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.
", "doi": "10.7907/rv8x-jm07", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17622", "collection": "thesis", "collection_id": "17622", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08132025-212958945", "primary_object_url": { "basename": "fnxfzgsvkxxdpgwywysdbcrbfcphkckp.pdf", "content": "final", "filesize": 9109118, "license": "other", "mime_type": "application/pdf", "url": "/17622/1/fnxfzgsvkxxdpgwywysdbcrbfcphkckp.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Nonlinear Disturbance Evolution in Boundary Layers Using the One-Way Navier-Stokes Equations", "author": [ { "family_name": "Sleeman", "given_name": "Michael Kenneth Elliott", "orcid": "0000-0001-5949-9289", "clpid": "Sleeman-Michael-Kenneth-Elliott" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Laminar-turbulent transition prediction for boundary-layer flows is a pacing item in engineering design. This work extends the one-way Navier Stokes (OWNS) equations to support nonlinear interactions between waves of different frequencies which enables nonlinear disturbance evolution in spatially-developing shear flows, with the goal of predicting transition for a reduced computational cost relative to direct numerical simulation (DNS). The OWNS approach linearizes the Navier-Stokes equations about a user-specified equilibrium solution, and then evolves disturbances to the equilibrium solution by solving a spatial initial-value problem in the frequency domain. OWNS yields a reduced computational cost compared to global linear stability analysis, while also conferring numerous advantages over the parabolized stability equations (PSE) that we seek to extend to nonlinear OWNS (NOWNS). We validate NOWNS for two- and three-dimensional disturbances to a low-speed Blasius boundary layer by comparing to DNS results from the literature. We further demonstrate that NOWNS can be used to for transition prediction since it accurately predicts the onset of laminar-turbulent transition in low-speed boundary-layer flows, relative to DNS. Subsequently, we extend the approach to high-speed boundary-layer flows, where we apply it to study oblique-wave breakdown of Mack's first and second modes. Finally, we formulate a greedy algorithm for choosing optimal OWNS recursion parameters, which achieves rapid error convergence and a net decrease in computational cost compared to previous approaches to recursion parameter selection.", "doi": "10.7907/wwv8-v242", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17653", "collection": "thesis", "collection_id": "17653", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08262025-221106238", "type": "thesis", "title": "Autoignition Modeling and a Generalized Hot Surface Ignition Criterion", "author": [ { "family_name": "Davis", "given_name": "Branson William", "orcid": "0000-0002-5288-7913", "clpid": "Davis-Branson-William" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/meyv-1317", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17277", "collection": "thesis", "collection_id": "17277", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272025-230901725", "primary_object_url": { "basename": "schoeffler_donner_2025.pdf", "content": "final", "filesize": 35837338, "license": "other", "mime_type": "application/pdf", "url": "/17277/1/schoeffler_donner_2025.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Measurement and Modeling of Detonation-Driven Shock Tube Flows", "author": [ { "family_name": "Schoeffler", "given_name": "Donner Thomas", "orcid": "0000-0002-1932-5986", "clpid": "Schoeffler-Donner-Thomas" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T-E" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/vedz-t661", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16619", "collection": "thesis", "collection_id": "16619", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08072024-203148023", "primary_object_url": { "basename": "HeidtLiam_CaltechThesis.pdf", "content": "final", "filesize": 32563876, "license": "other", "mime_type": "application/pdf", "url": "/16619/1/HeidtLiam_CaltechThesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Modal Analysis of Harmonically Forced Turbulent Flows with Application to Jets", "author": [ { "family_name": "Heidt", "given_name": "Liam Frank Raven", "orcid": "0000-0003-1967-6847", "clpid": "Heidt-Liam-Frank-Raven" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909" } ], "thesis_committee": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/e6fe-kz94", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16724", "collection": "thesis", "collection_id": "16724", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09132024-172910003", "type": "thesis", "title": "Cloudy with a Chance of Microphysics: Modeling Droplet Collisions for the Climate Scale", "author": [ { "family_name": "de Jong", "given_name": "Emily Katherine", "orcid": "0000-0002-5310-4554", "clpid": "de-Jong-Emily-Katherine" } ], "thesis_advisor": [ { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Fu", "given_name": "Xiaojing", "orcid": "0000-0001-7120-704X", "clpid": "Fu-Xiaojing" }, { "family_name": "Morrison", "given_name": "Hugh", "orcid": "0000-0001-8480-9787", "clpid": "Morrison-Hugh" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" } ], "local_group": [ { "literal": "3MT Competition (Caltech)" }, { "literal": "div_eng" } ], "abstract": "Feedbacks between a warming atmosphere, emission of aerosols, and clouds and precipitation are some of the most difficult aspects for climate models to accurately capture. While climate models operate at resolutions of tens or hundreds of kilometers, many of the physics that determine how and where clouds form or precipitate function at the micron droplet scale. Due to this disparity in physical scales, most of these cloud physics must be modeled with only a few approximate quantities and physical equations. These simplifications lead to large uncertainties about climate forcings such as the sensitivity of global warming to human-emitted aerosols.
\r\n\r\nThis work presents several promising new techniques for modeling and understanding hydrometeors in the climate system, with a particular focus on processes that involve collisions between droplets. First, I extend a high-complexity high-fidelity Lagrangian microphysics method to represent the process of breakup, in which colliding droplets fragment upon collision. Next, I introduce two new methods which attempt to reduce the assumptions inherent to modeling droplet coalescence, in which colliding droplets combine to form a larger drop. The first method uses a spectral finite element approach, while the second generalizes this technique using a method of moments to create a fully flexible microphysics scheme. Finally, I turn to remote observations of clouds, aerosols, and lightning over busy shipping regions to offer new techniques for quantifying aerosol-cloud interactions from creative data resources. This combination of high-fidelity modeling tools, observational data, and efficient numerical methods offers a path toward improving our understanding of the role of cloud microphysics in our climate system.
", "doi": "10.7907/yv2y-kg55", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16811", "collection": "thesis", "collection_id": "16811", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10232024-033952961", "primary_object_url": { "basename": "Huang_Yuting_2025.pdf", "content": "final", "filesize": 36962804, "license": "other", "mime_type": "application/pdf", "url": "/16811/1/Huang_Yuting_2025.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Linear and Non-Linear Interactions Involving Large-Scale Structures in Turbulence", "author": [ { "family_name": "Huang", "given_name": "Yuting", "orcid": "0000-0002-9457-7964", "clpid": "Huang-Yuting" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis performs a linear resolvent analysis (McKeon and Sharma, 2010), and a novel quantitative non-linear analysis of the triadic interactions, to study the largescale structures in wall-bounded turbulence.
\r\n\r\nFirst, resolvent analysis is applied to a flow over spanwise periodic roughness, to model the large-scale counter-rotating rolls. The experimental data (Wangsawijaya et al., 2020) is utilized to examine both the predictive and data compression capabilities of the resolvent. The improvements by the inclusion of an eddy viscosity and a crude boundary geometry model are also demonstrated. Standard resolvent is able to qualitatively predict the shape of the counter-rotating rolls. The inclusion of eddy viscosity improves the quantitative predictions and combined with the boundary geometry model is able to efficiently represent the data with small differences using only a fraction of the degree of freedom.
\r\n\r\nNext, we developed a novel framework to quantitatively analyze the triadic non-linear contributions in a turbulent channel. We incorporated the linear resolvent operator to provide the missing link from energy transfer between modes to the effect on the spectral turbulent kinetic energy. The coefficients highlight the importance of interactions involving large-scale structures, for both the large and small-scale forcing and response, providing a natural connection to the modeling assumptions of the quasi-linear (QL) and generalized quasi-linear (GQL) analyses. Specifically, it is revealed that QL and GQL are efficiently capturing important triadic interactions in the flow, and the inclusion of small amounts of wavenumbers into the GQL large-scale base flow quickly captures most of the important triadic interactions.
\r\n\r\nFinally, by performing spatio-temporal analyses of the triadic contributions to a single mode, we demonstrated the spatio-temporal nature of the triadic interactions and the effect of the resolvent operator. It is shown that the energetic triadic interactions are concentrated in temporal frequencies around a plane where all three wavespeeds are the same, allowing for a truncation of the important triadic interactions. We also demonstrated the linear amplification mechanism of the resolvent, allowing certain triadic interactions to generate a stronger response even with a weak forcing, underscoring the different perspectives offered by the inclusion of the linear resolvent operator into the analyses of the non-linear triadic interactions.
", "doi": "10.7907/739k-dj72", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16811", "collection": "thesis", "collection_id": "16811", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10232024-033952961", "primary_object_url": { "basename": "Huang_Yuting_2025.pdf", "content": "final", "filesize": 36962804, "license": "other", "mime_type": "application/pdf", "url": "/16811/1/Huang_Yuting_2025.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Linear and Non-Linear Interactions Involving Large-Scale Structures in Turbulence", "author": [ { "family_name": "Huang", "given_name": "Yuting", "orcid": "0000-0002-9457-7964", "clpid": "Huang-Yuting" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis performs a linear resolvent analysis (McKeon and Sharma, 2010), and a novel quantitative non-linear analysis of the triadic interactions, to study the largescale structures in wall-bounded turbulence.
\r\n\r\nFirst, resolvent analysis is applied to a flow over spanwise periodic roughness, to model the large-scale counter-rotating rolls. The experimental data (Wangsawijaya et al., 2020) is utilized to examine both the predictive and data compression capabilities of the resolvent. The improvements by the inclusion of an eddy viscosity and a crude boundary geometry model are also demonstrated. Standard resolvent is able to qualitatively predict the shape of the counter-rotating rolls. The inclusion of eddy viscosity improves the quantitative predictions and combined with the boundary geometry model is able to efficiently represent the data with small differences using only a fraction of the degree of freedom.
\r\n\r\nNext, we developed a novel framework to quantitatively analyze the triadic non-linear contributions in a turbulent channel. We incorporated the linear resolvent operator to provide the missing link from energy transfer between modes to the effect on the spectral turbulent kinetic energy. The coefficients highlight the importance of interactions involving large-scale structures, for both the large and small-scale forcing and response, providing a natural connection to the modeling assumptions of the quasi-linear (QL) and generalized quasi-linear (GQL) analyses. Specifically, it is revealed that QL and GQL are efficiently capturing important triadic interactions in the flow, and the inclusion of small amounts of wavenumbers into the GQL large-scale base flow quickly captures most of the important triadic interactions.
\r\n\r\nFinally, by performing spatio-temporal analyses of the triadic contributions to a single mode, we demonstrated the spatio-temporal nature of the triadic interactions and the effect of the resolvent operator. It is shown that the energetic triadic interactions are concentrated in temporal frequencies around a plane where all three wavespeeds are the same, allowing for a truncation of the important triadic interactions. We also demonstrated the linear amplification mechanism of the resolvent, allowing certain triadic interactions to generate a stronger response even with a weak forcing, underscoring the different perspectives offered by the inclusion of the linear resolvent operator into the analyses of the non-linear triadic interactions.
", "doi": "10.7907/739k-dj72", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16671", "collection": "thesis", "collection_id": "16671", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08272024-002246183", "primary_object_url": { "basename": "AlexandraBaumgart_Thesis.pdf", "content": "final", "filesize": 35984180, "license": "other", "mime_type": "application/pdf", "url": "/16671/1/AlexandraBaumgart_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Stability and Reduced Order Chemistry Modeling in Detonation Simulations", "author": [ { "family_name": "Baumgart", "given_name": "Alexandra Rose", "orcid": "0000-0001-8575-3404", "clpid": "Baumgart-Alexandra-Rose" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The coupling between shocks and chemistry in detonations poses a challenge for simulations. In this thesis, a simulation framework is developed to address key components of detonation modeling: numerical stability of shocks and discontinuities, and computational efficiency in chemistry modeling.
\r\n\r\nTo ensure numerical stability in the vicinity of shocks, a variety of methods have been used, including shock-capturing schemes such as weighted essentially non-oscillatory (WENO) schemes, as well as the addition of artificial diffusivities to the governing equations. In this work, all necessary viscous/diffusion terms are derived from first principles, and the performance of these analytical terms is demonstrated within a centered differencing framework. The physical Euler equations are spatially-filtered with a Gaussian-like filter. Sub-filter scale (SFS) terms arise in the momentum and energy equations. Analytical closure is provided for each of them by leveraging the jump conditions for a shock. No SFS terms are present in the continuity or species equations. For contact discontinuities, the analytical SFS terms are identically zero. However, numerically, the transport of a contact discontinuity may result in artificial oscillations due to dispersive errors. To treat contact discontinuities, a WENO-like correction term is applied to the enthalpy transport. Implemented within a centered difference code, this filtered framework performs well for a range of shock-dominated flows without introducing excessive diffusion. In addition to providing new insight into the placement and form of required diffusion terms in the governing equations, this framework is general and may be used with any numerical scheme.
\r\n\r\nChemistry modeling in detonations typically relies on two broad approaches: simplified models with one- or two-step chemistry, and detailed chemistry. These approaches require choosing between computational efficiency or physical accuracy. In detailed chemistry simulations, there are physical constraints that must be met when transporting species mass fractions; nonlinear transport schemes such as WENO do not satisfy these constraints automatically. A new method is presented to ensure that the sum of mass fractions equals 1, without penalizing inert species. The approach is better able to capture the physical instability expected for detonations. To reduce the cost of chemistry while maintaining accurate physics, tabulated chemistry has been used extensively for flames/deflagrations in the low Mach number framework. In the simplest tabulated chemistry model for premixed flames, a progress variable, describing the progress of reactions in the system, is transported in the simulation. This progress variable is then used to look up all other species, transport properties, and thermodynamic variables from a pre-computed table. Unfortunately, there is no existing tabulation approach designed specifically for detonations. As such, this work extends the tabulated chemistry method to detonations. To describe the enthalpy and specific heat capacity, the temperature is selected as a second table coordinate. The two table coordinates are able to capture virtually all variations in the progress variable source term. The Zel'dovich-von Neumann-D\u00f6ring (ZND) model is found to be the most appropriate one-dimensional problem for generation of the table. The ZND tabulation approach is validated for both one-dimensional stable and pulsating and two-dimensional regular and irregular detonations in various hydrogen-oxygen mixtures. The tabulated chemistry simulations are able to reproduce the detailed chemistry results in terms of propagation speed, cellular structures, and source term statistics at a reduced computational cost, demonstrating the benefits of this approach for predictive modeling of detonations.
", "doi": "10.7907/1vna-9r39", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17386", "collection": "thesis", "collection_id": "17386", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032025-000538732", "primary_object_url": { "basename": "PhD_Thesis-1.pdf", "content": "final", "filesize": 6919777, "license": "other", "mime_type": "application/pdf", "url": "/17386/1/PhD_Thesis-1.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Mixing-Driven Abyssal Ocean Circulation over Sloping Topography", "author": [ { "family_name": "Peterson", "given_name": "Henry Grant", "orcid": "0000-0003-3491-7688", "clpid": "Peterson-Henry-Grant" } ], "thesis_advisor": [ { "family_name": "Callies", "given_name": "J\u00f6rn", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J\u00f6rn" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Adkins", "given_name": "Jess F.", "orcid": "0000-0002-3174-5190", "clpid": "Adkins-J-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Purkey", "given_name": "Sarah", "orcid": "0000-0002-1893-6224", "clpid": "Purkey-Sarah" }, { "family_name": "Callies", "given_name": "J\u00f6rn", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J\u00f6rn" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The planetary-scale overturning circulation of the ocean is maintained by small-scale diapycnal mixing in the abyss. Recent theory and observations suggest that this turbulence is bottom-enhanced, confining the upwelling needed to close this circulation to thin bottom boundary layers (BLs) over sloping topography. Developing an understanding of how this mixing shapes the abyssal circulation, both locally and at the basin scale, is the unifying goal of this thesis.
\r\n\r\nThe local response of a water column to mixing has previously been understood using a one-dimensional model of a rotating, stratified fluid over a sloping seafloor. Canonically, this model assumes no cross- or along-slope variations of the flow, pressure, and buoyancy anomalies. At steady state, it predicts a peculiar form of the net cross-slope transport, however, failing to consider its coupling to the global circulation. For symmetric bathymetry without along-slope variations, for instance, this large-scale context implies that all cross-slope BL transport must be exactly returned in the interior. This interior downwelling is then turned by the Coriolis acceleration, rapidly spinning up along-slope flow in balance with a cross-slope barotropic pressure gradient. With these added physics, the one-dimensional model better captures the local response to mixing over an idealized ridge, for example. Using BL theory, we explicitly describe how the BL and interior communicate in this model. The up-slope transport of dense water in the bottom BL contributes a net downward flux of buoyancy, creating an effective bottom boundary condition on the interior. The coupling goes both ways, with the interior stratification at the top of the BL setting the strength of the BL transport. Variations across the slope then allow for BL--interior exchange.
\r\n\r\nUltimately, the net transport of the local response must conserve potential vorticity at the basin scale. To better understand this coupling for arbitrary topography, we develop a novel finite element model of the planetary geostrophic equations. Using a combination of simulations and BL theory, we then study the mixing-driven abyssal circulation in an idealized bowl-shaped basin. In the absence of wind forcing and the joint effect of baroclinicity and relief, the leading-order barotropic transport flows along f/H contours, where f is the Coriolis frequency and H is the depth. The local response to mixing is coupled to this barotropic circulation, simultaneously constrained by the barotropic circulation and forcing it via a bottom stress curl. For closed f/H contours, a strong along-contour barotropic circulation spins up, reminiscent of the local response described above. On the other hand, if these contours intersect the boundary, a case more typical in the real ocean, the barotropic transport is suppressed. This decouples the leading-order local response from the large-scale circulation and intensifies bottom BL upwelling. This work therefore suggests that the local abyssal stratification in the presence of bottom-enhanced mixing strongly depends on the large-scale context.
", "doi": "10.7907/94gy-cy80", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17321", "collection": "thesis", "collection_id": "17321", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302025-232647443", "type": "thesis", "title": "Beyond Symmetry: Normality-Based Analysis of Velocity Gradients in Turbulent Flows", "author": [ { "family_name": "Arun", "given_name": "Rahul", "orcid": "0000-0002-5942-169X", "clpid": "Arun-Rahul" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Lozano-Dur\u00e1n", "given_name": "Adri\u00e1n", "orcid": "0000-0001-9306-0261", "clpid": "Lozano-Duran-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "Dale Ian", "orcid": "0009-0007-5991-2863", "clpid": "Pullin-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/3py1-wj85", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17049", "collection": "thesis", "collection_id": "17049", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03102025-172015248", "type": "thesis", "title": "Resolvent Analysis of Non-Stationary Turbulent Flows and Transient Flow Phenomena", "author": [ { "family_name": "Ballouz", "given_name": "Eric", "orcid": "0009-0003-7034-1898", "clpid": "Ballouz-Eric" } ], "thesis_advisor": [ { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" } ], "thesis_committee": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Lozano-Duran", "given_name": "Adrian", "orcid": "0000-0001-9306-0261", "clpid": "Lozano-Duran-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this work, we develop a wavelet-based formulation of resolvent analysis in order to extend the method to transient phenomena and non-stationary flows. We apply this method in two ways: first, to analyze systems that were not previously amenable to traditional resolvent analysis, and second, to probe the limits of the resolvent forcing modes' \"optimality\" in a nonlinear simulation as well as investigate the mechanisms that suppress their effectiveness. In wavelet-based resolvent analysis, the Navier-Stokes equations are linearized about a mean profile, Fourier-transformed in the homogeneous directions, and wavelet-transformed in time. The nonlinear terms are represented as forcing terms acting on the system, and a maximally perturbing forcing mode and the response it produces are then computed for this linear system. The wavelet formulation enables the forcing and response modes to represent transient trajectories. By windowing the wavelet-based resolvent operator, we can also compute optimal forcing modes restricted to a time-localized pulse along with their transient response.
\r\n\r\nFor the first application of the method, we use the windowing approach to study bursting in channel flow. The optimal response mode grows and decays in time scales that match turbulent data, and we show that this optimal burst exploits the Orr mechanism.\r\nWe also study channel flow subjected to a spanwise pressure gradient. The corresponding resolvent modes mirror the mean flow and gradually realign themselves according to the new flow conditions. More interestingly, they exhibit a collapse of the lift-up mechanism during this realignment, which offers an explanation to the depletion of tangential Reynolds stresses in the turbulent system.
\r\n\r\nFor the second application of the method, we inject time-localized resolvent forcing modes for the minimal flow unit into a simulation of the system, at different intensities. The principal resolvent forcing mode is much more effective than a randomly generated forcing structure at amplifying the near-wall streak. For initial times and close to the wall, the turbulent minimal flow unit matches the principal response mode well, but due to nonlinear effects, the response decays prematurely. By computing the nonlinear energy transfer to secondary scales, we find that the breakdown of the actuated mode proceeds similarly across all forcing intensities: in the near-wall region, the induced streak forks into two branches, while in the outer region, the streak breaks up in the streamwise direction. In both regions, spanwise gradients account for the dominant share of nonlinear energy transfer.
", "doi": "10.7907/t9sw-b215", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17083", "collection": "thesis", "collection_id": "17083", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03202025-173020131", "primary_object_url": { "basename": "Thesis_Hou.pdf", "content": "final", "filesize": 21073399, "license": "other", "mime_type": "application/pdf", "url": "/17083/1/Thesis_Hou.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Fast Algorithms for Spanwise Periodic Incompressible External Flows: From Simulation to Analysis", "author": [ { "family_name": "Hou", "given_name": "Wei", "orcid": "0000-0001-8023-6395", "clpid": "Hou-Wei" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "External flows over spanwise-homogeneous geometries are ubiquitous in science and engineering applications. In this thesis, we propose algorithms to simulate and analyze these flows using the lattice Green's function (LGF) approach. The LGF is the analytical inverse of a discrete elliptic operator that automatically incorporates exact far-field boundary conditions and minimizes computational expense by allowing snug computational regions encompassing only vortical flow regions. By combining LGFs with adaptive mesh refinement (AMR) and immersed boundary (IB) methods, we present two numerical algorithms specially designed for spanwise periodic incompressible external flows: one to directly solve the nonlinear equations of motion and one to compute stability and resolvent analyses.
\r\n\r\nFor these algorithms, the LGFs of the screened Poisson equation must be computed at runtime. To enable efficient flow simulation and analysis algorithms, we propose a fast numerical algorithm to tabulate these LGFs. We derive convergence results for the algorithms and show that they are orders of magnitude faster than existing algorithms. Armed with the LGF for the screened Poisson equation, we further develop algorithms to solve the Navier-Stokes equations and associated linearized eigenvalue problems.
\r\n\r\nWe present two applications of these algorithms. We perform simulations to validate the starting vortex theory proposed by Pullin and Sader (2021), and we perform stability analyses of flow past a rotating cylinder with a control cylinder in its wake.
", "doi": "10.7907/eygj-k325", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16463", "collection": "thesis", "collection_id": "16463", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012024-234504692", "type": "thesis", "title": "Seismic Thermometry of the North Pacific and Equatorial Indian Oceans", "author": [ { "family_name": "Peng", "given_name": "Shirui", "orcid": "0000-0002-4616-4604", "clpid": "Peng-Shirui" } ], "thesis_advisor": [ { "family_name": "Callies", "given_name": "J\u00f6rn", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J\u00f6rn" } ], "thesis_committee": [ { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Batygin", "given_name": "Konstantin", "orcid": "0000-0002-7094-7908", "clpid": "Batygin-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Zhan", "given_name": "Zhongwen", "orcid": "0000-0002-5586-2607", "clpid": "Zhan-Zhongwen" }, { "family_name": "McPhaden", "given_name": "Michael J", "orcid": "0000-0002-8423-5805", "clpid": "McPhaden-Michael-J" }, { "family_name": "Callies", "given_name": "J\u00f6rn", "orcid": "0000-0002-6815-1230", "clpid": "Callies-J\u00f6rn" } ], "local_group": [ { "literal": "div_gps" } ], "abstract": "The ocean absorbs the majority of excess heat in the climate system. Ocean mixing is also critical in setting Earth's thermal inertia. Over the course of the past few decades, conventional observations like Argo floats have drastically improved the coverage of the global ocean. However, their temporal and spatial resolutions are still limited. Resolving trends and patterns of temperature variations in the ocean under climate change remains a challenging sampling problem. This dissertation seeks to reduce such sampling errors by developing seismic thermometry. It is an acoustic method that measures large-scale ocean temperature changes using sound waves generated by repeating earthquakes. The chapters in this thesis attempt to combine physical understanding with statistical analysis to improve and implement seismic thermometry in several ways. First, acoustic waves generated by earthquakes along the Japan Trench and received at Wake Island are used to constrain temperature variation in the Kuroshio Extension region. An inversion that combines these measurements for the time and azimuth dependence of the range-averaged deep temperatures reveals lateral and temporal variations due to Kuroshio Extension meanders, mesoscale eddies, and decadal water mass rearrangements. Second, a comprehensive covariance structure is proposed to represent variabilities due to stochastic mesoscale, regional trend, and large-scale seasonality. It demonstrates statistical consistency between conventional float data and seismic measurements, and shows quantitatively that seismic thermometry reduces basin-scale temperature uncertainty when combined with conventional measurements. Finally, seismic data are compared with ocean models in the equatorial Indian Ocean to study the vertical structure of biweekly Yanai waves. The comparison indicates qualitative agreements in biweekly variations, and regression analysis confirms their origin as west-propagating Yanai waves. Yet quantitative differences in the biweekly variance magnitude demand further calibrations in both models and the seismic inversion.", "doi": "10.7907/a814-nf75", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16383", "collection": "thesis", "collection_id": "16383", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05142024-215222393", "primary_object_url": { "basename": "nelsen_nicholas_2024_thesis.pdf", "content": "final", "filesize": 3824472, "license": "other", "mime_type": "application/pdf", "url": "/16383/1/nelsen_nicholas_2024_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Statistical Foundations of Operator Learning", "author": [ { "family_name": "Nelsen", "given_name": "Nicholas Hao", "orcid": "0000-0002-8328-1199", "clpid": "Nelsen-Nicholas-Hao" } ], "thesis_advisor": [ { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" } ], "thesis_committee": [ { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Owhadi", "given_name": "Houman", "orcid": "0000-0002-5677-1600", "clpid": "Owhadi-H" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis studies operator learning from a statistical perspective. Operator learning uses observed data to estimate mappings between infinite-dimensional spaces. It does so at the conceptually continuum level, leading to discretization-independent machine learning methods when implemented in practice. Although this framework shows promise for physical model acceleration and discovery, the mathematical theory of operator learning lags behind its empirical success. Motivated by scientific computing and inverse problems where the available data are often scarce, this thesis develops scalable algorithms for operator learning and theoretical insights into their data efficiency.
\r\n\r\nThe thesis begins by introducing a convergent operator learning algorithm that is implementable on a computer with controlled complexity. The method is based on linear combinations of function-valued random features, enjoys efficient training via convex optimization, and accurately approximates nonlinear solution operators of parametric partial differential equations. A statistical analysis derives state-of-the-art error bounds for the method and establishes its robustness to errors stemming from noisy observations and model misspecification. Next, the thesis tackles fundamental statistical questions about how problem structure, data quality, and prior information influence learning accuracy. Specializing to a linear setting, a sharp Bayesian nonparametric analysis shows that continuum linear operators, such as the integration or differentiation of spatially varying functions, are provably learnable from noisy input-output pairs. The theory reveals that smoothing operators are easier to learn than unbounded ones and that training with rough or high-frequency input data improves sample complexity. When only specific linear functionals of the operator\u2019s output are the primary quantities of interest, the final part of the thesis proves that the smoothness of the functionals determines whether learning directly from these finite-dimensional observations carries a statistical advantage over plug-in estimators based on learning the entire operator. To validate the findings beyond linear problems, the thesis develops practical deep operator learning architectures for nonlinear mappings that send functions to vectors, or vice versa, and shows their corresponding universal approximation properties. Altogether, this thesis advances the reliability and efficiency of operator learning for continuum problems in the physical and data sciences.
", "doi": "10.7907/0246-7574", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16435", "collection": "thesis", "collection_id": "16435", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292024-213954509", "primary_object_url": { "basename": "MatthewYao_PhD_Thesis.pdf", "content": "final", "filesize": 13823304, "license": "other", "mime_type": "application/pdf", "url": "/16435/1/MatthewYao_PhD_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Lean Premixed Hydrogen Flames: Turbulence, Chemistry, and Modelling", "author": [ { "family_name": "Yao", "given_name": "Matthew Xuhuai", "orcid": "0000-0001-6141-1477", "clpid": "Yao-Matthew-Xuhuai" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, a behaviour which is attributed to differential diffusion effects. In this thesis, the relationships between turbulence, chemistry, and modelling are studied through direct numerical simulation (DNS) and large eddy simulation (LES).
\r\n\r\nThe effect of turbulence on lean hydrogen combustion is studied through DNS using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. Neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.
\r\n\r\nThe structure of lean hydrogen flames, namely the species mass fraction dependence on the local temperature, differs significantly from that of unity Lewis number fuels due to thermodiffusive instabilities. When subjected to turbulence, the conditional mean species mass fraction profiles are observed to transition from the laminar mixture-averaged flamelet solution to the unity Lewis number flamelet solution. We assess the impact of Soret diffusion and integral length scales on an effective Lewis number model. The results show that the turbulent flame structure can be mapped onto laminar flamelets via the use of effective Lewis numbers, which are expressed by an a priori Karlovitz number model. Although the flame structure is altered by Soret diffusion, there is still strong agreement with previously derived Karlovitz number models for effective Lewis numbers. To map the turbulent flames onto laminar flames with effective Lewis numbers, the relative impact of Soret diffusion needs to be proportionally reduced.
\r\n\r\nTo assess the LES modelling of lean hydrogen flames, we simulate a low-swirl burner, an alternative means of clean energy generation. The LES modelling of these flows remains challenging because the transition of small-scale instabilities into large-scale turbulent structures cannot be modelled by conventional strategies. Traditional one-equation tabulated chemistry formulations require only a progress variable, and cannot capture differential diffusion and curvature effects. In this work, we study the effects of tabulating different conditional mean source terms. It is shown that tabulating the appropriate conditional mean source term leads to improvements in the flow field prediction, however, key features such as the main recirculation region are not reproduced. Then, a two-equation tabulated chemistry model which accounts for differential diffusion and curvature effects is tested. This model provides the best agreement with experimental results. The work is a first effort in evaluating the performance of the two-equation model in the LES framework.
", "doi": "10.7907/yjzw-vp60", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16208", "collection": "thesis", "collection_id": "16208", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10162023-141935060", "primary_object_url": { "basename": "Spratt_Thesis.pdf", "content": "final", "filesize": 28877365, "license": "other", "mime_type": "application/pdf", "url": "/16208/1/Spratt_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Simulations of Cavitating Bubbles in Elastic and Viscoelastic Materials for Biomedical Applications", "author": [ { "family_name": "Spratt", "given_name": "Jean-S\u00e9bastien Alexandre", "orcid": "0000-0002-1962-4214", "clpid": "Spratt-Jean-S\u00e9bastien-Alexandre" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The interactions of cavitating bubbles with elastic and viscoelastic materials play a central role in many biomedical applications. This thesis makes use of numerical modeling and data-driven approaches to characterize soft biomaterials at high strain rates via observation of bubble dynamics, and to model burst-wave lithotripsy, a focused ultrasound therapy to break kidney stones.
\r\n\r\nIn the first part of the thesis, a data assimilation framework is developed for cavitation rheometry, a technique that uses bubble dynamics to characterize soft, viscoelastic materials at high strain-rates. This framework aims to determine material properties that best fit observed cavitating bubble dynamics. We propose ensemble-based data assimilation methods to solve this inverse problem. This approach is validated with surrogate data generated by adding random noise to simulated bubble radius time histories, and we show that we can confidently and efficiently estimate parameters of interest within 5% given an iterative Kalman smoother approach and an ensemble- based 4D-Var hybrid technique. The developed framework is applied to experimental data in three distinct settings, with varying bubble nucleation methods, cavitation media, and using different material constitutive models. We demonstrate that the mechanical properties of gels used in each experiment can be estimated quickly and accurately despite experimental inconsistencies, model error, and noisy data. The framework is used to further our understanding of the underlying physics and identify limitations of our bubble dynamics model for violent bubble collapse.
\r\n\r\nIn the second part of the thesis, we simulate burst-wave lithotripsy (BWL), a non- invasive treatment for kidney stones that relies on repeated short bursts of focused ultrasound. Numerical approaches to study BWL require simulation of acoustic waves interacting with solid stones as well as bubble clouds which can nucleate ahead of the stone. We implement and validate a hypoelastic material model, which, with the addition of a continuum damage model and calibration of a spherically- focused transducer array, enables us to determine how effective various treatment strategies are with arbitrary stones. We present a preliminary investigation of the bubble dynamics occurring during treatment, and their impact on damage to the stone. Finally, we propose a strategy to reduce shielding by collapsing bubbles ahead of the stone via introduction of a secondary, low-frequency ultrasound pulse during treatment.
", "doi": "10.7907/g34e-6p65", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15270", "collection": "thesis", "collection_id": "15270", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012023-233342281", "primary_object_url": { "basename": "Dissertation_NJW_v2_final.pdf", "content": "final", "filesize": 28173009, "license": "other", "mime_type": "application/pdf", "url": "/15270/1/Dissertation_NJW_v2_final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Dynamics and Performance of Wind-Energy Systems in Unsteady Flow Conditions", "author": [ { "family_name": "Wei", "given_name": "Nathaniel James", "orcid": "0000-0001-5846-6485", "clpid": "Wei-Nathaniel-James" } ], "thesis_advisor": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/d9wh-pj98", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:16060", "collection": "thesis", "collection_id": "16060", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022023-192522565", "primary_object_url": { "basename": "Martin_Conor_2023_Thesis_Final.pdf", "content": "final", "filesize": 122081165, "license": "other", "mime_type": "application/pdf", "url": "/16060/5/Martin_Conor_2023_Thesis_Final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Experiments in Thermal Ignition: Influence of Natural Convection on Properties of Gaseous Explosions", "author": [ { "family_name": "Martin", "given_name": "Conor Daniel", "orcid": "0000-0003-2332-7383", "clpid": "Martin-Conor-Daniel" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/twcf-m219", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15277", "collection": "thesis", "collection_id": "15277", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022023-051815823", "type": "thesis", "title": "Computational Compensation for Model Imperfections in Photoacoustic Computed Tomography", "author": [ { "family_name": "Hu", "given_name": "Peng", "orcid": "0000-0002-2933-1239", "clpid": "Hu-Peng" } ], "thesis_advisor": [ { "family_name": "Wang", "given_name": "Lihong", "orcid": "0000-0001-9783-4383", "clpid": "Wang-Lihong" } ], "thesis_committee": [ { "family_name": "Yang", "given_name": "Changhuei", "orcid": "0000-0001-8791-0354", "clpid": "Yang-Changhuei" }, { "family_name": "Bouman", "given_name": "Katherine L.", "orcid": "0000-0003-0077-4367", "clpid": "Bouman-K-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Wang", "given_name": "Lihong", "orcid": "0000-0001-9783-4383", "clpid": "Wang-Lihong" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Photoacoustic computed tomography (PACT) images biological tissues\u2019 optical absorption through detection of photon-absorption-induced ultrasonic waves. Various systems have been proposed for PACT and they are described by different mathematical models to reconstruct from detected ultrasonic signals the photon-absorption-induced initial pressure, the main contrast in PACT. Accurate image reconstruction has high requirements for the system and the mathematical model, which is often imperfect in practice due to multiple factors, e.g., limited transducer bandwidth, finite transducer element size, sparse spatial sampling, partial-view detection, and tissue motion. The focus of this dissertation is on using computational methods to compensate for these model imperfections.
\r\n\r\nFirst, for a human breast imaging system based on a full-ring transducer array, we incorporate the limited transducer bandwidth into the model for spatiotemporal analysis to clarify the aliasing due to sparse spatial sampling and propose (1) two methods (radius-dependent spatiotemporal antialiasing and location-dependent spatiotemporal antialiasing) to mitigate these artifacts. Second, for an isotropic-resolution 3D PACT system formed by four arc arrays, we consider both the limited transducer bandwidth and the finite transducer element size and (2) compress the system matrix through singular value decomposition and fast Fourier transform for its efficient explicit expression. Enabled by this expression, we then propose (3) fast sparsely sampling functional imaging by incorporating a densely sampled prior image into the system matrix, which maintains the critical linearity while mitigating artifacts, and (4) intra-image nonrigid motion correction by incorporating the motion as subdomain translations into the system matrix and reconstructing the translations together with the image iteratively. Finally, for a single-shot 3D PACT system based on a single ultrasonic transducer, we propose (5) a fast implementation of the forward model by connecting traditional PACT with virtual detector responses through fast Fourier transform, and we iteratively reconstruct the image from signals with extremely compressed sensing and partial-view detection.
\r\n\r\nAll these proposed methods enable image reconstruction or significantly improve image quality in numerical simulations, phantom experiments, and in vivo experiments. Although they are demonstrated only for certain PACT systems, they are directly applicable to other systems and can be extended to other tomographic imaging modalities such as X-ray computed tomography (X-ray CT) and magnetic resonance imaging (MRI).
", "doi": "10.7907/6hdm-ar41", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15083", "collection": "thesis", "collection_id": "15083", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01162023-041217909", "primary_object_url": { "basename": "Kamal_thesis_final.pdf", "content": "final", "filesize": 47336253, "license": "other", "mime_type": "application/pdf", "url": "/15083/1/Kamal_thesis_final.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Optimal Receptivity and the Generalization of the One-Way Navier-Stokes (OWNS) Equations to Complex High-Speed Boundary Layers and Jets", "author": [ { "family_name": "Kamal", "given_name": "Omar", "orcid": "0000-0002-3431-2964", "clpid": "Kamal-Omar" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Prediction of the linear amplification of disturbances in hypersonic boundary layers is challenging due to the presence and interactions of discrete modes (e.g. Tollmien-Schlichting and Mack) and continuous modes (entropic, vortical, and acoustic). While direct numerical simulations (DNS) and global analysis can be used, the large grids required make the stability calculations expensive, particularly when a large parameter space is required. At the same time, parabolized stability equations are non-convergent and unreliable for problems involving multi-modal and non-modal interactions. We therefore apply the One-Way Navier-Stokes (OWNS) Equations to study transitional hypersonic boundary layers. OWNS is based on a rigorous, approximate parabolization of the equations of motion that removes disturbances with upstream group velocity using a higher-order recursive filter. We extend the original algorithm by considering non-orthogonal curvilinear coordinates and incorporate full compressibility with temperature-dependent fluid properties. The generalized OWNS methodology is validated by comparing to DNS data for flat plates and a sharp cone, and to linear stability theory results for local disturbances on the centerline of the Mach 6 HIFiRE-5 elliptic cone. OWNS provides DNS-quality results for the former flows at a small fraction of the computational expense. We further demonstrate the capability of OWNS to track fully 3D instabilities by applying the algorithm to a complex Mach 6 finned-cone geometry as well as a 3D Mach 1.5 turbulent jet.
\r\n\r\nIt is often desirable, especially for design purposes, to compute worst-case disturbances, i.e. solving the inverse problem, otherwise known as resolvent or input-output analysis. While DNS and global analysis can be used to compute optimal forced responses, their large computational expense render these tools less practical for large design parameter spaces. We address this issue by modifying the original OWNS framework to find the optimal forcing and responses using Lagrangian multipliers via an iterative, adjoint-based, space-marching technique that appreciably reduces the computational burden compared to the global approach that uses singular value decomposition without sacrificing accuracy. The input-output OWNS model is validated against optimal forcings and responses of a Mach 4.5 flat-plate boundary layer from literature and a Mach 1.5 turbulent jet. We then apply these equations to study worst-case disturbances on the centerline of the Mach 6 HIFiRE-5 elliptic cone and on a highly cooled Mach 6 flat-plate boundary layer.
\r\n \r\nAlthough the worst-case forcings are theoretically informative, they are not physically realizable. In natural receptivity analysis, disturbances are forced by matching local solutions within the boundary layer to outer solutions consisting of free-stream vortical, entropic, and acoustic disturbances. We pose a scattering formalism to restrict the input forcing to a set of realizable disturbances associated with plane-wave solutions of the outer problem. The formulation is validated by comparing with DNS of a Mach 4.5 flat-plate boundary layer. We show that the method provides insight into transition mechanisms by identifying those linear combinations of plane-wave disturbances that maximize energy amplification over a range of frequencies. We also discuss how the framework can be extended to accommodate scattering from shocks and in shock layers for supersonic flow.
", "doi": "10.7907/haet-h558", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15100", "collection": "thesis", "collection_id": "15100", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02042023-015312785", "primary_object_url": { "basename": "Oshima_EK_Dissertation.pdf", "content": "final", "filesize": 77968077, "license": "other", "mime_type": "application/pdf", "url": "/15100/1/Oshima_EK_Dissertation.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Experimental Studies of Flow Control Techniques for Future Aircraft", "author": [ { "family_name": "Oshima", "given_name": "Emile Kazuo", "orcid": "0000-0002-1689-3726", "clpid": "Oshima-Emile-Kazuo" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Bae", "given_name": "H. Jane", "orcid": "0000-0001-6789-6209", "clpid": "Bae-H-J" }, { "family_name": "Wygnanski", "given_name": "Israel J.", "orcid": "0009-0001-5711-7029", "clpid": "Wygnanski-I-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/fpcj-w268", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15111", "collection": "thesis", "collection_id": "15111", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02272023-213525351", "primary_object_url": { "basename": "Thesis_JMMelis-Final.pdf", "content": "final", "filesize": 55294232, "license": "other", "mime_type": "application/pdf", "url": "/15111/1/Thesis_JMMelis-Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Neural Network Model of an Insect's Wing Hinge Reveals How Steering Muscles Control Flight", "author": [ { "family_name": "Melis", "given_name": "Johan Matthijs", "orcid": "0000-0001-8966-9496", "clpid": "Melis-Johan-Matthijs" } ], "thesis_advisor": [ { "family_name": "Dickinson", "given_name": "Michael H.", "orcid": "0000-0002-8587-9936", "clpid": "Dickinson-M-H" } ], "thesis_committee": [ { "family_name": "Burdick", "given_name": "Joel Wakeman", "orcid": "0000-0002-3091-540X", "clpid": "Burdick-J-W" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dickinson", "given_name": "Michael H.", "orcid": "0000-0002-8587-9936", "clpid": "Dickinson-M-H" } ], "local_group": [ { "literal": "div_bbe" } ], "abstract": "The flight system of the fly is remarkable. A fly can execute an escape maneuver in milliseconds, compensate for wing damage when half of the wing is missing, fly in turbulent conditions, and migrate over large distances. While there are many factors that contribute to the robustness and versatility of insect flight, it is the mechanical encoding of wing motion in the wing hinge that allows flies to rapidly and accurately change wing motion over a large dynamic range. The wing hinge consists of several hardened skeletal elements, named sclerites, and a set of twelve steering muscles are attached to some of these components within the exoskeleton. Due to the anatomical complexity and minute size of the sclerites, the way in which the steering muscles alter the mechanical encoding of wing motion in the hinge is poorly understood.
\r\n\r\nUsing genetically encoded calcium indicators and high-speed videography, is is possible to simultaneously image steering muscle activity and wing motion. In order to extract wing pose from the high-speed video frames, an automated tracking algorithm was developed, that used a neural network and model fitting to accurately reconstruct the wing kinematics. The synchronous recordings of wing motion and steering muscle activity were used to train a convolutional neural network that learned to accurately predict the wing kinematics from muscle activity patterns. After training, the convolutional neural network was used to perform virtual experiments, revealing how the steering muscles regulate wing motion. Correlation analysis revealed that the 12 steering muscles have highly correlated activity. The correlation of muscle activity can be approximated well by a 12D-plane, in which all activity has to reside.
\r\n\r\nTo study the function of the sclerites, a bottleneck was introduced in the convolutional neural network. The bottleneck consists of five neurons, or latent parameters, four parameters corresponding to the state of the different sclerites, on which the steering muscles act, and one parameter representing the wingbeat frequency. This so called latent network predicts both the changes in wing motion and muscle activity patterns as a function of sclerite state. The predicted wing motion as a function of sclerite state matches with previous anatomy and electrophysiology studies for the basalare, first axillary and third axillary sclerites. The fourth axillary sclerite has not been studied before, but shows an antagonistic relationship between the hg1,2 and hg3,4 muscles, resulting in a strong decrease and increase, respectively, of stroke amplitude, deviation and wing pitch angles.
\r\n\r\nBy replaying the wing kinematics of the virtual experiments on a dynamically scaled robotic fly, a model of the aerodynamic and inertial control forces as a function of steering muscle activity was constructed. This control force model was subsequently integrated in a state-space system of fly flight, which in turn was integrated in a model predictive control simulation that was used to simulate free flight maneuvers. The body motion, steering muscle activity, and wing kinematics of the model predictive control simulations were strikingly similar to the recorded maneuvers of free-flying flies.
\r\n\r\nThe integrative, multi-disciplinary approach that was used to reveal the mechanical logic of the wing hinge, and the control problem that a fly needs to solve to stay airborne, are both unprecedented in prior literature. The methodologies and models of this study will be a valuable resource in future research on how the fly's nervous system controls the complex behavior that is flight.
", "doi": "10.7907/teej-tb66", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14520", "collection": "thesis", "collection_id": "14520", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03222022-135834919", "primary_object_url": { "basename": "Benedikt_Barthel_Caltech_PhD_Thesis_Final.pdf", "content": "final", "filesize": 12475488, "license": "other", "mime_type": "application/pdf", "url": "/14520/1/Benedikt_Barthel_Caltech_PhD_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "On the Variational Principles of Linear and Nonlinear Resolvent Analysis", "author": [ { "family_name": "Barthel", "given_name": "Benedikt", "orcid": "0000-0002-6890-5047", "clpid": "Barthel-Benedikt" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Chini", "given_name": "Gregory", "orcid": "0000-0001-5539-3364", "clpid": "Chini-Gregory" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Despite decades of research, the accurate and efficient modeling of turbulent flows remains a challenge. However, one promising avenue of research has been the resolvent analysis framework pioneered by McKeon and Sharma (2010) which interprets the nonlinearity of the Navier-Stokes equations (NSE) as an intrinsic forcing to the linear dynamics. This thesis contributes to the advancement of both the linear and nonlinear aspects of resolvent analysis (RA) based modeling of wall bounded turbulent flows. On the linear front, we suggest an alternative definition of the resolvent basis based on the calculus of variations. The proposed formulation circumvents the reliance on the inversion of the linear operator and is inherently compatible with any arbitrary choice of norm. This definition, which defines resolvent modes as stationary points of an operator norm, allows for more tractable analytical manipulation and leads to a straightforward approach to approximate the resolvent (response) modes of complex flows as expansions in any arbitrary basis. The proposed method avoids matrix inversions and requires only the spectral decomposition of a matrix of significantly reduced size as compared to the original system, thus having the potential to open up RA to the investigation of larger domains and more complex flow configurations. These analytical and numerical advantages are illustrated through a series of examples in one and two dimensions. The nonlinear aspects of RA are addressed in the context of Taylor vortex flow. Highly truncated and fully nonlinear solutions are computed by treating the nonlinearity not as an inherent part of the governing equations but rather as a triadic constraint which must be satisfied by the model solution. Our results show that as the Reynolds number increases, the flow undergoes a fundamental transition from a classical weakly nonlinear regime, where the forcing cascade is strictly down scale, to a fully nonlinear regime characterized by the emergence of an inverse (up scale) forcing cascade. It is shown analytically that this is a direct consequence of the structure of the quadratic nonlinearity of the NSE formulated in Fourier space. Finally, we suggest an algorithm based on the energy conserving nature of the nonlinearity of the NSE to reconstruct the phase information, and thus higher order statistics, from knowledge of solely the velocity spectrum. We demonstrate the potential of the proposed algorithm through a series of examples and discuss the challenges and potential applications to the study and simulation of turbulent flows.
", "doi": "10.7907/sy44-d841", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14649", "collection": "thesis", "collection_id": "14649", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292022-204424650", "type": "thesis", "title": "Foundations and Applications of Single-Cell RNA Sequencing", "author": [ { "family_name": "Booeshaghi", "given_name": "Ali Sina", "orcid": "0000-0002-6442-4502", "clpid": "Booeshaghi-Ali-Sina" } ], "thesis_advisor": [ { "family_name": "Pachter", "given_name": "Lior S.", "orcid": "0000-0002-9164-6231", "clpid": "Pachter-L" } ], "thesis_committee": [ { "family_name": "Greer", "given_name": "Julia R.", "orcid": "0000-0002-9675-1508", "clpid": "Greer-J-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Melsted", "given_name": "P\u00e1ll", "orcid": "0000-0002-8418-6724", "clpid": "Melsted-P\u00e1ll" }, { "family_name": "Pachter", "given_name": "Lior S.", "orcid": "0000-0002-9164-6231", "clpid": "Pachter-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Single-cell RNA-sequencing is an experimental technique for studying cellular gene expression, with a multitude of engineering challenges. These challenges transcend the boundaries of traditional academic disciplines and the field of mechanical engineering, that aims to address roadblocks in critical technologies towards engineering our environment, is central to this endeavor.
\r\n\r\nThis thesis addresses three engineering challenges that must be met in order to realize the goal of bringing single-cell RNA sequencing to the clinic. The first is scalable cellular isolation and sampling. Chapter 2 describes the poseidon and colosseum instruments that enable massive scale single-cell isolation and collection. They each have novel design elements that reduce cost and enable modularity, at a similar accuracy to expensive commercial alternatives.
\r\n\r\nThe second challenge is the rapid preprocessing of single-cell RNA-sequencing data. Chapter 3 describes the kallisto | bustools command-line tools that make scalable scRNAseq analysis fast and efficient. These tools implement novel algorithms for sequence read-alignment, barcode error correction, and molecular counting that helps resolve ambiguities in sequence mapping.
\r\n\r\nThe third challenge is refining gene expression data to the isoform level. This refinement is crucial for understanding transcriptional regulation and the effects of alternative splicing in biological processes. Towards that end, I have extended the kallisto | bustools workflow to process full-length scRNAseq data taking advantage of expectation maximization algorithm to disambiguate sequence alignments. Chapter four describes how I used these tools to assemble the first ever spatially-resolved single-cell isoform atlas, and in particular one of great interest in the neuroscience community (the mouse primary motor cortex) with data generated with three RNA-sequencing assays.
", "doi": "10.7907/ptbp-a779", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14377", "collection": "thesis", "collection_id": "14377", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09282021-234035965", "primary_object_url": { "basename": "LeeMarcus_Thesis.pdf", "content": "final", "filesize": 35706257, "license": "other", "mime_type": "application/pdf", "url": "/14377/1/LeeMarcus_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Instabilities in the Flow Over a Spinning Disk at Angle of Attack", "author": [ { "family_name": "Lee", "given_name": "Marcus Kuok Kuan", "orcid": "0000-0003-3972-843X", "clpid": "Lee-Marcus-Kuok-Kuan" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Micro air vehicles (MAVs) face stability issues, especially as they continue to decrease in size. A spinning disk is inherently robust to external disturbances due to its spin stabilization, and therefore is a potential design for stable MAV flight. However, controlled flight of a spinning disk requires a detailed understanding of the underlying flow structures that determine the aerodynamic behavior. A spinning disk acts to rotate and propel nearby flow tangentially outwards, while drawing in fluid from above. In this way, spin acts as an additional source of both angular and linear momentum from the disk's surface, which can alter the wake structure significantly. In this thesis, we explore how spin affects the aerodynamic forces on a disk and characterize several instabilities that occur. To this end, we use the immersed-boundary Lattice Green's function (IBLGF) method to simulate flow over a spinning disk at angle of attack for Reynolds numbers of O(102) and tip-speed ratios (non-dimensional spin rate) up to 3.
\r\n\r\nAt these Reynolds numbers, the steady flow first undergoes a bifurcation associated with wake instability, giving rise to vortex shedding. Increasing tip-speed ratio leads to monotonic increases in both lift and drag, although the lift-to-drag ratio remains fairly constant. We also identify several distinct wake regimes, including a region of vortex-shedding suppression, and the appearance of a distinct corkscrew-like short-wavelength instability in the advancing tip vortex. To understand the mechanism leading to suppression of vortex shedding, we study the streamlines and vortex lines in the wake. We show that the vorticity produced by the spinning disk strengthens the tip vortices, inducing a spanwise flow in the trailing edge vortex sheet. This helps to dissipate the vorticity, which in turn prevents roll up and thus suppresses vortex shedding. For the short-wavelength instability, we use spectral proper orthogonal decomposition (SPOD) to identify the most energetic modes and compare it to elliptic instabilities seen in counter-rotating vortex pairs with axial flow. The addition of vorticity from the disk rotation significantly alters the circulation and axial velocity in the tip vortices, giving rise to elliptic instability despite its absence in the non-spinning case. We also observe lock-in between the frequency of the elliptic instability and twice the spin frequency, indicating that disk rotation acts as an additional forcing for the elliptic instability. Many of these phenomena are consistent with observations in high Reynolds number studies and for other bluff body geometries. As a result, the mechanisms proposed here may serve as a basis for understanding and predicting the changing wake structures in more complex flow configurations.
", "doi": "10.7907/kmhn-7e49", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14567", "collection": "thesis", "collection_id": "14567", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04292022-213605156", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 12618424, "license": "other", "mime_type": "application/pdf", "url": "/14567/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Transport and Microrheology of Active Colloids", "author": [ { "family_name": "Peng", "given_name": "Zhiwei", "orcid": "0000-0002-9486-2837", "clpid": "Peng-Zhiwei" } ], "thesis_advisor": [ { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "thesis_committee": [ { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "orcid": "0000-0002-0291-4215", "clpid": "Shapiro-M-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Active colloids are micron-sized particles that self-propel through viscous fluids by converting energy extracted from their environment into mechanical motion. The origin or mechanism of their locomotion can be either biological or synthetic ranging from motile bacteria to artificial phoretic particles. Owing to their ability to self-propel, active colloids are out of thermodynamic equilibrium and exhibit interesting macroscopic or collective dynamics. In particular, active colloids exhibit accumulation at confining boundaries, upstream swimming in Poiseuille flow, and a reduced or negative apparent shear viscosity. My work has been focused on a theoretical and computational understanding of the dynamics of active colloids under the influence of confinement and external fluid flows, which are ubiquitous in biological processes. I consider the transport of active colloids in channel flows, the microrheology of active colloids, and lastly I propose and study a vesicle propulsion system based on the learned principles.
\r\n\r\nA generalized Taylor dispersion theory is developed to study the transport of active colloids in channel flows. I show that the often-observed upstream swimming can be explained by the biased upstream reorientation due to the flow vorticity. The longitudinal dispersion of active colloids includes the classical shear-enhanced dispersion and an active swim diffusivity. Their coupling results in a non-monotonic variation of the dispersivity as a function of the flow speed. To understand the effect of particle shape on the transport of active colloids, a simulation algorithm is developed that is able to faithfully resolve the inelastic collision between an ellipsoidal particle and the channel walls. I show that the collision-induced rotation for active ellipsoids can suppress upstream swimming. I then investigate the particle-tracking microrheology of active colloids. I show that active colloids exhibit a swim-thinning microrheology and a negative microviscosity can be observed when certain hydrodynamic effects are considered. I show that the traditional constant-velocity probe model is not suitable for the quantification of fluctuations in the suspension. To resolve this difficulty, a generalized microrheology model that closely mimics the experimental setup is developed. I conclude by proposing a microscale propulsion system in which active colloids are encapsulated in a vesicle with a semi-permeable membrane that allows water to pass through. By maintaining an asymmetric number density distribution, I show that the vesicle can self-propel through the surrounding viscous fluid.
", "doi": "10.7907/wa00-y892", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14441", "collection": "thesis", "collection_id": "14441", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12052021-002539806", "type": "thesis", "title": "Swimming in Potential Flow", "author": [ { "family_name": "Glisman", "given_name": "Alec Gregory", "orcid": "0000-0001-9677-1958", "clpid": "Glisman-Alec-Gregory" } ], "thesis_advisor": [ { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "thesis_committee": [ { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "local_group": [ { "literal": "div_chem" } ], "abstract": "Active bodies undergo self-propulsive motion in a fluid medium and span a broad range of length and time scales. This report focuses specifically on the motion at high Reynolds number, where inertial forces dominate the fluid dynamics. Many active systems spontaneously self-organize into visually striking structures: fish schooling, birds flocking, and bacterial colonies growing. Current models of emergent behavior in the inertial regime are mainly phenomenological and do not account for the fluid-mediated interactions between bodies. We seek to advance physical models of swimmers in high inertia environments. To this end, we explicitly model the hydrodynamics to discern what role the fluid medium plays in active group dynamics and whether it can reproduce the observed emergent phenomenon without the imposition of phenomenologically based interaction rules.
\r\n\r\nA minimal swimmer model consisting of three linked spheres is constructed, and we find self-propulsion without external forces or momentum transfer via vortex shedding. The inertial swimmer is also compared to an identical swimmer in the Stokes regime---where fluid inertia is neglected. The Stokes hydrodynamics are longer-ranged at leading order, and we demonstrate that the stronger hydrodynamic interactions lead to a greater center of mass translation after a period of articulation.
", "doi": "10.7907/6xkb-rs66", "publication_date": "2022", "thesis_type": "masters", "thesis_year": "2022" }, { "id": "thesis:14489", "collection": "thesis", "collection_id": "14489", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02062022-203334930", "primary_object_url": { "basename": "Ben_Stevens_Thesis (4).pdf", "content": "final", "filesize": 5499929, "license": "other", "mime_type": "application/pdf", "url": "/14489/1/Ben_Stevens_Thesis (4).pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Applications of Machine Learning to Finite Volume Methods", "author": [ { "family_name": "Stevens", "given_name": "Benjamin Carter", "orcid": "0000-0002-3410-5922", "clpid": "Stevens-Benjamin-Carter" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Yue", "given_name": "Yisong", "orcid": "0000-0001-9127-1989", "clpid": "Yue-Yisong" }, { "family_name": "Anandkumar", "given_name": "Anima", "orcid": "0000-0002-6974-6797", "clpid": "Anandkumar-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The finite volume method (FVM) has been one of the primary tools of computational fluid dynamics (CFD) for many decades. This method allows for the approximate solution of a partial differential equation (PDE) to be determined by breaking up a problem with no analytical solution into smaller pieces that can be solved together to get a physically realistic simulation. These algorithms can even be used for PDEs with discontinuous solutions, though they must be carefully designed for those situations because they cannot assume any level of smoothness in the solution. An FVM that has been designed for PDEs with discontinuous solutions is referred to as a shock-capturing method. For most of their history, FVM algorithms have been developed using rigorous mathematical arguments to formally maximize the order of convergence of the solution as the grid is refined. However, these arguments depend on the solution to the PDE being smooth, and therefore do not apply to shock-capturing methods. Instead, shock-capturing methods have traditionally been designed using human intuition to create algorithms that then perform well empirically. In this thesis, we instead follow a data-driven approach to train neural networks to use for enhanced FVM methods.
\r\n\r\nBy including a neural network in our FVM, we can use empirical data to optimize the algorithm. We can also utilize ideas from traditional FVM algorithms to create hybrid methods that have tunable parameters and maintain convergence guarantees present in FVMs that have been designed by hand. We explore these hybrid methods in a variety of settings. First, we create a general-purpose shock-capturing method WENO-NN by hybridizing the popular shock-capturing method WENO-JS with a neural network. Additionally, we develop a network architecture, called FiniteNet, that can be used to learn a coarse-graining model associated with a specific PDE and embed it into an FVM scheme. Finally, we also explore the idea of using transfer learning to further improve the WENO-NN for specific problems and name the resulting algorithm WENO-TL. We demonstrate experimentally that this hybrid approach results in methods that can offer similar error levels as traditional FVMs at less computational cost. Although the neural network increases the computational cost of one evaluation of our hybrid FVM, these methods also allow the simulation to be carried out on a coarser grid, leading to a net reduction in both simulation time and memory usage.
", "doi": "10.7907/41qn-7n22", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14097", "collection": "thesis", "collection_id": "14097", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03022021-005902351", "type": "thesis", "title": "Resolvent Modeling of Turbulent Jets", "author": [ { "family_name": "Pickering", "given_name": "Ethan Marcus", "orcid": "0000-0002-4485-6359", "clpid": "Pickering-Ethan-Marcus" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Optimal control of turbulent flows requires a detailed prediction of the unsteady, three-dimensional turbulent structures that govern quantities of interest like noise, drag, and mixing efficiency. There is a need for physics-based, reduced-order models of turbulent structure for those cases where direct simulation of the flow would be computationally prohibitive. In this thesis, we explore resolvent analysis as a framework for such models. Based on a linearization about the turbulent mean flow field, the resolvent finds optimal (highest gain) forcing functions that give rise, through linear amplification mechanisms, to energetic coherent structures. The forcing functions represent the nonlinear interactions between the coherent structures as well as with background incoherent turbulence. While the high-gain structures capture many characteristics of the observed turbulent coherent structures in both wall-bounded and free-shear flows, closures for the forcing function are required to make these models predictive and thus utilize them for flow control.
\r\n\r\nIn the first part of this thesis, we examine a linear model for the resolvent forcing by adapting the concept of a turbulent (eddy) viscosity from classical Reynolds-Averaged Navier--Stokes (RANS) turbulence modeling. We present a data-driven approach to identify an optimal eddy-viscosity field that best matches the resolvent prediction to the most energetic coherent structure educed via spectral proper orthogonal decomposition (SPOD) of data from high-fidelity simulations. We analyze the specific case of turbulent jets spanning a range of Mach numbers from subsonic to supersonic. We find the optimal eddy-viscosity field to be effective at matching both the shape and energy distribution of structures. More importantly, we find that calibrated eddy-viscosity fields predicted using standard eddy-viscosity models (utilizing only quantities available from RANS) yield results that are close to optimal.
\r\n\r\nWe use the resulting resolvent model together with the high-fidelity data to investigate the full spectrum of amplification mechanisms and coherent structures present in turbulent jets. The addition of a turbulence model provides a clear separation between two established mechanisms in turbulent jets (Kelvin-Helmholtz and Orr) and leads to the identification of a third mechanism known as lift-up. Lift-up becomes the dominant mechanism at low-frequency limits for nonzero azimuthal wavenumbers, generating elongated, streaky structures. We find these streaks to be the most energetic structures in the jet, and that their presence has implications for altering the mean flow and controlling noise.
\r\n\r\nFinally, we extend resolvent analysis to that of an acoustic analogy that relates the near-field forcing to the far-field acoustics 100 diameters from the nozzle. We again leverage high-fidelity data to produce an ensemble of realizations of the acoustic field and find that only a few resolvent modes are necessary for reconstruction. Ultimately, we find that a resolvent model based solely upon RANS quantities can reconstruct and predict the peak acoustic field at rank-1 to within 2 decibels for both the supersonic and transonic jets.
", "doi": "10.7907/szxb-f168", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14249", "collection": "thesis", "collection_id": "14249", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062021-094519451", "primary_object_url": { "basename": "RuanJ_Thesis_2021.pdf", "content": "final", "filesize": 6024161, "license": "other", "mime_type": "application/pdf", "url": "/14249/1/RuanJ_Thesis_2021.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Streamwise Homogeneous Turbulent Boundary Layers", "author": [ { "family_name": "Ruan", "given_name": "Joseph Y.", "orcid": "0000-0002-9110-0458", "clpid": "Ruan-Joseph-Y" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Boundary layers are everywhere and computing direct numerical simulations (DNS) of them is crucial for drag reduction. However, traditional DNS of flat-plate boundary layers are prohibitively expensive. Due to the streamwise inhomogeneity of the boundary layer, simulations of spatially growing boundary layer simulations require long domains and long convergence times. Current methods to overcome streamwise inhomogeneity (and allow for shorter streamwise domains) either suffer from a lack of stationarity or have difficult numerical implementation. The goal of this thesis is to develop and validate a more efficient method for simulating boundary layers that will be both statistically stationary and streamwise homogeneous.
\r\n\r\nThe current methodology is developed and validated for the flat plate, zero pressure gradient, incompressible boundary layer. The Navier-Stokes equations are rescaled by a boundary layer thickness to produce a new set of governing equations that resemble the original Navier-Stokes equations with additional source terms. Streamwise homogeneity and statistical stationarity are verified through non-periodic and periodic simulations, respectively. To test the accuracy of the methodology, a sweep of Reynolds number simulations is conducted in streamwise periodic domains for Re\u03b4*=1460-5650. The global quantities show excellent agreement with established empirical values: the computed shape factor and skin friction coefficient for all cases are within 3% and 1% of empirical values, respectively. Furthermore, to obtain accurate two-point correlations, it is sufficient to have a computational domain of length 14\u03b499 and width 5\u03b499, thus, leading to large computational savings by one-to-two orders of magnitude. This translates into increasing the largest possible Reynolds number one could simulate by about a factor of 3.
\r\n\r\nThanks to the streamwise homogeneous nature of the simulation results, it is now possible to apply cost-efficient data-driven techniques like spectral proper orthogonal decomposition (SPOD; Towne et al. 2018) to extract turbulent structures. Particular emphasis is place on identifying structures for waves in the inner and outer layers. To interpret these structures, 1D resolvent analysis (McKeon and Sharma 2010) is leveraged. The peak location for the extracted inner wave is captured by traditional resolvent analysis, assuming a parallel flow. However, the peak location for the extracted outer wave differs from that predicted by the classic 1D resolvent analysis by 20%. Recovering the peak location requires including in the resolvent operator the mean wall-normal velocity profile and the streamwise growth of the boundary layer.
\t\r\n\r\nThis methodology has natural extensions to slowly growing boundary layer flows, including thermal boundary layers, rough wall boundary layers and mild pressure gradient flows.
", "doi": "10.7907/qjfk-5q05", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14253", "collection": "thesis", "collection_id": "14253", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072021-162542722", "primary_object_url": { "basename": "Caltech_Thesis_Ke_Yu_final.pdf", "content": "final", "filesize": 13477246, "license": "other", "mime_type": "application/pdf", "url": "/14253/1/Caltech_Thesis_Ke_Yu_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Multi-Resolution Lattice Green's Function Method for High Reynolds Number External Flows", "author": [ { "family_name": "Yu", "given_name": "Ke", "orcid": "0000-0003-0157-4471", "clpid": "Yu-Ke" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work expands the state-of-the-art computational fluid dynamics (CFD) methods for simulating three-dimensional, turbulent, external flows by further developing the immersed boundary (IB) Lattice Green's function (LGF) method. \r\nThe original IB-LGF method applies an exact far-field boundary condition using fundamental solutions on regular Cartesian grids and allows active computational cells to be restricted to vortical flow regions in an adaptive fashion as the flow evolves. The combination of spatial adaptivity and regular Cartesian structure leads to superior efficiency, scalability, and robustness, but necessitates uniform grid spacing. However, the scale separation associated with thin boundary layers and turbulence at higher Reynolds numbers favors a more flexible distribution of elements/cells, which is achieved in this thesis by developing a multi-resolution LGF approach that permits block-wise grid refinement while maintaining the important properties of the original scheme. We further show that the multi-resolution LGF method can be fruitfully combined with the IB method to simulate external flows around complex geometries at high Reynolds numbers. This novel multi-resolution IB-LGF scheme retains good efficiency, parallel scaling as well as robustness (conservation and stability properties). DNS of bluff and streamlined bodies at Reynolds numbers O(104) are conducted and the new multi-resolution scheme is shown to reduce the total number of computational cells up to 99.87%.
\r\n\r\nWe also extended this method to large-eddy simulation (LES) with the stretched-vortex sub-grid-scale model. In validating the LES implementation, we considered an isolated spherical region of turbulence in free space. The initial condition is spherically windowed, isotropic homogeneous incompressible turbulence. We study the spectrum and statistics of the decaying turbulence and compare the results with decaying isotropic turbulence, including cases representing different low wavenumber behavior of the energy spectrum (i.e. k2 versus k4). At late times the turbulent sphere expands with both mean radius and integral scale showing similar time-wise growth exponents. The low wavenumber behavior has little effect on the inertial scales, and we find that decay rates follow Saffman's predictions in both cases, at least until about 400 initial eddy turnover times. The boundary of the spherical region develops intermittency and features ejections of vortex rings. These are shown to occur at the integral scale of the initial turbulence field and are hypothesized to occur due to a local imbalance of impulse on this scale.
", "doi": "10.7907/wkc8-se35", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14253", "collection": "thesis", "collection_id": "14253", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072021-162542722", "primary_object_url": { "basename": "Caltech_Thesis_Ke_Yu_final.pdf", "content": "final", "filesize": 13477246, "license": "other", "mime_type": "application/pdf", "url": "/14253/1/Caltech_Thesis_Ke_Yu_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Multi-Resolution Lattice Green's Function Method for High Reynolds Number External Flows", "author": [ { "family_name": "Yu", "given_name": "Ke", "orcid": "0000-0003-0157-4471", "clpid": "Yu-Ke" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work expands the state-of-the-art computational fluid dynamics (CFD) methods for simulating three-dimensional, turbulent, external flows by further developing the immersed boundary (IB) Lattice Green's function (LGF) method. \r\nThe original IB-LGF method applies an exact far-field boundary condition using fundamental solutions on regular Cartesian grids and allows active computational cells to be restricted to vortical flow regions in an adaptive fashion as the flow evolves. The combination of spatial adaptivity and regular Cartesian structure leads to superior efficiency, scalability, and robustness, but necessitates uniform grid spacing. However, the scale separation associated with thin boundary layers and turbulence at higher Reynolds numbers favors a more flexible distribution of elements/cells, which is achieved in this thesis by developing a multi-resolution LGF approach that permits block-wise grid refinement while maintaining the important properties of the original scheme. We further show that the multi-resolution LGF method can be fruitfully combined with the IB method to simulate external flows around complex geometries at high Reynolds numbers. This novel multi-resolution IB-LGF scheme retains good efficiency, parallel scaling as well as robustness (conservation and stability properties). DNS of bluff and streamlined bodies at Reynolds numbers O(104) are conducted and the new multi-resolution scheme is shown to reduce the total number of computational cells up to 99.87%.
\r\n\r\nWe also extended this method to large-eddy simulation (LES) with the stretched-vortex sub-grid-scale model. In validating the LES implementation, we considered an isolated spherical region of turbulence in free space. The initial condition is spherically windowed, isotropic homogeneous incompressible turbulence. We study the spectrum and statistics of the decaying turbulence and compare the results with decaying isotropic turbulence, including cases representing different low wavenumber behavior of the energy spectrum (i.e. k2 versus k4). At late times the turbulent sphere expands with both mean radius and integral scale showing similar time-wise growth exponents. The low wavenumber behavior has little effect on the inertial scales, and we find that decay rates follow Saffman's predictions in both cases, at least until about 400 initial eddy turnover times. The boundary of the spherical region develops intermittency and features ejections of vortex rings. These are shown to occur at the integral scale of the initial turbulence field and are hypothesized to occur due to a local imbalance of impulse on this scale.
", "doi": "10.7907/wkc8-se35", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13996", "collection": "thesis", "collection_id": "13996", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11102020-065104205", "type": "thesis", "title": "Numerical Investigation of Compressibility Effects in Reacting Subsonic Flows", "author": [ { "family_name": "Beardsell", "given_name": "Guillaume", "orcid": "0000-0001-7138-488X", "clpid": "Beardsell-Guillaume" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "Dale I.", "clpid": "Pullin-D-I" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Direct numerical simulations (DNS) of reacting flows are routinely performed either by solving the fully compressible Navier-Stokes equations or using the low Mach number approximation. The latter is obtained by performing a Mach number expansion of the Navier-Stokes equations for small Mach numbers. These two frameworks differ by their ability to capture compressibility effects, which can be broadly defined as phenomena that are not captured by the low Mach number approximation. These phenomena include acoustics, compressible turbulence, and shocks. In this thesis, we systematically isolate compressibility effects in subsonic flows by performing two sets of DNS: one using the fully compressible framework, and one using the low Mach number approximation. We are specifically interested in the interactions between turbulence, acoustics, and flames.
\r\n\r\nThe addition of detailed chemistry in the compressible flow solver required the development of a novel time integration scheme. This scheme combines an iterative semi-implicit method for the integration of the species transport equations, and the classical Runge-Kutta method for the integration of the other flow quantities. It is found to perform well, yielding time steps limited by the acoustic CFL only. Furthermore, the computational cost per iteration of this hybrid scheme is low, being comparable to the one for the classical Runge-Kutta method.
\r\n\r\nAfter extensive validation, the first application is the investigation of flame-acoustics interactions in laminar premixed flames. The thermodynamic fluctuations that accompany the acoustic wave are shown to significantly impact the flame response. Using the Rayleigh criterion, the flame-acoustics system is found to be thermo-acoustically unstable for various fuels, flow conditions, and acoustic frequencies. As expected, the low Mach number approximation and the fully compressible framework are in good agreement at low frequencies, since the flame is very thin compared to the acoustic wavelength. The two frameworks differ for very large acoustic frequencies only. In the high frequency limit, the gain reaches a plateau using the low Mach number approximation, while it goes to zero using the fully compressible framework. This is related to the spatial variations in the acoustic pressure field, which are not present in the low Mach number approximation. However, for practically relevant acoustic frequencies, the low Mach number framework is found to yield accurate results.
\r\n\r\nNext, a numerical methodology to simulate compressible flows in geometries that lack a natural turbulence generation mechanism is presented. It is found that, unlike in incompressible flows, special care must be taken regarding the energy equation and the presence of standing acoustic modes. When using periodic boundary conditions, forcing the dilatational velocity field promotes the growth of unstable modes. This is explained by extracting the eigenvalues of the linearized forced Navier-Stokes equations. Based on these observations, it is found necessary to force the solenoidal velocity field only. This methodology is applied first to simulations of subsonic homogeneous non-reacting turbulence. We present simulations results for turbulent Mach numbers varying from 0.02 to 0.65. The Mach number dependence of various quantities, such as the dilatational to solenoidal kinetic energy ratio, is extracted. The Mach number scaling of all quantities of interest is found to be readily explained by the low Mach number expansion, specifically the zeroth and first order sets of equations, for turbulent Mach numbers up to 0.1.
\r\n\r\nFinally, the interaction between subsonic compressible turbulence and premixed flames is investigated. Compressibility effects are isolated by comparing results obtained with the low Mach number approximation and the fully compressible framework, at the same flow conditions. Compressibility effects on chemistry are found to be limited for turbulent Mach numbers at least up to 0.4, especially when contrasted with the large impact of the Karlovitz number. Compressibility effects give rise to significant thermodynamic fluctuations away from the flame front, but these remain small compared to the large fluctuations due to the presence of the turbulent flame brush. The low Mach number approximation thus remains a valid framework for the Mach numbers considered, when the primary goal is to characterize the impact of turbulence on the chemical processes at play.
", "doi": "10.7907/dtfx-gy14", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14146", "collection": "thesis", "collection_id": "14146", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05132021-180953405", "primary_object_url": { "basename": "lawson_joel_2021.pdf", "content": "final", "filesize": 34790195, "license": "other", "mime_type": "application/pdf", "url": "/14146/1/lawson_joel_2021.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Focused Laser Differential Interferometry", "author": [ { "family_name": "Lawson", "given_name": "Joel Michael", "orcid": "0000-0002-3042-0909", "clpid": "Lawson-Joel-Michael" } ], "thesis_advisor": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/5thh-f652", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13721", "collection": "thesis", "collection_id": "13721", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05242020-045332969", "type": "thesis", "title": "Modifying Ultrasound Waveform Parameters to Control, Influence, or Disrupt Cells", "author": [ { "family_name": "Mittelstein", "given_name": "David Reza", "orcid": "0000-0001-8747-0483", "clpid": "Mittelstein-David-Reza" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "orcid": "0000-0002-0291-4215", "clpid": "Shapiro-M-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "orcid": "0000-0002-0291-4215", "clpid": "Shapiro-M-G" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Ultrasound can be focused into deep tissues with millimeter precision to perform non-invasive ablative therapy for diseases such as cancer. In most cases, this ablation uses high intensity ultrasound to deposit non-selective thermal or mechanical energy at the ultrasound focus, damaging both healthy bystander tissue and cancer cells. Here we describe an alternative low intensity pulsed ultrasound approach known as \u201concotripsy\u201d that leverages the distinct mechanical properties of neoplastic cells to achieve inherent cancer selectivity. We show that when applied at a specific frequency and pulse duration, focused ultrasound selectively disrupts a panel of breast, colon, and leukemia cancer cell models in suspension without significantly damaging healthy immune or red blood cells. Mechanistic experiments reveal that the formation of acoustic standing waves and the emergence of cell-seeded cavitation lead to cytoskeletal disruption, expression of apoptotic markers, and cell death. The inherent selectivity of this low intensity pulsed ultrasound approach offers a potentially safer and thus more broadly applicable alternative to non-selective high intensity ultrasound ablation.
\r\n\r\nIn this dissertation, I describe the oncotripsy theory in its initial formulation, the experimental validation and investigation of testable predictions from that theory, and the refinement of said theory with new experimental evidence. Throughout, I describe how careful modifications to the ultrasound waveform directly can significantly impact how the ultrasound bio-effects control, influence, or disrupt cells in a selective and controlled manner.
\r\n", "doi": "10.7907/71ak-w328", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11766", "collection": "thesis", "collection_id": "11766", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08262019-191842947", "primary_object_url": { "basename": "Caltech_Thesis_LaTeX_Template__without_logo_ (3).pdf", "content": "final", "filesize": 6365848, "license": "other", "mime_type": "application/pdf", "url": "/11766/1/Caltech_Thesis_LaTeX_Template__without_logo_ (3).pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Derivation of Realistic Forcing Schemes to Reproduce Turbulent Characteristics of Round Jets on Centerline", "author": [ { "family_name": "Rah", "given_name": "Kyupaeck Jeff", "orcid": "0000-0003-1898-2930", "clpid": "Rah-Kyupaeck-Jeff" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Turbulence forcing techniques are often required in the numerical simulation of statistically stationary turbulent flows. However, the existing forcing techniques are not based on physics, but rather arbitrary numerical methods that sustain the turbulent kinetic energy. In this work, a realistic forcing technique is devised to reproduce the centerline turbulent characteristics of round jets in a triply periodic box.
\r\n\r\nA velocity forcing term is derived from the Navier-Stokes equations by applying a Reynolds decomposition with the mean velocity of the axisymmetric jet. The result is an anisotropic linear forcing term. A series of direct numerical simulations (DNS) are performed over a range of Reynolds numbers by applying the derived velocity forcing term in a 3D cubic box. The budget of the terms in the kinetic energy equation is found to be very close to the experimental measurement on the centerline. The anisotropy ratio, kinetic energy, and dissipation rate of the simulations are also comparable to experimental values. Finally, the kinetic energy spectrum in the axial direction is presented. With appropriate normalizations, the spectrum agrees well with the round jet spectrum on its centerline.
\r\n\r\nA similar procedure is applied to passive scalars to derive a scalar forcing term to simulate the centerline mixing properties of round jets. The term is derived from the scalar transport equation using a Reynolds-like decomposition of the scalar field. The equation is closed by applying the known mean velocity and scalar profiles of axisymmetric jets. The result is a combination of a mean gradient term and a linear scalar term. DNS at different Reynolds numbers have been performed with these source terms for unity Schmidt number. Scalar flux values and scaling exponents of scalar energy spectra from simulations are comparable to experimental values. In addition, a dimensional analysis shows that the normalized scalar statistics, such as variance, flux, and dissipation rate, should only be a function of Reynolds number; indeed, such quantities computed from our simulations approach constant values as the Reynolds number increases. The effects of velocity forcing on scalar fields are also investigated; changing velocity forcing terms may result in unstable scalar fields even under the same scalar forcing.
\r\n\r\nMore computations on higher Schmidt number scalars are performed with the same velocity and scalar forcing terms. It is found that the scalar flux values decrease with increasing Schmidt number for low Reynolds number flows, and reach plateaus as the Schmidt number increases. The flux values also increase with the Reynolds number for all non-unity Schmidt numbers. The scaling exponents of scalar energy spectra are found to decrease with increasing Schmidt number for all Reynolds numbers.
", "doi": "10.7907/7SXH-V275", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11176", "collection": "thesis", "collection_id": "11176", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09072018-105527896", "type": "thesis", "title": "An EnKF-Based Flow State Estimator for Aerodynamic Problems", "author": [ { "family_name": "da Silva", "given_name": "Andre Fernando de Castro", "orcid": "0000-0002-8125-6010", "clpid": "da-Silva-Andre-Fernando-de-Castro" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Stuart", "given_name": "Andrew M.", "orcid": "0000-0001-9091-7266", "clpid": "Stuart-A-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Regardless of the plant model, robust flow estimation based on limited measurements remains a major challenge in successful flow control applications. Aiming to combine the robustness of a high-dimensional representation of the dynamics with the cost efficiency of a low-order approximation of the state covariance matrix, a flow state estimator based on the Ensemble Kalman Filter (EnKF) is applied to two-dimensional flow past a cylinder and an airfoil at high angle of attack and low Reynolds number. For development purposes, we use the numerical algorithm as both the estimator and as a surrogate for the measurements. In a perfect-model framework, a reduced number of either pressure sensors on the surface of the body or sparsely placed velocity probes in the wake are sufficient to accurately estimate the instantaneous flow state. Because the dynamics of these flows are restricted to a low-dimensional manifold of the state space, a small ensemble size is sufficient to yield the correct asymptotic behavior. The relative importance of each sensor location is evaluated by analyzing how they influence the estimated flow field, and optimal locations for pressure sensors are determined.
\r\n\r\nHowever, model inaccuracies are ubiquitous in practical applications. Covariance inflation is used to enhance the estimator performance in the presence of unmodeled freestream perturbations. A combination of parametric modeling and augmented state methodology is used to successfully estimate the forces on immersed bodies subjected to deterministic and random gusts. The robustness of high-dimensional representation of the dynamics to the choice of parameters such as the Reynolds number is inherited by the estimator, which was shown to successfully estimate the reference Reynolds number on the fly. Spatial and temporal discretization can constitute a second source of errors which can render numerical solutions a biased representation of reality. Left unaccounted for, biased forecast and observation models can lead to poor estimator performance. In this work, we propose a low-rank representation for the bias whose dynamics are represented by a colored-noise process. System state and bias parameters are simultaneously tracked online with the Ensemble Kalman Filter (EnKF) algorithm. The proposed methodology is demonstrated to achieve a 70% error reduction for the problem of estimating the state of the two-dimensional low-Re flow past a flat plate at high angle of attack using an ensemble of coarse-mesh simulations and pressure measurements at the surface of the body, compared to a bias-blind estimator. Strategies to determine the bias statistics and to deal with nonlinear observation functions in the context of ensemble methods are discussed.
", "doi": "10.7907/W327-VF41", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11181", "collection": "thesis", "collection_id": "11181", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09122018-201219035", "primary_object_url": { "basename": "Tosi_2018_Thesis.pdf", "content": "final", "filesize": 22012966, "license": "other", "mime_type": "application/pdf", "url": "/11181/1/Tosi_2018_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Fluid-Structure Instability in an Internal Flow Energy Harvester", "author": [ { "family_name": "Tosi", "given_name": "Lu\u00eds Phillipe Costa Ferreira", "orcid": "0000-0002-0819-4765", "clpid": "Tosi-Lu\u00eds-Phillipe-Costa-Ferreira" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Hall", "given_name": "Jeffery", "orcid": "0000-0002-8835-2830", "clpid": "Hall-J-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Access to reliable power sources in remote locations is a recurring engineering challenge for both large and small applications. The developing world struggles with power connectivity in remote villages, while sensor networks strain with power limitations of batteries or short-lived turbines. Energy harvesting based on fluid-induced vibration provides a potential robust alternative for in-situ power generation, furnishing means for a decades long supply of power. Yet, one of the main challenges in the design of flow energy harvesters is understanding the mechanisms that drive their motion. Fluid-structure interaction problems often span a large parametric space and require considerable computational resources to resolve the necessary dynamic details for reliable designs.
\r\n\r\nThis thesis aims to address this challenge for a piezoelectric internal flow energy harvester developed in conjunction with NASA Jet Propulsion Laboratory for in-well, deepwater sensor and actuator systems. Through exploratory experimentation, a configuration consisting of a piezoelectric beam within a converging-diverging channel in axial flow generated considerable power at moderate flow velocities when compared to other devices of the same size. The current device, though adapted to a more robust configuration based on flextensional actuators, still maintains the same fluid-structure interaction: the instability that ensues forces the system into self-sustained oscillations that produces consistent power output for flow rates above a critical threshold.
\r\n\r\nTo understand and quantify this behavior, we develop an analytical framework based on a leakage-flow type instability, which curtails the shortcomings of expensive numerical simulations once verified. The formulation consists of a quasi one-dimensional simplification of coupled fluid-structure equations, which are linearized for classical stability analysis. The stability boundary and critical property predictions are verified through a set of fully coupled fluid-structure immersed boundary direct numerical simulations. Experiments are carried out in tandem to quantify the dynamics of the harvester, specifically targeting the critical flow rate threshold. The analytical framework is expanded to include flow in the spanwise direction of the beam, and results to a simplified geometry of the harvester compared with those from experiments. Agreement between predicted critical values suggest that leakage-flow may be the principal mechanism for fluid-induced vibration within our device. The model can serve as the foundation of initial exploration of design parameters, and perhaps more powerful devices in future endeavors.
", "doi": "10.7907/Y0KG-Y197", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11395", "collection": "thesis", "collection_id": "11395", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02172019-174051312", "primary_object_url": { "basename": "Pressure_and_Stress_Transients_in_Autoinjector_Devices.pdf", "content": "final", "filesize": 29005814, "license": "other", "mime_type": "application/pdf", "url": "/11395/1/Pressure_and_Stress_Transients_in_Autoinjector_Devices.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Pressure and Stress Transients in Autoinjector Devices", "author": [ { "family_name": "Veilleux", "given_name": "Jean-Christophe", "orcid": "0000-0002-5420-9411", "clpid": "Veilleux-Jean-Christophe" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VJSH-TF65", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11453", "collection": "thesis", "collection_id": "11453", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04112019-234812867", "type": "thesis", "title": "Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer", "author": [ { "family_name": "Morgan", "given_name": "Jonathan Philip", "orcid": "0000-0003-2898-4868", "clpid": "Morgan-Jonathan-Philip" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/7RSR-3277", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11453", "collection": "thesis", "collection_id": "11453", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04112019-234812867", "type": "thesis", "title": "Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer", "author": [ { "family_name": "Morgan", "given_name": "Jonathan Philip", "orcid": "0000-0003-2898-4868", "clpid": "Morgan-Jonathan-Philip" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/7RSR-3277", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11707", "collection": "thesis", "collection_id": "11707", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-103225366", "primary_object_url": { "basename": "HuertasCerdeira_Cecilia_2019.pdf", "content": "final", "filesize": 53927544, "license": "other", "mime_type": "application/pdf", "url": "/11707/1/HuertasCerdeira_Cecilia_2019.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering", "author": [ { "family_name": "Huertas-Cerdeira", "given_name": "Cecilia", "orcid": "0000-0003-4553-0470", "clpid": "Huertas-Cerdeira-Cecilia" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/326X-M576", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11707", "collection": "thesis", "collection_id": "11707", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-103225366", "primary_object_url": { "basename": "HuertasCerdeira_Cecilia_2019.pdf", "content": "final", "filesize": 53927544, "license": "other", "mime_type": "application/pdf", "url": "/11707/1/HuertasCerdeira_Cecilia_2019.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering", "author": [ { "family_name": "Huertas-Cerdeira", "given_name": "Cecilia", "orcid": "0000-0003-4553-0470", "clpid": "Huertas-Cerdeira-Cecilia" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/326X-M576", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11710", "collection": "thesis", "collection_id": "11710", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-114300433", "primary_object_url": { "basename": "WangCong_Thesis2019.pdf", "content": "final", "filesize": 26132000, "license": "other", "mime_type": "application/pdf", "url": "/11710/1/WangCong_Thesis2019.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "On the Manipulation of a Turbulent Boundary Layer by Unsteady Boundary Conditions", "author": [ { "family_name": "Wang", "given_name": "Cong", "orcid": "0000-0002-8271-5637", "clpid": "Wang-Cong" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brown", "given_name": "Garry L.", "clpid": "Brown-Garry-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/FT8M-PM75", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11710", "collection": "thesis", "collection_id": "11710", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072019-114300433", "primary_object_url": { "basename": "WangCong_Thesis2019.pdf", "content": "final", "filesize": 26132000, "license": "other", "mime_type": "application/pdf", "url": "/11710/1/WangCong_Thesis2019.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "On the Manipulation of a Turbulent Boundary Layer by Unsteady Boundary Conditions", "author": [ { "family_name": "Wang", "given_name": "Cong", "orcid": "0000-0002-8271-5637", "clpid": "Wang-Cong" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brown", "given_name": "Garry L.", "clpid": "Brown-Garry-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/FT8M-PM75", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11737", "collection": "thesis", "collection_id": "11737", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06102019-185605511", "primary_object_url": { "basename": "Chandru_Dhandapani_2019_thesis.pdf", "content": "final", "filesize": 5043454, "license": "other", "mime_type": "application/pdf", "url": "/11737/1/Chandru_Dhandapani_2019_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations", "author": [ { "family_name": "Dhandapani", "given_name": "Chandru", "orcid": "0000-0002-7319-557X", "clpid": "Dhandapani-Chandru" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/FH31-4468", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11737", "collection": "thesis", "collection_id": "11737", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06102019-185605511", "primary_object_url": { "basename": "Chandru_Dhandapani_2019_thesis.pdf", "content": "final", "filesize": 5043454, "license": "other", "mime_type": "application/pdf", "url": "/11737/1/Chandru_Dhandapani_2019_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations", "author": [ { "family_name": "Dhandapani", "given_name": "Chandru", "orcid": "0000-0002-7319-557X", "clpid": "Dhandapani-Chandru" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/FH31-4468", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:10381", "collection": "thesis", "collection_id": "10381", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08182017-103752052", "primary_object_url": { "basename": "MGrivel_Thesis2018.pdf", "content": "final", "filesize": 136293795, "license": "other", "mime_type": "application/pdf", "url": "/10381/1/MGrivel_Thesis2018.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics", "author": [ { "family_name": "Grivel", "given_name": "Morgane Anne Marie", "orcid": "0000-0002-4391-799X", "clpid": "Grivel-Morgane-Anne-Marie" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/Z9736P2V", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10976", "collection": "thesis", "collection_id": "10976", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302018-181049042", "primary_object_url": { "basename": "symon_thesis_2018.pdf", "content": "final", "filesize": 9337476, "license": "other", "mime_type": "application/pdf", "url": "/10976/14/symon_thesis_2018.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Reconstruction and Estimation of Flows Using Resolvent Analysis and Data-Assimilation", "author": [ { "family_name": "Symon", "given_name": "Sean Pearson", "orcid": "0000-0001-9085-0778", "clpid": "Symon-Sean-Pearson" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/B4K7-K876", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10509", "collection": "thesis", "collection_id": "10509", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10122017-095438989", "primary_object_url": { "basename": "goza_andres_2017.pdf", "content": "final", "filesize": 16239607, "license": "other", "mime_type": "application/pdf", "url": "/10509/1/goza_andres_2017.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Methods for Fluid-Structure Interaction, and their Application to Flag Flapping", "author": [ { "family_name": "Goza", "given_name": "Andres Jared", "clpid": "Goza-Andres-Jared" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Sader", "given_name": "John E.", "orcid": "0000-0002-7096-0627", "clpid": "Sader-J-E" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis is divided into two parts. Part I is devoted to the development of numerical techniques for simulating fluid-structure interaction (FSI) systems and for educing important physical mechanisms that drive these systems\u2019 behavior; part II discusses the application of many of these techniques to investigate a specific FSI system.
\r\n\r\nWithin part I, we first describe a procedure for accurately computing the stresses on an immersed surface using the immersed-boundary method. This is a key step to simulating FSI problems, as the surface stresses simultaneously dictate the motion of the structure and enforce the no-slip boundary condition on the fluid. At the same time, accurate stress computations are also important for applications involving rigid bodies that are either stationary or moving with prescribed kinematics (e.g., characterizing the performance of wings and aerodynamic bodies in unsteady flows or understanding and controlling flow separation around bluff bodies). Thus, the method is first formulated for the rigid-body prescribed-kinematics case. The procedure described therein is subsequently incorporated into an immersed boundary method for efficiently simulating FSI problems involving arbitrarily large structural motions and rotations.
\r\n\r\nWhile these techniques can be used to perform high-fidelity simulations of FSI systems, the resulting data often involves a range of spatial and temporal scales in both the structure and the fluid and are thus typically difficult to interpret directly. The remainder of part I is therefore devoted to extending tools regularly used for understanding complex flows to FSI systems. We focus in particular on the application of global linear stability analysis and snapshot-based data analysis (such as dynamic mode decomposition and proper orthogonal decomposition) to FSI problems. To our knowledge, these techniques had not been applied to deforming-body problems in a manner that that accounts for both the fluid and structure leading up to this work.
\r\n\r\nThroughout part I, our methods are derived in the context of fairly general FSI systems and are validated using results from the literature for flapping flags in both the conventional configuration (in which the flag is pinned or clamped at its leading edge with respect to the oncoming flow) and the inverted configuration (in which the flag is clamped at its trailing edge). In part II, we apply many of the techniques developed in part I to uncover new physical mechanisms about inverted-flag flapping. We identify the instability-driving mechanism responsible for the initiation of flapping and further characterize the large-amplitude and chaotic flapping regimes that the system undergoes for a range of physical parameters.
", "doi": "10.7907/Z95T3HPB", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10570", "collection": "thesis", "collection_id": "10570", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11272017-152918929", "type": "thesis", "title": "Coherent Structures, their Interactions, and their Effects on Passive Scalar Transport and Aero-Optic Distortion in a Turbulent Boundary Layer", "author": [ { "family_name": "Saxton-Fox", "given_name": "Theresa Ann", "orcid": "0000-0003-1328-4148", "clpid": "Saxton-Fox-Theresa-Ann" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis focused on the characterization of coherent structures and their interactions in a turbulent boundary layer using data from particle image velocimetry (PIV) measurements performed at Caltech and from a direct numerical simulation (DNS) of Wu et al. (2017). Connections were identified between instantaneous and statistical descriptions of coherent velocity structures, through the analysis of representative models for their structures derived from the resolvent analysis of McKeon and Sharma (2010). The representative models were used in a novel conditional averaging technique to identify the average behavior of small scales about variations in the large-scale streamwise velocity field. Based upon the results of this analysis, a hypothesis for a scale interaction mechanism was proposed involving three-dimensional critical layers. The modeling and analysis methods were then applied to the aero-optic problem in which optical beams are observed to be distorted after passing through variable-density turbulent flows. Measurements using simultaneous PIV and an aero-optic sensor called a Malley probe (Malley, Sutton, and Kincheloe, 1992) were conducted in an incompressible, mildly-heated turbulent boundary layer with Prandtl number of 0.7. A conditional averaging analysis of the data identified that the nonlinear interaction of two scales was most correlated to the aero-optic distortion. The modeling of this interaction using resolvent modes led to new insights regarding the instantaneous relationship between the velocity and scalar fields over a range of Prandtl numbers.
", "doi": "10.7907/Z9W0943P", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10998", "collection": "thesis", "collection_id": "10998", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012018-114927289", "primary_object_url": { "basename": "rosenberg_kevin_2018.pdf", "content": "final", "filesize": 18140748, "license": "other", "mime_type": "application/pdf", "url": "/10998/1/rosenberg_kevin_2018.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Resolvent-Based Modeling of Flows in a Channel", "author": [ { "family_name": "Rosenberg", "given_name": "Kevin Thomas", "orcid": "0000-0001-6101-3823", "clpid": "Rosenberg-Kevin-Thomas" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/PHDW-Z389", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:11030", "collection": "thesis", "collection_id": "11030", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062018-163232775", "primary_object_url": { "basename": "Burali_Nicholas_2018.pdf", "content": "final", "filesize": 11110326, "license": "other", "mime_type": "application/pdf", "url": "/11030/16/Burali_Nicholas_2018.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Towards a priori Models for Differential Diffusion in Turbulent Non-Premixed Flames", "author": [ { "family_name": "Burali", "given_name": "Nicholas", "orcid": "0000-0002-0733-0577", "clpid": "Burali-Nicholas" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this work, progress is made towards the correct modeling of differential diffusion, both for resolved simulations, and for reduced-order combustion models. For resolved simulations, the validity and the limitations of the constant non-unity Lewis number approach in the description of molecular mixing in laminar and turbulent flames is studied. Three test cases are selected, including a lean, highly unstable, premixed hydrogen/air flame, a lean turbulent premixed n-heptane/air flame, and a laminar ethylene/air coflow diffusion flame. For the hydrogen flame, both a laminar and a turbulent configuration are considered. The three flames are characterized by Lewis numbers which are less than unity, greater than unity, and close to unity, respectively. For each flame, mixture-averaged transport simulations are carried out and used as reference data. The analysis suggests that, for numerous combustion configurations, the constant non-unity Lewis number approximation leads to small errors when the set of Lewis numbers is chosen properly. For the selected test cases and our numerical framework, the reduction of computational cost is found to be minimal. Two different methods of evaluating the Lewis numbers are tested, with both performing well, and neither consistently better than the other.
\r\n\r\nThe flamelet-based chemistry tabulation technique is a popular reduced-order chemical model for non-premixed turbulent flames. In this approach, the correct choice of the species Lewis numbers in the flamelet equations plays an important role. Experimental results have highlighted that, in turbulent non-premixed jet flames, turbulent transport becomes gradually dominant over molecular mixing with (i) increasing axial distance from the burner exit plane, and (ii) increasing jet Reynolds number. In the current work, this transition is characterized and a priori models for the effective species Lewis numbers in turbulent non-premixed flames are assessed.
\r\n\r\nFirst, a flamelet-based methodology is proposed to extract these effective Lewis numbers from data sets of turbulent non-premixed flames. This methodology is then applied to the Sandia non-premixed methane/air jet flames B, C, D, and E (R. Barlow, Int. Work. Meas. Comput. Turb. Non-Prem. Flames, 2003). The effective Lewis numbers are found to transition from their laminar values, close to the burner exit plane, to unity further downstream. Previously-suggested scalings for the effective Lewis numbers are then assessed.
\r\n\r\nTo overcome the limitations associated with the experimental data, a campaign of Direct Numerical Simulations (DNS) of Sandia flame B is carried out. A baseline grid is carefully designed, and grid independence is assessed through simulations using refined grids in the axial, radial and azimuthal directions. Radiation and differential diffusion effects are systematically isolated by considering radiating and unity Lewis number cases, respectively. The DNS database is then validated using available measured statistics for flame B, and comparisons to the higher Reynolds number flames are carried out. Effective Lewis numbers extracted from the DNS data are found to transition to unity with increasing downstream distance. Finally, the scalings for the effective Lewis numbers are re-computed from the DNS data base, and compared to the higher Reynolds number flames.
", "doi": "10.7907/N3VJ-BE39", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:11007", "collection": "thesis", "collection_id": "11007", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012018-165819361", "primary_object_url": { "basename": "thesis_maeda.pdf", "content": "final", "filesize": 16816964, "license": "other", "mime_type": "application/pdf", "url": "/11007/1/thesis_maeda.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Simulation, Experiments, and Modeling of Cloud Cavitation with Application to Burst Wave Lithotripsy", "author": [ { "family_name": "Maeda", "given_name": "Kazuki", "orcid": "0000-0002-5729-6194", "clpid": "Maeda-Kazuki" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Shapiro", "given_name": "Mikhail G.", "orcid": "0000-0002-0291-4215", "clpid": "Shapiro-M-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Modeling, numerical simulations, and experiments are used to investigate the dynamics of cavitation bubble clouds induced by strong ultrasound waves.
\r\n\r\nA major application of this work is burst wave lithotripsy (BWL), recently proposed method of lithotripsy that uses pulses (typically 10 wavelengths each) of highintensity, focused ultrasound at a frequency of O(100) kHz and an amplitude of O(1) MPa to break kidney stones. BWL is an alternative to standard shockwave lithotripsy (SWL), which uses much higher amplitude shock waves delivered at a typically much lower rate. In both SWL and BWL, the tensile component of the pressure can nucleate cavitation bubbles in the human body. For SWL, cavitation is a significant mechanism in stone communition, but also causes tissue injury. By contrast, little is yet known about cavitation in BWL.
\r\n\r\nTo investigate cloud cavitation in BWL, two numerical tools are developed: a model of ultrasound generation from a medical transducer, and a method of simulating clouds of cavitation bubbles in the focal region of the ultrasound. The numerical tools enable simulation of the cavitation growth and collapse of individual bubbles, their mutual interactions, and the resulting bubble-scattered acoustics. The numerics are implemented in a massively parallel framework to enable large-scale, three-dimensional simulations. Next, the numerical tools are applied to bubble clouds associated with BWL. Additionally, laboratory experiments are conducted in vitro in order to calibrate and validate the simulations. A major feature of the resulting bubble clouds is that the cloud size is similar to the ultrasound wavelength. This results in an anisotropic structure where the bubbles closest to the wave source grow to larger size and oscillate more rapidly. A new scaling parameter is introduced to characterize the nonlinear bubble cloud dynamics that generalizes the cloud interaction parameter of d'Agostino and Brennen (1989) defined for weak (linearized), bubble cloud dynamics excited uniformly by long-wavelength pressure waves. The mechanisms leading to the observed bubble dynamics are identified. The results further show that bubble clouds can scatter a large portion of incident ultrasound and consequently shield distal regions, including kidney stones, from irradiation. This energy shielding is quantified, and the simulations show that even a thin layer of bubbles can scatter up to 90% of the incident wave energy. A strong correlation is identified between the magnitude of energy shielding and the amplitude of the bubble-scattered acoustics. The correlation may be of use to control cavitation in the human body in real time by ultrasound monitoring for better outcomes of BWL.
", "doi": "10.7907/N7JK-F529", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10998", "collection": "thesis", "collection_id": "10998", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012018-114927289", "primary_object_url": { "basename": "rosenberg_kevin_2018.pdf", "content": "final", "filesize": 18140748, "license": "other", "mime_type": "application/pdf", "url": "/10998/1/rosenberg_kevin_2018.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Resolvent-Based Modeling of Flows in a Channel", "author": [ { "family_name": "Rosenberg", "given_name": "Kevin Thomas", "orcid": "0000-0001-6101-3823", "clpid": "Rosenberg-Kevin-Thomas" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/PHDW-Z389", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10976", "collection": "thesis", "collection_id": "10976", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302018-181049042", "primary_object_url": { "basename": "symon_thesis_2018.pdf", "content": "final", "filesize": 9337476, "license": "other", "mime_type": "application/pdf", "url": "/10976/14/symon_thesis_2018.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Reconstruction and Estimation of Flows Using Resolvent Analysis and Data-Assimilation", "author": [ { "family_name": "Symon", "given_name": "Sean Pearson", "orcid": "0000-0001-9085-0778", "clpid": "Symon-Sean-Pearson" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/B4K7-K876", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10219", "collection": "thesis", "collection_id": "10219", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302017-094600781", "primary_object_url": { "basename": "rufat_dzhelil_2017_thesis.pdf", "content": "final", "filesize": 39366010, "license": "other", "mime_type": "application/pdf", "url": "/10219/1/rufat_dzhelil_2017_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Spectral Exterior Calculus and its Implementation", "author": [ { "family_name": "Rufat", "given_name": "Dzhelil Sabahatin", "orcid": "0000-0001-8766-2338", "clpid": "Rufat-Dzhelil-Sabahatin" } ], "thesis_advisor": [ { "family_name": "Desbrun", "given_name": "Mathieu", "orcid": "0000-0003-3424-6079", "clpid": "Desbrun-M" } ], "thesis_committee": [ { "family_name": "Desbrun", "given_name": "Mathieu", "orcid": "0000-0003-3424-6079", "clpid": "Desbrun-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Porter", "given_name": "Frank C.", "orcid": "0000-0003-1948-8889", "clpid": "Porter-F-C" }, { "family_name": "Bellan", "given_name": "Paul Murray", "orcid": "0000-0002-0886-8782", "clpid": "Bellan-P-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Preserving geometric, topological and algebraic structures at play in partial differential equations has proven to be a fruitful guiding principle for computational methods in a variety of scientific fields. However, structure-preserving numerical methods have traditionally used spaces of piecewise polynomial basis functions with local support to interpolate differential forms. When solutions are known to be smooth, a spectral treatment is often preferred instead as it brings exponential convergence. While recent works have established spectral variants of discrete exterior calculus, no existing approach offers the full breadth of exterior calculus operators and a clear distinction between vectors and covectors. We present such a unified approach to spectral exterior calculus (SPEX) and provide detail on its implementation. Notably, our approach leverages Poincare duality through the use of a primal grid and its dual (with a natural handling of boundaries to facilitate the treatment of boundary conditions), and uses a twin representation of differential forms as both integrated and pointwise values. Through its reliance on the fast Fourier transform, the resulting framework enables computations in arbitrary dimensions that are both efficient and have excellent convergence properties.", "doi": "10.7907/Z9VX0DKV", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10194", "collection": "thesis", "collection_id": "10194", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232017-111820349", "primary_object_url": { "basename": "Hufstedler_Esteban_2017_final.pdf", "content": "final", "filesize": 15108414, "license": "other", "mime_type": "application/pdf", "url": "/10194/1/Hufstedler_Esteban_2017_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Experimental Generation and Modeling of Vortical Gusts and Their Interactions with an Airfoil", "author": [ { "family_name": "Hufstedler", "given_name": "Esteban Antonio Lemus", "orcid": "0000-0001-7162-920X", "clpid": "Hufstedler-Esteban-Antonio-Lemus" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/Z9Q52MN5", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10332", "collection": "thesis", "collection_id": "10332", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06092017-112408552", "type": "thesis", "title": "An Experimental and Theoretical Study of Active Flow Control", "author": [ { "family_name": "Hirsch", "given_name": "Damian George", "clpid": "Hirsch-Damian-George" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Wygnanski", "given_name": "Israel J.", "orcid": "0009-0001-5711-7029", "clpid": "Wygnanski-I-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/Z9N014KR", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9347", "collection": "thesis", "collection_id": "9347", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12232015-091951061", "primary_object_url": { "basename": "Thesis_MMW_final.pdf", "content": "final", "filesize": 100602731, "license": "other", "mime_type": "application/pdf", "url": "/9347/1/Thesis_MMW_final.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Fluid Transport by Aggregations of Small Swimming Organisms", "author": [ { "family_name": "Martinez-Ortiz", "given_name": "Monica Paola", "clpid": "Martinez-Ortiz-Monica-Paola" } ], "thesis_advisor": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Thompson", "given_name": "Andrew F.", "orcid": "0000-0003-0322-4811", "clpid": "Thompson-A-F" }, { "family_name": "Zenit Camacho", "given_name": "Jose Roberto", "orcid": "0000-0002-2717-4954", "clpid": "Zenit-Camacho-J-R" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Diel vertical migration of zooplankton has been proposed to affect global ocean circulation to a degree comparable to physical phenomena. Almost a decade after shipboard measurements showed high kinetic energy dissipation rates in the vicinity of migrating krill swarms, the hypothesis that biogenic mixing is relevant to ocean dynamics and local fluid transport has remained controversial due to the inability to directly measure the efficiency of this biological process. In situ field measurements of individual swimming jellyfish have demonstrated large-scale fluid transport via Darwinian drift, but it has remained an open question how this transport mechanism is manifested in smaller species of vertically-migrating zooplankton that are sufficient in number to be accountable in the dynamics. The goals of the present study are, first, to devise and implement experimental instruments and develop methodologies to investigate this biological process in a laboratory setting and, second, to determine whether efficient fluid transport mechanisms become available during vertical collective motion and, if so, analyze how energy is distributed within the flow. By leveraging the phototactic abilities of zooplankton, a multi-laser guidance system was developed to achieve controllable vertical migrations of A. salina concurrently with laser velocimetry of the surrounding flow. Measurements show that the hydrodynamic interactions between neighboring swimmers during vertical migration result in the development of a pronounced jet opposite to animal motion. In non-stratified fluid, this hydrodynamic feature is shown to trigger a Kelvin-Helmholtz instability that results in the generation of eddy-like structures with characteristic length scales much larger than the individual size of the organisms. Experiments in a thermally stratified water column also display the presence of a downward jet despite the strong stable stratification. Furthermore, overturning regions larger than the size of an individual organism are observed adjacent to the migrating aggregation, suggesting an alternate energy transfer route from the small scale of individual swimmers to significantly larger scales, at which mixing can be efficient via a Rayleigh-Taylor instability. The computed velocity spectrum is consistent with these findings and displays energy input at scales larger than the body length of a single swimmer. The mixing efficiency, inferred from the spectral energy distribution with and without stratification, matches experimentally achieved mixing efficiencies via a Rayleigh-Taylor instability within a stable stratification. According to our findings, biogenic mixing does have the potential to redistribute temperature, salinity and nutrients effectively. We propose the employment of laser control to examine additional species as well as alternative oceanic environments and interrogate its effect on the efficiency of biogenic mixing.\r\n", "doi": "10.7907/Z9057CX7", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9140", "collection": "thesis", "collection_id": "9140", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09042015-152813860", "type": "thesis", "title": "Dynamic Stall on Vertical Axis Wind Turbines", "author": [ { "family_name": "Dunne", "given_name": "Reeve", "clpid": "Dunne-Reeve" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this study the dynamics of flow over the blades of vertical axis wind turbines was investigated using a simplified periodic motion to uncover the fundamental flow physics and provide insight into the design of more efficient turbines. Time-resolved, two-dimensional velocity measurements were made with particle image velocimetry on a wing undergoing pitching and surging motion to mimic the flow on a turbine blade in a non-rotating frame. Dynamic stall prior to maximum angle of attack and a leading edge vortex development were identified in the phase-averaged flow field and captured by a simple model with five modes, including the first two harmonics of the pitch/surge frequency identified using the dynamic mode decomposition. Analysis of these modes identified vortical structures corresponding to both frequencies that led the separation and reattachment processes, while their phase relationship determined the evolution of the flow.
\r\n\r\nDetailed analysis of the leading edge vortex found multiple regimes of vortex development coupled to the time-varying flow field on the airfoil. The vortex was shown to grow on the airfoil for four convection times, before shedding and causing dynamic stall in agreement with 'optimal' vortex formation theory. Vortex shedding from the trailing edge was identified from instantaneous velocity fields prior to separation. This shedding was found to be in agreement with classical Strouhal frequency scaling and was removed by phase averaging, which indicates that it is not exactly coupled to the phase of the airfoil motion.
\r\n\r\nThe flow field over an airfoil undergoing solely pitch motion was shown to develop similarly to the pitch/surge motion; however, flow separation took place earlier, corresponding to the earlier formation of the leading edge vortex. A similar reduced-order model to the pitch/surge case was developed, with similar vortical structures leading separation and reattachment; however, the relative phase lead of the separation mode, corresponding to earlier separation, necessitated that a third frequency to be incorporated into the reattachment mode to provide a relative lag in reattachment.
\r\n\r\nFinally, the results are returned to the rotating frame and the effects of each flow phenomena on the turbine are estimated, suggesting kinematic criteria for the design of improved turbines.
", "doi": "10.7907/Z92Z13FX", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9303", "collection": "thesis", "collection_id": "9303", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12022015-023535926", "primary_object_url": { "basename": "DBA_PhDthesis_final.pdf", "content": "", "filesize": 27987407, "license": "other", "mime_type": "application/pdf", "url": "/9303/1/DBA_PhDthesis_final.pdf", "version": "v1.0.0" }, "type": "thesis", "title": "Aerodynamics of Vertical-Axis Wind Turbines in Full-Scale and Laboratory-Scale Experiments", "author": [ { "family_name": "Araya", "given_name": "Daniel Borsodi", "clpid": "Araya-Daniel-Borsodi" } ], "thesis_advisor": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/Z9VD6WC2 ", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9595", "collection": "thesis", "collection_id": "9595", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02292016-143116051", "primary_object_url": { "basename": "Duvvuri_Subrahmanyam_thesis_2016.pdf", "content": "final", "filesize": 19888081, "license": "other", "mime_type": "application/pdf", "url": "/9595/1/Duvvuri_Subrahmanyam_thesis_2016.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Non-Linear Scale Interactions in a Forced Turbulent Boundary Layer", "author": [ { "family_name": "Duvvuri", "given_name": "Subrahmanyam", "clpid": "Duvvuri-Subrahmanyam" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z9Z31WKP ", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9595", "collection": "thesis", "collection_id": "9595", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02292016-143116051", "primary_object_url": { "basename": "Duvvuri_Subrahmanyam_thesis_2016.pdf", "content": "final", "filesize": 19888081, "license": "other", "mime_type": "application/pdf", "url": "/9595/1/Duvvuri_Subrahmanyam_thesis_2016.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Non-Linear Scale Interactions in a Forced Turbulent Boundary Layer", "author": [ { "family_name": "Duvvuri", "given_name": "Subrahmanyam", "clpid": "Duvvuri-Subrahmanyam" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z9Z31WKP ", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9658", "collection": "thesis", "collection_id": "9658", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04062016-223108239", "primary_object_url": { "basename": "liska_sebastian_2016_thesis.pdf", "content": "final", "filesize": 15551488, "license": "other", "mime_type": "application/pdf", "url": "/9658/1/liska_sebastian_2016_thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Fast Lattice Green's Function Methods for Viscous Incompressible Flows on Unbounded Domains", "author": [ { "family_name": "Liska", "given_name": "Sebastian", "orcid": "0000-0003-4139-9364", "clpid": "Liska-Sebastian" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z9ZC80TG", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9361", "collection": "thesis", "collection_id": "9361", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01062016-163653523", "type": "thesis", "title": "Advancements in Jet Turbulence and Noise Modeling: Accurate One-Way Solutions and Empirical Evaluation of the Nonlinear Forcing of Wavepackets", "author": [ { "family_name": "Towne", "given_name": "Aaron S.", "orcid": "0000-0002-7315-5375", "clpid": "Towne-Aaron-S" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Jet noise reduction is an important goal within both commercial and military aviation. Although large-scale numerical simulations are now able to simultaneously compute turbulent jets and their radiated sound, lost-cost, physically-motivated models are needed to guide noise-reduction efforts. A particularly promising modeling approach centers around certain large-scale coherent structures, called wavepackets, that are observed in jets and their radiated sound. The typical approach to modeling wavepackets is to approximate them as linear modal solutions of the Euler or Navier-Stokes equations linearized about the long-time mean of the turbulent flow field. The near-field wavepackets obtained from these models show compelling agreement with those educed from experimental and simulation data for both subsonic and supersonic jets, but the acoustic radiation is severely under-predicted in the subsonic case. This thesis contributes to two aspects of these models. First, two new solution methods are developed that can be used to efficiently compute wavepackets and their acoustic radiation, reducing the computational cost of the model by more than an order of magnitude. The new techniques are spatial integration methods and constitute a well-posed, convergent alternative to the frequently used parabolized stability equations. Using concepts related to well-posed boundary conditions, the methods are formulated for general hyperbolic equations and thus have potential applications in many fields of physics and engineering. Second, the nonlinear and stochastic forcing of wavepackets is investigated with the goal of identifying and characterizing the missing dynamics responsible for the under-prediction of acoustic radiation by linear wavepacket models for subsonic jets. Specifically, we use ensembles of large-eddy-simulation flow and force data along with two data decomposition techniques to educe the actual nonlinear forcing experienced by wavepackets in a Mach 0.9 turbulent jet. Modes with high energy are extracted using proper orthogonal decomposition, while high gain modes are identified using a novel technique called empirical resolvent-mode decomposition. In contrast to the flow and acoustic fields, the forcing field is characterized by a lack of energetic coherent structures. Furthermore, the structures that do exist are largely uncorrelated with the acoustic field. Instead, the forces that most efficiently excite an acoustic response appear to take the form of random turbulent fluctuations, implying that direct feedback from nonlinear interactions amongst wavepackets is not an essential noise source mechanism. This suggests that the essential ingredients of sound generation in high Reynolds number jets are contained within the linearized Navier-Stokes operator rather than in the nonlinear forcing terms, a conclusion that has important implications for jet noise modeling.", "doi": "10.7907/Z99884XJ", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9611", "collection": "thesis", "collection_id": "9611", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03102016-211538603", "primary_object_url": { "basename": "thesis2_final.pdf", "content": "final", "filesize": 9406888, "license": "other", "mime_type": "application/pdf", "url": "/9611/1/thesis2_final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Small Scale Turbulence in High Karlovitz Number Premixed Flames", "author": [ { "family_name": "Bobbitt", "given_name": "Brock Douglas", "clpid": "Bobbitt-Brock-Douglas" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The purpose of this thesis is to characterize the behavior of the smallest turbulent scales in high Karlovitz number (Ka) premixed flames. These scales are particularly important in the two-way coupling between turbulence and chemistry and better understanding of these scales will support future modeling efforts using large eddy simulations (LES). The smallest turbulent scales are studied by considering the vorticity vector, \u03c9, and its transport equation.
\r\n\r\nDue to the complexity of turbulent combustion introduced by the wide range of length and time scales, the two-dimensional vortex-flame interaction is first studied as a simplified test case. Numerical and analytical techniques are used to discern the dominate transport terms and their effects on vorticity based on the initial size and strength of the vortex. This description of the effects of the flame on a vortex provides a foundation for investigating vorticity in turbulent combustion.
\r\n\r\nSubsequently, enstrophy, \u03c92 = \u03c9 \u2022 \u03c9, and its transport equation are investigated in premixed turbulent combustion. For this purpose, a series of direct numerical simulations (DNS) of premixed n-heptane/air flames are performed, the conditions of which span a wide range of unburnt Karlovitz numbers and turbulent Reynolds numbers. Theoretical scaling analysis along with the DNS results support that, at high Karlovitz number, enstrophy transport is controlled by the viscous dissipation and vortex stretching/production terms. As a result, vorticity scales throughout the flame with the inverse of the Kolmogorov time scale, \u03c4\u03b7, just as in homogeneous isotropic turbulence. As \u03c4\u03b7 is only a function of the viscosity and dissipation rate, this supports the validity of Kolmogorov\u2019s first similarity hypothesis for sufficiently high Ka numbers (Ka \u2273 100). These conclusions are in contrast to low Karlovitz number behavior, where dilatation and baroclinic torque have a significant impact on vorticity within the flame. Results are unaffected by the transport model, chemical model, turbulent Reynolds number, and lastly the physical configuration.
\r\n\r\nNext, the isotropy of vorticity is assessed. It is found that given a sufficiently large value of the Karlovitz number (Ka \u2273 100) the vorticity is isotropic. At lower Karlovitz numbers, anisotropy develops due to the effects of the flame on the vortex stretching/production term. In this case, the local dynamics of vorticity in the strain-rate tensor, S, eigenframe are altered by the flame. At sufficiently high Karlovitz numbers, the dynamics of vorticity in this eigenframe resemble that of homogeneous isotropic turbulence.
\r\n\r\nCombined, the results of this thesis support that both the magnitude and orientation of vorticity resemble the behavior of homogeneous isotropic turbulence, given a sufficiently high Karlovitz number (Ka \u2273 100). This supports the validity of Kolmogorov\u2019s first similarity hypothesis and the hypothesis of local isotropy under these condition. However, dramatically different behavior is found at lower Karlovitz numbers. These conclusions provides/suggests directions for modeling high Karlovitz number premixed flames using LES. With more accurate models, the design of aircraft combustors and other combustion based devices may better mitigate the detrimental effects of combustion, from reducing CO2 and soot production to increasing engine efficiency.
", "doi": "10.7907/Z9Z03649", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9808", "collection": "thesis", "collection_id": "9808", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-220450949", "primary_object_url": { "basename": "Jeesoon Choi Caltech Thesis (Jun 10 2016).pdf", "content": "final", "filesize": 18192088, "license": "other", "mime_type": "application/pdf", "url": "/9808/1/Jeesoon Choi Caltech Thesis (Jun 10 2016).pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Unsteady Aerodynamics and Optimal Control of an Airfoil at Low Reynolds Number", "author": [ { "family_name": "Choi", "given_name": "Jeesoon", "clpid": "Choi-Jeesoon" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "As opposed to conventional air vehicles that have fixed wings, small birds and insects are known to flap their wings at higher angles of attack. The vortex produced at the tip of the wing, known as the leading-edge vortex (LEV), plays an important role to enhance lift during its flight. In this thesis, we analyze the influence of these vortices on aerodynamic forces that could be beneficial to micro-air vehicle performance and efficiency. The flow structures associated with simple harmonic motions of an airfoil are first investigated. The characteristics of the time-averaged and fluctuating forces are explained by analyzing vortical flow features, such as vortex lock-in, leading-edge vortex synchronization, and vortex formation time. Specific frequency regions where the wake instability locks in to the unsteady motion of the airfoil are identified, and these lead to significant changes in the mean forces. A detailed study of the flow structures associated with the LEV acting either in- or out-of-phase with the quasi-steady component of the forces is performed to quantify the amplification and attenuation behavior of the fluctuating forces. An inherent time scale of the LEV associated with its formation and detachment (LEV formation time) is shown to control the time-averaged forces. With these results, several optimal flow control problems are formulated. Adjoint-based optimal control is applied to an airfoil moving at a constant velocity and also to a reciprocating airfoil with no forward velocity. In both cases, we maximize lift by controlling the pitch rate of the airfoil. For the former case, the static map of lift at various angles of attack is additionally examined to find the static angle that provides maximum lift and also to confirm whether the optimizations perform according to the static map. For the latter case, we obtain a solution of the optimized motion of the flapping airfoil which resembles that of a hovering insect.", "doi": "10.7907/Z9J1014Q", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9808", "collection": "thesis", "collection_id": "9808", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-220450949", "primary_object_url": { "basename": "Jeesoon Choi Caltech Thesis (Jun 10 2016).pdf", "content": "final", "filesize": 18192088, "license": "other", "mime_type": "application/pdf", "url": "/9808/1/Jeesoon Choi Caltech Thesis (Jun 10 2016).pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Unsteady Aerodynamics and Optimal Control of an Airfoil at Low Reynolds Number", "author": [ { "family_name": "Choi", "given_name": "Jeesoon", "clpid": "Choi-Jeesoon" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Williams", "given_name": "David R.", "clpid": "Williams-D-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "As opposed to conventional air vehicles that have fixed wings, small birds and insects are known to flap their wings at higher angles of attack. The vortex produced at the tip of the wing, known as the leading-edge vortex (LEV), plays an important role to enhance lift during its flight. In this thesis, we analyze the influence of these vortices on aerodynamic forces that could be beneficial to micro-air vehicle performance and efficiency. The flow structures associated with simple harmonic motions of an airfoil are first investigated. The characteristics of the time-averaged and fluctuating forces are explained by analyzing vortical flow features, such as vortex lock-in, leading-edge vortex synchronization, and vortex formation time. Specific frequency regions where the wake instability locks in to the unsteady motion of the airfoil are identified, and these lead to significant changes in the mean forces. A detailed study of the flow structures associated with the LEV acting either in- or out-of-phase with the quasi-steady component of the forces is performed to quantify the amplification and attenuation behavior of the fluctuating forces. An inherent time scale of the LEV associated with its formation and detachment (LEV formation time) is shown to control the time-averaged forces. With these results, several optimal flow control problems are formulated. Adjoint-based optimal control is applied to an airfoil moving at a constant velocity and also to a reciprocating airfoil with no forward velocity. In both cases, we maximize lift by controlling the pitch rate of the airfoil. For the former case, the static map of lift at various angles of attack is additionally examined to find the static angle that provides maximum lift and also to confirm whether the optimizations perform according to the static map. For the latter case, we obtain a solution of the optimized motion of the flapping airfoil which resembles that of a hovering insect.", "doi": "10.7907/Z9J1014Q", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9796", "collection": "thesis", "collection_id": "9796", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-150613633", "primary_object_url": { "basename": "Hsieh-Chen_Tsai_thesis_2016_final.pdf", "content": "final", "filesize": 13168932, "license": "other", "mime_type": "application/pdf", "url": "/9796/1/Hsieh-Chen_Tsai_thesis_2016_final.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Numerical Investigation of Vertical-Axis Wind Turbines at Low Reynolds Number", "author": [ { "family_name": "Tsai", "given_name": "Hsieh-Chen", "clpid": "Tsai-Hsieh-Chen" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis is aimed at numerically investigating the aerodynamics and the starting of a vertical-axis wind turbine at low Reynolds number using the immersed boundary method. The influence of the Coriolis effect on dynamic stall is isolated by comparing the rotating airfoil to one undergoing an equivalent planar motion that is composed of surging and pitching motions that produce an equivalent speed and angle of attack variation over a cycle. At lower tip-speed ratios, the Coriolis force leads to the capture of a vortex pair which results in a significant decrease in lift when the angle of attack of a rotating airfoil begins to decrease in the upwind half cycle. In the absence of the wake-capturing, the equivalent planar motion is a good approximation to a rotating blade in a vertical-axis wind turbine.
\r\n\r\nAnalysis on the starting torque shows that when the turbine solidity is lower than about 0.5, the starting torque distribution can be well-modeled by considering a single blade at different orientations, and starting torque distributions for multi-bladed turbines can be constructed by linearly combining the torques at the respective positions of the blades. Using this model, optimal configurations to start a multi-bladed low-solidity vertical-axis wind turbine is proposed.
\r\n\r\nA preliminary study is made to determine an optimal blade pitch for a single-bladed motor-driven turbine using optimal control theory. When the input power is minimized directly, the solution seems to converge to only a local minimum due to a lower input power reduction than that obtained by maximizing the mean tangential force. After a transient, both controls converge to time-invariant pitch angles of about the same magnitude but with opposite signs. The wake-capturing phenomenon observed in the uncontrolled case necessitates large input power. Under active control, the disappearance of wake-capturing and attendant changes in the flow field collectively result in a reduction of required input power.
", "doi": "10.7907/Z9SF2T5R", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9784", "collection": "thesis", "collection_id": "9784", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272016-105842881", "primary_object_url": { "basename": "thesis_Lapointe2016.pdf", "content": "final", "filesize": 17491660, "license": "other", "mime_type": "application/pdf", "url": "/9784/1/thesis_Lapointe2016.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Simulation of Premixed Hydrocarbon Flames at High Turbulence Intensities", "author": [ { "family_name": "Lapointe", "given_name": "Simon", "clpid": "Lapointe-Simon" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z90V89SW", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9764", "collection": "thesis", "collection_id": "9764", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262016-092840941", "primary_object_url": { "basename": "JCMeng_Thesis_Final.pdf", "content": "final", "filesize": 98083728, "license": "other", "mime_type": "application/pdf", "url": "/9764/1/JCMeng_Thesis_Final.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Numerical Simulations of Droplet Aerobreakup", "author": [ { "family_name": "Meng", "given_name": "Jomela Chen-Chen", "clpid": "Meng-Jomela-Chen-Chen" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z9KW5D09", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9755", "collection": "thesis", "collection_id": "9755", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252016-141702166", "primary_object_url": { "basename": "SchmidtBryan2016Thesis.pdf", "content": "final", "filesize": 12705650, "license": "other", "mime_type": "application/pdf", "url": "/9755/1/SchmidtBryan2016Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "On the Stability of Supersonic Boundary Layers with Injection", "author": [ { "family_name": "Schmidt", "given_name": "Bryan Eric", "orcid": "0000-0001-9193-7760", "clpid": "Schmidt-Bryan-Eric" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Austin", "given_name": "Joanna M.", "orcid": "0000-0003-3129-5035", "clpid": "Austin-J-M" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z93X84M6", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8758", "collection": "thesis", "collection_id": "8758", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01222015-234921548", "type": "thesis", "title": "Simulation of Shock-Induced Bubble Collapse with Application to Vascular Injury in Shockwave Lithotripsy", "author": [ { "family_name": "Coralic", "given_name": "Vedran", "clpid": "Coralic-Vedran" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shockwave lithotripsy is a noninvasive medical procedure wherein shockwaves are repeatedly focused at the location of kidney stones in order to pulverize them. Stone comminution is thought to be the product of two mechanisms: the propagation of stress waves within the stone and cavitation erosion. However, the latter mechanism has also been implicated in vascular injury. In the present work, shock-induced bubble collapse is studied in order to understand the role that it might play in inducing vascular injury. A high-order accurate, shock- and interface-capturing numerical scheme is developed to simulate the three-dimensional collapse of the bubble in both the free-field and inside a vessel phantom. The primary contributions of the numerical study are the characterization of the shock-bubble and shock-bubble-vessel interactions across a large parameter space that includes clinical shockwave lithotripsy pressure amplitudes, problem geometry and tissue viscoelasticity, and the subsequent correlation of these interactions to vascular injury. Specifically, measurements of the vessel wall pressures and displacements, as well as the finite strains in the fluid surrounding the bubble, are utilized with available experiments in tissue to evaluate damage potential. Estimates are made of the smallest injurious bubbles in the microvasculature during both the collapse and jetting phases of the bubble's life cycle. The present results suggest that bubbles larger than 1 \u03bcm in diameter could rupture blood vessels under clinical SWL conditions.", "doi": "10.7907/Z91N7Z26", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8851", "collection": "thesis", "collection_id": "8851", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05082015-184801592", "primary_object_url": { "basename": "MaxCubillosThesis.pdf", "content": "final", "filesize": 10176963, "license": "other", "mime_type": "application/pdf", "url": "/8851/1/MaxCubillosThesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "General-Domain Compressible Navier-Stokes Solvers Exhibiting Quasi-Unconditional Stability and High Order Accuracy in Space and Time", "author": [ { "family_name": "Cubillos-Moraga", "given_name": "Max Anton", "clpid": "Cubillos-Moraga-Max-Anton" } ], "thesis_advisor": [ { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" } ], "thesis_committee": [ { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Owhadi", "given_name": "Houman", "orcid": "0000-0002-5677-1600", "clpid": "Owhadi-H" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis presents a new class of solvers for the subsonic compressible Navier-Stokes equations in general two- and three-dimensional spatial domains. The proposed methodology incorporates: 1) A novel linear-cost implicit solver based on use of higher-order backward differentiation formulae (BDF) and the alternating direction implicit approach (ADI); 2) A fast explicit solver; 3) Dispersionless spectral spatial discretizations; and 4) A domain decomposition strategy that negotiates the interactions between the implicit and explicit domains. In particular, the implicit methodology is quasi-unconditionally stable (it does not suffer from CFL constraints for adequately resolved flows), and it can deliver orders of time accuracy between two and six in the presence of general boundary conditions. In fact this thesis presents, for the first time in the literature, high-order time-convergence curves for Navier-Stokes solvers based on the ADI strategy---previous ADI solvers for the Navier-Stokes equations have not demonstrated orders of temporal accuracy higher than one. An extended discussion is presented in this thesis which places on a solid theoretical basis the observed quasi-unconditional stability of the methods of orders two through six. The performance of the proposed solvers is favorable. For example, a two-dimensional rough-surface configuration including boundary layer effects at Reynolds number equal to one million and Mach number 0.85 (with a well-resolved boundary layer, run up to a sufficiently long time that single vortices travel the entire spatial extent of the domain, and with spatial mesh sizes near the wall of the order of one hundred-thousandth the length of the domain) was successfully tackled in a relatively short (approximately thirty-hour) single-core run; for such discretizations an explicit solver would require truly prohibitive computing times. As demonstrated via a variety of numerical experiments in two- and three-dimensions, further, the proposed multi-domain parallel implicit-explicit implementations exhibit high-order convergence in space and time, useful stability properties, limited dispersion, and high parallel efficiency.", "doi": "10.7907/Z9WW7FKW", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8904", "collection": "thesis", "collection_id": "8904", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282015-154709861", "primary_object_url": { "basename": "thesis_defended_Savard2015.pdf", "content": "final", "filesize": 13970787, "license": "other", "mime_type": "application/pdf", "url": "/8904/1/thesis_defended_Savard2015.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Characterization and Modeling of Premixed Turbulent n-Heptane Flames in the Thin Reaction Zone Regime", "author": [ { "family_name": "Savard", "given_name": "Bruno", "clpid": "Savard-Bruno" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/Z9GM858F", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8077", "collection": "thesis", "collection_id": "8077", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02142014-112419793", "type": "thesis", "title": "Large-Eddy Simulations of Fully Developed Turbulent Channel and Pipe Flows with Smooth and Rough Walls", "author": [ { "family_name": "Saito", "given_name": "Namiko", "clpid": "Saito-Namiko" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale I.", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/WKNJ-ET18", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8445", "collection": "thesis", "collection_id": "8445", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302014-140011538", "type": "thesis", "title": "Advancing EDL Technologies for Future Space Missions: From Ground Testing Facilities to Ablative Heatshields", "author": [ { "family_name": "Rabinovitch", "given_name": "Jason", "orcid": "0000-0002-1914-7964", "clpid": "Rabinovitch-Jason" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/XKM7-7368", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8394", "collection": "thesis", "collection_id": "8394", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262014-160059300", "primary_object_url": { "basename": "GDR_Thesis_Final.pdf", "content": "final", "filesize": 18421139, "license": "other", "mime_type": "application/pdf", "url": "/8394/1/GDR_Thesis_Final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "A Novel Methodology for Simulating Contact-Line Behavior in Capillary-Driven Flows", "author": [ { "family_name": "Della Rocca", "given_name": "Gerry V.", "clpid": "Della-Rocca-Gerry-V" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Despite the wide swath of applications where multiphase fluid contact lines exist, there is still no consensus on an accurate and general simulation methodology. Most prior numerical work has imposed one of the many dynamic contact-angle theories at solid walls. Such approaches are inherently limited by the theory accuracy. In fact, when inertial effects are important, the contact angle may be history dependent and, thus, any single mathematical function is inappropriate. Given these limitations, the present work has two primary goals: 1) create a numerical framework that allows the contact angle to evolve naturally with appropriate contact-line physics and 2) develop equations and numerical methods such that contact-line simulations may be performed on coarse computational meshes.
\r\n\r\nFluid flows affected by contact lines are dominated by capillary stresses and require accurate curvature calculations. The level set method was chosen to track the fluid interfaces because it is easy to calculate interface curvature accurately. Unfortunately, the level set reinitialization suffers from an ill-posed mathematical problem at contact lines: a ``blind spot'' exists. Standard techniques to handle this deficiency are shown to introduce parasitic velocity currents that artificially deform freely floating (non-prescribed) contact angles. As an alternative, a new relaxation equation reinitialization is proposed to remove these spurious velocity currents and its concept is further explored with level-set extension velocities.
\r\n\r\nTo capture contact-line physics, two classical boundary conditions, the Navier-slip velocity boundary condition and a fixed contact angle, are implemented in direct numerical simulations (DNS). DNS are found to converge only if the slip length is well resolved by the computational mesh. Unfortunately, since the slip length is often very small compared to fluid structures, these simulations are not computationally feasible for large systems. To address the second goal, a new methodology is proposed which relies on the volumetric-filtered Navier-Stokes equations. Two unclosed terms, an average curvature and a viscous shear VS, are proposed to represent the missing microscale physics on a coarse mesh.
\r\n\r\nAll of these components are then combined into a single framework and tested for a water droplet impacting a partially-wetting substrate. Very good agreement is found for the evolution of the contact diameter in time between the experimental measurements and the numerical simulation. Such comparison would not be possible with prior methods, since the Reynolds number Re and capillary number Ca are large. Furthermore, the experimentally approximated slip length ratio is well outside of the range currently achievable by DNS. This framework is a promising first step towards simulating complex physics in capillary-dominated flows at a reasonable computational expense.
", "doi": "10.7907/Z9CN71WW", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8481", "collection": "thesis", "collection_id": "8481", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042014-163735743", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 11570274, "license": "other", "mime_type": "application/pdf", "url": "/8481/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Velocity Resolved - Scalar Modeled Simulations of High Schmidt Number Turbulent Transport", "author": [ { "family_name": "Verma", "given_name": "Siddhartha", "clpid": "Verma-Siddhartha" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/PTD9-W004", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8481", "collection": "thesis", "collection_id": "8481", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06042014-163735743", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 11570274, "license": "other", "mime_type": "application/pdf", "url": "/8481/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Velocity Resolved - Scalar Modeled Simulations of High Schmidt Number Turbulent Transport", "author": [ { "family_name": "Verma", "given_name": "Siddhartha", "clpid": "Verma-Siddhartha" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/PTD9-W004", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8253", "collection": "thesis", "collection_id": "8253", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05212014-101703985", "primary_object_url": { "basename": "Thesis_PLCarroll_June2014_Final.pdf", "content": "final", "filesize": 8274577, "license": "other", "mime_type": "application/pdf", "url": "/8253/1/Thesis_PLCarroll_June2014_Final.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Towards Understanding the Mixing Characteristics of Turbulent Buoyant Flows", "author": [ { "family_name": "Carroll", "given_name": "Phares Lynn", "clpid": "Carroll-Phares-Lynn" } ], "thesis_advisor": [ { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This work proposes a new simulation methodology in which variable density turbulent flows can be studied in the context of a mixing layer with or without the presence of gravity. Specifically, this methodology is developed to probe the nature of non-buoyantly-driven (i.e. isotropically-driven) or buoyantly-driven mixing deep inside a mixing layer. Numerical forcing methods are incorporated into both the velocity and scalar fields, which extends the length of time over which mixing physics can be studied. The simulation framework is designed to allow for independent variation of four non-dimensional parameters, including the Reynolds, Richardson, Atwood, and Schmidt numbers. Additionally, the governing equations are integrated in such a way to allow for the relative magnitude of buoyant energy production and non-buoyant energy production to be varied.
\r\n\r\n\r\nThe computational requirements needed to implement the proposed configuration are presented. They are justified in terms of grid resolution, order of accuracy, and transport scheme. Canonical features of turbulent buoyant flows are reproduced as validation of the proposed methodology. These features include the recovery of isotropic Kolmogorov scales under buoyant and non-buoyant conditions, the recovery of anisotropic one-dimensional energy spectra under buoyant conditions, and the preservation of known statistical distributions in the scalar field, as found in other DNS studies.
\r\n\r\n\r\nThis simulation methodology is used to perform a parametric study of turbulent buoyant flows to discern the effects of varying the Reynolds, Richardson, and Atwood numbers on the resulting state of mixing. The effects of the Reynolds and Atwood numbers are isolated by looking at two energy dissipation rate conditions under non-buoyant (variable density) and constant density conditions. The effects of Richardson number are isolated by varying the ratio of buoyant energy production to total energy production from zero (non-buoyant) to one (entirely buoyant) under constant Atwood number, Schmidt number, and energy dissipation rate conditions. It is found that the major differences between non-buoyant and buoyant turbulent flows are contained in the transfer spectrum and longitudinal structure functions, while all other metrics are largely similar (e.g. energy spectra, alignment characteristics of the strain-rate tensor). Also, despite the differences noted between fully buoyant and non-buoyant turbulent fields, the scalar field, in all cases, is unchanged by these. The mixing dynamics in the scalar field are found to be insensitive to the source of turbulent kinetic energy production (non-buoyant vs. buoyant).
\r\n", "doi": "10.7907/RDHJ-3X60", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8077", "collection": "thesis", "collection_id": "8077", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02142014-112419793", "type": "thesis", "title": "Large-Eddy Simulations of Fully Developed Turbulent Channel and Pipe Flows with Smooth and Rough Walls", "author": [ { "family_name": "Saito", "given_name": "Namiko", "clpid": "Saito-Namiko" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale I.", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/WKNJ-ET18", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:7175", "collection": "thesis", "collection_id": "7175", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07102012-152431583", "primary_object_url": { "basename": "jacobi_thesis_july12.pdf", "content": "final", "filesize": 32035644, "license": "other", "mime_type": "application/pdf", "url": "/7175/1/jacobi_thesis_july12.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Structure of the Turbulent Boundary Layer under Static and Dynamic Impulsive Roughness Perturbation", "author": [ { "family_name": "Jacobi", "given_name": "Ian", "clpid": "Jacobi-Ian" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/H5WJ-RK31", "publication_date": "2013", "thesis_type": "phd", "thesis_year": "2013" }, { "id": "thesis:7900", "collection": "thesis", "collection_id": "7900", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06172013-144232973", "primary_object_url": { "basename": "Rebecca_Rought_Thesis.pdf", "content": "final", "filesize": 11692721, "license": "other", "mime_type": "application/pdf", "url": "/7900/1/Rebecca_Rought_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Identification of Thermally-Tagged Coherent Structures in the Zero Pressure Gradient Turbulent Boundary Layer", "author": [ { "family_name": "Rought", "given_name": "Rebecca Lynn", "clpid": "Rought-Rebecca-Lynn" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "abstract": "A zero pressure gradient boundary layer over a flat plate is subjected to step changes in thermal condition at the wall, causing the formation of internal, heated layers. The resulting temperature fluctuations and their corresponding density variations are associated with turbulent coherent structures. Aero-optical distortion occurs when light passes through the boundary layer, encountering the changing index of refraction resulting from the density variations. Instantaneous measurements of streamwise velocity, temperature and the optical deflection angle experienced by a laser traversing the boundary layer are made using hot and cold wires and a Malley probe, respectively. Correlations of the deflection angle with the temperature and velocity records suggest that the dominant contribution to the deflection angle comes from thermally-tagged structures in the outer boundary layer with a convective velocity of approximately 0.8U\u221e. An examination of instantaneous temperature and velocity and their temporal gradients conditionally averaged around significant optical deflections shows behavior consistent with the passage of a heated vortex. Strong deflections are associated with strong negative temperature gradients, and strong positive velocity gradients where the sign of the streamwise velocity fluctuation changes. The power density spectrum of the optical deflections reveals associated structure size to be on the order of the boundary layer thickness. A comparison to the temperature and velocity spectra suggests that the responsible structures are smaller vortices in the outer boundary layer as opposed to larger scale motions. Notable differences between the power density spectra of the optical deflections and the temperature remain unresolved due to the low frequency response of the cold wire.", "doi": "10.7907/Z2V0-9V82", "publication_date": "2013", "thesis_type": "engd", "thesis_year": "2013" }, { "id": "thesis:7065", "collection": "thesis", "collection_id": "7065", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05222012-183141047", "primary_object_url": { "basename": "minoue_thesis.pdf", "content": "final", "filesize": 3141841, "license": "other", "mime_type": "application/pdf", "url": "/7065/1/minoue_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Large-Eddy Simulation of the Flat-Plate Turbulent Boundary Layer at High Reynolds Numbers", "author": [ { "family_name": "Inoue", "given_name": "Michio", "clpid": "Inoue-Michio" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/PXTM-W616", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6759", "collection": "thesis", "collection_id": "6759", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12302011-185742249", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 100881217, "license": "other", "mime_type": "application/pdf", "url": "/6759/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Simulations of Compressible, Diffusive, Reactive Flows with Detailed Chemistry Using a High-Order Hybrid WENO-CD Scheme", "author": [ { "family_name": "Ziegler", "given_name": "John Lewis (Jack)", "orcid": "0000-0001-6127-5567", "clpid": "Ziegler-John-Lewis-Jack" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/ZKW8-ES97", "publication_date": "2012", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:7067", "collection": "thesis", "collection_id": "7067", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232012-142127799", "primary_object_url": { "basename": "LeHew_Jeff_2012_Thesis.pdf", "content": "final", "filesize": 10530698, "license": "other", "mime_type": "application/pdf", "url": "/7067/1/LeHew_Jeff_2012_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Spatio-Temporal Analysis of the Turbulent Boundary Layer and An Investigation of the Effects of Periodic Disturbances", "author": [ { "family_name": "LeHew", "given_name": "Jeffrey Allen", "clpid": "LeHew-Jeffrey-Allen" } ], "thesis_advisor": [ { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/20CM-EV70", "publication_date": "2012-06-15", "thesis_type": "phd", "thesis_year": "2012" }, { "id": "thesis:6053", "collection": "thesis", "collection_id": "6053", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-133354529", "type": "thesis", "title": "The Development of Low-Order Models for the Study of Fluid-Structure Interactions", "author": [ { "family_name": "Tchieu", "given_name": "Andrew Allen", "clpid": "Tchieu-Andrew-Allen" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/SYHX-8A77", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6053", "collection": "thesis", "collection_id": "6053", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09242010-133354529", "type": "thesis", "title": "The Development of Low-Order Models for the Study of Fluid-Structure Interactions", "author": [ { "family_name": "Tchieu", "given_name": "Andrew Allen", "clpid": "Tchieu-Andrew-Allen" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/SYHX-8A77", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6455", "collection": "thesis", "collection_id": "6455", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262011-141745737", "type": "thesis", "title": "Individual Particle Motion in Colloids: Microviscosity, Microdiffusivity, and Normal Stresses", "author": [ { "family_name": "Zia", "given_name": "Roseanna Nellie", "clpid": "Zia-Roseanna-Nellie" } ], "thesis_advisor": [ { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "thesis_committee": [ { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Bhattacharya", "given_name": "Kaushik", "orcid": "0000-0003-2908-5469", "clpid": "Bhattacharya-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Lauga", "given_name": "Eric", "orcid": "0000-0002-8916-2545", "clpid": "Lauga-E" }, { "family_name": "Wang", "given_name": "Zhen-Gang", "orcid": "0000-0002-3361-6114", "clpid": "Wang-Zhen-Gang" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Colloidal dispersions play an important role in nearly every aspect of life, from paint to biofuels to nano-therapeutics. In the study of these so-called complex \ufb02uids, a connection is sought between macroscopic material properties and the micromechanics of the suspended particles. Such properties include viscosity, diffusivity, and the osmotic pressure, for example. But many such systems are themselves only microns in size overall; recent years have thus seen a dramatic growth in demand for exploring microscale systems at a much smaller length scale than can be probed with conventional macroscopic techniques. Microrheology is one approach to such microscale interrogation, in which a Brownian \u201cprobe\u201d particle is driven through a complex \ufb02uid, and its motion tracked in order to infer the mechanical properties of the embedding material. With no external forcing the probe and background particles form an equilibrium microstructure that \ufb02uctuates thermally with the solvent. Probe motion through the dispersion distorts the microstructure; the character of this deformation, and hence its in\ufb02uence on probe motion, depends on the strength with which the probe is forced, F ext , compared to thermal forces, kT/b, defining a P\u00b4eclet number, P e = F ext /(kT /b), where kT is the thermal energy and b the bath-particle size. Both the mean and the \ufb02uctuating motion of the probe are of interest. Recent studies showed that the reduction in mean probe speed gives the e\ufb00ective material viscosity. But the velocity of the probe also \ufb02uctuates due to collisions with the suspended particles, causing the probe to undergo a random walk process. It is shown that the long-time mean-square \ufb02uctuational motion of the probe is diffusive and the effective diffusivity of the forced probe is determined for the full range of P\u00b4eclet number. At small Pe Brownian motion dominates and the diffusive behavior of the probe characteristic of passive microrheology is recovered, but with an incremental \ufb02ow-induced \u201cmicro-diffusivity\u201d that scales as Dmicro \u223c Da P e 2 \u03c6b , where viii \u03c6b is the volume fraction of bath particles and Da is the self-diffusivity of an isolated probe. At the other extreme of high P\u00b4eclet number the fuctuational motion is still diffusive, and the diffusivity becomes primarily force-induced , scaling as (F ext /\u03b7)\u03c6b , where \u03b7 is the viscosity of the solvent. The force-induced \u201cmicrodiffusivity\u201d is anisotropic, with diffusion longitudinal to the direction of forcing larger in both limits compared to transverse diffusion, but more strongly so in the high-P e limit.
\r\n\r\nPrevious work in microrheology defined a scalar viscosity; however, a tensorial expression for the suspension stress in microrheology was still lacking. The notion that diffusive \ufb02ux is driven by gradients in particle-phase stress leads to the idea that the microdiffusivity can be related directly to the suspension stress. In consequence, the anisotropy of the diffusion tensor may re\ufb02ect the presence of normal stress differences in non-linear microrheology. While the particle-phase stress tensor can be determined as the second moment of the deformed microstructure, in this study a connection is made between diffusion and stress gradients, and an analytical expression for particle-phase stress as a function of the microdiffusivity and microviscosity is obtained. The two approaches agree, suggesting that normal stresses and normal stress differences can be measured in active microrheological experiments if both the mean and mean-square motion of the probe are monitored. Owing to the axisymmetry of the motion about a spherical probe, the second normal stress difference is zero, while the \ufb01rst normal stress difference is linear in P e for P e \u226b 1 and vanishes as P e 3 for P e \u226a 1. An additional important outcome is that the analytical expression obtained for stress-induced migration can be viewed as a generalized non-equilibrium Stokes-Einstein relation.
\r\n\r\nStudies of steady-state dispersion behavior reveal the hydrodynamic and microstructural mechanisms that underlie non-Newtonian behaviors (e.g. shear-thinning, shear-thickening, and normal stress differences). But an understanding of how the microstructures evolve from the equilibrium state, and how non-equilibrium properties develop in time is much less well understood. Transient suspension behavior in the near-equilibrium, linear response regime has been studied via its connection to low-amplitude oscillatory probe forcing and the complex modulus; at very weak forcing, the microstructural response that drives viscosity is indistinguishable from equilibrium \ufb02uctuations. But important information about the basic physical aspects of structural development and relaxation ix in a medium are captured by start-up and cessation of the imposed deformation in the non-linear regime, where the structure is driven far from equilibrium. Here we study the evolution of stress and microstructure in a colloidal dispersion by tracking transient probe motion during start-up and cessation of a strong \ufb02ow. For large P e, steady state is reached when a boundary layer (in which advection balances diffusion) forms at particle contact on the timescale of the \ufb02ow, a/U , where a is the probe size and U its speed. On the other hand, relaxation following cessation occurs over several timescales corresponding to distinct physical processes. For very short times, the timescale for relaxation is set by the diffusion over the boundary-layer thickness. Nearly all stress relaxation occurs during this process, owing to the dependence of the bath-particle drag on the contact value of the microstructure. At longer times the collective diffusion of the bath particles acts to close the wake. In this long-time limit as structural isotropy is restored, the majority of the microstructural relaxation occurs with very little change in suspension stress. Theoretical results are presented and compared with Brownian dynamics simulation. Two regimes of probe motion are studied: an externally applied constant force and an imposed constant velocity. The microstructural evolution is qualitatively different for the two regimes, with a longer transient phase and a thinner boundary layer and longer wake at steady state in the latter case. The work is also compared to analogous results for sheared suspensions undergoing start-up and cessation.
\r\n\r\nThe study moves next to investigations of dual-probe microrheology. Motivated by the phenomenon of equilibrium depletion interactions, we study the interaction between a pair of probe particles translating with equal velocity through a dispersion with their line of centers transverse to the external forcing. The character of the microstructure surrounding the probes is determined both by the distance R by which the two probes are separated and by the strength of the external forcing, P e = U a/Db , where U is the constant probe velocity and Db the diffusivity of the bath particles. Osmotic pressure gradients develop as the microstructure is deformed, giving rise to an interactive force between the probes. This force is studied for a range of P e and R. For all separations R > 2a, the probes attract when P e is small. As the strength of the forcing increases, a qualitative change in the interactive force occurs: the probes repel each other. The probe separation R at which the x attraction-to-repulsion transition occurs decreases as P e increases, because the entropic depletion attraction becomes weak compared to the force-induced osmotic repulsion. The non-equilibrium interactive force is strictly repulsive for two separated probes.
\r\n\r\nBut non-linear microrheology provides far more than a microscale technique for interrogating complex \ufb02uids. In 1906, Einstein published the famous thought experiment in which he proposed that if a liquid were indeed composed of atoms, then the motion of a small particle suspended in the \ufb02uid would move with the same random trajectories as the solvent atoms. Combining the theories of kinetics, diffusion, and thermodynamics, he showed that the diffusive motion of a small particle is indeed evidence of the existence of the atom. Perrin con\ufb01rmed the theory with measurement in 1909. This is a profound conclusion, drawn by simply watching a particle move in a liquid. Here, we follow this example and watch a particle move in a complex \ufb02uid\u2014but now for a system that is not at equilibrium. In equilibrium systems, the relationship between \ufb02uctuation and dissipation is fundamental to our understanding of colloid physics. By studying \ufb02uctuations away from equilibrium, we have discovered an analogous non-equilibrium relation between \ufb02uctuation and dissipation\u2014and that the balance between the two is stored in the material stress. A \ufb01nal connection can be made between this stress and energy storage.
", "doi": "10.7907/2743-8W26", "publication_date": "2011-06-10", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6188", "collection": "thesis", "collection_id": "6188", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:11222010-114324484", "primary_object_url": { "basename": "GVenturini_thesis.pdf", "content": "final", "filesize": 6798002, "license": "other", "mime_type": "application/pdf", "url": "/6188/1/GVenturini_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Topics in Multiscale Modeling of Metals and Metallic Alloys", "author": [ { "family_name": "Venturini", "given_name": "Gabriela Natalia", "clpid": "Venturini-Gabriela-Natalia" } ], "thesis_advisor": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" } ], "thesis_committee": [ { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Lapusta", "given_name": "Nadia", "orcid": "0000-0001-6558-0323", "clpid": "Lapusta-N" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Marian", "given_name": "Jaime", "orcid": "0000-0001-9000-3405", "clpid": "Marian-J" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/D6YS-B365", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6211", "collection": "thesis", "collection_id": "6211", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12162010-115725941", "primary_object_url": { "basename": "ward_thesis.pdf", "content": "final", "filesize": 6831681, "license": "other", "mime_type": "application/pdf", "url": "/6211/1/ward_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Simulation of Shock- and Impact-Driven Flows with Mie-Gr\u00fcneisen Equations of State", "author": [ { "family_name": "Ward", "given_name": "Geoffrey M.", "clpid": "Ward-Geoffrey-M" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/8Q2Q-GT29", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:6211", "collection": "thesis", "collection_id": "6211", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12162010-115725941", "primary_object_url": { "basename": "ward_thesis.pdf", "content": "final", "filesize": 6831681, "license": "other", "mime_type": "application/pdf", "url": "/6211/1/ward_thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Simulation of Shock- and Impact-Driven Flows with Mie-Gr\u00fcneisen Equations of State", "author": [ { "family_name": "Ward", "given_name": "Geoffrey M.", "clpid": "Ward-Geoffrey-M" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/8Q2Q-GT29", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:5275", "collection": "thesis", "collection_id": "5275", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07312009-095021", "primary_object_url": { "basename": "ChiaChiehChu_Thesis.pdf", "content": "final", "filesize": 6832637, "license": "other", "mime_type": "application/pdf", "url": "/5275/1/ChiaChiehChu_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Multiscale Methods for Elliptic Partial Differential Equations and Related Applications", "author": [ { "family_name": "Chu", "given_name": "Chia-Chieh", "clpid": "Chu-Chia-Chieh" } ], "thesis_advisor": [ { "family_name": "Hou", "given_name": "Thomas Y.", "orcid": "0000-0001-6287-1133", "clpid": "Hou-T-Y" } ], "thesis_committee": [ { "family_name": "Hou", "given_name": "Thomas Y.", "orcid": "0000-0001-6287-1133", "clpid": "Hou-T-Y" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Multiscale problems arise in many scientific and engineering disciplines. A typical example is the modelling of flow in a porous medium containing a number of low and high permeability embedded in a matrix. Due to the high degrees of variability and the multiscale nature of formation properties, not only is a complete analysis of these problems extremely difficult, but also numerical solvers require an excessive amount of CPU time and storage. In this thesis, we study multiscale numerical methods for the elliptic equations arising in interface and two-phase flow problems. The model problems we consider are motivated by the multiscale computations of flow and transport of two-phase flow in strongly heterogeneous porous media. Although the analysis is carried out for simplified model problems, it does provide valuable insight in designing accurate multiscale methods for more realistic applications.
\r\n\r\nIn the first part, we introduce a new multiscale finite element method which is able to accurately capture solutions of elliptic interface problems with high contrast coefficients by using only coarse quasiuniform meshes, and without resolving the interfaces. The method is H\u00b9-conforming, and has an optimal convergence rate of O(h) in the energy norm and O(h\u00b2) in the L\u2082 norm, where h is the mesh diameter and the hidden constants in these estimates are independent of the \"contrast\" (i.e. ratio of largest to smallest value) of the PDE's coefficients. The new interior boundary conditions depend not only on the contrast of the coefficients, but also on the angles of intersection of the interface with the element edges. We conduct some numerical experiments to confirm the optimal rate of convergence of the proposed method and its independence from the aspect ratio of the coefficients.
\r\n\r\nIn the second part, we propose a flow-based oversampling method where the actual two-phase flow boundary conditions are used to construct oversampling auxiliary functions. Our numerical results show that the flow-based oversampling approach is several times more accurate than the standard oversampling method. A partial theoretical explanation is provided for these numerical observations.
\r\n\r\nIn the third part, we discuss \"metric-based upscaling\" for the pressure equation in two-phase flow problem. We show a compensation phenomenon and design a multiscale method for the pressure equation with highly oscillatory permeability.
\r\n", "doi": "10.7907/PFGA-YY17", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5857", "collection": "thesis", "collection_id": "5857", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-122004587", "primary_object_url": { "basename": "Full_Thesis.pdf", "content": "final", "filesize": 38221978, "license": "other", "mime_type": "application/pdf", "url": "/5857/1/Full_Thesis.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "The Role of Unsteady Hydrodynamics in the Propulsive Performance of a Self-Propelled Bioinspired Vehicle", "author": [ { "family_name": "Ruiz", "given_name": "Lydia Ann", "clpid": "Ruiz-Lydia-Ann" } ], "thesis_advisor": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" } ], "thesis_committee": [ { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Aquatic animals differ from typical engineering systems in their method of locomotion. In general, aquatic animals propel using unsteady dynamics producing vortex rings. Researchers have long shown interest in designing devices that resemble their shape and propulsive behavior. Traditional definitions of propulsive efficiency used to model these behaviors have not taken unsteady effects into account and are typically based on steady flow through propellers or rocket motors. Measurements of aquatic animals based on these quasi-steady metrics have suggested propulsive efficiencies over 80% when utilizing certain swimming kinematics. However, the mechanical efficiency of muscle-actuated biological propulsion has been found to be much lower, typically less than 20%. It is important to take into account the total efficiency of the system, the product of the mechanical and propulsive efficiency, when designing and implementing a biologically inspired propulsive device.
\r\n\r\nThe purpose of my research is to make a direct, experimental comparison between biological and engineering propulsion systems. For this study, I designed an underwater vehicle that has the capability of producing either a steady or unsteady jet for propulsion, akin to a squid and jellyfish, while utilizing the same mechanical efficiency. I show that it is unnecessary to take an approach that mimics animal shape and kinematics to achieve the associated propulsive performance. A bioinspired, propeller-based platform that mimics animal wake dynamics can be similarly effective.
\r\n\r\nA study on how vortex dynamics plays a key role in improving the propulsive efficiency of pulsed jet propulsion was conducted. Measurements of propulsive performance resulted in superior performance for the pulsed-jet configuration in comparison to the steady jet configuration particularly at higher motor speeds. The analysis demonstrated that vortex ring formation led to the acceleration of two classes of ambient fluid, entrained and added mass, and this consequently led to an increased total fluid impulse of the jet and propulsive performance. The first source of ambient fluid acceleration investigated was entrained mass. The magnitude of the entrainment ratio was measured and found to be smaller for the steady jet mode of propulsion in comparison to the pulsed jet mode of propulsion given comparable motor speeds. The role of the added mass effect was also investigated in increasing propulsive performance. A model developed by Krueger is used to determine the fraction of the total impulse imparted to the flow that was contributed by the added mass effect. Results demonstrated that the added mass effect associated with the acceleration of ambient fluid at the initiation of a starting jet provides an increase in the total impulse and is thus a source for increased propulsive performance. Last, a model was developed to investigate how an increase in the total fluid impulse due to vortex ring formation is related to the propulsive efficiency. Results obtained using the model are in agreement, within uncertainty, with previous experimental results for the measurement of propulsive efficiency. The results support that the additional force generated from the acceleration of two classes of ambient fluid are the source of increased propulsive efficiency for the pulsed jet configuration in comparison to the steady jet configuration. This model serves as an additional metric for determining the propulsive efficiency of a system utilizing pulsed jet propulsion.
\r\n", "doi": "10.7907/7JWD-TB88", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5859", "collection": "thesis", "collection_id": "5859", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-133830557", "primary_object_url": { "basename": "AndoCIT2010.pdf", "content": "final", "filesize": 2480169, "license": "other", "mime_type": "application/pdf", "url": "/5859/1/AndoCIT2010.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Effects of Polydispersity in Bubbly Flows", "author": [ { "family_name": "Ando", "given_name": "Keita", "orcid": "0000-0002-9572-8242", "clpid": "Ando-Keita" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis concerns the dynamics of bubbly flows with a distribution of equilibrium bubble sizes. The main goal is to formulate the physical and numerical models of continuum bubbly flows that enable us to efficiently compute the average mixture dynamics. Numerical simulations are conducted to quantify the effects of bubble size distributions on the averaged dynamics for several model flows.
\r\n\r\nFirst, the ensemble-averaged conservation laws for polydisperse bubbly flows are derived. One-way-coupled flow computations are conducted to illustrate that the different-sized bubbles can oscillate with different frequencies. The resulting phase cancellations can be regarded as an apparent damping of the averaged dynamics of polydisperse flows. A high-order-accurate finite-volume method is then developed to compute the flow, paying special attention to issues of wave dispersion and stiffness.
\r\n\r\nNext, computations of one-dimensional shock propagation through bubbly liquids are performed. The numerical experiments reveal that the bubble size distribution has a profound impact on the averaged shock structure. If the distribution is sufficiently broad, the apparent damping due to the phase cancellations can dominate over the single-bubble-dynamic dissipation (due to thermal, viscous, and compressibility effects) and the averaged shock dynamics become insensitive to the individual bubble dynamics. One-dimensional cloud cavitation caused by fluid-structure interaction is also solved to investigate the collapse of cavitation clouds with both monodisperse and polydisperse nuclei. The phase cancellations among the cavitation bubbles with broad nuclei size distributions are found to eliminate violent cloud collapse in the averaged dynamics.
\r\n\r\nFinally, shock propagation through a bubbly liquid-filled, deformable tube is considered. The quasi-one-dimensional conservation law that takes into account structural deformation is formulated and steady shock relations are derived. The results are compared to water-hammer experiments; the present shock theory gives better agreement with the measured wave speeds than linear theory. This indicates that the gas-phase nonlinearity needs to be included to accurately predict the propagation speeds of finite-amplitude waves in a deformable tube filled with a bubbly liquid.
", "doi": "10.7907/SW8K-Y135", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5866", "collection": "thesis", "collection_id": "5866", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272010-165618198", "type": "thesis", "title": "An Experimental and Numerical Study of Normal Particle Collisions in a Viscous Liquid", "author": [ { "family_name": "Li", "given_name": "Xiaobai", "clpid": "Li-Xiaobai" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "When two solid bodies collide in a liquid environment, the collision process is influenced by viscous effects and the increased pressure in the interstitial liquid layer between the two solid boundaries. A normal collision process is investigated for a range of impact Stokes numbers using both experimental and numerical methods. Experiments of a steel sphere falling under gravity and colliding with a Zerodur wall with Stokes number ranging from 5 to 100 are performed, which complement previous investigations of immersed particle-wall collision processes. The incompressible Navier-Stokes equations are solved numerically to predict the coupled motion of the falling particle and the surrounding fluid as the particle impacts and rebounds from the planar wall. The numerical method is validated by comparing the numerical simulations of a settling sphere with experimental measurements of the sphere trajectory and the accompanying flow-field. A contact model of the liquid-solid and solid-solid interaction is developed that incorporates the elasticity of the solids to permit the rebound trajectory to be simulated accurately. The contact model is applied when the particle is sufficiently close to the wall that it becomes difficult to resolve the thin lubrication layer. The model is calibrated with measured particle trajectories and is found to represent well the observed coefficient of restitution over a range of impact Stokes numbers from 1 to 1000. In addition, the model is modified to simulate the normal collision of two spheres. The effective coefficient of restitution obtained from the simulation shows a strong dependence on the binary Stokes number accordant with other researcher\u2019s experimental results. The unique behaviors of the two spheres at low binary Stokes number including target motion prior to contact and group motion after collision are simulated by the current work.", "doi": "10.7907/S5V0-3E25", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5886", "collection": "thesis", "collection_id": "5886", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05282010-150032204", "primary_object_url": { "basename": "jjk_thesis_final.pdf", "content": "final", "filesize": 2429194, "license": "other", "mime_type": "application/pdf", "url": "/5886/1/jjk_thesis_final.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Numerical Simulation of Wave Focusing and Scattering in Shock Wave Lithotripsy", "author": [ { "family_name": "Krimmel", "given_name": "Jeffrey James", "clpid": "Krimmel-Jeffrey-James" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this work we simulate shock wave focusing and scattering that occurs during shock wave lithotripsy, a noninvasive medical treatment for kidney stone disease. Shock waves are generated outside the body of the patient and are focused at the kidney stone with the intention of pulverizing the stone while it remains inside the patient. The patient can then ostensibly pass the debris naturally. We use a multidimensional second-order method of the Godunov type with slope limiters and shock capturing capability to solve the inviscid Euler equations. Because we begin with the fundamental statements of conservation of mass, momentum, and energy, we simulate all the relevant acoustics occurring during a typical treatment.
\r\n\r\nLithotripters, the machines that generate and focus shock waves, can be classified according to the mechanism of shock generation. In this work, we simulate three different types of lithotripters: electrohydraulic, piezoelectric, and electromagnetic. We choose one representative of each lithotripter type: the Dornier HM3, a research piezoelectric lithotripter array, and the XX-Es, respectively. We first study a model of the in vitro setting for each lithotripter, where shock waves are generated and focus in a bath of pure water. Next, we introduce different heterogeneous materials near the focus of the lithotripter to model the effect of the body of an actual patient, i.e., the in vivo condition. We use two approaches in this modeling effort. One approach is to use simple geometrical models for the body cavity and kidney that we created ourselves. The other approach is to import real anatomical data made available from the VOXEL-MAN Group.
\r\n\r\nIn studying the focal region acoustics, we specifically examine the maximum calculated pressures. These pressures represent the forces that will ultimately cause the kidney stone to break. We also study the pulse intensity integral, i.e., the energy density carried by the focusing shock wave. In addition to these pressures and energy densities, we are interested in investigating how soft tissue in the focal region may potentially be damaged by the resulting wavefields. We isolate two mechanisms that are thought to be important in soft tissue injury: shearing and cavitation. We calculate estimates for the maximum principal normal and shear strains in the focal region in addition to the corresponding strain rates and use these as metrics for the potential for damage via shearing. We study the calculated negative pressure fields in this region as a surrogate for potential damage caused by cavitation.
\r\n\r\nWe find that our simple geometrical anatomical models cause little deviation from the acoustics observed in a water bath. However, when the real anatomical data of the VOXEL-MAN Group is used, the fields of the various relevant flow quantities become more highly oscillatory and produce secondary extrema that could produce damage not predicted from the water bath case. In addition to the conclusions from our own work, we discuss how our results motivate future studies that will hopefully help elucidate specific mechanisms by which kidney stones break and soft tissue becomes damaged.
", "doi": "10.7907/XWED-RZ95", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5928", "collection": "thesis", "collection_id": "5928", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072010-131025711", "primary_object_url": { "basename": "JoeCaltechThesis2010.pdf", "content": "final", "filesize": 77167886, "license": "other", "mime_type": "application/pdf", "url": "/5928/1/JoeCaltechThesis2010.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Optimized Feedback Control of Vortex Shedding on an Inclined Flat Plate", "author": [ { "family_name": "Joe", "given_name": "Won Tae", "clpid": "Joe-Won-Tae" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" }, { "family_name": "MacMynowski", "given_name": "Douglas G.", "clpid": "MacMynowski-D-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Blanquart", "given_name": "Guillaume", "orcid": "0000-0002-5074-9728", "clpid": "Blanquart-G" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis examines flow control and the potentially favorable effects of feedback, associated with unsteady actuation in separated flows over airfoils. The objective of the flow control is to enhance lift at post-stall angles of attack by changing the dynamics of the wake vortices. We present results from a numerical study of unsteady actuation on a two-dimensional flat plate at post-stall angles of attack at Reynolds number (Re) of 300 and 3000. At Re=300, the control waveform is optimized and a feedback strategy is developed to optimize the phase of the control relative to the lift with either a sinusoidal or the optimized waveform, resulting in a high-lift limit cycle of vortex shedding. Also at Re=3000, we show that certain frequencies and actuator waveforms lead to stable (high-lift) limit cycles, in which the flow is phase locked to the actuation.
\r\n\r\nFirst, a two-dimensional flat plate model at a high angle of attack at a Re of 300 is considered. We design the feedback to slightly adjust the frequency and/or phase of actuation to lock it to a particular phase of the lift, thus achieving a phase-locked flow with the maximal period-averaged lift over every cycle of actuation.
\r\n\r\nWith the sinusoidal forcing and feedback, we show that it is possible to optimize the phase of the control relative to the lift in order to achieve the highest possible period-averaged lift in a consistent fashion. However, continuous sinusoidal forcing could be adding circulation when it is unnecessary, or undesirable. Thus we employ an adjoint-based optimization in order to find the waveform (time history of the jet velocity) that maximizes the lift for a given actuation amplitude. The adjoint of the linearized perturbed equations is solved backwards in time to obtain the gradient of the lift to changes in actuation (the jet velocity), and this information is used to iteratively improve the controls.
\r\n\r\nOptimal control provides a periodic control waveform, resulting in high lift shedding cycle with minimal control input. However, if applied in open loop, the flow fails to phase lock onto the optimal waveform, degrading the lift performance. Thus, the optimized waveform is also implemented in a closed-loop controller where the control signal is shifted or deformed periodically to adjust to the (instantaneous) frequency of the lift fluctuations. The feedback utilizes a narrowband filter and an Extended Kalman Filter to robustly estimate the phase of vortex shedding and achieve phase-locked, high lift flow states. Feedback control of the optimized waveform is able to reproduce the high-lift limit cycle from the optimization, but starting from an arbitrary phase of the baseline limit cycle.
\r\n\r\nFinally, we apply the tools developed and knowledge gained at Re=300 to a Re of 3000 on a thin airfoil with a thickness-to-chord ratio of 4%, which were chosen to match the experimental studies of Greenblatt et al. (2008). We consider more detailed time-dependent aspects of the lift and corresponding flow fields, particularly the flow structures at the minimum and maximum lift, and the phase of pulses relative to the lift, in order to more precisely compare different actuated flow fields and distinguish the differences responsible for higher or lower instantaneous lift, along with identifying different vortex evolutions. We consider two representative angles of attack, 10 and 20 degrees, and investigate the lift enhancement and which combinations of forcing frequency and duty cycle lead to phase-locked flow. Finally, we show that for certain frequencies and actuator waveforms, there occur stable limit cycles in which the flow is phase locked to the actuation.
", "doi": "10.7907/6D11-2Y92", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5275", "collection": "thesis", "collection_id": "5275", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07312009-095021", "primary_object_url": { "basename": "ChiaChiehChu_Thesis.pdf", "content": "final", "filesize": 6832637, "license": "other", "mime_type": "application/pdf", "url": "/5275/1/ChiaChiehChu_Thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Multiscale Methods for Elliptic Partial Differential Equations and Related Applications", "author": [ { "family_name": "Chu", "given_name": "Chia-Chieh", "clpid": "Chu-Chia-Chieh" } ], "thesis_advisor": [ { "family_name": "Hou", "given_name": "Thomas Y.", "orcid": "0000-0001-6287-1133", "clpid": "Hou-T-Y" } ], "thesis_committee": [ { "family_name": "Hou", "given_name": "Thomas Y.", "orcid": "0000-0001-6287-1133", "clpid": "Hou-T-Y" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Multiscale problems arise in many scientific and engineering disciplines. A typical example is the modelling of flow in a porous medium containing a number of low and high permeability embedded in a matrix. Due to the high degrees of variability and the multiscale nature of formation properties, not only is a complete analysis of these problems extremely difficult, but also numerical solvers require an excessive amount of CPU time and storage. In this thesis, we study multiscale numerical methods for the elliptic equations arising in interface and two-phase flow problems. The model problems we consider are motivated by the multiscale computations of flow and transport of two-phase flow in strongly heterogeneous porous media. Although the analysis is carried out for simplified model problems, it does provide valuable insight in designing accurate multiscale methods for more realistic applications.
\r\n\r\nIn the first part, we introduce a new multiscale finite element method which is able to accurately capture solutions of elliptic interface problems with high contrast coefficients by using only coarse quasiuniform meshes, and without resolving the interfaces. The method is H\u00b9-conforming, and has an optimal convergence rate of O(h) in the energy norm and O(h\u00b2) in the L\u2082 norm, where h is the mesh diameter and the hidden constants in these estimates are independent of the \"contrast\" (i.e. ratio of largest to smallest value) of the PDE's coefficients. The new interior boundary conditions depend not only on the contrast of the coefficients, but also on the angles of intersection of the interface with the element edges. We conduct some numerical experiments to confirm the optimal rate of convergence of the proposed method and its independence from the aspect ratio of the coefficients.
\r\n\r\nIn the second part, we propose a flow-based oversampling method where the actual two-phase flow boundary conditions are used to construct oversampling auxiliary functions. Our numerical results show that the flow-based oversampling approach is several times more accurate than the standard oversampling method. A partial theoretical explanation is provided for these numerical observations.
\r\n\r\nIn the third part, we discuss \"metric-based upscaling\" for the pressure equation in two-phase flow problem. We show a compensation phenomenon and design a multiscale method for the pressure equation with highly oscillatory permeability.
\r\n", "doi": "10.7907/PFGA-YY17", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5539", "collection": "thesis", "collection_id": "5539", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01242010-111852941", "type": "thesis", "title": "Instability Wave Models of Turbulent Jets from Round and Serrated Nozzles", "author": [ { "family_name": "Gu\u00f0mundsson", "given_name": "Kristj\u00e1n", "clpid": "Gu\u00f0mundsson-Kristj\u00e1n" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hussain", "given_name": "Fazle", "orcid": "0000-0002-2209-9270", "clpid": "Hussain-F" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this thesis we study pressure fluctuations associated with large-scale coherent structures in turbulent round and serrated jets. Linear disturbances to the turbulent mean flow of the round jet are modeled via linear stability analysis and the Parabolized Stability Equations (PSE). We show that PSE provides better agreement with near-field microphone-array data at low frequencies than previous models based on linear stability theory. We examine the extent to which microphone data is contaminated by fluctuations uncorrelated with large-scale structures. By filtering out the uncorrelated fluctuations, via the proper orthogonal decomposition (POD), better agreement between data and theory is obtained. We next extend the linear stability analysis of round jets to include the effects of azimuthal inhomogeneities of serrated jets. We solve the resulting system of equations and find new modes, associated with the streamwise vorticity of the serrated-jet mean flow. All unstable modes of the serrated jet are stabilized, potentially explaining the noise reduction achieved by such jets. We also compare these predictions to POD-filtered microphone measurements, generally finding good agreement.\r\n", "doi": "10.7907/BQH9-G487", "publication_date": "2010", "thesis_type": "phd", "thesis_year": "2010" }, { "id": "thesis:5221", "collection": "thesis", "collection_id": "5221", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06022009-183247", "primary_object_url": { "basename": "Thesis_Franck.pdf", "content": "final", "filesize": 3520640, "license": "other", "mime_type": "application/pdf", "url": "/5221/1/Thesis_Franck.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Large-Eddy Simulation of Flow Separation and Control on a Wall-Mounted Hump", "author": [ { "family_name": "Franck", "given_name": "Jennifer Ann", "clpid": "Franck-Jennifer-Ann" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Active flow control techniques such as synthetic jets have been successful in increasing the performance of naturally separating flows on post-stall airfoils, bluff body shedding, and internal flows such as wide-angle diffusers. However, in order to implement robust control techniques there is a need for accurate computational tools capable of predicting unsteady separation and control at high Reynolds numbers. This thesis developed a compressible large-eddy simulation (LES) and validated it by simulating the turbulent flow over a wall-mounted hump. The flow is characterized by an unsteady, turbulent recirculation region along the trailing edge of the geometry, and is simulated at a Reynolds number of 500,000. Active flow control is applied just before the natural separation point via steady suction and zero-net mass flux oscillatory forcing. The addition of control is shown to be effective in decreasing the size of the separation bubble and pressure drag. LES baseline and controlled results are validated against previously performed experiments by Seifert and Pack and those performed for the NASA Langley Workshop on Turbulent Flow Separation and Control. Three test cases are explored to determine the effect of explicit filtering and the Smagorinsky subgrid scale model on the average flow and turbulent statistics. The flow physics and the control effectiveness are investigated at two Mach numbers, M=0.25 and M=0.6. Compressibility is shown to increase the separation bubble length in the baseline case, but does not significantly change the effectiveness of the control. In terms of decreasing drag on the wall-mounted hump model, steady suction is more effective than oscillatory control, but both control techniques are effective in reducing the separation bubble length. Two-dimensional direct numerical simulations (DNS) of the wall-mounted hump flow are also presented, and the results show different baseline flow features than the 3D LES. However the controlled 2D flow gives an indication of the most receptive actuation frequencies around twice that of the natural shedding frequency. Two regimes of reduced actuation frequency are also explored with the 3D LES. It is found that the low frequency actuation is successful in reducing the separation bubble length, but high frequency actuation produces an average flow comparable to the baseline case, and does not result in drag or separation bubble length reduction.\r\n", "doi": "10.7907/DH38-D592", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:5273", "collection": "thesis", "collection_id": "5273", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-07212009-142144", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 4854568, "license": "other", "mime_type": "application/pdf", "url": "/5273/1/thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Continuum Modeling of Mixed Conductors: A Study of Ceria", "author": [ { "family_name": "Ciucci", "given_name": "Francesco", "orcid": "0000-0003-0614-5537", "clpid": "Ciucci-Francesco" } ], "thesis_advisor": [ { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Goodwin", "given_name": "David G.", "clpid": "Goodwin-D-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Haile", "given_name": "Sossina M.", "orcid": "0000-0002-5293-6252", "clpid": "Haile-S-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this thesis we have derived a new way to analyze the impedance response of mixed conducting materials for use in solid oxide fuel cells (SOFCs), with the main focus on anodic materials, in particular cerium oxides.
\r\n\r\nFirst we have analyzed the impact of mixed conductivity coupled to electrocatalytic behavior in the linear time-independent domain for a thick ceria sample. We have derived that, for a promising fuel cell material, Samarium Doped Ceria, chemical reactions are the determining component of the polarization resistance.
\r\n\r\nAs a second step we have extended the previous model to the time-dependent case, where we focused on single harmonic excitation, the impedance spectroscopy conditions. We extended the model to the case where some input diffusivities are spatially nonuniform. For instance we considered the case where diffusivities change significantly in the vicinity of the electrocatalytic region.
\r\n \r\nAs a third and final step we use to model to capture the two dimensional behavior of mixed conducting thin films, where the electronic motion from one side of the sample to the other is impeded. Such conditions are similar to those encountered in fuel cells where an electrolyte conducting exclusively oxygen ions is placed between the anode and the cathode. The framework developed was also extended to study a popular cathodic material, Lanthanum Manganite.
\r\n\r\nThe model is used to give unprecedented insight in SOFC polarization resistance analysis of mixed conductors. It helps elucidate rigorously rate determining steps and to address the interplay of diffusion with diffusion losses. Electrochemical surface losses dominate for most experimental conditions of Samarium Doped Ceria and they are shown to be strongly dependent on geometry.
\r\n", "doi": "10.7907/3TWK-W923", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:2274", "collection": "thesis", "collection_id": "2274", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292009-123828", "primary_object_url": { "basename": "dchung_thesis_090529.pdf", "content": "final", "filesize": 1744414, "license": "other", "mime_type": "application/pdf", "url": "/2274/1/dchung_thesis_090529.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Numerical Simulation and Subgrid-Scale Modeling of Mixing and Wall-Bounded Turbulent Flows", "author": [ { "family_name": "Chung", "given_name": "Daniel", "orcid": "0000-0003-3732-364X", "clpid": "Chung-Daniel" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/NE1Y-M812", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:2274", "collection": "thesis", "collection_id": "2274", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05292009-123828", "primary_object_url": { "basename": "dchung_thesis_090529.pdf", "content": "final", "filesize": 1744414, "license": "other", "mime_type": "application/pdf", "url": "/2274/1/dchung_thesis_090529.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Numerical Simulation and Subgrid-Scale Modeling of Mixing and Wall-Bounded Turbulent Flows", "author": [ { "family_name": "Chung", "given_name": "Daniel", "orcid": "0000-0003-3732-364X", "clpid": "Chung-Daniel" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "McKeon", "given_name": "Beverley J.", "orcid": "0000-0003-4220-1583", "clpid": "McKeon-B-J" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Bruno", "given_name": "Oscar P.", "orcid": "0000-0001-8369-3014", "clpid": "Bruno-O-P" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/NE1Y-M812", "publication_date": "2009", "thesis_type": "phd", "thesis_year": "2009" }, { "id": "thesis:2319", "collection": "thesis", "collection_id": "2319", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302008-140331", "primary_object_url": { "basename": "ManuelLombardini_PhDThesis_30May2008.pdf", "content": "final", "filesize": 34371895, "license": "other", "mime_type": "application/pdf", "url": "/2319/1/ManuelLombardini_PhDThesis_30May2008.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Richtmyer-Meshkov Instability in Converging Geometries", "author": [ { "family_name": "Lombardini", "given_name": "Manuel", "clpid": "Lombardini-Manuel" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/5SNE-4003", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2117", "collection": "thesis", "collection_id": "2117", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262008-152803", "primary_object_url": { "basename": "Matheou_G_2008_thesis.pdf", "content": "final", "filesize": 12870383, "license": "other", "mime_type": "application/pdf", "url": "/2117/1/Matheou_G_2008_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Large-Eddy Simulations of Molecular Mixing in a Recirculating Shear Flow", "author": [ { "family_name": "Matheou", "given_name": "Georgios", "orcid": "0000-0003-4024-4571", "clpid": "Matheou-Georgios" } ], "thesis_advisor": [ { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" } ], "thesis_committee": [ { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VFKF-SC30", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2117", "collection": "thesis", "collection_id": "2117", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262008-152803", "primary_object_url": { "basename": "Matheou_G_2008_thesis.pdf", "content": "final", "filesize": 12870383, "license": "other", "mime_type": "application/pdf", "url": "/2117/1/Matheou_G_2008_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Large-Eddy Simulations of Molecular Mixing in a Recirculating Shear Flow", "author": [ { "family_name": "Matheou", "given_name": "Georgios", "orcid": "0000-0003-4024-4571", "clpid": "Matheou-Georgios" } ], "thesis_advisor": [ { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" } ], "thesis_committee": [ { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VFKF-SC30", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2319", "collection": "thesis", "collection_id": "2319", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05302008-140331", "primary_object_url": { "basename": "ManuelLombardini_PhDThesis_30May2008.pdf", "content": "final", "filesize": 34371895, "license": "other", "mime_type": "application/pdf", "url": "/2319/1/ManuelLombardini_PhDThesis_30May2008.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Richtmyer-Meshkov Instability in Converging Geometries", "author": [ { "family_name": "Lombardini", "given_name": "Manuel", "clpid": "Lombardini-Manuel" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/5SNE-4003", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2409", "collection": "thesis", "collection_id": "2409", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06022008-170629", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 8111314, "license": "other", "mime_type": "application/pdf", "url": "/2409/1/Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Detonation Stability with Reversible Kinetics", "author": [ { "family_name": "Kao", "given_name": "Shannon Theresa", "clpid": "Kao-Shannon-Theresa" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "Detonation propagation is unsteady due to the innate instability of the reaction zone structure. Up until the present, investigations of detonation stability have been exclusively concerned with model systems using the perfect gas equation of state and primarily single-step irreversible reaction mechanisms.
\r\n\r\nThis study investigates detonation stability characteristics with reversible chemical kinetics models. To allow for more general kinetics models, we generalize the perfect gas, one-step irreversible kinetics, linear stability equations to a set of equations using the ideal gas equation of state and a general reaction scheme. We linearly perturb the reactive Euler equations following the method of Lee and Stewart (1990) and Short and Stewart (1998). Our implementation uses Cantera (Goodwin, 2005) to evaluate all thermodynamic quantities and evaluate generalized analytic derivatives of quantities dependent on the kinetics model.
\r\n\r\nThe computational domain is the reaction zone in the shock-fixed frame such that the left boundary conditions are the perturbed shock jump conditions which we have derived for a general equation of state and implemented for an ideal gas equation of state. At the right boundary, the system must satisfy a radiation condition requiring that all waves travel out of the domain. Unlike the case of a single reversible reaction, in a truly multistep kinetics model, the radiation boundary condition cannot be solved analytically. In this work, we provide a general methodology for satisfying the appropriate boundary condition.
\r\n\r\nWe then investigate the effects of reversibility on the characteristics of the instability in one and two dimensions. These characteristics are quantified by the unstable eigenvalues as well as the shape of the base flow and eigenfunctions. We show that there is an exchange of stability as a function of reversibility. To confirm the results our work, we have performed unsteady calculations. We show that we can match the frequencies predicted by our linear stability calculations near the stability threshold.
", "doi": "10.7907/H8JN-VS03", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:2409", "collection": "thesis", "collection_id": "2409", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06022008-170629", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 8111314, "license": "other", "mime_type": "application/pdf", "url": "/2409/1/Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Detonation Stability with Reversible Kinetics", "author": [ { "family_name": "Kao", "given_name": "Shannon Theresa", "clpid": "Kao-Shannon-Theresa" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "abstract": "Detonation propagation is unsteady due to the innate instability of the reaction zone structure. Up until the present, investigations of detonation stability have been exclusively concerned with model systems using the perfect gas equation of state and primarily single-step irreversible reaction mechanisms.
\r\n\r\nThis study investigates detonation stability characteristics with reversible chemical kinetics models. To allow for more general kinetics models, we generalize the perfect gas, one-step irreversible kinetics, linear stability equations to a set of equations using the ideal gas equation of state and a general reaction scheme. We linearly perturb the reactive Euler equations following the method of Lee and Stewart (1990) and Short and Stewart (1998). Our implementation uses Cantera (Goodwin, 2005) to evaluate all thermodynamic quantities and evaluate generalized analytic derivatives of quantities dependent on the kinetics model.
\r\n\r\nThe computational domain is the reaction zone in the shock-fixed frame such that the left boundary conditions are the perturbed shock jump conditions which we have derived for a general equation of state and implemented for an ideal gas equation of state. At the right boundary, the system must satisfy a radiation condition requiring that all waves travel out of the domain. Unlike the case of a single reversible reaction, in a truly multistep kinetics model, the radiation boundary condition cannot be solved analytically. In this work, we provide a general methodology for satisfying the appropriate boundary condition.
\r\n\r\nWe then investigate the effects of reversibility on the characteristics of the instability in one and two dimensions. These characteristics are quantified by the unstable eigenvalues as well as the shape of the base flow and eigenfunctions. We show that there is an exchange of stability as a function of reversibility. To confirm the results our work, we have performed unsteady calculations. We show that we can match the frequencies predicted by our linear stability calculations near the stability threshold.
", "doi": "10.7907/H8JN-VS03", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1712", "collection": "thesis", "collection_id": "1712", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05092008-171346", "primary_object_url": { "basename": "johnsen2007.pdf", "content": "final", "filesize": 7026493, "license": "other", "mime_type": "application/pdf", "url": "/1712/1/johnsen2007.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Numerical Simulations of Non-Spherical Bubble Collapse with Applications to Shockwave Lithotripsy", "author": [ { "family_name": "Johnsen", "given_name": "Eric", "orcid": "0000-0001-9530-408X", "clpid": "Johnsen-Eric" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shockwave lithotripsy (SWL) is a non-invasive medical procedure in which shockwaves are focused on kidney stones in an attempt to break them. Because the stones are usually immersed in liquid, cavitation occurs during the process. However, the stone comminution mechanisms and the bubble dynamics of SWL are not fully understood. In the present thesis, numerical simulations are employed to study axisymmetric Rayleigh collapse and shock-induced collapse of a single gas bubble in a free field and near a wall. A high-order accurate, quasi-conservative, shock- and interface-capturing scheme is developed to solve the multicomponent Euler equations.
\r\n\r\nThe primary contributions of the present work are the development of a new numerical framework to study compressible multicomponent flows, the characterization of the dynamics of non-spherical bubble collapse, and quantitative measurements of wall pressures generated by bubble collapse. Because of asymmetries in the flow field, a re-entrant jet develops and generates a large water-hammer pressure upon impact onto the distal side. Jet properties are calculated and, as an indication of potential damage, wall pressures are measured; pressures on the order of 1 GPa are achieved locally. In shock-induced collapse, the wall pressure is amplified by the presence of bubbles within several initial radii from the wall. Thus, the pressure generated by the bubble collapse is larger than the incoming shock. The results extended to SWL show that shock-induced collapse has tremendous potential for damage along the stone surface. Furthermore, the simulations are coupled to an elastic wave propagation code to show that bubble collapse may cause damage within kidney stones as well.
\r\n", "doi": "10.7907/WPQB-2W24", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1712", "collection": "thesis", "collection_id": "1712", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05092008-171346", "primary_object_url": { "basename": "johnsen2007.pdf", "content": "final", "filesize": 7026493, "license": "other", "mime_type": "application/pdf", "url": "/1712/1/johnsen2007.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Numerical Simulations of Non-Spherical Bubble Collapse with Applications to Shockwave Lithotripsy", "author": [ { "family_name": "Johnsen", "given_name": "Eric", "orcid": "0000-0001-9530-408X", "clpid": "Johnsen-Eric" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Shockwave lithotripsy (SWL) is a non-invasive medical procedure in which shockwaves are focused on kidney stones in an attempt to break them. Because the stones are usually immersed in liquid, cavitation occurs during the process. However, the stone comminution mechanisms and the bubble dynamics of SWL are not fully understood. In the present thesis, numerical simulations are employed to study axisymmetric Rayleigh collapse and shock-induced collapse of a single gas bubble in a free field and near a wall. A high-order accurate, quasi-conservative, shock- and interface-capturing scheme is developed to solve the multicomponent Euler equations.
\r\n\r\nThe primary contributions of the present work are the development of a new numerical framework to study compressible multicomponent flows, the characterization of the dynamics of non-spherical bubble collapse, and quantitative measurements of wall pressures generated by bubble collapse. Because of asymmetries in the flow field, a re-entrant jet develops and generates a large water-hammer pressure upon impact onto the distal side. Jet properties are calculated and, as an indication of potential damage, wall pressures are measured; pressures on the order of 1 GPa are achieved locally. In shock-induced collapse, the wall pressure is amplified by the presence of bubbles within several initial radii from the wall. Thus, the pressure generated by the bubble collapse is larger than the incoming shock. The results extended to SWL show that shock-induced collapse has tremendous potential for damage along the stone surface. Furthermore, the simulations are coupled to an elastic wave propagation code to show that bubble collapse may cause damage within kidney stones as well.
\r\n", "doi": "10.7907/WPQB-2W24", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1961", "collection": "thesis", "collection_id": "1961", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05222008-090253", "primary_object_url": { "basename": "ARAthesis.pdf", "content": "final", "filesize": 4584211, "license": "other", "mime_type": "application/pdf", "url": "/1961/1/ARAthesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Surface Deformation in a Liquid Environment Resulting from Single Particle Collisions", "author": [ { "family_name": "Ruiz-Angulo", "given_name": "Angel", "orcid": "0000-0002-7292-3002", "clpid": "Ruiz-Angulo-Angel" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Multiphase flows are fairly complex and they are usually studied as a bulk. In this thesis, these flows are approached by looking at single particle interactions (particle-particle and particle-wall). This work presents experimental measurements of the approach and rebound of a particle colliding with a ``deformable' surface in a viscous liquid. The complex interaction between the fluid and the solid phases is coupled through the dynamics of the flow as well as the deformation process. A simple pendulum experiment was used to produced single controlled collisions; steel particles were used to impact different aluminum alloy samples (Al-6061, Al-2024, and Al-7075) using different aqueous mixtures of glycerol and water as a viscous fluid. The velocity of the particle before and after the collision was estimated by post-processing the particle position recorded with a high speed camera. For the combination of materials proposed, the elastic limit is reached at relatively low velocities. The deformations produced by the collision were analyzed using an optical profilometer. The measurements showed that the size of the indentations is independent of the fluid media. It was found that the size of the indentations was the same for collisions in air than for the rest of the collisions using various viscous fluids. The results show that the plastic deformation is only a function of the impact velocity and the material properties. The normal coefficient of restitution and deformation parameters account for losses due to lubrication effect and inelasticity, identifying then, the dominant energy loss mechanism during the collision process.
\r\n\r\nAccording to the strain imposed in the samples due to the collision, the deformations were either elastic or elastic-plastic. The equivalent load due to the impact velocities used in this work did not reach the fully-plastic regime. For the collisions in air, different models were used to compare the experimental results showing that the elastic-plastic regime is not well characterized by only the material properties and the impact velocity. The time-resolved contact force was measured during the process of the indentation for the dry collision experiments using a quartz load transducer.
\r\n\r\nThe experiments clearly show four different regimes depending on the impact Stokes number: lubrication effect and elastic deformation, lubrication effect and elastic-plastic deformation, elastic deformation with no hydrodynamic effects, and elastic-plastic deformation with negligible lubrication effect. An analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements.
", "doi": "10.7907/WVJT-TG10", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1961", "collection": "thesis", "collection_id": "1961", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05222008-090253", "primary_object_url": { "basename": "ARAthesis.pdf", "content": "final", "filesize": 4584211, "license": "other", "mime_type": "application/pdf", "url": "/1961/1/ARAthesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Surface Deformation in a Liquid Environment Resulting from Single Particle Collisions", "author": [ { "family_name": "Ruiz-Angulo", "given_name": "Angel", "orcid": "0000-0002-7292-3002", "clpid": "Ruiz-Angulo-Angel" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Multiphase flows are fairly complex and they are usually studied as a bulk. In this thesis, these flows are approached by looking at single particle interactions (particle-particle and particle-wall). This work presents experimental measurements of the approach and rebound of a particle colliding with a ``deformable' surface in a viscous liquid. The complex interaction between the fluid and the solid phases is coupled through the dynamics of the flow as well as the deformation process. A simple pendulum experiment was used to produced single controlled collisions; steel particles were used to impact different aluminum alloy samples (Al-6061, Al-2024, and Al-7075) using different aqueous mixtures of glycerol and water as a viscous fluid. The velocity of the particle before and after the collision was estimated by post-processing the particle position recorded with a high speed camera. For the combination of materials proposed, the elastic limit is reached at relatively low velocities. The deformations produced by the collision were analyzed using an optical profilometer. The measurements showed that the size of the indentations is independent of the fluid media. It was found that the size of the indentations was the same for collisions in air than for the rest of the collisions using various viscous fluids. The results show that the plastic deformation is only a function of the impact velocity and the material properties. The normal coefficient of restitution and deformation parameters account for losses due to lubrication effect and inelasticity, identifying then, the dominant energy loss mechanism during the collision process.
\r\n\r\nAccording to the strain imposed in the samples due to the collision, the deformations were either elastic or elastic-plastic. The equivalent load due to the impact velocities used in this work did not reach the fully-plastic regime. For the collisions in air, different models were used to compare the experimental results showing that the elastic-plastic regime is not well characterized by only the material properties and the impact velocity. The time-resolved contact force was measured during the process of the indentation for the dry collision experiments using a quartz load transducer.
\r\n\r\nThe experiments clearly show four different regimes depending on the impact Stokes number: lubrication effect and elastic deformation, lubrication effect and elastic-plastic deformation, elastic deformation with no hydrodynamic effects, and elastic-plastic deformation with negligible lubrication effect. An analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements.
", "doi": "10.7907/WVJT-TG10", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:1990", "collection": "thesis", "collection_id": "1990", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05232008-124342", "primary_object_url": { "basename": "Taira_thesis.pdf", "content": "final", "filesize": 7582318, "license": "other", "mime_type": "application/pdf", "url": "/1990/3/Taira_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "The Immersed Boundary Projection Method and Its Application to Simulation and Control of Flows Around Low-Aspect-Ratio Wings", "author": [ { "family_name": "Taira", "given_name": "Kunihiko (Sam)", "orcid": "0000-0002-3762-8075", "clpid": "Taira-Kunihiko-Sam" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VSDD-P465", "publication_date": "2008", "thesis_type": "phd", "thesis_year": "2008" }, { "id": "thesis:65", "collection": "thesis", "collection_id": "65", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01082007-103832", "primary_object_url": { "basename": "L.Loumes-Thesis-All.pdf", "content": "final", "filesize": 4871165, "license": "other", "mime_type": "application/pdf", "url": "/65/12/L.Loumes-Thesis-All.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Multilayer Impedance Pump: A Bio-Inspired Valveless Pump with Medical Applications", "author": [ { "family_name": "Loumes", "given_name": "Laurence", "clpid": "Loumes-Laurence" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/MT2J-AR81", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:141", "collection": "thesis", "collection_id": "141", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01122007-114557", "primary_object_url": { "basename": "2007-01-12-rubel-thesis-final.pdf", "content": "final", "filesize": 1694954, "license": "other", "mime_type": "application/pdf", "url": "/141/1/2007-01-12-rubel-thesis-final.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Theory of Stationarity and Asymptotic Approach in Dissipative Systems", "author": [ { "family_name": "Rubel", "given_name": "Michael Thomas", "clpid": "Rubel-Michael-Thomas" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VWDE-GB16", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:141", "collection": "thesis", "collection_id": "141", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01122007-114557", "primary_object_url": { "basename": "2007-01-12-rubel-thesis-final.pdf", "content": "final", "filesize": 1694954, "license": "other", "mime_type": "application/pdf", "url": "/141/1/2007-01-12-rubel-thesis-final.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "A Theory of Stationarity and Asymptotic Approach in Dissipative Systems", "author": [ { "family_name": "Rubel", "given_name": "Michael Thomas", "clpid": "Rubel-Michael-Thomas" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VWDE-GB16", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:65", "collection": "thesis", "collection_id": "65", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01082007-103832", "primary_object_url": { "basename": "L.Loumes-Thesis-All.pdf", "content": "final", "filesize": 4871165, "license": "other", "mime_type": "application/pdf", "url": "/65/12/L.Loumes-Thesis-All.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Multilayer Impedance Pump: A Bio-Inspired Valveless Pump with Medical Applications", "author": [ { "family_name": "Loumes", "given_name": "Laurence", "clpid": "Loumes-Laurence" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Daraio", "given_name": "Chiara", "orcid": "0000-0001-5296-4440", "clpid": "Daraio-C" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/MT2J-AR81", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1515", "collection": "thesis", "collection_id": "1515", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04262007-095945", "primary_object_url": { "basename": "GBres_Thesis.pdf", "content": "final", "filesize": 11080622, "license": "other", "mime_type": "application/pdf", "url": "/1515/13/GBres_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Simulations of Three-Dimensional Instabilities in Cavity Flows", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume Alain", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-Guillaume-Alain" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Direct numerical simulations are performed to investigate the stability of compressible flow over three-dimensional open cavities for future control applications.
\r\n\r\nFirst, the typical self-sustained oscillations, commonly referred as shear-layer (Rossiter) modes, are characterized for two-dimensional cavities over a range of flow conditions. A linear stability analysis is then conducted to search for three-dimensional global instabilities of the 2D mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of cavity configurations. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately 1 cavity depth and oscillates with a frequency about an order-of-magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities reported in flows over backward-facing steps, lid-driven cavity flows, and Couette flows.
\r\n\r\nResults from three-dimensional simulations of the nonlinear compressible Navier-Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. These results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.
", "doi": "10.7907/Z96W988B", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:1515", "collection": "thesis", "collection_id": "1515", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04262007-095945", "primary_object_url": { "basename": "GBres_Thesis.pdf", "content": "final", "filesize": 11080622, "license": "other", "mime_type": "application/pdf", "url": "/1515/13/GBres_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Numerical Simulations of Three-Dimensional Instabilities in Cavity Flows", "author": [ { "family_name": "Br\u00e8s", "given_name": "Guillaume Alain", "orcid": "0000-0003-2507-8659", "clpid": "Br\u00e8s-Guillaume-Alain" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Direct numerical simulations are performed to investigate the stability of compressible flow over three-dimensional open cavities for future control applications.
\r\n\r\nFirst, the typical self-sustained oscillations, commonly referred as shear-layer (Rossiter) modes, are characterized for two-dimensional cavities over a range of flow conditions. A linear stability analysis is then conducted to search for three-dimensional global instabilities of the 2D mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of cavity configurations. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately 1 cavity depth and oscillates with a frequency about an order-of-magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities reported in flows over backward-facing steps, lid-driven cavity flows, and Couette flows.
\r\n\r\nResults from three-dimensional simulations of the nonlinear compressible Navier-Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. These results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.
", "doi": "10.7907/Z96W988B", "publication_date": "2007", "thesis_type": "phd", "thesis_year": "2007" }, { "id": "thesis:2108", "collection": "thesis", "collection_id": "2108", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262006-120244", "primary_object_url": { "basename": "Thesis_FLYang2.pdf", "content": "final", "filesize": 2765613, "license": "other", "mime_type": "application/pdf", "url": "/2108/1/Thesis_FLYang2.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Interaction Law for a Collision Between Two Solid Particles in a Viscous Liquid", "author": [ { "family_name": "Yang", "given_name": "Fu-Ling", "orcid": "0000-0002-6633-6311", "clpid": "Yang-Fu-Ling" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Zenit Camacho", "given_name": "Jose Roberto", "orcid": "0000-0002-2717-4954", "clpid": "Zenit-Camacho-J-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis addresses the problem of inter-particle collisions in a viscous liquid. Experimental measurements were made on normal and oblique collisions between identical and dissimilar pairs of solid spheres. The experimental evidence supports the hypothesis that the normal and the tangential component of motions are decoupled during a rapid collision.
\r\n\r\nThe relative particle motion in the normal direction is crucial to an immersed collision process and can be characterized by an effective coefficient of restitution and a binary Stokes number. The effective coefficient of restitution monotonically decreases with a diminishing binary Stokes number, indicating a particle motion with less inertia and higher hindering fluid forces. The correlation between the two parameters exhibits a similar trend to what is observed in a sphere-wall collision, which motivates a theoretical modeling.
\r\n\r\nThe collision model developed in the current work includes a flow model and a revised rebound scheme. The flow model considers the steady viscous drag, the added mass force, and the history force. How the presence of a second nearby solid boundary affects these forces is investigated. A flow model is proposed with wall-correction terms and is used to predict an immersed pendulum motion toward a solid wall. General agreement with the available experimental data validates the model. The rebound scheme considers the magnitude of the surface roughness and the minimum distance of approach resuling from an elastohydrodynamic contact.
\r\n\r\nThe performance of the collision model in predicting the effective coefficient of restitution is evaluated through comparisons with experimental measurements and an existing elastohydrodynamic collision model that the current work is based on.
\r\n\r\nBased on the current experimental findings, the tangential component of motion can be described by a dry collision model, provided that the material parameters are properly modified for the interstitial liquid. Two pertinent parameters are the normal effective coefficient of restitution and an effective friction coefficient.
", "doi": "10.7907/VFD0-C413", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1755", "collection": "thesis", "collection_id": "1755", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05122006-083011", "primary_object_url": { "basename": "Shadden06.pdf", "content": "final", "filesize": 16601102, "license": "other", "mime_type": "application/pdf", "url": "/1755/1/Shadden06.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Dynamical Systems Approach to Unsteady Systems", "author": [ { "family_name": "Shadden", "given_name": "Shawn Christopher", "orcid": "0000-0001-7561-1568", "clpid": "Shadden-Shawn-Christopher" } ], "thesis_advisor": [ { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" } ], "thesis_committee": [ { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Owhadi", "given_name": "Houman", "orcid": "0000-0002-5677-1600", "clpid": "Owhadi-H" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "For steady systems, interpreting the flow structure is typically straightforward because streamlines and trajectories coincide. Therefore the velocity field, or quantities derived from it, provide a clear description of the flow geometry. For unsteady flows, this is often not the case. A more natural choice is to understand the flow in terms of particle trajectories, i.e., the Lagrangian viewpoint. While the chaotic behavior of trajectories of unsteady systems makes direct interpretation difficult, more structured and frame-independent techniques have been developed. The method presented here uses finite-time Lyapunov exponent (FTLE) fields to locate Lagrangian Coherent Structures (LCS). LCS are co-dimension 1 separatrices that partition regions in phase space with dynamically different behavior. This method enables the detection of often non-obvious, time-dependent boundaries in complicated flows, which greatly elucidates the transport and mixing geometry.
\r\n\r\nThe first portion of this thesis deals with the theoretical development of LCS for two-, and then, n-dimensional systems, where n>2. Based on the definitions presented, some important properties of these structures are proven. It is shown that the flux across an LCS is typically very small and depends on the relative strength of the structure, the difference between the local rotation rate of the LCS with that of the Eulerian velocity field, and the integration time used to compute the FTLE field.
\r\n\r\nThe second portion of the thesis presents a series of numerical studies in which LCS are used to examine a range of interesting applications. This portion is bridged with the theoretical development presented in the first half by a brief chapter describing the numerical computation of FTLE fields and LCS. Applications presented in the second half of the thesis include the study of vortex rings in which LCS are used to define the unsteady vortex boundary to clarify the entrainment and detrainment processes; the computation of LCS in the ocean to provide mesoscale separatrices that help characterize the flow conditions and help navigate gliders or drifters used for sampling; flow over an airfoil where an LCS captures the unsteady separation profile; flow through a micro-mixing channel where LCS reveal the mechanism and geometry of chaotic mixing.
\r\n", "doi": "10.7907/BG86-YB12", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2108", "collection": "thesis", "collection_id": "2108", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05262006-120244", "primary_object_url": { "basename": "Thesis_FLYang2.pdf", "content": "final", "filesize": 2765613, "license": "other", "mime_type": "application/pdf", "url": "/2108/1/Thesis_FLYang2.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Interaction Law for a Collision Between Two Solid Particles in a Viscous Liquid", "author": [ { "family_name": "Yang", "given_name": "Fu-Ling", "orcid": "0000-0002-6633-6311", "clpid": "Yang-Fu-Ling" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Zenit Camacho", "given_name": "Jose Roberto", "orcid": "0000-0002-2717-4954", "clpid": "Zenit-Camacho-J-R" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis addresses the problem of inter-particle collisions in a viscous liquid. Experimental measurements were made on normal and oblique collisions between identical and dissimilar pairs of solid spheres. The experimental evidence supports the hypothesis that the normal and the tangential component of motions are decoupled during a rapid collision.
\r\n\r\nThe relative particle motion in the normal direction is crucial to an immersed collision process and can be characterized by an effective coefficient of restitution and a binary Stokes number. The effective coefficient of restitution monotonically decreases with a diminishing binary Stokes number, indicating a particle motion with less inertia and higher hindering fluid forces. The correlation between the two parameters exhibits a similar trend to what is observed in a sphere-wall collision, which motivates a theoretical modeling.
\r\n\r\nThe collision model developed in the current work includes a flow model and a revised rebound scheme. The flow model considers the steady viscous drag, the added mass force, and the history force. How the presence of a second nearby solid boundary affects these forces is investigated. A flow model is proposed with wall-correction terms and is used to predict an immersed pendulum motion toward a solid wall. General agreement with the available experimental data validates the model. The rebound scheme considers the magnitude of the surface roughness and the minimum distance of approach resuling from an elastohydrodynamic contact.
\r\n\r\nThe performance of the collision model in predicting the effective coefficient of restitution is evaluated through comparisons with experimental measurements and an existing elastohydrodynamic collision model that the current work is based on.
\r\n\r\nBased on the current experimental findings, the tangential component of motion can be described by a dry collision model, provided that the material parameters are properly modified for the interstitial liquid. Two pertinent parameters are the normal effective coefficient of restitution and an effective friction coefficient.
", "doi": "10.7907/VFD0-C413", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1755", "collection": "thesis", "collection_id": "1755", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05122006-083011", "primary_object_url": { "basename": "Shadden06.pdf", "content": "final", "filesize": 16601102, "license": "other", "mime_type": "application/pdf", "url": "/1755/1/Shadden06.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Dynamical Systems Approach to Unsteady Systems", "author": [ { "family_name": "Shadden", "given_name": "Shawn Christopher", "orcid": "0000-0001-7561-1568", "clpid": "Shadden-Shawn-Christopher" } ], "thesis_advisor": [ { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" } ], "thesis_committee": [ { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Owhadi", "given_name": "Houman", "orcid": "0000-0002-5677-1600", "clpid": "Owhadi-H" }, { "family_name": "Dabiri", "given_name": "John O.", "orcid": "0000-0002-6722-9008", "clpid": "Dabiri-J-O" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "For steady systems, interpreting the flow structure is typically straightforward because streamlines and trajectories coincide. Therefore the velocity field, or quantities derived from it, provide a clear description of the flow geometry. For unsteady flows, this is often not the case. A more natural choice is to understand the flow in terms of particle trajectories, i.e., the Lagrangian viewpoint. While the chaotic behavior of trajectories of unsteady systems makes direct interpretation difficult, more structured and frame-independent techniques have been developed. The method presented here uses finite-time Lyapunov exponent (FTLE) fields to locate Lagrangian Coherent Structures (LCS). LCS are co-dimension 1 separatrices that partition regions in phase space with dynamically different behavior. This method enables the detection of often non-obvious, time-dependent boundaries in complicated flows, which greatly elucidates the transport and mixing geometry.
\r\n\r\nThe first portion of this thesis deals with the theoretical development of LCS for two-, and then, n-dimensional systems, where n>2. Based on the definitions presented, some important properties of these structures are proven. It is shown that the flux across an LCS is typically very small and depends on the relative strength of the structure, the difference between the local rotation rate of the LCS with that of the Eulerian velocity field, and the integration time used to compute the FTLE field.
\r\n\r\nThe second portion of the thesis presents a series of numerical studies in which LCS are used to examine a range of interesting applications. This portion is bridged with the theoretical development presented in the first half by a brief chapter describing the numerical computation of FTLE fields and LCS. Applications presented in the second half of the thesis include the study of vortex rings in which LCS are used to define the unsteady vortex boundary to clarify the entrainment and detrainment processes; the computation of LCS in the ocean to provide mesoscale separatrices that help characterize the flow conditions and help navigate gliders or drifters used for sampling; flow over an airfoil where an LCS captures the unsteady separation profile; flow through a micro-mixing channel where LCS reveal the mechanism and geometry of chaotic mixing.
\r\n", "doi": "10.7907/BG86-YB12", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1490", "collection": "thesis", "collection_id": "1490", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04242006-172719", "primary_object_url": { "basename": "sjl_thesis.pdf", "content": "final", "filesize": 9105131, "license": "other", "mime_type": "application/pdf", "url": "/1490/1/sjl_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Proximal Bodies in Hypersonic Flow", "author": [ { "family_name": "Laurence", "given_name": "Stuart Jon", "orcid": "0000-0001-8760-8366", "clpid": "Laurence-Stuart-Jon" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VZJV-KJ48", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:1490", "collection": "thesis", "collection_id": "1490", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-04242006-172719", "primary_object_url": { "basename": "sjl_thesis.pdf", "content": "final", "filesize": 9105131, "license": "other", "mime_type": "application/pdf", "url": "/1490/1/sjl_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Proximal Bodies in Hypersonic Flow", "author": [ { "family_name": "Laurence", "given_name": "Stuart Jon", "orcid": "0000-0001-8760-8366", "clpid": "Laurence-Stuart-Jon" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VZJV-KJ48", "publication_date": "2006", "thesis_type": "phd", "thesis_year": "2006" }, { "id": "thesis:2326", "collection": "thesis", "collection_id": "2326", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05312005-111401", "primary_object_url": { "basename": "N_Ponchaut_Thesis.pdf", "content": "final", "filesize": 3581162, "license": "other", "mime_type": "application/pdf", "url": "/2326/1/N_Ponchaut_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Part I: 3DPTV: Advances and Error Analysis. Part II: Extension of Guderley's Solution for Converging Shock Waves", "author": [ { "family_name": "Ponchaut", "given_name": "Nicolas Frederic", "clpid": "Ponchaut-Nicolas-Frederic" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/09ZH-9M66", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2163", "collection": "thesis", "collection_id": "2163", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-165938", "primary_object_url": { "basename": "jf_cit_phdthesis.pdf", "content": "final", "filesize": 4884172, "license": "other", "mime_type": "application/pdf", "url": "/2163/1/jf_cit_phdthesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Coarse Analysis of Multiscale Systems: Diffuser Flows, Charged Particle Motion, and Connections to Averaging Theory", "author": [ { "family_name": "Fung", "given_name": "Jimmy", "orcid": "0000-0002-6612-2209", "clpid": "Fung-Jimmy" } ], "thesis_advisor": [ { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" } ], "thesis_committee": [ { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/wn0z-gn57", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2163", "collection": "thesis", "collection_id": "2163", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-165938", "primary_object_url": { "basename": "jf_cit_phdthesis.pdf", "content": "final", "filesize": 4884172, "license": "other", "mime_type": "application/pdf", "url": "/2163/1/jf_cit_phdthesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Coarse Analysis of Multiscale Systems: Diffuser Flows, Charged Particle Motion, and Connections to Averaging Theory", "author": [ { "family_name": "Fung", "given_name": "Jimmy", "orcid": "0000-0002-6612-2209", "clpid": "Fung-Jimmy" } ], "thesis_advisor": [ { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" } ], "thesis_committee": [ { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Ortiz", "given_name": "Michael", "orcid": "0000-0001-5877-4824", "clpid": "Ortiz-M" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/wn0z-gn57", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:533", "collection": "thesis", "collection_id": "533", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02072005-120514", "primary_object_url": { "basename": "SRH-1sided-print.pdf", "content": "final", "filesize": 4400606, "license": "other", "mime_type": "application/pdf", "url": "/533/1/SRH-1sided-print.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Wave Propagation in Granular Materials", "author": [ { "family_name": "Hostler", "given_name": "Stephen Richard", "orcid": "0000-0002-3160-1269", "clpid": "Hostler-Stephen-Richard" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Wave propagation is a fundamental property of all physical systems. The wave speed is directly related to the compressibility of the system and determines the rate at which local disturbances are propagated into the bulk of the material. The wave propagation characteristics of conventional forms of matter are well understood and well documented. In contrast, waves in granular materials are more complex due to the heterogeneous nature of these systems. The key element of the mechanics of a granular system is the force chain. It is along these preferentially stressed chains of particles that waves are transmitted. These nonlinear chains are heavily dependent on the geometry of the bed and are prone to rearrangement even by the slightest of forces.
\r\n\r\nResults from both experiments and simulations on wave propagation in granular materials are presented in the current study. The experiments measure the pressures at two points within the granular bed that result from the motion of a piston at one end of the bed. The simulations are a two-dimensional version of the experiments and use a discrete, soft-particle method to detect the wave at both the output of the simulated bed and at any point within it. In addition to examining wave propagation in a granular bed at rest, simulations and experiments are also performed for a granular bed undergoing agitation perpendicular to the direction of the wave input. Imposed agitation increases the granular temperature of the bed and allows for the exploration of the effect of granular state changes on the wave propagation characteristics. Such information may provide a means to diagnose the state of a flowing granular material.
\r\n\r\nMeasurements of the wave speed and attenuation in the bed reveal the unique properties of waves in granular systems that result from the nonlinearity of the bed and the heterogeneity of the force chains. Sinusoidal waves demonstrate the nondispersive nature of a granular bed and show the transient effects of force chain rearrangement. Pulsed waves display a semi-permanent shape qualitatively similar to predictions from nonlinear wave theory. In an agitated granular bed, measurements of the wave characteristics were found to be possible even in the presence of significant agitation. The prevailing confining pressure, which changes throughout the agitation cycle, was determined to be the system parameter that correlates best with changes to the wave speed.
", "doi": "10.7907/NQ1S-5E45", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:533", "collection": "thesis", "collection_id": "533", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02072005-120514", "primary_object_url": { "basename": "SRH-1sided-print.pdf", "content": "final", "filesize": 4400606, "license": "other", "mime_type": "application/pdf", "url": "/533/1/SRH-1sided-print.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Wave Propagation in Granular Materials", "author": [ { "family_name": "Hostler", "given_name": "Stephen Richard", "orcid": "0000-0002-3160-1269", "clpid": "Hostler-Stephen-Richard" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Wave propagation is a fundamental property of all physical systems. The wave speed is directly related to the compressibility of the system and determines the rate at which local disturbances are propagated into the bulk of the material. The wave propagation characteristics of conventional forms of matter are well understood and well documented. In contrast, waves in granular materials are more complex due to the heterogeneous nature of these systems. The key element of the mechanics of a granular system is the force chain. It is along these preferentially stressed chains of particles that waves are transmitted. These nonlinear chains are heavily dependent on the geometry of the bed and are prone to rearrangement even by the slightest of forces.
\r\n\r\nResults from both experiments and simulations on wave propagation in granular materials are presented in the current study. The experiments measure the pressures at two points within the granular bed that result from the motion of a piston at one end of the bed. The simulations are a two-dimensional version of the experiments and use a discrete, soft-particle method to detect the wave at both the output of the simulated bed and at any point within it. In addition to examining wave propagation in a granular bed at rest, simulations and experiments are also performed for a granular bed undergoing agitation perpendicular to the direction of the wave input. Imposed agitation increases the granular temperature of the bed and allows for the exploration of the effect of granular state changes on the wave propagation characteristics. Such information may provide a means to diagnose the state of a flowing granular material.
\r\n\r\nMeasurements of the wave speed and attenuation in the bed reveal the unique properties of waves in granular systems that result from the nonlinearity of the bed and the heterogeneity of the force chains. Sinusoidal waves demonstrate the nondispersive nature of a granular bed and show the transient effects of force chain rearrangement. Pulsed waves display a semi-permanent shape qualitatively similar to predictions from nonlinear wave theory. In an agitated granular bed, measurements of the wave characteristics were found to be possible even in the presence of significant agitation. The prevailing confining pressure, which changes throughout the agitation cycle, was determined to be the system parameter that correlates best with changes to the wave speed.
", "doi": "10.7907/NQ1S-5E45", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:424", "collection": "thesis", "collection_id": "424", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02012005-061553", "primary_object_url": { "basename": "chatelain_thesis_onesided_linkscolor.pdf", "content": "final", "filesize": 35192248, "license": "other", "mime_type": "application/pdf", "url": "/424/1/chatelain_thesis_onesided_linkscolor.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Contributions to the Three-Dimensional Vortex Element Method and Spinning Bluff Body Flows", "author": [ { "family_name": "Chatelain", "given_name": "Philippe", "orcid": "0000-0001-9891-5265", "clpid": "Chatelain-Philippe" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/YEDS-0V89", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2156", "collection": "thesis", "collection_id": "2156", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-145538", "primary_object_url": { "basename": "vwmain.pdf", "content": "final", "filesize": 3942640, "license": "other", "mime_type": "application/pdf", "url": "/2156/1/vwmain.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "On the Richtmyer-Meshkov Instability in Magnetohydrodynamics", "author": [ { "family_name": "Wheatley", "given_name": "Vincent", "orcid": "0000-0002-7287-7659", "clpid": "Wheatley-Vincent" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/N407-2B54", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2156", "collection": "thesis", "collection_id": "2156", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-145538", "primary_object_url": { "basename": "vwmain.pdf", "content": "final", "filesize": 3942640, "license": "other", "mime_type": "application/pdf", "url": "/2156/1/vwmain.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "On the Richtmyer-Meshkov Instability in Magnetohydrodynamics", "author": [ { "family_name": "Wheatley", "given_name": "Vincent", "orcid": "0000-0002-7287-7659", "clpid": "Wheatley-Vincent" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/N407-2B54", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2162", "collection": "thesis", "collection_id": "2162", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-163801", "primary_object_url": { "basename": "Faddy_PhD.pdf", "content": "final", "filesize": 14558318, "license": "other", "mime_type": "application/pdf", "url": "/2162/1/Faddy_PhD.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Flow Structure in a Model of Aircraft Trailing Vortices", "author": [ { "family_name": "Faddy", "given_name": "James Malcolm", "clpid": "Faddy-James-Malcolm" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/RC42-JY40", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2162", "collection": "thesis", "collection_id": "2162", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05272005-163801", "primary_object_url": { "basename": "Faddy_PhD.pdf", "content": "final", "filesize": 14558318, "license": "other", "mime_type": "application/pdf", "url": "/2162/1/Faddy_PhD.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Flow Structure in a Model of Aircraft Trailing Vortices", "author": [ { "family_name": "Faddy", "given_name": "James Malcolm", "clpid": "Faddy-James-Malcolm" } ], "thesis_advisor": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" } ], "thesis_committee": [ { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Schneider", "given_name": "Tapio", "orcid": "0000-0001-5687-2287", "clpid": "Schneider-T" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/RC42-JY40", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:2326", "collection": "thesis", "collection_id": "2326", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05312005-111401", "primary_object_url": { "basename": "N_Ponchaut_Thesis.pdf", "content": "final", "filesize": 3581162, "license": "other", "mime_type": "application/pdf", "url": "/2326/1/N_Ponchaut_Thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Part I: 3DPTV: Advances and Error Analysis. Part II: Extension of Guderley's Solution for Converging Shock Waves", "author": [ { "family_name": "Ponchaut", "given_name": "Nicolas Frederic", "clpid": "Ponchaut-Nicolas-Frederic" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/09ZH-9M66", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:424", "collection": "thesis", "collection_id": "424", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02012005-061553", "primary_object_url": { "basename": "chatelain_thesis_onesided_linkscolor.pdf", "content": "final", "filesize": 35192248, "license": "other", "mime_type": "application/pdf", "url": "/424/1/chatelain_thesis_onesided_linkscolor.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Contributions to the Three-Dimensional Vortex Element Method and Spinning Bluff Body Flows", "author": [ { "family_name": "Chatelain", "given_name": "Philippe", "orcid": "0000-0001-9891-5265", "clpid": "Chatelain-Philippe" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/YEDS-0V89", "publication_date": "2005", "thesis_type": "phd", "thesis_year": "2005" }, { "id": "thesis:5040", "collection": "thesis", "collection_id": "5040", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12182003-150738", "primary_object_url": { "basename": "twosided.pdf", "content": "final", "filesize": 5778819, "license": "other", "mime_type": "application/pdf", "url": "/5040/2/twosided.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Modeling Heat and Mass Transfer in Bubbly Cavitating Flows and Shock Waves in Cavitating Nozzles", "author": [ { "family_name": "Preston", "given_name": "Alastair Thomas", "clpid": "Preston-Alastair-Thomas" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Two problems are considered in this thesis: the modeling of heat and mass diffusion effects on the dynamics of spherical bubbles, and the computation of unsteady, bubbly cavitating flows in nozzles. The goal of Part I is to develop a reduced-order model that is able to accurately and efficiently capture the effect of heat and mass transfer on the dynamics of bubbles. Detailed computations of forced and oscillating bubbles including heat and mass diffusion show that the assumptions of polytropic behavior, constant vapor pressure, and an effective liquid viscosity do not accurately account for diffusive damping and thus do not accurately capture bubble dynamics. While the full bubble computations are readily performed for single bubbles, they are too expensive to implement into continuum models of complex bubbly flows where the radial diffusion equations would have to be solved at each grid point. Therefore reduced-order models that accurately capture diffusive effects are needed.
\r\n\r\nWe first develop a full bubble computation, where the full set of radial conservation equations are solved in the bubble interior and surrounding liquid. This provides insight as to which equations, or terms in equations, may be able to be neglected while still accurately capturing the bubble dynamics. Motivated by results of the full computations, we use constant heat and mass transfer coefficients to model the transfer at the bubble wall. In the resulting reduced-order model the heat and mass diffusion equations are each replaced by a single ordinary differential equation. The model is therefore efficient enough to implement into continuum computations. Comparisons of the reduced-order model to the full computations over a wide range of parameters indicate agreement that is superior to existing models.
\r\n\r\nIn Part II we investigate the effects of unsteady bubble dynamics on cavitating flow through a converging-diverging nozzle. A continuum model that couples the Rayleigh-Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. Flow regimes studied include those where steady state solutions exist, and those where steady state solutions diverge at the so-called flashing instability. These latter flows consist of unsteady bubbly shock waves traveling downstream in the diverging section of the nozzle. An approximate analytical expression is developed to predict the critical back pressure for choked flow. The results agree with previous barotropic models for those flows where bubble dynamics are not important, but show that in many instances the neglect of bubble dynamics cannot be justified. Finally the computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shock free flows, and good agreement with an experiment that measures the throat pressure and shock position for flows with bubbly shocks. In the model, damping of the bubble radial motion is restricted to a simple \"effective\" viscosity to account for diffusive effects. However, many features of the nozzle flow are shown to be independent of the specific damping mechanism. This is confirmed by the implementation of the more sophisticated diffusive modeling developed in Part I.
", "doi": "10.7907/ZHSN-D849", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2188", "collection": "thesis", "collection_id": "2188", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-130028", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 29774773, "license": "other", "mime_type": "application/pdf", "url": "/2188/1/thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Computation of Bubbly Cavitating Flow in Shock Wave Lithotripsy", "author": [ { "family_name": "Tanguay", "given_name": "Michel", "clpid": "Tanguay-Michel" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" }, { "family_name": "Crum", "given_name": "Lawrence A.", "clpid": "Crum-L-A" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Lithotripsy is at the forefront of treatment of kidney stones. By firing shock waves at the stone, it can be broken down into small fragments. Although the treatment is non-invasive, both short- and long-term side effects occur. In order to understand and rectify these shortcomings, lithotripsy has been the subject of ongoing research. Based on in vitro experiments, it has been ascertained that the cloud of cavitating bubble produced in the wake of the shock wave is a crucial element in the stone comminution process.
\r\n\r\nVarious solutions designed to maximize stone comminution and/or decrease tissue damage have been proposed over the years. However, the particulars of the comminution mechanism(s) are still undetermined. In this work, a numerical model of the two-phase flow inside an electrohydraulic lithotripter was used to provide additional insight in the behavior of the bubble cloud. The numerical model is based on an ensemble averaged two-phase flow model for a compressible liquid. The differential equations were discretized following the WENO shock capturing scheme in prolate spheroidal and cylindrical coordinate systems. The initial conditions for the flow field are estimated based on empirical observations and then validated by comparing the predicted pressure measurements and bubble cloud behavior against experimental values.
\r\n\r\nIn order to gain additional insight in the mechanism for stone comminution, a variety of relevant initial conditions were modeled. The following lithotripter configurations were analyzed: free-field, dual-pulse and single-pulse with an artificial stone at the focus. The impact of parameters such as the intensity of the initial shock wave and the pulse rate frequency (PRF) has been investigated. Based on an energy argument, conclusions regarding the efficiency of stone comminution are presented. In addition, based on these conclusions, avenues for improvement of the numerical model are highlighted.
", "doi": "10.7907/VQXV-Y948", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2185", "collection": "thesis", "collection_id": "2185", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-030854", "primary_object_url": { "basename": "barbaTHES.pdf", "content": "final", "filesize": 8809795, "license": "other", "mime_type": "application/pdf", "url": "/2185/1/barbaTHES.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Vortex Method for Computing High-Reynolds Number Flows: Increased Accuracy with a Fully Mesh-Less Formulation", "author": [ { "family_name": "Barba", "given_name": "Lorena A.", "orcid": "0000-0001-5812-2711", "clpid": "Barba-Lorena-A" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/TSR5-DE67", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2185", "collection": "thesis", "collection_id": "2185", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05282004-030854", "primary_object_url": { "basename": "barbaTHES.pdf", "content": "final", "filesize": 8809795, "license": "other", "mime_type": "application/pdf", "url": "/2185/1/barbaTHES.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Vortex Method for Computing High-Reynolds Number Flows: Increased Accuracy with a Fully Mesh-Less Formulation", "author": [ { "family_name": "Barba", "given_name": "Lorena A.", "orcid": "0000-0001-5812-2711", "clpid": "Barba-Lorena-A" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Beck", "given_name": "James L.", "clpid": "Beck-J-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/TSR5-DE67", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:2513", "collection": "thesis", "collection_id": "2513", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06082004-151101", "primary_object_url": { "basename": "thesis_HongyuRan.pdf", "content": "final", "filesize": 22723806, "license": "other", "mime_type": "application/pdf", "url": "/2513/1/thesis_HongyuRan.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Numerical Study of the Dynamics and Sound Generation of a Turbulent Vortex Ring", "author": [ { "family_name": "Ran", "given_name": "Hongyu", "clpid": "Ran-Hongyu" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In the present study, Direct Numerical Simulations (DNS) of the fully compressible, three-dimensional Navier-Stokes equations are used to generate an axisymmetric vortex ring to which three-dimensional stochastic disturbances are added. The radiated acoustic field is computed directly in the near field, and by solving the wave equation in a spherical coordinate system in the far field.
\r\n\r\nAt high Reynolds number, a vortex ring will undergo an instability to azimuthal waves. The instability produces higher azimuthal modes and induces nonlinear interaction between the modes, and will cause the vortex ring to break down and transition to turbulence. The early stages of the simulation agree well with the linear instability theory. Nonlinear stage of instability, transition, formation of axial flow and streamwise vorticity are analyzed and compared with experimental results. After turbulent transition, the evolution of statistical quantities becomes independent of viscosity and the initial geometry, and the flow become self-similar. The temporal evolution of quantities including total circulation, axial velocity profile, vortex ring displacement and vorticity profile agrees well with the self-similarity law. Turbulent energy spectrum, Reynolds stresses and turbulence production are also presented.
\r\n\r\nThe unsteady vorticity field generates acoustic waves with higher azimuthal modes, each mode with a distinctive spectrum and directivity. The ensemble averaged peak frequency, bandwidth, and the sound pressure level agrees qualitatively with reported experimental results. The directivity of each azimuthal mode is compared with predictions of vortex sound theory. The sound generation consists of three stages. The first is a deterministic stage when linear instability waves emerge and grow and generate relatively weak sound. The second stage is nonlinear interaction and vortex breakdown; at this stage the sound pressure level reaches a peak value. The third stage is the turbulent asymptotic decay of the acoustic field. Based on the self-similar decay of the turbulent near field, the self-similar decay of the sound field is investigated. Connection between the acoustic field and the vortex ring oscillations is also studied with vortex sound theory. Finally, we note some similarities between the sound radiated by a train of de-correlated vortex rings and turbulent jet noise. The sound pressure level, spectrum, and directivity of the train of vortex rings is similar to the sound field from a jet with similar Reynolds number and Mach number.
", "doi": "10.7907/ASHK-JV07", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" }, { "id": "thesis:610", "collection": "thesis", "collection_id": "610", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02122003-152525", "primary_object_url": { "basename": "arienti_PhD_thesis.pdf", "content": "final", "filesize": 47552719, "license": "other", "mime_type": "application/pdf", "url": "/610/1/arienti_PhD_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Numerical and Analytical Study of Detonation Diffraction", "author": [ { "family_name": "Arienti", "given_name": "Marco", "orcid": "0000-0001-8166-0016", "clpid": "Arienti-Marco" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/MAGN-R628", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:859", "collection": "thesis", "collection_id": "859", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03042003-102221", "primary_object_url": { "basename": "matveev_thesis.pdf", "content": "final", "filesize": 2117824, "license": "other", "mime_type": "application/pdf", "url": "/859/1/matveev_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Thermoacoustic Instabilities in the Rijke Tube: Experiments and Modeling", "author": [ { "family_name": "Matveev", "given_name": "Konstantin Ivanovich", "clpid": "Matveev-Konstantin-Ivanovich" } ], "thesis_advisor": [ { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" } ], "thesis_committee": [ { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" }, { "family_name": "Pickar", "given_name": "Kenneth A.", "clpid": "Pickar-K-A" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Thermoacoustic instability can appear in thermal devices when unsteady heat release is coupled with pressure perturbations. This effect results in excitation of eigen acoustic modes of the system. These instabilities are important in various technical applications, for instance, in rocket motors and thermoacoustic engines.
\r\n\r\nA Rijke tube, representing a resonator with a mean flow and a concentrated heat source, is a convenient system for studying the fundamental physics of thermoacoustic instabilities. At certain values of the main system parameters, a loud sound is generated through a process similar to that in real-world devices prone to thermoacoustic instability. Rijke devices have been extensively employed for research purposes. The current work is intended to overcome the serious deficiencies of previous investigations with regard to estimating experimental errors and the influence of parameter variation on the results. Also, part of the objective here is to account for temperature field non-uniformity and to interpret nonlinear phenomena. The major goals of this study are to deliver accurate experimental results for the transition to instability and the scope and nature of the excited regimes, and to develop a theory that explains and predicts the effects observed.
\r\n\r\nAn electrically heated, horizontally oriented, Rijke tube is used for the experimental study of transition to instability. The stability boundary is quantified as a function of major system parameters with measured uncertainties for the data collected. Hysteresis in the stability boundary is observed for certain operating regimes of the Rijke tube.
\r\n\r\nAn innovative theory is developed for modeling the Rijke oscillations. First, linear theory, incorporating thermal analysis that accurately determines the properties of the modes responsible for the transition to instability, is used to predict the stability boundary. Then, a nonlinear extension of the theory is derived by introducing a hypothesis for a special form of the nonlinear heat transfer function. This nonlinear modeling is shown to predict the hysteresis phenomenon and the limit cycles observed during the tests.
\r\n\r\nA new, reduced-order modeling approach for combustion instabilities in systems with vortex shedding is derived using the developed analytical framework. A hypothesis for the vortex detachment criterion is introduced, and a kicked oscillator model is applied to produce nonlinear results characteristic for unstable combustion systems.
\r\n\r\nThe experimental system and the mathematical model, developed in this work for the Rijke tube, are recommended for preliminary design and analysis of real-world thermal devices, where thermoacoustic instability is a concern.
", "doi": "10.7907/3X6A-6D11", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:859", "collection": "thesis", "collection_id": "859", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03042003-102221", "primary_object_url": { "basename": "matveev_thesis.pdf", "content": "final", "filesize": 2117824, "license": "other", "mime_type": "application/pdf", "url": "/859/1/matveev_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Thermoacoustic Instabilities in the Rijke Tube: Experiments and Modeling", "author": [ { "family_name": "Matveev", "given_name": "Konstantin Ivanovich", "clpid": "Matveev-Konstantin-Ivanovich" } ], "thesis_advisor": [ { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" } ], "thesis_committee": [ { "family_name": "Culick", "given_name": "Fred E. C.", "clpid": "Culick-F-E-C" }, { "family_name": "Pickar", "given_name": "Kenneth A.", "clpid": "Pickar-K-A" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Thermoacoustic instability can appear in thermal devices when unsteady heat release is coupled with pressure perturbations. This effect results in excitation of eigen acoustic modes of the system. These instabilities are important in various technical applications, for instance, in rocket motors and thermoacoustic engines.
\r\n\r\nA Rijke tube, representing a resonator with a mean flow and a concentrated heat source, is a convenient system for studying the fundamental physics of thermoacoustic instabilities. At certain values of the main system parameters, a loud sound is generated through a process similar to that in real-world devices prone to thermoacoustic instability. Rijke devices have been extensively employed for research purposes. The current work is intended to overcome the serious deficiencies of previous investigations with regard to estimating experimental errors and the influence of parameter variation on the results. Also, part of the objective here is to account for temperature field non-uniformity and to interpret nonlinear phenomena. The major goals of this study are to deliver accurate experimental results for the transition to instability and the scope and nature of the excited regimes, and to develop a theory that explains and predicts the effects observed.
\r\n\r\nAn electrically heated, horizontally oriented, Rijke tube is used for the experimental study of transition to instability. The stability boundary is quantified as a function of major system parameters with measured uncertainties for the data collected. Hysteresis in the stability boundary is observed for certain operating regimes of the Rijke tube.
\r\n\r\nAn innovative theory is developed for modeling the Rijke oscillations. First, linear theory, incorporating thermal analysis that accurately determines the properties of the modes responsible for the transition to instability, is used to predict the stability boundary. Then, a nonlinear extension of the theory is derived by introducing a hypothesis for a special form of the nonlinear heat transfer function. This nonlinear modeling is shown to predict the hysteresis phenomenon and the limit cycles observed during the tests.
\r\n\r\nA new, reduced-order modeling approach for combustion instabilities in systems with vortex shedding is derived using the developed analytical framework. A hypothesis for the vortex detachment criterion is introduced, and a kicked oscillator model is applied to produce nonlinear results characteristic for unstable combustion systems.
\r\n\r\nThe experimental system and the mathematical model, developed in this work for the Rijke tube, are recommended for preliminary design and analysis of real-world thermal devices, where thermoacoustic instability is a concern.
", "doi": "10.7907/3X6A-6D11", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:610", "collection": "thesis", "collection_id": "610", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-02122003-152525", "primary_object_url": { "basename": "arienti_PhD_thesis.pdf", "content": "final", "filesize": 47552719, "license": "other", "mime_type": "application/pdf", "url": "/610/1/arienti_PhD_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Numerical and Analytical Study of Detonation Diffraction", "author": [ { "family_name": "Arienti", "given_name": "Marco", "orcid": "0000-0001-8166-0016", "clpid": "Arienti-Marco" } ], "thesis_advisor": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Cohen", "given_name": "Donald S.", "clpid": "Cohen-D-S" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "Explosion Dynamics Laboratory" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/MAGN-R628", "publication_date": "2003", "thesis_type": "phd", "thesis_year": "2003" }, { "id": "thesis:4742", "collection": "thesis", "collection_id": "4742", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12032004-075012", "primary_object_url": { "basename": "cwr_thesis.pdf", "content": "final", "filesize": 10860896, "license": "other", "mime_type": "application/pdf", "url": "/4742/1/cwr_thesis.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Modeling, Simulation, and Control of Cavity Flow Oscillations", "author": [ { "family_name": "Rowley", "given_name": "Clarence Worth, III", "clpid": "Rowley-Clarence-Worth-III" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Marsden", "given_name": "Jerrold E.", "clpid": "Marsden-J-E" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This thesis involves the modeling of self-sustained oscillations in the flow past a rectangular cavity. The emphasis is on developing low-dimensional models that are suitable for analysis using tools from dynamical systems and control theory. Two-dimensional direct numerical simulations are performed, and indicate the presence of a \u201cwake mode,\u201d which has been observed previously in experiments, but which is much less well understood than the \u201cshear-layer mode\u201d usually observed. We characterize the flow in both shear-layer mode and wake mode, and provide a criterion for predicting the onset of wake mode, as a function of the various geometrical and flow-related parameters. We focus on the modeling of shear-layer mode, and employ two distinct modeling approaches: first, we use the method of Proper Orthogonal Decomposition (POD) and Galerkin projection to reduce the Navier-Stokes equations to a lowdimensional system of ordinary differential equations (ODEs). We extend the method to compressible flows, using approximations that are valid for cold flows at moderate Mach number. In a compressible flow, both the kinematic and thermodynamic variables contribute to the total energy, and an inner product is introduced which respects this, and allows one to use vector-valued POD modes for the Galerkin projection. We obtain models in the form of ODEs with between 2 and 60 states, and compare models based on scalar-valued and vector-valued POD modes. All of the models work well for short times (a few periods of oscillation), but the models based on scalar-valued modes deviate for longer times, while in general the models based on vector-valued modes retain qualitatively correct dynamical behavior. In the second modeling approach, we model the underlying physical mechanisms separately (shear-layer amplification, acoustic scattering, acoustic propagation), and obtain linear models that are suitable for control design and analysis. We design a controller which stabilizes the model, and implement a similar control law on an experiment, demonstrating significant reduction in the amplitude of the oscillations, but revealing some limitations of feedback control.", "doi": "10.7907/G4ZX-KH73", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:5147", "collection": "thesis", "collection_id": "5147", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12282004-113953", "primary_object_url": { "basename": "jeffthesis.pdf", "content": "final", "filesize": 3360828, "license": "other", "mime_type": "application/pdf", "url": "/5147/1/jeffthesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Dilating Vortex Particle Method for Compressible Flow with Applications to Aeroacoustics", "author": [ { "family_name": "Eldredge", "given_name": "Jeffrey D.", "orcid": "0000-0002-2672-706X", "clpid": "Eldredge-Jeffrey-D" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Vortex methods have become useful tools for the computation of incompressible fluid flow. In the present work, a vortex particle method for the simulation of unsteady two-dimensional compressible flow is developed and applied to several problems. The method is the first Langrangian simulation method for the full compressible Navier-Stokes equations. By decomposing the velocity into irrotational and solenoidal parts, and using particles that are able to change volume and that carry vorticity, dilation, enthalpy, entropy, and density, the equations of motion are satisfied. A general deterministic treatment of spatial derivatives in particle methods is developed by extending the method of particle strength exchange through the construction of higher-order-accurate, non-dissipative kernels for use in approximating arbitrary differential operators. The application of this technique to wave propagation problems is thoroughly explored. A one-sided operator is developed for approximating derivatives near the periphery of particle coverage; the operator is used to enforce a non-reflecting boundary condition for the absorption of acoustic waves at this periphery. Remeshing of the particles and the smooth interpolation of their strengths are addressed, and a criterion for the frequency of remeshing is developed on the principle axes of the rate-of-strain tensor. The fast multipole method for the fast summation of the velocity field is adapted for use with compressible particles. The new vortex method is applied to co-rotating and leapfrogging vortices in compressible flow, with the acoustic field computed using a two-dimensional Kirchoff surface, and the results agree will with those of previous work or analytical prediction. The method is also applied to the baroclinic generation of vorticity, and to the steepening of waves in the one-dimensional Burgers\u2019 equation, with favorable results in both cases.
", "doi": "10.7907/7EYY-0S65", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:5147", "collection": "thesis", "collection_id": "5147", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-12282004-113953", "primary_object_url": { "basename": "jeffthesis.pdf", "content": "final", "filesize": 3360828, "license": "other", "mime_type": "application/pdf", "url": "/5147/1/jeffthesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Dilating Vortex Particle Method for Compressible Flow with Applications to Aeroacoustics", "author": [ { "family_name": "Eldredge", "given_name": "Jeffrey D.", "orcid": "0000-0002-2672-706X", "clpid": "Eldredge-Jeffrey-D" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Vortex methods have become useful tools for the computation of incompressible fluid flow. In the present work, a vortex particle method for the simulation of unsteady two-dimensional compressible flow is developed and applied to several problems. The method is the first Langrangian simulation method for the full compressible Navier-Stokes equations. By decomposing the velocity into irrotational and solenoidal parts, and using particles that are able to change volume and that carry vorticity, dilation, enthalpy, entropy, and density, the equations of motion are satisfied. A general deterministic treatment of spatial derivatives in particle methods is developed by extending the method of particle strength exchange through the construction of higher-order-accurate, non-dissipative kernels for use in approximating arbitrary differential operators. The application of this technique to wave propagation problems is thoroughly explored. A one-sided operator is developed for approximating derivatives near the periphery of particle coverage; the operator is used to enforce a non-reflecting boundary condition for the absorption of acoustic waves at this periphery. Remeshing of the particles and the smooth interpolation of their strengths are addressed, and a criterion for the frequency of remeshing is developed on the principle axes of the rate-of-strain tensor. The fast multipole method for the fast summation of the velocity field is adapted for use with compressible particles. The new vortex method is applied to co-rotating and leapfrogging vortices in compressible flow, with the acoustic field computed using a two-dimensional Kirchoff surface, and the results agree will with those of previous work or analytical prediction. The method is also applied to the baroclinic generation of vorticity, and to the steepening of waves in the one-dimensional Burgers\u2019 equation, with favorable results in both cases.
", "doi": "10.7907/7EYY-0S65", "publication_date": "2002", "thesis_type": "phd", "thesis_year": "2002" }, { "id": "thesis:7469", "collection": "thesis", "collection_id": "7469", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02072013-122723580", "primary_object_url": { "basename": "Zuhal_lr_2001.pdf", "content": "final", "filesize": 30962342, "license": "other", "mime_type": "application/pdf", "url": "/7469/1/Zuhal_lr_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Formation and Near-Field Dynamics of a Wing Tip Vortex", "author": [ { "family_name": "Zuhal", "given_name": "Lavi Rizki", "clpid": "Zuhal-Lavi-Rizki" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VNJW-6592", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:1143", "collection": "thesis", "collection_id": "1143", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-112838", "primary_object_url": { "basename": "Duttweiler_me_2001.pdf", "content": "final", "filesize": 20766786, "license": "other", "mime_type": "application/pdf", "url": "/1143/1/Duttweiler_me_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Surge Instability on a Cavitating Propeller", "author": [ { "family_name": "Duttweiler", "given_name": "Mark Edward", "clpid": "Duttweiler-Mark-Edward" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The present study details results from experiments investigating a surge instability on a cavitating propeller. Initially, the stable behavior of the propeller is explored, and the nature and extent of the cavitation is documented at various experimental conditions, including propeller yaw. The cavitation surge instability is first explored through visual observation of the cavitation on the propeller blades and in the tip vortices. Particular note is made of similarities between the behavior of the re-entrant jets and that noted by other investigators. It is also observed that the nature of the instability is closely related to the partial cavity instability observed on single, two-dimensional hydrofoils.
\r\n\r\nThe flow conditions that lead to instability are determined and it is shown that onset corresponds to a specific configuration of attached cavity lengths on an individual propeller blade. Pressure measurements are obtained from transducers within the experimental facility, and the acoustic signature of the instability is identified. The magnitude of the fluctuating pressures is very large, presumably capable of producing severe hull vibration. A simple model is developed based on cavity volume estimates obtained from high speed video footage, and the predictions of the model are compared with the experimentally obtained pressures.
\r\n\r\nTo assess the significance of the surrounding facility in initiating and sustaining the instability, a model is developed for the experimental facility dynamics. The predictions of this model are then compared with an experimentally determined facility response to a volumetric excitation imposed by an oscillating piston. To quantify the response of the cavitation to fluctuations in test section conditions, quasistatic estimates are obtained for the cavitation compliance and mass flow gain factor of the propeller. These parameters have previously been employed in developing system transfer functions for cavitating pumps.
\r\n\r\nFinally, a model is developed for the complete system, incorporating both the cavitation and facility dynamics. The model predicts active system dynamics and therefore potentially unstable behavior for two distinct frequency ranges, and one such range is hypothesized to correspond to the observed instability. The ability of the model to predict the observed characteristics of the instability is then evaluated.
", "doi": "10.7907/07RB-NX02", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6192", "collection": "thesis", "collection_id": "6192", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12022010-081219787", "type": "thesis", "title": "Rotordynamic Forces Generated by Annular Leakage Flows in Centrifugal Pumps", "author": [ { "family_name": "Hsu", "given_name": "Yun", "clpid": "Hsu-Yun" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fluid-induced rotordynamic forces in pumping machinery are well documented but poorly understood. The present research focuses on the rotordynamics due to fluid flow in annuli, in particular, the discharge-to-suction leakage flow in centrifugal pumps. There are indications that the contribution of the front shroud leakage flow can be of the same order of magnitude as contributions from the nonuniform pressure acting on the impeller discharge. Previous investigations have established some of\r\nthe basic traits of these flows. This work furthers the experimental and computational approach to quantify and predict the shroud contribution to the rotordynamic\r\nstability of pumping machinery.\r\nChilds' bulk flow model for leakage paths is carefully examined, and convective relations for vorticity and total pressure are deduced. This analysis leads to a new\r\nsolution procedure for the bulk flow equations which does not resort to linearization or assumed harmonic forms of the flow variables.\r\nExperimental results presented show the contributions of the inlet swirl velocities\r\nto the rotordynamic forces. Antiswirl devices are evaluated for their effectiveness in reducing instability. Additional tests measuring the pressure distributions and the inlet swirl velocities of the leakage flow confirm some of the predictions by numerical analysis.", "doi": "10.7907/fr51-ft92", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:7469", "collection": "thesis", "collection_id": "7469", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:02072013-122723580", "primary_object_url": { "basename": "Zuhal_lr_2001.pdf", "content": "final", "filesize": 30962342, "license": "other", "mime_type": "application/pdf", "url": "/7469/1/Zuhal_lr_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Formation and Near-Field Dynamics of a Wing Tip Vortex", "author": [ { "family_name": "Zuhal", "given_name": "Lavi Rizki", "clpid": "Zuhal-Lavi-Rizki" } ], "thesis_advisor": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.
", "doi": "10.7907/VNJW-6592", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:966", "collection": "thesis", "collection_id": "966", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03162005-130412", "primary_object_url": { "basename": "Lokhandwalla_m_2001.pdf", "content": "final", "filesize": 5622602, "license": "other", "mime_type": "application/pdf", "url": "/966/1/Lokhandwalla_m_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Damage Mechanisms in Shock Wave Lithotripsy (SWL)", "author": [ { "family_name": "Lokhandwalla", "given_name": "Murtuza", "clpid": "Lokhandwalla-Murtuza" } ], "thesis_advisor": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" } ], "thesis_committee": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/VZWS-7Z85", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:966", "collection": "thesis", "collection_id": "966", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03162005-130412", "primary_object_url": { "basename": "Lokhandwalla_m_2001.pdf", "content": "final", "filesize": 5622602, "license": "other", "mime_type": "application/pdf", "url": "/966/1/Lokhandwalla_m_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Damage Mechanisms in Shock Wave Lithotripsy (SWL)", "author": [ { "family_name": "Lokhandwalla", "given_name": "Murtuza", "clpid": "Lokhandwalla-Murtuza" } ], "thesis_advisor": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" } ], "thesis_committee": [ { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Ravichandran", "given_name": "Guruswami", "orcid": "0000-0002-2912-0001", "clpid": "Ravichandran-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/VZWS-7Z85", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:3168", "collection": "thesis", "collection_id": "3168", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08192001-143746", "primary_object_url": { "basename": "thesis1side.pdf", "content": "final", "filesize": 2807272, "license": "other", "mime_type": "application/pdf", "url": "/3168/1/thesis1side.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Passive Hypervelocity Boundary Layer Control Using an Ultrasonically Absorptive Surface", "author": [ { "family_name": "Rasheed", "given_name": "Adam", "clpid": "Rasheed-Adam" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/EFZZ-X764", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:3168", "collection": "thesis", "collection_id": "3168", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-08192001-143746", "primary_object_url": { "basename": "thesis1side.pdf", "content": "final", "filesize": 2807272, "license": "other", "mime_type": "application/pdf", "url": "/3168/1/thesis1side.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Passive Hypervelocity Boundary Layer Control Using an Ultrasonically Absorptive Surface", "author": [ { "family_name": "Rasheed", "given_name": "Adam", "clpid": "Rasheed-Adam" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/EFZZ-X764", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6192", "collection": "thesis", "collection_id": "6192", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:12022010-081219787", "type": "thesis", "title": "Rotordynamic Forces Generated by Annular Leakage Flows in Centrifugal Pumps", "author": [ { "family_name": "Hsu", "given_name": "Yun", "clpid": "Hsu-Yun" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fluid-induced rotordynamic forces in pumping machinery are well documented but poorly understood. The present research focuses on the rotordynamics due to fluid flow in annuli, in particular, the discharge-to-suction leakage flow in centrifugal pumps. There are indications that the contribution of the front shroud leakage flow can be of the same order of magnitude as contributions from the nonuniform pressure acting on the impeller discharge. Previous investigations have established some of\r\nthe basic traits of these flows. This work furthers the experimental and computational approach to quantify and predict the shroud contribution to the rotordynamic\r\nstability of pumping machinery.\r\nChilds' bulk flow model for leakage paths is carefully examined, and convective relations for vorticity and total pressure are deduced. This analysis leads to a new\r\nsolution procedure for the bulk flow equations which does not resort to linearization or assumed harmonic forms of the flow variables.\r\nExperimental results presented show the contributions of the inlet swirl velocities\r\nto the rotordynamic forces. Antiswirl devices are evaluated for their effectiveness in reducing instability. Additional tests measuring the pressure distributions and the inlet swirl velocities of the leakage flow confirm some of the predictions by numerical analysis.", "doi": "10.7907/fr51-ft92", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:1143", "collection": "thesis", "collection_id": "1143", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-112838", "primary_object_url": { "basename": "Duttweiler_me_2001.pdf", "content": "final", "filesize": 20766786, "license": "other", "mime_type": "application/pdf", "url": "/1143/1/Duttweiler_me_2001.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Surge Instability on a Cavitating Propeller", "author": [ { "family_name": "Duttweiler", "given_name": "Mark Edward", "clpid": "Duttweiler-Mark-Edward" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Murray", "given_name": "Richard M.", "orcid": "0000-0002-5785-7481", "clpid": "Murray-R-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "The present study details results from experiments investigating a surge instability on a cavitating propeller. Initially, the stable behavior of the propeller is explored, and the nature and extent of the cavitation is documented at various experimental conditions, including propeller yaw. The cavitation surge instability is first explored through visual observation of the cavitation on the propeller blades and in the tip vortices. Particular note is made of similarities between the behavior of the re-entrant jets and that noted by other investigators. It is also observed that the nature of the instability is closely related to the partial cavity instability observed on single, two-dimensional hydrofoils.
\r\n\r\nThe flow conditions that lead to instability are determined and it is shown that onset corresponds to a specific configuration of attached cavity lengths on an individual propeller blade. Pressure measurements are obtained from transducers within the experimental facility, and the acoustic signature of the instability is identified. The magnitude of the fluctuating pressures is very large, presumably capable of producing severe hull vibration. A simple model is developed based on cavity volume estimates obtained from high speed video footage, and the predictions of the model are compared with the experimentally obtained pressures.
\r\n\r\nTo assess the significance of the surrounding facility in initiating and sustaining the instability, a model is developed for the experimental facility dynamics. The predictions of this model are then compared with an experimentally determined facility response to a volumetric excitation imposed by an oscillating piston. To quantify the response of the cavitation to fluctuations in test section conditions, quasistatic estimates are obtained for the cavitation compliance and mass flow gain factor of the propeller. These parameters have previously been employed in developing system transfer functions for cavitating pumps.
\r\n\r\nFinally, a model is developed for the complete system, incorporating both the cavitation and facility dynamics. The model predicts active system dynamics and therefore potentially unstable behavior for two distinct frequency ranges, and one such range is hypothesized to correspond to the observed instability. The ability of the model to predict the observed characteristics of the instability is then evaluated.
", "doi": "10.7907/07RB-NX02", "publication_date": "2001", "thesis_type": "phd", "thesis_year": "2001" }, { "id": "thesis:6121", "collection": "thesis", "collection_id": "6121", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10082010-114955709", "primary_object_url": { "basename": "Mohseni_k_2000.pdf", "content": "final", "filesize": 5587886, "license": "other", "mime_type": "application/pdf", "url": "/6121/1/Mohseni_k_2000.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "A: Universality in Vortex Formation. B: Evaluation of Mach Wave Radiation Mechanisms in a Supersonic Jet", "author": [ { "family_name": "Mohseni", "given_name": "Kamran", "clpid": "Mohseni-Kamran" } ], "thesis_advisor": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" } ], "thesis_committee": [ { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "In this thesis two distinct features of coherent structures are investigated. In Part I a model for the pinch-off process in vortex ring formation is developed. The predicted nondimensional stroke length L/D (referred to as \"formation number\") satisfactorily matches experimental observations. The model introduces two nondimensional parameters that govern the limiting value of the formation number: a nondimensional energy and circulation, E_(nd) and \u0393_(nd), respectively. The predicted value of E_(nd) also matches well with the experimental data. The limiting value for the new nondimensional circulation is predicted to be in the range 1.77 \u2272 \u0393_(nd) \u2272 2.07. We perform detailed computations of vortex ring formation by nonconservative forcing. The validity of the assumptions in our model is verified in these computations. Some techniques for generating fat rings are successfully investigated, resulting in generation of vortex rings with Hill's like properties. We consider thermodynamics of the vorticity density field (w/r), and we develop a statistical equilibrium theory for axisymmetric flows. It is shown that the statistical equilibrium of an axisymmetric flow is the state that maximizes an entropy functional constrained to the invariants of motion. Furthermore, it is shown that the final equilibrium state satisfies a variational principle similar to Kelvin's variational principle. In Part II Mach wave radiation mechanisms in a fully expanded supersonic jet is studied. We compare a direct numerical simulation (DNS) of a 1.92 Mach number jet with a linearized Navier-Stokes (LNS) simulation. The numerical integration technique, inflow boundary conditions, and grid distributions are the same in both simulations. We found that the generated noise in the DNS calculation is dominated by the first two azimuthal modes, and contributions from all other azimuthal modes were limited to less than 1.5 dB in the acoustic field. The total directivity of the sound field in the LNS matches reasonably well with the sound field of the DNS data. At the peak Strouhal frequency, particularly for the azimuthal mode n = 1, the amplification of flow variables in the LNS closely matches that of the DNS data. However, for frequencies away from the peak Strouhal number the DNS data shows amplification rates comparable to those of the peak Strouhal number, while in the LNS data any disturbances away from the peak Strouhal number are highly damped. These extra noise sources in the DNS data have the characteristics of a nonlinear interaction among various modes. The noise generated by the first two modes in the linearized computation is substantially weaker than in the DNS. For example, in the near acoustic field, at a distance of 6 jet diameters from the jet centerline, the sound pressure level in the linearized computation is as much as 8 db smaller than the DNS results. We observed that the maximum amplification rate for the DNS data occurs at an axial location further downstream than for the LNS data, which corresponds to regions around and beyond the end of the potential core. Our results indicate that the missing sound generation mechanisms in the LNS computation at the frequencies considered in this study can be attributed to the non-linear sound generation mechanisms, that are not captured in linear theories.", "doi": "10.7907/MB7E-7950", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:6110", "collection": "thesis", "collection_id": "6110", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10062010-112425905", "primary_object_url": { "basename": "Li_y_2000.pdf", "content": "final", "filesize": 7420970, "license": "other", "mime_type": "application/pdf", "url": "/6110/1/Li_y_2000.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Tsunamis: Non-Breaking and Breaking Solitary Wave Run-Up", "author": [ { "family_name": "Li", "given_name": "Ying", "clpid": "Li-Ying" } ], "thesis_advisor": [ { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" } ], "thesis_committee": [ { "family_name": "Wu", "given_name": "Theodore Yao-tsu", "clpid": "Wu-T-Y-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This study considers the run-up of non-breaking and breaking solitary waves on a smooth sloping beach. A non-linear theory and a numerical model solving the non-linear shallow water equations (NLSW) were developed to model this physical process. Various experiments to obtain wave amplitude time-histories, water particle velocities, wave free-surface profiles, and maximum run-up were conducted and the results were compared with the analytical and numerical models.\r\nA higher order theoretical solution to the non-linear shallow water equations, which describes the non-breaking wave characteristics on the beach, was sought and presented in this study. The solution was obtained analytically by using the Carrier and Greenspan (1958) hodograph transformation. It was found that the non-linear theory agreed well with experimental results. The maximum run-up predicted by the non-linear theory is larger than that predicted by Synolakis (1986) at the order of the offshore relative wave height for a given slope. This correction for non-breaking waves on beach decreases as the beach slope steepens, and increases as the relative incident solitary wave height increases.\r\nA unique run-up gage that consists of a laser and a photodiode camera was developed in connection with this study to measure the time-history of the tip of the run-up tongue of a non-breaking solitary wave as it progresses up the slope. The results obtained with this run-up gage agree well with other measurements and provides a simple and reliable way of measuring run-up time histories.\r\nThe run-up of breaking solitary waves was studied experimentally and numerically since no fully theoretical approach is possible. The wave characteristics such as wave shape and shoaling characteristics, and, for plunging breakers, the shape of the jet produced are presented. The experimental results show that wave breaking is such a complicated process that even sophisticated numerical models cannot adequately model its details.\r\nTwo different plunging wave breaking and resultant run-up were found from the experiments. The point, where the tip of the incident jet produced by the plunging breaking wave impinges determines the characteristics of the resulting splash-up. If the jet impinges on a dry slope, no splash-up occurs and the plunging breaker simply collapses. If the impingement point is located on the free-surface, splash-up including a reflected jet is formed, which further increases the turbulence and energy dissipation associated with wave breaking. It is hypothesized that both clockwise and counter clockwise vortices may be generated by the impinging plunging jet and the reflected jet associated with the splash-up when the jet impinges on the front face of a breaking wave or on the still water surface in front of the wave.\r\nIf only the run-up process and maximum run-up are of interest, the wave and the water flow produced after breaking can be simplified as a propagating bore, which is analogous to a shock wave in gas dynamics. A numerical model using this bore structure to treat the process of wave breaking and propagation was developed. The non-linear shallow water equations were solved using the weighted essentially non-oscillatory (WENO) shock capturing scheme employed in gas dynamics. Wave breaking and propagation is handled automatically 1w this scheme and no ad-hoc term is required. A computational domain mapping technique proposed by Zhang (1996) is used in the numerical scheme to model the shoreline movement. This numerical scheme is found to provide a somewhat simple and reasonably good prediction of various aspects of the run-up process. The numerical results agree well with the experiments corresponding to the run-up on a. relatively steep slope (1:2.08) as well as on a more gentle slope (1:19.85).\r\nA simple empirical estimate of maximum run-up based on energy conservation considerations is also presented where the energy dissipation associated with wave breaking was estimated using the results from the numerical model. This approach appears to be useful and the maximum run-up predicted agrees reasonably well with the experimental results.\r\nThe splash-up of a solitary wave on a vertical wall positioned at different locations on a gentle slope was also investigated in this study to understand the degree of protection from tsunamis afforded by seawalls. It was found that the effect of breaking wave kinematics offshore of the vertical wall on the splash-up is of critical importance to the maximum splash-up. The maximum slope of the front face of the wave upon impingement of the wave on the wall, which represents the maximum water particle acceleration, was important in defining the maximum sheet splash-up as well as the trend for splash-up composed of drops and spray.\r\n", "doi": "10.7907/kake-7p89", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:6110", "collection": "thesis", "collection_id": "6110", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10062010-112425905", "primary_object_url": { "basename": "Li_y_2000.pdf", "content": "final", "filesize": 7420970, "license": "other", "mime_type": "application/pdf", "url": "/6110/1/Li_y_2000.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Tsunamis: Non-Breaking and Breaking Solitary Wave Run-Up", "author": [ { "family_name": "Li", "given_name": "Ying", "clpid": "Li-Ying" } ], "thesis_advisor": [ { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" } ], "thesis_committee": [ { "family_name": "Wu", "given_name": "Theodore Yao-tsu", "clpid": "Wu-T-Y-T" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "This study considers the run-up of non-breaking and breaking solitary waves on a smooth sloping beach. A non-linear theory and a numerical model solving the non-linear shallow water equations (NLSW) were developed to model this physical process. Various experiments to obtain wave amplitude time-histories, water particle velocities, wave free-surface profiles, and maximum run-up were conducted and the results were compared with the analytical and numerical models.\r\nA higher order theoretical solution to the non-linear shallow water equations, which describes the non-breaking wave characteristics on the beach, was sought and presented in this study. The solution was obtained analytically by using the Carrier and Greenspan (1958) hodograph transformation. It was found that the non-linear theory agreed well with experimental results. The maximum run-up predicted by the non-linear theory is larger than that predicted by Synolakis (1986) at the order of the offshore relative wave height for a given slope. This correction for non-breaking waves on beach decreases as the beach slope steepens, and increases as the relative incident solitary wave height increases.\r\nA unique run-up gage that consists of a laser and a photodiode camera was developed in connection with this study to measure the time-history of the tip of the run-up tongue of a non-breaking solitary wave as it progresses up the slope. The results obtained with this run-up gage agree well with other measurements and provides a simple and reliable way of measuring run-up time histories.\r\nThe run-up of breaking solitary waves was studied experimentally and numerically since no fully theoretical approach is possible. The wave characteristics such as wave shape and shoaling characteristics, and, for plunging breakers, the shape of the jet produced are presented. The experimental results show that wave breaking is such a complicated process that even sophisticated numerical models cannot adequately model its details.\r\nTwo different plunging wave breaking and resultant run-up were found from the experiments. The point, where the tip of the incident jet produced by the plunging breaking wave impinges determines the characteristics of the resulting splash-up. If the jet impinges on a dry slope, no splash-up occurs and the plunging breaker simply collapses. If the impingement point is located on the free-surface, splash-up including a reflected jet is formed, which further increases the turbulence and energy dissipation associated with wave breaking. It is hypothesized that both clockwise and counter clockwise vortices may be generated by the impinging plunging jet and the reflected jet associated with the splash-up when the jet impinges on the front face of a breaking wave or on the still water surface in front of the wave.\r\nIf only the run-up process and maximum run-up are of interest, the wave and the water flow produced after breaking can be simplified as a propagating bore, which is analogous to a shock wave in gas dynamics. A numerical model using this bore structure to treat the process of wave breaking and propagation was developed. The non-linear shallow water equations were solved using the weighted essentially non-oscillatory (WENO) shock capturing scheme employed in gas dynamics. Wave breaking and propagation is handled automatically 1w this scheme and no ad-hoc term is required. A computational domain mapping technique proposed by Zhang (1996) is used in the numerical scheme to model the shoreline movement. This numerical scheme is found to provide a somewhat simple and reasonably good prediction of various aspects of the run-up process. The numerical results agree well with the experiments corresponding to the run-up on a. relatively steep slope (1:2.08) as well as on a more gentle slope (1:19.85).\r\nA simple empirical estimate of maximum run-up based on energy conservation considerations is also presented where the energy dissipation associated with wave breaking was estimated using the results from the numerical model. This approach appears to be useful and the maximum run-up predicted agrees reasonably well with the experimental results.\r\nThe splash-up of a solitary wave on a vertical wall positioned at different locations on a gentle slope was also investigated in this study to understand the degree of protection from tsunamis afforded by seawalls. It was found that the effect of breaking wave kinematics offshore of the vertical wall on the splash-up is of critical importance to the maximum splash-up. The maximum slope of the front face of the wave upon impingement of the wave on the wall, which represents the maximum water particle acceleration, was important in defining the maximum sheet splash-up as well as the trend for splash-up composed of drops and spray.\r\n", "doi": "10.7907/kake-7p89", "publication_date": "2000", "thesis_type": "phd", "thesis_year": "2000" }, { "id": "thesis:965", "collection": "thesis", "collection_id": "965", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03162004-133652", "primary_object_url": { "basename": "Shiels_D_1998.pdf", "content": "final", "filesize": 8923372, "license": "other", "mime_type": "application/pdf", "url": "/965/1/Shiels_D_1998.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Simulation of Controlled Bluff Body Flow with a Viscous Vortex Method", "author": [ { "family_name": "Shiels", "given_name": "Douglas G.", "clpid": "Shiels-Dpouglas-G" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/49A0-VA12", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:965", "collection": "thesis", "collection_id": "965", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03162004-133652", "primary_object_url": { "basename": "Shiels_D_1998.pdf", "content": "final", "filesize": 8923372, "license": "other", "mime_type": "application/pdf", "url": "/965/1/Shiels_D_1998.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Simulation of Controlled Bluff Body Flow with a Viscous Vortex Method", "author": [ { "family_name": "Shiels", "given_name": "Douglas G.", "clpid": "Shiels-Dpouglas-G" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Pullin", "given_name": "Dale Ian", "clpid": "Pullin-D-I" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Gharib", "given_name": "Morteza", "orcid": "0000-0003-0754-4193", "clpid": "Gharib-M" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/49A0-VA12", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:1173", "collection": "thesis", "collection_id": "1173", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03272007-111205", "primary_object_url": { "basename": "Uy_rv_1998.pdf", "content": "final", "filesize": 6627831, "license": "other", "mime_type": "application/pdf", "url": "/1173/1/Uy_rv_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Studies of Rotordynamic Forces Generated by Annular Flows", "author": [ { "family_name": "Uy", "given_name": "Robert Vincent", "clpid": "Uy-Robert-Vincent" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fluid-induced rotordynamic forces in pumping machinery are well documented but poorly understood. The present research focuses on the rotordynamics due to fluid flow in annuli, in particular, the discharge-to-suction leakage flow in centrifugal pumps. There are indications that the contribution of the front shroud leakage flow can be of the same order of magnitude as contributions from the nonuniform pressure acting on the impeller discharge. Previous investigations have established some of the basic traits of these flows. This work further elaborates both the experimental and computational approach to quantify and predict the shroud contribution to the rotordynamic stability of pumping machinery.\t\r\n\r\nExperimental results presented show the contributions of the curvature of the leakage path to the rotordynamics both with and without inlet swirl. The effect of different inlet swirl rates at constant flow rate is examined. Anti-swirl devices are evaluated for their effectiveness in reducing instability. Geometrical changes to the high-pressure and low-pressure seals for the leakage path are quantified. All results are in good agreement with other reported measurements.\r\n\r\nChilds' bulk flow model for leakage paths is carefully examined, and convective relations for vorticity and total pressure are deduced. This analysis suggests a new solution procedure of the bulk flow equations which does not resort to linearization or assumed harmonic forms of the flow variables.", "doi": "10.7907/9nq0-qp03", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:1173", "collection": "thesis", "collection_id": "1173", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03272007-111205", "primary_object_url": { "basename": "Uy_rv_1998.pdf", "content": "final", "filesize": 6627831, "license": "other", "mime_type": "application/pdf", "url": "/1173/1/Uy_rv_1998.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Studies of Rotordynamic Forces Generated by Annular Flows", "author": [ { "family_name": "Uy", "given_name": "Robert Vincent", "clpid": "Uy-Robert-Vincent" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Fluid-induced rotordynamic forces in pumping machinery are well documented but poorly understood. The present research focuses on the rotordynamics due to fluid flow in annuli, in particular, the discharge-to-suction leakage flow in centrifugal pumps. There are indications that the contribution of the front shroud leakage flow can be of the same order of magnitude as contributions from the nonuniform pressure acting on the impeller discharge. Previous investigations have established some of the basic traits of these flows. This work further elaborates both the experimental and computational approach to quantify and predict the shroud contribution to the rotordynamic stability of pumping machinery.\t\r\n\r\nExperimental results presented show the contributions of the curvature of the leakage path to the rotordynamics both with and without inlet swirl. The effect of different inlet swirl rates at constant flow rate is examined. Anti-swirl devices are evaluated for their effectiveness in reducing instability. Geometrical changes to the high-pressure and low-pressure seals for the leakage path are quantified. All results are in good agreement with other reported measurements.\r\n\r\nChilds' bulk flow model for leakage paths is carefully examined, and convective relations for vorticity and total pressure are deduced. This analysis suggests a new solution procedure of the bulk flow equations which does not resort to linearization or assumed harmonic forms of the flow variables.", "doi": "10.7907/9nq0-qp03", "publication_date": "1998", "thesis_type": "phd", "thesis_year": "1998" }, { "id": "thesis:2427", "collection": "thesis", "collection_id": "2427", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06032005-161139", "primary_object_url": { "basename": "Papalexandris_mv_1997.pdf", "content": "final", "filesize": 11607111, "license": "other", "mime_type": "application/pdf", "url": "/2427/1/Papalexandris_mv_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Unsplit Numerical Schemes for Hyperbolic Systems of Conservation Laws with Source Terms", "author": [ { "family_name": "Papalexandris", "given_name": "Miltiadis Vassilios", "clpid": "Papalexandris-Miltiadis-Vassilios" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/HW7S-AR36", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:2427", "collection": "thesis", "collection_id": "2427", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-06032005-161139", "primary_object_url": { "basename": "Papalexandris_mv_1997.pdf", "content": "final", "filesize": 11607111, "license": "other", "mime_type": "application/pdf", "url": "/2427/1/Papalexandris_mv_1997.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Unsplit Numerical Schemes for Hyperbolic Systems of Conservation Laws with Source Terms", "author": [ { "family_name": "Papalexandris", "given_name": "Miltiadis Vassilios", "clpid": "Papalexandris-Miltiadis-Vassilios" } ], "thesis_advisor": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" } ], "thesis_committee": [ { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Meiron", "given_name": "Daniel I.", "orcid": "0000-0003-0397-3775", "clpid": "Meiron-D-I" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Dimotakis", "given_name": "Paul E.", "clpid": "Dimotakis-P-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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.", "doi": "10.7907/HW7S-AR36", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:112", "collection": "thesis", "collection_id": "112", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01102008-131126", "primary_object_url": { "basename": "Kedia_r_1997.pdf", "content": "final", "filesize": 5591584, "license": "other", "mime_type": "application/pdf", "url": "/112/1/Kedia_r_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Investigation of Velocity and Temperature Fields in Taylor-Couette Flows", "author": [ { "family_name": "Kedia", "given_name": "Rajesh", "clpid": "Kedia-Rajesh" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Moser", "given_name": "Robert", "clpid": "Moser-R" }, { "family_name": "Sabersky", "given_name": "Rolf H.", "clpid": "Sabersky-R-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nIn many experiments, especially those investigating aspects of fluid flow, it is common to observe time series data exhibiting chaos. Chaos lies in the realm of nonlinear dynamics, and specialized methods are available for the analysis of nonlinear time series. One particular method, called time delay analysis, is particularly useful for extracting information from time series representing measurements at a single point in space. In this thesis, hot-wire anemometry is used to obtain velocity time series from experiments on isothermal Taylor-Couette flow. For R/R[subscript c]=1.6, a simple limit cycle is observed, yielding an attractor of dimension of 1. For R/R[subscript c]=11.1, the attractor dimension increases, and the reconstructed attractor exhibits features characteristic of a transition to turbulence. In addition, various other states and transitions of the Taylor-Couette system are studied as well.\r\n\r\nDirect numerical simulations (DNS) have also been performed to study the effects of the gravitational and the centrifugal potentials on the stability of heated, incompressible Taylor-Couette flow. The flow is confined between two differentially heated, concentric cylinders and the inner cylinder is allowed to rotate. The Navier-Stokes equations and the coupled energy equation are solved using a spectral method. To validate the code, comparisons are made with existing linear stability analysis and with experiments. The code is used to calculate the local and average heat transfer coefficients for a fixed Reynolds number (R=100) and a range of Grashof numbers. The variation of the local coefficients of heat transfer on the cylinder surface is investigated, and maps showing different stable states of the flow are presented. Calculations of the time and space averaged equivalent conductivity show that the heat transfer decreases with Grashof number in axisymmetric Taylor vortex flow regime and increases with Grashof number after the flow becomes non-axisymmetric.\r\n\r\nThe numerical simulations also demonstrate the existence of a hysteresis loop in heated Taylor-Couette flow, obtained by slowly varying the Grashof number. Two different stable states with same heat transfer are found to exist at the same Grashof number. The validity of Colburn's correlation is investigated as well; the Prandtl number dependence is found to be slightly different from Pr[...] for the range of Reynolds number investigated. Finally, a time delay analysis of the radial velocity and the local heat transfer coefficient time series obtained from the numerical simulation of the radially heated Taylor-Couette flow is performed. The two-dimensional projection of the reconstructed attractor shows a limit cycle for Gr[...]-1700. The limit cycle behavior disappears at Gr[...]-2100, and the reconstructed attractor becomes irregular. The attractor dimension increases to about 3.2 from a value of 1 for the limit cycle case.\r\n", "doi": "10.7907/0AAS-AW20", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:112", "collection": "thesis", "collection_id": "112", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01102008-131126", "primary_object_url": { "basename": "Kedia_r_1997.pdf", "content": "final", "filesize": 5591584, "license": "other", "mime_type": "application/pdf", "url": "/112/1/Kedia_r_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "An Investigation of Velocity and Temperature Fields in Taylor-Couette Flows", "author": [ { "family_name": "Kedia", "given_name": "Rajesh", "clpid": "Kedia-Rajesh" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "thesis_committee": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Leonard", "given_name": "Anthony", "clpid": "Leonard-A" }, { "family_name": "Moser", "given_name": "Robert", "clpid": "Moser-R" }, { "family_name": "Sabersky", "given_name": "Rolf H.", "clpid": "Sabersky-R-H" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.\r\n\r\nIn many experiments, especially those investigating aspects of fluid flow, it is common to observe time series data exhibiting chaos. Chaos lies in the realm of nonlinear dynamics, and specialized methods are available for the analysis of nonlinear time series. One particular method, called time delay analysis, is particularly useful for extracting information from time series representing measurements at a single point in space. In this thesis, hot-wire anemometry is used to obtain velocity time series from experiments on isothermal Taylor-Couette flow. For R/R[subscript c]=1.6, a simple limit cycle is observed, yielding an attractor of dimension of 1. For R/R[subscript c]=11.1, the attractor dimension increases, and the reconstructed attractor exhibits features characteristic of a transition to turbulence. In addition, various other states and transitions of the Taylor-Couette system are studied as well.\r\n\r\nDirect numerical simulations (DNS) have also been performed to study the effects of the gravitational and the centrifugal potentials on the stability of heated, incompressible Taylor-Couette flow. The flow is confined between two differentially heated, concentric cylinders and the inner cylinder is allowed to rotate. The Navier-Stokes equations and the coupled energy equation are solved using a spectral method. To validate the code, comparisons are made with existing linear stability analysis and with experiments. The code is used to calculate the local and average heat transfer coefficients for a fixed Reynolds number (R=100) and a range of Grashof numbers. The variation of the local coefficients of heat transfer on the cylinder surface is investigated, and maps showing different stable states of the flow are presented. Calculations of the time and space averaged equivalent conductivity show that the heat transfer decreases with Grashof number in axisymmetric Taylor vortex flow regime and increases with Grashof number after the flow becomes non-axisymmetric.\r\n\r\nThe numerical simulations also demonstrate the existence of a hysteresis loop in heated Taylor-Couette flow, obtained by slowly varying the Grashof number. Two different stable states with same heat transfer are found to exist at the same Grashof number. The validity of Colburn's correlation is investigated as well; the Prandtl number dependence is found to be slightly different from Pr[...] for the range of Reynolds number investigated. Finally, a time delay analysis of the radial velocity and the local heat transfer coefficient time series obtained from the numerical simulation of the radially heated Taylor-Couette flow is performed. The two-dimensional projection of the reconstructed attractor shows a limit cycle for Gr[...]-1700. The limit cycle behavior disappears at Gr[...]-2100, and the reconstructed attractor becomes irregular. The attractor dimension increases to about 3.2 from a value of 1 for the limit cycle case.\r\n", "doi": "10.7907/0AAS-AW20", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:1138", "collection": "thesis", "collection_id": "1138", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-103727", "primary_object_url": { "basename": "Sivo_jm_1997.pdf", "content": "final", "filesize": 5564535, "license": "other", "mime_type": "application/pdf", "url": "/1138/1/Sivo_jm_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Rotordynamic Forces Due to Annular Leakage Flows in Shrouded Centrifugal Pumps", "author": [ { "family_name": "Sivo", "given_name": "Joseph Michael", "clpid": "Sivo-Joseph-Michael" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Zukoski", "given_name": "Edward E.", "clpid": "Zukoski-E-E" } ], "thesis_committee": [ { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Zukoski", "given_name": "Edward E.", "clpid": "Zukoski-E-E" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Previous experimental and analytical results have shown that discharge-to-suction leakage flows in the annulus of a shrouded centrifugal pump contribute substantially to the fluid induced rotordynamic forces (Adkins, 1988). Experiments conducted in the Rotor Force Test Facility (RFTF) at Caltech on an impeller undergoing a prescribed circular whirl have indicated that the leakage flow contribution to the normal and tangential forces can be as much as 70% and 30% of the total, respectively (Jery, 1986). Recent experiments at Caltech have examined the rotordynamic consequences of leakage flows and have shown that the rotordynamic forces are functions not only of the whirl ratio but also of the leakage flow rate and the impeller shroud to pump housing clearance. The forces were found to be inversely proportional to the clearance and a region of forward subsynchronous whirl was found for which the average tangential force was destabilizing. This region decreased with flow coefficient (Guinzburg, 1992).
\r\n\r\nThe present research is a continuation of the previous experimental work and has been motivated by the rotordynamic stability problems with the recently developed Alternate Turbopump Design (ATD) of the Space Shuttle High Pressure Oxygen Turbopump. The present study investigates the influence of swirl brakes, installed in the annular leakage path, as a means of reducing the undesirable rotordynamic forces over a range of flow rates. Also, the present study evaluates the effect on the rotordynamic forces of tip leakage restrictions at discharge used by the ATD for establishing axial thrust balance. As a first step to understanding the flow field in the leakage annulus, the region is probed with a laser velocimeter to provide basic information on these unsteady turbulent three-dimensional leakage flows and to serve as a standard of comparison for approximate theoretical models as well as applications of computational fluid dynamics.
", "doi": "10.7907/bpfm-0a24", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:1138", "collection": "thesis", "collection_id": "1138", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-103727", "primary_object_url": { "basename": "Sivo_jm_1997.pdf", "content": "final", "filesize": 5564535, "license": "other", "mime_type": "application/pdf", "url": "/1138/1/Sivo_jm_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Rotordynamic Forces Due to Annular Leakage Flows in Shrouded Centrifugal Pumps", "author": [ { "family_name": "Sivo", "given_name": "Joseph Michael", "clpid": "Sivo-Joseph-Michael" } ], "thesis_advisor": [ { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Zukoski", "given_name": "Edward E.", "clpid": "Zukoski-E-E" } ], "thesis_committee": [ { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Zukoski", "given_name": "Edward E.", "clpid": "Zukoski-E-E" }, { "family_name": "Caughey", "given_name": "Thomas Kirk", "clpid": "Caughey-T-K" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Previous experimental and analytical results have shown that discharge-to-suction leakage flows in the annulus of a shrouded centrifugal pump contribute substantially to the fluid induced rotordynamic forces (Adkins, 1988). Experiments conducted in the Rotor Force Test Facility (RFTF) at Caltech on an impeller undergoing a prescribed circular whirl have indicated that the leakage flow contribution to the normal and tangential forces can be as much as 70% and 30% of the total, respectively (Jery, 1986). Recent experiments at Caltech have examined the rotordynamic consequences of leakage flows and have shown that the rotordynamic forces are functions not only of the whirl ratio but also of the leakage flow rate and the impeller shroud to pump housing clearance. The forces were found to be inversely proportional to the clearance and a region of forward subsynchronous whirl was found for which the average tangential force was destabilizing. This region decreased with flow coefficient (Guinzburg, 1992).
\r\n\r\nThe present research is a continuation of the previous experimental work and has been motivated by the rotordynamic stability problems with the recently developed Alternate Turbopump Design (ATD) of the Space Shuttle High Pressure Oxygen Turbopump. The present study investigates the influence of swirl brakes, installed in the annular leakage path, as a means of reducing the undesirable rotordynamic forces over a range of flow rates. Also, the present study evaluates the effect on the rotordynamic forces of tip leakage restrictions at discharge used by the ATD for establishing axial thrust balance. As a first step to understanding the flow field in the leakage annulus, the region is probed with a laser velocimeter to provide basic information on these unsteady turbulent three-dimensional leakage flows and to serve as a standard of comparison for approximate theoretical models as well as applications of computational fluid dynamics.
", "doi": "10.7907/bpfm-0a24", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:57", "collection": "thesis", "collection_id": "57", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01072008-111636", "primary_object_url": { "basename": "Adam_ph_1997.pdf", "content": "final", "filesize": 5940757, "license": "other", "mime_type": "application/pdf", "url": "/57/1/Adam_ph_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Enthalpy Effects on Hypervelocity Boundary Layers", "author": [ { "family_name": "Adam", "given_name": "Philippe H.", "clpid": "Adam-Philippe-H" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/YF0K-HK91", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:57", "collection": "thesis", "collection_id": "57", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-01072008-111636", "primary_object_url": { "basename": "Adam_ph_1997.pdf", "content": "final", "filesize": 5940757, "license": "other", "mime_type": "application/pdf", "url": "/57/1/Adam_ph_1997.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Enthalpy Effects on Hypervelocity Boundary Layers", "author": [ { "family_name": "Adam", "given_name": "Philippe H.", "clpid": "Adam-Philippe-H" } ], "thesis_advisor": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" } ], "thesis_committee": [ { "family_name": "Hornung", "given_name": "Hans G.", "orcid": "0000-0002-4903-8419", "clpid": "Hornung-H-G" }, { "family_name": "Ingersoll", "given_name": "Andrew P.", "orcid": "0000-0002-2035-9198", "clpid": "Ingersoll-A-P" }, { "family_name": "Sturtevant", "given_name": "Bradford", "clpid": "Sturtevant-B" }, { "family_name": "Shepherd", "given_name": "Joseph E.", "orcid": "0000-0003-3181-9310", "clpid": "Shepherd-J-E" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "GALCIT" }, { "literal": "div_eng" } ], "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", "doi": "10.7907/YF0K-HK91", "publication_date": "1997", "thesis_type": "phd", "thesis_year": "1997" }, { "id": "thesis:1145", "collection": "thesis", "collection_id": "1145", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-131335", "primary_object_url": { "basename": "McKenney_ea_1995.pdf", "content": "final", "filesize": 13166737, "license": "other", "mime_type": "application/pdf", "url": "/1145/1/McKenney_ea_1995.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Study of Tip Vortices and Cavitation on a Propeller in a Non-Uniform Flow Field", "author": [ { "family_name": "McKenney", "given_name": "Elizabeth Anne", "clpid": "McKenney-Elizabeth-Anne" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Antonsson", "given_name": "Erik K.", "clpid": "Antonsson-E-K" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Unsteady lifting surface flows are important subjects for study, both for the purposes of improving propulsive or lifting efficiency and also for mitigating the destructive effects and noise caused by cavitation. Some progress may be made by selecting a simple type of unsteadiness for closer study. In the present work, this tactic was implemented in two ways: the operation of a propeller at an angle of yaw to the freestream and the pitching oscillation of a finite-span hydrofoil.\r\n\r\nA new facility was designed and constructed to set a propeller at an angle of yaw to the freestream, creating a fairly simple non-uniformity in the propeller inflow. Tip vortex cavitation inception measurements were made for a range of yaw angles and freestream velocities, and photographs of the cavitation were taken to illustrate the effects of the yaw angle.\r\n\r\nThe unsteady tip vortex flow field was measured on an oscillating finite aspect ratio hydrofoil using Particle Image Velocimetry (PIV), revealing how the circulation varied during a typical oscillation cycle. The results were compared with unsteady infinite-span theory, and also with recent measurements using LDV techniques on the same foil.\r\n\r\nThe hydrofoil was also the focus of a study of surface cavitation. High-speed motion pictures of the cavitation cycle helped to separate the process into its component stages, and variations with cavitation number and reduced frequency of oscillation were observed. The acoustic signals generated by the cavity collapse were correlated with the motion pictures, providing insights into the correspondence between the flow structures involved in the cavity collapse process and the sound generated by them.\r\n\r\nThe results from these studies provide valuable insights into the effects of unsteadiness in lifting surface flows.\r\n", "doi": "10.7907/7THC-NG74", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" }, { "id": "thesis:1145", "collection": "thesis", "collection_id": "1145", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-03262007-131335", "primary_object_url": { "basename": "McKenney_ea_1995.pdf", "content": "final", "filesize": 13166737, "license": "other", "mime_type": "application/pdf", "url": "/1145/1/McKenney_ea_1995.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "A Study of Tip Vortices and Cavitation on a Propeller in a Non-Uniform Flow Field", "author": [ { "family_name": "McKenney", "given_name": "Elizabeth Anne", "clpid": "McKenney-Elizabeth-Anne" } ], "thesis_advisor": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" } ], "thesis_committee": [ { "family_name": "Brennen", "given_name": "Christopher E.", "clpid": "Brennen-C-E" }, { "family_name": "Acosta", "given_name": "Allan J.", "clpid": "Acosta-A-J" }, { "family_name": "Antonsson", "given_name": "Erik K.", "clpid": "Antonsson-E-K" }, { "family_name": "Raichlen", "given_name": "Fredric", "clpid": "Raichlen-F" }, { "family_name": "Hunt", "given_name": "Melany L.", "orcid": "0000-0001-5592-2334", "clpid": "Hunt-M-L" }, { "family_name": "Colonius", "given_name": "Tim", "orcid": "0000-0003-0326-3909", "clpid": "Colonius-T" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Unsteady lifting surface flows are important subjects for study, both for the purposes of improving propulsive or lifting efficiency and also for mitigating the destructive effects and noise caused by cavitation. Some progress may be made by selecting a simple type of unsteadiness for closer study. In the present work, this tactic was implemented in two ways: the operation of a propeller at an angle of yaw to the freestream and the pitching oscillation of a finite-span hydrofoil.\r\n\r\nA new facility was designed and constructed to set a propeller at an angle of yaw to the freestream, creating a fairly simple non-uniformity in the propeller inflow. Tip vortex cavitation inception measurements were made for a range of yaw angles and freestream velocities, and photographs of the cavitation were taken to illustrate the effects of the yaw angle.\r\n\r\nThe unsteady tip vortex flow field was measured on an oscillating finite aspect ratio hydrofoil using Particle Image Velocimetry (PIV), revealing how the circulation varied during a typical oscillation cycle. The results were compared with unsteady infinite-span theory, and also with recent measurements using LDV techniques on the same foil.\r\n\r\nThe hydrofoil was also the focus of a study of surface cavitation. High-speed motion pictures of the cavitation cycle helped to separate the process into its component stages, and variations with cavitation number and reduced frequency of oscillation were observed. The acoustic signals generated by the cavity collapse were correlated with the motion pictures, providing insights into the correspondence between the flow structures involved in the cavity collapse process and the sound generated by them.\r\n\r\nThe results from these studies provide valuable insights into the effects of unsteadiness in lifting surface flows.\r\n", "doi": "10.7907/7THC-NG74", "publication_date": "1995", "thesis_type": "phd", "thesis_year": "1995" } ]