an integer, which takes values 0, 1, 2, ... etc. depending upon the mode of oscillation.\r\n\r\nStability calculations of the measured mean velocity profile were made by numerically integrating the governing equation of motion. These numerical results were used to compute the phase and the integrated amplification of the growing disturbances, through the cavity shear layer. Finally, the mode of cavity oscillation can be predicted for a given cavity flow by studying simultaneously the phase and integrated amplification of various disturbance frequencies through the shear layer and applying the mode relation." }, { "name": "Smith, Gordon Carl", "degree": "PhD", "year": "1975", "title": "An Experimental Investigation of the Dynamic Fracture of a Brittle Material", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042006-082245", "creators": [ { "name": { "family": "Smith", "given": "Gordon Carl" }, "id": "Smith-Gordon-Carl", "display_name": "Smith, Gordon Carl" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/35AK-XJ82", "abstract": "A new method of experimentally investigating the behavior of a crack in a stress wave environment has been developed and used to study the initial stages of dynamic crack propagation in Homalite 100, a polyester. The experimental method, which employs an electromagnetic loading device, permitted the application of pressure pulses to the surfaces of an 18 inch crack. The amplitude (51 psi to 1020 psi) and duration (~ 200 \u00b5sec) of the pulses were highly repeatable. The experimental configuration simulates, in the vicinity of the crack tip, a tension wave impacting a stationary semi-infinite crack in an infinite two-dimensional body where the wave front is parallel to the crack. A high speed framing camera, synchronized with the loading device, was used to record the time required for the crack to begin to propagate and its subsequent extension and velocity. The experimental results were analyzed within the bounds of linearly elastic fracture mechanics and a correlation was made between the dynamic stress intensity factor and the time at which the cracks began to propagate. It was found that the critical stress intensity factor increased dramatically with increasing loading rates at very high rates, which contrasts with quasi-static loading where the critical stress intensity factor decreases with an increasing loading rate. A simple model suggests that temperature effects at the crack tip may account for the observed increase in the critical stress intensity factor at high loading rates. Crack branching of running cracks was observed but unlike previous reports of branching, the cracks in this study branched mostly into three separate fast-running cracks." }, { "name": "Knight, Doyle Dana", "degree": "PhD", "year": "1974", "title": "An Analytical Investigation of Turbulent Flow Over a Wavy Boundary", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-151736905", "creators": [ { "name": { "family": "Knight", "given": "Doyle Dana" }, "id": "Knight-Doyle-Dana", "display_name": "Knight, Doyle Dana" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T9PD-TM13", "abstract": "The linearized, two-dimensional flow of an incompressible fully turbulent fluid over a sinusoidal boundary is solved using the method of matched asymptotic expansions in the limit of vanishing skin friction.\r\nA phenomenological turbulence model due to Saffman\r\n(1970, 1974) is utilized to incorporate the effects of the wavy boundary on the turbulence structure.\r\nArbitrary lowest order wave speed is allowed in order to consider both the stationary wavy wall, and the water wave moving with arbitrary positive or negative velocity.\r\nGood agreement is found with measured tangential velocity profiles and surface normal stress coefficients. The phase shift of the surface normal stress exhibits correct qualitative behavior with both positive and negative wave speeds, although predicted values are low.\r\n" }, { "name": "Rizk, Magdi Hanna", "degree": "PhD", "year": "1974", "title": "Nonsteady Fluid Mechanics of Vehicles in Tubes", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-152813759", "creators": [ { "name": { "family": "Rizk", "given": "Magdi Hanna" }, "id": "Rizk-Magdi-Hanna", "display_name": "Rizk, Magdi Hanna" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8PJF-8H92", "abstract": "A theoretical study is performed to investigate the drag\r\nexperienced by vehicles travelling in tubes, the pressure distribution and flow velocities resulting from the vehicle's motion. The study deals with both the cases of vehicles accelerating from rest in the tube and vehicles entering a tube at finite speed. The effect of having a vent in the tube is also studied.\r\n\r\nThe unsteady compressible equations are used to describe\r\nthe flow in the tube. Before the boundary layer fills the tube, an inviscid core-boundary layer formulation is used. However, it is found that the simpler one-dimensional formulation is adequate for describing the flow in the tube. The quasi-steady near-field assumption agrees well with the unsteady near-field solution except for the initial period of low velocities.\r\n\r\nThe solution is compared to that in which compressibility\r\nis neglected. It is found that the effect of compressibility is not large for short tubes, low blockage ratios and small velocities. However, its importance increases as the values of these parameters increase. Compressibility may not be neglected during the initial period in the case of vehicles entering tubes at finite speeds.\r\n" }, { "name": "Uehara, Sachio", "degree": "PhD", "year": "1974", "title": "Theoretical Investigation of Minimum Time Loop Maneuvers of Jet Aircraft", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-111949185", "creators": [ { "name": { "family": "Uehara", "given": "Sachio" }, "id": "Uehara-Sachio", "display_name": "Uehara, Sachio" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZTAZ-J396", "abstract": "Minimum time loop maneuvers of high performance jet aircraft have been investigated by means of the calculus of variations. A number of simplifying assumptions have been made in the atmospheric conditions, aerodynamic parameters, and the number of controls and their upper and lower bounds, in order to obtain general features and basic characteristics of the problem. The optimal control (lift coefficient and thrust) has been determined as a function of the state variables and Lagrange multipliers. It is found that subarcs with variable thrust, or with variable lift coefficient and minimum thrust do not occur on time optimal paths. Possible transitions among the five optimal subarcs have been established by applying the corner conditions of variational calculus. These relationships are applicable to any minimum time maneuver in the vertical plane. The effects of the magnitudes of maximum lift coefficient and maximum thrust on the control program, maneuver time, final speed, and final. horizontal distance for minimum time loop maneuvers are explored through numerical computation. It is found that the control history in lift and thrust and the minimum time required for a loop maneuver depend strongly on the magnitudes of maximum lift coefficient and maximum thrust. A limited numerical exploration using more realistic aerodynamic and atmospheric parameters and a state-dependent maximum thrust yielded results in qualitative agreement with the more extensive analysis based on simplified parameters. Normal acceleration constraints are analyzed by considering the maximum lift coefficient to be a function of altitude and speed. New design criteria for the inlet, duct and engine are suggested by consideration of the problem of engine surge." }, { "name": "Bhatia, Prem", "degree": "PhD", "year": "1973", "title": "Buckling of Imperfect Circular Cylindrical Shells", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechTHESIS:08302010-162439924", "creators": [ { "name": { "family": "Bhatia", "given": "Prem" }, "id": "Bhatia-Prem", "display_name": "Bhatia, Prem" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FS7M-8D46", "abstract": "In this project, the buckling of imperfect circular cylindrical shells under uniform axial compression has been investigated. The imperfection considered is prismatic and is in the form of flat spots along the complete length of the shell. The problem is solved by considering it as an interaction problem between curved and flat panels. Shell equations are satisfied in the curved portions of the shell while flat plate equations are used in the flat spot regions. At the common edge between two adjacent panels, forces and displacements are matched to arrive at the eigenvalue problem for the critical load of the shell. Two flat spot configurations have been studied. In the first case, a single flat spot along the complete length of the shell is considered. Curved and flat panels join to form sharp corners at their common edges. Numerical results are presented to show the effect of the width of the flat spot, thickness ratio and length to radius ratio of the shell, on the buckling load of the shell. In the second case, the imperfection is in the form of two or more identical flat spots distributed uniformly along the circumference of the shell. Between two consecutive flat spots is a uniform radius cylindrical panel joining smoothly to the flat panels. This analysis is valid for any integral number of identical flat spots. Numerical results are presented for 2, 3, 4, 8-flat spots. The effect of width of flat spots, radius of curved panels, thickness ratio of the shell has been investigated. The results, presented in this paper, clearly demonstrate a very significant reduction in the buckling load of the shell below its classical value. This is true even in the presence of relatively small flat spots. This reduction becomes more severe with a decrease in the thickness of the shell or an increase in the width of the flat spot. " }, { "name": "Cummings, John Chester", "degree": "PhD", "year": "1973", "title": "I. Development of a Cryogenic Shock Tube. II. Experimental Investigation of the Interaction of a Shock Wave with Liquid Helium and I and II", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechTHESIS:10072010-095321580", "creators": [ { "name": { "family": "Cummings", "given": "John Chester" }, "id": "Cummings-John-Chester", "display_name": "Cummings, John Chester" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AM0F-DB64", "abstract": "I. Development of a cryogenic shock tube
\r\n\r\nA cryogenic shock tube has been developed as a tool for research in fluid mechanics and low temperature physics. The shock tube is designed to operate with the test section immersed in a cryogenic liquid. A unique diaphragm changing mechanism makes this shock tube an economical and practical device.
\r\n\r\nThere are several advantages in operating a shock tube at cryogenic temperatures. Shock waves of very large Mach number can be produced. The flow field can be accurately calculated using ideal shock tube - perfect gas theory. Boundary layer effects are decreased so that long test times are possible.
\r\n\r\nThe cases which have been studied are test gas temperatures of 300, 77, 4.2, and 2.3\u00b0K. Helium is used as both test and driver gas. The largest Mach numbers which have been observed range from 2.4 at 300\u00b0K to 32 at 2.3\u00b0K (several runs at 1.46\u00b0K have produced Mach 40 shocks). As the test gas temperature is decreased the observed Mach numbers approach those calculated using the ideal shock tube equation. The observed test times can be interpreted using laminar or turbulent boundary layer theory if the effects of shock formation distance and wall temperature rise are taken into account.
\r\n\r\nAs a laboratory tool the cryogenic shock tube may be applied in many areas and modified for use in even more. Large Mach number shocks and large Reynolds number flows can be produced with this device. The rapid increase in temperature and pressure across a shock wave is useful for studies of sublimation, evaporation, or chemical reactions. Quantum mechanical effects in cryogenic materials, superconductors, or superfluid helium can also be investigated.
\r\n\r\nII. Experimental investigation of the interaction of a shock wave with liquid helium I and II
\r\n\r\nThe flow field produced by a shock wave reflecting from a helium gas -liquid interface has been investigated using a new cryogenic shock tube. Incident and reflected shock waves have been observed in the gas; transmitted first and second sound shocks have been observed in the liquid. Wave diagrams have been constructed to compare the data to theoretical wave trajectories. Qualitative agreement between data and theory has been shown. Quantitative differences between data and theory indicate a need for further analysis of both the gas-liquid interface and the propagation of nonlinear waves in liquid helium.
\r\n\r\nThis work is essentially a first step in the experimental investigation of a very complex nonequilibrium state. The well controlled jump in temperature and pressure across the incident shock wave provides unique initial conditions for the study of dynamic phenomena in superfluid helium.
\r\n\r\nThe results clearly demonstrate the usefulness of the cryogenic shock tube as a research tool.
" }, { "name": "Ikawa, Hideo", "degree": "PhD", "year": "1973", "title": "Turbulent Mixing Layer Experiment in Supersonic Flow", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechTHESIS:09272010-154352889", "creators": [ { "name": { "family": "Ikawa", "given": "Hideo" }, "id": "Ikawa-Hideo", "display_name": "Ikawa, Hideo" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4MBK-D880", "abstract": "A supersonic, two-dimensional, turbulent free mixing layer\r\nwas experimentally investigated in the GALCIT Supersonic wind tunnel to clarify the Mach number dependence of this flow field. The tests were conducted with a nominal Mach number of 2.47 with an ambient stagnation condition. Self similarities of the mean and turbulent flow fields were established.\r\n\r\nThe mean and fluctuating flow measurements were made. In\r\naddition, the actual entrainment rate of the turbulent free mixing layer through the low speed interface was also measured and it was verified that this quantity equals the growth rate of momentum thickness. The spreading rate, the entrainment rate and the maximum shear stress were appreciably smaller than the respective incompressible\r\nvalues. Velocity profile scaled to an incompressible form.\r\nby a linear transformation of the lateral coordinate. The maximum streamwise velocity fluctuation was approximately 1/3 of the incompressible value. The flow field of turbulent free mixing layer was found to be highly dependent upon the supersonic compressibility\r\neffect.\r\n" }, { "name": "Lee, Peter Hoong-Yee", "degree": "PhD", "year": "1973", "title": "An Investigation of Collisionless Plasma Beam Interaction with a Nonhomogeneous Magnetic Field", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechTHESIS:08262010-104719284", "creators": [ { "name": { "family": "Lee", "given": "Peter Hoong-Yee" }, "id": "Lee-Peter-Hoong-Yee", "display_name": "Lee, Peter Hoong-Yee" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XNF6-9D90", "abstract": "The interaction between a rarefied flowing plasma and an externally imposed nonhomogeneous magnetic field is studied experimentally in a collisionless plasma wind tunnel, where a collisionless plasma beam is directed through a current loop. The collisionless plasma is generated by an electron bombardment engine. Nominal values of ion flow speed, number density, electron temperature and magnetic induction of the loop are U ~10^4 m/s, N ~5 x 10^7/cc, kT_e ~0.2 eV and B ~10 Gauss, respectively. Due to lack of probe theories in the presence of nonhomogeneous magnetic fields and failure of conventional Langmuir probe methods, a new, simple method of diagnostics is developed. This method employs two probes of different geometry and obtains information on the ion density and flow speed from the ion-saturation region of the probe characteristic. Radial density profiles in the wake of the current loop mapped by the \"two-probe\" method indicate annular density \"peaks\" at certain radial positions. In order to understand this non-uniform \"pinching\" process, a theoretical analysis is attempted. It is found that the experimentally observed phenomenon can be qualitatively described by the collision-less two-fluid equations, but turbulent \"friction\" is required to improve the two-fluid model. A heuristic turbulent model is used, and evidence of turbulence in the flow field is also obtained through measurements of the fluctuating probe currents." }, { "name": "Rebollo-Rebollo, Manuel", "degree": "PhD", "year": "1973", "title": "Analytical and Experimental Investigation of a Turbulent Mixing Layer of Different Gases in a Pressure Gradient", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechTHESIS:10072010-091455869", "creators": [ { "name": { "family": "Rebollo-Rebollo", "given": "Manuel" }, "id": "Rebollo-Rebollo-Manuel", "display_name": "Rebollo-Rebollo, Manuel" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6YB5-CJ15", "abstract": "An analytical and experimental study has been made of the turbulent mixing layer in a pressure gradient. Theory predicts the possible existence of equilibrium flows, and this was confirmed experimentally for turbulent shear layers between streams of helium and nitrogen. \r\nThe only case for which similarity is possible is for P2 U_2^2 = P1 U_1^2 since then P_2 (x) = P_1 (x). These equilibrium flows are of the form U_1 ~x^ \u03b1 and \u03b4~X, where \u03b1=x/U_1 dU_1/dx is non \u2013dimensional pressure gradient parameter. \r\nThe experimental investigation was conducted in the facility designed by Brown to produce turbulent flows at pressures up to 10 atmospheres. The adjustable walls of the test section of the apparatus were modified in order to set the pressure gradient. \r\nShadowgraphs of the mixing zone for \u03b1 = 0 and \u03b1 = - 0.18, at different Reynolds numbers, revealed a large scale structure noticeably different for each \u03b1. \r\nThe similarity properties of the shear layer were established from mean profiles of total head and density. In addition, the rms density fluctuations were found to be self-preserving. From the mean profiles, the spreading rate, turbulent mass diffusion, Reynolds stress and Schmidt number distributions were calculated from the equations of motion. \r\nThe experimental results show that the spreading rate for the adverse pressure gradient is 60% A greater than for the \u03b1 = 0 case. The maximum shearing stress is 70% larger and the maximum value of the turbulent mass diffusion is 20% larger than their \u03b1 = 0 counterparts. The maximum rms density fluctuations are approximately 0.2 in both flows. \r\nSurprisingly low values of turbulent Schmidt numbers were found; e. g., at the dividing streamline Sc_t = 0.16 for \u03b1 = 0 and Sc_t = 0. 33 for \u03b1 = - 0. 18.\r\n" }, { "name": "Rupert, Viviane Claude", "degree": "PhD", "year": "1973", "title": "Experimental Study of Shock Wave Strengthening by a Positive Density Gradient in a Cryogenic Shock Tube", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechTHESIS:08272010-083731520", "creators": [ { "name": { "family": "Rupert", "given": "Viviane Claude" }, "id": "Rupert-Viviane-Claude", "display_name": "Rupert, Viviane Claude" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZXBM-7809", "abstract": "An experimental investigation of the strengthening of a shock wave propagating through an isobaric region of increasing density is presented. A new experimental configuration consisting of a pressure-driven shock tube mounted vertically with the test section partially immersed in a cryogenic bath is used. The resulting test gas density distribution consists of a uniform region of low density near the shock tube diaphragm, then a strong local gradient followed by another uniform region of high density. The\r\nMach number of the shock initiated at the diaphragm is determined as the shock emerges from the gradient from velocity and temperature measurements for various initial conditions.\r\nThe experimental data are compared with predictions from approximate theoretical models and a numerical integration of the exact flow equations for the shock-gradient interaction. The measured Mach numbers are considerably higher than these predictions indicating that the models are not adequate to represent the experimental configuration. Calculations show that the additional strengthening of the shock results from multiple interactions between waves generated within the gradient and flow nonuniformities due to the shock formation mechanism.\r\n" }, { "name": "Storm, Erik", "degree": "PhD", "year": "1973", "title": "Part I. Investigation of Strong Shock Waves in a Conical Convergent Channel. Part II. Spectroscopic Investigation of Strong Shockwaves in a Conical, Convergent Channel", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-09042007-104952", "creators": [ { "name": { "family": "Storm", "given": "Erik" }, "id": "Storm-Erik", "display_name": "Storm, Erik" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TJFS-6964", "abstract": "ABSTRACT OF PART I:
\r\n\r\nThe behavior of an initially plane, strong shock wave propagating into a conical convergence is investigated experimentally and theoretically. In the experiment a 10\u00b0 half-angle cone is mounted on the end of a pressure-driven shock tube. Shock waves with initial Mach numbers varying from 6.0 to 10.2 are generated in argon at a pressure of 1.5 torr. During each run local shock velocities at several positions along the cone axis are measured using a thin, multi-crystal piezoelectric probe inserted from the vertex. This technique produces accurate velocity data for both the incident and reflected shock waves. In the corresponding analysis, a simplified characteristics method is used to obtain an approximate solution of the axisymmetric diffraction equations derived by Whitham (1959).
\r\n\r\nBoth the shock velocity measurements and the axisymmetric diffraction solution confirm that the incident shock behavior is dominated by cyclic diffraction processes which originate at the entrance of the cone. Each diffraction cycle is characterized by Mach reflection on the cone wall followed by Mach reflection on the axis. These cycles evidently persist until the shock reaches the cone vertex, where the measured velocity has increased by as much as a factor of three. Real-gas effects, enhanced in the experiment by increasing the initial Mach number and decreasing the pressure, apparently alter the shock wave behavior only in the region near the vertex. Velocity measurements for the reflected shock within the cone show that the shock velocity is nearly constant throughout most of the convergence length.
\r\n\r\nABSTRACT OF PART II:
\r\n\r\nThe thermodynamic conditions behind the incident and reflected shock wave close to the vertex of a convergent channel are investigated spectroscopically. The investigation was initiated in order to better determine the possible uses of such a geometrical device as a tool for high temperature plasma research. Using argon at an initial pressure of 1.5 torr, the shock Mach number prior to the entrance of the cone is 10.2. Two windows are mounted at x/1 = 0.9 in the cone, where the Mach number has increased to 24, and the emitted radiation is monitored for both time-resolved (Monochromator - Photomultiplier) and time-integrated (Spectrograph) analysis. The relative line intensity method is used to measure the electron temperature. The Stark broadened profile of the 6965.4 \u00c5 neutral argon line, and continuum intensity measurements are used to determine the electron number density.
\r\n\r\nFrom initial values of 13200\u00b0K and 1.4 x 10\u00b9\u2077 cm\u207b\u00b3, both the electron temperature and number density profiles behind the incident shock are dominated by the previous shock diffraction processes. The general trend is a gradual increase, presumably due to the continuing compression of the gas shocked at successively earlier times. Superimposed upon this is the effect of hot slugs of gas from previous localized regions of very high Mach number. The reflected shock wave heats and compresses the gas even further. The subsequent expansion results in a series of rapid exponential decreases in temperature, density and pressure. Immediately after the reflected shock wave has passed, the gas appears to be in a nonequilibrium state with a population inversion among the upper excited atomic energy levels. There is an indication of the presence of a second reflected wave. The effects of self-absorption on Stark broadened lines is studied. An equation is derived, demonstrating the effect of individual corrections that are necessary before accurate interpretations of measured quantities can be made. Simple self-absorption correction schemes are demonstrated and shown to be self-consistent.
" }, { "name": "Chen, Jay-Chung", "degree": "PhD", "year": "1972", "title": "Nonlinear Vibration of Cylindrical Shells", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-085617930", "creators": [ { "name": { "family": "Chen", "given": "Jay-Chung" }, "id": "Chen-Jay-Chung", "display_name": "Chen, Jay-Chung" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CBC4-MP23", "abstract": "The large amplitude vibrations of a thin-walled cylindrical shell are analyzed using the Donnell's shallow-shell equations. A perturbation method is applied to reduce the nonlinear partial differential equations into a system of linear partial differential equations. The simply-supported boundary condition and the circumferential periodicity condition are satisfied. The resulting solution indicates that in addition to the fundamental modes, the response contains asymmetric modes as well as axisymmetric modes with the frequency twice that of the fundamental modes. In the previous investigations in which the Galerkins procedure was applied, only the additional axisymrnetric modes were assumed.\r\nVibrations involving a single driven mode response are investigated. The results indicate that the nonlinearity is either softening or hardening depending on the mode. The vibrations involving both a driven mode and a companion mode are also investigated. The region where the companion mode participates in the vibration is obtained and the effects due to the participation of the companion mode are studied.\r\nAn experimental investigation is also conducted. The\r\nresults are generally in agreement with the theory. \"Non-stationary4 response is detected at some frequencies for large amplitude response where the amplitude drifts from one value to another. Various nonlinear phenomena are observed and quantitative comparisons with the theoretical results are made.\r\n" }, { "name": "Davis, Joseph Eugene", "degree": "PhD", "year": "1972", "title": "Non-Planar Wings in Non-Planar Ground Effect", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechTHESIS:09272010-162542308", "creators": [ { "name": { "family": "Davis", "given": "Joseph Eugene" }, "id": "Davis-Joseph-Eugene", "display_name": "Davis, Joseph Eugene" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NNF7-GV39", "abstract": " A numerical method is developed for solving the problem of a wing in arbitrary non-planar ground effect. The linearized equations of unsteady motion for an arbitrary non-planar wing in non-planar ground effect are presented. Numerical calculations were made to determine all the aerodynamic characteristics and stability derivatives for various thin, uncambered planar and non-planar wings (including two wings connected in tandem) in planar and non-planar ground effect. These calculations were incorporated into the equations of unsteady motion to determine the dynamic stability characteristics of these wings in the various ground effect situations. Several wings were found to be longitudinally stable, but only in rare cases were they found to be laterally stable.\r\n \r\n An experiment was conducted to compare some of the numerical calculations with reality. The agreement was reasonable.\r\n" }, { "name": "Logan, Samuel Ernest", "degree": "PhD", "year": "1972", "title": "Laser Velocimeter Measurement of Reynolds Stress and Turbulence in Dilute Polymer Solutions", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechTHESIS:08202010-110214903", "creators": [ { "name": { "family": "Logan", "given": "Samuel Ernest" }, "id": "Logan-Samuel-Ernest", "display_name": "Logan, Samuel Ernest" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BD39-BV19", "abstract": "Measurements of Reynolds stress and axial and transverse\r\nturbulence intensities have been made in drag-reducing\r\nturbulent pipe flow of a dilute solution of high molecular\r\nweight polymer and compared to measurements made with pure\r\nwater. A newly developed laser velocimeter capable of\r\nmeasuring these turbulence parameters has been utilized and\r\nis described in detail.\r\n\r\nAxial turbulence intensities measured in polymer solution\r\nare consistent with previous polymer results and\r\nviscous sublayer thickening is observed. New results include\r\ndemonstration that the turbulent shearing stress is\r\nreduced in the turbulent core by an amount proportional to\r\nthe observed decrease in pressure gradient at the wall, and\r\nextrapolates to a wall value in agreement with calculated\r\nlocal wall shear. Near the wall polymer solution Reynolds\r\nstress is reduced below that measured for water consistent\r\nwith observed velocity profiles. Polymer radial turbulence\r\nintensities are comparable with those for water in the\r\nturbulent core, but exhibit similar dramatic suppression\r\nnear the wall. These and other recent results strongly\r\nsuggest that dilute polymer solution drag reduction is\r\nprimarily a wall phenomenon. Polymers appear to have little\r\nor no effect on turbulent flow away from a solid boundary\r\nwhere turbulent velocities scale with u_\u03c4, the shear\r\nvelocity based on the observed wall shear.\r\n" }, { "name": "Palaniswamy, Karuppagounder", "degree": "PhD", "year": "1972", "title": "Crack Propagation under General In-Plane Loading", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechTHESIS:08252010-104555143", "creators": [ { "name": { "family": "Palaniswamy", "given": "Karuppagounder" }, "id": "Palaniswamy-Karuppagounder", "display_name": "Palaniswamy, Karuppagounder" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M7WW-G964", "abstract": "The problem of crack extension in brittle materials under\r\ngeneral loading conditions is investigated. Methods of solution of the related two-dimensional elasto-static boundary value problem are discussed. Using Kolosov-Muskhelishvili stress functions, an approximate solution is obtained. The effect of the approximation on the results is estimated by solving two related problems exactly. Then using two postulates the critical loads and crack extension direction are determined under loading conditions\r\nunsymmetrical to the crack axis. Results are compared with those obtained using a different set of postulates.\r\n" }, { "name": "Pu\u010dik, Thomas Antone", "degree": "PhD", "year": "1972", "title": "Elastostatic Interaction of Cracks in the Infinite Plane", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechTHESIS:09282010-083924490", "creators": [ { "name": { "family": "Pu\u010dik", "given": "Thomas Antone" }, "id": "Pu\u010dik-Thomas-Antone", "display_name": "Pu\u010dik, Thomas Antone" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BVJ3-BV94", "abstract": " The stress boundary value problem of an infinite, planar region with embedded rectilinear cracks is investigated from the viewpoint of two-dimensional, static, linear elasticity theory (plane strain or generalized stress). Any finite number of cracks may be considered. Their orientation may be arbitrary, so long as they do not intersect. Boundary loadings may take the form of quite general in-plane tractions along the crack surfaces, together with a bounded in-plane stress field at infinity.\r\n\r\n Using Muskhelishvili\u2019s solution for colinear cracks, the problem is reduced to a set of one-dimensional Fredholm integral equations. A simple numerical technique is presented for the approximate solution of these equations. The method is established to possess an extremely high rate of convergence.\r\n\r\n Results are presented for a number of two-crack interaction problems. As expected, the interaction of the cracks generally tends to reduce the fracture strength of a material, relative to the strength that would exist with either crack acting independently. However, for certain orientations, it is found that the interaction phenomenon can actually increase the resistance to fracture." }, { "name": "Setchell, Robert Earle", "degree": "PhD", "year": "1972", "title": "Shock Tube Investigations of Strong Shock Waves in a Convergent Channel", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-11212003-154300", "creators": [ { "name": { "family": "Setchell", "given": "Robert Earle" }, "id": "Setchell-Robert-Earle", "display_name": "Setchell, Robert Earle" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RVJ0-B836", "abstract": "The behavior of the incident and reflected shock waves in a convergent channel is investigated in order to determine if such a geometrical device could be used as a means of producing high-enthalpy gas samples. A 10\u00b0 half-angle conical convergence is mounted on the end of a pressure-driven, six-inch shock tube. Using argon at an initial pressure of 1.5 torr, initial shock Mach numbers are varied from 6.0 to 10.2. During each run local shock velocities at several positions along the cone axis are measured using a small, multi-crystal, axial piezoelectric probe inserted from the cone vertex.
\r\n\r\nThe incident shock velocity profiles show that the shock behavior is dominated by multiple diffraction processes which originate at the cone entrance. Sudden increases in shock velocity at certain positions along the axis are observed, corresponding to the intersection of stemshocks formed by Mach reflection on the cone wall. These increases are separated by regions of deceleration and acceleration, corresponding to the growth and decline of a center shock formed by Mach reflection on the cone axis. Near the vertex the shock velocity has increased by as much as a factor of three, indicating that high temperatures and pressures are generated. By varying the initial Mach number and pressure, real gas effects are found to influence the diffraction process only in a region near the vertex.
\r\n\r\nReflected shock profiles show that the shock velocity is nearly constant for much of the convergence length, in contrast to the power-law decline predicted by the similarity solution. During this period the shock propagates into fluid originally set into steady, uniform motion outside the cone entrance. Small variations in the velocity result from weak interactions with localized nonuniformities and secondary waves. Beyond the cone entrance the shock decelerates towards the velocity corresponding to reflection from a plane end wall. A departure from ionization equilibrium is likely near the vertex during the rapid expansion which occurs behind the reflected shock.
" }, { "name": "Steinhilper, Eric Anthony", "degree": "PhD", "year": "1972", "title": "Electron Beam Measurements of the Shock Wave Structure: Part I. The Inference of Intermolecular Potentials from Shock Wave Experiments. Part II. The Influence of Accommodation on Reflecting Shock Waves", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-09042007-094927", "creators": [ { "name": { "family": "Steinhilper", "given": "Eric Anthony" }, "id": "Steinhilper-Eric-Anthony", "display_name": "Steinhilper, Eric Anthony" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AWTA-PA12", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA technique has been developed in which accurate measurements of shock wave structure and an exact molecular theory of shock waves are used to determine intermolecular potentials. Shock wave density profiles in neon, argon, krypton, and xenon are measured in the GALCIT 17-inch diameter shock tube. The theory is a numerical molecular simulation technique (developed by G. Bird of the University of Sidney) in which the only adjustable parameter is the intermolecular potential. Parameters for the exp-6 and Lennard-Jones potentials are determined by matching the experimental shock wave density profiles with those predicted by the Monte Carlo simulation technique. The experimental data are taken at shock Mach number of about 8; consequently, these results fall in an energy range midway between the molecular beam measurements and low temperature transport property results.\r\n\r\nAfter the potentials for neon, argon, krypton, and xenon have been determined, they are tested for conformity to the Law of Corresponding States. Plots of the potentials in corresponding states coordinates, [...] vs.[...], show that the exp-6 potential model issuperior to the Lennard-Jones. This is an important result, because for the first time this statement can be made on the basis of one set of measurements. Previously it had been necessary to adduce molecular beam results in order toprove that the inverse twelfth-power repulsive part of the Lennard-Jones potential is too strong. Comparisons show that the exp-6, Lennard-Jones, simple repulsive, and hard sphere molecular potentials predict the experimental shock structure with successively decreasing accuracy. However, their accuracy is sufficient that any one of the potentials would predict any flow accurately enough to give an indication of the relative importance of the parameters governing the flow. This point is emphasized by the need for both the most precise experimental measurements and the use of the Law of Corresponding States in order to provide the basis for ranking the potentials. Moreover, changing the potentials has given a better understanding of the mechanisms by which intermolecular forces influence shock structure.\r\n\r\nMeasurements of density profiles during the reflection of thick shock waves in argon from the end wall of the GALCIT 17-inch diameter shock tube were reported previously. A mass balance using these profiles had revealed that as much as 20% of the gas which should have been between the end wall and the reflected shock was simply not present. Comparison with theory was not possible because no theory incorporated a loss of mass. Currently available theories for the reflection process include a Monte Carlo flow simulation technique for a thermally accommodating wall.\r\n\r\nIt is found that this technique can correctly predict either the reflected shock trajectory or the thermal layer near the wall, but the inability to duplicate both implies that there is a second important effect which we assume to be adsorption.\r\n\r\nAdditional experiments are conducted in neon which has a lower thermal accommodation coefficient than argon. If thermal accommodation is the only wall boundary condition, then according to the Monte Carlo calculations the shock should reflect faster in neon, and the thermal layer should be thinner. However, the measured density profiles show that the reflected shock trajectory is nearly the same as in argon but that there is only half as much \"missing\" mass. Thus, the neon results provide the most significant confirmation of adsorption.\r\n\r\nBecause this unexpected violation of the continuity equation was observed, a comprehensive review of instrumental effects and the data reduction technique is made. Several hypothetical effects are shown to have no influence on the loss of mass. However, improving the mass balance calculations accounts for approximately 25% of the missing mass. Correcting for multiple scattering of the electron beam accounts for another 10%, but this correction applies only at the highest densities. Therefore, the \"missing\" mass of the previous experiment is verified but is reduced somewhat in magnitude." }, { "name": "Trijonis, John Charles, Jr.", "degree": "PhD", "year": "1972", "title": "An Economic Air Pollution Control Model-Application: Photochemical Smog in Los Angeles County in 1975", "advisor": "List, E. John", "url": "https://resolver.caltech.edu/CaltechTHESIS:06262014-113258422", "creators": [ { "name": { "family": "Trijonis", "given": "John Charles, Jr." }, "id": "Trijonis-John-Charles", "display_name": "Trijonis, John Charles, Jr." } ], "advisors": [ { "name": { "family": "List", "given": "E. John" }, "id": "List-E-J", "role": "advisor", "display_name": "List, E. John" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "envreng" ], "doi": "10.7907/hxbp-pc89", "abstract": "An economic air pollution control model, which determines the least cost of reaching various air quality levels, is formulated. The model takes the form of a general, nonlinear, mathematical programming problem. Primary contaminant emission levels are the independent variables. The objective function is the cost of attaining various emission levels and is to be minimized subject to constraints that given air quality levels be attained.
\r\n\r\nThe model is applied to a simplified statement of the photochemical smog problem in Los Angeles County in 1975 with emissions specified by a two-dimensional vector, total reactive hydrocarbon, (RHC), and nitrogen oxide, (NOx), emissions. Air quality, also two-dimensional, is measured by the expected number of days per year that nitrogen dioxide, (NO2), and mid-day ozone, (O3), exceed standards in Central Los Angeles.
\r\n\r\n\r\nThe minimum cost of reaching various emission levels is found by a linear programming model. The base or \"uncontrolled\" emission levels are those that will exist in 1975 with the present new car control program and with the degree of stationary source control existing in 1971. Controls, basically \"add-on devices\", are considered here for used cars, aircraft, and existing stationary sources. It is found that with these added controls, Los Angeles County emission levels [(1300\r\ntons/day RHC, 1000 tons /day NOx) in 1969] and [(670 tons/day RHC, 790 tons/day NOx) at the base 1975 level], can be reduced to 260 tons/day RHC (minimum RHC program) and 460 tons/day NOx (minimum NOx program).
\r\n\r\n\"Phenomenological\" or statistical air quality models provide the relationship between air quality and emissions. These models estimate the relationship by using atmospheric monitoring data taken at one (yearly) emission level and by using certain simple physical assumptions, (e. g., that emissions are reduced proportionately at all points in space and time). For NO2, (concentrations assumed proportional to NOx emissions), it is found that standard violations in\r\nCentral Los Angeles, (55 in 1969), can be reduced to 25, 5, and 0 days per year by controlling emissions to 800, 550, and 300 tons /day, respectively. A probabilistic model reveals that RHC control is much more effective than NOx control in reducing Central Los Angeles ozone. The 150 days per year ozone violations in 1969 can be reduced to 75, 30, 10, and 0 days per year by abating RHC emissions to 700, 450, 300, and 150 tons/day, respectively, (at the 1969 NOx emission level).
\r\n\r\nThe control cost-emission level and air quality-emission level relationships are combined in a graphical solution of the complete model to find the cost of various air quality levels. Best possible air quality levels with the controls considered here are 8 O3 and 10 NO2 violations per year (minimum ozone program) or 25 O3 and 3 NO2 violations per year (minimum NO2 program) with an annualized cost of $230,000,000 (above the estimated $150,000,000 per year for the new car control program for Los Angeles County motor vehicles in 1975).
\r\n" }, { "name": "Barcelo, Brian Thomas", "degree": "PhD", "year": "1971", "title": "Electron Beam Measurements of the Shock Wave Structure. Part 1. Determination of the Interaction Potential of the Noble Gases from Shock Wave Structure Experiments. Part II. Feasibility of a Modified Electron Beam Densitometer Technique to Measure Diffusive Separation in Shock Waves in Helium-Argon Mixtures", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-04022009-092506", "creators": [ { "name": { "family": "Barcelo", "given": "Brian Thomas" }, "id": "Barcelo-Brian-Thomas", "display_name": "Barcelo, Brian Thomas" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "chair", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "member", "display_name": "Roshko, Anatol" }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QH9Z-BH10", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nShock wave experiments were conducted in the GALCIT* seventeen-inch low density shock tube to measure the interaction potential of the following nonradioactive noble gases: neon, argon, krypton, and xenon. The experimental shock profiles obtained by employing the electron beam densitometer technique were compared to the Monte Carlo numerical simulation of the shock wave structure. The comparison determined the intermolecular potential for these gases, a potential assumed to be of the form [...] = const/r[superscript s]. The values resulting for the free parameter s in the inverse repulsive power law were 9, 10, 11, and 12 for xenon, krypton, argon, and neon, respectively.\r\n\r\nIn a second phase of the experiments, the feasibility of a modified electron beam densitometer technique was investigated for measuring the shock wave structure in a binary mixture of helium and argon. It was desired to obtain both the argon and helium density profiles through the shock wave by varying the electron beam energy in two experiments of identical shock conditions (gas concentrations, Mach number, initial pressure, etc.). Theoretical calculations of the collision cross-section indicated only a slight possibility of separating the density profiles of the two species with the range of electron energy possible in the experimental apparatus (7000 to 15000 volts). Experiments conducted with initial gas concentrations of 10%, 20%, 50%, and 80% argon in shock waves with a Mach number of approximately four confirmed these suspicions but permitted qualitative conclusions in agreement with other similar investigations.\r\n\r\n*Graduate Aeronautical Laboratories California Institute of Technology." }, { "name": "Davey, Robert Frederick", "degree": "PhD", "year": "1971", "title": "An Experimental Investigation of the Effect of a Density Gradient on Shear Layer Instability", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-04022009-131624", "creators": [ { "name": { "family": "Davey", "given": "Robert Frederick" }, "id": "Davey-Robert-Frederick", "display_name": "Davey, Robert Frederick" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YJF7-DV47", "abstract": "Measurements of mass flow rate and mean density have been made in separated laminar boundary layers having large transverse density gradients. A 3/8 in. by 1 1/2 in. rectangular half-jet was used to generate a two-dimensional shear layer and the density heterogeneity was produced in an incompressible flow by exhausting one gas into a reservoir of another gas having a different molecular weight. Two Freons were used having a density ratio of 1.98 and unique properties which permitted the measurement of the mass flow rate in all mixtures of the gases with a single hot wire.\r\n\r\nMean density and mass flow rate profiles were compared to the Holmboe model used in theoretical hydrodynamic stability analyses. Fluctuations in the mass flow rate were analyzed and the frequency, wave number and amplification rate of the most unstable oscillation were measured and compared to theoretically predicted values. The oscillations were found to have a higher amplification rate, lower wave number and lower frequency than homogeneous flow when the lighter gas flowed into the heavier one, in agreement with the theoretical predictions. Opposite trends were observed with the heavy gas flowing.\r\n\r\nThe development of harmonic frequency oscillations in the non-linear region is discussed briefly." }, { "name": "Elachi, Charles", "degree": "PhD", "year": "1971", "title": "Electromagnetic Wave Propagation and Source Radiation in Space-Time Periodic Media", "advisor": "Papas, Charles Herach", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-143115", "creators": [ { "name": { "family": "Elachi", "given": "Charles" }, "id": "Elachi-Charles", "display_name": "Elachi, Charles" } ], "advisors": [ { "name": { "family": "Papas", "given": "Charles Herach" }, "id": "Papas-C-H", "role": "advisor", "display_name": "Papas, Charles Herach" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "eleceng" ], "doi": "10.7907/WYFR-GR84", "abstract": "The electromagnetic wave equations for the fields, potentials and Hertz vectors are derived and a Lorentz gauge is given for space-time dependent media. Electromagnetic wave propagation, electric and magnetic dipole radiation, and Cerenkov and transition radiation in sinusoidally space-time periodic dielectric, plasma and uniaxial plasma are studied and numerous radiation patterns are given. A special radiation effect in the uniaxial plasma is investigated. Finally the study is extended to general space-time periodic media (i.e., \u03b5 = \u03b50\u03b5r [1 + \u03b51 f(Kz - \u03a9t)] where f(\u03be) is a periodic function)." }, { "name": "Ramaswamy, Mathagondapally A.", "degree": "PhD", "year": "1971", "title": "Experimental Investigation of the Effect of Cooling on Near Wake of Circular Cylinder at Mach Number Six", "advisor": "Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechTHESIS:04152011-112031398", "creators": [ { "name": { "family": "Ramaswamy", "given": "Mathagondapally A." }, "id": "Ramaswamy-Mathagondapally-A", "display_name": "Ramaswamy, Mathagondapally A." } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8ZZ8-PC49", "abstract": "An experimental investigation has been conducted to study the near wake of a two-dimensional circular cylinder of 0.2 in. diameter at M_\u221e = 6. Mean flow properties were determined from Pitot pressure, static pressure, and hot-wire recovery temperature measurements at free stream Reynolds number of 0.905 X 10^4 and 2.95 X 10^4 for both adiabatic and cooled models, the latter at 0.19 T_o.
\r\n\r\nThe near-wake was laminar for the adiabatic model at both\r\nthe Reynolds numbers tested. For the cold model, the near-wake was laminar for the lower Reynolds number and transition occurred in the near wake at the higher Reynolds number. The wake shocks, the shear layer edge and the thermal layer edge moved closer to the wake centerline with cooling and with increase in Reynolds number. The base pressure decreased with cooling and the sonic point moved closer to the model on cooling. In the recirculating region, the total temperature distributions exhibited a minimum close to the dividing stream line for all the cases, and the total temperature on the centerline was nearly constant and equal to the value at the rear stagnation point (0.5 T_o for the cold models) indicating that the heat transfer in this region was mainly by convection. The existence of a thin thermal layer on the base was evident for the cold models.
\r\n\r\nPreliminary experiments on the two-dimensionality of the\r\nflow and an emperical formulation for the viscous corrections to the measured Pitot pressure have been included in the Appendices.
\r\n" }, { "name": "Shollenberger, Carl Alvin", "degree": "PhD", "year": "1971", "title": "An Investigation of a Two-Dimensional Propulsive Lifting System", "advisor": "Lissaman, Peter B. S.; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechTHESIS:08312010-081235652", "creators": [ { "name": { "family": "Shollenberger", "given": "Carl Alvin" }, "id": "Shollenberger-Carl-Alvin", "display_name": "Shollenberger, Carl Alvin" } ], "advisors": [ { "name": { "family": "Lissaman", "given": "Peter B. S." }, "id": "Lissaman-P-B-S", "role": "advisor", "display_name": "Lissaman, Peter B. S." }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GZNT-1X61", "abstract": " Several aspects of the nonhomogeneous flow associated with a system combining lifting and propulsive requirements of an aircraft are considered in detail by analytical and experimental methods. The basic geometry of the problem is that of two lifting surfaces with an actuator disk located between them. The resulting flow consists of two regions of different total energies.\r\n\r\n Propulsive lift systems are prototypes of many similar multi-energy flow problems. The principles governing flow with energy addition are examined. Basic equations and boundary conditions are developed for the complete inviscid and incompressible analysis for the two-dimensional case. The corresponding flow singularities are discussed and the integral equations which completely specify the system are derived.\r\n \r\n The two special cases of small and large energy addition are considered in detail including solutions.\r\n\r\n A numerical procedure is developed to solve the full problem including allowance for the wake deflection. Appropriate vorticity forms are used to represent the entire system. Wake vorticity is provided the freedom to move in the plane. An iterative scheme is presented which rapidly converges to a solution for the magnitude and location of the system vorticity distributions. Forces and moments are evaluated on the propulsive lift system.\r\n\r\n Analytical results are given from the numerical solution for various values of the geometric and energy parameters. Comparison of the numerical result with the solutions for extreme values of energizing is given.\r\n\r\n Results from a wind tunnel study of the two-dimensional propulsive-lift system provide a check on the importance of real effects. Comparison of the analytical and experimental results is given in detail. The experimentally determined wake development is observed to be similar to the predicted shape. In addition, the lift augmentation is similar for the theoretical and experimental cases. Further, the airfoil pressure distributions and resulting pitching moments are seen to exhibit the behavior expected from the calculations.\r\n" }, { "name": "Sigal, Asher", "degree": "PhD", "year": "1971", "title": "An Experimental Investigation of the Turbulent Boundary Layer Over a Wavy Wall", "advisor": "Lees, Lester; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechTHESIS:09232010-101022816", "creators": [ { "name": { "family": "Sigal", "given": "Asher" }, "id": "Sigal-Asher", "display_name": "Sigal, Asher" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VK9A-XA44", "abstract": " An experimental investigation of turbulent boundary layer flow over wavy surfaces was conducted at low speed.\r\n\r\n Two models with the ratio of the amplitude to the wave length a/\u03bb = 0.03 and wave lengths \u03bb = 6\" and 12\" were tested in an open-circuit wind tunnel. The free stream velocity was 15.4 m/sec, giving Reynolds number Re = 2.54 X 10^4 per inch. Boundary-layer thickness varied from \u03b4 = 1.5\" to \u03b4 = 4. 1\" by means of boundary-layer trips of various height, in order to change the ratio \u03bb/6.\r\n \r\n The following measurements were taken:\r\n * Wall pressure distribution\r\n * Average velocity and turbulence level, using a single element hot-wire probe\r\n * Wall stress distribution, using Preston's tube\r\n * Static and total pressures\r\n * Turbulence intensities and shear stress using X-array hot-wire probe.\r\n\r\n An appreciable modulation of all the flow quantities, imposed by the wavy boundary, is observed throughout the investigation. Wall pressure is much lower than predicted by uniform, inviscid theory and is slightly non-symmetric. Wall stress distribution has a peak with C_f/C_fo = 1.2 upstream of the crest and a dip of C_f/C_fo = 0. 6 upstream of the trough. Static pressure decays exponentially in the outer layer while its gradient is decreased toward the surface in the wall layer.\r\n\r\n The turbulence intensities and shear stress distributions near the wall show oscillatory modulation superimposed on the reference flat plate profiles. The amplitude of the oscillations decay exponentially toward the edge of the layer, so that in the outer part of the layer the turbulence quantities are practically independent of the longitudinal position.\r\n\r\n It was found that Coles' Law of the Wall does not apply in the present situation because of the modulation of the slope of the semi-logarithmic portion of the velocity profiles. A presentation of velocity profiles is suggested through the use of total velocity defined by U^t = (U^2 + 2(p\u2013p_\u221e)/p)^(1/2). This quantity obeys the Law of the Wake. Mixing length and eddy viscosity profiles based on the derivative \u2202U^t/\u2202y are reduced into one curve which is the reference flat plate distribution.\r\n" }, { "name": "Blumenthal, Donald Lawrence", "degree": "PhD", "year": "1970", "title": "Experimental Study of Satellite Wakes in a Simulated Ionospheric Plasma", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechTHESIS:07092010-100552106", "creators": [ { "name": { "family": "Blumenthal", "given": "Donald Lawrence" }, "id": "Blumenthal-Donald-Lawrence", "display_name": "Blumenthal, Donald Lawrence" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PYM3-H107", "abstract": "Wakes of simple bodies (discs, strips) were investigated using an electrostatically accelerated stream of argon \r\n\r\nions and electrons. Typical conditions are: beam ion energy is 80 eV, ion density is 10^7-10^8 cm^(-3), electron \r\n\r\ntemperature 1-3 eV, ion thermal speed very small compared to mean ion velocity. The dimensionless parameters \r\n\r\nclosely approximate satellite flight conditions, with the exception of the electron-ion temperature ratio, which \r\n\r\nis near unity in flight and large in these experiments.\r\n\r\nThe dependence of principal near wake features (such as the large ion current peak on the centerline behind the \r\n\r\nbody) on the shape of the body was investigated systematically. All trends can be explained qualitatively by \r\n\r\nrecognizing the dominant role of those portions of the sheath where the free stream ion velocity is tangential to \r\n\r\nthe body.\r\n\r\nThe far wake of a strip (downstream of the ion current peak) displays a decaying radial distribution on the scale \r\n\r\nof the body size, somewhat similar to what is expected from a neutral gas. For axial symmetric models, the far \r\n\r\nwake displays a small structure on the scale of the ion current peak. The evolution of these disturbances\r\nis qualitatively explained by a simple, linearized two fluid theory. These features are initiated by the \r\n\r\ninteraction of the inwardly deflected ion streams behind the body. At least in certain regions of the flow field, \r\n\r\nthis interaction involves two-humped ion distribution functions, which may play a role in the further development \r\n\r\nof the far wake.\r\n\r\nThe effects on the simulation of varying the vacuum chamber background pressure was also examined in detail.\r\n" }, { "name": "Cassady, Phillip Earl", "degree": "PhD", "year": "1970", "title": "An Investigation of Non-Equilibrium Effects in an Argon Free-Jet Plasma", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechTHESIS:10012010-092851271", "creators": [ { "name": { "family": "Cassady", "given": "Phillip Earl" }, "id": "Cassady-Phillip-Earl", "display_name": "Cassady, Phillip Earl" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WHYP-RZ81", "abstract": "The non-equilibrium effects present in the formation of a strong normal shock wave in a low density, slightly ionized argon flow field, particularly as evidenced by the appearance of a dark region upstream of the shock wave, have been analyzed both theoretically and experimentally. A model for the flow through the shock wave was formulated which incorporates a quantum mechanical theory to explain the existence of the dark region, and the problem was solved numerically to yield flow field property distributions. A precursor region of high electron temperature was found to exist upstream of the main body of the heavy-particle shock wave.\r\n\r\nAn experimental investigation of the phenomenon was carried out in an arc heated free jet flow field. A test facility was constructed in which the goal has been to attain operation at low enough enthalpies to allow precise and extensive diagnostic testing while still high enough to exhibit the interesting non-equilibrium effects. Extensive study was carried out on the effect of electrode design and gas flow phenomena on the stability of the arc discharge. The completed unit was instrumented fully for measurement of the operating parameters and a computer program was developed to monitor its operation as a supply of slightly ionized argon for free-jet experiments.\r\n\r\nThe non-equilibrium aspects of the free-jet were analyzed both theoretically and experimentally. A theoretical model was developed and numerically solved for the free-jet expansion of slightly ionized argon. Pitot pressure measurements were completed and compared favorably with predictions calculated from this theoretical model.\r\n\r\nElectron temperature and ion density profiles were measured both along the axis of the empty free-jet and through the normal shock wave in front of a cooled blunt body using a new type of cooled Langmuir probe, the operation of which was theoretically analyzed. The existence of a region of electron temperature in front of a strong normal shock wave coincident with the observed dark region was experimentally verified." }, { "name": "Chu, Billie Mae", "degree": "PhD", "year": "1970", "title": "Rheological Properties of Soft Extensible Animal Tissue in Both Living and Excised States", "advisor": "Wayland, J. Harold", "url": "https://resolver.caltech.edu/CaltechTHESIS:06072010-153606301", "creators": [ { "name": { "family": "Chu", "given": "Billie Mae" }, "id": "Chu-Billie-Mae", "display_name": "Chu, Billie Mae" } ], "advisors": [ { "name": { "family": "Wayland", "given": "J. Harold" }, "id": "Wayland-J-H", "role": "advisor", "display_name": "Wayland, J. Harold" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/48T9-AD89", "abstract": "A mechanical characterization, over a wide range of response, \r\nof a particular soft extensible animal tissue, the mesentery of the cat, \r\nis presented. The structure of the mesenteric tissue is made up of a \r\ncomplicated array of components and the mechanical response is \r\ninfluenced not only by local factors, but also by adjustments of higher \r\ncontrol centers of the animal. Certain individual aspects of the \r\nresponse of living mesenteric membrane per se have been studied and \r\ncontrasted with (1) membrane strongly influenced by or containing \r\nlarge blood vessels, (2) excised membrane and (3) membrane after \r\ncirculatory collapse and accompanying sustained gut contraction.\r\n\r\n\tA freely floating segment of mesentery exists in a state of \r\ntension which can be demonstrated by making an incision through the \r\nplane of the mesentery. A technique has been developed to determine \r\nthe magnitude of this tension and also the corresponding stretch which \r\nis designated here as the initial configuration length. The tension \r\nlevel in the tissue at the initial configuration length is not unique but \r\ncan vary significantly according to the activity of the components of \r\nthe membrane per se as well as the state of the gut and the large \r\nblood vessels. The most nearly unique length of the tissue which can \r\nbe detected by these experimental methods is a relaxed length determined \r\nby excising a piece of tissue of known dimensions and \r\nmeasuring the freely floating (in a physiologic solution) dimensions\r\nto which the tissue relaxes. There is no marked material anisotropy\r\nin the plane of the membrane, i.e. the two principal dimensions in the \r\nplane of the mesentery do not vary by more than five percent even with \r\nwide history variations just prior to excision.\r\n\r\n\tThe temperature of the test preparation was monitored during \r\nthe course of the tests and maintained at the level of the core temperature \r\nof the animal. Since this temperature could drop as much as \r\nthree or four \u00b0C as a result of the anesthesia, the influence of \r\ntemperature variations on the force-stretch response of the tissue was \r\nstudied and was found to be less significant than the influence of \r\nmechanical degradation in successive loading cycles.\r\n\r\n\tA theoretical characterization that correlates rather well with \r\nthe data of the loading curves for the various tissues has been \r\nproposed. The limitations and assumptions incorporated in this \r\ntreatment have been discussed and when appropriate additional experimental \r\ndata are procured then the analytical treatment can likewise be \r\nextended to a more adequate characterization.\r\n\r\n\tPhotographic materials on pp. 53, 54, 55, 56, 58, 59, 60, 61, \r\n63, 64, 66, 67, 71, 72, 83, 89, 90 and 93 are essential and will not \r\nreproduce clearly on Xerox copies. Photographic copies should be \r\nordered.\r\n" }, { "name": "El Raheb, Michel Sadek", "degree": "PhD", "year": "1970", "title": "Some Approximations in the Dynamic Shell Equations", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechTHESIS:02212013-152121344", "creators": [ { "name": { "family": "El Raheb", "given": "Michel Sadek" }, "id": "El-Raheb-Michel-Sadek", "display_name": "El Raheb, Michel Sadek" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ejzn-dy90", "abstract": "A theoretical analysis was performed on the linear dynamic\r\nequations of thin cylindrical shells to find the error committed by the\r\nDonnell assumption and the neglect of inplane inertia.
\r\n\r\nThe Donnell approximation was found to be valid at high\r\nfrequencies as compared to the ring frequencies, for all admissible\r\nsets of boundary conditions for finite length thin shells.
\r\n\r\nThe error from neglecting tangential inertia is appreciable for\r\nlong circumferential and axial wave lengths, independent of shell\r\nthickness.
\r\n\r\nThe effect of boundary conditions was investigated from an\r\nexact solution of the linear eigenvalue problem. The inplane boundary\r\nconditions proved to be very influential in the neighborhood of the\r\nminimum frequency. An approximate technique which accounts for the\r\ninplane boundary conditions was then developed and shown to be in good\r\nagreement with the exact solution.
\r\n\r\nFinally, the effect of an elastic end ring on the eigenfrequencies\r\nwas studied. The out-of-plane and torsional rigidities of the ring were\r\nfound to govern the overall shell stiffness. Considerable mode inter action\r\nwas noticed at low circumferential wave numbers for low values\r\nof ring stiffness. The computed eigenfrequencies were found to be in\r\ngood agreement with the experimental results.
\r\n\r\n" }, { "name": "Fisher, Raymond Kurt", "degree": "PhD", "year": "1970", "title": "Resonance Cones in the Field Pattern of a Short Radio Frequency Probe in a Warm Anisotropic Plasma", "advisor": "Gould, Roy Walter", "url": "https://resolver.caltech.edu/CaltechTHESIS:06062012-143738636", "creators": [ { "name": { "family": "Fisher", "given": "Raymond Kurt" }, "id": "Fisher-Raymond-Kurt", "display_name": "Fisher, Raymond Kurt" } ], "advisors": [ { "name": { "family": "Gould", "given": "Roy Walter" }, "id": "Gould-R-W", "role": "advisor", "display_name": "Gould, Roy Walter" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "physics" ], "doi": "10.7907/909M-7S46", "abstract": "An experimental investigation of the angular field pattern of a small radio frequency probe in a plasma in a magnetic field is described. The field is observed to become very large along a resonance cone whose axis is parallel to the static magnetic field and whose opening angle is observed to vary with incident probe frequency, electron cyclotron frequency, and electron plasma frequency in agreement with simple cold plasma dielectric theory. The use of the resonance cone angle as a diagnostic tool to measure the plasma density in a plasma in a magnetic field is discussed. It is noted that similar cones might be expected near the ion cyclotron frequency.
\r\n\r\nThe relationship of these cones to the limiting phase- and group-velocity cones which appear in the theory of plane wave propagation is discussed. The necessity for examining the allowed directions of the group velocity rather than the allowed directions of the phase velocity and customary phase velocity plots when determining whether propagation between two remote points in a plasma is possible, is emphasized.
\r\n\r\nThe addition of electron thermal velocities to the theory is examined in the limit of a large static magnetic field. The resonance cone angle is shifted to a slightly smaller angle than that predicted by cold plasma theory, and a fine structure appears inside the cones and is shown to result from an interference between a fast electromagnetic wave and a slow plasma wave. The interference structure is observed experimentally, and measurements of the angular interference spacing are shown to agree with the warm plasma theory.
" }, { "name": "Govindaraju, Shakkottai P.", "degree": "PhD", "year": "1970", "title": "Part I. An Analysis of Reiner's Centripetal Pump. Part II. A Study of Some Turbulent Flows Using a Model for Inhomogeneous Turbulence", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03312009-150404", "creators": [ { "name": { "family": "Govindaraju", "given": "Shakkottai P." }, "id": "Govindaraju-Shakkottai-P", "display_name": "Govindaraju, Shakkottai P." } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/C143-TT24", "abstract": "Part I:\tA centripetal pump consists of a self-aligning rotor in rotation close to a fixed plane stator. In such an apparatus Reiner observed an excess air pressure in the gap between the discs over ambient and a consequent repulsive force between the discs. Reiner interpreted this repulsive force to be the result of non-Newtonian properties of air. Since this hypothesis is in contradiction with known behavior of air in other similar situations, we try to explain it as due to imperfections of the apparatus used and consider three possibilities: dynamic unbalance of the rotor, instability of the rotor and vibrations of the stator. The results of the analyses show that the third possibility can explain Reiner's observations under some reasonable assumptions. It is concluded that Reiner's hypothesis is unjustified.
\r\n\r\nPart II: A model for inhomogeneous turbulence, due to Saffman, describes turbulence in terms of two scalar densities governed by nonlinear diffusion equations. Using this model some turbulent flows are studied to assess the value of the model as a basis for analyzing turbulent flows. The specific problems studied include the two-dimensional wake, the two-dimensional jet, the turbulent couette flow, flow in a channel and the turbulent trailing vortex. Predictions based on the model agree fairly well with experimental results except for the turbulent trailing vortex. Some shortcomings of the model as evident from the application are discussed.
" }, { "name": "Starnes, James Herbert, Jr.", "degree": "PhD", "year": "1970", "title": "The Effect of a Circular Hole on the Buckling of Cylindrical Shells", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-04012009-110903", "creators": [ { "name": { "family": "Starnes", "given": "James Herbert, Jr." }, "id": "Starnes-James-Herbert-Jr", "display_name": "Starnes, James Herbert, Jr." } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VGHN-EQ52", "abstract": "An experimental and theoretical investigation of the effect of a circular hole on the buckling of thin cylindrical shells under axial compression was carried out. The experimental program consisted of tests performed on seamless electroformed copper shells and Mylar shells with a lap joint seam. The copper shells were tested in a controlled displacement testing machine equipped with a noncontacting surface displacement measuring device. Three-dimensional surface plots obtained in this manner showed the changes in the displacement field over the entire shell, including the hole region, as the applied load was increased. The Mylar shells were tested in a controlled load testing machine and demonstrated the effect of increasing the hole radius on the buckling loads of the cylinder.
\r\n\r\nThe theoretical solution was based on a Rayleigh-Ritz approximation. The solution provided an upper bound for the buckling stresses of the cylinders tested for hole radii less than ten per cent of the shell radii. The theoretical solution also identified the governing parameter of the problem as being related to the hole radius, the shell radius, and the shell thickness.
\r\n\r\nThe theoretical part of the investigation showed that even a small hole should significantly reduce the buckling stresses of circular cylinders. Experimentally, it was found that the effect of a small hole is masked by the effects of initial deformations but, at larger hole radii, the reduction in buckling stress took the form predicted by the theory. The experimental results also showed that the character of the shell buckling was dependent on the hole size. For very small holes the shell buckled into the general collapse configuration and there was no apparent effect of the hole on the buckling mode of the shell. For slightly larger holes the shell still buckled into the general collapse configuration, but the buckling stresses of the shell were sharply reduced as the hole size increased. For still larger holes the buckling stresses did not decrease as sharply as the hole size increased and the shell buckled into a stable local buckling configuration.
\r\n" }, { "name": "Wilcox, David Clinton", "degree": "PhD", "year": "1970", "title": "The Motion of a Disc at Angle of Attack in a Rapidly Rotating Fluid", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04032009-113521", "creators": [ { "name": { "family": "Wilcox", "given": "David Clinton" }, "id": "Wilcox-David-Clinton", "display_name": "Wilcox, David Clinton" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EJBR-X182", "abstract": "The motion of a thin disc in a slightly viscous incompressible rotating fluid is studied. The axis of rotation is termed the vertical axis and the fluid and disc are in a container which is bounded by horizontal planes. Nonlinear inertia terms and unsteady effects are assumed small relative to the Coriolis acceleration and hence neglected. Of most importance is the fact that the disc is inclined to the container walls at an angle, \u03b1, which is not necessarily small. The angle is assumed to be large enough so that there are no closed geostrophic contours between the disc and the walls.\r\n\r\nSince the equations of motion are linear, the motions in the six degrees of freedom are considered independently. In all cases, a Taylor column is present although, in all but one case, there is fluid flowing across the boundary of the column. The detailed structure of the shear column is examined for infinitesimal angle of incidence. It is shown that it is possible to solve for the geostrophic flow without actually doing the detailed solution for the shear column structure.\r\n\r\nA static stability study is done and the disc is found to be unstable to small disturbances.\r\n\r\nThe motion of an elliptical plate at finite angle of attack for which the Taylor column is circular is studied. Using the techniques developed for infinitesimal \u03b1, an equation relating the geostrophic flow inside and outside the Taylor column is proposed. This equation is general enough to be used for arbitrary motion of any thin plate. However, only the solution for horizontal translation in a specific direction of the elliptical plate is done.\r\n" }, { "name": "Cheung, Man Cheong", "degree": "PhD", "year": "1969", "title": "The Static and Dynamic Stability of Clamped Shallow Circular Arches", "advisor": "Babcock, Charles D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11162005-081618", "creators": [ { "name": { "family": "Cheung", "given": "Man Cheong" }, "id": "Cheung-Man-Cheong", "display_name": "Cheung, Man Cheong" } ], "advisors": [ { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "advisor", "display_name": "Babcock, Charles D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8MXG-DE89", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental investigation of the relationship between the dynamic buckling and the static buckling phenomenon was carried out.\r\n\r\nThe first half of the paper studies the stability of shallow arches subjected to lateral forces. Concentrated static and step loads were applied to circular arches with geometric parameters 7 < [...] < 20. Equilibrium path and the dynamic response at the loading position were recorded. Dynamic buckling is distinct. Supercritical and subcritical responses were separated by [...] of the total load. The sensitivity of the arch to loading imperfection was studied. The effect was pronounced in the static tests but moderate in the dynamic responses.\r\n\r\nThe second half of the paper studies the response of clamped circular arches under lateral impulse. The motion of the arch was recorded by a high speed camera and the deflected arch shape was measured in detail. The response was expressed in terms of the average displacement and three generalized displacement coordinates representing the amplitudes of the assumed mode shapes. The critical impulse is defined as the impulse level at the inflection point of the curve of maximum response versus impulse." }, { "name": "Collins, Donald James", "degree": "PhD", "year": "1969", "title": "The Near Wake of a Two-Dimensional Hypersonic Blunt Body with Mass Addition", "advisor": "Roshko, Anatol; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-10072002-144356", "creators": [ { "name": { "family": "Collins", "given": "Donald James" }, "id": "Collins-Donald-James", "display_name": "Collins, Donald James" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZHBC-A772", "abstract": "An experimental investigation of the steady, laminar nearwake flow field of a two-dimensional, adiabatic, circular cylinder with surface mass transfer has been made at a free-stream Mach number of 6. 0, and free-stream Reynolds numbers Re\u221e,d=0.9 and 3.0x104.
\r\n\r\nA flush-mounted porous section was used to transfer argon, nitrogen or helium into the near wake of the circular cylinder to determine the flow field associated with the addition of a passive scalar. Two cases were studied: mass transfer from the forward stagnation region, and mass transfer from the base. The pressure field was mapped by standard Pitot- and static-pressure measurements. The mass-concentration field was monitored by a continuous sampling mass-spectrometer system which utilized the output of a single mass peak to determine the relative mass-concentration levels.
\r\n\r\nFor mass addition from the base, a recirculating vortex remains in the near-wake flow and the characteristic near-wake pressure is the pressure at the stagnation point created by the interaction of the reversed flow with the injected fluid. This pressure, and the entire near-wake flow field, correlates with the ratio of the momentum flux of the injected fluid to the momentum flux in the cylinder boundary layer upstream of separation, and not the mass flow of the injected fluid as predicted by Chapman.
\r\n\r\nFor mass addition from the base, the axial mass concentration decays rapidly away from the base as a consequence of the countercurrent diffusion of mass into the oncoming recirculating flow. In addition, strong transverse mass-concentration gradients exist in the region between the two stagnation points and a local maximum occurs in the vicinity of the u = 0 locus for those cases in which ReSc > 0(1) for the reversed flow.
\r\n\r\nWith moderate mass addition from the forward stagnation region, the near-wake pressure field is unperturbed. In addition, because there is no source in the base region, the near-wake mass-concentration field is nearly uniform in the region of reversed flow. Bounding the uniform region, in the vicinity of the viscous shear layers, narrow diffusion layers govern the transport of mass into the outer flow.
\r\n\r\nIn the intermediate-wake region, immediately downstream of the neck, the mass-concentration fields for both forward and base injection are explained by a single model which incorporates the influence both of the accelerating axial velocity and of an assumed Gaussian distribution for the mass-concentration of argon. This model predicts the axial decay of mass concentration in the intermediate wake, and establishes the location of the virtual origin of the asymptotic far wake in terms of the mass-concentration profile parameters at the neck.
\r\n" }, { "name": "Fernandez, Fernando Lawrence", "degree": "PhD", "year": "1969", "title": "Two-Dimensional Viscous Flows with Large Distributed Surface Injection. Part I. Boundary Layer Flows with Large Injection and Heat Transfer. Part II. Experiments in Supersonic Turbulent Flow with Large Distributed Surface Injection. Part III. The Effect of Finite Plate Length", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-155216", "creators": [ { "name": { "family": "Fernandez", "given": "Fernando Lawrence" }, "id": "Fernandez-Fernando-Lawrence", "display_name": "Fernandez, Fernando Lawrence" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TCCK-HT69", "abstract": "This report is concerned primarily with the effect of surface injection on viscous two-dimensional flows. More precisely, the investigation centers on surface injection rates where the wall shear has been considerably reduced below the no-injection value, but where the momentum of the injectant is still negligible compared to that in the free stream. Three separate problems are investigated to try to obtain an understanding of the physical mechanisms which control the flow.
\r\n\t\r\nFor the case of laminar boundary-layer flow, asymptotic solutions are obtained for large injection and heat transfer. It is found in this case that the boundary layer may be divided into two regions: (1) an inner region adjacent to the surface where viscous mixing plays a minor role; (2) a viscous layer where the transition occurs from the inner solution to the inviscid flow outside the boundary layer. In the case of the insulated wall the viscous layer contributes only small corrections to the boundary-layer properties. For the highly-cooled wall the boundary layer is strongly influenced by the viscous mixing between the inviscid outer flow and the high density low-speed gas adjacent to the wall.
\r\n\t\r\nFor turbulent flow, experiments with constant distributed surface injection at M\u221e=2.6 have been performed. These show that large injection leads to a constant pressure self-similar flow with linear growth. The experimental results are shown to be in good agreement with low Mach number experiments when the normal coordinate is stretched by using a Howarth-Dorodnitsyn transformation at the same value of the ratio of wall mass flow per unit area to that in the free stream.
\r\n\t\r\nFinally, the third part considers the upstream effect of the termination of injection on the flow in the \"blown\" layer. An analysis, using an integral approach is presented which agrees with the experimentally observed effects. In particular, as injection rates approaching the maximum value which can be entrained by a constant pressure mixing layer are approached, the analysis predicts that virtually the entire porous region experiences a falling pressure. It is postulated that this effect provides for a smooth transition from a boundary-layer flow to one where mixing is negligible, except in a thin layer near the streamline which divides the injected and freestream gas. Therefore, the analysis provides the step which gives a quantitative estimate for the range of injection rates in turbulent flow where the effect of mixing can be neglected and inviscid flow models utilized.
" }, { "name": "Foster, Michael Ralph", "degree": "PhD", "year": "1969", "title": "I. The Rotation of a Gravitating Sphere in a Monatomic Gas. II. The Drag of a Body Moving Transversely in a Confined Stratified Fluid", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01132006-084851", "creators": [ { "name": { "family": "Foster", "given": "Michael Ralph" }, "id": "Foster-Michael-Ralph", "display_name": "Foster, Michael Ralph" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/89KW-B656", "abstract": "Part I - The Rotation of a Gravitating Sphere in a Monatomic Gas
\r\n\r\nThe flow resulting from the steady rotation of a gravitating sphere in a monatomic gas at rest is studied in a variety of special cases. The low speed rotation problem involves the solution of non-uniform Stokes equations and exhibits the interesting property that, if the field is large enough to make a \"scale height\" very small compared to the sphere radius, the motion is very weak and occurs primarily in a thin boundary layer on the sphere. The asymptotic theory for the gravitational field strength very large with arbitrary rotation speed shows essentially the same boundary layer, regardless of Reynolds number; the perturbation theory presents some interesting mathematical problems as well. The high speed rotation case is finally considered, and solutions have been obtained only for a gas with small Prandtl number. Even then, the flow structure is very complex. Depending on the relative sizes of the Prandtl number and inverse Reynolds number, there are six possibilities. In every case, there is a thin Prandtl boundary layer on the surface of the sphere and an essentially incompressible jet in the equatorial plane. In some cases, a thermal layer outside the Prandtl boundary layer is required to adjust the temperature, and in every case but one, it is necessary to infer the existence of still another layer, which is inviscid but rotational, that adjusts the uniform flow into the layer required by the strong hydrostatic constraints on the outer flow to that necessary for Prandtl boundary layer entrainment. In some cases these layers are unstable to small disturbances if the temperature on the sphere is sufficiently large.
\r\n\r\nPart II - The Drag of a Body Moving Transversely in a Confined Stratified Fluid
\r\n\r\nThe motion of a body through a stratified fluid bounded by vertical plates is studied in the case when the motion of the body is sufficiently slow to make the inertia of the fluid negligible. The case studied is for a very small coefficient of diffusion (for salt in water, for example). The density changes are quite large, and the drag is quite easily computed without appeal to the structure of any boundary layers or shear layers, depending only on changes of potential energy of the fluid. The solution exhibits regions where the fluid is unstably stratified, and hence mixes. Depending upon how complete the mixing process is, the body might experience a thrust!
\r\n\r\nThe equations for boundary layers are given, but details of their solution are not dealt with here, because of their quasi-linear nature. The horizontal shear layers consist of a simple density adjustment layer surrounded by a thicker and quite complicated non linear dynamical layer. The more conventional Stewartson layers do not appear here, these layers, because of the non linearities, are quite complex, and details of their structure have not yet been fully worked out.
" }, { "name": "Ko, Denny Ru-sue", "degree": "PhD", "year": "1969", "title": "I. Supersonic Laminar Boundary Layer Along a Two-Dimensional Adiabatic Curved Ramp. II. Non-Linear Stability Theory for a Laminar, Incompressible Wake", "advisor": "Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-11102003-093156", "creators": [ { "name": { "family": "Ko", "given": "Denny Ru-sue" }, "id": "Ko-Denny-Ru-sue", "display_name": "Ko, Denny Ru-sue" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DM2V-N712", "abstract": "In Part I, the integral method of Lees and Reeves is applied to study a supersonic laminar boundary layer along a two-dimensional adiabatic curved ramp. The present method of solution requires no prior knowledge of the separation point and can be used to treat relatively weak interaction, including a fully attached flow. It starts with small perturbations of the self-induced interaction on a flat plate; consequently, it can be applied to flows with the hypersonic interaction parameter \u03a7\u0305, based on the distance of the beginning station of interaction to the leading edge, of the order 1. The effect of the radius of curvature on the separation phenomena is then investigated using this method. The effect of finite ramp length on the interaction is examined by making use of the characteristics of the singularities associated with the set of moment equations. Satisfactory agreement with the theory is obtained for the corresponding experiments conducted in the Mach 6 wind tunnel at the Graduate Aeronautical Laboratories of the California Institute of Technology.
\r\n\r\nIn Part II, a non-linear theory for the stability of the laminar wake behind a flat plate in an incompressible flow is presented. An integral method is used to investigate the effects of a finite amplitude disturbance on the flow. The flow is decomposed into a mean part, which is independent of time and a fluctuating part, which has a zero time average. The mean flow is assumed to be characterized by two parameters: the centerline velocity defect w[subscript c] and the wake half-width b. By using a two-length expansion procedure, the assumption of local, parallel mean flow is justified for the solution of the fluctuating component to the order considered in the present study. The fluctuation is assumed to be represented by an ascending power series of the amplitude A. The coefficients of the power series, as functions of the radial distance y, are then obtained in terms of the two mean flow parameters wc and b. The three unknowns b, wc and A are then obtained by solving the integral conservation equations of mean momentum, mean energy and fluctuation energy. In this integral method, the higher-order effects are introduced systematically by truncating the expansion for the fluctuation at various orders. The coupling between the mean flow and the fluctuation is found to be the most important mechanism in limiting the fluctuation amplitude and determining the mean flow. Satisfactory agreements with the experiment of Sato-Kuriki in the mean flow quantities and the relative development of the fluctuations are obtained, including the observed effect of free-stream Reynolds number.
" }, { "name": "Lake, Bruce Meno", "degree": "PhD", "year": "1969", "title": "Velocity Measurements Ahead of a Semi-Infinite Body in Magnetohydrodynamic Flow with Aligned Fields", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12282005-142805", "creators": [ { "name": { "family": "Lake", "given": "Bruce Meno" }, "id": "Lake-Bruce-Meno", "display_name": "Lake, Bruce Meno" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PN2X-JB97", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nExperiments are described in which velocities were measured ahead of a semi-infinite Rankine body moving parallel to a uniform magnetic field in a conducting fluid. The flow disturbance in front of the body is found to increase in length as [...], where N is the interaction parameter. In most of the experiments this parameter was varied from 4 to about 50. Measurements made along the axis of symmetry in the flow show that there is a relatively short region of stagnant fluid directly ahead of the body. The major part of the disturbance is found to consist of a much longer region in which the flow undergoes transition from conditions in the freestream to conditions near the body. Velocity profiles across the flow in this region show that for increased N, at a fixed distance ahead of the body, the velocity defect increases and the disturbance becomes more confined radially. Although the radial gradients in the flow increase with N, they are found to be much smaller than would be expected in a flow containing thin current layers. A physical model of the flow which has currents and pressures consistent with these results is discussed." }, { "name": "Parthasarathy, Shakkottai P.", "degree": "PhD", "year": "1969", "title": "I. The Transient Boundary Layer Produced by a Sink on a Plane Wall. II. Flow of Dusty Gases", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04012009-085458", "creators": [ { "name": { "family": "Parthasarathy", "given": "Shakkottai P." }, "id": "Parthasarathy-Shakkottai-P", "display_name": "Parthasarathy, Shakkottai P." } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GAFK-2R04", "abstract": "Part I: The solution for the problem of the transient boundary layers generated by a sink on a plane wall is obtained by an integral method. The incompressible flow is similar and the similarity solutions are obtained for the two dimensional and axisymmetric cases. The velocity layer reaches a steady state and the thermal layer does not. For large times, when the thermal layer is much thicker than the velocity layer, a solution for the temperature field is obtained ignoring the velocity layer. With some approximations to the flow near the sink, similar solutions for compressible flow are also obtained.\r\n\r\nPart IIa: By using the integrated equations of motion, the development of a laminar, two-dimensional, dusty jet issuing from a slit is considered. The solutions are simple in the limits \u03c4 \u2192 0 and \u03c4 \u2192 \u221e, where \u03c4 is the particle relaxation time. For arbitrary \u03c4, a numerical example is given. With some assumptions, the turbulent dusty jet is also considered.\r\n\r\nPart IIb: There are three parameters in the problem of steady motion of a dusty gas around a sphere. These are the Reynolds number R, particle parameter \u03c3 and the mass concentration of dust f[subscript \u221e]. Solutions are obtained by the perturbation method by expanding in terms of R with \u03c3 or \u03c3/R fixed, in the limit R \u2192 0. Solutions are also obtained for the limit R tending to infinity with f < < 1. In both cases critical values of \u03c3 exist, below which the sphere does not capture dust. The efficiency of capture as a function of \u03c3 is calculated in both cases." }, { "name": "Robinson, William McKinley, Jr.", "degree": "PhD", "year": "1969", "title": "Mass Spectrometric Studies of Ionization Precursors Ahead of Strong Shock Waves", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11222005-155153", "creators": [ { "name": { "family": "Robinson", "given": "William McKinley, Jr." }, "id": "Robinson-William-McKinley-Jr", "display_name": "Robinson, William McKinley, Jr." } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0QN4-MY03", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental study was conducted to identify the nature and origin of precursor ions produced by photoionization ahead of strong shock waves in xenon. A magnetic mass spectrometer is mounted at the end of a hydrogen-driven shock tube. Ions produced upstream of a shock wave are sampled by the spectrometer and the collected current provides a continuous time history of a particular ionic species. A mass spectrum is obtained in the precursor region for all impurities found in the shock tube. The incident shock Mach number is varied from 11.9 to 21.3, the initial pressure is varied from 0.050 to 0.500 torr, and the impurity level is altered. In all the conditions studied, the dominant ion present in the precursor is Xe+, although in certain instances, the impurity ion currents are found to be of the same order of magnitude as the xenon ion current. For small impurity levels, photoionization processes in xenon and impurities are apparently independent. Independent double probe measurements determine the total ion density to be about [...] at the shock front, the observable precursor extending about 150 cm from the shock wave.\r\n\r\nA theoretical model accounting for one-step and multi-step photoionization of xenon and impurities is used to find the ionization level ahead of a shock wave. The calculated ion density profiles agree well with experimental observations at the low pressures, where it appears that one-step photoionization predominates. Lack of agreement at high pressures, where, apparently, multi-step ionization is more efficient than the single-step process, suggests inadequacies in the treatment of photoexcitation and multi-step photoionization. Additional areas for experimental study are suggested.\r\n\r\nThe mass spectrometric data yield a better understanding of the role of radiation in shock structure, of the kinetics of photoionization processes in rare gases, and of the influence of impurities in the experimental facility on the radiation mechanisms." }, { "name": "Rose, James Robert", "degree": "PhD", "year": "1969", "title": "An Analysis of the Incompressible Two-Dimensional Jet Ejector", "advisor": "Lissaman, Peter B. S.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-093813", "creators": [ { "name": { "family": "Rose", "given": "James Robert" }, "id": "Rose-James-Robert", "display_name": "Rose, James Robert" } ], "advisors": [ { "name": { "family": "Lissaman", "given": "Peter B. S." }, "id": "Lissaman-P-B-S", "role": "advisor", "display_name": "Lissaman, Peter B. S." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q1EZ-AW49", "abstract": "A general method for calculating the performance of two-dimensional jet ejector systems is presented, with special emphasis on those qualities desired in thrust augmenter devices. The nature of the viscous-inviscid interaction problem is examined and a \"frictionless\" model is defined. The equations are solved for the particular parametric problem of a single jet in a finite length shroud of uniform height. The flow in the \"recovery\" or confined-mixing region is discussed for this model and techniques are developed which may be applied to devices with diffusing systems.\r\n\r\nThe solutions presented for the free-mixing or interaction region are shown to agree with the simple channel flow studied analytically and experimentally by Curtet." }, { "name": "Tombach, Ivar Harald", "degree": "PhD", "year": "1969", "title": "Velocity Measurements with a New Probe in Inhomogeneous Turbulent Jets", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-03312009-110134", "creators": [ { "name": { "family": "Tombach", "given": "Ivar Harald" }, "id": "Tombach-Ivar-Harald", "display_name": "Tombach, Ivar Harald" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/V2CR-0Q19", "abstract": "Velocity profiles have been measured in subsonic, inhomogeneous, axisymmetric turbulent jets, using a new velocity measuring probe. This probe creates a train of heat pulses at one point in the flow and measures the time interval between the time a pulse is created and the time at which it is convected by the flow past a sensing wire a short distance downstream. In a turbulent flow the detected pulses are highly disturbed, but reconstruction of a mean pulse, by a digital computer, from several hundred pulses enabled calculation of a mean velocity and a velocity fluctuation level.\r\n\r\nMeasurements were made with this probe from near the nozzle to 48 diameters downstream in several combinations of jet gas and ambient gas (air-air, air-He, He-air, and He-SF[subfield 6]) having a range of jet/ambient density ratios from 0.03 to 7.25. Axial profiles of the variation with the density ratio of the jet width, mean velocity, and turbulence level were obtained from these measurements, showing the entrainment and approach to homogeneity of the inhomogeneous jet. A jet which is less dense than the ambient fluid is seen to entrain the ambient fluid more vigorously than the denser jet, as is demonstrated by its higher turbulence levels, more rapid growth, and more rapid axial decay of the mean velocity." }, { "name": "Victoria, Keith Jordis", "degree": "PhD", "year": "1969", "title": "The Hypersonic Laminar Boundary Layer Near a Sharp Expansion Corner", "advisor": "Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-06172009-151827", "creators": [ { "name": { "family": "Victoria", "given": "Keith Jordis" }, "id": "Victoria-Keith-Jordis", "display_name": "Victoria, Keith Jordis" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EZX3-M918", "abstract": "The integral moment method for treating interactions between a laminar boundary layer and an external supersonic flow is applied to the problem of the hypersonic laminar boundary layer near sharp and slightly rounded convex (expansion) corners. The general features of this type of interacting flow are established by an analytical solution of the integral equations using the method of matched asymptotic expansions for the case of small interaction parameter. Numerical solutions are obtained for flows for which the interaction parameter can no longer be considered small.
\r\n\r\nAn experimental study is carried out in the GALCIT Mach 8 hypersonic wind tunnel in order to study the two-dimensional laminar boundary layer expansion. Major emphasis is placed on the acquisition of detailed data near the corner region. The basic measurements consist of the model surface pressure distribution and pitot pressure surveys of the boundary layer and inviscid flow field between the boundary layer and the leading edge shock wave both upstream and downstream of the corner region. The surface pressure measurements illustrate the striking departure of the flow field at hypersonic speeds from the classical Prandtl-Meyer description.
\r\n\r\nThese data with appropriate assumptions made regarding the static pressure and temperature fields at points away from the model surface allow calculation of the distributions of profile functions defined in the integral moment method formulation. These distributions along with the surface pressure distribution are compared directly with solutions of the moment equations.
\r\n" }, { "name": "Arbocz, Johann", "degree": "PhD", "year": "1968", "title": "The Effect of General Imperfections on the Buckling of Cylindrical SheIls.", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-11292005-080202", "creators": [ { "name": { "family": "Arbocz", "given": "Johann" }, "id": "Arbocz-Johann", "display_name": "Arbocz, Johann" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DSCT-G640", "abstract": "An experimental and theoretical investigation of the effect of general imperfections on the buckling load of a circular cylindrical shell under axial compression was carried out.\r\n\r\nA non-contact probe has been used to make complete imperfection surveys on electroformed copper shells before and during the loading process up to the buckling load. The data recording process has been fully automated and the data reduction was done on an IBM 7094. Three-dimensional plots were obtained of the measured initial imperfection surfaces and of the growth of these imperfections under increasing axial load. The modal components of the measured imperfection surfaces were also obtained.\r\n\r\nThe theoretical solution located the limit points of the post-buckled states. A simplified imperfection model was used consisting of one axisymmetric and one asymmetric component. For global buckling the correlation between the theoretical buckling loads and the experimental values was found to be good." }, { "name": "Crespo Mart\u00ednez, Antonio", "degree": "PhD", "year": "1968", "title": "I. Theoretical Investigation of the Reflection of Ionizing Shocks. II. Theoretical Study of Sound and Shock Waves in a Two-Phase Flow", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05102005-094142", "creators": [ { "name": { "family": "Crespo Mart\u00ednez", "given": "Antonio" }, "id": "Crespo-Mart\u00ednez-Antonio", "display_name": "Crespo Mart\u00ednez, Antonio" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3XYJ-AC18", "abstract": "PART I\r\n\r\nThe reflection of an ionizing shock from the end wall of a shock tube is studied theoretically following the experimental model of J. Smith. The observed perturbations in the wall pressure history are found to agree with this theory. To describe the first perturbation, a decrease in pressure due to the ionization part in the reflected shock structure, the flow equations are linearized but the rate equations are used in nonlinear form. The second perturbation, an increase in pressure due to the ionization part of the incident shock structure, is studied using Whitham's theory and assuming equilibrium behind the reflected shock.\r\n\r\nPART II\r\n\r\nThe propagation of sound and shock waves in a two-phase medium is studied theoretically using the flow equations for each component. It is shown that the assumption of constant mass ratio during the sound propagation, used previously in the literature for the case of bubbles suspended in a liquid, is only valid for low frequencies. For high frequencies a larger sound speed is obtained. These two sound speeds give two different Mach numbers. It is found that when both Mach numbers are larger than one, the shock structure in a liquid containing bubbles is given by an initial increase of the pressure, followed by a region in which it oscillates around its final equilibrium value. When the low frequency Mach number is larger than one, and the high frequency Mach number smaller than one, the oscillations disappear and the transition is uniform.\r\n\r\nThe speed of sound of the mixture is also calculated by evaluating the scattering by the suspended phase." }, { "name": "Fersht, Rena Scher", "degree": "PhD", "year": "1968", "title": "Buckling of Cylindrical Shells with Random Imperfections", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-11142005-105632", "creators": [ { "name": { "family": "Fersht", "given": "Rena Scher" }, "id": "Fersht-Rena-Scher", "display_name": "Fersht, Rena Scher" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KF7B-XA89", "abstract": "The buckling stability analysis of long cylindrical shells with random imperfections subjected to axial load is treated using two different approaches. The first study is based on a Lyapunov method which enables one to establish sufficient conditions for buckling stability of a long cylindrical shell with axisymmetric random imperfections. A perturbed system of equations in the neighborhood of the prebuckling solution is investigated. By reducing the problem to a system of integral equations, it is observed that the stability boundary value problem of a long shell is similar to that of a dynamical system with random parametric excitations.\r\n\r\nInitial imperfections were assumed to have Gaussian distribution and an exponential cosine correlation function. The critical load was obtained as a function of the root mean square of the imperfections. Results obtained are qualitatively similar to those of Koiter for a periodic imperfection (Ref. 1).\r\n\r\nThe second part is based on the approximate method of truncated hierarchy. The prebuckling state of equilibrium for asymmetric imperfections is found by a successive substitution technique. A homogeneous variational system of equations is set up in order to examine the existence of bifurcation in the neighborhood of the equilibrium state. These last equations involve random parametric terms. The truncated hierarchy method is applied and characteristic equations are obtained. Various exponential cosine correlation functions associated with asymmetric imperfections are examined numerically. Qualitatively the results obtained are as anticipated." }, { "name": "Mueller, Hans-Karl Christian Alfred", "degree": "PhD", "year": "1968", "title": "Stable Crack Propagation in a Viscoelastic Strip", "advisor": "Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11152005-142505", "creators": [ { "name": { "family": "Mueller", "given": "Hans-Karl Christian Alfred" }, "id": "Mueller-Hans-Karl-Christian-Alfred", "display_name": "Mueller, Hans-Karl Christian Alfred" } ], "advisors": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MGSD-R362", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA crack of length 2a which propagates with small, constant speed through a viscoelastic strip of width 2b is considered. The strip is strained by displacing its shearfree edges. Linear theory is applied. The stress on the line of crack advancement and the shape of the crack surface are calculated for a state of plane stress.\r\n\r\nThe stress intensity factor which is independent of material properties is given as a function of a/b. It exhibits a maximum at [...]. For a/b > 1.5 the stress intensity factor becomes essentially independent of crack length. The crack surface deflection is obtained in the form of a superposition integral and is a function of material properties and crack speed.\r\n\r\nThe energy which is released when the crack extends a small distancee is calculated. This crack energy depends on the crack speed and involves the creep function of the material. A characteristic length enters in the course of its derivation. This length does not appear in the case of an elastic material and is considered as an additional material property necessary to describe viscoelastic crack propagation.\r\n\r\nThe energy conservation equation is established by considering a small control volume surrounding the crack tip. A relationship emerges from this equation which implicitly gives a stable crack speed as a function of applied strain, temperature, and material properties. The creep function is the controlling factor in this equation.\r\n\r\nThe relevant material properties are discussed and presented for a Polyurethane rubber (Solithane 113 - 50/50). The lower bound of the surface energy is determined from fracture tests on the swollen material. The results of the material characterization are used to calculate the crack speed as a function of applied strain and temperature. Good agreement is found to exist between theory and experiment." }, { "name": "Piva, Hugo de Oliveira", "degree": "PhD", "year": "1968", "title": "Electron Beam Measurements of Density in Shock Waves Reflecting From a Cold Wall", "advisor": "Sturtevant, Bradford", "url": "https://resolver.caltech.edu/CaltechETD:etd-11292005-131010", "creators": [ { "name": { "family": "Piva", "given": "Hugo de Oliveira" }, "id": "Piva-Hugo-de-Oliveira", "display_name": "Piva, Hugo de Oliveira" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EDZS-EA78", "abstract": "The normal shock wave is a rarefied-gas flow in which large departures from thermodynamic equilibrium and strong non-linearities occur and it is the simplest such flow due to its one-dimensionality and the absence of solid boundaries. Because of this, both theoretical and experimental studies of the structure of normal shock waves have been used successfully in recent years to get a better understanding of such phenomena. The present work is an extension of the earlier studies to the more complicated problem of shock-wave reflection. The density distribution of a normal shock wave in argon is measured during its reflection from a heat-conducting wall using an electron beam densitometer in the GALCIT 17-in diameter shock tube at incident Mach numbers 6.00, 4.00 and 2.96.\r\n\r\nDuring each run a density history is obtained at a certain distance from the wall by measuring electron-beam current as a function of time. By defining a consistent zero of time for all runs at the same conditions, these histories are converted by a cross-plot to families of density profiles at different times. x-t diagrams are obtained from the density history plots, and a wall-density history is obtained from the density profiles. Measured in terms of the ideal conditions behind the incident shock wave, the distances extend from 0.25 to 56 mean free paths from the wall and the times to 160 mean collision times after the beginning of the reflection.\r\n\r\nThe results presented here give both quantitative and qualitative information about the interaction of the incident shock wave with the wall, the effects of the wall heat transfer and accommodation on the density near the wall, the formation of the reflected shock wave, its strength and trajectory on the x-t diagram, and the nature of the flow field lying between the thermal layer and the reflected shock wave at large times." }, { "name": "Alber, Irwin Emanuel", "degree": "PhD", "year": "1967", "title": "Integral Theory for Turbulent Base Flows at Subsonic and Supersonic Speeds", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-141417", "creators": [ { "name": { "family": "Alber", "given": "Irwin Emanuel" }, "id": "Alber-Irwin-Emanuel", "display_name": "Alber, Irwin Emanuel" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "chair", "display_name": "Lees, Lester" }, { "name": { "family": "Babcock", "given": "Charles D." }, "id": "Babcock-C-D", "role": "member", "display_name": "Babcock, Charles D." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "member", "display_name": "Coles, Donald Earl" }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "member", "display_name": "Saffman, Philip G." }, { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Wu-T-Y-T", "role": "member", "display_name": "Wu, Theodore Yao-tsu" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0YEZ-H062", "abstract": "The integral near wake analysis of Reeves and Lees developed for supersonic laminar base flows is extended to the case of fully turbulent separated adiabatic flow behind a rearward facing step at both subsonic and supersonic speeds. A turbulent eddy viscosity model is formulated for the shear stress scaling of the dissipation integral in the mechanical energy equation. It is shown that the eddy viscosity can be described simply by one incompressible constant (valid for both shear layers and wakes) and one reference density \u03c1r. Using a compressibility transformation, theoretical solutions for the spreading rates of free shear layers are found to agree with experiment when the reference density is chosen to be the centerline density for the wake flow.
\r\n\r\nTwo alternate methods are presented for joining the wake flow solution to the body first, through a turbulent free shear layer mixing solution, and then through the use of a two parameter family of velocity profiles valid near the body. A simple conservation model is presented to relate the viscous sublayer after expansion to the initial boundary layer ahead of the step.
\r\n\r\nFor free stream Mach numbers M1 \u2264 2.3, the integral theory is found to give good estimates for the length scales and centerline pressure variations measured experimentally for both wake flows and step flows (where reattachment is to a solid surface).
\r\n\r\nAn iterative method of solution for the incompressible wake flow problem is presented as an extension of the work of Green. The calculation proposes the proper criteria for obtaining a convergent solution. The base pressure coefficient is found to be equal to the difference between the momentum thicknesses in the far wake and at the base.
\r\n" }, { "name": "Batt, Richard George", "degree": "PhD", "year": "1967", "title": "Experimental Investigation of Wakes Behind Two-Dimensional Slender Bodies at Mach Number Six", "advisor": "Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12292005-132450", "creators": [ { "name": { "family": "Batt", "given": "Richard George" }, "id": "Batt-Richard-George", "display_name": "Batt, Richard George" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3WW3-B746", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental investigation has been conducted to determine mean flow properties for both near and far wakes behind several two-dimensional slender bodies at M[...] = 6. Three adiabatic wall models consisting of a flat plate model and two 20 [degree] included angle wedge models (H = .15\", H = .3\") were tested. The effect of wall temperature on wake properties was examined by cooling the larger of these two wedge models with the internal flow of liquid nitrogen ([...] = .19). Free stream Reynolds numbers were varied from [...] to [...] for each of these four configurations. In the far wake, measurements of total temperature, as determined with hot wire probes, and Pitot and static pressures were used to derive all other mean flow properties. The effect of transition on these far wake data was determined. Near wake flows were laminar for all adiabatic wall tests and at least for the two lowest test Reynolds numbers of the cold wall wedge. Base region flow field mappings and shear layer profiles were obtained for the .3\"H wedge model by combining Pitot pressure data with hot wire measurements of total temperature and mass flux. These results illustrated that for slender bodies with flat bases, the basic structure for laminar near wakes is appreciably more complex for hypersonic than for supersonic flow primarily because, in hypersonic flow, the corner expansion fan extends into the separated shear layers and base region shocks now become imbedded within the viscous portion of the shear layers." }, { "name": "Flandro, Gary Arnold", "degree": "PhD", "year": "1967", "title": "Rotating Flows in Acoustically Unstable Rocket Motors", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02272004-154250", "creators": [ { "name": { "family": "Flandro", "given": "Gary Arnold" }, "id": "Flandro-Gary-Arnold", "display_name": "Flandro, Gary Arnold" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BYWJ-5Q96", "abstract": "One of the most interesting manifestations of acoustic combustion instability in solid propellant rocket motors is the formation of strong vortices in the combustion chamber. A single vortex filament stretching along the motor axis from the head-end has been observed in several experiments in association with gas oscillations in the frequently occurring traveling tangential mode of instability. These flows are sometimes accompanied by a quite noticeable axial torque on the motor itself, and this effect has given rise to flight performance difficulties in a number of instances. Previous theoretical studies of the vortex generation effect have been inadequate in several respects. The present work is an attempt to place the theory on a more firm base and to clarify the connection between traveling wave motions and the generation of vortices and torques.
\r\n\r\nIt is readily shown that traveling waves transport momentum, and in the case of traveling tangential waves in a cylindrical combustion chamber this represents a steady axial component of angular momentum in the gas. This observation gives rise to a simple conceptual model of the vortex generation effect. Thus the presence of a steady mass flux about the axis implies the existence of a layer of vorticity at the chamber walls which may be represented by a vortex sheet composed of axially oriented bound vortex filaments. In the three-dimensional case these vortices are shed either at the end of propellant grain or at the periphery of the nozzle; the other ends of the filaments traverse the fore-end closure to the center and are combined and shed in an intense vortex filament along the symmetry axis of the motor.
\r\n\r\nDue to the production of gas at the chamber wall, tangential forces at the wall are produced by the interaction of this mean flow with the bound vortices. Angular momentum arguments must be used in this conceptual mechanism to estimate the strength of the axial vortex filament, and it is readily shown that the sense of the vortex must be opposite to the direction of travel of the waves. The direction and magnitude of the torque on the motor depend on the mean flow Mach number at the wall and must be established by calculation of the wall shear stresses.
\r\n\r\nThe detailed calculations are guided by the mechanism just outlined. All physical features of the problem which appear to be significant are simulated mathematically. In particular, the effects of the three dimensional mean flow pattern in the chamber and the pressure-sensitive combustion region at the burning surface are represented. Also considered are the effects of freedom of motion of the rocket motor in the plane normal to the symmetry axis. Both inviscid and viscous theories are developed using multi-parameter asymptotic perturbation expansion techniques. It is proved that traveling tangential waves are subject to amplification under conditions existing in typical solid propellant rockets, and that a steady transport of gas about the chamber axis accompanies this motion as a second-order perturbation. The equations of motion admit of only a vortex-like steady second-order azimuthal solution. This must be superimposed on the acoustic wave motions in such a way that angular momentum is conserved (due consideration being given to body forces on the gas and tangential forces at the wall). Thus the net pattern of steady circumfer vential mass flux at a given motor cross-section consists of a drift of fluid in the direction of the wave adjacent to the wall with a rapid transition to an oppositely spinning vortex flow as the longitudinal axis is approached. Introduction of the viscous corrections gives rise to a boundary. condition which sets the vortex strength, and a formal connection with the classical acoustic streaming effect is established. Since momentum is dissipated in the shear region at the wall, a torque appears on the chamber itself. This roll moment is opposite in sense to the wave travel during amplification of the acoustic waves, and numerical calculations give torque magnitudes which are in agreement with experimental data from several sources.
\r\n" }, { "name": "Harstad, Kenneth Gunder", "degree": "PhD", "year": "1967", "title": "Steady Laminar Compressible Magneto-Fluid-Dynamic Gas Flows in Channels.", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09172002-144944", "creators": [ { "name": { "family": "Harstad", "given": "Kenneth Gunder" }, "id": "Harstad-Kenneth-Gunder", "display_name": "Harstad, Kenneth Gunder" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5T8X-8S80", "abstract": "Numerical computations are carried out for the core flow of subsonic MFD generator channels with a large length-to-height ratio and fine electrode segmentation. The working fluid is taken as potassium seeded argon. Variable transport properties and radiation effects are considered. It is shown that transverse variations in fluid properties are very important in Faraday generators; a one-dimensional analysis of the flow is not adequate. Axial currents in nonequilibrium flows can be kept low if the right value of the Hall parameter can be obtained; this also depends critically on the Mach number and load parameter. Mach numbers much less than one and high load parameters are to be avoided. Attainment of very large Hall parameters and fields cannot be expected." }, { "name": "Hoffman, Alan Lowell", "degree": "PhD", "year": "1967", "title": "Magnetohydrodynamic Shock Production and Current Sheet Diffusion", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12292005-135853", "creators": [ { "name": { "family": "Hoffman", "given": "Alan Lowell" }, "id": "Hoffman-Alan-Lowell", "display_name": "Hoffman, Alan Lowell" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4WVH-W290", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nCurrent sheets in inverse pinch MHD shock tubes exhibit the strange property of forming shocks in the very rear of the sheet when accelerating heavy gases. When accelerating light gases, shocks are formed further to the front in the sheet, but in no case do the shocks separate from the driving current sheet. This \"piston dragging shock\" effect is explained on the basis of a single-fluid model with variable conductivity. Shocks are shown to always form within current sheets which move at supersonic speeds with respect to the driven gas. The relevant parameters for determining the shock position are the Mach number and the magnetic Reynolds number. Large magnetic Reynolds numbers and small Mach numbers enhance forward shock formation. These conditions are obtained in light gases with high speeds of sound. Similarity methods are developed to estimate gas conductivities, electron temperatures, and degrees of ionization for the experiments which are conducted. In hydrogen typical electron temperatures of 4 ev are produced by the ohmic heating, but twice this value is shown necessary to achieve separation at the current sheet speeds of 2-3 [...] used. Higher current sheet speeds produce shocks in the rear of the current sheet where separation can never occur. The correct method of procedure and the relevant design parameters to achieve separation are given. The success of single-fluid methods in explaining plasma phenomena is especially notable, and these methods can be extended to other similar problems. Based on these methods, multiple-fluid and microscopic effects are easily detectable and can be accounted for." }, { "name": "Klein, Alan Frank", "degree": "PhD", "year": "1967", "title": "I. The Use of a Large Conventional Shock Tube as a Pre-Ionizer for an Inverse Pinch Shock Tube. II. The Application of Thin-Film Heat Transfer Gauges and Flush Electrostatic Probes to Partially Ionized Flows in Shock Tubes", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12292005-133945", "creators": [ { "name": { "family": "Klein", "given": "Alan Frank" }, "id": "Klein-Alan-Frank", "display_name": "Klein, Alan Frank" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/G7GV-GX76", "abstract": "Part 1.
\r\n\r\nA large diameter (17\"), conventional shock tube has been used as the pre-ionizer for an inverse pinch shock tube in an attempt to achieve separation of the shock front and the current sheath in the inverse pinch. The inverse pinch was mounted in the endwall of the shock tube and was operated without an anode, either behind the incident or reflected shock wave generated by the pre-ionizer shock tube. The test gases used were Xenon, Argon, and Helium. Separation was not achieved, but in Argon the pressure front did move closer to the front of the current sheath. In Xenon, no improvement in the performance of the inverse pinch was observed as a result of the pre-ionization, and in one case it was noticeably degraded, with the piston appearing to leak excessively. Because of test time limitations it was only possible to operate the inverse pinch behind the incident shock wave in Xenon. By measuring the ionization relaxation time in Xenon it was found that for all the conditions of the present experiments, ionization equilibrium was not attained in the times available. Therefore, the inverse pinch was being operated in a slightly ionized, relaxing gas. The electrical conductivity of such a gas was calculated for Xenon and Argon and the results in Argon were found to be in good agreement with previous shock tube measurements of the conductivity. The relaxation time measurements, conducted primarily in the GALCIT 6\" shock tube, show that P\u2081\u03c4, the product of the initial pressure and the relaxation time behind the incident shock, depends strongly upon the magnitude of P\u2081, especially for P\u2081 < .5 mm Hg of Xenon. The dependence decreases as the Mach number is increased in the range 10 < Ms < 20.6.
\r\n\r\nPart 2.
\r\n\r\nPrevious shock tube observations of \"spurious\" signals in the output of thin-film heat gauges at Mach number for which the shocked gas becomes partially ionized are summarized. It is shown that these effects, and those observed the the present experiments in Xenon, cannot be explained in terms of a shorting gauge model. It is demonstrated that the effects are due to the gauge acting more as an electrostatic probe than as a heat gauge. Under these conditions it is shown that the heat gauge provides an accurate measurement of the ionization relaxation time as well as still being useful for determining the shock velocity. The thin-film is also operated as a flush electrostatic probe to measure the ion density in the shock tube wall boundary layer, and the experimental results are in good agreement with the theoretical predictions of two solutions of the boundary layer problem: one based on an approximate solution of the transformed boundary layer equations, and the other based on the solution of the equivalent Couette flow problems. The applicability of these solutions is found to be limited to conditions for which the ionization relaxation time is either very long or very short. Because of the computational simplifications involved it is seen that the Couette flow solution is preferred under most conditions.
" }, { "name": "Lewis, John Eldon", "degree": "PhD", "year": "1967", "title": "Experimental Investigation of Supersonic, Laminar, Two-Dimensional, Boundary Layer Separation in a Compression Corner with and without Cooling", "advisor": "Lees, Lester; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-11162005-102455", "creators": [ { "name": { "family": "Lewis", "given": "John Eldon" }, "id": "Lewis-John-Eldon", "display_name": "Lewis, John Eldon" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/R0FB-DD20", "abstract": "An experimental investigation of the boundary layer separation associated with a compression corner was conducted in the GALCIT Mach 6 wind tunnel, and a supplementary study was performed in the JPL supersonic wind tunnel. Special emphasis was placed on the development of a wind tunnel model which approximated true two-dimensional flow, and which could be run in either a highly cooled or an adiabatic configuration. The basic measurements consist of the model surface pressure and temperature, and Pitot surveys of the boundary layer. The surface pressure distributions for the adiabatic wall configurations are compared with the theory of Lees and Reeves (modified by Klineberg and Lees). The surface pressure distribution for the cold wall was compared with the adiabatic configuration for a laminar interaction, and the dependence on Reynolds number for both laminar and transitional interactions are observed. The \"free interaction\" similarity suggested by Chapman is empirically tested and found to be a good approximation for the adiabatic configuration, but it fails to correlate the cooled with the adiabatic case. The scaling suggested by Curle was tested and found to eliminate this deficiency." }, { "name": "Smith, Jerome Allan", "degree": "PhD", "year": "1967", "title": "An Experimental Investigation of the Structure of Reflecting, Ionizing Shocks in Xenon", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-10092002-160549", "creators": [ { "name": { "family": "Smith", "given": "Jerome Allan" }, "id": "Smith-Jerome-Allan", "display_name": "Smith, Jerome Allan" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KSPH-2V57", "abstract": "The results of an experimental investigation of the reflection of strong shocks in xenon from the end wall of the GALCIT 6\" shock tube are presented. The reflection of the incident shock structure, consisting of a frozen shock front, a region of relatively uniform frozen flow, and an ionization front, was observed with a fast-rise (0.3 [mu] sec) pressure gauge mounted in the shock tube end wall. The interaction between the reflecting shock and the ionizing gas in the incident shock structure was of particular interest. This interaction produces a complicated series of shock and rarefaction waves; those waves that propagate back to the end wall were observed with the pressure gauge. The incident shock Mach number was varied from 11 to 20, and the initial pressure was varied from 0.1 to 1.5 mm Hg.\r\n\r\nA simple model which includes the gross features of the shock reflection process is used to calculate end wall pressures. The calculated pressures agree well with the experimental observations. In addition, ionization relaxation times for xenon behind the incident and reflected shocks are determined from this simple model and the measured pressure histories. The relaxation time data yield a better understanding of the ionization relaxation process in monatomic gases and provide an estimate for the electron-atom, inelastic excitation cross section for xenon." }, { "name": "Tuan, Dorothy Yung-Hsun", "degree": "PhD", "year": "1967", "title": "Part I. Interaction of DNA and Histone in Native Nucleohistone. Part II. Dormancy Associated with the Repression of Genetic Activity", "advisor": "Bonner, James Frederick", "url": "https://resolver.caltech.edu/CaltechETD:etd-10042002-162144", "creators": [ { "name": { "family": "Tuan", "given": "Dorothy Yung-Hsun" }, "id": "Tuan-Dorothy-Yung-Hsun", "display_name": "Tuan, Dorothy Yung-Hsun" } ], "advisors": [ { "name": { "family": "Bonner", "given": "James Frederick" }, "id": "Bonner-J-F", "role": "advisor", "display_name": "Bonner, James Frederick" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "bioch" ], "doi": "10.7907/X7X3-QH34", "abstract": "Part I. Interaction of DNA and histone in nucleohistone \r\n\r\nChapter 1. SELECTIVE DISSOCIATION OF HISTONE FROM NUCLEOHISTONE\r\n\r\nWith increasing concentration of NaCl solution, an increasing amount of histone is dissociated from dissolved nucleohistone. The dissociated histone fractions are identified by gel electrophoresis. The lysine rich histone fraction I is dissociated from nucleohistone in the range 0.3-0.5 F NaCl; slightly lysine rich histone II in the range 0.8-1.6 F NaCl; arginine rich histone III+IV in the range 0.9-1.6 F NaCl. The results suggest that both electrostatic and non-electrostatic interactions contribute to the strength of binding between DNA and histones.\r\n\r\nChapter 2. OPTICAL ROTATORY DISPERSION STUDIES ON HISTONES\r\n\r\nThe optical rotatory dispersion spectra of histones free and in reconstituted nucleohistone (in which histone is complexed to DNA) are recorded. By the criterion of optical rotatory dispersion at wavelengths below 220 mu, histone I is the least helical of the histones, histone II the most helical. The helicity of DNA-bound histones in reconstituted nucleohistone is greater than that of free histones, but the order, histone II most helical and histone I least, is still preserved.\r\n\r\nChapter 3. OPTICAL ROTATORY DISPERSION STUDIES ON THE DNA OF NATIVE NUCLEOHISTONE AND OF PARTIALLY DEHISTONIZED NUCLEOHISTONES\r\n\r\nThe conformation of DNA in native nucleohistone is altered by the DNA-histone interaction. The dissociation of histone I does not produce significant conformational change in DNA of nucleohistone but removal of histone II and of histone III+IV bring about changes which cause the conformation of DNA in nucleohistone to resume virtually that characteristic of free DNA. The possibility of DNA supercoiling in nucleohistone is discussed.\r\n\r\nPart II. Dormancy associated with repression of genetic activity\r\n\r\nChapter 1. THE DORMANCY OF POTATO BUDS\r\n\r\nChromatin of the buds of dormant potato tubers is almost totally incapable of the support of DNA-dependent RNA synthesis in the presence of added exogenous RNA polymerase. The chromatin of non-dormant buds of potato tubers (in which dormancy has been broken by treatment with ethylene chlorohydrin) is highly effective in the support of DNA-dependent RNA synthesis by added exogenous RNA polymerase. It is therefore concluded that the genetic material of the buds of dormant potato tubers is largely in a repressed state, and that the breaking of dormancy is accompanied by derepression of the genetic material.\r\n\r\nChapter 2. THE DORMANCY OF ONION BULBS\r\n\r\nThe chromosomal material of non-growing and non-dividing onion buds possesses template activity in support of in vitro DNA-dependent RNA synthesis. If we define dormancy not only by the absence of visible growth and mitotic division, but also by the lack of ability to direct in vitro DNA-dependent RNA synthesis, potato buds are then dormant but onion buds are not. And the block to onion bud growth must lie somewhere else than in the repression of genetic material.\r\n\r\nChapter 3. ISOLATION OF GLADIOLUS CHROMATIN\r\n\r\nGladiolus corms contain very little isolatable chromatin material and the isolated chromatin is highly contaminated by the presence of starchy material." }, { "name": "Tyson, Thomas Janney", "degree": "PhD", "year": "1967", "title": "Laminar Boundary Layers in the Neighborhood of Abrupt Spatial Disturbances", "advisor": "Sechler, Ernest Edwin; Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-111459", "creators": [ { "name": { "family": "Tyson", "given": "Thomas Janney" }, "id": "Tyson-Thomas-Janney", "display_name": "Tyson, Thomas Janney" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/S72R-AM75", "abstract": "Supersonic, steady laminar boundary layers exhibiting \"strong\" local interaction with the outer flow are considered. The general behavior of such flows on a flat adiabatic plate are studied by means of the \"moment method\" equations and by finite difference solutions of the full boundary layer equations, including the transverse momentum equation. A one-parameter family of \"free interaction\" solutions is generated with the finite difference approach. These include separated reverse flow solutions. The infinite plate solution is established from the leading edge through weak interaction by both techniques. Expansive corner flow solutions are developed using both methods. In the \"moment method\" study the nature of the leading edge, Blasius point and \"critical\" line singularities is developed by numerical investigation." }, { "name": "Walsh, Myles Alexander, III", "degree": "PhD", "year": "1967", "title": "On the Turbulent Flow of Dilute Polymer Solutions", "advisor": "Blatz, Paul J.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-140727", "creators": [ { "name": { "family": "Walsh", "given": "Myles Alexander, III" }, "id": "Walsh-Myles-Alexander", "display_name": "Walsh, Myles Alexander, III" } ], "advisors": [ { "name": { "family": "Blatz", "given": "Paul J." }, "id": "Blatz-P-J", "role": "advisor", "display_name": "Blatz, Paul J." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6NTP-AH77", "abstract": "This paper is concerned with the problem of explaining the anomalous decrease in turbulent skin friction observed in the turbulent flow of very dilute polymer solutions.\r\n\r\nThe experimental evidence for dilute solutions is summarized.\r\n\r\nThe polymer molecule in solution is examined from a theoretical point of view, using the Rouse model. It is found that the model predicts that the molecule will locally store energy as a function of the local strain rate of the solution.\r\n\r\nThe experimental evidence is reexamined and it is concluded that the anomalous decrease in turbulent momentum transport results because the molecules manage to alter the energy balance of the small disturbances at the edge of the viscous sublayer. By slightly altering this balance the molecules allow viscous dissipation to destroy disturbances which would have had sufficient energy to grow had the molecules not been present. By decreasing the number of small disturbances which grow per unit area and time and move out from the edge of the viscous sublayer, the addition of polymer molecules ultimately changes the structure of the turbulence in the outer part of the boundary layer. This change results in lower Reynolds stresses and hence lower turbulent momentum transport.\r\n\r\nWith the help of the relation for local energy storage derived from the Rouse model, parameters are developed to characterize the phenomenon. These parameters appear to be useful in understanding the experimental evidence to date." }, { "name": "Wang, Charles Chang-Ping", "degree": "PhD", "year": "1967", "title": "Mass Spectrometric Studies of Ionization in Shock Heated Gas Mixtures", "advisor": "Sturtevant, Bradford; Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11142005-104254", "creators": [ { "name": { "family": "Wang", "given": "Charles Chang-Ping" }, "id": "Wang-Charles-Chang-Ping", "display_name": "Wang, Charles Chang-Ping" } ], "advisors": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-B", "role": "advisor", "display_name": "Sturtevant, Bradford" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GN3Q-Y356", "abstract": "The initial stage of thermal ionization in gas mixtures behind strong normal shock waves has been studied using a mass spectrometer. The sampling process, the unique role of mass spectrometric analysis, the ionization mechanism and the effect of impurities on the ionization relaxation time of the inert gases are discussed. A detailed analysis of ion diffusion through the end wall thermal boundary layer to investigate the effect of the sampling process on the reaction and reaction products are described. The experiments were conducted in argon and xenon at temperatures of from 10,000 \u00baK to 30,000 \u00baK and pressures from 15 mmHg to 80 mmHg. A large number of different ions were detected in \"pure\" (the estimated level of naturally occurring impurities was 50 ppm) argon. H+ and O+ were found in much greater amounts than any of the other products. The roles of H and O were studied further by mixing a small amount of H2 or O2 in argon. The activation energy and the effective cross section for ionization were obtained from the measured ionization rate. For argon, the activation energy agrees within experimental accuracy with the result of Kelly (1966). For hydrogen the activation energy is about 10 ev, and this value coincides with its first excited state. The effective cross section is a thousand times larger than the corresponding effective cross section for the ionization of argon. This explains why small amounts of hydrogen affect the argon ionization relaxation time." }, { "name": "Wesseling, Pieter", "degree": "PhD", "year": "1967", "title": "The Driving Mechanism of Strongly Developed Taylor Vortex Flow", "advisor": "Lagerstrom, Paco A.; Saffman, Philip G.; Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechTHESIS:06132025-213106117", "creators": [ { "name": { "family": "Wesseling", "given": "Pieter" }, "id": "Wesseling-Pieter", "display_name": "Wesseling, Pieter" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." }, { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/00y0-xt20", "abstract": "The purpose of this thesis is to explain how strongly\r\ndeveloped Taylor vortex flow is kept motion. On the basis of this\r\nexplanation approximate torque calculations for high Taylor numbers\r\nhave been made. Agreement with experiment is satisfactory. Axial\r\nsymmetry is assumed throughout." }, { "name": "Witte, Arvel Benjamin", "degree": "PhD", "year": "1967", "title": "Part I. Experimental Investigation of an Arc-Heated Supersonic Free Jet. Part II. Analysis of One-Dimensional Isentropic Flow for Partially Ionized Argon", "advisor": "Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-12022005-082626", "creators": [ { "name": { "family": "Witte", "given": "Arvel Benjamin" }, "id": "Witte-Arvel-Benjamin", "display_name": "Witte, Arvel Benjamin" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/STEK-4N18", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nI. Experimental Investigation of an Arc-Heated Supersonic Free Jet.\r\n\r\nAn experimental investigation of the flow field of a highly ionized supersonic free jet has been carried out in a continuous-flow test facility. Measurements of impact pressure, mass flux, total enthalpy and stagnation point heat transfer profile were made in this flow field with two water cooled probes.\r\n\r\nArgon gas, at a flowrate of 0.5 gm/sec, was heated in a magneto-plasma-dynamic arc heater without an external magnetic field operating from between 200 amp and 40 volts to 1000 amp and 25 volts. The total pressure ranged from between 20 and 35 mm Hg, at constant flowrate, and the atom-ion number density was approximately [...] cm[...] at the exit plane. The average total enthalpy calculated from a heat balance ranged from between about 5,000 to 10,000 BTU/[...], while the probe measurements showed that the peak total enthalpy on the jet centerline near the exit plane was about three times the average total enthalpy.\r\n\r\nThe impact and mass flux measurements showed that the flow was hypersonic, source-like, chemically frozen, and in other details very much like the under-expanded free jet flow of a perfect gas. By combining these measurements with the total enthalpy measurements it was shown that the fraction of the total energy contained in ionization was about 0.6 which is quite close to the equilibrium stagnation value. For equilibrium stagnation conditions, the total temperature ranges from between 12,000\u00b0 to 20,000\u00b0K. The species mass fraction ranges from 0.2 for the atoms and 0.8 for the singly-ionized ions, to 0.8 for the singly-ionized ions and 0.2 for doubly-ionized ions.\r\n\r\nExamination of the electron energy equation showed that within a few diameters from the exit plane the electrons become energetically isolated from the ions and the electron heat conduction term dominates.\r\n\r\nA preliminary attempt to correlate the stagnation point heat transfer measurements along the axis shows that the electron temperature (TE [...] TI in general) plays an important role.\r\n\r\nII. Analysis of One-Dimensional Isentropic Flow for Partially Ionized Argon.\r\n\r\nOne-dimensional isentropic-flow variables of partially ionized argon have been calculated by coupling the isentropic flow equations with the partition-function method of deriving equilibrium thermodynamic properties. Tabulated gas properties and flow variables are presented for stagnation conditions of 0.1, 0.5, 1.0, 2.0, and 3.0 atm pressure and temperatures from 6,000 to 14,000\u00b0K in 1,000\u00b0K increments. The gas properties computed for this flow process include the sound speed, entropy, enthalpy, electron concentration, ionization fraction, electrical conductivity and static-to-stagnation ratios of temperature, pressure, and density. Flow variables include velocity, mass flux, area ratio, and Reynolds number per centimeter. Compared to results obtainable from perfect gas relationships (neglecting excitation and ionization), the results indicate that electronic excitation, and especially ionization effects, significantly alter the flow variables, particularly at the lower stagnation pressures and higher temperatures considered. However, with the exception of the effect of one excited state (the first excited state of the ion) on the equilibrium-composition equation, the thermodynamic properties calculated by neglecting excitation yielded results which were within 1% of those predicted by including excitation." }, { "name": "Zien, Tse-Fou", "degree": "PhD", "year": "1967", "title": "A Class of Three-Dimensional Optimum Wings in Hypersonic Flow", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11302005-135648", "creators": [ { "name": { "family": "Zien", "given": "Tse-Fou" }, "id": "Zien-Tse-Fou", "display_name": "Zien, Tse-Fou" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/X1J8-JK85", "abstract": "The idea of using streamlines of a certain known flow field to construct generally three-dimensional lifting surfaces together with the method of evaluating the aerodynamic forces on the surfaces, developed by Nonweiler, Jones and Woods, has been extended and applied to axisymmetric hypersonic flow fields associated with a class of slender power-law shock waves of the form r ~ \u03c4xn in the limit of infinite free stream Mach number. For this purpose, the basic flow fields associated with concave shocks (n > 1) have first been calculated numerically at a fixed value of the ratio of specific heats \u03b3 = 1.40, and the results are presented in tabulated form, covering a wide range of values of n. The method of constructing a lifting surface either by prescribing its leading edge shape on the basic shock or by specifying its trailing edge shape in the plane x = 1 is then discussed. Expressions for lift and drag on the surface are derived. A class of optimum shapes giving minimum pressure drag at a fixed value of lift has been determined for every basic flow field with n ranging from 1/2 to 10 at \u03b3 = 1.40.\r\n" }, { "name": "Beebe, Wayne Metcalf", "degree": "PhD", "year": "1966", "title": "An Experimental Investigation of Dynamic Crack Propagation in Plastic and Metals", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09262005-152806", "creators": [ { "name": { "family": "Beebe", "given": "Wayne Metcalf" }, "id": "Beebe-Wayne-Metcalf", "display_name": "Beebe, Wayne Metcalf" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PC0B-4P13", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nCrack propagation experiments were conducted in polyester resin sheets containing a central crack. Uniaxial tension loading at several loading rates was applied perpendicular to the crack direction. Two types of experiments were conducted: (1) High loading rate tests at 24\u00b0C and -45\u00b0C, with a constant loading rate to study the acceleration characteristics of cracks running in a glassy material, and (2) high temperature-low loading rate tests to study slow crack propagation when appreciable viscous dissipation could occur.\r\n\r\nDuring crack propagation, full frame photographs were taken of the photoviscoelastic isochromatic patterns and crack tip position at framing rates from 250 to 100,000 frames per second. The principal conclusions were as follows:\r\n\r\n1. Even at loading rates exceeding [...] psi per sec, isochromatic patterns prior to crack propagation compare closely with static patterns.\r\n\r\n2. Constant crack velocities were achieved in the high loading rate tests and it was found that the isochromatic patterns compare closely with the theoretical solution of Broberg.\r\n\r\n3. During the crack acceleration period, the experimental data could not be represented adequately by the Berry elastic theory.\r\n\r\n4. For the early phase of the slow (viscous) crack growth period, the crack length could be predicted using a simple theory proposed by Schapery and Williams.\r\n\r\nSeveral tests were conducted on silicon-iron metal sheets; it was concluded that the same testing technique can be applied to the study of crack growth in metals." }, { "name": "Behrens, Hermann Wilhelm", "degree": "PhD", "year": "1966", "title": "Flow Field and Stability of the Far Wake Behind Cylinders at Hypersonic Speeds", "advisor": "Lees, Lester; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechTHESIS:08172010-151639253", "creators": [ { "name": { "family": "Behrens", "given": "Hermann Wilhelm" }, "id": "Behrens-Hermann-Wilhelm", "display_name": "Behrens, Hermann Wilhelm" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K0KH-9W58", "abstract": "An experimental study of the mean wake flow field and its stability has been carried out in the far wake of circular cylinders at a Mach number of 6. The Reynolds numbers ranged from 200 to 4000 with a few measurements at higher Reynolds number. Pitot pressure, static pressure and mean flow hot wire measurements were done at many axial stations behind cylinders up to (x/d) = 2400.
\r\n\r\nThe inner wake formed from the cylinder boundary layers is laminar and loses its identity within the first 60 diameters or less depending on the Reynolds numbers so that only the outer wake, caused by the bow shock, has to be considered. Within a certain region the experimental results compare well with linear laminar theory, but the wake profiles are not similar up to the farthest downstream station (x/d = 2400). At four Reynolds numbers strong deviations from steady laminar behavior were observed far behind the cylinder, indicating breakdown of the flow because of non-linear instability effects.
\r\n\r\nIn the instability study hot wire fluctuation measurements were made over the whole frequency range (f = 1 - 320 KC) and also at particular frequencies in a band width of 1 KC up to x/d = 12000 at the lowest Reynolds number. Two instability regions were found and investigated: the linear growth region and the non-linear region. In the linear region there is quite a close correspondence with linear stability theory. The onset of non-linearity is characterized by the simultaneous strong deviation of the mean flow from laminar steady behavior, the increase of the fundamental frequency fluctuation component on the wake axis and the sudden rise of the first harmonic frequency component. The non-linear region is compared with the non-linear wake region at low speeds behind a flat plate. On the basis of these measurements a tentative picture is given of the onset of non-linearity and/or transition in the inner and outer wake behind blunt bodies at hypersonic speeds.
\r\n" }, { "name": "Bowman, Robert Marcus", "degree": "PhD", "year": "1966", "title": "Investigation of Shock Front Topography in Shock Tubes", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-105038", "creators": [ { "name": { "family": "Bowman", "given": "Robert Marcus" }, "id": "Bowman-Robert-Marcus", "display_name": "Bowman, Robert Marcus" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XPM1-ZZ53", "abstract": "An experimental investigation of the shape of shock waves in a circular shock tube is conducted. It is found that there are three distinct regimes governed, in a given tube, by the initial pressure in the test section.
\r\n\r\nAt very low pressures, where the shock thickness is greater than about half the tube radius, the axial extent (deviation from a plane) of the shock is roughly constant and dominated by the viscous interaction between the \"shock\", the boundary layer, and the driving piston of gas. This range of pressures is called the viscosity-dominated regime.
\r\n\r\nAt intermediate pressures, the shape of the shock is very nearly that predicted by the theory of de Boer, the shock curvature being produced by the boundary layer and the axial extent being roughly inversely proportional to the square root of the initial pressure. This is the boundary layer regime. de Boer's work is extended and the shock shapes for both the two-dimensional and axisymmetric cases are computed and plotted.
\r\n\r\nAt high pressures, the shape of the shock is complex and varies periodically down the tube. This shape is determined by transverse waves produced at the diaphragm (or other upstream disturbance) and reflecting back and forth across the tube, decaying with the square root of the distance down the tube. In this transverse wave regime, the axial extent of the shock is essentially independent of initial pressure and is much greater than had been expected.
\r\n\r\nThe square root decay of the transverse wave disturbances is in contrast to the 3/2 power decay predicted by Freeman and apparently verified by Lapworth. The experimental data of Lapworth is re-plotted and it is shown that if this data is analyzed in a slightly different manner it appears to exhibit square root decay.
\r\n\r\nIt is shown that the shock perturbations which exist in the transverse wave regime are absent at lower pressures. The transition region where these disturbances suddenly disappear seems to correspond approximately to the initial pressure at which the boundary layer (appropriately defined) at the disturbance fills the tube.
\r\n\r\nA rule of thumb is developed from which it should be possible to predict the transition initial pressure (which separates the transverse wave and boundary layer regimes) in any given shock tube. This pressure occurs when the quantity L/p1R2 is of order one, the tube dimensions being in millimeters and the initial pressure in millimeters of mercury. This rule of thumb is used to analyze the results of several shock tube experiments published by other researchers.
\r\n\r\nUsing this rule of thumb as an important constraint, a low pressure shock tube design chart is developed, from which, given the type of experiments contemplated and the nature of the instrumentation available, the proper shock tube dimensions and operating pressures may be determined.
\r\n\r\nFinally, avenues of future research are suggested, wherein it may be possible to design a new type of \"hi-fi\" shock tube, capable of producing more nearly plane shock fronts for use in shock structure and relaxation time studies, especially where methods such as integrated schlieren, optical reflectivity, or electron beam scattering are to be used.
\r\n" }, { "name": "Crow, Steven Collins", "degree": "PhD", "year": "1966", "title": "1. The Spanwise Perturbation of Two-Dimensional Boundary Layers. 2. The Turbulent Rayleigh Problem. 3. The Propagation of Free Turbulence in a Mean Shear Flow", "advisor": "Saffman, Philip G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11172005-144841", "creators": [ { "name": { "family": "Crow", "given": "Steven Collins" }, "id": "Crow-Steven-Collins", "display_name": "Crow, Steven Collins" } ], "advisors": [ { "name": { "family": "Saffman", "given": "Philip G." }, "id": "Saffman-P-G", "role": "advisor", "display_name": "Saffman, Philip G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/75J2-GM72", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\n1. The Spanwise Perturbation of Two-Dimensional Boundary Layers.\r\n\r\nLarge spanwise variations of boundary-layer thickness have recently been found in wind tunnels designed to maintain two-dimensional flow. Bradshaw argues that these variations are caused by minute deflections of the free-stream flow rather than an intrinsic boundary-layer instability. The effect of a small, periodic transverse flow on a flat-plate boundary layer is studied in this chapter. The transverse flow is found to produce spanwise thickness variations whose amplitude increases linearly with distance downstream.\r\n\r\n2. The Turbulent Rayleigh Problem.\r\n\r\nRayleigh flow is the non-steady motion of fluid above a flat plate accelerated suddenly into motion. Laminar Rayleigh flow is closely analogous to laminar boundary-layer flow but does not involve the analytical difficulty of non-linear convection. In this chapter, turbulent Rayleigh flow is studied to illuminate physical ideas used recently in boundary-layer theory. Boundary layers have nearly similar profiles for certain rates of pressure change. The Rayleigh problem is shown to have a class of exactly similar solutions. Townsend's energy balance argument for the wall layer and Clauser's constant eddy viscosity assumption for the outer layer are adapted to the Rayleigh problem to fix the relation between shear and stress. The resulting non-linear, ordinary differential equation of motion is solved exactly for constant wall stress, analogous to zero pressure gradient in the boundary-layer problem, and for zero wall stress, analogous to continuously separating flow. Finally, the boundary-layer equations are expanded in powers of the skin friction parameter [...], and the zeroth order problem is shown to be identical to the Rayleigh problem. The turbulent Rayleigh problem is not merely an analogy, but is a rational approximation to the turbulent boundary-layer problem.\r\n\r\n3. The Propagation of Free Turbulence in a Mean Shear Flow\r\n\r\nThis chapter begins with the assumption that the propagation of turbulence through a rapidly shearing flow depends primarily on random stretching of mean vorticity. The Reynolds stress [...] acting on a mean flow [...] in the x direction is computed from the linearized equations of motion. Turbulence homogeneous in x, z and concentrated near y = 0 was expected to catalyze the growth of turbulence further out by stretching mean vorticity, but [...] is found to become steady as [...]. As far as Reynolds stress is a measure of turbulent intensity, random stretching of mean vorticity alone cannot yield steadily propagating turbulence.\r\n\r\nThe problem is simplified by assuming that all flow properties are independent of x. Eddy motion in the y, z plane is then independent of the x momentum it transports, and the mean speed U(y,t) is diffused passively. The equations of motion are partially linearized by neglecting convection of eddies in the y, z plane, and wave equations for [...] and U(y,t) are derived. The solutions are worthless, however, for large times. Turbulence artificially steady in the y, z plane forces the mean speed gradient steadily to zero. In a real flow the eddies disperse as fast as U diffuses.\r\n\r\nNumerical experiments are designed to find how quickly concentrated vortex columns parallel to x disperse over the y, z plane and how effectively they diffuse U. It is shown that unless a lower limit on the distance between any two vortices is imposed, computational errors can dominate the solution no matter how small a time increment is used. Vortices which approach closely must be united. Uniting vortices during the computations is justified by finding a capture cross section for two vortices interacting in a strain field. The experiments confirm the result that columnar eddies disperse as fast as they transport momentum." }, { "name": "Krane, Stanley Garson", "degree": "PhD", "year": "1966", "title": "Studies on the Biosynthesis of Studies of \u00d8X174 Coat Protein", "advisor": "Sinsheimer, Robert L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09272002-131251", "creators": [ { "name": { "family": "Krane", "given": "Stanley Garson" }, "id": "Krane-Stanley-Garson", "display_name": "Krane, Stanley Garson" } ], "advisors": [ { "name": { "family": "Sinsheimer", "given": "Robert L." }, "id": "Sinsheimer-R-L", "role": "advisor", "display_name": "Sinsheimer, Robert L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "bioch" ], "doi": "10.7907/1MXF-6632", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA serum blocking power (SBP) assay for [phi]X174 coat protein, which depends on the ability of whole phage or certain phage components to combine with antiphage serum, was developed. This assay registers complete [phi]X, both viable and nonviable, and 70S particles, but not chemically prepared 5S subunits.\r\n\r\nStudies on the growth of wild type [phi]X, at 37[degrees], showed that SBP synthesis begins at about 5 min, under conditions such that the eclipse ends at 6 to 8 min. Although the curves for SBP and intracellular phage growth have the same shape, the titer of SBP phage equivalents exceeds that of infectious progeny particles at all times. The excess SBP is found in the form of 70S particles, complete but noninfectious phage, and subunits that sediment at about 15S.\r\n\r\nChloramphenicol, 5-fluorouracil deoxyriboside, and phleomycin when present at the time of infection each produce essentially complete inhibition of SBP synthesis. These results suggest that at least one complete RF molecule (the double stranded form of [phi]X DNA) must be made in order for SBP synthesis to occur.\r\n\r\nSeveral temperature sensitive [...] mutants were studied for their ability to produce SBP at 40[degrees]C (nonpermissive conditions for production of infectious phage). Most of the twenty-six mutants examined did make SBP at 40[degrees]C; however, mutant [...] 79 definitely did not.\r\n\r\nStudies were made on the physical state of the SBP synthesized, at 40[degrees]C, by the mutants [...] Y, 9, and [...] 4 by the technique of sucrose gradient velocity centrifugation. The results showed that [...] Y, and [...] 9 make their SBP principally in the form of subunits that sediment at approximately 15S. The mutant [...] 4 makes its SBP principally in the form of particles that sediment at 71S. The [...] 4 71S particles may contain both DNA and protein." }, { "name": "Lee, Jen-shih", "degree": "PhD", "year": "1966", "title": "Application of Finite Elastic Theory to the Deformation of Cylindrical Tubes", "advisor": "Fung, Yuan-cheng; Frasher, Wallace G.; Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-12052005-150440", "creators": [ { "name": { "family": "Lee", "given": "Jen-shih" }, "id": "Lee-Jen-shih", "display_name": "Lee, Jen-shih" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" }, { "name": { "family": "Frasher", "given": "Wallace G." }, "id": "Frasher-W-G", "role": "advisor", "display_name": "Frasher, Wallace G." }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HM2E-AX28", "abstract": "In Part I three types of symmetrical deformations of thin cylindrical rubber tubes are discussed. In the first type a rubber tube is deformed into another circular cylindrical tube of different length and diameter by simultaneous inflation and extension of the tube. This deformation is useful in determining the mechanical properties of tube-like material and it was found that Rivlin-Saunder form of strain-energy fitted a particular latex rubber used in our experiments. The second and third types of deformation are a tube deformed by a longitudinal stretching or an increase in internal pressure to a curved surface of revolution. A number of numerical examples were worked out with a view toward designing experiments to determine mechanical properties of short cylindrical tubes.
\r\n\r\nIn Part II experimental studies on the overall mechanical properties of large blood vessels are presented. Two Lagrangian stresses and two extension ratios are used to describe the stress and strain states of the vessels subjected to symmetrical deformations. The interested deformation range is about ten to twenty percent in the neighborhood of the natural state.
\r\n\r\nTests consisted of (1) a longitudinal stretching while the diameter of the vessels was maintained, (2) a lateral distension with the length of the vessels unchanged, and (3) repeated stretching of the vessels at low frequency.
\r\n\r\nThe first two tests show that the stress-strain law of the vessels tested is highly nonlinear and the vessels behave more rigidly in the longitudinal direction than in the lateral direction. The last test shows that the vessels are more likely to behave as a plastic elastic metal and a higher tangential modulus was observed for the vessels stretched at a smaller oscillation amplitude.
" }, { "name": "Letcher, John Seymour", "degree": "PhD", "year": "1966", "title": "Transverse Hydrodynamic Forces on Slender Bodies in Free-Surface Flows at Low Speed", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-11112005-154228", "creators": [ { "name": { "family": "Letcher", "given": "John Seymour" }, "id": "Letcher-John-Seymour", "display_name": "Letcher, John Seymour" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M8NJ-KC21", "abstract": "The forces and moments on a moving body partially immersed in the surface of a deep ocean of heavy fluid are considered in the limit of small Froude number, F. Asymptotic expressions for velocity potential and free surface elevation are developed. The choice of the first terms of the asymptotic sequence is indicated by the behavior, at small F, of the classical results of \"small disturbance theory\" - analysis starting from the linearized free-surface boundary conditions. It is found that the leading terms depend on the local disturbance, which can be expanded as a power series in F. The wave pattern contributes higher-order terms which are not analytic about F = 0; only estimates of the order of these terms are obtained. Consequently the present work does not estimate drag but is confined to consideration of transverse forces and moments.\r\n\r\nOnce the asymptotic sequence is assumed, perturbation of the exact equations and boundary conditions about F = 0 is straightforward. The zero-order potential is that of the \"reflection-plane\" model of Davidson. For a restricted class of shapes, the slender body theory is applied to the zero-order and first-order problems. A general method is developed using conformal mapping to solve the first order problem for sufficiently slender shapes of arbitrary cross-section. This method is applied to two particular shapes, viz. a wing of zero thickness and a half-submerged body of revolution, both in sideslip. The correction to the reflection plane model is found to be generally quite small in the range of F for which this theory is expected to apply." }, { "name": "Lindsey, Gerald Herbert", "degree": "PhD", "year": "1966", "title": "Hydrostatic Tensile Fracture of a Polyurethane Elastomer", "advisor": "Williams, Max L.; Knauss, Wolfgang Gustav", "url": "https://resolver.caltech.edu/CaltechETD:etd-11232005-113650", "creators": [ { "name": { "family": "Lindsey", "given": "Gerald Herbert" }, "id": "Lindsey-Gerald-Herbert", "display_name": "Lindsey, Gerald Herbert" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." }, { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-W-G", "role": "advisor", "display_name": "Knauss, Wolfgang Gustav" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K12J-X907", "abstract": "The investigation of fracture of polymeric materials in hydrostatic tensile fields constitutes an avenue of approach to the study of fracture in more general three-dimensional environments. The advantages created by the symmetry of the stress field are considerable and, in one of the cases studied, facilitates a theoretical treatment that includes large deformations, which are characteristic of this class of materials.\r\n\r\nThe analysis is developed through the concept of fracture originating from a flaw, which in this instance is taken to be a spherical cavity. Through the application of energy principles, a theoretical prediction of ultimate strength is made for hydrostatic tensile fields.\r\n\r\nExperiments were conducted to demonstrate the existence of such flaws and to evaluate the theory. Results of the tests on specimens containing both residual flaws and artificially inserted ones indicate a fundamental difference in behavior as contrasted with cracks.\r\n\r\nAn explanation is given linking experimental results and theoretical predictions. It is based on the concept that a flaw \"grows\" in the material under load using the cavity as a nucleating point. Upon this hypothesis is built a theory of rupture in which planar cracks grow radially from the center of the cavity in the form of Saturn-ring cracks." }, { "name": "Lissaman, Peter Barry Stuart", "degree": "PhD", "year": "1966", "title": "A Linear Solution for the Jet Flap in Ground Effect", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-09302002-123622", "creators": [ { "name": { "family": "Lissaman", "given": "Peter Barry Stuart" }, "id": "Lissaman-Peter-Barry-Stuart", "display_name": "Lissaman, Peter Barry Stuart" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "co-advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PWFF-E933", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. \r\n\r\nThe paper presents the solution of the problem of the Jet Flap airfoil in a plane inviscid flow in the presence of the ground.\r\n\r\nThe basic flow equations are derived and the non-linearity of the boundary conditions are discussed. The problem is then linearized as in thin airfoil theory. By a conformal transformation the flow field is mapped into one having very simple geometry. The singularities of the mapping are identified and the asymptotic character of the flow fields derived. The basic integro-differential equation is developed; this has singular Hilbert type kernels and discontinuous boundary conditions.\r\n\r\nBy considerations of the second order effects, significant relationships between the lift slopes with angle of attack, with jet angle and jet coefficient are developed. These are further simplified by introduction of a new geometrical parameter developed from the mapping.\r\n\r\nThe lift coefficient of the airfoil is expressed in three parts, of which two may be evaluated in simple closed form. The remaining part depends on the solution of the integro-differential equation. This equation is then solved at N points by assuming a piecewise smooth velocity distribution and generating an N x N matrix: numerical results are obtained from an IBM 7094 computer. It is proved that this approximation converges to the exact solution.\r\n\r\nThe limiting cases, when the height to chord ratio, h/c, or jet coefficient, C[subscript J], approach zero or infinity are developed, exploiting the decomposition of the lifting components; and an asymptotic result for small C[subscript J] is presented.\r\n\r\nA linearized theory for wake blockage is given, which also gives an indication of the restrictions on the various parameters implied by the basic linear approach.\r\n\r\nThe results for [...] correlate excellently with Spence's solution for [...]. For low values of h/c the results agree quite well with the limited applicable test data." }, { "name": "Murthy, K. R. Ananda", "degree": "PhD", "year": "1966", "title": "A Possible Analytical Explanation for the Micrometeorite Concentration near the Earth", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-11152005-144237", "creators": [ { "name": { "family": "Murthy", "given": "K. R. Ananda" }, "id": "Murthy-K-R-Ananda", "display_name": "Murthy, K. R. Ananda" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/D8XP-4Q06", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe Keplerian motion of a single particle in a dissipating medium, such as air, is analysed and a theory of micrometeorite capture and resulting orbital lifetime is developed. The equations of motion are derived for a planar, two-dimensional model, and all orbital variables are assumed to be perturbed slightly from their Keplerian values. The equations are then linearized and solved. Then a statistical model of the interplanetary micrometeorite flux is developed in which the distribution of velocities at infinity relative to the earth and masses of the particles are taken into account. The velocity distribution is taken to be [...]. The distribution of the masses is taken as a constant number of particle flux M(m) at infinity. Finally, this statistical model is combined with the theory of capture and lifetime to furnish a possible explanation for the micrometeorite concentration near the earth." }, { "name": "Olson, Mervyn Daniel", "degree": "PhD", "year": "1966", "title": "Supersonic Flutter of Circular Cylindrical Shells", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-12022005-075701", "creators": [ { "name": { "family": "Olson", "given": "Mervyn Daniel" }, "id": "Olson-Mervyn-Daniel", "display_name": "Olson, Mervyn Daniel" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/32AP-C463", "abstract": "Various experimental and theoretical studies on the supersonic flutter of circular cylindrical shells are discussed.
\r\n\r\nResults of experiments in the Mach number range 2.5 - 3.5 are presented. Three shells with radius-to-thickness ratios of 2,000 were subjected to radial external pressure loadings and to combinations of axial compressive loading and internal pressurization while in the presence of an external axially-directed supersonic flow.
\r\n\r\nSmall amounts of internal pressurization were very stabilizing with respect to flutter, but moderate amounts reduced stability to the unpressurized level. However, high internal pressures completely stabilized the shells. The axial compressive loading was slightly destabilizing for moderate amounts of internal pressurization.
\r\n\r\nThe flutter modes (which were standing waves in the axial direction with zero, one or two circumferential nodal lines) contained many waves around the circumference (of the order of 20) that travelled in the circumferential direction. This circumferentially travelling wave phenomenon possibly results from the nonlinear nature of cylindrical shells.
\r\n\r\nModel integrity was not threatened by even the most violent flutter which occurred just prior to buckling under radial external pressure loading and just after buckling under axial compressive loading. Buckled portions of a shell did not flutter. It appears that the large local curvatures encountered in the buckling of a cylindrical shell tend to stabilize the shell locally. However, it also appears that the localized buckling usually encountered in practice reduces the stability of any unbuckled regions of the shell.
\r\n\r\nThe experimental flutter boundaries are compared with various theoretical predictions. Following Voss a modal analysis which satisfies the so-called freely supported shell boundary conditions is used in conjunction with different aerodynamic approximations - namely piston theory and the potential theory of Leonard and Hedgepeth. It was found that the pressurized cylindrical shells fluttered at a lower level of free stream energy than predicted by the theory. Of the two results, that using piston theory appears to correspond closest to the experiment both in stability boundary and in critical values of circumferential wave number. Both predictions yield a larger stabilizing influence of the shell internal pressure than observed in the experiment.
\r\n\r\nAn analysis is presented for calculating the final limiting amplitudes of flutter based on a two-mode, piston theory approximation. A Galerkin procedure is used to reduce the nonlinear shallow shell equations of Marguerre to two coupled nonlinear ordinary differential equations for the modal amplitudes. An approximate limit cycle solution to these equations is obtained by the method of Krylov and Bogoliubov. The results indicate that for practical purposes cylindrical shell flutter does not occur below the stability boundary for infinitesimal disturbances. The limit cycle amplitudes predicted by this analysis seem to agree very well with the experimental ones. The results further indicate that the flutter amplitude, frequency and mode shape should change discontinuously (or jump) as the aerodynamic pressure is increased beyond the value for first flutter.
" }, { "name": "Sorrell, Furman Yates, Jr.", "degree": "PhD", "year": "1966", "title": "On the Generation of Shock Waves in an Inverse Pinch", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03312009-155713", "creators": [ { "name": { "family": "Sorrell", "given": "Furman Yates, Jr." }, "id": "Sorrell-Furman-Yates", "display_name": "Sorrell, Furman Yates, Jr." } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZCDM-SF32", "abstract": "A problem inherent in magnetic shock tubes is the difficulty of achieving separation of the driving current sheet and the shock wave. If such devices are to be applied to produce shock waves for experimentation, then separation will usually be a necessary requirement. In the present experiments in an inverse pinch shock tube, preliminary measurements showed that not only was separation not achieved, but under certain conditions the shock was actually found to be located far behind the front of the current sheet. This appeared to be a paradoxical case of the shock wave pushing the piston. Moreover, measurements of the current sheet velocity indicated that the interaction of the current sheet with the gas should be strong enough to sweep up all the gas encountered by the current sheet and thus to produce a shock wave moving ahead of it. In order to find explanations for the absence of separation and for some other puzzling aspects of these early experiments, further measurements were made to study in more detail the processes taking place in the device. These included measurements of the radial electric field with electrostatic probes and of the ionization levels by the technique of spectral line broadening. The results of these measurements show that the degree of ionization is surprisingly low and that the amount of gas leaking through the current sheet is significantly high in some cases. The conclusion is then reached that although the so-called \"snowplow model\" is successful in predicting the current sheet velocity, it does not lead to the correct picture of the physical processes taking place. Finally, conditions for which separation may be achievable are inferred from the experiments." }, { "name": "Tong, Pin", "degree": "PhD", "year": "1966", "title": "Liquid Sloshing in an Elastic Container", "advisor": "Fung, Yuan-cheng; Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechTHESIS:08202010-092311496", "creators": [ { "name": { "family": "Tong", "given": "Pin" }, "id": "Tong-Pin", "display_name": "Tong, Pin" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WYZD-Q384", "abstract": "The motion of a liquid in a flexible container is important for rocket structural dynamics. The purpose of this paper is to study the dynamic response of the liquid, the sloshing frequencies and the stability of the free surface of the liquid in an elastic container.
\r\n\r\nThe variational principle for the problem of an incompressible, inviscid fluid in an elastic container is presented by considering the pressure energy of the fluid, the surface energy, and the Lagrangian of the elastic thin shell. The corresponding linearized equations are studied in terms of eigenvalues and eigenfunctions.
\r\n\r\nThe effects of the gravitation, the surface tension, the rigidity of the container, the free surface contact angle and its dynamic variation, on the natural frequencies and the stability of the free surface are discussed.
\r\n\r\nIt is found that the flexibility of the container always lowers the natural frequencies and also induces a mean oscillatory motion of the liquid that creates an oscillatory force on the container in the vertical direction. The equilibrium contact angle and its dynamic variation have an important effect on the limit of stability.
\r\n\r\nThe motion of a liquid in a circular cylindrical container with a flat flexible bottom is worked out in detail analytically by means of eigenfunctions. Some results are presented graphically. A numerical scheme using finite elements method is developed for an arbitrary container. Methods for improving the solution systematically are indicated.
\r\n" }, { "name": "Williams, Richard R.", "degree": "PhD", "year": "1966", "title": "Application of the Two Variable Expansion Procedure to the Commensurable Planar Restricted Three-Body Problem", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11162005-082219", "creators": [ { "name": { "family": "Williams", "given": "Richard R." }, "id": "Williams-Richard-R", "display_name": "Williams, Richard R." } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PYNV-5723", "abstract": "The nearly commensurable case of the planar restricted three-body problem is treated by application of the two variable expansion procedure. The polar angle of the infinitesimal body, rather than the time, is taken as the independent variable. A set of four coupled first order differential equations, which govern the long-period behavior of the orbital elements, is obtained by imposing the requirement that the assumed form of the expansions must be self-consistent. The independent variable in these equations is the \"slow variable\". It is then found that the short-period perturbations of the motion of the infinitesimal body do not contain small divisors or secular terms.\r\n\r\nApproximate solutions for the orbital elements are given, for two different cases. Both libratory and non-libratory solutions are found, depending upon the initial conditions. Numerical results are calculated from these solutions, and are compared to numerical computations recently reported in the literature." }, { "name": "Peck, Jerry Clifford", "degree": "PhD", "year": "1965", "title": "Plane-Strain Diffraction of Transient Elastic Waves by a Circular Cavity", "advisor": "Miklowitz, Julius", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142004-144633", "creators": [ { "name": { "family": "Peck", "given": "Jerry Clifford" }, "id": "Peck-Jerry-Clifford", "display_name": "Peck, Jerry Clifford" } ], "advisors": [ { "name": { "family": "Miklowitz", "given": "Julius" }, "id": "Miklowitz-J", "role": "advisor", "display_name": "Miklowitz, Julius" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/GDNE-E586", "abstract": "The plane-strain problem of the diffraction of a transient plane dilatation wave by a circular cavity in an elastic medium is treated. The method used determines the (total) solution only in the shadow zone, i.e., those points which cannot be connected to the source of disturbance by straight-line rays. Numerical results are obtained for the velocities and displacements on the \"back\" surface of the cavity caused by a step-stress incident wave.\r\n\r\nThe analysis is based on a method devised by Friedlander (see his book Sound Pulses, Cambridge, 1958) for the analogous acoustic diffraction problem. This method converges most rapidly at short time, in contrast to Fourier series methods. The Friedlander method essentially employs integral transforms on both time and [Theta], the circumferential coordinate. In the shadow zone, the [Theta]-inversion can be performed by residue theory, the residues resulting from poles at the roots of a \"frequency equation.\" The roots are infinite in number, and may be regarded as forming a dispersion spectrum relating the frequencies and angular wave numbers of a series of circumferential propagation modes. The time-transform inversion is carried out by contour integration and subsequent numerical evaluation.\r\n\r\nThe transient response results are found to compare well with the Fourier-series solutions at moderate to long times, but at short time the differences are marked, as would be expected. The fact that the present technique yields good long-time results suggests it is even more powerful than might be expected. The major limitation of the numerical method is its restriction to the shadow zone." }, { "name": "Swedlow, Jerold Lindsay", "degree": "PhD", "year": "1965", "title": "The Thickness Effect and Plastic Flow in Cracked Plates", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12122003-100109", "creators": [ { "name": { "family": "Swedlow", "given": "Jerold Lindsay" }, "id": "Swedlow-Jerold-Lindsay", "display_name": "Swedlow, Jerold Lindsay" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0WVE-W364", "abstract": "Over a range of plate thickness, it is well known that the fracture behavior of flat plates is substantially different from that predicted by classical fracture analyses. Finiteness of the plate thickness causes a variety of failure mechanisms to occur, and qualitative features of the associated stress and strain fields may be deduced. It is indicated that both the three-dimensional nature of the stress field and the plastic deformations will be needed for an accurate prediction of the thickness effect.\r\n\r\nAs a contribution to the three-dimensional analysis, an appropriate elastic boundary value problem is given limited consideration. It is observed that the three in-plane stresses can be singular, in accord with the two-dimensional results, but the transverse components appear to be bounded at the crack tip.\r\n\r\nEquations which include plastic behavior are outlined, and a plane stress problem is solved using numerical methods. Comparison with analytical and experimental results is made and found to be satisfactory. One important result indicates that, compared to the elastic solution, the intensity of stress at the crack point decreases with load, while that for strain increases.\r\n\r\nThe results do not include determination of a fracture stress, as this further step requires the development of an elastoplastic fracture criterion. Comments on this extension are included, together with other aspects of future work." }, { "name": "Van Atta, Charles William", "degree": "PhD", "year": "1965", "title": "Spiral Turbulence in Circular Couette Flow", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-01262004-102054", "creators": [ { "name": { "family": "Van Atta", "given": "Charles William" }, "id": "Van-Atta-Charles-William", "display_name": "Van Atta, Charles William" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/73GQ-A629", "abstract": "Under certain conditions, the fluid motion between counter-rotating concentric cylinders is made up of alternate helical stripes of laminar and turbulent flow. The present experiments show that, over a large range of cylinder Reynolds numbers, this helical pattern of turbulence rotates steadily at very nearly the mean angular velocity of the two cylinders. When the speed of the outer cylinder is held fixed and the speed of the inner cylinder increases from rest in the opposite direction, spiral turbulence follows a catastrophic breakdown of the toroidal vortices arising from Taylor instability.\r\n\r\nThe ultimate objective of the present experiments is to measure the local rate of energy transfer between the turbulence and the mean motion in a typical spiral turbulent flow. The working fluid is air, and the instrumentation consists of hot-wire anemometers together with a variety of devices for operating on the resulting signals. Each cycle of the mixed laminar-turbulent flow (as observed by a probe mounted on one or the other cylinder) is treated as a member of an ensemble of realizations. The energy transfer (in a coordinate system rotating with the mean velocity of the turbulence) can be determined by averaging over a large number of instantaneous velocity samples taken at corresponding points in successive cycles of the turbulence.\r\n\r\nAfter some exploratory measurements, particular flow was selected for which the laminar-turbulent interfaces were sharply delineated across the entire annular gap and for which the dispersion in interface location was a minimum. This flow is about half laminar and half turbulent: it is characterized by a nose of turbulence associated with the leading interface and projecting into the laminar region near the outer cylinder, while a corresponding tail near the inner cylinder is associated with the trailing interface. The helical pattern is lefthanded and makes an angle of about 62 degrees with the axis of the cylinders.\r\n\r\nFor the flow in question, analog voltage signals from a calibrated array of four hot wires were first recorded on magnetic tape. Several thousand cycles of turbulence were recorded at each of 17 different radial positions. Sampled values from these signals were then obtained and stored in digital form on magnetic tape, and were finally processed by a large electronic computer to restore the voltage data to the original laboratory units. The total amount of didital information available to describe the turbulence is roughly 180,000,000 bits. The amount of noise introduced into the individual sampled voltages by the recording, playback, digitizing, and processing operations so far carried out is believed to be no more than 3 parts in 10,000.\r\n" }, { "name": "Wu, Jain-Ming", "degree": "PhD", "year": "1965", "title": "A Satellite Theory and its Applications", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142004-104844", "creators": [ { "name": { "family": "Wu", "given": "Jain-Ming" }, "id": "Wu-Jain-Ming", "display_name": "Wu, Jain-Ming" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2XH7-1828", "abstract": "A theory of an Earth satellite has been developed by considering the Earth's bulge, atmospheric drag and the rotation of the atmosphere simultaneously. The equations of motion have been set up on a tilted equatorial plane coordinate system. All of the orbital variables have been expanded in a series in terms of a perturbative force parameter based on the Keplerian orbit. These equations have been linearized and then solved. By means of geometrical arguments, all of the above solutions have been expressed in the form of conventional orbital elements. In the limiting case, these solutions agree with the classical values. One previously neglected effect, the rotation of the line of apsides by drag, is identified and evaluated. The results have been used to show the correction due to the effect of the above-mentioned forces on the Earth's gravitational potential." }, { "name": "Arenz, Robert James", "degree": "PhD", "year": "1964", "title": "Theoretical and Experimental Studies of Wave Propagation in Viscoelastic Materials", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09072002-150755", "creators": [ { "name": { "family": "Arenz", "given": "Robert James" }, "id": "Arenz-Robert-James", "display_name": "Arenz, Robert James" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E8KM-6M98", "abstract": "The phenomenon of wave propagation in viscoelastic materials is investigated both theoretically and experimentally, with attention directed to two areas. First, analytical methods of solution are developed for certain wave propagation problems in one and two dimensions utilizing realistic material properties. This is accomplished by use of time-dependent material property characterization through a Dirichlet series representation to overcome the limitations of the widely-used simple spring and dashpot models involving two or three elements. The Laplace transformed solutions are then inverted by an extension of the Schapery collocation method to dynamic situations.\r\nThe second topic deals with dynamic photoelasticity applied to viscoelastic materials. It is shown that the relationships between stress optic and strain optic coefficients for linearly viscoelastic materials can be formulated. Then the time-dependent birefringence characteristics of a typical low modulus polymer material are determined from constant strain rate tests for a full range of dynamic loading rates by taking advantage of the time-temperature shift phenomenon. Much recent work in dynamic photoviscoelasticity has been based on static calibrations only. Hence to put the technique on a firm foundation and indicate the general necessity of including the time dependency in treatment of material properties, a comparison is made of predicted fringe patterns with experimental results for both one- and two-dimensional situations. The cases considered are the rod and semi-infinite plate geometries under quasistep pressure inputs, for which viscoelastic solutions are obtained from the wave propagation analysis in the first part of the thesis. The results indicate the feasibility of quantitative photoviscoelasticity for dynamic stress analysis." }, { "name": "Baganoff, Donald", "degree": "PhD", "year": "1964", "title": "Experiments on the Wall-Pressure History in Shock Reflection Processes", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-09122002-083236", "creators": [ { "name": { "family": "Baganoff", "given": "Donald" }, "id": "Baganoff-Donald", "display_name": "Baganoff, Donald" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XK5D-HZ55", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe normal reflection of a plane shock from a plane wall is investigated experimentally using a pressure gauge which has a risetime of 0.1 [....] and no overshoot. Pressure histories of 5 [.....] duration can be obtained with this gauge. The experiments were conducted on the end wall of the GALCIT 17-in.-diameter shock tube.\r\n\r\nExperimental results for three aspects of the reflection process are discussed: (1) profile for the reflected shock front, (2) effect of a cold wall, and (3) effect of a real gas.\r\n\r\nIt is concluded that the effect of a cold wall must play a comparatively minor role in the reflection process since the thickness of the recorded profile is comparable to the thickness of the incident shock, and the pressure jump across the profile is about 85 per cent of the ideal value. Also, the pressure history immediately behind the reflected shock can be approximated by boundary layer theory. The effect of a real gas was studied in carbon dioxide and it is shown that the relaxation process behind the incident shock produces a large effect on the recorded pressure history which provides a method of measuring the vibrational relaxation time in carbon dioxide for high temperatures." }, { "name": "Bush, William B.", "degree": "PhD", "year": "1964", "title": "On the Viscous Hypersonic Blunt-Body Problem", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09102002-143008", "creators": [ { "name": { "family": "Bush", "given": "William B." }, "id": "Bush-William-B", "display_name": "Bush, William B." } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QCBK-ZD50", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. \r\n\r\nThe viscous hypersonic flow past an axisyrnmetric blunt-body is analyzed based upon the Navier-Stokes equations for a perfect gas having constant specific heats, a constant Prandtl number, P, whose numerical value is of order one, and a viscosity varying as a power, [...], of the absolute temperature, as the free-stream Mach number, M, and the freestream Reynolds number based on the body nose radius, R, go to infinity, and [...] (where [gamma] is the ratio of the specific heats) and [...] go to zero.\r\n\r\nThrough the use of strict asymptotic expansions, the behavior of the flow in the three distinct regions that comprise the interior of the \"shock structure\" is found, as well as for the one, two, or three regions that make up the \"shock layer\" depending on whether the quantity [...] is equal to [...], equal to [...], or greater than [...], respectively." }, { "name": "Evensen, David Arthur", "degree": "PhD", "year": "1964", "title": "Non-Linear Flexural Vibrations of Thin Circular Rings", "advisor": "Sechler, Ernest Edwin; Caughey, Thomas Kirk", "url": "https://resolver.caltech.edu/CaltechETD:etd-09262002-165232", "creators": [ { "name": { "family": "Evensen", "given": "David Arthur" }, "id": "Evensen-David-Arthur", "display_name": "Evensen, David Arthur" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "role": "advisor", "display_name": "Caughey, Thomas Kirk" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7D5J-3C40", "abstract": "The non-linear flexural vibrations of thin circular rings are analyzed by means of the appropriate \"shallow shell\" equations. These partial differential equations are reduced to non-linear ordinary differential equations by assuming vibration modes and applying Galerkin's procedure. Vibrations involving primarily a single bending mode are investigated for three distinct cases, and the results indicate that the basic features of the problem are exhibited by an inextensional analysis.\r\n\r\nThis information is then applied to simplify the analysis of vibrations in which several modes participate. A study of \"self-coupled\" bending modes shows that the single mode solution is valid only for certain combinations of amplitude and frequency: when the single mode exceeds a \"critical amplitude\", its companion mode is parametrically excited and participates in the motion.\r\n\r\nThe general inextensional case (involving an infinite number of modes) is examined for two important sets of forces, and possible solutions are shown to be the excitation of primarily one or two bending modes. Stability analyses of these solutions indicate that when certain restrictions are met, all other bending modes play only a minor part in the vibration.\r\n\r\nAn experimental study of the problem was also conducted. Theory and experiment both indicate a non-linearity of the softening type, the presence of ultraharmonic responses, and the appearance of the companion mode. Measurements of the steady-state response are in good agreement with the calculated values, and the experimentally determined mode shapes agree with the form of the assumed deflection.\r\n\r\nThe analytical and experimental results exhibit several features that are common to the non-linear vibration of axisymmetric systems in general and to circular cylindrical shells in particular." }, { "name": "Grimes, Charles Kenneth", "degree": "PhD", "year": "1964", "title": "Studies on the Propagation of Elastic Waves in Solid Media", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09192002-160252", "creators": [ { "name": { "family": "Grimes", "given": "Charles Kenneth" }, "id": "Grimes-Charles Kenneth", "display_name": "Grimes, Charles Kenneth" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EW57-KB61", "abstract": "Several aspects of three basic problems concerned with the propagation of elastic waves in solid media are explored.\r\n\r\nStress and displacement correction terms resulting from application of a subsonically moving point load to the free surface of the infinite half-space are obtained using Fourier transform techniques (the load moves subsonically with respect to the longitudinal and transverse wave speeds). It is shown, for the supersonically travelling point load, that the solution is given, in the limit as the load velocity becomes large, by the well known solution of Sauter for the impulsive point load.\r\n\r\nAnalytic function theory is used to predict the existence of Rayleigh waves on the free surface of the infinite halfspace and Stoneley waves along the welded interface between two dissimilar solid media. A brief analysis shows that free-running waves are also possible on the interior surface of an infinitely long cylindrical cavity. These waves are dispersive, however, because of the introduction of a characteristic length.\r\n\r\nThe early and long time approximations for the hoop stress generated through scattering of a plane dilatational wave by a cylindrical cavity in an infinite medium are developed. Use is made of Friedlandler's Riemann surface representation (early time) and expansion in Fourier series (long time)." }, { "name": "Hegemier, Gilbert Arthur", "degree": "PhD", "year": "1964", "title": "Stability of Thin Cylindrical Shells Subjected to a Class of Axially Symmetric Moving Loads", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-10112002-121702", "creators": [ { "name": { "family": "Hegemier", "given": "Gilbert Arthur" }, "id": "Hegemier-Gilbert-Arthur", "display_name": "Hegemier, Gilbert Arthur" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4P8K-YN54", "abstract": "The stability of an infinite-length cylindrical shell subjected to a broad class of axially symmetric moving loads with constant velocity is studied by utilizing a large deflection Donnell theory. Special cases of the general loading function include the moving ring, step and decayed step loads.\r\n\r\nStability is defined on the basis of the boundedness or divergence of an infinitesimal nonsymmetric disturbed motion about an initial nonlinear steady-state symmetric response. Following the determination of the symmetric response, under this concept of stability, the analysis is reduced to a study of a system of linear partial differential equations or so-called variational equations; these are analyzed by use of a double Laplace transform technique and the original stability problem is replaced by a simpler one of determining the location of the poles of a certain function. A scheme for accomplishing this task is outlined. Extension of the method to include more exact equations of motion and to a class of static problems involving finite length shells is discussed.\r\n\r\nA related problem concerning a moving concentrated load on a nonlinear elastic cylindrical membrane (nonlinearity in both geometric and constitutive relations) and a string on a nonlinear foundation is discussed in an appendix to the text. Interesting analogies in both analysis and physical behavior of the string and shell systems are found." }, { "name": "Levinson, Mark", "degree": "PhD", "year": "1964", "title": "Variational Principles and Applications in Finite Elastic Strain Theory", "advisor": "Blatz, Paul J.; Knowles, James K.", "url": "https://resolver.caltech.edu/CaltechETD:etd-09272002-145537", "creators": [ { "name": { "family": "Levinson", "given": "Mark" }, "id": "Levinson-Mark", "display_name": "Levinson, Mark" } ], "advisors": [ { "name": { "family": "Blatz", "given": "Paul J." }, "id": "Blatz-P-J", "role": "advisor", "display_name": "Blatz, Paul J." }, { "name": { "family": "Knowles", "given": "James K." }, "id": "Knowles-J-K", "role": "advisor", "display_name": "Knowles, James K." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PK58-HQ71", "abstract": "The variational principles of finite elastostatic strain theory are presented in a unified manner for both compressible and incompressible bodies. Whereas the principle of stationary potential energy, a restricted case of the general principle of Hu and Washizu, is valid for any elastic deformation, it is found that the principle of stationary complementary energy is valid only for infinitesimal elastic strains. Consequently, Reissner's Theorem is the appropriate stationary principle to use in finite elastic strain theory when the complementary strain energy density is to be the argument function.\r\n\r\nThe potential energy principle is applied to several problems dealing with the finite straining of a neo-Hookean material. All but one of these problems are concerned with plane strain deformations; the one other problem, in a spherical geometry, involves an unusual stability question. Approximate solutions are obtained for some mixed boundary value problems which are not amenable to the semi-inverse methods of solution frequently used in finite elastic strain theory.\r\n\r\nAnother plane strain problem, requiring more detailed stress information than can be obtained from the potential energy principle, is studied approximately by means of Reissner's Theorem." }, { "name": "Liu, Joseph Tsu Chieh", "degree": "PhD", "year": "1964", "title": "Problems in Particle-Fluid Mechanics", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10172002-122957", "creators": [ { "name": { "family": "Liu", "given": "Joseph Tsu Chieh" }, "id": "Liu-Joseph-Tsu-Chieh", "display_name": "Liu, Joseph Tsu Chieh" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/11Y4-2H39", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe continuum equations describing the motion of a fluid containing small solid particles are discussed and stated. The examples considered fall into two categories: (1) when the fluid is incompressible and viscous, with simultaneous occurrence of particle-fluid momentum relaxation and fluid viscous diffusion; and (2) when the fluid can be considered as \"inviscid\" but compressible, with simultaneous occurrence of coupled particle-fluid momentum and thermal relaxations and fluid compressibility.\r\n\r\nUnder (1), the low Mach-number Rayleigh problem is studied. Many of the physical features of the non-linear steady (constant pressure) laminar boundary-layer problem are recovered from appropriate expansions from this exact solution. One obtains answers to questions about the modifications on the boundary layer growth and skin friction; particularly their transition from the \"frozen\" value near the leading edge, where the viscous layer is \"thin\" and the fluid viscous diffusion behaves as if in the absence of particles with the ordinary fluid kinematic viscosity,[.....], to the ultimate \"equilibrium\" value far downstream where the mixture then behaves as a single heavier fluid and viscous diffusion takes place with the \"equilibrium\" kinematic viscosity augmented by the particle density [.....].The uncoupled thermal Rayleigh problem (small relative temperature differences) is directly inferred, and this answers questions about the modifications on the surface heat-transfer rates and particularly about the possibility of similarity with the velocity boundary layer. Similarity of the two boundary layers is possible when, in addition to lateral diffusion effects being similar as indicated by Prandtl number unity, the streamwise relaxation processes must also be similar. The infinite flat plate oscillating in its own plane is studied, and appropriate expansions from the exact solutions point out how approximate treatment of periodic boundary layers in the absence of a mean flow may be made.\r\n\r\nUnder (2), the first-order small perturbation theory is discussed, leading from the equation for acoustic propagation to that for linearized supersonic flow. The two-dimensional steady case, or the Ackeret problem, is considered in detail. The Mach wave structure induced by a thin obstacle is deduced and shows a rapid damping of the disturbance along the \"frozen\" Mach wave (based on the sound speed of a gas in the absence of particles), both damping and diffusiveness along an intermediary Mach wave, and diffusiveness along the \"equilibrium\" Mach wave (based on the sound speed of an equilibrium mixture of gas and particles) and along which the bulk of the disturbance is carried to regions far from the obstacle. An exact form of the pressure coefficient is obtained for any surface shape (consistent with the linear theory), and involves a convolution integral of two Bessel functions with imaginary argument which is analytically evaluated. When the particle-fluid density ratio is small, the \"frozen\" and \"equilibrium\" Mach waves are very closely clustered together. A \"boundary layer technique\", based on the fact that changes across the Mach waves are rapid compared to changes along Mach waves, is then applied to obtain a simplified version of the linearized equation that describes Mach waves inclined toward the downstream direction only. While the Mach wave structure is consistent with the exact treatment, the pressure coefficient takes on the much simpler form of decreasing exponentials. The transition is, again, from the \"frozen\" value at the leading edge towards the \"equilibrium\" value in the downstream direction insofar as the surface shape permits.\r\n" }, { "name": "Tracy, Richard Ripley", "degree": "PhD", "year": "1964", "title": "Hypersonic Flow Over a Yawed Circular Cone", "advisor": "Lees, Lester; Kubota, Toshi", "url": "https://resolver.caltech.edu/CaltechETD:etd-10042002-161203", "creators": [ { "name": { "family": "Tracy", "given": "Richard Ripley" }, "id": "Tracy-Richard-Ripley", "display_name": "Tracy, Richard Ripley" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" }, { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RMWT-TH67", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA 10[degree] semi-apex, circular cone is tested in air at Mach 8 and at yaw angles to 24[degrees]; surface pressure, heat transfer, and pitot-pressure throughout the flow field are presented. The nominal surface temperature is 40% of the free stream stagnation temperature, and the Reynolds number, based on cone generator length, is varied from 0.5 x 10[^5] to 4.2 x 10[^5]. Heat transfer is measured at higher surface temperature ratios (up to 56%) and Reynolds numbers (up to 7.3 x 10[^5]) by reducing the free stream stagnation temperature. All raw data consist of continuous circumferential distributions of each quantity and are included in a supplement.\r\n\r\nThe surface pressure data are compared with the theories of Stone-Kopal and Cheng; Reshotko's theory of heat transfer to the windward generator is compared with experiment. The probe data delineate the boundary between viscous and inviscid flow and determine the shape of the outer shock wave as well as the secondary shocks which appear in the flow field at large yaw. The probe data are sufficient to determine the flow field in the plane of symmetry and permit an approximate representation of the Mach number profiles of the separated viscous flow in the leeward meridian plane beyond a moderate yaw angle." }, { "name": "Ahlstrom, Harlow Garth", "degree": "PhD", "year": "1963", "title": "Experiments on the Upstream Wake in Magneto-Fluid Dynamics", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-111323", "creators": [ { "name": { "family": "Ahlstrom", "given": "Harlow Garth" }, "id": "Ahlstrom-Harlow-Garth", "display_name": "Ahlstrom, Harlow Garth" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "chair", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "member", "display_name": "Fung, Yuan-cheng" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Lauritsen", "given": "Thomas" }, "id": "Lauritsen-T", "role": "member", "display_name": "Lauritsen, Thomas" }, { "name": { "family": "Whitham", "given": "Gerald Beresford" }, "id": "Whitham-G-B", "role": "member", "display_name": "Whitham, Gerald Beresford" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NAR6-M485", "abstract": "Measurements have been made of the perturbation magnetic field in front of a semi-infinite Rankine body moving parallel to a uniform impressed magnetic field in a conducting fluid. The purpose of these experiments was to investigate the so-called upstream wake effect which has been predicted by theory. It is believed that these are the first experiments in which the upstream wake was observed. Although the wake was found to exist as predicted when the Alfv\u00e9n number is greater than one, its decay behavior was remarkably different from that which was predicted. The solutions for infinite medium predicted that in the wake the perturbations should decay inversely as the distance from the body. However the experiments showed that the perturbations decayed exponentially. It was finally shown that this change in the decay behavior was an effect of the walls and the conducting material surrounding the fluid.
\r\n" }, { "name": "Anderson, William Judson", "degree": "PhD", "year": "1963", "title": "Studies in Panel Flutter at Higher Mach Numbers: I. Flat and Slightly Curved Panels at Mach Number 2.81. II. Cylindrical Shells with Boundary Layer", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-12152005-081824", "creators": [ { "name": { "family": "Anderson", "given": "William Judson" }, "id": "Anderson-William-Judson", "display_name": "Anderson, William Judson" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E3ZX-JG81", "abstract": "Part I.
\r\n\r\nTwo series of panel flutter tests were carried out in the Jet Propulsion Laboratory's 12 inch supersonic wind tunnel. Flat and slightly curved panels were tested at Mach number 2.81.
\r\n\r\nThe flat, rectangular panels were designed to study two-dimensional flutter. They were clamped at front and rear with free sides which extended into the boundary layer at the sides of the tunnel. These panels fluttered in a two-dimensional mode which occurred at a thickness ratio approximately 15 per cent different from the predictions of existing theory. One of the panels exhibited a three-dimensional \"rocking\" flutter which has not been observed or discussed before. A theory is developed for this type of flutter.
\r\n\r\nThe slightly curved panels were shallow circular cylindrical shells with the generators perpendicular to the flow direction. These panels were all of aspect ratio one. It was found that the effect of curvature was destabilizing and that the effect of internal pressurization was stabilizing.
\r\n\r\nPart II.
\r\n\r\nThe effect of a boundary layer on the flutter of a cylindrical shell is studied. The aerodynamic forces are developed for a shell of infinite length. The boundary layer is idealized as an annular region of uniform subsonic flow surrounding the shell. This boundary layer is of constant thickness along the shell and has a constant velocity distribution through its thickness. The external supersonic flow is also taken to be of uniform velocity, resulting in a \"stepped\" velocity profile through the boundary layer. Small perturbation theory is used in the boundary layer region and linear piston theory is used for the supersonic flow.
\r\n\r\nIn order to replace a physical boundary layer with an idealization for calculations, a procedure is developed for choosing the boundary layer parameters of velocity, pressure, etc. in a consistent way.
\r\n\r\nThe forces which are found through this boundary layer theory are compared with those obtained using piston theory directly. It is found that the forces on a mode with many circumferential waves are much smaller than the forces given by piston theory - - a reduction in amplitude of 95 per cent is possible. Phase changes also occur. The effect of the boundary layer on axisymmetric modes is not so great.
\r\n\r\nFlutter boundaries are obtained for axisymmetric flutter under several conditions and illustrate the effect of boundary layer thickness and structural damping.
\r\n" }, { "name": "Dewey, Clarence Forbes", "degree": "PhD", "year": "1963", "title": "Measurements in Highly Dissipative Regions of Hypersonic Flows. Part I. Hot-Wire Measurements in Low Reynolds Number Hypersonic Flows. Part II. The Near Wake of a Blunt Body at Hypers0nic Speeds", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12212005-083759", "creators": [ { "name": { "family": "Dewey", "given": "Clarence Forbes" }, "id": "Dewey-Clarence-Forbes", "display_name": "Dewey, Clarence Forbes" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XG0Q-ZA42", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nPart I:\r\n\r\nMeasurements were made of the heat loss and recovery temperature of a fine hot-wire at a nominal Mach number of 5.8. Data were obtained over an eight-fold range of Reynolds numbers in the transitional regime between continuum and free-molecule flow. At high Reynolds numbers, the heat transfer data agree well with the results of Laufer and McClellan, which were obtained at lower Mach numbers. At lower Reynolds numbers, the results indicate a monotonic transition between continuum and free molecule heat transfer laws. The slope of the heat transfer correlation also appears to vary monotonically, with Nu [...] at high Reynolds numbers and Nu ~ Re for Re< < 1.\r\n\r\nData on the wire recovery temperature (corresponding to zero net heat transfer) were obtained for free-stream Knudsen numbers between 0.4 and 3.0. Comparison with previous supersonic data suggests that for Mach numbers greater than about two the normalized variation of recovery temperature in the transitional regime is a unique function of the free-stream Knudsen number. The recent data of Vrebalovich (33) suggests that the relation between the normalized recovery temperature and Knudsen number found in this investigation also applies to subsonic and transonic flow.\r\n\r\nThe steady-state hot-wire may be used to obtain two thermodynamic measurements: the rate of heat transfer from the wire and the wire recovery temperature. An illustrative experiment was performed in the wake of a transverse cylinder, using both hot-wire and pressure instruments in a redundant system of measurements. It was shown that good accuracy may be obtained with a hot-wire even when the Reynolds number based on wire diameter is small.\r\n\r\nPart II:\r\n\r\nA theoretical model of the near wake is derived following the ideas of Chapman. This model is based on the postulates of mass conservation in the base flow region, thin viscous shear layers, and a recompression process which is independent of Reynolds number. The analysis, which includes the effects of initial shear layer thickness and base flow temperature, shows that the characteristics of the near wake (base pressure, shear layer angle, etc.) are independent of Reynolds number, and that the shear layer and initial wake thicknesses are proportional to Re[...].\r\n\r\nA series of experiments are presented which show that the postulate of thin shear layers is invalid for Reynolds numbers less than about [...]. At higher Reynolds numbers, the theory is qualitatively incorrect if the Mach number [...] external to the shear layer is large. Detailed measurements with a steady-state hot-wire in the near wake of a two-dimensional circular cylinder indicate that the compression process at the neck is not isentropic, and that the maximum pressure rise occurs downstream of the stagnation point formed by the merging shear layers. Comparison between the experimental and theoretical results points out the importance of the base flow temperature and the initial shear layer profile in determining the observable characteristics of the near wake." }, { "name": "Fourney, Michael Eugene", "degree": "PhD", "year": "1963", "title": "I. On the Application of a Laser to High Speed Photography. II. Torsional Magnetoelastic Waves in a Circular Cylinder", "advisor": "Ellis, Albert T.; Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-131705", "creators": [ { "name": { "family": "Fourney", "given": "Michael Eugene" }, "id": "Fourney-Michael-Eugene", "display_name": "Fourney, Michael Eugene" } ], "advisors": [ { "name": { "family": "Ellis", "given": "Albert T." }, "id": "Ellis-A-T", "role": "advisor", "display_name": "Ellis, Albert T." }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6M9E-V056", "abstract": "The first part of this thesis deals with the application of a ruby laser to high speed photography. The light that is emitted from a ruby laser is monochromatic, collimated, coherent, linearly polarized, and highly intense. It has been demonstrated that these properties make the laser a very useful tool for research in areas involving photographic techniques.\r\n\r\nA method has been devised by which the output of the laser is controlled and is emitted in the form of a series of light pulses. The rate at which these pulses are generated can be controlled and repetition rates of over 1.6 Mc/sec have been achieved. The 30 nsec duration of an individual pulse represents the exposure time for a high speed laser camera which has been developed. Pictures have been taken at rates of over 1.2 million frames per second. This exposure time represents a reduction of an order of magnitude over that previously possible, with an increase of three orders of magnitude in intensity. In applications where a particular characteristic of the laser light is required, such as the monochromatic nature desired in photoelasticity, this improvement of four orders of magnitude is further increased.\r\n\r\nWhen a series of light pulses is generated in the manner described above it is found that the amplitude of the pulse train becomes stable above a certain critical frequency. This critical frequency is determined to be a function of the laser cavity length. The amplitude of the stabilized pulse train is a function of the repetition rate and the cavity length. The nature of this variation is established and a mechanistic explanation of the phenomena involved is presented." }, { "name": "Gold, Harris", "degree": "PhD", "year": "1963", "title": "Stability of Laminar Wakes", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12212005-152513", "creators": [ { "name": { "family": "Gold", "given": "Harris" }, "id": "Gold-Harris", "display_name": "Gold, Harris" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/X1TJ-MG72", "abstract": "This investigation deals with the effects of compressibility on the hydrodynamic stability of wake flows. It is found that the effect of temperature is two-fold: (1), as the wake core temperature increases, the range of Mach numbers over which neutral and self-excited subsonic disturbances can exist also increases; (2) as long as the relative Mach number is below the critical Mach number the neutral inviscid wave number will decrease with increasing core temperature, implying that a hot wake will be more stable than a cool one.\r\n\r\nThe analysis of Batchelor and Gill for the inviscid stability of axi-symmetric incompressible jets has been extended to the more general problem of compressible wakes and jets. It is shown that the results are directly analogous to those obtained for the two-dimensional problem. The sinuous (n = 1) mode is the most unstable allowable mode. This unstable mode is observed in a hypersonic wake." }, { "name": "Harwell, Kenneth Edwin", "degree": "PhD", "year": "1963", "title": "Initial Ionization Rates in Shock-Heated Argon, Krypton, and Xenon", "advisor": "Jahn, Robert G.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12072005-144544", "creators": [ { "name": { "family": "Harwell", "given": "Kenneth Edwin" }, "id": "Harwell-Kenneth-Edwin", "display_name": "Harwell, Kenneth Edwin" } ], "advisors": [ { "name": { "family": "Jahn", "given": "Robert G." }, "id": "Jahn-R-G", "role": "advisor", "display_name": "Jahn, Robert G." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HW2S-MW55", "abstract": "The rates of ionization behind strong shock waves in argon, krypton, and xenon are observed by a transverse microwave probe over a range of electron densities low enough that atom-atom inelastic collisions are the rate determining mechanism. Shocks of Mach number 7.0 to 10.0 propagate down a 5 cm square aluminum shock tube into ambient gases at pressures of 3 to 17 mm Hg, heating them abruptly to atomic temperatures of 5500\u00b0K to 9600\u00b0K. The subsequent relaxation toward ionization equilibrium is examined in its early stages by the reflection, transmission, and phase shifts of a 24.0 Kmc (1.25 cm) transverse microwave beam propagating between two rectangular horns abreast a glass test section. The data yield effective activation energies of 11.9 +/- 0.5 ev for argon, 10.4 +/- 0.5 ev for krypton, and 8.6 +/- 0.6 ev for xenon. These coincide, within experimental error, with the first excitation potentials, rather than the ionization potentials of the gases, indicating that in this range, ionization proceeds via a two-step process involving the first excited electronic state. Within experimental error, the pressure dependence is found to be proportional to the number density squared." }, { "name": "Knauss, Wolfgang Gustav", "degree": "PhD", "year": "1963", "title": "Rupture Phenomena in Viscoelastic Materials", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11052003-091542", "creators": [ { "name": { "family": "Knauss", "given": "Wolfgang Gustav" }, "id": "Knauss-Wolfgang-Gustav", "display_name": "Knauss, Wolfgang Gustav" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KZSV-0Y32", "abstract": "A failure theory for high polymers is developed from the hypothesis that weak regions exist in the material. Defects nucleate in these regions through bond rupture until the defects reach a size which is critical for the applied boundary loading. This critical condition is based on energy balance considerations.\r\n\r\nBy considering the relaxation of the polymer chain in terms of the phenomenological stress-strain behavior and the rupture of chemical bonds in terms of an Arrhenius type rate law, the theory is able to accommodate an arbitrary stress or strain history, and shows reasonably good agreement with experiments which cover a large range of conditions.\r\n\r\nIn addition the stress analysis of a special crack geometry is presented. The geometry consists of a thin infinite strip containing a semi-infinite crack. For a uniform separation of the infinite boundaries an infinitesimal elasticity solution is obtained with the help of the Fourier transform and Wiener-Hopf techniques. The effect of large strains on the stresses near the crack tip is studied experimentally and a surprising correlation with the infinitesimal elasticity solution is found." }, { "name": "Ko, William L.", "degree": "PhD", "year": "1963", "title": "Application of Finite Elastic Theory to the Behavior of Rubber-Like Materials", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03012004-143718", "creators": [ { "name": { "family": "Ko", "given": "William L." }, "id": "Ko-William-L", "display_name": "Ko, William L." } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WMS4-A521", "abstract": "In Part I, methods for determining the strain energy function and the associated constitutive stress-deformation law for rubber-like materials is undertaken and the mechanics of data reduction needed to determine some parameters of the theory are displayed. Experiments were performed in four different stress fields on a foamed polyurethane rubber (dilatable rubber) and on several kinds of continuum rubbers. A new strain energy function and the associated stress-deformation law for a foamed rubber are generated which correlate most of the data to a high degree of accuracy. A parameter appearing in the functional expression for a foam rubber has the same significance as Poisson's ratio in infinitesimal elastic theory. For continuum rubbers, the isotropic Neo-Hookean representations of quasi-static behavior is found to be sufficient over most of the whole range of extension.\r\n\r\nIn Part II, geometrical representations of an isotropic failure surface based on various criteria are depicted both in principal stress and principal stretch spaces for elastic materials. The experimental data are compared with all criteria and the results are discussed.\r\n\t\r\nIn Part III, finite elastic theory is used to determine the stress and deformation fields around the base of a radial crack in an infinitely long rubber log opened by a facially bonded rigid wedge-shaped bellow.\r\n\t\r\nIn the last Part, the topology of interstices idealized as closest packed spherical holes (idealized foam structure) is investigated. Equivalent elastic constants are calculated for rubbery interstices of both hexagonal and face-centered cubic closest packings under small displacement." }, { "name": "Petty, James Sibley", "degree": "PhD", "year": "1963", "title": "Linearized Transonic Flow About Non-Lifting, Thin Symmetric Airfoils", "advisor": "Royce, Winston W.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12072005-135856", "creators": [ { "name": { "family": "Petty", "given": "James Sibley" }, "id": "Petty-James-Sibley", "display_name": "Petty, James Sibley" } ], "advisors": [ { "name": { "family": "Royce", "given": "Winston W." }, "id": "Royce-W-W", "role": "advisor", "display_name": "Royce, Winston W." } ], "committee": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "chair", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Royce", "given": "Winston W." }, "id": "Royce-W-W", "role": "member", "display_name": "Royce, Winston W." }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "member", "display_name": "Cole, Julian D." }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "member", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3ZC0-7Y26", "abstract": "Transonic flow about symmetric, non-lifting airfoils is treated by solving an approximate linear differential equation of mixed type in place of the exact small-perturbation equations. The pressure distribution and drag coefficient are obtained in closed form for power series airfoils. The technique of local linearization is also applied to improve the accuracy of the results, particularly near the leading edge where the linearizing approximation is found to be invalid. Numerical results are obtained for the parabolic arc and single wedge airfoils and are found to compare favorably with experimental data and with previous theoretical results.\r\n" }, { "name": "Schmidt, Louis Vincent", "degree": "PhD", "year": "1963", "title": "Measurements of Fluctuating Air Loads on a Circular Cylinder", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-06102008-110237", "creators": [ { "name": { "family": "Schmidt", "given": "Louis Vincent" }, "id": "Schmidt-Louis-Vincent", "display_name": "Schmidt, Louis Vincent" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8714-PJ73", "abstract": "Measurements were made of the unsteady air loads, both lift and drag, developed on a circular cylinder when exposed to flow in the supercritical Reynolds number range from 0.38 x 106 to 0.75 x 106. The three dimensional nature of the flow, which is a consequence of the flow separation effects, is recognized. The statistical character of spatially varying random type fluctuating air loads is presented in the form of cross-spectral densities and cross-correlation coefficients. Information of this type has application to the problem of determining the response of a cylindrical structure when exposed to air loads.
\r\n\r\nThe extreme sensitivity of the flow to surface irregularities in the supercritical Reynolds number region is reported including the effects upon the local and spatial character of the fluctuating loads. Local values of steady state lift could be induced by suitably orienting surface disturbances on the forward portion of the cylinder.
\r\n" }, { "name": "Shi, Yun-Yuan", "degree": "PhD", "year": "1963", "title": "Low Reynolds Number Flow Past Finite Cylinders of Large Aspect Ratio", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10172005-152936", "creators": [ { "name": { "family": "Shi", "given": "Yun-Yuan" }, "id": "Shi-Yun-Yuan", "display_name": "Shi, Yun-Yuan" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JA8T-4N67", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nLow Reynolds number flow past finite cylinders of large aspect ratio is considered in this thesis. The first cylinder under consideration consists of a finite cylinder of constant radius [...] with two hemispherical caps attached to each end. The axis of the cylinder is perpendicular to uniform flow at infinity and the half length of the cylinder is L. Therefore, two Reynolds numbers can be formed in the present study, namely, Re=[...] and, [...] . The low Reynolds number flow is studied in the limit [...] for [...] fixed. This clearly shows that the body is of large aspect ratio.\r\n\r\nThe other cylinder under consideration is an ellipsoid of revolution whose half-axis parallel to the flow is [...] and whose half-axis perpendicular to the flow is L. The same limiting case as that for the first finite cylinder is studied.\r\n\r\nAsymptotic expansions of the solution valid for the limiting case [...] are obtained by applying singular perturbation procedures with proper use of the idea of the intermediate solution. The nature of the end source variation and the order of magnitude of various effects are clarified in the present study. In addition, certain general remarks have been made about the difference between the end effects for \"tapered\" and \"untapered\" bodies. It is found that the \"taper\" at the ends plays an essential role." }, { "name": "Vlases, George Charpentier", "degree": "PhD", "year": "1963", "title": "Experiments in a Cylindrical Magnetic Shock Tube", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechTHESIS:10092012-110749746", "creators": [ { "name": { "family": "Vlases", "given": "George Charpentier" }, "id": "Vlases-George-Charpentier", "display_name": "Vlases, George Charpentier" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1EHZ-AZ30", "abstract": "An investigation has been conducted with the two-fold purpose\r\nof producing very high Mach number shock waves and studying their\r\ninteraction with an external magnetic field parallel to the shock front.\r\nBy means of the technique of electromagnetic driving, stable reproducible, outward-going cylindrical shock waves in the Mach number\r\nrange from 20 to 100 have been produced and studied.
\r\n\r\nTheory predicts fundamental differences between the interaction\r\nof a magnetic field with a shock moving into a highly conducting\r\nfluid and the interaction of a field with a strong gas-ionizing shock.\r\nIn the former case a true mhd shock is produced. In the latter the\r\nfield interacts directly only with the piston and the shock remains\r\nan ordinary one. The effect of a conducting wall surrounding the\r\nchamber also differs substantially in the two cases.
\r\n\r\nDetailed experiments have been carried out on gas-ionizing\r\nshocks. While the overall motion is very nearly that predicted by the\r\ntheory, anomalies have arisen in the details of the flow and are\r\nexplained in a qualitative manner.
\r\n\r\nMethods of producing sufficient initial conductivity to obtain a\r\nthin magnetohydrodynamic shock are discussed, together with some\r\npreliminary experiments along these lines.
" }, { "name": "Wu, Ying-Chu Lin Susan", "degree": "PhD", "year": "1963", "title": "Flow Generated by Suddenly Heated Flat Plate", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-10052004-163535", "creators": [ { "name": { "family": "Wu", "given": "Ying-Chu Lin Susan" }, "id": "Wu-Ying-Chu-Lin-Susan", "display_name": "Wu, Ying-Chu Lin Susan" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "chair", "display_name": "Lees, Lester" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Plesset", "given": "Milton S." }, "id": "Plesset-M-S", "role": "member", "display_name": "Plesset, Milton S." }, { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "member", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Lauritsen", "given": "Thomas" }, "id": "Lauritsen-T", "role": "member", "display_name": "Lauritsen, Thomas" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CPQD-RQ29", "abstract": "By employing the two-sided Maxwellian in Maxwell's moment method a kinetic theory description is obtained of the flow generated by a step-function increase in the temperature of an infinite flat plate. Four moments are employed in order to satisfy the three conservation equations, plus one additional equation involving the heat flux in the direction normal to the plate. For a small temperature rise the equations are linearized, and closed-form solutions are obtained for small and large time in terms of the average collision time.
\r\n\r\nInitially the disturbances propagate along two distinct characteristics, but the discontinuities across these waves damp out as time increases. At large time the main disturbance propagates with the isentropic sound speed. Solutions for mean normal velocity and temperature show the transition from the nearly collision-free regime to the Navier-Stokes-Fourier regime, which is characterized by a boundary layer near the plate surface merging into a diffuse \"wave\". The classical continuum equations, plus a temperature jump boundary condition, seem to be perfectly adequate to describe the flow beyond a few collision times, provided one accounts properly for the interaction between the inner thermal layer and the outer diffuse wave.
" }, { "name": "Babcock, Charles Dwight", "degree": "PhD", "year": "1962", "title": "The Buckling of Cylindrical Shells with an Initial Imperfection Under Axial Compression Loading", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-02242004-151013", "creators": [ { "name": { "family": "Babcock", "given": "Charles Dwight" }, "id": "Babcock-Charles-Dwight", "display_name": "Babcock, Charles Dwight" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0P4A-SA39", "abstract": "An experimental and theoretical investigation of the effect of a specific type of initial imperfection on the buckling load of a circular cylindrical shell under axial compression loading was carried out. The imperfection studied was axially symmetric in shape and had the form of a half sine wave in the length direction. Both inward and outward bowing imperfections were considered.\r\n\r\nThe experiments were carried out with shells fabricated by a copper electroforming process. The shells had no longitudinal seams and had unintended imperfections of the order of the wall thickness. The buckling stress for the intended imperfection studied was only slightly influenced over a considerable range of imperfection amplitudes.\r\n\r\nThe theoretical solution located the bifurcation points of the prebuckled axially symmetric state. The solution showed that outward bowing shells should have the same buckling stress as a perfect cylindrical shell and inward bowing shells should have a lower buckling stress than the perfect cylinder." }, { "name": "Cummings, Benjamin Edgar", "degree": "PhD", "year": "1962", "title": "Some Nonlinear Vibration and Response Problems of Cylindrical Panels and Shells", "advisor": "Fung, Yuan-cheng; Caughey, Thomas Kirk", "url": "https://resolver.caltech.edu/CaltechETD:etd-12222005-115144", "creators": [ { "name": { "family": "Cummings", "given": "Benjamin Edgar" }, "id": "Cummings-Benjamin-Edgar", "display_name": "Cummings, Benjamin Edgar" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" }, { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "role": "advisor", "display_name": "Caughey, Thomas Kirk" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/AXC2-6P94", "abstract": "Large amplitude vibrations and forced responses of curved panels and shells are studied by the application of the shallow shell equation. The Galerkin procedure is used to reduce the nonlinear partial differential equations to ordinary nonlinear equations. Marked differences are found between the behavior of curved panels and cylindrical shells. Relations for the dependence of the free vibration period on amplitude are given. A two mode analysis of the cylindrical shell problem is included.
\r\n\r\nThe curved panel is found to exhibit a buckling phenomenon for the simple \"breathing modes\". Shock response methods are used to predict dynamic buckling of the curved panel and the predictions are verified by numerical integration.
\r\n\r\nThe cylindrical shell vibrations and responses are found to be governed by Duffing's equation and certain of the well-known properties of Duffing's equation are applied to the cylindrical shell dynamics.
\r\n\r\nThe two mode analysis of the cylindrical shell is shown to exhibit weak coupling, allowing the separate excitation of the coupled modes.
\r\n\r\nSome numerical results are given.
" }, { "name": "Hoult, David Parks", "degree": "PhD", "year": "1962", "title": "Magnetohydrodynamic Surface Waves", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03262008-082838", "creators": [ { "name": { "family": "Hoult", "given": "David Parks" }, "id": "Hoult-David- Parks", "display_name": "Hoult, David Parks" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RVM0-1T80", "abstract": "This is an experimental and theoretical study of deep water gravity-like waves which are induced in a liquid metal by a changing magnetic field. The dominant feature of such waves is the emission of Alfven waves from the free surface. A linearized theory is derived and compared with experiments." }, { "name": "Liu, Chung-Yen", "degree": "PhD", "year": "1962", "title": "Part I. Kinetic Theory Description of Plane, Compressible Couette Flow. Part II. Kinetic Theory Description of Conductive Heat Transfer from a Fine Wire", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12092005-133941", "creators": [ { "name": { "family": "Liu", "given": "Chung-Yen" }, "id": "Liu-Chung-Yen", "display_name": "Liu, Chung-Yen" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q16R-JR92", "abstract": "PART I: \r\n \r\nBy utilizing the two-stream Maxwellian in Maxwell's integral equations of transfer we are able to find a closed-form solution of the problem of compressible plane Couette flow over the whole range of gas density from free molecule flow to atmospheric. The ratio of shear stress to the product of ordinary viscosity and velocity gradient, which is unity for a Newtonian fluid, here depends also on the gas density, the plate temperatures and the plate spacing. For example, this ratio decreases rapidly with increasing plate Mach number when the plate temperatures are fixed. On the other hand, at a fixed Mach number based on the temperature of one plate, this ratio approaches unity as the temperature of the other plate increases. Similar remarks can be made for the ratio of heat flux to the product of ordinary heat conduction coefficient and temperature gradient.\r\n\r\nThe effect of gas density on the skin friction and heat transfer coefficients is described in terms of a single rarefaction parameter, which amounts to evaluating gas properties at a certain \"kinetic temperature\" defined in terms of plate Mach number and plate temperature ratio. One interesting result is the effect of plate temperature on velocity \"slip\". In the Navier-Stokes regime most of the gas follows the hot plate, because the gas viscosity is larger there. As the gas density decreases the situation is reversed, because the velocity slip is larger at the hot plate than at the cold plate. In the limiting case of a highly rarefied gas most of the gas follows the cold plate.\r\n\r\nLimitations of the present six-moment approximation at high plate Mach numbers are discussed and it is concluded that an eight-moment approximation would eliminate these difficulties. The results obtained in this simple geometry suggest certain conclusions about hypersonic flow over solid bodies when the surface temperature is much lower than the kinetic temperature.\r\n\r\nPART II:\r\n\r\nThe Maxwell moment method utilizing the two-sided Maxwellian distribution function is applied to the problem of conductive heat transfer between two concentric clylinders at rest. Analytical solutions are obtained for small temperature differences between the cylinders. The predicted heat transfer agrees very well with experiments performed by Bomelburg, Schafer-Rating and Eucken. Comparison with results given by the Grad's thirteen moment equations, and with those given by Fourier's \"law\" plus Maxwell-Smoluchowski temperature-jump boundary condition shows that the two-sided character in the distribution function is a crucial factor in problems involving surface curvature." }, { "name": "Malmuth, Norman David", "degree": "PhD", "year": "1962", "title": "Perturbations on Hypersonic Wedge Flow", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12152005-133232", "creators": [ { "name": { "family": "Malmuth", "given": "Norman David" }, "id": "Malmuth-Norman-David", "display_name": "Malmuth, Norman David" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KE9Q-XN81", "abstract": "The hypersonic inviscid flow over a configuration representing a small perturbation about a two-dimensional wedge is analyzed. Equations and boundary conditions are obtained for a class of general perturbations within the framework of Hypersonic Small Disturbance Theory.\tA specialization of this formulation is made to the case where the resultant perturbation consists of semi-infinite flat plates of slightly different incidence to the freestream. The flow over such a shape is divided into an outlying uniform region and a central conefield. Here, the outlying, uniform region solution is found to be trivial. The determination of the conefield gives rise to an elliptic boundary value problem which is solved with the aid of the Tschaplygin transformation and other conformal mappings.\r\n\r\nCalculations are presented using the Fourier series solution for the perturbation pressure indicating the surface loads associated with the perturbation as well as the shock distortion function. Integral representations are obtained for the downwash and sidewash perturbations using the pressure solution.\r\n\r\nThe results are compared qualitatively with an analogous linear supersonic flow.\r\n\r\nFinally, a solution for more general profiles is obtained under the further restriction that the specific heat ratio [gamma] is close to one. This solution is specialized to the case considered previously and a qualitative evaluation of the physical significance of the results is made." }, { "name": "Marlotte, Gary Lynn", "degree": "PhD", "year": "1962", "title": "An Experimental Investigation of the Effect of a Transverse Hypersonic Flow Velocity upon a Low-Density D.C. Electrical Discharge in Air", "advisor": "Demetriades, Anthony; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12132005-105051", "creators": [ { "name": { "family": "Marlotte", "given": "Gary Lynn" }, "id": "Marlotte-Gary-Lynn", "display_name": "Marlotte, Gary Lynn" } ], "advisors": [ { "name": { "family": "Demetriades", "given": "Anthony" }, "id": "Demetriades-A", "role": "advisor", "display_name": "Demetriades, Anthony" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3BR3-ZT02", "abstract": "The low-density D.C. electrical discharge in a uniform gas stationary with respect to the electrodes has been studied extensively. However, when the gas moves at a hypersonic speed transverse to the electrodes, several completely new effects are introduced. Experiments were carried out with air in the GALCIT 5-inch by 5-inch hypersonic wind tunnel with a nominal Mach number of 5.8. D.C. breakdown voltages and steady-state sub-normal glow voltages were measured across a channel formed by two sharp-edged insulating flat plates in which flat-plate \"Rogowski\" electrodes were embedded. Segmented electrodes were then used in the normal glow regime to measure current distributions at each electrode for various electrode segment combinations, total currents, and densities.\r\n\r\nSome important results of the present study are the following. For the characteristic dimensions and speeds involved, the explicit dependence of electrical breakdown upon the velocity of the stream is small compared to the effect of boundary layer density defects. A theoretical treatment of breakdown is given and qualitative agreement with experiments is obtained. In the normal glow regime using segmented electrodes, an unmistakable explicit flow velocity effect was observed, with the discharge current paths being displaced downstream compared to static bell-jar tests at equivalent densities." }, { "name": "McCarthy, John Francis", "degree": "PhD", "year": "1962", "title": "Hypersonic Wakes", "advisor": "Kubota, Toshi; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-092458", "creators": [ { "name": { "family": "McCarthy", "given": "John Francis" }, "id": "McCarthy-John-Francis", "display_name": "McCarthy, John Francis" } ], "advisors": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-T", "role": "advisor", "display_name": "Kubota, Toshi" }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/63NK-AG38", "abstract": "An experimental investigation was made of the flow field behind a two-dimensional circular cylinder at a nominal Mach number of 5.7. The free-stream Reynolds number based on the cylinder diameter was varied over a range from 4300 to 66, 500 by changing both the diameter of the cylinder and the stagnation pressure of the wind tunnel. Pitot-pressure, static-pressure, and total-temperature measurements were made at various distances behind the cylindrical rod in order to determine the state properties in the wake. Base-pressure measurements were also taken at various Reynolds numbers.\r\n\r\nFrom these measurements, the transition from laminar to turbulent flow in the wake was determined and successfully correlated with other data. A transition Reynolds number based on local conditions and the length of laminar run was determined. Extensive comparison of the experimental data with Kubota's theory for laminar flow was then made. A satisfactory comparison was made between theory and experiment. Because of the nature of the tests conducted, only a qualitative comparison was made with the theory of Lees and Hromas for turbulent flow." }, { "name": "Schapery, Richard Allan", "degree": "PhD", "year": "1962", "title": "Irreversible Thermodynamics and Variational Principles with Applications to Viscoelasticity", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechTHESIS:08092011-111909246", "creators": [ { "name": { "family": "Schapery", "given": "Richard Allan" }, "id": "Schapery-Richard-Allan", "display_name": "Schapery, Richard Allan" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QEB0-N308", "abstract": "A unified theory of the thermo-mechanical behavior of\r\nviscoelastic media is developed from studying the thermodynamics of irreversible processes, and includes discussions of the general equations of motion, crack propagation, variational principles, and approximate methods of stress analysis.\r\n\r\nThe equations of motion in terms of generalized coordinates\r\nand forces are derived for systems in the neighborhood of a stable equilibrium state. They represent a modification of Biot's theory in that they contain explicit temperature dependence, and a thermodynamically consistent inclusion of the time-temperature superposition principle for treating media with temperature-dependent viscosity coefficients. The stress-strain-temperature and energy equations for viscoelastic solids follow immediately from the general\r\nequations and, along with equilibrium and strain-displacement relations, they form a complete set for the description of the thermomechanical behavior of media with temperature-dependent viscosity. In addition, an energy equation for crack propagation is derived and examined briefly for its essential features by applying it to a specific problem.\r\n\r\nThe thermodynamic equations of motion are then used to\r\ndeduce new variational principles for generalized coordinates and forces, employing convolution-type functionals. Anticipating various engineering applications, the formulation is phrased alternately in terms of mechanical displacement, stresses, entropy displacement, and temperature in thermally and mechanically linear solids. Some special variational principles are also suggested for applications wherein the nonlinear thermal effects of temperature dependent viscosity and dissipation may be important. \r\n\r\nBuilding upon the basic variational formulation, it is next\r\nshown that when these convolution functionals are Laplace-transformed with respect to time, some convenient minimum principles result which can be employed for the approximate calculation of transformed, viscoelastic responses. The characteristic time dependence of exact and approximate solutions is then derived and used in relating error\r\nin approximate viscoelastic solutions to error in the associated elastic solutions.\r\n\r\nThe dissertation is concluded with a study of some approximate methods of viscoelastic analysis. First, the important problem of inverting complicated Laplace transforms to physical time-dependent solutions is resolved by advancing two easily applied, approximate methods of transform inversion. These inversion methods and\r\nvariational principles are then used in some illustrative, numerical, examples of stress and heat conduction analysis.\r\n" }, { "name": "Zukerman, Abraham", "degree": "PhD", "year": "1962", "title": "Analytical Approximations to the Solutions of the Equations of Motion in the Earth-Moon Space", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-091409", "creators": [ { "name": { "family": "Zukerman", "given": "Abraham" }, "id": "Zukerman-Abraham", "display_name": "Zukerman, Abraham" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZFV9-4018", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\n\t\r\nTwo methods of obtaining approximate solutions of the equations of motion in the Earth-Moon space are derived. The first method - asymptotic expansions of the solutions of the equations of motion - is a power series expansion of the solutions in powers of the inverse maximum velocity [...]. A comparison of the results of numerical integration with the asymptotic expansions is presented, which shows the range of applicability of this method.\r\n\r\nThe second method is similar to the small perturbation approach. In this method the zeroth order solution is a Keplerian orbit about the Earth (the Moon's effect being neglected). The first order solution corrects for the lunar gravity effects on the zeroth order trajectory. To demonstrate the computational difficulties involved in the application of this method, a straight line Keplerian trajectory was used as the zeroth order solution. Several applications of the solutions are discussed." }, { "name": "Ai, Daniel Kwoh-i", "degree": "PhD", "year": "1961", "title": "Part I. Cylindrical Couette Flow in a Rarefied Gas According to Grad's Equation. Part II. Small Perturbations in the Unsteady Flow of a Rarefied Gas Based on Grad's Thirteen Moment Approximation", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12092005-131707", "creators": [ { "name": { "family": "Ai", "given": "Daniel Kwoh-i" }, "id": "Ai-Daniel-Kwoh-i", "display_name": "Ai, Daniel Kwoh-i" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "chair", "display_name": "Lees, Lester" }, { "name": { "family": "Goetz", "given": "Alexander" }, "id": "Goetz-Alexander", "role": "member", "display_name": "Goetz, Alexander" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "member", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Royce", "given": "Winston W." }, "id": "Royce-W-W", "role": "member", "display_name": "Royce, Winston W." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9P2N-HF23", "abstract": "Part I
\r\n\r\nGrad's thirteen moment method is applied to the problem of the shear flow and heat conduction between two concentric, rotating cylinders of infinite length. In order to concentrate on the effects of curvature the problem is linearized by requiring that the Mach number is small compared with unity, and that the temperature difference between the two cylinders is small compared with the mean temperature. The solutions of the linearized Grad equations show a qualitatively correct transition of the cylinder drag from free-molecule flow to the classical Navier-Stokes regime. However the magnitude of the curvature effect on the drag in rarefied flow is not given correctly, because Grad's distribution function ignores the wedge-like domains of influence of the two cylinders.
\r\n\r\nThe solution obtained for the heat transfer rate is physically unrealistic in the free-molecule flow limit, and this result is produced by a cross-coupling between the normal stresses and the radial heat flux imposed by Grad's distribution function. In this simple problem the difficulty can be eliminated by taking the normal stresses to be identically zero and employing a truncated moment method. However, in general this device cannot be utilized in problems involving curved solid boundaries, or when dissipation is considered. One concludes that the choice of the distribution function to be employed in Maxwell's moment equations is dictated by the requirements imposed in the limiting case of highly rarefied gas flows, as well as in the Navier-Stokes regime.
\r\n\r\nPart II
\r\n\r\nIn this paper, the unsteady one-dimensional flow of a compressible, viscous and heat conducting fluid is treated, based on linearized Grad's thirteen moment equations. The fluid, initially at rest, is set into motion by some small external disturbances. Our interest is to examine the nature of all the responses. The fluid field extends to infinity in both directions; thus no length is involved, and also there is no solid wall boundary existing in the problem. The nature of the external disturbances is restricted to having a unit impulse in the momentum equation and a unit heat addition in the energy equation. The disturbances are located on an infinite plane normal to the flow direction; and the responses induced correspond to fundamental solutions of the problem. The method of Laplace transforms is applied, and the inverse transforms of all quantities are obtained in integral form. Because of the complicated expressions of the integrands involved, we consider only certain limiting cases which correspond to small and large times from the start of the motion, compared to the average time between molecular collisions. In order to study these limiting cases, it is essential to understand the behavior of the integrand in the complex plane; hence all singularities and branch points are obtained.
\r\n\r\nWhen t is small, the integrand is expanded in powers of t to obtain a wave front approximation. All discontinuities are propagated along the characteristics of the linearized system, and a damping term also appears.
\r\n\r\nAt large values of time, the integrand gets its main contribution around the branch points, and these solutions are identical to those obtained from the Navier-Stokes equation. The fundamental solution of the one-dimensional unsteady flow, idealized as it seems to be, offers itself as a tool to understand other related problems. The piston problem, as well as the normal quantities in Rayleigh's problem (e.g., normal velocity, normal stress, and thermodynamical quantities), are governed by the same set of equations. Hence, certain parts of the fundamental solutions can be applied directly to these problems. The limiting forms of the normal quantities in Rayleigh's problem are expected to be worked out in another paper in the near future.
" }, { "name": "Cann, Gordon Lawrence", "degree": "PhD", "year": "1961", "title": "Energy Transfer Processes in a Partially Ionized Gas", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-12202005-133300", "creators": [ { "name": { "family": "Cann", "given": "Gordon Lawrence" }, "id": "Cann-Gordon-Lawrence", "display_name": "Cann, Gordon Lawrence" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XV8R-MM73", "abstract": "The following paper is divided into three more or less separate sections. The first section (Chapters II - VI) deals with an analysis of the transport properties of a partially ionized gas subject to the constraint that the average random energy of all constituent particles is exactly equal (equipartition of energy). This constraint is necessary so that the formal Chapman-Enskog solution of Boltzman's equation can be used to evaluate the various transport coefficients. Subject to this constraint, a set of tractable equations describing the mass and energy diffusion in a partially ionized gas is obtained that includes all terms correct to the order of the square root of the ratio of the electron to atom mass compared to one. The transport coefficients are evaluated for helium and argon over the complete range of partial ionization assuming that the species particle densities are quite close to their equilibrium values.\r\n\r\nThe analysis indicates that the electron and ion diffusion velocities are more closely coupled than the equations of Chapman and Cowling show. The added coupling implicitly applies the constraint of zero mass velocity to the gas locally. Because of this constraint a current in the direction of (E x B) x B occurs in addition to the direct and Hall currents.\r\n\r\nIt is shown that the only part of the thermal conductivity that can be influenced by a magnetic field is that part of the energy carried by the diffusion of the charged particles. For this reason, magnetic fields, in general, cannot be nearly as effective in reducing heat transfer rates as was previously thought, e. g., a magnetic field will have no influence on the thermal conductivity in a fully ionized gas, except through its influence on the current density and the thermal diffusion.\r\n\r\nChapters VII - IX comprise the second section of this paper and deal with the development of a similarity solution for axially symmetric electric discharges. A number of parameters are obtained and discussed. The solution is evaluated for a discharge in argon gas at one atmosphere pressure in which the temperature on the axis of the discharge varies from 6,000\u00b0K to 19,000\u00b0K. The current-voltage characteristic obtained from this solution is compared with an experimentally determined curve of H. Maecker.\r\n\r\nThe third section of this paper (Chapters X - XIII) is concerned with the mechanisms of energy transfer in arc jet devices. Use is made of the previous sections of the paper to determine the relative magnitude of the amount of energy that is transferred to the gas in the various parts of the electric discharge. The various possible electrode configurations are discussed in detail and compared. The design and performance of an annular electrode arc heater with a rotating arc is next described and discussed. Because of a number of undesirable performance characteristics of this type of electrode configuration, a modified heater was constructed with the cathode emission occurring along the axis of the applied magnetic field. Details of the unexpectedly good performance of this configuration are given. It is shown that the arc potential drop depends primarily on the strength of the applied magnetic field and the gas enthalpy downstream of the arc. The dependence of the arc potential drop on the arc current and the ambient pressure is shown to be weak over the ranges tested, e. g., 50 to 300 amperes for the current and 1 to 4 atmospheres for the pressure. Some heat transfer measurements taken with this equipment are presented.\r\n\r\nAppendix I is concerned with the evaluation of the transport coefficients in a partially ionized gas. Formulae are developed for determining the viscosity, thermal conductivity, and electric conductivity of the plasma. These coefficients are computed for argon and helium at one atmosphere pressure and over the temperature range of partial ionization." }, { "name": "Childress, William Stephen", "degree": "PhD", "year": "1961", "title": "Asymptotic Expansion of Navier-Stokes Solutions in Three Dimensions for Large Distances", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-113027", "creators": [ { "name": { "family": "Childress", "given": "William Stephen" }, "id": "Childress-William-Stephen", "display_name": "Childress, William Stephen" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/A77G-P071", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThis thesis studies the stationary flow field at large distances from a finite obstacle moving uniformly in a viscous, incompressible fluid. The principal results consist of asymptotic expansions, uniformly valid for large distance, of the velocity and the pressure of the flow field.\r\n\r\nThe expansion procedure employed is based upon the introduction of a small, extraneous parameter; the construction is thus recast as a perturbation for small values of the parameter. Owing to the presence of a viscous wake, the perturbation is in general a singular one, and is treated accordingly, using methods developed for related hydrodynamical problems.\r\n\r\nThe calculated results include the following: for the case of axially-symmetric flow, a uniformly valid expansion of the velocity to order [...] inclusive, and of the pressure to order [...] inclusive, r being the distance from the obstacle; for the general case, an expansion of the velocity to order [...] and of the pressure to order [...], inclusive." }, { "name": "Christiansen, Walter Henry", "degree": "PhD", "year": "1961", "title": "Development and Calibration of a Cold Wire Probe for Use in Shock Tubes", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-12142005-124939", "creators": [ { "name": { "family": "Christiansen", "given": "Walter Henry" }, "id": "Christiansen-Walter-Henry", "display_name": "Christiansen, Walter Henry" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QGS9-WZ66", "abstract": "The use of a fine unheated wire for making shock tube flow measurements is investigated. The operation of the instrument depends on the transient nature of the shock tube flow. The wire is referred to here as a cold wire; it operates in a non-steady manner which is completely different from the usual hot wire operation. This report describes the construction and calibration of the cold wire.\r\n\r\nThe experimental law for the rate of gain of heat to the wire in air is determined over a range of Mach numbers from 0.4 to 1.9 and a range of Reynolds numbers from 0.035 to 3,500 based on the wire diameter and the conditions in the hot flow following the initial shock wave. Similar measurements are reported for argon. The heat transfer measurements cover the continuum region, the slip and transitional regions, and extend into the free-molecule flow region. The dimensionless results are compared with hot wire measurements obtained in wind tunnels and are found to differ slightly. A difference exists because the cold wire gains heat from the fluid while the hot wire loses heat to the fluid. The measurements are very repeatable and self-consistent, and they indicate that the wire can be used to give an accurate flow measurement in the shock tube.\r\n\r\nSome potential applications of the wire for the study of shocktube flows are presented. It is concluded that the fine unheated wire is a versatile tool that can be used to great advantage in the shock tube." }, { "name": "Kevorkian, Jirair Kevork", "degree": "PhD", "year": "1961", "title": "The Uniformly Valid Asymptotic Approximations to the Solutions of Certain Non-Linear Ordinary Differential Equations", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12222005-092728", "creators": [ { "name": { "family": "Kevorkian", "given": "Jirair Kevork" }, "id": "Kevorkian-Jirair-Kevork", "display_name": "Kevorkian, Jirair Kevork" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K8NE-5X16", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThis work deals with the application of an expansion procedure in terms of two independent time variables for the uniform asymptotic representation of solutions representing certain mechanical systems.\r\n\r\nThe method is first applied to systems governed by the equation [...] where [...] is a small parameter, and f has the character of a damping (i. e. y is a bounded function of t for all t [...] 0).\r\n\r\nIt is shown that the physical problems which can be brought to the above non-dimensional form possess two characteristic time scales, one associated with the oscillatory behavior of the solution, while the other measures the time interval in which the effects of the non-linear term become apparent.\r\n\r\nThe dependence of the solution on these time scales is not simple, in the sense that an asymptotic representation of the exact solution which is valid for large times cannot be obtained by a limit process in which a non-dimensional time variable is held fixed. This fact has motivated the introduction of an expansion procedure in functions of two time variables, and it is shown that with the use of certain simple boundedness criteria a uniform asymptotic representation can be derived.\r\n\r\nIn addition to the above mentioned class of problems a variety of examples possessing certain boundedness properties is studied by this method, including, for example, the Mathieu equation.\r\n\r\nThe main emphasis of this paper is on the constructive rather than general approach to the solutions of specific examples. These examples are introduced in turn to illustrate the underlying ideas of the method, whose main advantage is its simplicity especially for computing the higher approximations." }, { "name": "Lock, Malcolm Harvey", "degree": "PhD", "year": "1961", "title": "A Study of Two-Dimensional Panel Flutter", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-12142005-124410", "creators": [ { "name": { "family": "Lock", "given": "Malcolm Harvey" }, "id": "Lock-Malcolm-Harvey", "display_name": "Lock, Malcolm Harvey" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NZ2V-H267", "abstract": "An investigation of the problem of the flutter of two-dimensional flat panels is undertaken. The research is largely devoted to investigating the adequacy of the ideal flutter theory that has been employed to predict flutter boundaries for such panels. A series of panel flutter experiments carried out in the GALCIT 4\u201d x 10\u201d transonic wind tunnel at Mach numbers up to 1.5 are described in detail. Before the results of these experiments are compared with the predictions of the theory some further analytical studies of the flutter problem are presented that enable a more critical comparison of theory and experiment to be made. These analyses treat some aspects of the problems of transonic and supersonic panel flutter. The nature of the energy exchange at flutter is also considered. This latter study throws considerable light upon the flutter process as described by the ideal theory and also clarifies the breakdown of certain approximate unsteady aerodynamic theories in the low supersonic flow region (1 < M < 1.5). Comparison of theory and experiment reveals considerable differences between the theoretical and experimental flutter boundaries at the lower supersonic Mach numbers. The agreement between theory and experiment improves at Mach numbers above about 1.4. The possible sources of the apparent inadequacy of the theory at the lower supersonic Mach numbers are discussed." }, { "name": "Lynn, Yen-Mow", "degree": "PhD", "year": "1961", "title": "Isentropic Plane Waves in Magnetohydrodynamics", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-134709", "creators": [ { "name": { "family": "Lynn", "given": "Yen-Mow" }, "id": "Lynn-Yen-Mow", "display_name": "Lynn, Yen-Mow" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/K49C-2N76", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nPlane waves propagating in a perfectly electrically conducting polytropic gas of otherwise uniform state in the presence of an arbitrarily oriented uniform magnetic field are studied; they correspond to plane simple waves in magnetohydrodynamics. Riemann invariants across finite amplitude waves in ordinary gasdynamics are generalized herein to take into account all possible magnetohydrodynamics effects. There exist totally seven types of waves, namely, contact surfaces, forward and backward facing transverse simple waves and forward and backward facing coupled (fast and slow) simple waves. But of these only coupled waves are genuinely nonlinear and receive most of our attention. The mathematical theory of simple waves is discussed first to give a general picture of the underlying structure of solutions. Contact surfaces and transverse simple wave solutions are given next with particular emphasis on the case of the contact surface adjacent to a vacuum, region. An exact analytical solution of coupled waves for gases of arbitrary value of [...] is obtained in terms of generalized Riemann invariants; some of these invariants are expressed in terms of definite integrals of a parameter [...]. The invariant relations among several physical quantities are thus expressed in a parametric form. An alternative method of solving coupled waves by graphical means is proposed and some detailed calculations are presented. General properties of physical variables across coupled waves are mentioned. For the special case of gas in a purely transverse magnetic field, a scheme of solving arbitrary flow problems is discussed briefly. The corresponding case of coupled wave solutions is given in terms of a hypergeometric function. Finally, some examples are shown to illustrate the application of the solutions to actual physical problems." }, { "name": "Narasimha, Roddam", "degree": "PhD", "year": "1961", "title": "Some Flow Problems in Rarefied Gas Dynamics", "advisor": "Liepmann, Hans Wolfgang; Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11042003-095050", "creators": [ { "name": { "family": "Narasimha", "given": "Roddam" }, "id": "Narasimha-Roddam", "display_name": "Narasimha, Roddam" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1S8T-QA38", "abstract": "This thesis discusses three rather loosely connected problems in free molecule and nearly free molecule flow. First the expansion of a gas cloud into perfect vacuum is considered on the basis of the collision-less Boltzmann equation, and it is shown that if the initial distribution is an isothermal Maxwellian, the density obeys a diffusion equation with a diffusion coefficient proportional to the time. This leads to the description of the free expansion of symmetric clouds in terms of a thick 'diffusion front' traveling at the initial isothermal speed of sound. The expansion of asymmetric clouds and the flow due to sources and jets are also studied.\r\n\r\nSecond, a method of iteration proposed by Willis for calculating nearly free molecular flow is extended to general unsteady flows; it is then applied to the flow through an orifice to show that the correction to the mass flow is of the first order in the inverse Knudsen number. The coefficient, estimated by making some reasonable assumptions about the three-dimensional nature of the flow, is found to agree quite well with Liepmann's measurements.\r\n\r\nFinally a physical basis is suggested for Krook's collision model used in the above calculations. Several consequences of the model are then derived, including the important one that, in the Navier-Stokes limit, the model implies a Stokesian gas with a Prandtl number of unity. The value to be given to the parameter in the model is also discussed at some length." }, { "name": "Russell, David Allison", "degree": "PhD", "year": "1961", "title": "Studies of the Effects of Cross-Sectional Area Change and Boundary-Layer Growth on the Motion of a Shock Wave", "advisor": "Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-11212003-155010", "creators": [ { "name": { "family": "Russell", "given": "David Allison" }, "id": "Russell-David-Allison", "display_name": "Russell, David Allison" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HXQP-VQ30", "abstract": "Three problems concerned with the motion of a shock wave are discussed. The first is an analytical and experimental study of the performance of a shock tube with area change near the diaphragm. Interesting results of this section are the development of a simple shock-speed control through the use of area change, and the fact that a spread-out dissipation region was shown to exist for a configuration where a non-stationary secondary shock wave was originally expected. A general discussion of the use of this type of area change is also included.\r\n\r\nThe second problem is a study of the effects of boundary-layer growth on the motion of a shock wave. A simple theory for predicting the attenuation of a shock wave on entering an upstream-facing tube is developed from the analysis of Spence and Woods. When simple laminar boundary-layer approximations are applied, the theory shows good agreement with measurements.\r\n\r\nThe final problem is an experimental study of the motion of a shock wave downstream of a finite-length area contraction. Normalized results are presented which show that the shock wave emerges from the area change at a speed close to the \"linearized\" theory value, and is then attenuated by second-order disturbances until it reaches the speed predicted for the steady-state configuration. The results are presented in a form which is shown to be insensitive to both the incident shock Mach number and the amount of the area reduction." }, { "name": "Stearman, Ronald Oran", "degree": "PhD", "year": "1961", "title": "Part I. The Stability of a Membrane in a Subsonic Flow. Part II. The Stability of a Grid of Panels in a Supersonic Flow", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-12212005-160627", "creators": [ { "name": { "family": "Stearman", "given": "Ronald Oran" }, "id": "Stearman-Ronald-Oran", "display_name": "Stearman, Ronald Oran" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TE64-4652", "abstract": "Part I.
\r\n\r\nAn experimental investigation has been made on the stability of small aspect ratio rectangular membranes in a subsonic flow. The leading and trailing edges of the membrane were attached to rigid streamlined supports while the two streamwise edges were free. Both surfaces of the membrane were exposed to the airstream, and the membrane tension was applied through the trailing edge.
\r\n\r\nThe results of the test show that two types of flutter (instability) occur. The first to appear as the wind speed was increased from zero, with a fixed tension level in the membrane, was a small amplitude flutter which has a shallow wave like motion traveling in the streamwise direction. At higher wind speeds this motion was damped out. A narrow equilibrium zone or boundary existed which separated the first type of flutter from a second type of motion having a traveling wave of larger amplitude and greater speed. This second type of flutter had no tendency to damp out, but became more violent as the wind speed was increased.
\r\n\r\nThe span of the slender membrane is the physical parameter that uniquely determines and controls the first flutter boundary; its mass plays no part here, but does affect the equilibrium zones.
\r\n\r\nAppendix A contains an obvious formulation of the slender membrane flutter problem.
\r\n\r\nPart II.
\r\n\r\nA theoretical investigation has been made on the stability of a grid of panels in a supersonic flow. The problem is formulated by considering this structure as a limiting case of a more general configuration composed of a ring of panels (i.e. an axially stiffened cylindrical shell) whose outer surface is exposed to a supersonic flow parallel to its axis. It is shown that the stability analysis of this more general configuration can be reduced to the analysis of an \"equivalent\" single panel using the circulant matrix idea. The reduction procedure, applicable to most cyclic configurations, allows for all types of inter-element (panel) coupling and is subject to the sole restriction that the dynamic phenomenon be satisfactorily described by linear theory.
\r\n\r\nIt is shown that at least five different multi-panel configurations can be obtained from this general problem by taking the appropriate limiting process. The stability (flutter) analysis of one of these limiting cases is discussed for high Mach number flows where only an elastic coupling exists between neighboring panels.
" }, { "name": "Fleming, Edward Richard", "degree": "PhD", "year": "1960", "title": "Non-Linear Flutter", "advisor": "Fung, Yuan-cheng; Caughey, Thomas Kirk", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-133349", "creators": [ { "name": { "family": "Fleming", "given": "Edward Richard" }, "id": "Fleming-Edward-Richard", "display_name": "Fleming, Edward Richard" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" }, { "name": { "family": "Caughey", "given": "Thomas Kirk" }, "id": "Caughey-T-K", "role": "advisor", "display_name": "Caughey, Thomas Kirk" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/CSNK-SA02", "abstract": "The problem of two degree of freedom flutter in the presence of structural non-linearities is investigated. The specific problem chosen for investigation is that of bending-torsion flutter of a two-dimensional airfoil in a supersonic flow. The Kryloff-Bogoliuboff assumption of nearly sinusoidal response with slowly varying amplitude and phase is made and aerodynamic piston theory is used throughout the analysis.
\r\n\r\nFormulas for flutter limit cycles are developed in terms of general structural non-linearities. Necessary and sufficient conditions are developed for the existence of stable flutter limit cycles in the case of an airfoil with torsional stiffness. Several numerical examples of this case are given including a case which exhibits flutter for large disturbances but is stable at all airspeeds for small disturbances.
\r\n\r\nAn analog computer investigation of flutter dependence on initial conditions is given.
\r\n" }, { "name": "Glick, Herbert Seymour", "degree": "PhD", "year": "1960", "title": "Modified Crocco-Lees Mixing Theory for Supersonic Separated and Reattaching Flows", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12082005-132722", "creators": [ { "name": { "family": "Glick", "given": "Herbert Seymour" }, "id": "Glick-Herbert-Seymour", "display_name": "Glick, Herbert Seymour" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0V7K-QS61", "abstract": "Re-examination of the Crocco-Lees method has shown that the previous quantitative disagreement between theory and experiment in the region of flow up to separation was caused primarily by the improper C(K) relation assumed. A new C(K) correlation, based on low-speed theoretical and experimental data and on supersonic experimental results, has been developed and found to be satisfactory for accurate calculation of two-dimensional laminar supersonic flows up to separation.\r\n\r\nA study of separated and reattaching regions of flow has led to a physical model which incorporates the concept of the \"dividing\" streamline and the results of experiment. According to this physical model, viscous momentum transport is the essential mechanism in the zone between separation and the beginning of reattachment, while the reattachment process is, on the contrary, an essentially inviscid process. This physical model has been translated into Crocco-Lees language using a semi-empirical approach, and approximate C(K) and F(K) relations have been determined for the separated and reattaching regions. The results of this analysis have been applied to the problem of shock wave-laminar boundary layer interaction, and satisfactory quantitative agreement with experiment has been achieved." }, { "name": "Gorecki, Jacek Piotr", "degree": "PhD", "year": "1960", "title": "An Investigation of Temperature Fluctuations on Bluff Bodies", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-02242006-083641", "creators": [ { "name": { "family": "Gorecki", "given": "Jacek Piotr" }, "id": "Gorecki-Jacek-Piotr", "display_name": "Gorecki, Jacek Piotr" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KV3S-CJ64", "abstract": "Temperature fluctuations and recovery temperatures on the surface of a circular cylinder (with axis normal to a subsonic compressible flow) and the field of flow about the cylinder, particularly the wake area, are investigated experimentally in range between M = 0.35, Re = 117,000 and M = 0.70, Re = 201,000.\r\n\r\nSpectral analysis of fluctuations on body surface and other evidence indicate that formation of discrete vortex cores from the separated shear layers is initially an impulsive random process (of the generalized \"shot effect\" type), although the wake farther downstream from the model has a definitely periodic structure.\r\n\r\nImpulsive formation of vortex cores can be enhanced by wind tunnel resonance or by a high turbulence level in the free stream and is accompanied by abnormal cooling of the model surface in the separated area - the mechanism of these effects is also investigated." }, { "name": "Hunter, Herbert Erwin", "degree": "PhD", "year": "1960", "title": "Application of Asymptotic Expansion Procedures to Low Reynolds Number Flows about Infinite Bodies", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12092005-134820", "creators": [ { "name": { "family": "Hunter", "given": "Herbert Erwin" }, "id": "Hunter-Herbert-Erwin", "display_name": "Hunter, Herbert Erwin" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5PBX-0J36", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nSeveral limiting cases for viscous incompressible flow are considered for two examples. The first example considered is that of the flow past an expanding infinite cylinder at an angle of attack. The time dependence of the radius of the cylinder is given by the power law R = [...]. The second example considered is the flow past a semi-infinite power law body of revolution (i. e. R = [...]) at zero angle of attack. Both examples are considered for the limiting case of small Reynolds number. The Reynolds number is based on a characteristic length obtained from the parameters in the expression for the radius. The second example is also considered for the limiting case of the flow far down stream.\r\n\r\nAsymptotic expansions of the solution valid for the limiting cases considered (i. e, low Reynolds number or flow far down stream) are obtained by applying singular perturbation procedures. These expansions are obtained for 0 <= n < 1 for the first example and for 0 <= n <= 1/2 for the second example. For the second example the terms in the low Reynolds number expansion are not obtained in closed form, except for n = 1/2. For n < 1/2 the low Reynolds number expansion of the Navier-Stokes equations is expressed in terms of the solution of the corresponding Stokes flow problem. The expansions obtained for the flow far down stream on the power law body of revolution have the character of a very viscous flow although they are valid for any fixed Reynolds number." }, { "name": "MacGillivray, Dean", "degree": "PhD", "year": "1960", "title": "Motion of a Current Element Through a Fluid of Low Electrical Conductivity", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12072005-131316", "creators": [ { "name": { "family": "MacGillivray", "given": "Dean" }, "id": "MacGillivray-Dean", "display_name": "MacGillivray, Dean" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/B0AW-TR02", "abstract": "Two-dimensional flow of an incompressible, viscous, electrically conducting fluid past a current element is studied. A solution in the form of an asymptotic development is obtained, valid as a certain dimensionless parameter (essentially the product of the electrical conductivity and the current) tends to zero. An expression for the drag on the current element is computed, and is found to be independent of viscosity." }, { "name": "Reshotko, Eli", "degree": "PhD", "year": "1960", "title": "Stability of the Compressible Laminar Boundary Layer", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-01042006-141741", "creators": [ { "name": { "family": "Reshotko", "given": "Eli" }, "id": "Reshotko-Eli", "display_name": "Reshotko, Eli" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M6HG-WW29", "abstract": "In previous theoretical treatments of the stability of the compressible laminar boundary layer the effect of the temperature fluctuations on the \"viscous\" (rapidly varying) disturbances is either ignored (Lees-Lin), or is accounted for incompletely (Dunn-Lin). A thorough reexamination of this problem shows that temperature fluctuations have a profound influence on both the \"inviscid\" (slowly varying) and viscous disturbances above a Mach number of about 2.0. The present analysis includes the effect of temperature fluctuations on the viscosity and thermal conductivity, and also introduces the viscous dissipation term that was dropped in the earlier theoretical treatments.\r\n\r\nSome important results of the present study are: (1), the rate of conversion of energy from the mean flow to the disturbance flow through the action of viscosity in the vicinity of the wall increases with Mach number; (2), instead of being nearly constant across the boundary layer, the amplitude of inviscid pressure fluctuations for Mach numbers greater than 3 decreases markedly with distance outward from the plate surface. This behavior means that the jump in magnitude of the Reynolds stress in the neighborhood of the critical layer is greatly reduced; (3), at Mach numbers less than about 2 dissipation effects are minor, but they become extremely important at higher Mach numbers since for neutral disturbances they must compensate for the generally destabilizing effects of items (1) and (2).\r\n\r\nNumerical examples illustrating the effects of compressibility (including neutral stability characteristics) are obtained and are compared with the experimental results of Laufer and Vrebalovich at M = 2.2, and of Demetriades at M = 5.8." }, { "name": "Sturtevant, Bradford", "degree": "PhD", "year": "1960", "title": "The Effusion of Charged Particles from a Shock Heated Gas", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12302004-145356", "creators": [ { "name": { "family": "Sturtevant", "given": "Bradford" }, "id": "Sturtevant-Bradford", "display_name": "Sturtevant, Bradford" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PCNN-DW03", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental and theoretical investigation is made of the application of a molecular beam type sampling device for studying low density shock tube flows to the case of slowly ionizing argon behind a reflected shock wave. The flux of charged particles from a gas heated to about 10,OOO[degrees]K and 20 mm. Hg. through a small orifice in the shock tube end wall is measured. The processes determining this flux are the initial stages of ionization in argon and the diffusion of charged particles to a cold metallic wall. Providing the diffusion process is understood, the measurements constitute a direct observation of incipient ionization ([...]).\r\n\r\nThe transient charge diffusion mechanism is studied in detail theoretically, avoiding the assumption of ambipolar diffusion. It is concluded that the major problem lies in the understanding of the wall-gas interaction as represented by boundary conditions at the wall. An approximate relation for charge effusion is derived.\r\n\r\nIt is concluded from the experimental results that the initial ionization can not be due to a single step, electron-atom collision process but must result from a series of several atom-atom collisions resulting in the ionization of argon atoms." }, { "name": "Chang, I-Dee", "degree": "PhD", "year": "1959", "title": "Navier-Stokes Solutions at Large Distances from a Finite Object", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02062006-083016", "creators": [ { "name": { "family": "Chang", "given": "I-Dee" }, "id": "Chang-I-Dee", "display_name": "Chang, I-Dee" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SMPS-TA29", "abstract": "The asymptotic expansion of the Navier-Stokes solutions at fixed Reynolds numbers and large distances from a finite object for an incompressible, stationary and two-dimensional flow is studied. The expansion is a coordinate-type expansion and differs in many mathematical aspects from the more familiar parameter-type expansions for large and small Reynolds number flows. These differences are noted and discussed in some detail. The technique chosen for dealing with the problem is that of the use of an artificial parameter. This is one possible method for using some of the techniques of parameter-type expansions. In particular, at large distances from the object one may distinguish a viscous wake region and a potential (\"outer\") flow region. The relation between these regions is very similar to the relation between the viscous boundary layer and the potential flow region for flow at large Reynolds numbers.\r\n\r\nSeveral terms of the expansion are computed. However, the main emphasis is placed on discussing the methods for deriving these terms. The special features of expansions in artificial parameters are discussed in detail. The role of various properties of Navier-Stokes solutions, such as validity of integral theorems and rapid decay of vorticity is also brought out.\r\n\r\nThe original motivation of the study was an attempt to understand the Filon's paradox which historically was an error in evaluating the momentum, integral of the asymptotic flow field. The present study, however, deals with the general problem of the flow at large distances from a finite object, and, more generally, with expansion techniques for similar problems. The author's explanation of Filon's paradox is only an incidental result." }, { "name": "Royce, Winston Walker", "degree": "PhD", "year": "1959", "title": "Transonic Flow Over a Non-Lifting Slender Body of Revolution", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01092006-153522", "creators": [ { "name": { "family": "Royce", "given": "Winston Walker" }, "id": "Royce-Winston-Walker", "display_name": "Royce, Winston Walker" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NFVH-YH92", "abstract": "Sonic flow past a non-lifting, slender body of revolution is investigated by the use of small disturbance theory. An approximation for the local Mach number distribution is used to linearize the transonic potential equation. Solutions for the velocity components, pressure distribution, and drag are obtained in terms of simple integrals involving the body geometry. An extension to other Mach numbers in the transonic range is given. The theoretical pressure distribution and drag are found to give good agreement with experimental data." }, { "name": "Weyers, Paul Frederik Robert", "degree": "PhD", "year": "1959", "title": "The Vibration and Acoustic Radiation of Thin-Walled Cylinders Caused by Internal Turbulent Flow", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-02062006-134627", "creators": [ { "name": { "family": "Weyers", "given": "Paul Frederik Robert" }, "id": "Weyers-Paul-Frederick-Robert", "display_name": "Weyers, Paul Frederik Robert" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QTM4-9Z19", "abstract": "The investigation concerned noise produced by turbulent flow adjacent to a flexible wall. Measurements of the spectrum and intensity of the pressure field outside thin-walled Mylar cylinders containing turbulent pipe flow have been made. The resulting spectra could be interpreted in relation to the elastic properties of the cylinders and the character of the turbulent fluctuations inside the flow. The eigen frequencies of the cylinders could be identified and similarity parameters for the spectra were established. The effect of cylinder wall thickness on the spectrum and intensity of the pressure fluctuations was investigated. It was found that the intensity of the external pressure field scaled with the fifth power of the velocity at the center of the pipe.
\r\n\r\nFor one particular case the spectrum and intensity of the pressure fluctuations exerted by the turbulent flow on the wall were measured. The intensity of the pressure fluctuations at the wall scaled with the fourth power of the velocity as expected. The ratio of the root-mean-square wall pressure to the dynamic pressure was found to be independent of Mach number and equal to a constant (0.0078). Similarity laws for the spectra of the wall pressure fluctuations were also confirmed.
" }, { "name": "Ang, Dang Dinh", "degree": "PhD", "year": "1958", "title": "Some Radiation Problems in Elastodynamics", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-10072004-092112", "creators": [ { "name": { "family": "Ang", "given": "Dang Dinh" }, "id": "Ang-Dang-Dinh", "display_name": "Ang, Dang Dinh" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/TF74-2977", "abstract": "Three elastodynamic problems are studied. The first deals with waves generated by instantaneous and uniform closure of a semi-infinite crack, while in the second, a semi-infinite crack is suddenly initiated in a continuous medium initially subjected to uniform tension. The last of the three deals with a force moving at uniform velocity along a semi-infinite crack, starting from the edge. The problems are solved by means of the Wiener-Hopf integral methods. The characteristic wave patterns and stress singularities are discussed." }, { "name": "Bell, Richard William", "degree": "PhD", "year": "1958", "title": "The Elastic Instability of Thin Cantilever Struts on Elastic Supports with Axial and Transverse Loads at the Free End", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-10072004-093255", "creators": [ { "name": { "family": "Bell", "given": "Richard William" }, "id": "Bell-Richard-William", "display_name": "Bell, Richard William" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5FEX-RT33", "abstract": "An analysis is made of the elastic instability of thin, tapered cantilever struts subjected to a general concentrated load acting in the plane of the strut at its tip. The strut is supported at its root on a structure permitting elastic rotations of the root section in the buckled mode. The influence of the support on the minimum buckling load is one of the main points of interest. It is shown that the general linearized problem can be formulated in one second order differential equation with variable coefficients, and two associated boundary conditions. This homogeneous eigenvalue system constitutes a simplified statement of the problem which permits the easy extension of exact linear theory to a wide class of taper functions, including the effect of elastic supports. The solution emerges in terms of a generalized deflection parameter, rather than of either the torsional or the bending components of the coupled buckling mode, which are governed respectively by third and fourth order differential equations.\r\n\r\nSpecific solutions are derived for some \"natural\" taper forms of the strut. The general solutions for the deflection mode are power series, which are rapidly convergent for certain limiting geometries. The problems of convergence of the series, some singular physical aspects associated with pointed tips, and the increasing numerical difficulty for large taper ratio are correlated with the behavior of the singular points of the equation. Numerical results showing the effects of the elastic supports on minimum buckling loads are presented for the uniform strut and for a simple case of the tapered strut. The series solutions for more general cases are given in a form which can be applied to digital computers.\r\n" }, { "name": "Demetriades, Anthony", "degree": "PhD", "year": "1958", "title": "An Experimental Investigation of the Stability of the Hypersonic Laminar Boundary Layer", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-09232004-093013", "creators": [ { "name": { "family": "Demetriades", "given": "Anthony" }, "id": "Demetriades-Anthony", "display_name": "Demetriades, Anthony" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7SR9-9M59", "abstract": "NOTE: text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental investigation of the stability of the hypersonic laminar boundary layer was carried out for the case of a flat insulated surface at zero angle of attack. The stream-wise amplitude variation of both \"natural\" disturbances (i.e., flow fluctuations existing naturally in the boundary layer) and of disturbances artificially excited with a \"siren\" mechanism was studied with the aid of a hot-wire anemometer. In both cases it was found that such small fluctuations amplify for certain ranges of the fluctuation frequency and the Reynolds number [...], and damp for others. The demarcation boundaries for the amplification (instability) zone were found to resemble the corresponding boundaries of boundary layer instability at lower speeds. A \"line of maximum amplification\" of disturbances was also found. The amplification rates and hence the degree of \"selectivity\" of the hypersonic layer were found, however, to be considerably lower than those at the lower speeds. The disturbances selected by the layer for maximum amplification have a wavelength estimated at about twenty times the boundary-layer thickness [...], which is appreciably longer than the corresponding wave-lengths for low-speed boundary-layer flow." }, { "name": "Heimer, Harry James", "degree": "PhD", "year": "1958", "title": "Balanced Flap Type Supersonic Control Surfaces", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-11012004-142134", "creators": [ { "name": { "family": "Heimer", "given": "Harry James" }, "id": "Heimer-Harry-James", "display_name": "Heimer, Harry James" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YVYN-ZD39", "abstract": "The purpose of this investigation is to qualitatively explain the flow phenomena which occur on a balanced flap type control surface at supersonic speeds and to find means of calculating the pressure distributions occurring. The popular and generally useful linearized flow theory, and in fact any inviscid theory, cannot be used since viscosity plays a predominant part in the determination of the flow. It is shown that the flow often involves shock induced separation and reflection of shock waves by these separated wakes. Experimental data in the form of pressure distributions and shadowgraph pictures of the flow are used to arrive at a description of the flow. By use of the empirical results given, a method of predicting the pressure distributions for this type of surface is derived. This procedure is then used to calculate several pressure distributions which are compared with those experimentally determined.\r\n\r\nThe separation effects that occur at the larger angles of attack and/or control surface deflection angles cause the control effectiveness and hinge moments to be very non-linear. It is shown that the use of a gap between the wing and control surface delays or eliminates separation depending on the size of the gap used. Data are presented for a typical control surface, in which the hinge moments were reduced by a factor of 8 or 10 by using a gap between the wing and control surface. A method is given for estimating the size gap necessary to achieve this reduction." }, { "name": "McMahon, Howard Martin", "degree": "PhD", "year": "1958", "title": "An Experimental Study of the Effect of Mass Injection at the Stagnation Point of a Blunt Body", "advisor": "Lees, Lester; Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-01262006-142812", "creators": [ { "name": { "family": "McMahon", "given": "Howard Martin" }, "id": "McMahon-Howard-Martin", "display_name": "McMahon, Howard Martin" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PWSC-6787", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental study of the effect of the injection of nitrogen and helium coolant gases at the stagnation point of a blunt body was carried out in the GALCIT Hypersonic Wind Tunnel at a Mach number of 5.8. The gases were injected straight out of the stagnation point and also tangential to the body surface. The model was also fitted with flow separation spikes.\r\n\r\nThe injection of the coolant gas resulted in a marked reduction in the model equilibrium temperature, and this cooling effect persisted over the entire length of the model. For the same mass flow, helium was a better coolant than nitrogen.\r\n\r\nThe average heat transfer near the nose of the body was reduced almost to zero by injecting a mass of helium as small as 1/2 per cent of the mass flow of free-stream air contained in the \"capture\" area [...] of the spherical nose.\r\n\r\nSeparation near the spike tip was observed up to a ratio of spike length to spherical nose diameter of 1.78 and a free-stream Reynolds number based on nose diameter of 2.84 x 10(5), resulting in a value of the foredrag coefficient which was one-third the value with no spike attached." }, { "name": "Thorman, Herman Carl", "degree": "PhD", "year": "1958", "title": "Boundary Layer Measurements on an Axisymmetric Body with Spin and Yaw", "advisor": "Stewart, Homer Joseph; Millikan, Clark Blanchard; Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-01192006-132244", "creators": [ { "name": { "family": "Thorman", "given": "Herman Carl" }, "id": "Thorman-Herman-Carl", "display_name": "Thorman, Herman Carl" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/R54D-S906", "abstract": "A directionally-sensitive hot-wire anemometer was developed for measuring the three-dimensional boundary-layer flow on a spinning, axisymmetric body in a low-speed wind tunnel. Boundary-layer velocity profiles at numerous positions around the body are presented for several spin rates at incidence angles of zero and 3 degrees. Displacement thickness distributions, based on integration of the velocity profiles, are also included.\r\n\r\nThe asymmetries in the azimuthal distribution of the boundary layer around the spinning body at a small angle of incidence (3\u00b0) are examined by means of a Fourier analysis of the experimental data. The experimental results are compared with the results of certain theoretical analyses (BRL 870 (Revised), NAVORD 5036) which provide a method for determining Magnus force (and also normal force) due to the displacement effect of the boundary layer." }, { "name": "Turcotte, Donald Lawson", "degree": "PhD", "year": "1958", "title": "An Experimental Investigation of Flame Stabilization in a Heated Turbulent Boundary Layer", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-07162004-110746", "creators": [ { "name": { "family": "Turcotte", "given": "Donald Lawson" }, "id": "Turcotte-Donald-Lawson", "display_name": "Turcotte, Donald Lawson" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XT75-T174", "abstract": "For a number of years the significant parameters governing flame stabilization in moving streams have been known. In high speed applications the chemical time delay plays a fundamental role. For the low speed problems the normal flame speed and quenching distance govern stabilization. Unfortunately the transition region between the two groups of problems has not been investigated. Also the actual relation between these parameters and the properties of the combustible mixture has not been established.\r\n\r\nTo investigate these fundamental questions an experiment was set up to study flame stabilization in heated turbulent boundary layers. For wall temperatures above about 1700[degrees]F. the chemical time delay, represented by the length of the heated flame holder wall required for stabilization, was found to be a systematic and reproducible variable. A rational explanation was made for the transition from the low speed stabilization mechanism known to be applicable in unheated turbulent boundary layers and heated laminar boundary layers to the ignition mechanism applicable in heated turbulent boundary layers.\r\n\r\nAn attempt was made to relate the observed stabilization measurements to a theoretical solution based on ignition in a laminar sub-layer. The present methods of solution for such problems were found to be inadequate. A similarity solution yielded an interesting result which agreed fairly well with experiments." }, { "name": "Benenson, David Maurice", "degree": "PhD", "year": "1957", "title": "Characteristics of Propagating Stall in Axial-Flow Compressors", "advisor": "Rannie, W. Duncan", "url": "https://resolver.caltech.edu/CaltechETD:etd-07082004-133942", "creators": [ { "name": { "family": "Benenson", "given": "David Maurice" }, "id": "Benenson-David-Maurice", "display_name": "Benenson, David Maurice" } ], "advisors": [ { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q6J5-C011", "abstract": "Both small and large amplitude propagating stalls have been observed experimentally and their characteristics distinguished. The characteristics of the small amplitude propagating stall are such that the phenomenon can be described well by linearized theory. The characteristics of the large amplitude stall (which is the propagating stall phenomenon generally found in compressors) are such that the linearized theory is not adequate to describe the stall.\r\n\r\nThe propagating stall speeds determined from the small amplitude propagating stall experiments are in good agreement with those predicted by a particular formulation of the linearized theory." }, { "name": "Goddard, Frank Eber", "degree": "PhD", "year": "1957", "title": "The Effect of Uniformly Distributed Roughness on Turbulent Skin Friction Drag at Supersonic Speeds", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-07132004-115342", "creators": [ { "name": { "family": "Goddard", "given": "Frank Eber" }, "id": "Goddard-Frank-Eber", "display_name": "Goddard, Frank Eber" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BA5G-7H43", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn experimental program was carried out in the 18 x 20-inch supersonic wind tunnel of the Jet Propulsion Laboratory at the California Institute of Technology to determine the effect of uniformly distributed sand-grain roughness on the skin friction drag of a body of revolution for the case of a turbulent boundary layer. The Mach-number range covered was 1.98 to 4.54 and the Reynolds number varied from about [...] to [...]. Some data were also obtained at a Mach-number of 0.70.\r\n\r\nAt speeds up to a Mach number of 5 and for roughness sizes such that the quadratic resistance law holds, the compressibility effect is indirect and the skin friction drag is a function only of the roughness Reynolds number, [...], just as in the incompressible case.\r\n\r\nThe critical roughness below which the surface is hydraulically smooth is [...] and this is equal to the thickness of the laminar sublayer for a smooth surface for both compressible and incompressible flows.\r\n\r\nOver the range of roughness sizes considered here there appears to be no wave drag associated with the drag due to roughness.\r\n\r\nThe shift in the turbulent velocity profile [...] for a rough surface at supersonic speeds is a function only of the roughness Reynolds number, [...], and quantitatively follows identically the same law as in the incompressible case." }, { "name": "Hartwig, Frederic William", "degree": "PhD", "year": "1957", "title": "Development and Application of a Technique for Steady State Aerodynamic Heat Transfer Measurements", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-07132004-161800", "creators": [ { "name": { "family": "Hartwig", "given": "Frederic William" }, "id": "Hartwig-Frederic-William", "display_name": "Hartwig, Frederic William" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Y2JY-A341", "abstract": "A technique was developed for measuring steady state heat transfer on a hemisphere cylinder and the results are compared with theory. The instrumentation consisted of a miniaturized thermopile of silver-constantan thermocouples approximately 1/8\" x 1/16\" x 1/100\". The repeatability of readings with this device was found to be excellent. These heat measuring devices, or heat meters, were installed in both a ceramic hemisphere cylinder and in a similar metal one. There were obtained three different heat flow rates at each of six different combinations of tunnel pressure and temperature.
\r\n\r\nThe results compared very well with a theory developed by Lester Lees based upon the assumption of local similarity.
\r\n" }, { "name": "Kubota, Toshi", "degree": "PhD", "year": "1957", "title": "Investigation of Flow Around Simple Bodies in Hypersonic Flow", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-07142004-143403", "creators": [ { "name": { "family": "Kubota", "given": "Toshi" }, "id": "Kubota-Toshi", "display_name": "Kubota, Toshi" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZVPP-FK96", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA theoretical analysis of the flow around slender blunt-nosed bodies was made by applying the flow similarity concept to the hypersonic small-disturbance equations. The flow field around a class of bodies of the form [...] exhibits a certain similarity in the sense that the pressure, density and transverse velocity are described by relations of the form Q(x,r)/Q(R) = f(r/R), where R is the distance from the axis to the shock wave. This similarity holds when the Mach number is infinitely large, and when the exponent in the equation defining the body shape lies in the range [...] for axially-symmetric bodies and in the range [...] for two-dimensional bodies. For large but finite Mach numbers a second approximation was obtained by expanding solutions in series of powers of [...].\r\n\r\nAn experimental investigation of the flow around \"similar-flow\" bodies of revolution was conducted at Mach number 7.7 in the GALCIT hypersonic wind tunnel. The surface pressure distributions agreed closely with the theoretical predictions, after a simplified correction was made for the boundary-layer displacement effect. The results indicated that the boundary layer interaction effect needs a further investigation." }, { "name": "Messiter, Arthur Frederic", "degree": "PhD", "year": "1957", "title": "Expansion Procedures and Similarity Laws for Transonic Flow", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-07152004-091254", "creators": [ { "name": { "family": "Messiter", "given": "Arthur Frederic" }, "id": "Messiter-Arthur-Frederic", "display_name": "Messiter, Arthur Frederic" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4HK0-K279", "abstract": "The transonic flow past slender bodies and thin wings is investigated with the use of a general theory of expansion procedures. It is assumed that the solutions for the velocity components possess asymptotic expansions of a very general form, and the differential equations and boundary conditions for the first and higher approximations are obtained by applying appropriate limiting procedures to the full equations. The following cases are treated: 1) bodies of revolution at zero incidence; 2) bodies of nearly circular cross-section, at zero incidence; 3) bodies of revolution at an angle of attack; and 4) thin wings at zero incidence. Certain first-order similarity laws are derived for these problems, and the order of magnitude of the error is stated in each case." }, { "name": "Rabinowicz, Josef", "degree": "PhD", "year": "1957", "title": "Aerodynamic Studies in the Shock Tube", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-09152004-142813", "creators": [ { "name": { "family": "Rabinowicz", "given": "Josef" }, "id": "Rabinowicz-Josef", "display_name": "Rabinowicz, Josef" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MXH9-5H80", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nIn order to utilize the shock tube for quantitative investigations of some aerodynamic problems a thin platinum film resistance thermometer was developed for heat transfer rate measurements. The present report describes the construction and calibration of the heat transfer gage. Since the experimental technique presents a major problem this investigation has been carried out in the straight section of the shock tube where the flow conditions are well defined and readily measured. These flow conditions were calculated utilizing the most recent NBS data on air properties at high temperatures. The flow conditions were also measured utilizing the heat transfer gage and, several independent experimental techniques, and good agreement was found with the equilibrium flow calculations after an initial period of 30 - 50 [...]sec. Measurements of the heat transfer rate at the forward stagnation point and on the circumference of a circular cylinder are reported and compared with the theoretical calculations of L. Lees. A method for deduction of surface pressure distribution from the laminar boundary-layer heat transfer data is also presented." }, { "name": "Richmond, Ronald LeRoy", "degree": "PhD", "year": "1957", "title": "Experimental Investigation of Thick, Axially Symmetric Boundary Layers on Cylinders at Subsonic and Hypersonic Speeds", "advisor": "Coles, Donald Earl", "url": "https://resolver.caltech.edu/CaltechETD:etd-08242004-093622", "creators": [ { "name": { "family": "Richmond", "given": "Ronald LeRoy" }, "id": "Richmond-Ronald-LeRoy", "display_name": "Richmond, Ronald LeRoy" } ], "advisors": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-D-E", "role": "advisor", "display_name": "Coles, Donald Earl" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/A8P6-BA17", "abstract": "An experimental investigation of the transverse curvature effect on laminar and turbulent axially symmetric boundary layers was conducted in two subsonic wind tunnels and in the GALCIT 5 x 5 inch hypersonic wind tunnel.\r\n\r\nSubsonic turbulent skin friction coefficients were estimated from velocity profiles with axial flow on a 0.024 inch diameter cylinder and a 1.00 inch diameter cylinder. A considerable increase over the flat plate skin friction coefficient at the same momentum thickness Reynolds number was found.\r\n\r\nHypersonic laminar and turbulent skin friction coefficients with axial flow on an insulated 0.250 inch diameter cylinder were measured by the floating element technique and indicated, respectively, several times and 1.5 times the laminar and turbulent flat plate skin friction coefficients at the same momentum thickness Reynolds numbers. Turbulent skin friction coefficients were estimated from pitot profiles with axial flow on a 0.064 inch diameter cylinder and on a 0.024 inch diameter cylinder at M1 = 5.8 and indicate double the value to be found for an insulated flat plate at the same momentum thickness Reynolds number." }, { "name": "Campbell, George Stuart", "degree": "PhD", "year": "1956", "title": "Turbulence in the Wake of a Thin Airfoil at Low Speeds", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03192004-142218", "creators": [ { "name": { "family": "Campbell", "given": "George Stuart" }, "id": "Campbell-George-Stuart", "display_name": "Campbell, George Stuart" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T35K-QF86", "abstract": "Experiments have been made to determine the nature of turbulence in the wake of a two-dimensional airfoil at low speeds. The experiments were motivated by the need for data which can be used for analysis of the tail-buffeting problem in aircraft design. Turbulent intensity and power spectra of the velocity fluctuations were measured at a Reynolds number 1.6 x 10[superscript 5] for several angles of attack. Total-head measurements were also obtained in an attempt to relate steady and fluctuating wake properties.\r\n\r\nMean-square downwash was found to have nearly the same dependence on vertical position in the wake as that shown by total-head loss. For this particular wing, turbulent intensity, integrated across the wake, increased roughly as the 3 /2 power of the drag coefficient.\r\n\r\nPower-spectrum measurements indicated a decrease in frequency as wing angle of attack was increased. The average frequency in the wake was proportional to the ratio of mean wake velocity to wake width." }, { "name": "Dooley, Donald Allen", "degree": "PhD", "year": "1956", "title": "Combustion in Laminar Mixing Regions and Boundary Layers", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03222004-162749", "creators": [ { "name": { "family": "Dooley", "given": "Donald Allen" }, "id": "Dooley-Donald-Allen", "display_name": "Dooley, Donald Allen" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MGTH-TS83", "abstract": "The governing relations for gaseous flow systems with chemical reaction are briefly discussed. A mixture of mechanically similar Maxwellian molecules is assumed and the general relations are then reduced to the simplified forms appropriate for reacting, laminar boundary layer type flow systems. One-step unopposed, \"global\" reactions following first-order, second-order, and third-order kinetics are considered.\r\n\r\nThe simplified governing relations are transformed to an equivalent constant density plane by application of the Howarth transformation. A similarity function relating the specie concentrations to the local temperature is found for the case of equal Prandtl and Schmidt numbers. The similarity function is shown to be equal to the dimensionless streamwise velocity when the Prandtl and Schmidt numbers are both equal to unity. The remaining governing relations are then transformed to the Blasius plane in which the velocity field has known solutions. The energy equation is solved by an iteration process; a general analytic solution for the N'th approximation is obtained.\r\n\r\nThe analytic techniques developed are applied to the problems of combustion within laminar mixing regions and boundary layers. Temperature and concentration profiles are calculated and the dependence of the characteristic stay time upon the parameters of the system is determined. The application of the similarity techniques to the problem of chemical reaction within a hypersonic boundary layer is briefly discussed in an Appendix." }, { "name": "Eisley, Joe Griffin", "degree": "PhD", "year": "1956", "title": "Panel Flutter in Supersonic Flow", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-03222004-163442", "creators": [ { "name": { "family": "Eisley", "given": "Joe Griffin" }, "id": "Eisley-Joe-Griffin", "display_name": "Eisley, Joe Griffin" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZF04-TK14", "abstract": "The problem of panel flutter in a supersonic flow is treated in three parts. In the first the flutter of a simply supported rectangular plate is studied. Only small deflections are considered so that linear plate theory may be used. The flutter mode is described by a series expansion in terms of the normal modes of oscillation of the plate in a vacuum. Linearized aerodynamic theory is used. The exact aerodynamic solution as well as two simplifications--strip theory and quasi-steady theory--are discussed. Numerical calculations were made to determine flutter boundaries for plates of varying aspect ratio using strip theory aerodynamics for M = 2 and M = \u221a2. The flutter mode was described by considering only two or three normal modes in the calculations.
\r\n\r\nThe flutter of a two-dimensional buckled panel with clamped edges is studied both theoretically and experimentally. The flutter mode is described by a series expansion of functions which satisfy the boundary conditions for clamped edges. Quasi-steady linearized aerodynamics is used. Large deflections of the panel are considered. Numerical calculations have been made considering only the first two terms of the series expansion. The theoretical and experimental results are compared.
" }, { "name": "Hartlieb, Robert Joseph", "degree": "PhD", "year": "1956", "title": "The Cancellation of Random Disturbances in Automatic Control Systems", "advisor": "Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-03232004-132254", "creators": [ { "name": { "family": "Hartlieb", "given": "Robert Joseph" }, "id": "Hartlieb-Robert-Joseph", "display_name": "Hartlieb, Robert Joseph" } ], "advisors": [ { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/SCBZ-3H18", "abstract": "A disturbance-cancelling feedback transfer function is proposed in a preliminary study for linear systems with constant coefficients. This idea is then experimentally demonstrated.\r\n\r\nIn the main theory for more general systems, expressions are obtained for a computer which generates a cancelling input from measurements of the disturbances. The cases where there are fewer measurements than disturbances and also noisy measurements are treated. Three schemes and a basis for comparison are given. An example is calculated." }, { "name": "Karamcheti, Krishnamurty", "degree": "PhD", "year": "1956", "title": "Sound Radiation from Surface Cutouts in High Speed Flow", "advisor": "Roshko, Anatol; Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03262004-114458", "creators": [ { "name": { "family": "Karamcheti", "given": "Krishnamurty" }, "id": "Karamcheti-Krishnamurty", "display_name": "Karamcheti, Krishnamurty" } ], "advisors": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1CRR-9Y28", "abstract": "In an experimental investigation of subsonic and supersonic flows of air past rectangular cavities cut into a flat surface it was discovered that the cavities emit a strong acoustic radiation.\r\n\r\nThe frequency of the sound-producing oscillations measured by a hot wire in the cavity was found to be inversely proportional to the breadth for fixed depth. For fixed breadth the frequency was found to increase, though not systematically, with a decrease in depth.\r\n\r\nA non-dimensional frequency S is defined in terms of the frequency of emission, the gap breadth, and the free stream velocity. The dependence of S on the various parameters in the problem, such as Mach number, Reynolds number and ratio of the boundary layer thickness to a dimension of the cavity, is discussed in light of appropriate experiments.\r\n\r\nAn estimate of the intensity of the radiation was obtained by means of an optical interferometer of the Mach-Zehnder type. For points located at 3 to 4 cavity breadths from the cavity, intensities of the order of 100 - 150 decibels were found for sound fields from cavities 0.1\" deep and 0.1 to 0.2 inch broad at Mach numbers 0.7 to 0.85.\r\n\r\nPossible mechanisms for the sound production by the cavities are discussed." }, { "name": "Kendall, James Madison", "degree": "PhD", "year": "1956", "title": "An Experimental Investigation of Leading Edge Shock Wave-Boundary Layer Interaction at Hypersonic Speeds", "advisor": "Nagamatsu, Henry T.; Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-03252004-153340", "creators": [ { "name": { "family": "Kendall", "given": "James Madison" }, "id": "Kendall-James-Madison", "display_name": "Kendall, James Madison" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." }, { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6XVK-B331", "abstract": "The boundary layer on a slender body tends to be very thick at hypersonic speeds. It interacts with the external flow by producing larger flow deflections near the leading edge than those due to the body alone. The increased shock strength affects the boundary layer growth. The flow around the boundary layer gives rise to an induced pressure with a negative gradient which thins the boundary layer and increases the skin friction with respect to the zero pressure gradient value.
\r\n\r\nExperiments on a flat plate with a sharp leading edge (Re\u209c < 100) have been performed in the GALCIT 5 x 5 inch Mach 5.8 hypersonic wind tunnel. The induced pressure was measured by means of orifices in the plate surface. Profiles of Mach number, velocity, mass flow, pressure, and momentum deficiency were calculated from impact pressure surveys normal to the plate surface made at various distances from the leading edge.
\r\n\r\nThe results are as follows: (1) The induced pressures are 25 per cent higher than the weak interaction theory. (2) The boundary layer and the external flow are distinctly separate for Re\u2093 as low as 6000. (3) The shock wave location is in good agreement with that predicted by the Friedrichs theory for a body shape equivalent to the observed boundary layer displacement thickness. (4) Expansion waves reflected from the shock are weak. (5) The average skin friction coefficient tends toward and nearly matches the zero pressure gradient value downstream, but increases to approximately twice that value as the leading edge is approached.
" }, { "name": "Lamson, Philip", "degree": "PhD", "year": "1956", "title": "Measurements of Lift Fluctuations Due to Turbulence", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03262004-115115", "creators": [ { "name": { "family": "Lamson", "given": "Philip" }, "id": "Lamson-Philip", "display_name": "Lamson, Philip" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2ZDV-9T79", "abstract": "The fluctuating lift of a rigid wing in turbulent flow is studied. The power spectra of the lift and of the turbulent fluctuations are measured. From these measurements the aerodynamic admittance of the wing is obtained.\r\n\r\nThe ratio of span/scale of turbulence is varied by means of movable end plates. For a distance between the end plates of the order of the scale of turbulence the aerodynamic admittance is expected to approach the computed values of Sears.\r\n\r\nThis is shown to be the case if the reduced frequencies are larger than k = 0.8. For smaller k the experimental admittance falls below Sears' values. For large ratios of span/scale of turbulence the aerodynamic admittance is decreased for all frequencies and becomes nearly independent of frequency in the investigated range 0.2 <= k <= 2.\r\n\r\nIn general the measurements support the simplifying assumptions made in the statistical approach to gust loads and buffeting initiated by Clementson and by Liepmann." }, { "name": "Lindley, Charles Alexander", "degree": "PhD", "year": "1956", "title": "Secondary Flow in Compressor Cascades", "advisor": "Rannie, W. Duncan; Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04022009-083759", "creators": [ { "name": { "family": "Lindley", "given": "Charles Alexander" }, "id": "Lindley-Charles-Alexander", "display_name": "Lindley, Charles Alexander" } ], "advisors": [ { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2RYC-QB23", "abstract": "An experimental study was made of the secondary flow in a compressor cascade. Detailed surveys of the entire flow channel at various operating conditions are presented. The chief difference of the experimental results from those predicted by linearized theory was the presence of a turbulent separation region at the corner of the wall and the low pressure surface of the blade. While the actual losses involved were small, the effects on the succeeding stages of a turbomachine might be more severe." }, { "name": "Rethorst, Scott Carson", "degree": "PhD", "year": "1956", "title": "Characteristics of an Airfoil Extending Through a Circular Jet", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-153000", "creators": [ { "name": { "family": "Rethorst", "given": "Scott Carson" }, "id": "Rethorst-Scott-Carson", "display_name": "Rethorst, Scott Carson" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7PS7-T859", "abstract": "A method has been derived for the determination of the downwash in the field of an airfoil extending through a circular jet. This solution has been applied particularly to the region aft of the lifting line in the plane of the airfoil, to determine the lift distribution on a wing extending through a circular jet.\r\n\r\nThe method of solution is essentially based on a division of the flow induced by the jet boundary into parts which are even and odd with respect to the direction of flow. The analysis due to the even part alone is similar to previous theories, which in effect disregarded the odd part. Such previous results based on the even part alone differ considerably from the experimental values. The results based on the total of the even and odd parts show good agreement with the experiments, illustrating the necessity of including the odd part of the flow when the segment of the wing immersed in the jet is of low aspect ratio.\r\n\r\nThe problem has been solved in parametric form, so that the results may be employed to determine the characteristics of any geometry wing-jet combination at any jet velocity ratio." }, { "name": "Rodriguez, Alexander Martin", "degree": "PhD", "year": "1956", "title": "Theorems Concerning the Reduction of Drag for Supersonic Aircraft", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-08302006-133358", "creators": [ { "name": { "family": "Rodriguez", "given": "Alexander Martin" }, "id": "Rodriguez-Alexander-Martin", "display_name": "Rodriguez, Alexander Martin" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/R1JD-WT10", "abstract": "In this report the problem of drag reduction or minimization for aircraft in supersonic flight is investigated within the framework of linearized theory of inviscid flow for fixed flight Mach number.\r\n\r\nGeneral theorems applicable to complete aircraft configurations are developed. These theorems state defining properties of distributions of thickness and normal force restricted to a particular region of the aircraft configurations that minimize the drag of the complete aircraft under the condition that the distribution of thickness and normal force on the remainder of the aircraft is specified in advance. These optimum distributions are further required to satisfy some of the various types of constraints that are commonly specified for aircraft.\r\n\r\nThe problem of finding the optimum distribution of thickness, lift and sideforce on a slender body of revolution is also studied under the assumption that the body carries no total lift or sideforce and can be represented by placing the distributions along the body axis. The case for which the Mach envelope of the body does not include all of the remaining portion of the aircraft configuration, upon with the thickness and normal force are specified, is solved. This solution covers the previously known case for which the Mach envelope of the body includes the entire aircraft." }, { "name": "Shen, Yung-chung", "degree": "PhD", "year": "1956", "title": "Similarity Solution for Transonic Flow Past a Cone", "advisor": "Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06282004-095155", "creators": [ { "name": { "family": "Shen", "given": "Yung-chung" }, "id": "Shen-Yung-chung", "display_name": "Shen, Yung-chung" } ], "advisors": [ { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/74PX-HS89", "abstract": "By applying a transonic expansion procedure to a conical flow field, a system of approximate transonic equations, boundary conditions, and shock relations is derived. A similarity law for the pressure coefficient on the surface of slender cones is established. The surface pressure is computed by solving the approximate equations.\r\n\r\nBy use of similarity, the second order differential equations of the first two steps of the approximation scheme are reduced to first order equations. The solution of the first step is carried out numerically in great detail for different transonic parameters; the procedure for solving the latter is explained in the Appendix.\r\n\r\nThe results are compared with the exact solution, and a highly satisfactory agreement is reached." }, { "name": "Statler, Irving Carl", "degree": "PhD", "year": "1956", "title": "The Effects of Nonstationary Aerodynamics on the Rigid-Body Dynamic Stability of an Airplane", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-06302004-095128", "creators": [ { "name": { "family": "Statler", "given": "Irving Carl" }, "id": "Statler-Irving-Carl", "display_name": "Statler, Irving Carl" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1EHQ-B547", "abstract": "A first order in frequency theory is developed for the aerodynamic loads on a harmonically oscillating thin wing of finite aspect ratio in a subsonic compressible flow. The downwash in the vicinity of a horizontal tail behind such a wing is also evaluated to the same order in frequency. The results are then used to determine the stability derivatives of a conventional-type airplane and to set up the stick-free longitudinal equations of motion including the unsteady flow effects.\r\n\r\nAn important conclusion of this study is that, within the limitations of a \"lifting-strip\" theory, the airloads on the oscillating finite span wing are linear in frequency in the neighborhood of zero frequency. This is in contrast with the two-dimensional results which show a logarithmic singularity there.\r\n\r\nAs an example of a practical application, calculations are made of the frequency, damping and transient responses of the stick-free longitudinal motion of an F-80A airplane and the results compared with those obtained using quasisteady aerodynamic coefficients. The indications are that, while nonsteady flow considerations show considerable influence upon the control surface motion, they have a negligibly small effect upon the airplane motion." }, { "name": "Berndt, Sune Bertil", "degree": "PhD", "year": "1955", "title": "On the Influence of Wall Boundary Layers in Closed Transonic Test Sections", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-12052003-143311", "creators": [ { "name": { "family": "Berndt", "given": "Sune Bertil" }, "id": "Berndt-Sune-Bertil", "display_name": "Berndt, Sune Bertil" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T014-DR86", "abstract": "The boundary layers at the test section walls of a transonic wind tunnel are known to reduce the wall interference. In the present paper this effect is studied by means of small perturbation theory, assuming viscosity to be negligible when perturbing a turbulent boundary layer. An approximation for thin boundary layers leads to a modified boundary condition at the wall of the test section, expressing the normal streamline slope induced by changes in mass flow density and crossflow within the boundary layer. This boundary condition is applied to the linearized equations of subsonic flow and to the non-linear transonic equations at choking, the cases of plane and circular test sections only being treated in detail.\r\n\r\nThe results of linear theory show that all corrections except the three-dimensional angle-of-attack correction are considerably reduced by the presence of the boundary layers at Mach numbers greater than 0.9, the essential part of their influence being due to the change of mass flow density with pressure. In the case of choking the analysis indicates that the presence of boundary layers will increase the maximum model size for which the flow can be interpreted as corresponding to Mach number one in free flight. Finally, the technique of using artificial thickening of the wall boundary layers for reduction of wall interference is considered, though without reaching a definite conclusion as to its value compared to other techniques." }, { "name": "Burggraf, Odus Roy", "degree": "PhD", "year": "1955", "title": "A Theory of Stall Propagation in Axial Compressors on the Basis of Airfoil Characteristics", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11242003-103851", "creators": [ { "name": { "family": "Burggraf", "given": "Odus Roy" }, "id": "Burggraf-Odus-Roy", "display_name": "Burggraf, Odus Roy" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HQV0-7S12", "abstract": "The process of stall propagation in an axial flow compressor is represented by nonlinear airfoil lift and drag characteristics, with a time lag associated with the stalling mechanism. A pair of nonlinear integro-differential equations express the lift and drag as a function of time for a given airfoil in an isolated plane cascade representing an annulus with only a finite number of blades. Approximate solutions of these integro-differential equations are obtained by considering only the fundamental frequency in the Fourier series representing the blade loadings. Qualitative results are obtained for three cases: (a) only blade circulation is considered to be of importance in the mechanism of propagating stall, (b) blade drag is of predominant importance, and (c) combined effects of lift and drag are considered. Comparisons are made of the propagating speeds calculated for a finite number of blades with the values obtained by the approximation of an infinite number of blades. The magnitudes of the fluctuations in lift and drag are calculated as well as limiting angles of attack for which stall propagation can occur." }, { "name": "Chase, Robert Apperson", "degree": "PhD", "year": "1955", "title": "Pressurized Fuselage Stress Analysis", "advisor": "Williams, Max L.; Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-12022003-161233", "creators": [ { "name": { "family": "Chase", "given": "Robert Apperson" }, "id": "Chase-Robert-Apperson", "display_name": "Chase, Robert Apperson" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZCGR-N460", "abstract": "Theoretical methods for obtaining the complete stress analysis of a pressurized, stiffened circular cylinder of special geometry are presented. In certain limiting cases, the exact solutions are tractable, but in general the solutions lead to approximate results. There are practical cases for which none of the solutions is applicable. Accompanying the theoretical analysis is a short experimental program consisting of the strain gage instrumentation and testing of a Lucite and cellulose acetate model of typical aircraft structural geometry. The results compare favorably with the theoretical analysis." }, { "name": "Lal, Shankar", "degree": "PhD", "year": "1955", "title": "Heat Transfer in Compressible Laminar Boundary-Layers", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042003-095744", "creators": [ { "name": { "family": "Lal", "given": "Shankar" }, "id": "Lal-Shankar", "display_name": "Lal, Shankar" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BRKX-WV81", "abstract": "This report is concerned with the investigation of skin-friction and heat-transfer in the two-dimensional flow of a viscous compressible fluid.
\r\n\r\nThe boundary-layer equations are first transformed by the Howarth-Stewartson transformation and then it is shown that for fluids of Prandtl Number unity, if the Chapman viscosity law be assumed to hold, then any boundary-layer problem with the free stream Mach Number different from zero can be formally reduced to a problem for which the free stream Mach Number is equal to zero.
\r\n\r\nThe momentum method is then used to solve the boundary-layer equations in the Howarth-Stewartson form, for the case when the free stream Mach Number is zero. The basic equations developed are first used to solve the case of those specific pressure gradients which lead to \"similarity flows\". Other investigators have solved the exact equation for these flows on the differential analyser. The results obtained in this report, with the aid of very simple methods, agree to within a few percent with these more exact but laborious computations.
\r\n\r\nThe use of the method for the case of arbitrary pressure gradients is then developed. Three ways of solving the resulting equations are discussed. In particular, an integral solution for the square of the momentum thickness, analogous to the one existing for incompressible fluids but with different exponents, is given. The application of the method is demonstrated by solving an illustrative example.
" }, { "name": "Magnus, Richard Jeffrey", "degree": "PhD", "year": "1955", "title": "An Experimental Investigation of the Transfer of Heat from Small Wires to a Viscous Compressible Fluid", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042003-100200", "creators": [ { "name": { "family": "Magnus", "given": "Richard Jeffrey" }, "id": "Magnus-Richard-Jeffrey", "display_name": "Magnus, Richard Jeffrey" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4ZYH-PF56", "abstract": "A steam tunnel, suitable for making experimental measurements of the heat transfer from fine wires to a viscous compressible fluid, was developed and constructed.\r\n\r\nMeasurements of Nusselt numbers and recovery temperatures were carried out using small-diameter (0.00038 to 0.00254 cm.) tungsten wires in steam flow with Reynolds numbers ranging from about 1 to 12 and with nominal Mach numbers of 0.5 to 1.7.\r\n\r\nConsiderable difference was found between the Nusselt numbers for wires in subsonic and supersonic flow at corresponding Reynolds numbers. The results could be fairly well represented by an available theory based on the assumption that a temperature discontinuity of the fluid existed at the wire surface; however, they did not agree very well with other available data in the same range of Mach and Reynolds numbers.\r\n\r\nIn supersonic flow, the wire recovery temperatures were found to be consistently higher than the tunnel stagnation temperature.\r\n\r\nAn experimental procedure for making end-loss corrections to the heat transfer and temperature recovery tests was used and found to give satisfactory correlation of data taken with wires ranging in aspect ratio from 220 to 3040. Some experiments were performed to check the validity of the simple linear theory which is usually used to calculate end loss corrections; the theory was found to be adequate in the experimental range covered." }, { "name": "Ormsbee, Allen Ives", "degree": "PhD", "year": "1955", "title": "Conical Techniques for Incompressible Non-Viscous Flow", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-12052003-093114", "creators": [ { "name": { "family": "Ormsbee", "given": "Allen Ives" }, "id": "Ormsbee-Allen-Ives", "display_name": "Ormsbee, Allen Ives" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/77MG-6Z64", "abstract": "An analytical investigation was made of incompressible potential flow fields in which the velocity components are homogeneous of order zero. Superpositions of such fields were then made and expressions were derived for the flow fields associated with constant-strength source and vortex sheets of finite extent.\r\n\r\nThe constant strength source-sheets were then applied to the construction of aerodynamic models of thin non-lifting wings of polygonal platform and airfoil section.\r\n\r\nBy use of the constant strength vortex-sheets, several approximate aerodynamic models were constructed for the determination of the pressure distribution on thin lifting wings at small angles of attack." }, { "name": "Skinner, George Tolmie", "degree": "PhD", "year": "1955", "title": "A Time Correlator for Problems in Aerodynamics", "advisor": "Liepmann, Hans Wolfgang; Lagerstrom, Paco A.; Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-12122003-092906", "creators": [ { "name": { "family": "Skinner", "given": "George Tolmie" }, "id": "Skinner-George-Tolmie", "display_name": "Skinner, George Tolmie" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6B8H-3139", "abstract": "An instrument, of fairly simple design, for measuring time correlation functions of two stationary random electrical signals is discussed. It is intended primarily for use in problems connected with aerodynamically produced acoustic fields, but has suitable properties for application to a rather wide range of aerodynamic problems involving turbulent fields. It has been designed and constructed with a view to economy, and simplicity of operation, and makes extensive use of the general statistical properties of the problems for which it is intended.\r\n\r\nA few experimentally determined auto-correlation functions are given in order to indicate the degree of accuracy achieved, and the Fourier transform of the auto-correlation function of a random input is compared with the power-spectrum of the same function.\r\n\r\nSome practical points of general interest are discussed." }, { "name": "Yang, Hsun-Tiao", "degree": "PhD", "year": "1955", "title": "Rayleigh's Problem at Lo\\w Mach Number According to the Kinetic Theory of Gases", "advisor": "Lees, Lester", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142004-105335", "creators": [ { "name": { "family": "Yang", "given": "Hsun-Tiao" }, "id": "Yang-Hsun-Tiao", "display_name": "Yang, Hsun-Tiao" } ], "advisors": [ { "name": { "family": "Lees", "given": "Lester" }, "id": "Lees-L", "role": "advisor", "display_name": "Lees, Lester" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2VK2-FC14", "abstract": "Rayleigh's problem of an infinite flat plate set into uniform motion impulsively in its own plane is studied by using Grad's equations and boundary conditions developed from the kinetic theory of gases. For a heat insulated plate and a small impulsive velocity (low Mach number), only tangential shear stress and velocity and energy (heat) flow parallel to the plate are generated, while the pressure, density, and temperature of the gas remain unchanged. Moreover, no normal velocity, normal stress, or normal energy flow is developed. Near the start of the motion the flow behaves like a \"free-molecule flow\", and all physical quantities are analytic functions of the flow parameters and time. The results obtained for \"large time\", however, add to the growing lack of confidence in the Burnett-type series expansions in powers of mean free path. Although such expansions are obtained here, they are poorly convergent and inappropriate to the problem. To replace these unsatisfactory solutions, approximate closed-form solutions valid for all values of the time are developed, which agree with the free-molecule values for small time and the classical Rayleigh solution for large time. This technique may be useful in studying more general flow problems within the framework of the kinetic theory of gases." }, { "name": "Adamson, Thomas Charles, Jr.", "degree": "PhD", "year": "1954", "title": "Ignition and Combustion in a Laminar Mixing Zone", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-11202003-135437", "creators": [ { "name": { "family": "Adamson", "given": "Thomas Charles, Jr." }, "id": "Adamson-Thomas-Charles", "display_name": "Adamson, Thomas Charles, Jr." } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "chair", "display_name": "Marble, Frank E." }, { "name": { "family": "Bohnenblust", "given": "Henri Frederic" }, "id": "Bohnenblust-H-F", "role": "member", "display_name": "Bohnenblust, Henri Frederic" }, { "name": { "family": "Felberg", "given": "Fredrick H." }, "id": "Felberg-Fredrick-H", "role": "member", "display_name": "Felberg, Fredrick H." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Penner", "given": "Stanford S." }, "id": "Penner-S-S", "role": "member", "display_name": "Penner, Stanford S." }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "member", "display_name": "Tsien, Hsue Shen" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q9A7-T088", "abstract": "The equations describing combustion in a flow field are modified for use in laminar flows where the so called boundary layer approximations may be employed. These equations are transformed into a corresponding incompressible flow with the Howarth transformation.
\r\n\r\nAs an example of the use of boundary layer concepts this analysis considers the ignition and combustion in the laminar mixing zone between two parallel moving gas streams. One stream consists of a cool combustible mixture, the second is hot combustion products. The two streams come into contact at a given point and a laminar mixing process follows in which the velocity distribution is modified by viscosity, and the temperature and composition distributions by conduction, diffusion and chemical reaction. The decomposition of the combustible stream is assumed to follow first-order reaction kinetics with temperature dependence according to the Arrhenius law. For a given initial velocity, composition, and temperature distribution, the questions to be answered are: (1) Does the combustible material ignite and (2) How far downstream of the initial contact point does the flame appear and what is the detailed process of development?
\r\n\r\nSince the hot stream is of infinite extent it is found that ignition always takes place at some point of the stream. However when the temperature of the hot stream drops below a certain value, the distance required for ignition increases so enormously that it essentially does not occur in a physical apparatus of finite dimension. The complete development of the laminar flame front is computed using an approximation similar to the von Karman integral technique in boundary layer theory.
\r\n" }, { "name": "Berger, Howard Martin", "degree": "PhD", "year": "1954", "title": "A New Approach to the Analysis of Large Deflections of Plates", "advisor": "Williams, Max L.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042003-163054", "creators": [ { "name": { "family": "Berger", "given": "Howard Martin" }, "id": "Berger-Howard-Martin", "display_name": "Berger, Howard Martin" } ], "advisors": [ { "name": { "family": "Williams", "given": "Max L." }, "id": "Williams-M-L", "role": "advisor", "display_name": "Williams, Max L." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FA0E-1X79", "abstract": "As a result of the assumption that the strain energy due to the second invariant of the middle surface strains can be neglected when deriving the differential equations for a flat plate with large deflections, simplified equations are derived that can be solved readily. Computations using the solution of these simplified equations are carried out for the deflection of uniformly loaded circular and rectangular plates with various boundary conditions. Comparisons are made with available numerical solutions of the exact equations. The deflections found by this approach are then used to obtain the stresses, and the resulting stresses are compared with existing solutions. In all the cases where comparisons could be made, the deflections and stresses agree with the exact solutions within the accuracy required for engineering purposes." }, { "name": "Dailey, Charles Lee", "degree": "PhD", "year": "1954", "title": "Supersonic Diffuser Instability", "advisor": "Liepmann, Hans Wolfgang; Stewart, Homer Joseph; Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-12102003-101604", "creators": [ { "name": { "family": "Dailey", "given": "Charles Lee" }, "id": "Dailey-Charles-Lee", "display_name": "Dailey, Charles Lee" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/V9XM-W683", "abstract": "Steady operation of supersonic diffusers near critical mass flow is interrupted by a transient process known as buzz. This phenomenon consists of a random sequence of individual relaxation cycles. Mass flow entering the diffuser during steady operation is suddenly cut off by a strong interaction between the subcritical shock and boundary layer on the surface of the external compression generator, which blocks the inlet. Air in the plenum chamber, stored at high pressure, then ?blows down? until the inlet can restart. The subsequent supercritical flow entering the diffuser exceeds the flow rate at the exit and the plenum chamber is re-charged to the original condition.\r\n\r\nA distinction is drawn between this phenomenon and a high frequency wave-type resonance noticed at low mass flows and during an individual buzz cycle after the diffuser shock system has been expelled. For the large diffuser tested here, this high frequency oscillation compares well to the 8th closed-end organ pipe mode of the diffuser at low mass flows and to the 9th mode during the shock-expelled phase of the buzz cycle.\r\n\r\nIt is shown that burning almost always ceases in the presence of buzz. When burning was maintained during buzz, it was found to have no qualitative effect on the buzz cycle." }, { "name": "Diederich, Franklin Wolfgang", "degree": "PhD", "year": "1954", "title": "The Response of an Airplane to Random Atmospheric Disturbances", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-12102003-105523", "creators": [ { "name": { "family": "Diederich", "given": "Franklin Wolfgang" }, "id": "Diederich-Franklin-Wolfgang", "display_name": "Diederich, Franklin Wolfgang" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XPKW-QH24", "abstract": "The statistical approach to the gust-loads problem is extended by considering the aerodynamic forces due to the lateral and longitudinal variation of instantaneous gust intensity and using them in dynamic analyses of rigid and flexible airplanes free to move vertically, in pitch, and in roll, based on the assumptions of stationarity of the process (in the statistical sense) and of linearity of the forces involved. The effect on the wing stresses of the interaction of longitudinal, vertical and lateral gusts is considered.\r\n\r\nThe method of analyzing the rigid-body motions is similar to that used for analyses of the dynamic stability of airplanes, in that the equations of motion are referred to stability axes and expressed in terms of conventional stability derivatives. The method of analyzing the dynamic effects of structural flexibility consists of an extension of a numerical-integration approach to the static aeroelastic problem and is in a form which offers the possibility of calculating divergence and flutter speeds with relatively little additional effort.\r\n\r\nThe mean-square values, correlation functions and power spectra of some of the aerodynamic forces required in this type of analysis are calculated for certain special correlation functions of the atmospheric turbulence and certain special lift distributions. It is shown, for instance, that the mean-square lift is substantially reduced due to the difference in instantaneous intensity of the turbulent velocity along the span if the span is relatively large compared to the integral scale of turbulence, but that the mean-square pitching moment is substantially increased if the tail length is relatively large. Also, the wing stresses due to vertical, horizontal and side gusts are shown to be statistically independent under certain conditions." }, { "name": "Hakkinen, Raimo Jaakko", "degree": "PhD", "year": "1954", "title": "Measurements of Skin Friction in Turbulent Boundary Layers at Transonic Speeds", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12152003-084052", "creators": [ { "name": { "family": "Hakkinen", "given": "Raimo Jaakko" }, "id": "Hakkinen-Raimo Jaakko", "display_name": "Hakkinen, Raimo Jaakko" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WAX3-QV41", "abstract": "The first part of this report describes the design and construction of a floating element skin friction balance. This instrument, which is essentially an improved version of Dhawan's balance, was applied to measurements of local skin friction in the turbulent boundary layer of a smooth flat plate at high subsonic Mach numbers and supersonic Mach numbers up to M = 1.75. The measured skin friction coefficients are consistent with the results of other investigations at subsonic and also at high supersonic speeds. The principal difficulties which exist in comparing skin friction coefficients at various Mach numbers are discussed.\r\n\r\nThe second part of this report describes the application of the Stanton tube technique to skin friction measurements near the base of a shock wave impinging upon a turbulent boundary layer. The floating element technique is inherently difficult to apply for skin friction measurements in non-uniform flow. Hence, a Stanton tube is calibrated by means of a floating element balance in a uniform flow field and then used to measure skin friction near the base of an impinging shock. Oblique shock waves were produced by two wedges of 2.5[degree] and 4.6[degree] semi-angles and a normal shock was generated by a choked channel. Skin friction and velocity profiles were obtained for these three cases at a free stream Mach number of 1.48." }, { "name": "Kaplan, Abner", "degree": "PhD", "year": "1954", "title": "Finite Deflections and Buckling of Slightly Curved Beams and Shallow Spherical Shells under Lateral Loads", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-12032003-110343", "creators": [ { "name": { "family": "Kaplan", "given": "Abner" }, "id": "Kaplan-Abner", "display_name": "Kaplan, Abner" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Q1HV-T474", "abstract": "This research consists in the experimental and theoretical investigation of the finite deflection and buckling of two similar structures; the low arch or slightly curved beam and the shallow spherical dome, both subjected to lateral loads. These structures are of interest because the large interaction between bending and axial forces causes their load-deflection behavior to become nonlinear at very low values of the deflection. Due to the wide difference in the methods of solution of these two problems they are separated into two parts, each having its own abstract.
\r\n\r\n[Part I]
\r\n\r\nWhen a low arch (a thin curved beam of small curvature) is subjected to a lateral loading acting toward the center of curvature, the axial thrust induced by the bending of the arch may cause the arch to buckle so that the curvature becomes suddenly reversed. The critical lateral loading depends on the dimensions and rigidity of the arch, the elasticity of the the end fixation, the type of load distribution, and the initial curvature of the arch. A general solution of the problem is given in this paper, using the classical buckling criterion which is based on the stability with respect to infinitesimal displacements about the equilibrium positions.
\r\n\r\nFor a sinusoidal arch under sinusoidal loading, the critical load can be expressed exactly as a simple function of the beam dimension parameters. For other arch shapes and load distributions, approximate values of the critical load can be obtained by summing a few terms of a rapidly converging Fourier series. The effect of initial end thrust and axial and lateral elastic support are discussed.
\r\n\r\nThe buckling load based on the energy criterion of Karman and Tsien is also calculated. The results for both the classical and the energy criteria are compared with experiments made on a series of centrally loaded, pin-ended arches. For larger values of a dimensionless parameter [gamma subscript l], which is proportional to the ratio of the arch rise to the arch thickness, the experimental critical buckling loads agreed quite well with the classical criterion, but, for smaller values of [gamma subscript l], the experimental critical loads were appreciably below those calculated from the classical criterion, although they were always above those obtained from the energy criterion.
\r\n\r\n[Part II] The shallow spherical dome subjected to lateral pressure is a structure for which the deformation departs appreciably from the linear theory at very small values of the deflection amplitude. It is also. one for which the buckling process is characterized by a rapid decrease in the equilibrium load once the buckling load has been surpassed. For structures having this type of buckling characteristic, the question arises as to whether the proper buckling criterion to apply is the classical criterion, which considers equilibrium with respect to infinitesimal displacements or the finite displacement \"energy criterion\" proposed by Tsien.
\r\n\r\nIn this paper the problem of the finite displacement and buckling of a shallow spherical dome is investigated both theoretically and experimentally. In the theoretical approach the nonlinear equations are converted into a sequence of linear equations by expanding all of the variables in powers of the center deflection and then equating the coefficients of equal powers. The basic parameter for the shallow dome is proportional to the ratio of the central height of the dome, h, to its thickness, t. For small values of this ratio the expansions converge rapidly and enough terms are computed to determine the buckling load. For higher values of h/t, convergence deteriorates rapidly and the buckling, load is not computed. However, even for these higher values of h/t the deflection shapes are determined for deflection amplitudes below the amplitude at which buckling occurs. These deflection shapes are characterized by their rapid change as h/t increases and by the fact that, over most of the range of h/t studied, the maximum deflection does not occur at the center of the dome.
\r\n\r\nThe experimental program was carried out on a series of clamped-edge, eight inch base diameter shells, subjected to uniform pressure. The deflection shapes and the buckling loads agreed quite well with the values computed theoretically. It was also found that there was no significant difference between the buckling loads observed using air pressure and those observed using oil pressure. Thus it is concluded that for the shallow domes studied the classical buckling criterion holds rather than the \"energy criterion\" proposed by Tsien.
" }, { "name": "Kaplun, Saul", "degree": "PhD", "year": "1954", "title": "The Role of Coordinate Systems in Boundary Layer Theory", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12032003-111930", "creators": [ { "name": { "family": "Kaplun", "given": "Saul" }, "id": "Kaplun-Saul", "display_name": "Kaplun, Saul" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9QZM-8W36", "abstract": "The boundary layer approximation to a given flow problem is not invariant if different coordinate systems are used in the approximation process. However, a correlation theorem (Theorem 1) is given, which states that the boundary layer solution with respect to any given system can be found, by a simple substitution, from that with respect to any other system. On the basis of this theorem, the dependence of the solution on the choice of coordinates is investigated in detail. The skin friction is invariant, but the flow field is not invariant. At large distances from the wall, the flow field given by boundary layer theory depends almost entirely on the choice of coordinates, rather than on the physical problem.\r\n\r\nThis dependence may be used to obtain a complete matching between the boundary layer solution and the external flow, in the following sense: Theorem 2 states how a coordinate system can be found such that the boundary layer solution with respect to this system is valid as an approximation for the entire flow field. It contains the external flow and the flow due to displacement thickness.\r\n\r\nThe discussion is restricted to steady, two-dimensional, incompressible flow without separation. These restrictions, however, are not essential for many of the results." }, { "name": "Knuth, Eldon Luverne", "degree": "PhD", "year": "1954", "title": "The Mechanics of Film Cooling", "advisor": "Rannie, W. Duncan", "url": "https://resolver.caltech.edu/CaltechETD:etd-12032003-112656", "creators": [ { "name": { "family": "Knuth", "given": "Eldon Luverne" }, "id": "Knuth-Eldon-Luverne", "display_name": "Knuth, Eldon Luverne" } ], "advisors": [ { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/G4AT-5P75", "abstract": "Thin liquid wall films flowing under the influence of high-velocity turbulent gas streams were studied for the purpose of obtaining an understanding of the mechanics of film cooling. Conditions which insure liquid-film attachment to solid surfaces without loss of unevaporated liquid to the gas stream when simple radial-hole injectors are used were found; the maximum allowable coolant-flow rate for a stable coolant film was determined (a stable coolant film is obtained when no unevaporated coolant is entrained by the gas stream as the result of interfacial disturbances); and a method for calculating the evaporation rate and the surface temperature for a stable inert coolant film was found." }, { "name": "Korkegi, Robert Hani", "degree": "PhD", "year": "1954", "title": "Transition Studies and Skin Friction Measurements on an Insulated Flat Plate at a Hypersonic Mach Number", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01072004-114554", "creators": [ { "name": { "family": "Korkegi", "given": "Robert Hani" }, "id": "Korkegi-Robert-Hani", "display_name": "Korkegi, Robert Hani" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/X3B9-VW73", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nAn investigation of transition and skin friction on an insulated flat plate, 5 x 26 inches, was made in the GALCIT 5 x 5 inch Hypersonic Wind Tunnel, Leg No. 1, at a nominal Mach number of 5.8.\r\n\r\nThe phosphorescent lacquer technique was used for transition detection and was found to be in good agreement with total-head rake measurements along the plate surface and pitot boundary layer surveys. It was found that the boundary layer was laminar at Reynolds numbers of at least 5 x 10[superscript 6]. It was also observed that transverse contamination due to the turbulent boundary layer on the tunnel sidewall originated far downstream of the flat plate leading edge at Reynolds numbers of 1.5 to 2 x 10[superscript 6], and spread at a uniform angle of 5 1/2[degrees] compared with 9 1/2[degrees] in low speed flow.\r\n\r\nThe effect of two-dimensional and local disturbances was investigated. The technique of air injection into the boundary layer as a means of stimulating transition was extensively used. It was observed that, although the onset of transition occurred at Reynolds numbers down to 10[superscript 6], a fully developed turbulent boundary layer was not obtained at Reynolds numbers much below 2 x 10[superscript 6] regardless of the amount of air injected.\r\n\r\nA qualitative discussion of these results is given with emphasis on the possibility of a greater stability of the laminar boundary layer in hypersonic flow than at lower speeds.\r\n\r\nDirect skin friction measurements were made by means of the floating element technique incorporating a null system using chain loading, over a range of Reynolds numbers (based on distance from leading edge) from 10[superscript 6] to 4 x 10[superscript 6]. Without artificial tripping, the boundary layer was verified as being laminar over the complete range. With air injection, turbulent shear was obtained only for Reynolds numbers greater than 2 x 10[superscript 6] , this value being in good agreement with earlier results of this investigation. The turbulent skin friction coefficient was found to be approximately 0.40 of that for incompressible flow for a constant value of R[subscript theta], and 0.46 for an effective Reynolds number between 5 and 6 x 10[superscript 6]." }, { "name": "Lampert, Seymour", "degree": "PhD", "year": "1954", "title": "Homogeneous Flow Fields of Degree Greater than Zero", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-01092004-094836", "creators": [ { "name": { "family": "Lampert", "given": "Seymour" }, "id": "Lampert-Seymour", "display_name": "Lampert, Seymour" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/W215-QX89", "abstract": "Solutions to the Prandtl-Glauert differential equation expressed in terms of polynomial type Lame functions can be applied to the problem of the thin delta wings with subsonic leading edges in a supersonic flow field. It is demonstrated how these functions of different species and degrees of homogeneity may be employed to obtain previously known results for certain lifting cases. For the non-lifting or thickness case which is treated in detail in this paper it is shown that a large class of thickness distributions with blunt leading edges my be obtained by systematically studying the Lame functions of the first species. In particular these functions have been investigated up to, and including, n = 5. It is further shown by the methods of this paper that the prescription of the pressure distribution in problems of this sort is not always sufficient to determine the thickness distribution uniquely." }, { "name": "Mark, Richard Muin", "degree": "PhD", "year": "1954", "title": "Laminar Boundary Layers on Slender Bodies of Revolution in Axial Flow", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142004-155216", "creators": [ { "name": { "family": "Mark", "given": "Richard Muin" }, "id": "Mark-Richard-Muin", "display_name": "Mark, Richard Muin" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4H6Z-K569", "abstract": "An exact similar solution of the modified boundary layer equations has been obtained for the axial incompressible flow past paraboloids of revolution. It has been shown that the usual boundary layer assumptions are justified and that the local skin friction increases as the boundary layer thickness becomes large compared with the body radius.\r\n\r\nAn approximate method for obtaining the local skin friction on arbitrary slender bodies of revolution in axial incompressible flow has been developed. A comparison of the approximate results with the exact solutions for paraboloids of revolution and circular cylinders shows good agreement.\r\n\r\nThe existence of energy integrals of the modified compressible boundary layer equations is established. Similarity of the governing equations for the axial compressible flow past paraboloids of revolution has been shown; for the same bodies, a hypersonic similarity law is deduced.\r\n\r\nAn approximate method for obtaining the local skin friction on arbitrary slender insulated bodies of revolution in axial compressible flow has been developed. The results show that compressibility counterbalances the rise in local skin friction due to curvature at high Reynolds numbers (based on a characteristic length of the body) and increases the local skin friction at sufficiently low Reynolds numbers.\r\n\r\nVelocity profiles on a slender ogive-cylinder have been obtained experimentally at a Mach number of 5.8 and at different Reynolds numbers. The results indicate a curvature effect when compared with flat plate results." }, { "name": "Marschner, Bernard Walter", "degree": "PhD", "year": "1954", "title": "The Flow over a Body in a Choked Wind Tunnel and in a Sonic Free-Jet", "advisor": "Guderley, Karl Gottfried", "url": "https://resolver.caltech.edu/CaltechETD:etd-12042003-100833", "creators": [ { "name": { "family": "Marschner", "given": "Bernard Walter" }, "id": "Marschner-Bernard-Walter", "display_name": "Marschner, Bernard Walter" } ], "advisors": [ { "name": { "family": "Guderley", "given": "Karl Gottfried" }, "id": "Guderley-Karl-Gottfried", "role": "advisor", "display_name": "Guderley, Karl Gottfried" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/P4DG-RV54", "abstract": "The pressure distribution over a double wedge airfoil under free flight conditions with Mach number one is compared with the pressure distribution over the same airfoil in a choked closed wind tunnel and in a sonic free-jet.\r\n\r\nThe computation is carried out as a development with respect to a parameter which indicates the deviation from free flight conditions with Mach number one. The results are of interest for the question of wind tunnel wall influences. It is found that the deviatiors of the pressure distribution for a sonic free-jet from the distribution in an infinite air flow are somewhat larger than the deviations in a closed wind tunnel under choked flow conditions. For a specific exam of a wedge of a length of 13% of the tunnel height and a thickness ratio of 10%, the deviation of the pressure distribution does not go much beyond the usual experimental scatter.\r\n\r\nThe results are quite encouraging for the application of closed throat wind tunnels in transonic testing although the axial symmetric case may not show entirely the same desirable behavior." }, { "name": "Valluri, Sitaram Rao", "degree": "PhD", "year": "1954", "title": "Some Studies in the Fundamental Parameters of Fatigue", "advisor": "Sechler, Ernest Edwin; Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-01292004-104442", "creators": [ { "name": { "family": "Valluri", "given": "Sitaram Rao" }, "id": "Valluri-Sitaram-Rao", "display_name": "Valluri, Sitaram Rao" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/5Y4Z-CW45", "abstract": "A torsional fatigue testing machine of the resonant vibrator type has been designed in order to investigate the fatigue and internal friction properties of 3S-O aluminum. This machine uses an elastic restraint on an a-c motor to create resonant conditions and uses a photocell electronic system for the measurement of internal friction by the method of measuring the logarithmic decrement of free oscillations.\r\n\r\nIt has been established that for 3S-O aluminum at torsion stress levels below 112 psi, the stress history does not affect damping and that, for an annealed specimen, the material exhibits a maximum value of internal friction at 375[degrees]F. \"Temporary mobility\" aspects of the slip bands have been investigated and in addition, variation of internal friction as a function of stress amplitude in repeated torsional loading, temperature, and number of reversals has been obtained. At room temperature (75[degrees]F) the internal friction increases with the number of stress reversals in the 0 to 10[superscript 5] range. This increment in general increases with increasing amplitude of stress. At test temperatures of 225[degrees] and 525[degrees]F it was found that this increment does not show any regular variation with stress. In addition, however, it was found that at 375[degrees]F the internal friction decreases with stress reversals in the 0 to 10[superscript 5] range for all stresses. The variation of internal friction with stress history after a large number of stress reversals of the order 10[superscript 6] cycles, is quite complex, giving rise to random patterns of increase and decrease.\r\n\r\nThis work is exploratory in nature and suggestions for further work are indicated." }, { "name": "Vrebalovich, Thomas", "degree": "PhD", "year": "1954", "title": "The Development of Direct and Alternating Current Glow Discharge Anemometers for the Study of Turbulence Phenomena in Supersonic Flow", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-01222004-111758", "creators": [ { "name": { "family": "Vrebalovich", "given": "Thomas" }, "id": "Vrebalovich-Thomas", "display_name": "Vrebalovich, Thomas" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/95XT-RN93", "abstract": "A direct current glow discharge anemometer (D.C. glow) was designed and constructed. This instrument was calibrated in low speed flow. Shock tube experiments with the D.C. glow indicated that its frequency response was greater than 50 kc. However, the shortcomings of the D.C. glow such as sputtering and asymmetric burning properties of the discharge became apparent. Therefore, a 700 kc. alternating current (A.C.) glow was designed and constructed. The time stability properties of this instrument were found to be much better than those of the D.C. glow. Since no frequency compensation circuits were used with the A.C. glow, the signal to noise ratio was much higher than that of a hot wire. This A.C. glow was used to survey the profile of the fluctuations in a turbulent boundary layer in supersonic flow at Mach numbers between 1.3 and 4. Power spectrum measurements of the fluctuations in this boundary layer were also made with the A.C. glow. These measurements indicated that there was energy in the spectrum above 100 kc. Finally, measurements of frequencies in excess of 100 kc. were made by the A.C. glow in a sound field produced by a source of single frequency ultrasonic sound waves in supersonic flow." }, { "name": "Willmarth, William Walter", "degree": "PhD", "year": "1954", "title": "The Lift of Thin Airfoils at High-Subsonic Speeds", "advisor": "Liepmann, Hans Wolfgang; Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01222004-114509", "creators": [ { "name": { "family": "Willmarth", "given": "William Walter" }, "id": "Willmarth-William-Walter", "display_name": "Willmarth, William Walter" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DFFC-8041", "abstract": "Experimental results are presented for the lift characteristics of thin, two-dimensional airfoils at high-subsonic speeds and small angles of attack. Symmetrical airfoils with different locations of maximum thickness were investigated using a surface pressure probe technique which should find use in other applications.\r\n\r\nThe flow fields over each airfoil are discussed and the quantitative results for the lift and location of the center of lift are compared with theory whenever possible. The effects of flow separation caused by boundary-layer shock-wave interaction are noted and discussed. In particular, the possibility of the forced oscillation of control surfaces due to boundary layer separation is mentioned." }, { "name": "Yoler, Yusuf Amon", "degree": "PhD", "year": "1954", "title": "The Hypersonic Shock Tube", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02112004-115540", "creators": [ { "name": { "family": "Yoler", "given": "Yusuf Amon" }, "id": "Yoler-Yusuf-Amon", "display_name": "Yoler, Yusuf Amon" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/853G-K555", "abstract": "The feasibility of using a shock tube for quantitative investigations of hypersonic flow phenomena at temperatures simulating free flight conditions is studied theoretically and experimentally. In the theoretical part, various aspects of the hypersonic shock tube problem are treated in logical order. Methods of producing high Mach numbers, limitations on the test section Mach number, methods of generating strong shock waves, flows with variable specific heats and dissociation, types of problems amenable to study with the hypersonic shock tube are discussed.\r\n\r\nTo verify and supplement some of the theoretical results, a shock tube of a somewhat unconventional design has been built. The bulk of the experimental investigations undertaken to date have dealt with pressure studies using piezoelectric gages, and schlieren studies of the flow, The results obtained so far with flow Mach numbers in excess of six, stagnation temperatures up to 9000[degrees]R and stagnation pressures up to 200 psi, have not only contributed to a much greater understanding of this relatively new field of application of the shock tube, but have indicated a well defined course along Which future investigations will continue." }, { "name": "Zukoski, Edward Edom", "degree": "PhD", "year": "1954", "title": "Flame Stabilization on Bluff Bodies at Low and Intermediate Reynolds Numbers", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01142004-105717", "creators": [ { "name": { "family": "Zukoski", "given": "Edward Edom" }, "id": "Zukoski-Edward-Edom", "display_name": "Zukoski, Edward Edom" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E9V0-GM76", "abstract": "The problem of attaining stable combustion in ramjet power plants and in afterburners of turbojet engines has led to rather extensive studies of the processes involved in stabilizing flames on bluff bodies. One of the results of these studies was to indicate that the flame stabilization mechanism changes significantly at low Reynolds Numbers; the velocity at which flames may be stabilized drops abruptly as the Reynolds Number is decreased, and the mixture ratio for which maximum velocity of flame holding occurs shifts markedly from the stoichiometric value.\r\n\r\nThis abrupt change in the mechanism of flame stabilization is investigated through photographic studies of the flame front near the flame holder. A transition from a laminar to a turbulent surface of the flame front immediately downstream from the flame holder is shown to account for the change in flame stabilization characteristics. This transition was found to occur independently of fuel type and flame holder geometry.\r\n\r\nThe behavior of the low Reynolds Number stabilization limits is attributed to a diffusion process; in particular, the shift found for the mixture ratio corresponding to the maximum blowoff velocity is explained on the basis of the difference in the diffusion rate of fuel and oxygen. Detailed experiments including blowoff results, chemical analysis of gas taken from the flame holder wake, and measurements of wake temperatures are shown to confirm the suggested diffusion mechanism.\r\n\r\nOnce the transition phenomenon is appreciated, re-examination of high Reynolds Number blowoff data is found possible. The results of previous experiments are found to show that the blowoff velocity depends on the square root of the characteristic dimension if the transition Reynolds Number is exceeded and if the flame holder is a bluff body of small fineness ratio." }, { "name": "Chuan, Raymond Lu-Po", "degree": "PhD", "year": "1953", "title": "The Development and Utilization of Some Equipment for Low Reynolds Number Supersonic Flow Research", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-04292003-103202", "creators": [ { "name": { "family": "Chuan", "given": "Raymond Lu-Po" }, "id": "Chuan-Raymond-Lu-Po", "display_name": "Chuan, Raymond Lu-Po" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/B6T3-QB91", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA low Reynolds number supersonic wind tunnel is a useful tool for the investigation of viscous effects in a high-speed flow. In order to avoid some of the difficulties inherent in a conventional wind tunnel system when operating at low Reynolds numbers, a two-phase cycle is proposed. By examining the thermodynamics of the wind tunnel the relative merits of two methods of operation -- the one-phase cycle and the two-phase cycle -- are compared, indicating certain advantages the two-phase cycle possesses over the conventional one-phase cycle when the tunnel is operated at high-speed and low Reynolds number. The design, construction and operation of a small (about 5 cm.[superscript 2] test section area) supersonic (M[.....]2) wind tunnel using a two-phase cycle with water (in liquid and vapor phases) as the working medium are described.\r\n\r\nDifficulties in pressure measurement and flow visualization, due to the nature of the working medium and the low density of the flow, are encountered. Means of meeting these difficulties are proposed, including a critical analysis of the schlieren technique, from which is evolved a workable arrangement for visualizing the low Reynolds number flow.\r\n\r\nAs an example of the possible utilizations of the tunnel, the curvature, due to viscosity, of the attached shock wave on a wedge in uniform supersonic flow is investigated, using schlieren photography. The effective shape of the wedge is deduced from the shape of the shock wave, and is compared to theory with satisfactory qualitative agreement." }, { "name": "Coles, Donald Earl", "degree": "PhD", "year": "1953", "title": "Measurements in the Boundary Layer on a Smooth Flat Plate in Supersonic Flow", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-05092003-180051", "creators": [ { "name": { "family": "Coles", "given": "Donald Earl" }, "id": "Coles-Donald-Earl", "display_name": "Coles, Donald Earl" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0DDW-9S38", "abstract": "In Section I available measurements in low-speed turbulent boundary layer flow are compared with a simple analysis based on functional similarity, and the boundary layer is found to be unique within the accuracy of the experimental data. Some consequences of the mean equations of motion are obtained, including the distribution of shearing stress through the boundary layer, and an attempt is made to generalize the relationship known as the law of the wall to flows with variable density.
\r\n\r\nIn Section II some problems encountered in the development and use of the floating surface element and other instrumentation are discussed in detail.
\r\n\r\nIn Section III are presented measurements of mean and local surface friction carried out on a flat plate model in the 20-inch supersonic wind tunnel at the Jet Propulsion Laboratory. The boundary layer flow is studied for free stream Mach numbers of 2.0, 2.6, 3.7, and 4.5. The experiments, which involve nominal Reynolds numbers from 2 x 105 to 9 x 106, include a few measurements in laminar flow, but emphasize transition and the turbulent regime. The effectiveness of various tripping devices is mentioned, and the problem of defining an effective Reynolds number for the fully turbulent flow is discussed at length. Finally, turbulent boundary layer profile measurements are examined for consistency with low-speed data, with a generalized mixing length theory, and with the analysis of Section I of the present report.
" }, { "name": "DeLauer, Richard Daniel", "degree": "PhD", "year": "1953", "title": "Experimental Heat Transfer at Hypersonic Mach Number", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04222003-111626", "creators": [ { "name": { "family": "DeLauer", "given": "Richard Daniel" }, "id": "DeLauer-Richard-Daniel", "display_name": "DeLauer, Richard Daniel" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/59SJ-TR03", "abstract": "An experimental investigation was conducted in Leg 1 of the GALCIT 5 x 5 inch Hypersonic Wind Tunnel to determine the heat transfer coefficients of the laminar boundary layer on a cooled flat plate at a nominal Mach number of 5.8. As a consequence of the investigation, flat plate recovery factors were determined and the effect of condensation on heat transfer was noted. In addition qualitative results as to the laminar boundary layer transition and separation are also presented.
\r\n\r\nThe tests were conducted with a ratio of wall temperature to free stream temperature (Tw/T\u03b4) of approximately 6.2; but under stagnation temperature conditions ranging from 200\u00b0F to 285\u00b0F. The stagnation pressure range of 60 psia to 115.5 psia provided a maximum Reynolds number of 2.1 x 106.
\r\n\r\nA flat plate temperature recovery factor of .858 \u00b1 .004 was determined, and it was concluded that the temperature recovery factor range of Mach number independence could be extended to a Mach number of 5.8. The independence of the recovery factor on Reynolds number up to the beginning of the laminar boundary layer transition was also substantiated.
\r\n\r\nThe heat transfer coefficients were obtained for a negative temperature gradient over a considerable portion of the plate. The effect of these gradients produced values considerably higher than would be expected for an isothermal surface. These results, when related the constant temperature case by a theoretical calculation, were in good agreement, with the theoretical results and the results of a friction investigation carried out at the same Mach number. The accuracy of the results was estimated to be \u00b110% from a value of Nu/Re1/2Pr1/3 - .285.
\r\n \r\nThere was no apparent effect on the heat transfer coefficient by condensation, but the adiabatic wall temperature appeared to be 2% lower than for the condensation free flow. Due to a step increase in thickness of the model at the ten inch station, the shock wave-boundary layer interaction appears to produce laminar boundary layer transition at a Reynolds number of 1.3 x 106, and upon reducing the Reynolds number further, the transition point is subjected to an adverse pressure gradient which results in a boundary layer separation.
" }, { "name": "Eimer, Manfred", "degree": "PhD", "year": "1953", "title": "Direct Measurement of Laminar Skin Friction at Hypersonic Speeds", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04292003-112020", "creators": [ { "name": { "family": "Eimer", "given": "Manfred" }, "id": "Eimer-Manfred", "display_name": "Eimer, Manfred" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "role": "member", "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MK2E-1P76", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA direct measurement of flat plate laminar local skin friction was undertaken at M = 5.8 in the GALCIT Hypersonic Wind Tunnel, Leg No. 1. A new balance particularly suited to the requirements of hypersonic experimentation was designed.\r\n\r\nBy means of the fluorescent lacquer technique for indicating boundary layer transition, unexpectedly high natural transition Reynolds numbers were observed. The several methods of forcing transition which were used were unsuccessful in producing transition at a Reynolds number of two million.\r\n\r\nObservation of the leading edge shock wave and boundary layer by means of a schlieren system indicated that at M = 5.8 the shock wave and boundary layer are separated by appreciable distances wherever the boundary layer equations hold.\r\n \r\nSkin friction force measurements were made at five Reynolds numbers in condensation-free flow. The discrepancies between the observed low values of C[subscript f][...]Re and existing laminar boundary layer calculations are reconciled by means of a qualitative analysis.\r\n\r\nA theory describing the properties of the viscous boundary layer for flows with condensation in the free stream is presented. It is predicted that for a specified nozzle geometry and stagnation temperature, skin friction remains unchanged in the presence of moderate amounts of condensation, while heat transfer for a given wall temperature is affected by the presence of condensation which produces major changes in the adiabatic wall temperature." }, { "name": "Holmquist, Carl Oreal", "degree": "PhD", "year": "1953", "title": "An Approximate Method of Calculating Three-Dimensional Compressible Flow in Axial Turbomachines", "advisor": "Rannie, W. Duncan; Tsien, Hsue Shen; Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04232003-115022", "creators": [ { "name": { "family": "Holmquist", "given": "Carl Oreal" }, "id": "Holmquist-Carl-Oreal", "display_name": "Holmquist, Carl Oreal" } ], "advisors": [ { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "advisor", "display_name": "Rannie, W. Duncan" }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3FB2-G132", "abstract": "The two principal existing methods of calculating axially-symmetric compressible flow in turbomachines are: (1) a simplified one-dimensional analysis, and (2) numerical methods using the complete or linearized flow equations. The first is not satisfactory for multi-stage turbines with appreciable wall divergence; the second is very tedious and time consuming. The purpose of this investigation is to extend the approximate methods, successfully used in calculating incompressible flow in compressors with constant blade height, to the analysis of compressible flow in turbomachines with variable blade height. Assuming that the blades can be completely defined by the exit flow angle, and neglecting the influence of downstream blades, the analysis is made considering the flow between successive blade rows only. With these restrictions, subsonic and isentropic supersonic flow patterns can be determined for arbitrary boundary shapes as long as separation does not occur. Average losses can be accounted for by the use of a polytropic law, and the effect of radial variations in stagnation temperature can be included without difficulty. Examples illustrating the flexibility and practical value of the iteration method, and the rapid convergence of successive solutions are given." }, { "name": "Mager, Artur", "degree": "PhD", "year": "1953", "title": "Laminar Boundary Layer Problems Associated with Flow Through Turbomachines", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05122003-100849", "creators": [ { "name": { "family": "Mager", "given": "Artur" }, "id": "Mager-Artur", "display_name": "Mager, Artur" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/49CS-2731", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThis analysis deals with three-dimensional boundary layer flows which are of particular interest in the design of turbomachinery. By assuming only small lateral pressure gradients and applying perturbation procedure to the steady, laminar boundary layer equations of motion a set of zeroth and first order equations is found. While the zeroth order equations are just the two-dimensional ones for flow over flat plate, the first order equations retain the characteristic Blasius similarity for a family of external flows expressible by [?] (where U* [and] W* are the perturbation velocities, x [and] z are the coordinates and A [and] B are arbitrary constants). For various types of such external flows (which may or may not be rotational) boundary layer velocity distributions were found by a numerical solution.\r\n\r\nThe investigation is divided into two parts, the first one dealing with boundary layer flows over plane surfaces and the second one considering such flows over surfaces with very sharply varying lateral curvature. In order to obtain solutions in the second part it was necessary to expand the appropriate equations in terms of the product of the local surface curvature and the boundary layer thickness. In addition, the effects of compressibility and rotation (of the surface) on the flows over a plane surface were quantitatively evaluated.\r\n\r\nComparison of the present results with the more exact solutions of other investigators in certain special cases, and with the visual studies of three-dimensional boundary layer flow in cascades, indicates a fair qualitative agreement." }, { "name": "Meghreblian, Robert Vartan", "degree": "PhD", "year": "1953", "title": "Thermodynamic Functions of Polyelectronic Atoms at Very High Temperatures", "advisor": "Penner, Stanford S.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04252003-104736", "creators": [ { "name": { "family": "Meghreblian", "given": "Robert Vartan" }, "id": "Meghreblian-Robert-Vartan", "display_name": "Meghreblian, Robert Vartan" } ], "advisors": [ { "name": { "family": "Penner", "given": "Stanford S." }, "id": "Penner-S-S", "role": "advisor", "display_name": "Penner, Stanford S." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/88PE-S208", "abstract": "A convergent series representation for the internal partition function of polyelectronic atoms is obtained by assuming a covolume equation of state for the gas as previously applied by Fermi and Urey to the hydrogen atom.
\r\n\r\nThe present investigation is limited to those cases wherein only extranuclear electronic excitation occurs. The contribution of these electronic states to the thermodynamic functions is obtained from an acceptable approximation to the sum of the convergent series for the partition function.
\r\n\r\nIt is shown that at relatively low temperatures (3000 degrees K), the customary method of evaluating the internal partition function (based on the assumption of an ideal gas) agrees to within a few percent with the results obtained from the covolume treatment. However, at higher temperatures the increase in size of the excited atoms, along with the appearance of charged particles produced by ionization, render the ideal gas treatment inadequate. Since the interaction potentials of charged particles are not known in general, an approximate procedure, which neglects these interactions, is suggested for analyzing a system wherein ions and free electrons constitute a small fraction of the total population. This procedure should be useful for treating gaseous mixtures to temperatures of about 10,000 degrees K.
" }, { "name": "Ponsford, Henry Thomas", "degree": "PhD", "year": "1953", "title": "The Effects of Stiffness on the Buckling of Cylinders with Moderate Wall Thickness", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-05082003-160210", "creators": [ { "name": { "family": "Ponsford", "given": "Henry Thomas" }, "id": "Ponsford-Henry-Thomas", "display_name": "Ponsford, Henry Thomas" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JAYX-PF29", "abstract": "A series of 25 complete cylinders was tested experimentally to determine the effects of stiffening elements on the buckling of cylinders with moderate wall thickness, and to present a physical basis for an understanding of the mechanism of cylinder buckling.\r\n\r\nIt was found that both axial and circumferential stiffeners spaced at distances comparable to the buckle wave length of the unstiffened cylinder will raise the buckling stress, reduce the wave length, and alter the shape of the buckling waves. It was shown that for cylinders of D/t = 400, the addition of axial stiffening as little as 2% or 3% of the cylinder wall material can raise the buckling stress by 30% over the unstiffened case.\r\n\r\nThe experiments of this study lend support to a theory developed by Donnell and Wan which ascribes the reduction of the buckling stress of a real cylinder below the classical theoretical value to the initial imperfections of construction in the real cylinder. Conversely, the results cast doubt on the validity of the theory which explains the \"premature\" failure on the basis of external energy disturbances present in the testing laboratory.\r\n\r\nIt was shown that the buckling waves of a reasonably well-made cylinder develop with extreme rapidity without the necessity of a change in cylinder length, and are fully developed in their lateral dimensions throughout the buckling process. Some previous results of Kanemitsu and Nojima which exhibited an essentially different buckling mechanism were explained as the consequence of excessive initial imperfection.\r\n\r\nA brief study of the vibrations of a cylinder under axial load demonstrated an approximate correlation between the vibration and buckling modes of the cylinder." }, { "name": "Solomon, George Edward", "degree": "PhD", "year": "1953", "title": "Transonic Flow Past Cone-Cylinders", "advisor": "Liepmann, Hans Wolfgang; Cole, Julian D.; Roshko, Anatol", "url": "https://resolver.caltech.edu/CaltechETD:etd-05122003-103200", "creators": [ { "name": { "family": "Solomon", "given": "George Edward" }, "id": "Solomon-George-Edward", "display_name": "Solomon, George Edward" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." }, { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-A", "role": "advisor", "display_name": "Roshko, Anatol" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DE4W-ZJ43", "abstract": "Experimental results are presented for transonic flow past cone-cylinder, axially symmetric bodies. The drag coefficient, surface Mach number, etc. are studied as the free stream Mach number is varied and, wherever possible, the experimental results are compared with theoretical predictions. Interferometric results for several typical flow configurations are shown and an example of shock-free supersonic to subsonic compression is experimentally demonstrated.\r\n\r\nThe theoretical problem of transonic flow past finite cones is discussed briefly and an approximate solution of the axially symmetric transonic equations, valid for a semi-infinite cone, is presented." }, { "name": "Solverson, Robert Richard", "degree": "PhD", "year": "1953", "title": "Stress Concentrations in Fillets", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-05122003-105323", "creators": [ { "name": { "family": "Solverson", "given": "Robert Richard" }, "id": "Solverson-Robert-Richard", "display_name": "Solverson, Robert Richard" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/M29Q-K453", "abstract": "The stress concentrations arising in the fillets of fan and turbine blades, tie down rods, and bolt heads from axially symmetric centrifugal or static force loadings are treated two dimensionally by photoelastic and theoretical methods. The effects of the fillet radius, the height of the base, the mode of application of retention forces, and the distance between the retention reactions and fillet tangencies are considered. The stress concentration factor, K, was found to increase with decreasing radius and decreasing base height and, for small radii, to decrease at first with the distance between the reaction and fillet tangency but, in general, to increase with this distance. Comparisons with other experiments and stress concentration configurations are also made.\r\n\r\nAn approximate theoretical solution is derived by selecting a convenient region from the whole base and replacing, where necessary, the exact boundary conditions with relaxed or integral conditions. The problem is formulated in terms of the classical Airy stress function. Agreement between theory and experiment is reasonable." }, { "name": "Stoolman, Leo", "degree": "PhD", "year": "1953", "title": "Investigation of an Instability Phenomena Occurring in Supersonic Diffusors", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-05122003-115303", "creators": [ { "name": { "family": "Stoolman", "given": "Leo" }, "id": "Stoolman-Leo", "display_name": "Stoolman, Leo" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EG52-8V58", "abstract": "Experimental investigations of supersonic normal shock type diffusors have shown the existence of self-excited oscillations that occur as the internal mass flow is reduced somewhat below its maximum value. There is a lower bound of free stream Mach number (of the order of 1.8) below which no instability could be observed. However, as free stream Mach number was increased above this lower bound, instability occurred at increasing values of the internal mass flow. Also, the frequency at instability was of the order of the natural frequency of the internal duct acting as organ pipe.\r\n\r\nFirst-order theoretical investigations of the above phenomena indicate that the instability may (in part) be interpreted as intrinsic, that is, independent of viscous effects at the duct inlet or within the diffusor. The fundamental cause of the instability is shown to be due to the nature of the oscillatory inlet flow conditions that occur as a consequence of the external compression from the shock wave to the inlet, and the type of reflections suffered at the shock wave by upstream traveling pressure waves." }, { "name": "Acosta, Allan James", "degree": "PhD", "year": "1952", "title": "An Experimental and Theoretical Investigation of Two-Dimensional Centrifugal Pump Impellers", "advisor": "Hollander, Aladar", "url": "https://resolver.caltech.edu/CaltechETD:etd-04072003-091659", "creators": [ { "name": { "family": "Acosta", "given": "Allan James" }, "id": "Acosta-Allan-James", "display_name": "Acosta, Allan James" } ], "advisors": [ { "name": { "family": "Hollander", "given": "Aladar" }, "id": "Hollander-Alandar", "role": "advisor", "display_name": "Hollander, Aladar" } ], "committee": [ { "name": { "family": "Plesset", "given": "Milton S." }, "id": "Plesset-M-S", "role": "chair", "display_name": "Plesset, Milton S." }, { "name": { "family": "Erd\u00e9lyi", "given": "Arthur" }, "id": "Erd\u00e9lyi-Arthur", "role": "member", "display_name": "Erd\u00e9lyi, Arthur" }, { "name": { "family": "Hollander", "given": "Aladar" }, "id": "Hollander-Aladar", "role": "member", "display_name": "Hollander, Aladar" }, { "name": { "family": "Hudson", "given": "Donald E." }, "id": "Hudson-D-E", "role": "member", "display_name": "Hudson, Donald E." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Rannie", "given": "W. Duncan" }, "id": "Rannie-W-D", "role": "member", "display_name": "Rannie, W. Duncan" }, { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "member", "display_name": "Stewart, Homer Joseph" } ], "option_major": [ "mecheng" ], "doi": "10.7907/159P-SA35", "abstract": "An experimental and theoretical investigation on a series of three centrifugal pump impellers has been made in order to determine the usefulness and validity of tyro-dimensional potential theory for the description of the flow. Computed values of the developed head and distribution of pressure on the vane surfaces are compared with measurements an two-, four-, and six- bladed impellers which have 30\u00b0 logarithmic spiral vanes and a radius ratio of about one-half.
\r\n\r\nIt is found that for operating points where the influence on the flow into the impeller by the inlet turn is least, the agreement between the observed and predicted values is reasonably good, while for other flow rates large discrepancies occur. Although the impeller efficiency is relatively high when the flow is least disturbed by the inlet, the slope of the work coefficient line is steeper than the theoretically predicted value. This deviation is attributed to boundary layers which are observed on the vane surfaces.
" }, { "name": "Arthur, Paul David", "degree": "PhD", "year": "1952", "title": "Effects of Impurities on the Supersaturation of Nitrogen in a Hypersonic Wind Tunnel", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03232009-152948", "creators": [ { "name": { "family": "Arthur", "given": "Paul David" }, "id": "Arthur-Paul-David", "display_name": "Arthur, Paul David" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PBSD-2V70", "abstract": "An experimental investigation was conducted to determine the effects of additives on the supersaturation of commercial bottled nitrogen expanded in a hypersonic nozzle. In particular, enough oxygen was added to duplicate air proportions. A stainless steel two-dimensional source-flow nozzle of one-inch width was used to conduct the tests.\r\n\r\nCommercially pure nitrogen, expanded from room temperature and 8-1/3 atm. pressure, was found to supersaturate by approximately 18\u00b0 K or 1.2 Mach number. The supersaturation of the nitrogen was decreased by the addition of impurities, and only a fraction of a percent of carbon dioxide or water vapor was required to eliminate completely all supersaturation. Addition of argon and oxygen was found to be much less effective in decreasing the supersaturation. For the synthetic air, the supersaturation was 16\u00b0 K or 0.9 Mach number based on air vapor pressure values.\r\n\r\nDuring the collapse of the supersaturated state, the static pressure gradually increased above the isentropic value because of the heat release of the condensing gas. As has been shown before, there was no evidence of condensation shock with nitrogen. The impact pressure was only slightly changed from the isentropic value by the presence of condensation in the flow. After the collapse of the supersaturated state, the flow approximated that of a condensation shock.\r\n\r\nFrom these tests it is concluded that condensation of nitrogen, containing slightly more impurities than present in the commercial nitrogen, and of air of the same purity principally caused by foreign impurities, not by spontaneous self-nucleation." }, { "name": "Buhler, Rolf Dietrich", "degree": "PhD", "year": "1952", "title": "Condensation of Air Components in Hypersonic Wind Tunnels: Theoretical Calculations and Comparison with Experiment", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01162007-152444", "creators": [ { "name": { "family": "Buhler", "given": "Rolf Dietrich" }, "id": "Buhler-Rolf-Dietrich", "display_name": "Buhler, Rolf Dietrich" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0J8Q-9Z82", "abstract": "The effect of condensation on the flow in hypersonic wind tunnels is bracketed by equilibrium saturated expansion and by instantaneous condensation. By calculation of shock waves with evaporation, direct comparison of theoretical and measured pressures is made possible. Satisfactory agreement between saturated expansion theory and experiment is obtained after the collapse of the supersaturated state.\r\n\r\nThe droplet growth theory (for free molecule regime) is reexamined, and a good approximate solution is obtained for the nonsteady case (i.e., rapidly changing vapor properties). Limits of validity of the quasi-steady theory are defined, and an upper limiting (zero growth) drop size given for expanding flow.\r\n\r\nA simplified method is presented for calculating the pressure time history of the collapse of the supersaturated state in nozzles. From this, most effective nucleus sizes for given total mass of impurities are calculated. Thus the earliest possible collapse in a nozzle due to impurities is estimated theoretically for low impurity concentrations. The agreement of the predicted trend with experimental results in nitrogen appears to justify the assumed mechanism of the collapse, which is condensation on existing foreign nuclei formed upstream of the collapse.\r\n" }, { "name": "Grey, Jerry", "degree": "PhD", "year": "1952", "title": "The Effects of Air Condensation on Properties of Flow and their Measurement in Hypersonic Wind Tunnels", "advisor": "Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-061157", "creators": [ { "name": { "family": "Grey", "given": "Jerry" }, "id": "Grey-Jerry", "display_name": "Grey, Jerry" } ], "advisors": [ { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HFHE-HR77", "abstract": "Some of the fundamental problems encountered in the measurement of flow properties in condensing air have been discussed, and were investigated experimentally in the GALCIT 9 Hypersonic Wind Tunnel. The saturated expansion theory of flow in a condensing fluid as developed by Buhler was corroborated, and some of the unknown properties of the theory have been clarified by analysis of the wind tunnel tests. Several experimental techniques for the measurement of two-phase fluid properties were developed and the results were used to supply additional information concerning the nature of phenomena such as supersaturation and normal and oblique shock waves. No definite conclusions could be reached with regard to future application of condensed air wind tunnel data on the basis of these tests, but the subject of similarity parameters comparable to the Mach number in flow of a perfect gas has been discussed at some length." }, { "name": "MacCready, Paul Beattie", "degree": "PhD", "year": "1952", "title": "Investigation of Atmospheric Turbulence", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-104542", "creators": [ { "name": { "family": "MacCready", "given": "Paul Beattie" }, "id": "MacCready-Paul-Beattie", "display_name": "MacCready, Paul Beattie" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F8AR-VH67", "abstract": "Satisfactory measurement techniques were developed which overcame the four principal experimental problems arising in the investigation of atmospheric turbulence.\r\n\r\n1) The data are made reproducible for statistical analyses by recording them on a portable magnetic tape recorder.\r\n\r\n2) Measurements at elevations up to several hundred meters are made feasible by the use of a multi-cable tethering system for a balloon which supports the measuring devices.\r\n\r\n3) and 4) The problems of measuring wind velocity fluctuations over broad ranges (frequencies between 0 and 50 cycles per second, and magnitudes between 1/4 and 20 meters per second) are solved by using hot wire anemometers together with non-linear amplifiers.\r\n\r\nThe statistical analysis of recordings made under various meteorological conditions indicates that Kolmogoroff's similarity hypothesis, concerning the isotropy of turbulence in a certain inertial sub-range of eddy sizes, is directly applicable to eddies of sizes ranging from under 10 cm. to dimensions larger than the elevation of measurement. Over this range the \"2/3 law\" holds for the autocorrelation coefficient and the \"-5/3 law\" holds for the energy spectrum, within the accuracy of the measurements. The large eddy limit of the range of the \"2/3 law\" appears to be considerably extended by strong thermal instability. Heat flux measurements from the correlation between temperature and vertical velocity agree reasonably with the heat flux measured with heat meters. At 70 cm. under moderate wind conditions over half the heat flux is due to eddies of periods less than two seconds. Momentum flux measurements from velocity correlations were inconclusive." }, { "name": "Parkin, Blaine Raphael", "degree": "PhD", "year": "1952", "title": "Scale Effects in Cavitating Flow", "advisor": "Plesset, Milton S.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03302009-081459", "creators": [ { "name": { "family": "Parkin", "given": "Blaine Raphael" }, "id": "Parkin-Blaine-Raphael", "display_name": "Parkin, Blaine Raphael" } ], "advisors": [ { "name": { "family": "Plesset", "given": "Milton S." }, "id": "Plesset-M-S", "role": "advisor", "display_name": "Plesset, Milton S." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DWKE-J388", "abstract": "Scale effects in cavitating flow are considered for the so-called limited cavitation flow regime. The roles of nuclei and air diffusion in ordinary water and the kinetic theory of liquids for pure water are considered as to their bearing on cavitation scale effects.\r\n\r\nThe attack on the problem is concentrated in three general areas. First, dynamic similarity considerations for individual bubble growth show that no useful scaling laws can be established from such arguments. Aside from changes due to Reynolds number, it is concluded that scale effects are dependent upon the time required for a nucleus to grow from its original microscopic size to a macroscopic size. Second, a series of experiments shows that the cavitation behaves in a systematic way as the scale of the immersed body is changed. In certain instances, the inception of cavitation depends on both model size and free stream velocity. Third, a theoretical study is made to gain insight into the relationships that must hold between the parameters which affect the inception of cavitation. A simplified theory gives only rough qualitative agreement with experiment." }, { "name": "Roshko, Anatol", "degree": "PhD", "year": "1952", "title": "On the Development of Turbulent Wakes from Vortex Streets", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-144124", "creators": [ { "name": { "family": "Roshko", "given": "Anatol" }, "id": "Roshko-Anatol", "display_name": "Roshko, Anatol" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4WDN-9807", "abstract": "Wave development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low speed wind tunnel. Standard hot-wire techniques were used to study the velocity fluctuations.\r\n\r\nThe Reynolds number range of periodic vortex \"shedding\" is divided into two distinct sub-ranges. At R = 40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. R = 150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300.\r\n\r\nSpectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream.\r\n\r\nIt was found that in the stable range the vortex street has a periodic spanwise structure.\r\n\r\nThe dependence of shedding frequency on velocity was successfully used to measure flow velocity.\r\n\r\nMeasurements in the wake of a ring showed that an \"annular\" vortex street is developed." }, { "name": "Wu, Theodore Yao-tsu", "degree": "PhD", "year": "1952", "title": "On Problems of Heat Conduction in a Compressible Fluid", "advisor": "Lagerstrom, Paco A.; Cole, Julian D.", "url": "https://resolver.caltech.edu/CaltechETD:etd-06032004-135328", "creators": [ { "name": { "family": "Wu", "given": "Theodore Yao-tsu" }, "id": "Theodore-Yao-tsu", "display_name": "Wu, Theodore Yao-tsu" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "advisor", "display_name": "Cole, Julian D." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/G8DW-K988", "abstract": "The present work starts with a study of heat conduction in a non-viscous compressible fluid based on a linearized theory which is similar to that used in the theory of sound. Important features of exact equations of motion and their corresponding linearized equations are studied briefly. For this linear system, which preserves many of the features of the original non-linear system, the fundamental solutions are found and discussed. The additional role played by viscosity in the heat conduction problem is then investigated. The fundamental solutions for this compressible, viscous, heat-conducting flow problem are found and compared with the non-viscous case. The problem of heat conduction in a two-dimensional stationary flow of a viscous compressible fluid is further studied by finding the fundamental solutions and discussing the result in some detail. As an example proposed to show how a superposition of these fundamental solutions can be used to solve a boundary value problem, the problem of the anemometry of a heated flat plate is solved for both large and small values of the Reynolds number. The result obtained herein is discussed and compared with some existing theories and experiments. The causes of the discrepancy resulting from this linearized theory are briefly explained." }, { "name": "Bleviss, Zegmund Oscar", "degree": "PhD", "year": "1951", "title": "Interference Effects in Supersonic Flow", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03182009-105100", "creators": [ { "name": { "family": "Bleviss", "given": "Zegmund Oscar" }, "id": "Bleviss-Zegmund-Oscar", "display_name": "Bleviss, Zegmund Oscar" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YDMK-MS28", "abstract": "A study is made of interference problems with emphasis on low-aspect-ratio supersonic missile configurations. The configurations are composed of slender pointed bodies with circular cross section, plane or cruciform delta wings, and tails which are not specified. For fin-fin interference (no body) general multi-fin delta wings are studied for the roll problems.\r\n\r\nThree types of interferences are studied. They are (1) fin-fin interference, (2) fin-fin and wing-body interferences combined, and (3) wing-body-tail interference. Where possible, three aerodynamic problems are studied under each of these interferences. They are (1) lift and incidence, (2) roll due to aileron deflection, and (3) damping in roll.\r\n\r\nA survey of the theoretical work on supersonic interference problems is included.\r\n\r\nLinearized theory is used throughout for all problems treated by the writer. Work by other writers which fits naturally into the scheme of the present work is summarized briefly. For most of the problems it is not possible to obtain exact linearized solutions without excessive labor. Where possible, approximations to or estimates of the exact solutions are obtained. It is hoped that these results will be useful for engineering estimates of the interference effects.\r\n\r\nTheoretical results for the roll problems--exact, approximate, and estimated--are presented for fin-fin interference for cruciform and more general multi-fin delta wings with subsonic and supersonic leading edges.\r\n\r\nFor fin-fin and wing-body interferences combined, theoretical results which bracket the exact solutions are obtained for the roll problems.\r\n\r\nFor wing-body-tail interference, the vorticity distributions, rolled-up vortex strengths, and initial vortex positions at the trailing edges are estimated for lift, incidence, aileron deflection, and damping in roll problems. Both plane and cruciform delta wing body configurations are studied.\r\n\r\nA qualitative discussion of some of the nonlinear, viscous, and gap effects is included.\r\n\r\nRecommendations for future research are made." }, { "name": "Bryson, Arthur Earl", "degree": "PhD", "year": "1951", "title": "An Experimental Investigation of Transonic Flow Past Two-Dimensional Wedge and Circular Arc Sections Using a Mach-Zehnder Interferometer", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11062003-114932", "creators": [ { "name": { "family": "Bryson", "given": "Arthur Earl" }, "id": "Bryson-Arthur-Earl", "display_name": "Bryson, Arthur Earl" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/S45R-FG17", "abstract": "Interferometer measurements are given of the flow fields near two-dimensional wedge and circular arc sections at zero angle of attack at high subsonic and low subsonic velocities. Both subsonic flow with local supersonic zone and supersonic flow with detached shock wave have been investigated. Pressure distributions and drag coefficients as functions of Mach number have been obtained. The wedge data are compared with the theoretical work on flow past wedge sections of Guderley and Yoshihara, Vincenti and Wagoner, and Cole.
\r\n\r\nIt is shown that the local Mach number at any point on the surface of a finite three-dimensional body or an unswept two-dimensional body, moving through an infinite fluid, is a stationary value at Mach number one and, in fact, remains nearly constant for a range of speeds below and above Mach number one. On the basis of this concept and the experimental data, pressure distributions and drag coefficients for the wedge and circular arc sections are presented throughout the entire transonic range of velocities.
" }, { "name": "Dhawan, Satish", "degree": "PhD", "year": "1951", "title": "Direct Measurements of Skin Friction", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-09262002-155504", "creators": [ { "name": { "family": "Dhawan", "given": "Satish" }, "id": "Dhawan-Satish", "display_name": "Dhawan, Satish" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1S03-8631", "abstract": "A device has been developed to measure local skin friction on a flat plate by measuring the force exerted upon a very small movable part of the surface of a flat plate. These forces, which range from about 1 milligram to about 100 milligrams, are measured by means of a reluctance measuring device. The apparatus was first applied to measurements in the low-speed range, both for laminar and turbulent boundary layers. The measured skin friction coefficients show excellent agreement with Blasius' and Karman's results respectively. The device was then applied to high-speed subsonic flow and the turbulent skin friction coefficients were determined up to a Mach number of about 0.8. A few measurements in supersonic floor were also made.
\r\n\r\nThe paper describes the design and construction of the device and the results of the measurements.
" }, { "name": "Dixon, Howard Henry", "degree": "PhD", "year": "1951", "title": "Stresses and Deflections of Unswept and Swept Thin-Walled Beams", "advisor": "Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-03232009-093729", "creators": [ { "name": { "family": "Dixon", "given": "Howard Henry" }, "id": "Dixon-Howard-Henry", "display_name": "Dixon, Howard Henry" } ], "advisors": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/7D5X-ZN05", "abstract": "A method is presented for the determination of the stresses and deflections of unswept and swept, thin-walled beams of uniform closed cross section. The cross section, loading distribution and boundary conditions are assumed to be arbitrary. The method is based on the differential equation governing the behavior of orthogonal elastic shells. The differential equation is transformed into a difference equation and the solution obtained by the relaxation technique. A comparison of the theoretical solution and experimental data for a swept back wing with a carry through bay under symmetrical bending showed good agreement.
\r\n\r\nA tapered wing may be treated by approximating the variation by a series of spanwise steps.
\r\n\r\nAs the difference equations are a system of simultaneous algebraic equations, they may be solved by automatic calculating equipment or by electric analogue computers as well as by the relaxation technique.
\r\n" }, { "name": "Hipsh, Harold Marvin", "degree": "PhD", "year": "1951", "title": "Harmonic Oscillations of a Narrow Delta Wing in Supersonic Flow", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03112009-125900", "creators": [ { "name": { "family": "Hipsh", "given": "Harold Marvin" }, "id": "Hipsh-Harold-Marvin", "display_name": "Hipsh, Harold Marvin" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VRVK-HB35", "abstract": "A theory is presented for the calculation of the velocity potential of a harmonically oscillating delta wing having subsonic leading edges in a supersonic flow. The velocity potential is expanded in a power series in powers of the reduced frequency. Two modes of oscillation, plunging and pitching, are considered. For both modes the analysis is carried through the term linear in reduced frequency, this being generally sufficient for dynamic stability analyses. The results thus obtained for the pitching mode verify those of Miles (Ref. 9) obtained by an integral transformation of the steady-state solution. In addition, the term that is quadratic in the reduced frequency is presented for the plunging mode to illustrate the general procedure.\r\n\r\nLift and pitching moment coefficients are calculated from the velocity potential and numerical results valid for low frequency oscillations are presented." }, { "name": "Jensen, Arnold Axtell", "degree": "PhD", "year": "1951", "title": "A Slender Cone Starting Impulsively", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03102009-074636", "creators": [ { "name": { "family": "Jensen", "given": "Arnold Axtell" }, "id": "Jensen-Arnold-Axtell", "display_name": "Jensen, Arnold Axtell" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/19KD-1M64", "abstract": "The problem of a semi-infinite slender cone which starts impulsively from rest so that it suddenly has a constant supersonic velocity is considered. It is treated by using the acoustic wave equation for the air at rest at infinity. The problem is reduced to that of dealing with the radial velocity in two conical variables in space-time.\r\n\r\nIt is shown that there are three fundamental regions from the physical or mathematical standpoint. The boundary conditions and equations for each of these regions are developed so that a numerical solution of the problem may be obtained for a given Mach number and cone angle. From the solution of the radial velocity the potential and thence the pressure on the cone are obtained.\r\n\r\nAn approximation to the pressure far back on the cone where the curvature is small is obtained as an improvement on the piston value for zero curvature. This is done by suppressing variations in the axial direction and solving the resulting equation by Riemann's integration method.\r\n\r\nAn attempt to solve the problem by distributing sources on the axis with resulting difficulties is discussed." }, { "name": "Lo, Shih-Chun", "degree": "PhD", "year": "1951", "title": "Oscillating Airfoil in Parallel Streams Separated by an Interface", "advisor": "Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-10012002-100524", "creators": [ { "name": { "family": "Lo", "given": "Shih-Chun" }, "id": "Lo-Shih-Chun", "display_name": "Lo, Shih-Chun" } ], "advisors": [ { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HR3R-5994", "abstract": "A new approach to tail buffeting is made by studying the problem of a thin airfoil performing a periodic oscillation of small amplitude in the presence of an interface across which the flow undergoes a constant change in density and velocity. A general solution to the problem is found. Lift and moment for some special cases are obtained in simple forms and are plotted in Figs. 3 and 4 for the two basic modes of oscillation: bending and torsion. A typical application to flutter analysis is made and it is found that tail flutter at low speeds is possible for the tail lying in the wake of the wing." }, { "name": "Michelson, Irving", "degree": "PhD", "year": "1951", "title": "I. On the Dust Devils. II. Linearized Theory of Conical Turbomachines", "advisor": "Stewart, Homer Joseph; Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03042009-142746", "creators": [ { "name": { "family": "Michelson", "given": "Irving" }, "id": "Michelson-Irving", "display_name": "Michelson, Irving" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "co-advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PZKM-9280", "abstract": "Dust devils are small-scale atmospheric motions of instability arising from the development of large thermal stratifications in the lowest layers above the surface of the earth. A theory is proposed in Part I to describe the conditions of formation of these motions and depends on the new result that shear provides a powerful stabilizing influence even in non-viscous fluid motions in which denser fluid is situated above less dense. Those features of the flow which can be predicted by the theory and compared with observations are found to be in reasonably good agreement, and it is therefore indicated that the theory, which is based on a highly simplified model of flow, furnishes at least a qualitatively correct correlation of the basic ideas involved in the stable flow of very slightly viscous fluids containing density inversions. Applications to technically interesting flows of this type, in large-scale atmospheric motions as well as in high speed aerodynamic boundary layers, are indicated but not analyzed in detail.\r\n\r\nIn Part II the perfect fluid flow is determined for a turbomachine of conical shape and prescribed blade loading. On the basis of the assumption that the stream surfaces are conical in shape, a linear, elliptic partial differential equation of the second order is obtained. The associated boundary value problem is of the Sturm-Liouville type and is solved completely. An asymptotic representation of the solution is determined which is convenient for computational purposes." }, { "name": "Monroe, Gerald Morgan", "degree": "PhD", "year": "1951", "title": "A Study of Compressible Perfect Fluid Motion in Turbomachines with Infinitely Many Blades", "advisor": "Marble, Frank E.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03182009-151729", "creators": [ { "name": { "family": "Monroe", "given": "Gerald Morgan" }, "id": "Monroe-Gerald-Morgan", "display_name": "Monroe, Gerald Morgan" } ], "advisors": [ { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "advisor", "display_name": "Marble, Frank E." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HQD3-G406", "abstract": "A study is made of compressible perfect fluid motion in turbomachines having infinitely many blades and a general theory is developed. An underlying concept of the theory is that force fields which represent the action of infinitely many blades belong to a special class described as pseudo-conservative and can be expressed as the product of a scalar function and the gradient of a potential. The scalar function is simply the rate at which energy is imparted to the fluid by the blades, and the potential is simply the family of the equations for the blade surfaces. The introduction of these two functions to express the force field casts an entirely new light on problems of mixed-flow turbomachines having infinitely many blades of arbitrary shape.\r\n\r\nIn the formulation of the problem the non-linear action of rotationality and compressibility is regarded as a force tending to displace the streamsurfaces from their irrotational, incompressible position. It is shown that the character of the problem is determined by a governing velocity: the velocity relative to the blades where blades are present, or the meridional velocity, where blades are not present. Where the governing velocity is subsonic the problem is essentially elliptic, where supersonic, hyperbolic.\r\n\r\nThe theory and the examples lead to conclusions which are believed to explain in part the unexpected efficiencies observed for compressors having transonic governing velocities. These conclusions, which indicate that transonic compressors could perhaps be profitably developed, are as follows: The deflection of the streamsurfaces induced by a given strength of vorticity at a certain point in the flow has one sense when the governing velocity at the point is subsonic, the opposite sense when it is supersonic, and becomes zero as it becomes sonic. The deflection of the streamsurfaces brought about by a given distribution of vorticity in a region is less when the governing velocity in the region is transonic than when it is entirely subsonic or entirely supersonic.\r\n\r\nExamples of incompressible flow through a mixed flow compressor with prescribed blades, and subsonic and transonic flow through actuator disks, were solved by the method of finite differences, applying simultaneously the relaxation technique and an iteration process." }, { "name": "Morgan, Antony John Andrew", "degree": "PhD", "year": "1951", "title": "A General Similarity Theory of Partial Differential Equations and its Use in the Solution of Problems in Aeronautics", "advisor": "Michal, Aristotle D.; Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-03192009-091419", "creators": [ { "name": { "family": "Morgan", "given": "Antony John Andrew" }, "id": "Morgan-Antony-John-Andrew", "display_name": "Morgan, Antony John Andrew" } ], "advisors": [ { "name": { "family": "Michal", "given": "Aristotle D." }, "id": "Michal-A-D", "role": "advisor", "display_name": "Michal, Aristotle D." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Michal", "given": "Aristotle D." }, "id": "Michal-A-D", "role": "chair", "display_name": "Michal, Aristotle D." }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "member", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "member", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "member", "display_name": "Fung, Yuan-cheng" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9142-KZ49", "abstract": "A general similarity theory of systems of partial differential equations of any order in any number of independent variables is developed with the aid of the theory of continuous one-parameter groups of transformations. The theory is illustrated by means of several known examples of similarity equations, previously given without motivation, in Hydrodynamics. With the aid of the theory two new examples of similarity equations, one in Elasticity and one in Fluid Mechanics, have been found; these are discussed in the text.\r\n" }, { "name": "Nielsen, Jack Norman", "degree": "PhD", "year": "1951", "title": "Supersonic Wing-Body Interference", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-03252009-150359", "creators": [ { "name": { "family": "Nielsen", "given": "Jack Norman" }, "id": "Nielsen-Jack-Norman", "display_name": "Nielsen, Jack Norman" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E6Z2-SM31", "abstract": "A method of solving wing-body problems for circular bodies employing wings with supersonic edges has been developed. The method is based on decomposing the wing-body combination into a wing alone plus a number of Fourier component wing-body combinations corresponding to the Fourier series for the normal velocity induced at the body surface by the wing alone. The problem is then solved for each component by a method based on Laplace transform theory, and the method is then shown to be equivalent to a distributed-solution method analogous to that used by Karman and Moore to solve problems of bodies of revolution at supersonic speeds. Two sets of universal functions are presented. The first set is used to obtain the strength distribution of the fundamental solutions distributed along the body axis, from which the entire interference pressure field can be obtained. The second set permits a direct determination of the pressures acting on the body.\r\n\r\nAs an example in the use of the theory, calculations are carried out for the technologically important case of a flat rectangular wing mounted at zero incidence on a body at zero angle of attack. The calculations are carried out for four Fourier components. It was found that all four components were necessary to get good accuracy in determining the pressures at some points in the field, while only one component was required to get a fair determination of the span loading of the combination. From the example much insight into the mechanism of wing-body interference was obtained. The use of the universal functions to obtain pressures due to protuberances on nearly cylindrical bodies is discussed." }, { "name": "Oswald, Telford Wilbert", "degree": "PhD", "year": "1951", "title": "The Influence of Variable Air Density and of Nonlinear Aerodynamic Characteristics on Dynamic Behavior at Supersonic Speeds", "advisor": "Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-02242006-160420", "creators": [ { "name": { "family": "Oswald", "given": "Telford Wilbert" }, "id": "Oswald-Telford-Wilbert", "display_name": "Oswald, Telford Wilbert" } ], "advisors": [ { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VG6Q-C753", "abstract": "The effect of the variable density of the Standard Atmosphere on the dynamic stability of a missile in vertical flight is considered. The analysis is restricted to small disturbances from steady rectilinear flight. The exponential decrease of density with altitude characteristic of the Standard Stratosphere is introduced into the equations of motion and a stability criterion for the dynamic behavior immediately following a small disturbance is found. Alternatively, a hyperbolic variation of density with altitude is used to approximate the Standard Atmosphere and the identical stability criterion is obtained.\r\n\r\nThe effect of non-linear pitching moment and lift variations with angle of attack on the dynamic response to a sudden change in angle of attack is considered. An approximate solution to the non-linear equation of motion is developed. Several numerical examples are considered, and the results of the approximate solution are compared with the very accurate results of numerical integration as well as the classical linearized solution. The effect of a non-linear moment curve on the determination of stability derivatives from flight test data is discussed in the light of these examples." }, { "name": "Parkinson, Geoffrey Vernon", "degree": "PhD", "year": "1951", "title": "Unit-Step Motion of a Wide Delta-Wing", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-05052006-090936", "creators": [ { "name": { "family": "Parkinson", "given": "Geoffrey Vernon" }, "id": "Parkinson-Geoffrey-Vernon", "display_name": "Parkinson, Geoffrey Vernon" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/73TW-ZE47", "abstract": "The effect on a wide delta wing in supersonic flight of the sudden imposition of a small velocity perturbation normal to the plane of the wing is considered. The resulting pressure field on the wing is found in closed form in terms of elementary functions. The corresponding solution for the infinite swept wing with supersonic edges is obtained as a by-product, and for this the terms of the transient pressure coefficient are shown to have geometric significance. The force and moment coefficients are obtained by means of a method of descent, which simplifies the calculations and shows the nature of the dependence of these coefficients on the wing planform. Because of the short duration and moderate strength of the transient effects of the unit-step motion, it is considered to be of little practical importance. However, because of the simplicity of the solutions for the motion, it is also considered to be of possible theoretical value as a basis for Duhamel integration to obtain solutions for more general time-dependent motions of the wide delta wing." }, { "name": "Rannie, William Duncan", "degree": "PhD", "year": "1951", "title": "Heat Transfer in Turbulent Shear Flow", "advisor": "Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-03172004-135406", "creators": [ { "name": { "family": "Rannie", "given": "William Duncan" }, "id": "Rannie-William-Duncan", "display_name": "Rannie, William Duncan" } ], "advisors": [ { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/9G0F-9R87", "abstract": "A new and relatively simple description is proposed for the velocity profile in turbulent flow close to a smooth wall. Heat transfer coefficients are calculated from the description and are shown to agree better with experiment than other theories. The analysis is extended to transport processes in liquids where the viscosity has a large variation close to the wall.\r\n" }, { "name": "Wylly, Alexander", "degree": "PhD", "year": "1951", "title": "A Second-Order Solution for an Oscillating, Two-Dimensional, Supersonic Airfoil", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03232009-084121", "creators": [ { "name": { "family": "Wylly", "given": "Alexander" }, "id": "Wylly-Alexander", "display_name": "Wylly, Alexander" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/YA8A-4R06", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\n\r\nIn this paper a second-order solution, for the forces and moments produced by an oscillating two-dimensional airfoil of arbitrary cross section, has been determined. This solution was obtained by means of an iteration procedure. In the iteration procedure it was necessary to have a linearized solution of simple, closed form which was valid throughout the whole x, y plane. Existing solutions did not satisfy these requirements, thus, it was first necessary to develop a new linearized or first-order velocity potential. This potential was developed as a power series approximation, in frequency, to the exact linearized solution. Six terms of this series were developed and this sixth-order solution shown to be within a few percent of the exact linearized solution for reduced frequencies [...] less than 1.3.\r\n\r\nThe first two terms of the series approximation were then used in the iteration process to produce the second-order solution in thickness. This solution which is valid to second-order in thickness and frequency has been determined for an oscillating airfoil of general cross section.\r\n\r\nThe second-order terms were found to have a relatively strong influence on the final solution, particularly for the pitching moment. It will be seen in Section V that in many cases the second-order terms are larger in magnitude than the corresponding first order-terms and thus reverse the tendencies indicated by first-order theory. In particular, it was shown that the theoretical instability predicted by linearized theory for an airfoil of zero thickness is completely eliminated for an airfoil having a thickness ratio as small as three percent." }, { "name": "Alperin, Morton", "degree": "PhD", "year": "1950", "title": "A Study of Detached Shock Waves in Two-Dimensions", "advisor": "Stewart, Homer Joseph; Nagamatsu, Henry T.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02022009-081306", "creators": [ { "name": { "family": "Alperin", "given": "Morton" }, "id": "Alperin-Morton", "display_name": "Alperin, Morton" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Nagamatsu", "given": "Henry T." }, "id": "Nagamatsu-H-T", "role": "advisor", "display_name": "Nagamatsu, Henry T." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NW30-HY03", "abstract": "The present report contains results of an experimental and theoretical investigation of the detached shock wave phenomenon. The experimental phase of this study was actually carried out at the Jet Propulsion Laboratory at California Institute of Technology, in a two-dimensional wind tunnel which is briefly described in Section I.
\r\n\r\nSection II contains a description of the experiments on circular cylinders. The circular cylinder was used in this series of tests primarily because of its simplicity. The investigation discussed in II-1 required a large variation of model shapes and would have required much more time had it been based on a more complicated body shape. In addition to data on the shock wave position and shape, the pressure distribution was also obtained at M=1.546 for a two-dimensional circular cylinder. From this pressure distribution, the drag was calculated.
\r\n\r\nAlthough the theoretical knowledge of flow involving detached shock waves is in a rather primitive state, a review of the existing theoretical work and comparison with experimental data is made in section III.
\r\n\r\nIn section IV a method is presented for finding the stream function or velocity potential for the subsonic region behind the detached shock wave. This method depends upon the hypothesis that the flow can be considered to be irrotational in this region without introducing a serious error. The results appear to be in good agreement with the experiments although the example carried out does not apply strictly to the circular cylinder body shape used in the experiments.
\r\n\r\nA general discussion of the existing theories and their comparison with experimental data is presented in section V.
" }, { "name": "Benscoter, Stanley Urner", "degree": "PhD", "year": "1950", "title": "Secondary Stresses in Thin-Walled Beams with Closed Cross-Sections", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-02022009-084840", "creators": [ { "name": { "family": "Benscoter", "given": "Stanley Urner" }, "id": "Benscoter-Stanley-Urner", "display_name": "Benscoter, Stanley Urner" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/XKAT-E346", "abstract": "An accurate method of determining secondary stresses in thin-walled, uniform beams of closed cross-section is herein presented. The cross-sections are assumed to be preserved by closely spaced rigid diaphragms. In part I the integro-differential equation governing axial displacements is formulated and solved for a beam without longitudinal stiffeners. In Part II the corresponding summation-difference equation is developed and solved for a beam with stiffeners (flanges and stringers). The cross-section, loading distribution and end conditions are assumed to be arbitrary.\r\n\r\nBy introducing generalized difference equations the mathematical analysis for the stiffened beam may be performed in a manner exactly analogous to the process used for the unstiffened beam. A separation of variables in the homogeneous equation leads to the natural stress or displacement modes for a cross-section. The solution of the non-homogeneous equation is then expressed as an expansion in terms of the natural stress modes. Particular attention is given to cross-sections with single symmetry and double symmetry." }, { "name": "Chang, Chieh-Chien", "degree": "PhD", "year": "1950", "title": "The Linearized Wing Theory of the Supersonic Flow with the Karman's Fourier Integral Method", "advisor": "von K\u00e1rm\u00e1n, Theodore; Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-03042009-110100", "creators": [ { "name": { "family": "Chang", "given": "Chieh-Chien" }, "id": "Chang-Chieh-Chien", "display_name": "Chang, Chieh-Chien" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/67DX-1V06", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nPart I gives a short introduction and some physical interpretation of von armAn's Fourier integral method applied to the supersonic wing theory. A short review of other current contributions to the Linearized supersonic wing theory is also given.\r\n\r\nPart II presents the general formulation of the von [...] method from the view-point of the elementary harmonic sources and doublets. First, the disturbance potential and the velocity components of a general flat body with symmetrical airfoil are derived. Next, the disturbance potential of the lifting surface is presented. In contrast to the well-known conical flow method, the von [...] Fourier integral method can treat a complicated plan-form as a whole, without considering the detailed geometry, as long as the airfoil sections are similar.\r\n\r\nPart III applies the method to the investigation of the wave drag of the non-lifting wing in supersonic flight. A general solution of the wave drag is obtained for the wing with a diamond- shaped airfoil. This solution allows a free choice of a number of the important geometrical parameters. For instance, the wing may be swept forward or backward, tapered or reversed tapered to any ratio. A number of the limiting cases are also investigated. For the practical aerodynamic problems, two useful families of wing plan-form with the fixed taper ratios 0.2 and 0.5, any swept angle, aspect ratio and Mach number are shown in the graphs. A particular application is demonstrated.\r\n\r\nThe reversed flow theorem on wave drag as shown by von [...] and Hayes checks well with the consequence of the general solution. This method shows a certain elegance as no conical flow assumption is needed, and the mathematics is powerful enough to obtain a general solution covering all possible geometrical arrangements without detailed considerations.\r\n\r\nWhile in recent years, the direct problem of finding the lift distribution with given angle of attack on the wing has been well solved by the method of conical flow and others, the present treatment in Part IV, on the other hand, investigates the inverse problem, i.e., to find the downwash distribution in the plane of the wing with a pre- assigned lift distribution. This is particularly favorable with the present method. The general solution of the downwash of the tapered swept wings is derived for the case that a constant lift distribution on the wing is pre-assigned. Of course, the method may be applied to any lift or pressure distribution along the wing chord and span. The corresponding angle of attack on the wing and the downwash can be determined everywhere in the plane of the wing. To demonstrate the downwash distribution as given by the general solution, graphs are given to show the downwash of a number of wings including a swept-back tapered wing with supersonic trailing edge and a delta wing.\r\n" }, { "name": "Chuang, Feng-Kan", "degree": "PhD", "year": "1950", "title": "On the Statistical Theory of Turbulence", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-154021", "creators": [ { "name": { "family": "Chuang", "given": "Feng-Kan" }, "id": "Chuang-Feng-Kan", "display_name": "Chuang, Feng-Kan" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2TJM-BK44", "abstract": "The present work starts with a study of isotropic turbulence which was introduced by G. I. Taylor in 1935. The different notions of averages are critically examined. The notion of stochastic average is then introduced and the general transport equation is developed. After a detailed study of kinematics of turbulence, the concept of correlation and spectrum, the correspondence between the Karman-Howarth equation and the spectrum equation is made. The turbulence decay is studied. A theory for turbulence decay at large Reynolds number is proposed. In the study of turbulence spectrum, different assumptions on the transfer function are critically discussed and the solution using Heisenberg's assumption is obtained explicitly. The spectrum is further studied by trying to fit the turbulence phenomenon into a general scheme of stochastic processes. In the second part of the work, an entirely different approach to the statistical theory is made. Linearized vorticity transport theory is developed and finally the non-linear effects in turbulence are studied." }, { "name": "DeGroff, Harold Miller", "degree": "PhD", "year": "1950", "title": "Aerodynamic Forces on a Propeller in Non-Stationary Motion", "advisor": "Stewart, Homer Joseph; Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-02272009-083100", "creators": [ { "name": { "family": "DeGroff", "given": "Harold Miller" }, "id": "DeGroff-Harold-Miller", "display_name": "DeGroff, Harold Miller" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "chair", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "member", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "member", "display_name": "Fung, Yuan-cheng" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DYT6-1W82", "abstract": "The non-stationary lift and moment of an oscillating propeller blade element are determined. The solutions are obtained in the form of double definite integrals which are evaluated for one numerical example in Appendix A.
\r\n\r\nThe three-dimensional nature of the problem is accounted for by determination of the induced velocity field due to an approximate vorticity distribution in the propeller wake. The corresponding blade element circulation is calculated by means of the classical Munk integral theorem. The two dimensional results for non-stationary lift and moment, expressed in terms of the circulation, are then used to obtain the results of this paper. Derivations of the lift and moment equations are included.
\r\n\r\nThe resultant forces on the blade element are resolved into thrust and torque. Also, a qualitative discussion of the effects of compressibility is made based upon the Prandtl-Glauert transformation.
\r\n\r\nFinally, the results are compared with two dimensional theory and a discussion of the application to problems of flutter and forced oscillations of propellers is made. The discussion is illustrated by means of the numerical example.
\r\n" }, { "name": "Froehlich, Jack Edward", "degree": "PhD", "year": "1950", "title": "Non-Stationary Motion of Purely Supersonic Wings", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-02122009-155907", "creators": [ { "name": { "family": "Froehlich", "given": "Jack Edward" }, "id": "Froehlich-Jack Edward", "display_name": "Froehlich, Jack Edward" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/FCGZ-7P23", "abstract": "A general theory is presented for the calculation of the total forces acting on purely supersonic wings. The method applies to wings having an arbitrary downwash distribution (stationary or non-stationary) and is valid whenever all of the wing edges are supersonic. The general three-dimensional non-stationary problem is reduced to an equivalent two-dimensional problem. In the case of harmonic oscillations the aerodynamic coefficients are expressed in terms of known or tabulated functions. The specific example of an oscillating delta wing is considered and values of the aerodynamic coefficients for plunging, pitching, and rolling oscillations are calculated for two Mach numbers." }, { "name": "Li, Ting-Yi", "degree": "PhD", "year": "1950", "title": "Periodic Supersonic Motions of a Thin Wing of Finite Span", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03172009-110456", "creators": [ { "name": { "family": "Li", "given": "Ting-Yi" }, "id": "Li-Ting-Yi", "display_name": "Li, Ting-Yi" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/VZX8-E374", "abstract": "The present paper supplies some general theorems with which periodic supersonic motions of a thin wing of fairly general planform may be analyzed to yield valuable three-dimensional results. It is shown that the method developed by Evvard (Refs. 1,2) for treating the steady supersonic motion of a thin wing with subsonic leading or side edges is valid for an oscillating wing of similar planform. Illustrations of the application of these general theorems are furnished by a careful study of several types of periodic oscillations of a rectangular wing. The present report includes a complete analysis for the case of plunging oscillations. Important steps have also been taken towards solution of the cases of pitching and rolling oscillations. The essential results are presented in a number of vector diagrams giving the magnitudes and phase angles of the lift and moment. Computations are made for several aspect ratios at two Mach numbers (M=10/7,2) when the reduced frequency (k) ranges from 0 to 2.0. It is found that the lift and moment vectors acting on a rectangular wing with supersonic plunging oscillations have positive phase angles within certain ranges of Mach numbers and aspect ratios, while the corresponding vectors acting on a wing of infinite span with the same kind of motion have negative phase angles for every Mach number. This new discovery indicates strongly the necessity of revising present day wing flutter calculations.\r\n" }, { "name": "Lurie, Harold", "degree": "PhD", "year": "1950", "title": "Lateral Vibrations as Related to Structural Stability", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-07092007-131041", "creators": [ { "name": { "family": "Lurie", "given": "Harold" }, "id": "Lurie-Harold", "display_name": "Lurie, Harold" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/0P9M-XM21", "abstract": "The apparently different physical problems of lateral vibration and elastic stability are limiting cases of a single phenomenon, the most general expression being the mode of vibration with end thrust. The theory of straight beams and flat plates is discussed in detail, and it is shown that the square of the frequency of lateral vibration is approximately linearly related to the end load. The linear relationship is exact if the mode of free vibrations is identical to the buckling mode. In all cases, the load corresponding to zero frequency is the critical buckling load. The analysis is valid only if the boundary conditions do not change with load.\r\n\r\nExperimental tests were conducted on elastically restrained columns in the form of rigid rectangular frames. It is found that the relationship between the square of the frequency and the load is practically linear, and that the extrapolated load corresponding to zero frequency coincides with the buckling load. Determining the critical load by frequency measurements seems to have the advantage of predicting that load corresponding to the actual boundary conditions which prevail, whereas a theoretical calculation may unjustifiably assume certain conditions which are not exactly realized.\r\n\r\nIn the case of flat plates, tests showed that the linear relationship is not achieved in practice. It is shown that this is probably due to the fact that the linear plate equations are not valid due to initial curvatures in the plate.\r\n\r\nRigid-joint trusses were also tested. Due to the change of end restraint with load, in some cases the relationship between the square of the frequency and the load deviates considerably from linearity. The amount of deviation appears to depend on the section properties of the members of the truss." }, { "name": "Martin, Harold Clifford", "degree": "PhD", "year": "1950", "title": "Elastic Instability of Cantilever Struts Under Combined Axial and Transverse Forces at the Free End", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-02272009-130215", "creators": [ { "name": { "family": "Martin", "given": "Harold Clifford" }, "id": "Martin-Harold-Clifford", "display_name": "Martin, Harold Clifford" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F3YJ-3D63", "abstract": "This investigation considers the elastic instability of cantilever struts under applied axial and transverse forces at the free end. Fig.1 shows the general case of such a strut.\r\n\r\nFirst the strut of uniform depth and without sweep is studied. This is shown in Fig. 2. A derivation is given for the governing differential equation and boundary conditions. These are then solved for the minim coupled eigenvalues, which correspond to the critical load combinations. Fig. 10 is a plot of these calculated critical loadings.\r\n\r\nNext an experimental investigation, whose main purpose was to provide a check on the above theoretical calculations, is presented. Various difficulties are discussed in addition to the techniques finally adopted. Experimental values are shown to check theory within several per cent. See Fig. 16. Also Southwell\u2019s experimental procedure for determining instability loading is shown to apply to this case of coupled loading.\r\n\r\nThe theory is then extended to include the problem of the tapered strut. Equations and boundary conditions are given for the arbitrary taper case and a solution presented for the limiting strut having complete taper. These results are given in Fig. 24.\r\n\r\nIn the concluding Part some of the more important unsolved problems are discussed in detail. These include the strut with arbitrary taper, the swept strut, and the strut which buckles inelastically.\r\n\r\nThe Appendix derives the differential equation for the non-tapered strut by variational procedure." }, { "name": "Rasof, Bernard", "degree": "PhD", "year": "1950", "title": "Supersonic Source Flow Past Thin Air Foils", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-03052009-084956", "creators": [ { "name": { "family": "Rasof", "given": "Bernard" }, "id": "Rasof-Bernard", "display_name": "Rasof, Bernard" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NCKK-3A63", "abstract": "In this thesis the supersonic source flow over a thin sharp- edged airfoil is formulated as a linearized problem. A new potential equation is derived, using a system of spherical coordinates centered at the source; as a simplification only wings symmetrical about the z axis are considered, and of these only the limiting cases of ring- and annular- airfoils are treated.
\r\n\r\nAfter transforming to characteristic coordinates in the hodograph plane, the potential equation (which has variable coefficients) is shown to be approximated by two classical equations -- one holding for the ring wing and the other applying to the annular wing. The flow over a specific annular wing is computed by en application of the Riemann method to the telegraph equation, which is the appropriate approximation to the governing equation for this case.
\r\n\r\nThe linearized potential equation is also solved by the Method of Characteristics, using a numerical equivalent of the Mono, procedure for quasilinear partial differential equations. A complete set of compatibility equations is exhibited, allowing the computation of the perturbation velocity components at any point of the zone of influence of an airfoil set in the supersonic source flow. Two numerical examples are presented, illustrating the application to the computation of the flow over each of a ring- and annular- wing.
\r\n\r\nFinally in an appendix the usually powerful method of separation of variables is shown to be unsuitable as a procedure for solving the potential equation governing the present problem.
\r\n" }, { "name": "Williams, Max Lea", "degree": "PhD", "year": "1950", "title": "The Plate Problem for a Cantilever Sector of Uniform Thickness", "advisor": "Sechler, Ernest Edwin; Fung, Yuan-cheng", "url": "https://resolver.caltech.edu/CaltechETD:etd-07202004-095445", "creators": [ { "name": { "family": "Williams", "given": "Max Lea" }, "id": "Williams-Max-Lea", "display_name": "Williams, Max Lea" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng", "role": "advisor", "display_name": "Fung, Yuan-cheng" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/WFM9-RR55", "abstract": "As one approach to the problem of analyzing missile wings of approximately delta configuration for stress and deflection characteristics, a uniformly thin plate of sector planform clamped along one radial edge has been considered.\r\n\r\nIt is shown that an infinite set of deflection functions, resulting from a product solution to the double Laplacian, may be generated, but practical utility is impeded because the functions are non-orthogonal. It is believed that should the importance of the solution warrant, the deflection of a sector under normal loading may be found by using a combination of the deflection functions, the Trefftz variational method, and high speed computing machinery.\r\n\r\nAnother section of the report is devoted to a study of the stress along the clamped edge in the vicinity of the corner, and it is shown that the stress varies from zero to a mathematical infinity as the opening angle of the sector increases from zero through ninety degrees with the stress singularity becoming progressivev stronger as the opening angle is increased. Experimental data are included that show engineering agreement with the theoretical results, for the case of a delta plate of thirty degree opening angle and varying trailing edge angle.\r\n\r\nIn conclusion, some remarks are made upon the application of the sector results to swept rectangular plates by mans of a hydrodynamic analogy wherein the possibility of obtaining approximate overall stress distributions is indicated.\r\n" }, { "name": "Cole, Julian David", "degree": "PhD", "year": "1949", "title": "Problems in Transonic Flow", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-04202004-105506", "creators": [ { "name": { "family": "Cole", "given": "Julian David" }, "id": "Cole-Julian-David", "display_name": "Cole, Julian David" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "chair", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "De Prima", "given": "Charles R." }, "id": "De-Prima-C-R", "role": "member", "display_name": "De Prima, Charles R." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QS66-WA09", "abstract": "No Abstract." }, { "name": "Head, Richard Moore", "degree": "PhD", "year": "1949", "title": "Investigations of Spontaneous Condensation Phenomena", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechTHESIS:04212011-134533762", "creators": [ { "name": { "family": "Head", "given": "Richard Moore" }, "id": "Head-Richard-Moore", "display_name": "Head, Richard Moore" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/06R8-C778", "abstract": "The results of a systematic wind tunnel investigation into the attainment and ultimate collapse of the supersaturated state of water vapor are presented. These results, together with those of other recent investigations, are collected and compared with the theory. It is found that the deviations from the quasi-stationary conditions upon which the theory is based are very pronounced in the supersonic wind tunnel. A much higher degree of supersaturation can, therefore, be attained before condensation occurs than is predicted theoretically.\r\n\r\nMeasurements at low temperatures indicate that if the water vapor reaches a temperature of about 153 \u00b0K, without the occurrence of condensation, the vapor will not condense upon further expansion, regardless of how highly supersaturated it becomes. This observation is in agreement with some recent Wilson Cloud Chamber investigations.\r\n\r\nThe shock relations for flow-involving condensation are discussed. It is shown that two types of discontinuities can occur; the condensation shock and the shock with condensation (or vaporization). The latter solution has been disregarded in the past, but it is shown that the shock with vaporization is of importance and can result in appreciable errors in Mach number determination when droplets are present in the flow.\r\n\r\nVarious techniques of measurement of the condensation processes in supersonic flow are considered.\r\n" }, { "name": "Mettler, Ruben Fred", "degree": "PhD", "year": "1949", "title": "The Anemometric Application of an Electrical Glow Discharge in Transverse Air Streams", "advisor": "Lindvall, Frederick C.; Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11032003-110318", "creators": [ { "name": { "family": "Mettler", "given": "Ruben Fred" }, "id": "Mettler-Ruben-Fred", "display_name": "Mettler, Ruben Fred" } ], "advisors": [ { "name": { "family": "Lindvall", "given": "Frederick C." }, "id": "Lindvall-F-C", "role": "advisor", "display_name": "Lindvall, Frederick C." }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "eleceng" ], "doi": "10.7907/RTBA-4692", "abstract": "The possibility of using an electrical glow discharge for quantitative turbulence measurements is experimentally investigated. It is found that a glow discharge is stable in a transverse air stream throughout the subsonic velocity range, and at supersonic air velocities up to a Mach number of 1.5, with no indication that this Mach number represents an upper velocity limit. A calibration procedure is developed and used in measuring the decay of turbulence behind a grid at low subsonic velocities. Comparison with decay measurements made independently with a hot wire anemometer under similar flow conditions shows that the glow discharge data is as yet quite badly scattered and somewhat inconsistent.\r\n\r\nA quantitative theory of the dark current anemometer is presented and gives results which agree in form with reported experimental results. A qualitative theory of the mechanism of the glow discharge anemometer and the first steps of the corresponding quantitative analysis are also presented." }, { "name": "Morikawa, George Kiyoshi", "degree": "PhD", "year": "1949", "title": "The Wing-Body Problem for Linearized Supersonic Flow", "advisor": "Stewart, Homer Joseph; Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-01282009-112820", "creators": [ { "name": { "family": "Morikawa", "given": "George Kiyoshi" }, "id": "Morikawa-George-Kiyoshi", "display_name": "Morikawa, George Kiyoshi" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "co-advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/PJZP-YG35", "abstract": "This work is concerned with one of the important problems remaining in the theory of linearized supersonic flow: the study of non-planar systems dealing with configurations which cannot be completely solved with the existing theory - in particular, the study of interaction or interference between fuselage and lifting or control surfaces in supersonic flow.\r\n\r\nIn Sections 1.2 and 1.3 the non-planar problems are classified and the problem considered to be the fundamental wing body problem for linearized supersonic flow is presented. In Part II, this and related problems are formulated in a manner suitable for Laplace transform methods and subsequently the transformed solutions are presented in a general form by the Green's function method.\r\n\r\nDue to the inherent difficulties arising in non-planar problems, related planar problems are solved in Part III. In Part IV, the fundamental wing-body problem is discussed in detail and in the light of the results of Part III an approximate solution (in terms of the pressure) in the region of greatest interest is presented; and a quantitative estimate of the increase in lift due to the interaction between wing and body is indicated." }, { "name": "Nagamatsu, Henry Takeshi", "degree": "PhD", "year": "1949", "title": "Theoretical Investigation of Detached Shock Waves", "advisor": "Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-02092009-130132", "creators": [ { "name": { "family": "Nagamatsu", "given": "Henry Takeshi" }, "id": "Nagamatsu-Henry-Takeshi", "display_name": "Nagamatsu, Henry Takeshi" } ], "advisors": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HW36-ZP15", "abstract": "The problems associated with the detached shock wave are considered from the analytical standpoint in this report. For considering the general case for the detached shock wave, the nonstationary isentropic differential equation is derived. In general the stationary detached shock wave is curved and thus the flow back of the shock is rotational. The effect of rotational flow upon the velocity and pressure distribution over a circular cylinder is analyzed for a parabolic velocity distribution in the disturbed region.\r\n\r\nThe basic equations for both normal and oblique shock waves are presented and the significance of these equations to the problem of detached shock is discussed. The condition for the shock wave to be detached are presented and the mathematical formulation of the Tricomi type of differential equation for the detached shock wave is given.\r\n\r\nThe first approximation to the location of the detached shock wave is derived and the analytical results are correlated with the perimental data for spheres obtained from the supersonic wind tunnel and the ballistic range. The agreement was found to be satisfactory.\r\n\r\nThe existence and uniqueness of a potential solution for an infinite wedge with normal detached shock wave moving at constant velocity is presented. It is shown that, even for an infinite wedge with normal detached shock wave the potential solution does not exist." }, { "name": "Peterson, Norman Charles", "degree": "PhD", "year": "1949", "title": "Noncircular Fuselages in Supersonic Flow", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-02032009-125355", "creators": [ { "name": { "family": "Peterson", "given": "Norman Charles" }, "id": "Peterson-Norman-Charles", "display_name": "Peterson, Norman Charles" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JCZX-0749", "abstract": "The applicability of linearized theory to the aerodynamic study of slender, three-dimensional bodies in supersonic flow is considered in detail, and figures are presented which show the limitations of body shape and mach number to be observed if quantitatively reliable results are to be achieved. Then methods are developed and evaluated for calculating the supersonic flow about slender noncircular bodies, other than wings.\r\n\r\nSections I and II are concerned with the velocity and pressure predictions of the linearized theory. It is shown that these quantities do not converge to the corresponding predictions of the exact solutions for vanishing disturbance, and the reason therefore is found. In Section III the inapplicability of wing theory methods and the theory of slender circular bodies to the present study is reviewed, and the problem is carried to the fundamental nonrotationally symmetrical solutions of the wave equation; the properties of the noncircular functions are developed in Section IV. Section V contains a description of the Lorentz transformation for obtaining solutions singular on a yawed line, and Section VI a simple statement of the Gothert transformation for changing the Mach number.\r\n\r\nSections VII and VIII contain an exposition of the use of the theory developed in the previous Sections. Bodies in supersonic flow are classified according to size and orientation, an appropriate methods for each are presented and evaluated.\r\n\r\nThe Appendices present tables of the functions used in the analysis, along with sample computations." }, { "name": "Puckett, Allen Emerson", "degree": "PhD", "year": "1949", "title": "Supersonic Wave Drag of Thin Airfoils", "advisor": "von K\u00e1rm\u00e1n, Theodore; Stewart, Homer Joseph", "url": "https://resolver.caltech.edu/CaltechETD:etd-06232004-141931", "creators": [ { "name": { "family": "Puckett", "given": "Allen Emerson" }, "id": "Puckett-Allen-Emerson", "display_name": "Puckett, Allen Emerson" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "advisor", "display_name": "Stewart, Homer Joseph" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HYJJ-3241", "abstract": "The linearization of the equations of motion for the supersonic flow of a perfect fluid is discussed, and methods of solution using elementary source sink solutions are developed. These methods are applied to the calculation of performance of several types of three-dimensional supersonic airfoils; in particular, the drag at zero lift of a family of almost triangular, symmetrical wings is calculated. The significance of the results is discussed.\r\n" }, { "name": "Van Dyke, Milton Denman", "degree": "PhD", "year": "1949", "title": "A Study of Second-Order Supersonic Flow", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechTHESIS:12062017-085319714", "creators": [ { "name": { "family": "Van Dyke", "given": "Milton Denman" }, "id": "Van-Dyke-Milton-Denman", "display_name": "Van Dyke, Milton Denman" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MMKH-KT11", "abstract": "An attempt is made to develop a second approximation to the solution of problems of supersonic flow which can be solved by existing first-order theory. The method of attack adopted is an iteration procedure using the linearized solution as the first step.
\r\n\r\nSeveral simple problems are studied first in order to understand the limitations of the method. These suggest certain conjectures regarding convergence. A second-order solution is found for the cone which represents a considerable improvement over the linearized result.
\r\n\r\nFor plane and axially-symmetric flows it is discovered that a particular integral of the iteration equation can be written down at once in terms of the first-order solution. This reduces the second-order problem to the form of the first-order problem, so that it is effectively solved. Comparison with solutions by the method of characteristics indicates that the method is useful for bodies of revolution which have continuous slope.
\r\n\r\nFor full three-dimensional flow, only a partial particular integral has been found. As an example of a more general problem, the solution is derived for a cone at an angle. The possibility of treating other bodies of revolution at angle of attack and three-dimensional wings is discussed briefly.
" }, { "name": "Chapman, Dean Roden", "degree": "PhD", "year": "1948", "title": "Base Pressure at Supersonic Velocities", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12122008-143236", "creators": [ { "name": { "family": "Chapman", "given": "Dean Roden" }, "id": "Chapman-Dean-Roden", "display_name": "Chapman, Dean Roden" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F2ZC-4B47", "abstract": "The existing theories of base pressure are described in detail and are shown to be unsatisfactory. An \"exact\" analysis is then made of the base pressure in an inviscid fluid, both for two-dimensional and axially-symmetric flow. It is shown that for a given body there are, in general, an infinite number of possible solutions satisfying all necessary boundary conditions. For the particular case of inviscid flow about projectile-shaped bodies only one solution is possible, but it corresponds to zero base drag. This latter result is generalized and the following conjecture made: it is impossible for a steady axially-symmetric inviscid supersonic flow to converge toward, and to meet the axis at a finite (non-zero) angle.\r\n\r\nSince the inviscid-fluid theory does not adequately describe the conditions in a real fluid, an approximate theory for base pressure in a viscous fluid is developed. This latter theory is based in part on the inviscid-flow calculations and in part on dimensional analysis. It includes the effects of Mach number, Reynolds number, body shape, and type of boundary-layer flow. A comparison of the theory with the available experimental data indicates satisfactory agreement.\r\n\r\nIt is shown that under certain conditions the airfoil contour for minimum profile drag in a viscous fluid necessarily has a blunt trailing edge. Approximate calculations indicate that very substantial reductions in profile drag are possible by designing airfoils with blunt trailing edges. Consideration is briefly given to the interference of a support rod on base pressure measurements in a supersonic wind tunnel.\r\n" }, { "name": "Fung, Yuan-cheng", "degree": "PhD", "year": "1948", "title": "Elastostatic and Aereolastic Problems Relating to Thin Wings of High Speed Airplanes", "advisor": "Sechler, Ernest Edwin", "url": "https://resolver.caltech.edu/CaltechETD:etd-05222003-165709", "creators": [ { "name": { "family": "Fung", "given": "Yuan-cheng" }, "id": "Fung-Yuan-cheng-B", "display_name": "Fung, Yuan-cheng" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JK8Q-RH35", "abstract": "This report is concerned with the statics and dynamics of very thin wings of high speed airplanes. With the modern tendency towards sweepback, which is necessary for supersonic airplanes, the wing construction tend more and more to an ideal structure, hence for the static problem of this report, the wing is idealized to a thin cantilever elastic plate.
\r\n\r\nPart I gives a general formulation of the fundamental equations of deformation of thin elastic plates and the direct methods of solution. For small deflection of plates, the equations and boundary conditions are derived from the three-dimensional equations of elasticity developed in power series of the thickness of the plate. It is shown that the classical Poisson-Kirchhoff theory is coincident with the first approximation in this development. These equations are then transformed into oblique coordinates for treating problems concerning swept plates. Since the problem of the cantilever plate is very difficult to solve from the standpoint of biharmonic analysis, emphasis is laid on the direct methods of solution which lead to useful approximate solutions with desired accuracy. Section 1.21 discusses the relation between plate problems and equivalent variational problems. Section 1.22 contains a systematic review of the Rayleigh-Ritz method of relaxation of boundary conditions, including the Trefftz method as one instance.
\r\n\r\nPart II discusses the general aeroelastic problems of high speed airplanes. For airplanes accelerating or decelerating through the transonic region, the coefficients in the aeroelasticity equations are of transient nature. Such transient perturbations are new phenomena in aeronautics but are sufficiently important to warrant detailed investigation. A general mathematical treatment is given, though due to lack of aerodynamic data at present, no specific example is included. A general solution is obtained and this solution is expanded into a generalized power series which proves to be particularly useful when the transient perturbation is small. The present result includes the ordinary small perturbation theory for finite degrees of freedom as a particular case. Several results regarding small perturbations are given in section 2.6.
\r\n\r\nThe next two parts give a detailed computation on the deflection of and stresses in cantilever plates. The deflection of rectangular cantilever plates is solved both by the Rayleigh-Ritz method and the method of relaxation of boundary conditions. For swept plates the Rayleigh-Ritz method is used. A theory of stress approximation without using the intermediate deflection function is developed in Part IV, and is applied to rectangular plates.
" }, { "name": "Laufer, John", "degree": "PhD", "year": "1948", "title": "Investigation of Turbulent Flow in a Two-Dimensional Channel", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-09152005-133021", "creators": [ { "name": { "family": "Laufer", "given": "John" }, "id": "Laufer-John", "display_name": "Laufer, John" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6ZYC-HJ88", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nA detailed exploration of the field of mean and fluctuating quantities in a two-dimensional turbulent channel flow is presented. The measurements were repeated at three Reynolds numbers, 1.23 x 10\u2075, 3.08 x 10\u2075 and 6.16 x 10\u2075, based on the half width of the channel and the maximum velocity. A channel of 5\" width and 12:1 aspect ratio was used for the investigation.
\r\n\r\nMean speed and axial fluctuation measurements were made well within the laminar sublayer. The semi-theoretical predictions concerning the extent of the laminar sublayer were confirmed. It was found that the viscosity has a more profound influence on the fluctuations than on the mean velocity, the region of influence being approximately four times as wide.
\r\n\r\nFluctuations perpendicular to the flow direction v', w' and the correlation coefficient [...] were measured, and the turbulent shear distribution calculated. Shear calculations from independent methods using the measured velocity gradient at the wall and pressure gradient along the channel furnished a good check on the values of the shearing stress in all cases with the exception of the highest Reynolds number where \u03a4 obtained from the fluctuation measurements is approximately 25% lower. All mean fluctuating quantities were found to decrease with increasing Reynolds number. Measurements of the scales Ly, Lz and micro-scales of turbulence \u03bby, \u03bbz across the channel are presented and their variation with Reynolds number is discussed. Using a new technique, values for \u03bbx were obtained; a method for estimating Lx is also given.
\r\n\r\nThe energy balance in the turbulent flow field was calculated from the measured quantities. From this calculation it is possible to give a descriptive picture of turbulent energy diffusion in the center portion of the channel cross-section.
" }, { "name": "Marble, Frank Earl", "degree": "PhD", "year": "1948", "title": "Some Problems Concerning the Rotational Motion of a Perfect Fluid", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-104144", "creators": [ { "name": { "family": "Marble", "given": "Frank Earl" }, "id": "Marble-Frank-Earl", "display_name": "Marble, Frank Earl" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2JGF-0Z67", "abstract": "In an effort to obtain some understanding of the processes involved in the rotational motion of a perfect fluid several particular linearized examples of rotational flow are solved in detail. The first part discusses some types of boundary value problem which arise. The solution of the non-linear partial differential equation by a particular iteration process is considered and the process is shown to converge for an extended version of the problem when the vorticity distribution is sufficiently smooth. The first step of the iteration process may constitute a good approximation in these cases and is taken as the basis of linearized solutions studied in the remainder of the work.\r\n\r\nThe process of straightening a non-uniform velocity profile by means of an idealized screen is considered in Part II as a problem in rotational motion of an ideal fluid with the screen replaced by an appropriate non-conservative force field. The detailed solution is given for both the linearized problem and the second approximation, The complete second order correction is less than 6 percent of the local velocity given by the linear solution for a rather severe case, The corrections arising from the various physical processes involved are analyzed and found to exceed 6 percent in same cases but are inherently compensating.\r\n\r\nThe two-dimensional rotational flow about a closed body is obtained in Part III by utilizing the Green's function method of solving the inhomogeneous differential equation involved. The conformal transformation which maps the given contour into a circle is used to find the appropriate Green's function for the contour. Solutions are then written down for any body, the Riemann mapping function of which is known, The Blasius force and moment formula are extended to include the case of general rotational motion, the relations of Kuo appearing as special forms where the vorticity distribution is uniform.\r\n\r\nIn the final part the theory of the three-dimensional. flow through an axial turbomachine, associated with variation of circulation along the blade length, is described as an extension of the classical theory of finite wings and is simplified to a problem in axially symmetric rotational fluid motion by considering an infinite number of blades in each row. The linearized problem is solved for the radial, tangential, and axial velocity components induced by a single row of stationary or rotating blades with finite chord and prescribed loading. The particular case for which the blade chord approaches zero, and the tangential velocity changes discontinuously, is associated with the theory of the Prandtl lifting line for finite wings, The complete solution is given for a single stationary or rotating blade row of given loading with a hub/tip ratio of 0.6 and blade aspect ratio of 2. The corresponding discontinuous approximation is compared with the more nearly exact solution and is shown to constitute a useful approximation to the solution for a finite blade chord when the discontinuity is located appropriately. An exponential approximation for the velocity components, deduced from the analysis, allows rapid estimation of the rate at which the equilibrium velocity profiles develop ahead of and behind a blade row and, using the superposition principle, provides a simple means or approximating the velocity distribution in a multistage turbomachine and of discussing mutual interference of blade rows." }, { "name": "Trilling, Leon", "degree": "PhD", "year": "1948", "title": "Investigation into the Flow of a Viscous Heat Conducting Compressible Fluid", "advisor": "Lagerstrom, Paco A.", "url": "https://resolver.caltech.edu/CaltechETD:etd-12242008-101248", "creators": [ { "name": { "family": "Trilling", "given": "Leon" }, "id": "Trilling-Leon", "display_name": "Trilling, Leon" } ], "advisors": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "advisor", "display_name": "Lagerstrom, Paco A." } ], "committee": [ { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "chair", "display_name": "Lagerstrom, Paco A." }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Marble", "given": "Frank E." }, "id": "Marble-F-E", "role": "member", "display_name": "Marble, Frank E." }, { "name": { "family": "Cole", "given": "Julian D." }, "id": "Cole-J-D", "role": "member", "display_name": "Cole, Julian D." } ], "option_major": [ "aeronautics" ], "doi": "10.7907/14DR-5V68", "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nThe present investigation is concerned with the effect of a small viscosity and heat conduction coefficient on the flow of a compressible fluid. It is well known that, in the case of an incompressible fluid, such an investigation leads to the boundary layer theory.\r\n\r\nThe chief purpose of this paper is to determine whether the main result of boundary layer theory, namely, that viscosity plays a negligible part in the flow outside a very narrow region in the immediate vicinity of any solid boundary in the fluid, is still valid for a compressible fluid. To investigate that point, a very simple type of flow is selected: the flow past a semi-infinite two-dimensional flat plate parallel to the main stream direction. The problem is further simplified as follows: on the basis of experimental results, the existence of a layer influenced by viscosity is assumed, and the boundary conditions are applied near the outer edge of this layer. This allows a linearization of the equation of motion, and gives information on the interaction between the outer edge of this layer and the main field of flow.\r\n\r\nThe analysis is carried out by the methods based on the theory of the Laplace Transformation. The results are essentially, that if the flow is subsonic, the boundary layer theory developed for incompressible fluids may be extended without qualitative changes. However, in a supersonic flow, one must expect two related effects: one finds the boundary layer, which, as a first approximation, in similar to the boundary layer of an incompressible fluid, and a shock-wave along the Mach line which starts at the leading edge of the flat palate, and whose strength is given by the expression:\r\n[...] where [...] is the normal velocity across the shock, M is the free stream Mach number, [...] is the distance from the leading edge of the flat plate along the shock, [...] is the distance normal to the shock, [...] is sonic velocity of the free stream and [...] is the mean effective free stream kinematic viscosity of the fluid." }, { "name": "Ballhaus, William Francis", "degree": "PhD", "year": "1947", "title": "Aerodynamic and Geometric Parameters Affecting Aircraft Weight", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-10302003-152006", "creators": [ { "name": { "family": "Ballhaus", "given": "William Francis" }, "id": "Ballhaus-William-Francis", "display_name": "Ballhaus, William Francis" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/T60T-F120", "abstract": "A number of single engine conventional aircraft are studied to investigate the possibility of applying statistical methods to the problem of aircraft weight estimation. It is shown that the statistical treatment is definitely useful. It is also shown that, without proper care and judgment, such a statistical treatment leads to somewhat misleading results. The need for structural and aerodynamic training and experience together with essential weight estimating experience is evident. Such experience is requisite to arriving at proper weight estimates when basing these estimates upon the weights of aircraft which have previously been designed and built.
\r\n\r\nIt was hoped that the effects of all of the important aerodynamic and geometric parameters upon aircraft weight would be found from this study of successful single engine aircraft. It was further hoped that the results of this study would be applicable to the difficult problem of estimating the weight of new aircraft. The number of aircraft in the sample for which pertinent information was available was not large enough to permit the evaluation of the effects of all of the parameters although most of the important parameters and their essential effects have been indicated. Lack of complete information due to the restricted or confidential nature of the data was one of the most troublesome handicaps. The latter problem of obtaining weight estimating procedure has, however, been solved since satisfactory weight estimating formulas have been developed for use in basic design weight estimates.
\r\n\r\nIt is this latter result for which the entire study was made. That is, the study was initiated to offer assistance to practicing preliminary design engineers by providing usable information concerning the effects of such factors as gross weight, load factor, and wing span upon aircraft weight. Such information should be invaluable to assist in arriving at the optimum aircraft design with respect to performance, maneuverability, and utility.
\r\n\r\nAll the useful estimating formulas are summarized on page 43. Also included are the calculated probable errors of estimate.
" }, { "name": "Corrsin, Stanley", "degree": "PhD", "year": "1947", "title": "I. Extended Applications of the Hotwire Anemometer. II. Investigations of the Flow in Round, Turbulent Jets", "advisor": "Liepmann, Hans Wolfgang", "url": "https://resolver.caltech.edu/CaltechETD:etd-12092008-105044", "creators": [ { "name": { "family": "Corrsin", "given": "Stanley" }, "id": "Corrsin-Stanley", "display_name": "Corrsin, Stanley" } ], "advisors": [ { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "advisor", "display_name": "Liepmann, Hans Wolfgang" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/NB53-K411", "abstract": "Part 1:\r\n\r\nTwo new fields of application of the hot-wire anemometer are proposed, and the appropriate response equations and measuring procedures are developed.\r\n\r\nThe first analysis leads to a method for the measurement of physically significant statistical quantities in a turbulent flow with heat transfer: for example the turbulence levels, the temperature fluctuation level, the turbulent heat transfer coefficient, the velocity scale, the temperature scale and some spectra.\r\n\r\nThe second analysis involves the use of the hot-wire in the turbulent isothermal mixing of two appropriately different gases. If the thermal conductivity of the mixture is known and is a monotonic function of the relative concentration, it is possible to measure the mean velocity and mean concentration at any point. If no data are available on the thermal conductivity of the mixture, this additional unknown can be determined by an additional measurement. Furthermore, it is also possible to measure the various statistical functions of the fluctuating velocities and the local concentration fluctuation, provided, again, that the thermal conductivity is a known monotonic function of the concentration.\r\n\r\nAlthough the details of the present analysis are dependent upon the accuracy of King's equation for the rate of heat loss from fine wires, the general approach is equally valid for any (possibly more accurate) equation that may be deduced.\r\n\r\nPart 2: \r\n\r\nA detailed investigation has been made of the flow in a round turbulent air jet, heated slightly to permit measurement of mean temperature.\r\n\r\nOscillograms of the velocity fluctuation plus direct measurement of the turbulent shear both show that the flow in a fully developed \"turbulent\" jet is completely turbulent only out to approximately the radius at which the extreme outer edge is in the nature of a \"laminar collar\", with predominantly radial (inward) mean velocity, and in between the turbulent core and the laminar collar is a rather wide annular transition region.\r\n\r\nA study of the downstream history of the radial distribution of turbulent velocity shows that the fully developed state of this round jet is reached between 15 and 20 diameters. This conclusion is corroborated by examination of the partition between turbulent motion and mean motion, of total kinetic energy crossing planes perpendicular to the axis in unit time.\r\n\r\nThe directly measured shear distribution is checked roughly by a computation of the same quantity from the mean velocity distribution.\r\n\r\nA measurement of the double correlation function between points symmetrical about the jet axis shows considerable similarity to the corresponding function in isotropic turbulence, and permits calculation of scale and microscale.\r\n\r\nWith the assumption of constant microscale across a section, a rough estimate is made of the energy balance distribution of production, dissipation and diffusion of turbulent energy.\r\n\r\nA comparison with momentum transfer, modified vorticity transfer and constant exchange coefficient theories show that none of them is satisfactory.\r\n\r\nA comparison of mean velocity and temperature distributions verifies Ruden's result that the lateral rate of heat transfer in turbulent shear flow is appreciably greater that the lateral rate of momentum transfer.\r\n\r\nThe use of considerably increased heating, in a second jet unit, has permitted direct measurement of the temperature fluctuation level. Velocity fluctuations were also measured in this case for comparison, and they were found to be the same order of magnitude.\r\n\r\nThe final result is the direct measurement of temperature-velocity correlation and of velocity correlation in the hot jet. This gives a direct measure of the turbulent heat transfer and momentum transfer in the jet, and directly verifies the fact that the former is appreciably greater than the latter.\r\n" }, { "name": "Hayes, Wallace Dean", "degree": "PhD", "year": "1947", "title": "Linearized Supersonic Flow", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-04132007-131650", "creators": [ { "name": { "family": "Hayes", "given": "Wallace Dean" }, "id": "Hayes-Wallace-Dean", "display_name": "Hayes, Wallace Dean" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "chair", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Liepmann", "given": "Hans Wolfgang" }, "id": "Liepmann-H-W", "role": "member", "display_name": "Liepmann, Hans Wolfgang" }, { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-H-J", "role": "member", "display_name": "Stewart, Homer Joseph" }, { "name": { "family": "Lagerstrom", "given": "Paco A." }, "id": "Lagerstrom-P-A", "role": "member", "display_name": "Lagerstrom, Paco A." } ], "option_major": [ "physics" ], "doi": "10.7907/D7N0-4412", "abstract": "This thesis is a presentation of the methods and concepts of the theory of linearized supersonic flow. The fundamental theory which serves as a basis for this investigation is discussed in the first two chapters. Special emphasis is placed upon the study of planar systems.\r\n\r\nA system of conical coordinates is introduced in which the method of separation of variables is applied. The resultant solutions have the Mach cone as a natural boundary and involve a family of hypergeometric functions related to the Legendre functions.\r\n\r\nBasic integral relations for planar systems are obtained between the normal velocity component and the component giving the pressure. The behavior of planar systems relative to the planform configuration is discussed and the concept of problems of the first and second kind is introduced. The lift problem is treated with particular reference to the behavior of the leading edge singularity and to the concept of the Kutta condition as applied to a planform in supersonic flow.\r\n\r\nThe nature of drag in linearized supersonic systems is investigated and the separation of the drag into types is discussed. For planar systems the drag may be divided into basic and induced parts. For general systems the basic division may be made into wave drag and vortex drag. Two fundamental reversed flow theorems are obtained which state that the drag of a system is the same as that of the system with the flow reversed in direction.\r\n\r\nThe theory of conical flow as applied to planar systems is developed and the results for a basic thickness distribution and various lifting triangles are presented.\r\n\r\nThe method of the separation of the lateral variable is investigated using Schlomilch series.\r\n\r\nThe flow about bodies of revolution is discussed and the application of the Riemann method to the problem is given." }, { "name": "Schamberg, Richard", "degree": "PhD", "year": "1947", "title": "The Fundamental Differential Equations and the Boundary Conditions for High Speed Slip-Flow, and their Application to Several Specific Problems", "advisor": "Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-12272004-161736", "creators": [ { "name": { "family": "Schamberg", "given": "Richard" }, "id": "Schamberg-Richard", "display_name": "Schamberg, Richard" } ], "advisors": [ { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/F44D-FG36", "abstract": "The differential equations of motion and the associated boundary conditions for the slip-flow regime of fluid mechanics are derived from the point of view of the kinetic theory of non-uniform gases. The slip-flow regime comprises the flow of gases whose molecular mean free path is smaller than but not negligible relative to the macroscopic dimension characterizing the gas flow.\r\n\r\nA systematic review is presented of the methods of Hilbert and Burnett for obtaining a successive approximation solution to the Boltzmann integro-differential equation. The complete second approximation to the molecular velocity distribution function is calculated for the molecular model of Maxwell. This molecular distribution function is employed for the derivation of the macroscopic differential equations of motion and the associated boundary conditions. It is shown that the same number of boundary conditions are required for slip flows as for gas-dynamical flows, although the differential equations of motion for slip flows are of higher order than those of continuum gas-dynamics. Expressions for the second approximations to the slip velocity and temperature jump are obtained.\r\n\r\nThe general equations obtained are applied to three specific problems: the propagation of sound waves in rarefied gases, high-speed Couette flow of a rarefied gas, and slip-flow between concentric cylinders in relative rotary motion. It is found that the rarefaction of a gas increases the damping of sound waves, whereas the propagation speed differs from the ordinary adiabatic sound velocity by less than two percent. The Couette flow solution indicates that the slippage of gas and the temperature discontinuity at a solid boundary may reduce the gas-dynamical friction coefficient and heat transfer, respectively, by ten percent under approximate conditions. When applied to the flight of aircraft through the rarefied atmosphere, the theory presented is applicable to an altitude range from 100,000 to 300,000 feet.\r\n" }, { "name": "Charyk, Joseph Vincent", "degree": "PhD", "year": "1946", "title": "Condensation Phenomena in Supersonic Flows", "advisor": "von K\u00e1rm\u00e1n, Theodore; Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-11062003-165637", "creators": [ { "name": { "family": "Charyk", "given": "Joseph Vincent" }, "id": "Charyk-Joseph-Vincent", "display_name": "Charyk, Joseph Vincent" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RSQH-W442", "abstract": "With the very rapid development of jet propulsion systems, the attainment of speeds which seemed to be well beyond reach a few years ago now appears to be at hand. The war has produced many examples of guided missiles traveling at supersonic speeds such as the famed German A-4 rocket, more commonly referred to in this country as the V-2. Today a supersonic airplane is no longer a designer's dream but practically an accomplished fact. Despite the impressive array of symbols of apparent mastery of high speed flight, there exists a large gap of fundamental knowledge that the theoretician working with the experimentalist must fill before true mastery of transonic and supersonic speeds can be said to be at hand. It was only due to the efforts of pioneers in high speed fluid mechanics like de Laval, Riemann, Hugoniot, Lord Rayleigh and Tschaplygin (see for example Ref. 1 to 4) and later the applications of such basic knowledge to the new field of high speed aerodynamics by men with foresight such as Prandtl, Ackeret, von Karman Taylor and Busemann (see for example Ref. 5 to 9) that tools for the engineer and designer were available when the need for them suddenly arose.
\r\n\r\nToday the emphasis of the aeronautical profession is on the basic problems of transonic and supersonic flows. One of the important and at first mystifying phenomena that emerged from experimental investigations in supersonic wind tunnels was the condensation shock. Later such shocks were noticed in the flow over an airfoil in experiments conducted at the California Institute of Technology by Kate Liepmann in 1941. In more recent years their appearance has been noted in actual flight at high speeds. The importance of such shocks in connection with the aerodynamic characteristics of airfoils in supercritical transonic flow has been pointed out by Tsien and Fejer (Ref. 10). Apparently, however, no detailed investigation of the phenomenon has been made with a view to studying the fundamental aspects of the condensation shock in order to develop practical methods for predicting the occurrence, location, strength and effect of such shocks. It has been the basic purpose of this research to study the detailed aspects of this problem and to endeavor to develop a means of accomplishing the aims noted. It is felt that though crude in many respects, the results of this investigation can provide practical knowledge of basic importance in understanding and treating the problems of condensation shocks when they appear and can point the way towards more refined and detailed future analyses of this problem.
\r\n\r\nIn attacking this problem, an examination of the phenomenon of the sudden collapse of the supersaturated state of the moist air is first made. The assumptions necessary for the determination of the critical stability limit of the supersaturated air are analyzed and the necessity for further investigation, especially from the kinetic point of view, is pointed out. The study reveals that the temperature of air at which this collapse occurs is approximately a function only of the amount of water contained in the air and does not depend upon the pressure. This enables an important simplification in the analysis to be made.
\r\n\r\nThe condition for collapse of the supersaturated state is then applied to the special case of normal condensation shocks. Because of this relatively small amount of water present in air, the effect of the presence of the water on the properties of the air can be neglected except at the shock where the release of the latent heat of vaporization upon condensation is of vital importance. A simple consideration of this heating process yields the interesting result that the flow after the shock must always be supersonic. An important simplification in treating the general condensation problem is an approximation to the actual saturation vapor pressure versus temperature curve by means of an exponential curve. Mathematically this means an approximate integration of the Clausius-Clapeyron equation in the sense that the specific volume of the fluid phase is neglected as compared to the specific volume of the vapor phase. This simplification enables a closed form solution to be obtained.
\r\n\r\nThe oblique condensation shock is then analyzed and its application to the flow over an airfoil or other body in a stream of moist air is treated.
\r\n\r\nThe possibility of a continuous condensation instead of an abrupt condensation of a combination of the two is discussed for the case of a one-dimensional flow. Certain interesting results emerge from such a consideration and experimentation will be required to determine whether under certain conditions such a condensation process can take place.
\r\n\r\nA considerable number of charts are provided which may be of use in making calculations in practical cases. In instances where a different range of values is necessary, additional charts can readily be constructed.
" }, { "name": "Fej\u00e9r, Andrej (Andrew)", "degree": "PhD", "year": "1945", "title": "Lifting Line Theory in Linearly Varying Flow", "advisor": "von-K\u00e1rm\u00e1n, Theodore; Tsien, Hsue Shen", "url": "https://resolver.caltech.edu/CaltechETD:etd-11202008-133148", "creators": [ { "name": { "family": "Fej\u00e9r", "given": "Andrej (Andrew)" }, "id": "Fej\u00e9r-Andrej", "display_name": "Fej\u00e9r, Andrej (Andrew)" } ], "advisors": [ { "name": { "family": "von-K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von-K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-H-S", "role": "advisor", "display_name": "Tsien, Hsue Shen" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EJJN-XF31", "abstract": "The problem of a wing placed in a flow that is nonuniform in the spanwise direction is of interest to the aerodynamicist as well as to the designer of rotating machinery. The portion of a wing located in the slip stream of the propeller, the blades of propellers and fans, wind-tunnel models in nonuniform airstreams are some of the examples. Prandtl's three dimensional wing theory assumes a uniform undisturbed flow and thus it is not valid in these cases. Some authors tried to modify Prandtl's theory in order to make it applicable to such problems. The work of F. Vandrey (Ref. 1) and K. Bausch (Ref. 2) may be mentioned in this connection. A discussion of their work is given in Section 7 of Part I of this analysis. Lately Th. von Karman and H. S. Tsien presented a general solution of the problem, of a wing placed in a flow with the velocity varying in both directions normal to and parallel with the wing span (Ref. 3). In the following the author will utilize the results of von Karman and Tsien for the simpler case of the velocity varying linearly and in the direction of the span only and solve the \"third problem of airfoil theory\" (finding the lift distribution for an airfoil of given shape) for a finite wing in an infinite fluid and for the case of a wing between two parallel walls. The wing in an infinite fluid is treated in Part I while Part II contains the solution of the problem of the wing between walls.\r\n" }, { "name": "Kuo, Yung-Huai", "degree": "PhD", "year": "1945", "title": "Two-Dimensional Irrotational Mixed Subsonic and Supersonic Flow of a Compressible Fluid and the Upper Critical Mach Number", "advisor": "von-K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechTHESIS:08172010-111715951", "creators": [ { "name": { "family": "Kuo", "given": "Yung-Huai" }, "id": "Kuo-Yung-Huai", "display_name": "Kuo, Yung-Huai" } ], "advisors": [ { "name": { "family": "von-K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von-K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/D9ME-DD61", "abstract": "No abstract." }, { "name": "Boehnlein, Charles Thompson", "degree": "PhD", "year": "1944", "title": "Aerodynamic Theory of the Oscillating Wing-Aileron of Finite Span", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechTHESIS:04142011-111534450", "creators": [ { "name": { "family": "Boehnlein", "given": "Charles Thompson" }, "id": "Boehnlein-Charles-Thompson", "display_name": "Boehnlein, Charles Thompson" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KJZ7-FG07", "abstract": "No abstract." }, { "name": "Lin, Chia-Chiao", "degree": "PhD", "year": "1944", "title": "On the Development of Turbulence", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-06232004-140148", "creators": [ { "name": { "family": "Lin", "given": "Chia-Chiao" }, "id": "Lin-Chia-Chiao", "display_name": "Lin, Chia-Chiao" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZF3F-AN21", "abstract": "The stability of two-dimensional parallel flows of an incompressible fluid is investigated, based upon a study of the equation of Orr and Sommerfeld along the lines initiated by Heisenberg. The theory of Heisenberg is carefully examined and further developed to obtain several general and specific results on hydrodynamic stability. Most of the disputes in the existing theories are clearly brought out and carefully settled. It is further shown that all symmetrical and all boundary-layer types of velocity distributions are unstable above a certain minimum critical Reynolds number, whose approximate value can be easily calculated from equations (12.24) and (12.25) respectively. General characteristics of the curve of neutral stability are obtained (Fig. 9). Complete numerical calculations of this curve have been carried through for the plane Poiseuille flow and the Blasius flow. In the first case, the minimum critical Reynolds number is found to be 16000, based upon the maximun velocity and the width of the channel. In the second case, the number is 400, based upon the free stream velocity and the displacement thickness of the boundary layer. Physical interpretations of the results obtained are given, based upon the conservation of vorticity in a perfect fluid and its diffusion by viscous forces. Indications are also given to connect the stability theory with Taylor's theory of transition to turbulence. It is hoped that this work may remove all the doubts of applying the theory of small oscillations to the treatment of hydrodynamic stability using Navier-Stokes equations for an incompressible fluid.\r\n" }, { "name": "Yuan, Shao Wen", "degree": "PhD", "year": "1942", "title": "Thin Cylindrical Shells Subjected to Various Types of Concentrated Loads", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-04232008-152804", "creators": [ { "name": { "family": "Yuan", "given": "Shao Wen" }, "id": "Yuan-Shao-Wen", "display_name": "Yuan, Shao Wen" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QSE5-ZP85", "abstract": "The bending of thin cylinder shells based on the general theory of elasticity is of interest not only to the mathematician but also to the engineer. The general theory of the shells has recently been developed to the point that it is now being used by engineers as a basis for the design of this type of structure.\r\n" }, { "name": "Dunn, Louis Gerhardus", "degree": "PhD", "year": "1940", "title": "An Investigation of Sheet-Stiffener Panels Subjected to Compression Loads with Particular Reference to Torsionally Weak Stiffeners", "advisor": "Sechler, Ernest Edwin; von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-04202004-154649", "creators": [ { "name": { "family": "Dunn", "given": "Louis Gerhardus" }, "id": "Dunn-Louis-Gerhardus", "display_name": "Dunn, Louis Gerhardus" } ], "advisors": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/84RM-ZH50", "abstract": "A total of 183 panel specimens of 24ST alloy with nominal thicknesses of 0.020, 0.025, and 0.040 inch with extruded bulb-angle sections of 12 shapes spaced 4 and 5 inches as stiffeners were tested to obtain the buckling stress and the amplitude of the maximum wave when buckled. Bulb angles from 3 to 27-1/2 inches long were tested as pin-end columns. The experimental data are presented as stress-strain and column curves and in tabular form. Some comparisons with theoretical results are presented.
\r\n\r\nAnalytical methods are developed that make it possible for the designer to predict with reasonable accuracy the buckling stress and the maximum-wave amplitude of the sheet in stiffened-panel combinations. The scope of the test was insufficient to formulate general design criteria but the results are presented as a guide for design and an indictaion of the type of theoretical and experimental work needed.
" }, { "name": "Malina, Frank Joseph", "degree": "PhD", "year": "1940", "title": "Characteristics of the Rocket Motor and Flight Analyses of the Sounding Rocket", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechTHESIS:08202010-143142441", "creators": [ { "name": { "family": "Malina", "given": "Frank Joseph" }, "id": "Malina-Frank-Joseph", "display_name": "Malina, Frank Joseph" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/MF80-9W14", "abstract": "No abstract." }, { "name": "Pai, Shih-I", "degree": "PhD", "year": "1940", "title": "On Turbulent Flow Between Rotating Cylinders", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-01092006-143245", "creators": [ { "name": { "family": "Pai", "given": "Shih-I" }, "id": "Pai-Shih-I", "display_name": "Pai, Shih-I" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6VD9-HY80", "abstract": "The present work had as its purpose the investigation of the velocity and turbulence distributions in the turbulent flow between two co-axial cylinders, the inner of which rotated. The radius of the inner cylinder was 7-7/8\". Two aspect ratios were used, one 10\" to 1-1/16\", the other 10\" to 17/32\". All velocity and turbulence measurements were made with hot-wire technique, using a special holder which reduced spindle interference to a minimum.\r\n\r\nThe circumferential velocity of the inner rotating cylinder was several hundred times the critical speed for instability of the laminar flow as given by G. I. Taylor.\r\n\r\nIt was found that velocity distributions in which the circumferential velocity of the fluid increases outward actually exist and cannot be explained by the shadow effect of the measuring instrument as G. I. Taylor suggested. Also the flow between the rotating cylinders is three-dimensional. The mean velocity distribution depends on whether the transfer of shear is carried out by turbulent fluctuations or by secondary motion. The regions, where the secondary motion is negligible, the mean velocity distribution is logarithmic which was shown by Dr. von Karman to be the distribution in Couette's case of turbulent flow.\r\n\r\nThe analysis of the measurements indicated that the secondary flow can be described by assuming ring-shaped vortices between the cylinders. It was known that such ring-shaped vortices introduce the instability of the laminar flow (see Ref. 1); but it was not known that such vortices exist at speeds as high as several hundred times the critical speed for instability, although their shapes are distorted. The sizes of the vortices and their numbers depend on the speed of the inner cylinder. The vortices can only exist in pairs, and at certain critical speeds the flow pattern may change suddenly due to the loss or gain of a pair of vortices. At high speeds the number of the vortices tends to decrease.\r\n\r\nThe two-dimensional theories for turbulent flow between rotating cylinders are discussed. They can not even be taken as a first approximation for this three dimensional vortex-ring flow." }, { "name": "Stewart, Homer Joseph", "degree": "PhD", "year": "1940", "title": "The Effect of Shear Instability on the Transverse Circulation in the Atmosphere", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-05222003-084939", "creators": [ { "name": { "family": "Stewart", "given": "Homer Joseph" }, "id": "Stewart-Homer-Joseph", "display_name": "Stewart, Homer Joseph" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/1FW3-YN51", "abstract": "In this paper it is shown that the shear fields on either side of the westerlies are dynamically unstable and will roll up to form discrete eddies. A study of the stable vortex systems into which these eddies might collect shows that a \"vortex street\" is stable for a certain range of the ratio of width to vortex spacing. It is also shown that a formation with vortices placed on the corners of a regular polygon is stable if the number of vortices is less than seven. This still holds if the effect of the shear field north of the westerlies is stimulated by a fixed polar cyclone. It further appears that with a strong general circulation, i.e. strong westerlies, only two or three high pressure cells should be found while the numbers, up to six, should be found with weaker circulations. This conclusion is verified by Northern Hemisphere mean pressure charts." }, { "name": "Atsumi, John Shoichi", "degree": "PhD", "year": "1939", "title": "Correlation and Decay of Turbulence Produced by Various Grids in a Wind Tunnel", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechTHESIS:12092024-180737116", "creators": [ { "name": { "family": "Atsumi", "given": "John Shoichi" }, "id": "Atsumi- John-Shoichi", "display_name": "Atsumi, John Shoichi" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/yq2d-wt47", "abstract": "No abstract." }, { "name": "Howland, Walter Lavern", "degree": "PhD", "year": "1939", "title": "Strength of Thin-Walled Elliptical Cylinders Supported at the Minor Axis", "advisor": "von K\u00e1rm\u00e1n, Theodore; Sechler, Ernest Edwin; Millikan, Clark Blanchard; Klein, Arthur Louis", "url": "https://resolver.caltech.edu/CaltechTHESIS:10012015-161848918", "creators": [ { "name": { "family": "Howland", "given": "Walter Lavern" }, "id": "Howland-Walter-Lavern", "display_name": "Howland, Walter Lavern" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "advisor", "display_name": "Sechler, Ernest Edwin" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Klein", "given": "Arthur Louis" }, "id": "Klein-A-L", "role": "advisor", "display_name": "Klein, Arthur Louis" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/KQK7-V396", "abstract": "In this investigation it was found that the\r\ninstability failure of curved sheet is nearly\r\nindependent of the type of loading and is primarily\r\na function of the maximum stress, radius-thickness\r\nration and modulus of elasticity. A method of\r\ncorrelating the critical stress of thin sheet\r\nunder several different types of loading is given.\r\nAn explanation for the experimental critical stress\r\nof thin walled cylinders under bending being\r\ngreater than that for pure compression is given.\r\nThe strength of unstiffened thin walled circular\r\nnose sections under pure bending was found to be \r\ncontrolled by local instability of the section,\r\nrather than a large scale instability. The equation\r\nof local instability of curved sheet gives values\r\nwhich are in fair agreement with those found experimentally.
\r\n\r\nThe strength of elliptical cylinders supported\r\nat the minor axis under bending plus shear loads is\r\ngoverned primarily by the bending strength, and is\r\nlittle effected by the sheer force unless the amount\r\nof shear is quite large with respect to the moment.\r\nThe effect of increasing the amount of elliptically\r\ngreatly reduces the bending and shear strength of nose\r\nsections. Under torsional loads the stress at\r\nbuckling falls off as the ration of the major to minor \r\naxis increases but the failure stress decreases\r\nat a slower rate than the buckling stress. The\r\nlength effect of semi-circular sections under torsion\r\nis similar to that of a circular tube, and can be\r\nobtained by Donnell's theoretical equation.
\r\n" }, { "name": "Knoblock, Frederick Delbridge", "degree": "PhD", "year": "1939", "title": "Investigations on the Applications of the Hot Wire Anemometer for Turbulence Measurements", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-04252008-094935", "creators": [ { "name": { "family": "Knoblock", "given": "Frederick Delbridge" }, "id": "Knoblock-Frederick-Delbridge", "display_name": "Knoblock, Frederick Delbridge" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/DKG6-T043", "abstract": "The principle reasons for using the hot wire type of anemometer for turbulence measurements are outlined and some of the objectionable features limiting the applicability of the conventional type anemometer are pointed out. A brief historical background of the hot wire anemometer is followed by a consideration of various possibilities of improving the method of measuring the intensity of turbulence. This analysis purposes to show why such improvements can only be realized by a direct calibration of the hot wire\u2019s response to velocity fluctuations.\r\n\r\nThe consequent development of a new technique, referred to as the \u201cVibrator Method,\u201d is then explained. A general discussion of the results of a series of investigations using the Vibrator method is then presented and includes: a comparison with the orthodox method; the response of the hot wire to periodic fluctuations relative to the validity of the theory of hot wire lag; an extension of the method to correlation measurements using two hot wires, and other applications.\r\n\r\nThe paper concludes with a detailed description of various hot wire anemometer equipment which has been developed for use with the Vibrator method of measuring turbulence intensity.\r\n" }, { "name": "Lombard, Albert Eaton", "degree": "PhD", "year": "1939", "title": "An Investigation of the Conditions for the Occurrence of Flutter in Aircraft and the Development of Criteria for the Prediction and Elimination of Such Flutter", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-07182007-091133", "creators": [ { "name": { "family": "Lombard", "given": "Albert Eaton" }, "id": "Lombard-Albert-Eaton", "display_name": "Lombard, Albert Eaton" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/80H8-TB52", "abstract": "A review is presented of the work done by other investigators on the effects of inertia couplings in producing flutter in control surfaces that are not mass balanced. The conclusion is reached that for the prevention of such flutter complete dynamic balance should always maintain.\r\n\r\nFlexural-torsional flutter is investigated in considerable detail from the consideration of the dynamical equations for steady state forced oscillations of the two dimensional case. A complete set of response curves for two typical cases are included to show the types of responses that should be observed in flight with vibration pick-up equipment. The important fact is brought out that the response and behavior of the wing at its natural bending frequency has little or no correlation with the behavior of the wing at the stability limit of flutter. Curves are presented to show that, for normal airplanes, the most important parameters which determine flutter in this mode are (a) the position of the inertia axis, (b) the torsional frequency, end (c) the radius of gyration of the wing mass about the inertia axis.\r\n\r\nThe dynamical equations are set up for the cases of flexural-aileron, torsional-aileron, and flexural-torsional-aileron flutter in the two-dimensional case and an example is given of the determination of the stability limit of a specific example of the first of these modes.\r\n\r\nAn extension of the two-dimensional case to the three-dimensional case is presented with particular reference to determining the flexural-torsional flutter speed of a tail surface with vertical surfaces on the tips of the horizontals. The method of attack is outlined for the calculation of natural frequencies at zero airspeed to use in determining the flutter speed.\r\n\r\nStatistical data in a graphical form show the variations of natural frequencies of the various components of airplanes with the size of such airplanes.\r\n\r\nThe conclusion is reached that the speed of airplanes should be restricted to two-thirds of the critical speed for any mode of flutter, divergence, or aileron reversal." }, { "name": "Schairer, Robert Sorg", "degree": "PhD", "year": "1939", "title": "Unsymmetrical Lift Distributions on a Stalled Monoplane Wing", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-12282005-110200", "creators": [ { "name": { "family": "Schairer", "given": "Robert Sorg" }, "id": "Schairer-Robert-Sorg", "display_name": "Schairer, Robert Sorg" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/3WX9-JZ33", "abstract": "The purpose of the research was to find if it is possible to have an unsymmetrical span load distribution on a monoplane wing at the stall without a rolling velocity. The investigation is based upon the Prandtl first order wing theory. It is shown that it is possible to have an unsymmetrical lift distribution at the stall if a certain shape is assumed for the curve of section lift coefficient vs. effective angle of attack. Two methods of determining such a solution for any particular wing shape are discussed. Curves are presented showing unsymmetrical solutions for a number of different wing shapes and also showing the resulting lift and rolling moment coefficients. The important conclusions are as follows:\r\n\r\n1. An unsymmetrical lift distribution across the span of a monoplane wing at the stall is possible.\r\n\r\n2. The unsymmetrical solution produces a rolling moment of the same order of magnitude as that caused by a fully deflected aileron.\r\n\r\n3. The unsymmetrical distribution can occur only in a very small range of angles of attack after the wing begins to stall.\r\n\r\n4. The magnitude of the maximum rolling moment coefficient and the difference between the angle of attack at which the maximum rolling moment occurs and the angle of attack for first stall are nearly the same for all of the wing shapes investigated." }, { "name": "Tsien, Hsue Shen", "degree": "PhD", "year": "1939", "title": "Problems in Motion of Compressible Fluids and Reaction Propulsion", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-01122004-105646", "creators": [ { "name": { "family": "Tsien", "given": "Hsue Shen" }, "id": "Tsien-Hsue-Shen", "display_name": "Tsien, Hsue Shen" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/2698-R671", "abstract": "none" }, { "name": "Baker, Ralph Doris", "degree": "PhD", "year": "1938", "title": "The Effect of Surface Roughness on Skin Friction and Turbulence in Two Dimensional Flow", "advisor": "Unknown, Unknown", "url": "https://resolver.caltech.edu/CaltechETD:etd-02092006-154408", "creators": [ { "name": { "family": "Baker", "given": "Ralph Doris" }, "id": "Baker-Ralph-Doris", "display_name": "Baker, Ralph Doris" } ], "advisors": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6BXC-TP03", "abstract": "This investigation is divided into two parts. Part I deals with the effect of roughness and pressure drop and skin friction, and Part II covers the effect of surface roughness and the turbulent velocity fluctuations, and the correlation between these fluctuations in the direction of the mean flow and those normal to the channel walls.
\r\n\r\nThe roughness for both investigations was the same, and consisted of corrugated paper glued to the inside walls of a channel of 4.9 cm. wide by 85 cm. (inside dimensions). The roughness was changed by removing every other one of the corrugations, as illustrated in Fig. 6.
\r\n\r\nPressure and velocity distribution measurements were made at various stations and the results compared to Karman\u2019s equation, u/v* = Ar + B log y/k for rough walls, where u is the velocity in the channel, v* is the friction velocity = \u221a\u03c4o/\u2113, y = distance from wall, \u03c5 = the kinematic viscosity, k = the roughness height.
\r\n\r\nKarman\u2019s analysis showed B to be a universal constant independent of the wall roughness. Some investigators have given slightly different values to this constant. Their results were carried out in pipes or channels of near square cross section. These results appear to fall within 3% of those of Nikuradse as far as the value of B is concerned. However, a value of B a little less than 5.75 would fit the points better.
\r\n\r\nThese experiments were undertaken to add some information to this subject with flow in deep narrow channels and with other types of rough surfaces which had not been investigated.
\r\n\r\nThe turbulence measurements were determined with a hot-wire anemometer using a vibrating wire to calibrate it with. These results show that the correlation coefficient u'v'/(u')2 increases with speed for smooth walls and varies materially with the type of roughness with no definite systematic trend as to this change.
" }, { "name": "Charters, Alexander Crane", "degree": "PhD", "year": "1938", "title": "The Effect of Curvature on Boundary Layer Transition", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Clark Blanchard", "url": "https://resolver.caltech.edu/CaltechETD:etd-04292008-110647", "creators": [ { "name": { "family": "Charters", "given": "Alexander Crane" }, "id": "Charters-Alexander-Crane", "display_name": "Charters, Alexander Crane" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/Z13V-F628", "abstract": "Discrepancies in the boundary layer profiles of NACA T.N. 613, * and recent work done at Cambridge University, England, on the effect of inclination on transition required the reinvestigation of the effect of curvature on boundary layer transition using a more fundamentally sound experimental technique than previously. This technique consisted in comparing velocity profiles taken at different speeds at each measuring station with the universal Blasius laminar profile; a further indication of transition was obtained by measuring the local shear coefficient with a small total head tube pressed against the surface. Agreement between the two methods was complete in all but one case. The effect of inclination was investigated by rotating the working section of the tunnel as a whole around a fixed pivot. Unfavorable inclination produced a decrease in transition Reynolds number on either side of the sheet but a neutral region was found to exist in which inclination had no effect on the transition and in which the effect of curvature alone acted. The effect of the curvature could be expressed as a function of the single variable 1000\u03b4m/rc. As to the exact nature of the (R\u03b4m)T vs 1000\u03b4m/rc curve, little as yet can be said since a sufficient range of 1000\u03b4m/rc has not been investigated as yet. The results of T.N. 613 were thus validated qualitatively but the values of transition given there were found to be too large.
\r\n\r\n* National Advisory Committee for Aeronautics Technical Note.
\r\n" }, { "name": "Sears, William Rees", "degree": "PhD", "year": "1938", "title": "A Systematic Presentation of the Theory of Thin Airfoils in Non-Uniform Motion", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-06252004-134954", "creators": [ { "name": { "family": "Sears", "given": "William Rees" }, "id": "Sears-William-Rees", "display_name": "Sears, William Rees" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/EM5X-CZ66", "abstract": "The basic conceptions of the circulation theory of airfoils are reviewed briefly, and the mechanism by which a \"wake\" of vorticity is produced by an airfoil in non-uniform motion is pointed out. After a calculation of the induction effects of a wake vortex, it is shown how the lift and moment acting upon an airfoil in the two-dimensional case may be calculated directly from simple physical considerations of momentum and moment of momentum. Formulae for the lift and moment are then obtained which are applicable to all cases of motion of a two-dimensional thin airfoil in which the wake produced is approximately flat; i.e., in which the movement of the airfoil normal to its mean path is small.
\r\n\t\r\nThe general results are applied first to the case of an oscillating airfoil, and vector diagrams giving the magnitudes and phase angles of the lift and moment are obtained. The results of a sudden change of angle of attack are then determined, and a general method for handling transient cases is set up. This method is applied to the calculation of the lift and moment acting on an airfoil entering sharp-edged and graded gusts. The case of a series of sinusoidal gusts is also considered.
\r\n\t\r\nA method of calculating the distribution of forces over the airfoil chord is then shown, and it is applied to the steady-state oscillation. The paper concludes with a discussion of the applicability of certain results to the explanation of observed phenomena beyond the stall.
\r\n" }, { "name": "Clauser, Francis Hettinger", "degree": "PhD", "year": "1937", "title": "New Methods of Solving the Equations for the Flow of a Compressible Fluid", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-175229", "creators": [ { "name": { "family": "Clauser", "given": "Francis Hettinger" }, "id": "Clauser-Francis-Hettinger", "display_name": "Clauser, Francis Hettinger" }, { "name": { "family": "Clauser", "given": "Milton Ure" }, "id": "Clauser-Milton-Ure", "display_name": "Clauser, Milton Ure" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZAWH-VK93", "abstract": "The usual equations for the flow of a compressible fluid are non-linear in character and difficult to solve. It has been found that if the components of velocity are taken as independent variables, the resulting equations are linear. These equations are developed and new forms introduced.
\r\n\r\nA new function, \u0393 is introduced and a method advanced for effecting the transformation from the plane where the velocity components are coordinates, to the physical plane where x and y are coordinates.
\r\n\r\nA new way of finding plausible solutions to investigate is given and the case of flow in a corner is worked out in detail. The flow is found to have an anomalous behavior, the reason for which is explained. This solution is applied to the flow behind a curved shock wave.
" }, { "name": "Clauser, Milton Ure", "degree": "PhD", "year": "1937", "title": "The Effect of Curvature on the Transition from Laminar to the Turbulent Boundary Layer", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Robert Andrews", "url": "https://resolver.caltech.edu/CaltechETD:etd-11122003-174320", "creators": [ { "name": { "family": "Clauser", "given": "Milton Ure" }, "id": "Clauser-Milton-Ure", "display_name": "Clauser, Milton Ure" }, { "name": { "family": "Clauser", "given": "Francis Hettinger" }, "id": "Clauser-Francis-Hettinger", "display_name": "Clauser, Francis Hettinger" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Robert Andrews" }, "id": "Millikan-R-A", "role": "advisor", "display_name": "Millikan, Robert Andrews" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/E5EQ-DA15", "abstract": "In the flow over the upper surface of a wing, a discrepancy between the predicted and actual point of transition from laminar to turbulent boundary layer had been found. This effect may be due to the comparatively small radius of curvature of the upper surface of the wing. The present tests were undertaken to investigate this effect.
\r\n\r\nAs no available channel was suitable for this work, a new channel with two working sections was built. One working section had a wall with a twenty inch radius of curvature and the other section had a flat wall.
\r\n\r\nThree types of measurements were made: a. Traverses were made with a total head tube to determine the character of the boundary layer at various Reynolds numbers. b. The turbulence distribution in the boundary layer was investigated by means of a hot wire and a vacuum tube amplifier. c. A similar investigation of the mean velocity distribution in the boundary layer was made by means of a hot wire anemometer.
\r\n\r\nIt was found that by using an abbreviated form of the turbulence level traverses, critical Reynolds Numbers of the transitions could be established. These critical Reynolds are plotted as a function of x/r (x being the distance of the transition from the leading edge of the plate; r being radius of curvature of the plate) for both the convex and concave side of the plate.
" }, { "name": "Bollay, William", "degree": "PhD", "year": "1936", "title": "A New Theory for Wings of Small Aspect Ratio", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-01112006-081734", "creators": [ { "name": { "family": "Bollay", "given": "William" }, "id": "Bollay-William", "display_name": "Bollay, William" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/8BV2-QH84", "abstract": "Experiments on wings have shown that a very different kind of flow takes place for very small aspect ratios than for large aspect ratios. The lift curve continues up to about 45\u00b0 before stalling occurs. During this range it has a concave curvature upward rather than downward as the lifting line or lifting surface theories predict. No theoretical explanation of this effect has yet been given since it was generally supposed to be a stalling phenomenon and thus not adaptable to perfect fluid theories. The present paper shows that this curvature effect is due to the fact that the trailing vortices leave at an angle \u03b1 to the plate. For the limiting case of a plate with finite span and infinite chord it is shown that the bound vorticity and induced downwash are constant across the span, and the trailing vortices leave the wing at the half-angle of attack, \u03b1=\u03b8/2. These results are carried over into the assumptions for the analysis of the finite rectangular flat plate of very small aspect ratio. A surface distribution of vorticity over the plate is assumed, constant across the span, and varying according to the formula \u03b3 = \u03b30\u221at/2-x/t/2+x along the chord. Straight trailing vortices are assumed leaving the plate at an undetermined angle \u03b1. The boundary condition assumed is that the mean value of the induced velocity along the center line of the span is equal to the normal component of the free-stream velocity. This determines the constant \u03b30 and thus the normal force coefficient CN as a function of \u03b8. The parameter \u03b1 is still undetermined; however, its limits are given. For very small aspect ratios \u03b1=\u03b8/2, for large aspect ratios it approaches \u03b8. Winter\u2019s experiments on a wing of aspect ratio \u03ba=1/30 are checked very closely by this theory assuming \u03b1=\u03b8/2. At larger aspect ratios up to about \u03ba=1 the experimental curves lie between the theoretically predicted curves corresponding to \u03b1=\u03b8/2 and \u03b1=\u03b8, moving toward the latter limit at \u03ba=1." }, { "name": "Lipp, James Everett", "degree": "PhD", "year": "1935", "title": "Strength of Thin Walled Cylinders Subjected to Combined Compression and Torsion", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-01192006-141156", "creators": [ { "name": { "family": "Lipp", "given": "James Everett" }, "id": "Lipp-James-Everett", "display_name": "Lipp, James Everett" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "chair", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Klein", "given": "Arthur Louis" }, "id": "Klein-A-L", "role": "member", "display_name": "Klein, Arthur Louis" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-E-E", "role": "member", "display_name": "Sechler, Ernest Edwin" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/6Z0W-XJ59", "abstract": "This thesis is an extension of the work which was started by Dr. Donnell and Lieutenant Bridget on the problem of thin metal cylinders under combined torsion and direct stress. An explanation of the shape of the compression vs. torsion curve is found by calculating the yield failure of an element of sheet considered as a column with both transverse and end loads. Several causes of variation in that curve shape have been brought to light, but must wait for very complete and accurate experiments or a complete theoretical treatment for verification.
\r\n\r\nExperiments have been extended to several new materials, and have given results which parallel the previous work on steel. In order to cover as wide a range of cylinders as possible, the data of reference no. 1 have been included with the present series of tests.
" }, { "name": "Mills, Roscoe Harlan", "degree": "PhD", "year": "1935", "title": "The Boundary Layer for Some Axial Symmetric Flows", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-05022008-122728", "creators": [ { "name": { "family": "Mills", "given": "Roscoe Harlan" }, "id": "Mills-Roscoe-Harlan", "display_name": "Mills, Roscoe Harlan" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "chair", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Klein", "given": "Arthur Louis" }, "id": "Klein-A-L", "role": "member", "display_name": "Klein, Arthur Louis" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "member", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Tollmien", "given": "Walter" }, "id": "Tollmien-W", "role": "member", "display_name": "Tollmien, Walter" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/R7Q5-V769", "abstract": "The following thesis consists of two sections. Part 1. deals with \"The Boundary Layer for Some Axial Symmetric Flows\", Part 2. with \"Preliminary Experiments on the Flow between Two Circular Disks\"." }, { "name": "Moore, Norton Bartlett", "degree": "PhD", "year": "1934", "title": "The Boundary Layer and Skin Friction for a Body of Revolution at Large Reynold's Numbers", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-02072006-152924", "creators": [ { "name": { "family": "Moore", "given": "Norton Bartlett" }, "id": "Moore-Norton-Bartlett", "display_name": "Moore, Norton Bartlett" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/JZW9-JZ15", "abstract": "Clark Millikan\u2019s work on the boundary layer and skin friction for a figure of revolution is extended, with particular reference to the completely turbulent boundary layer, in two ways:
\r\n\r\na) His expressions for the completely turbulent regime are generalized so as to hold for the assumption of a one n-th power law for the velocity distribution in the boundary layer;
\r\n\r\nb) Von Karman\u2019s logarithmic velocity distribution is introduced into the analysis.
\r\n\r\nb){sic} leads to a practical method by which the drag of a full-scale dirigible can be predicted from wind tunnel tests.
\r\n\r\nComparison of the theory as gotten from b) with experiments leads to the conclusion that the present theory can be safely used to predict drags at large Reynolds\u2019 numbers, whereas drags predicted on the basis of a one seventh power law may be from 20% to 30% low.
" }, { "name": "Sechler, Ernest Edwin", "degree": "PhD", "year": "1934", "title": "The Ultimate Compressive Strength of Thin Sheet Metal Panels", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-05212003-155534", "creators": [ { "name": { "family": "Sechler", "given": "Ernest Edwin" }, "id": "Sechler-Ernest-Edwin", "display_name": "Sechler, Ernest Edwin" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/HX3P-3Y69", "abstract": "No abstract. " }, { "name": "Wattendorf, Frank Leslie", "degree": "PhD", "year": "1933", "title": "A Study of the Effect of Curvature on Fully Developed Turbulent Flow", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-08182006-154158", "creators": [ { "name": { "family": "Wattendorf", "given": "Frank Leslie" }, "id": "Wattendorf-Frank-Leslie", "display_name": "Wattendorf, Frank Leslie" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "chair", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Klein", "given": "Arthur Louis" }, "id": "Klein-A-L", "role": "member", "display_name": "Klein, Arthur Louis" }, { "name": { "family": "Tollmien", "given": "Walter" }, "id": "Tollmien-W", "role": "member", "display_name": "Tollmien, Walter" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/RWZE-DP61", "abstract": "In aeronautics we are especially interested in the flow of air adjacent to surfaces, such as airfoils. There are two main types of flow of real fluids, laminar and turbulent and it is turbulent flow which is of practical importance in aeronautics. We should like to be able to predict the skin friction and flow conditions for any surface of any shape. There has recently been much success with the problem of predicting flow along a flat plate parallel to the direction of flow, and the problem was attacked by investigation of fully developed turbulent flow in straight channels, and direct application of the semi-empirical laws obtained, to the flow along a flat plate. However, surfaces met with in practice are, in general, curved, so that it would be important to be able to predict the effect of curvature on turbulent flow. Most of the previous work in curved flow, however, has been with curved pipes and channels where the behavior of the flow was complicated by secondary vortices.
\r\n\r\nThe present work had the purposes of isolating as far as possible the effect of curvature on a fully developed turbulent flow, with two dimensional mean motion. The curved channels used were 5 cm. in breadth and 90 cm. in depth, and had straight entrance sections over 60 x breadth in length to produce a fully developed straight flow before subjecting it to the effect of curvature. Channel I had inner radius 45 cm. and outer radius 50 cm., while channel II had inner radius 20 cm. and outer 25 cm. In addition, measurements were made in an appratus consisting of two concentric cylinders, the inner one of radius 20 cm. and rotating, the outer of radius 25.4 cm. and fixed. The curvature was made of the same order as channel II for purpose of comparison.
\r\n\r\nMeasurements on the channels consisted of pressure drop along the channel walls at several speeds, velocity distribution at 30\u00b0 intervals around the curved portion, velocity distributions at several speeds, and for channel II, determination of the shearing stress at the walls of one of the curved sections.
\r\n\r\nMeasurements on the cylinders consisted of velocity distributions at two speeds and determination of shearing stress at the outer wall.
\r\n\r\nEvaluation of results included: calculation of resistance law, calculation of the shearing stress, distribution in radial direction across the curved portion, determination of the exponential law for the velocity distribution near the walls in the various cases, calculation of the \"mixing length\" 1, from turbulent exchange theory, and several dimensionless methods of plotting velocity distributions to show similarity between measurements in the channels and in the concentric cylinders.
\r\n\r\nAlso included are calculations of the laminar flow distribution in a curved channel, and a discussion of Rayleigh's stability criterion.
\r\n\r\nIt appears that the distribution of centrifugal force has a strong influence on the stability of the flow, and affects materially the velocity distribution. The fact that similarity can be obtained for several cases by proper dimensionless reduction based on the effective breadth of the mixing region looks hopeful, and it remains for future investigations to determine more facts about the effective breadth of the mixing region.
" }, { "name": "Biot, Maurice A.", "degree": "PhD", "year": "1932", "title": "Transient Oscillations in Elastic Systems", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-11252003-113841", "creators": [ { "name": { "family": "Biot", "given": "Maurice A." }, "id": "Biot-Maurice-A", "display_name": "Biot, Maurice A." } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/ZHSH-T443", "abstract": "No abstract." }, { "name": "Brahtz, John Henry Augustus", "degree": "PhD", "year": "1932", "title": "Stresses at Two-Dimensional Corners for Various Force Distributions", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-09062007-140731", "creators": [ { "name": { "family": "Brahtz", "given": "John Henry Augustus" }, "id": "Brahtz-John-Henry-Augustus", "display_name": "Brahtz, John Henry Augustus" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/4QRC-A816", "abstract": "This paper deals with the stress distribution under plain strain in a corner of any angular magnitude, i.e., a plane with an angular incision or notch.
\r\n\r\nThe Introduction contains a brief statement of the method employed by Dr. Theodor von Karman in his exact treatment of a beam in bending (Aachen Abhandlungen, Heft 7, 1927).
\r\n\r\nIn Part I a generalization of this method is outlined which is applicable to the corner for any force distribution over the straight boundaries. Solutions are found in the 3/4-plane for:
\r\n1. Concentrated load at any point of the straight boundaries.
\r\n2. Uniform distribution between the vertex and a point of the boundary.
\r\n3. Linear distribution in the same region.
\r\n4. Superposition of 2 and 3.
Certain stresses are determined and plotted and shown to be infinite at the vertex for partial loadings of the boundaries.
\r\n\r\nIn Part II an alternate method is given to obtain a solution for case 1.
\r\n\r\nThe discussion points out the very interesting paradox that stresses may be finite for certain continuous loadings, but become infinite if a portion of the load is removed.
" }, { "name": "Folsom, Richard Gilman", "degree": "PhD", "year": "1932", "title": "An Experimental Investigation of the Phenomena Produced by the Highly Turbulent Flow of Water Past a Series of Sharp Obstacles", "advisor": "von K\u00e1rm\u00e1n, Theodore", "url": "https://resolver.caltech.edu/CaltechETD:etd-06042004-104603", "creators": [ { "name": { "family": "Folsom", "given": "Richard Gilman" }, "id": "Folsom-Richard-Gilman", "display_name": "Folsom, Richard Gilman" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/BJ70-1607", "abstract": "A knowledge of the resistance to flow of a body placed in a stream of fluid is necessary for many problems in hydraulics, aeronautics, and ventilation. A large amount of experimental and theoretical effort has been expended investigating the drag characteristics of single bodies and small groups of similar bodies. Only a small amount of incomplete data is available regarding the performance of a long series of similar obstacles. This latter material was obtained for air flowing past supporting timber sets in mines. Since the dimensions of the mine shafts varied at different sections, the results of the experiments merely indicated the possibilities of obtaining different pressure losses depending on the size, shape, and spacing of the timber sets. Therefore, more data are needed regarding the drag characteristics of long series of obstacles." }, { "name": "Oswald, W. Bailey", "degree": "PhD", "year": "1932", "title": "The Transverse Force Distribution on Ellipsoidal and Nearly Ellipsoidal Bodies Moving in an Arbitrary Potential Flow", "advisor": "von K\u00e1rm\u00e1n, Theodore; Millikan, Clark Blanchard; Tollmien, Walter", "url": "https://resolver.caltech.edu/CaltechETD:etd-09062007-141717", "creators": [ { "name": { "family": "Oswald", "given": "W. Bailey" }, "id": "Oswald-W-Bailey", "display_name": "Oswald, W. Bailey" } ], "advisors": [ { "name": { "family": "von K\u00e1rm\u00e1n", "given": "Theodore" }, "id": "von-K\u00e1rm\u00e1n-Th", "role": "advisor", "display_name": "von K\u00e1rm\u00e1n, Theodore" }, { "name": { "family": "Millikan", "given": "Clark Blanchard" }, "id": "Millikan-C-B", "role": "advisor", "display_name": "Millikan, Clark Blanchard" }, { "name": { "family": "Tollmien", "given": "Walter" }, "id": "Tollmien-W", "role": "advisor", "display_name": "Tollmien, Walter" } ], "committee": [ { "name": { "family": "Unknown", "given": "Unknown" }, "display_name": "Unknown, Unknown" } ], "option_major": [ "aeronautics" ], "doi": "10.7907/QK4H-C181", "abstract": "The forces acting on an airship moving in a certain field of flow and the resulting path of motion are the result of numerous aerodynamic factors. These factors are principally skin friction drag, form drag, induced drag, aerodynamic transverse force distribution, lateral and longitudinal inertial and static force distribution, lift, and moments arising from all above factors. With the exception of aerodynamic transverse force distribution, lift, and induced drag, these factors are either small or may be determined by methods now available." } ]