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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:39:04 +0000A Study of Charged D* Mesons Produced in e⁺e⁻ Annihilation at E꜀ₘ = 29 GeV
https://resolver.caltech.edu/CaltechTHESIS:04152019-093410294
Authors: {'items': [{'id': 'Yamamoto-Hitoshi', 'name': {'family': 'Yamamoto', 'given': 'Hitoshi'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/nms8-7v38
<p>Charged D* mesons produced in e<sup>+</sup>e<sup>-</sup> annihilation at a center-of-mass energy of 29 GeV have been studied with the DELCO detector at the PEP storage ring. The selection criteria of D* candidates exploit the π/K separation capability in the momentum range from 2.6 GeV/c to 9.2 Ge VIc provided by the gas Čerenkov counter. The data correspond to an integrated luminosity of 147 pb<sup>-1</sup>.</p>
<p>We have measured the total production cross section of D*<sup>±</sup> to be [0.16±0.02(statistical)±0.02(systematic)] nb [x ≡ P<sub>D*</sub>/(E<sup>2</sup><sub>beam</sub>-M<sup>2</sup><sub>D*</sub>)<sup>1/2</sup> > 0.35], and (0.18 ± 0.02 ± 0.03) nb (x > 0) if the contribution from bottom quarks is subtracted. The branching fractions used are Br(D*<sup>+</sup> → D<sup>0</sup>π<sup>+</sup>) = 64% and Br(D<sup>0</sup> → K<sup>-</sup>π<sup>+</sup>) = 3%. The systematic errors due to the branching ratios are not included in the errors. With Br(D<sup>0</sup> → K<sup>-</sup>π<sup>+</sup>) = 4.9%, which is a recent measurement by the MARK III group, the above two cross sections become 0.10±0.02±0.02 nb (x > 0.35) and 0.11±0.02±0.02 nb (x > 0 and after the subtraction of the contribution from b quarks).</p>
<p>The charm fragmentation function is harder than that for light quarks, and the shape is found to be consistent with the prediction of the string model with a uniform string-breaking probability. Assuming the string model, the string-breaking probability is determined to be (0.019 ± 0.05 ± 0.09) GeV<sup>2</sup>.</p>
<p>We have also determined the lifetime of D<sup>0</sup> meson which is detected in the D* decay, with the result τ<sub>D<sup>0</sup></sub> = (5.3 ± 1.7<sup>+0.6</sup><sub>-0.5</sub>) x 10<sup>-13</sup> sec. Together with the semileptonic branching fraction of D<sup>0</sup> measured elsewhere, the semileptonic decay rate of D<sup>0</sup> is estimated to be (1.4 ± 0.5) x 10<sup>11</sup> sec<sup>-1</sup>, which corresponds to an effective charm quark mass of (1.54 ± 0.12) GeV/c<sup>2</sup>.</p>
<p>Using part of the D* candidates, we have set an upper limit on D<sup>0</sup>-D̅<sup>0</sup> mixing: r ≡ P(D<sup>0</sup> → D̅<sup>0</sup>)/P(D<sup>0</sup> → D<sup>0</sup> or D̅<sup>0</sup>) < 8.3% (90% c.l.), leading to a stringent limit on charm-changing neutral currents.</p>https://thesis.library.caltech.edu/id/eprint/11460Transition Between Regular Reflection and Mach Reflection in the Dual-Solution Domain
https://resolver.caltech.edu/CaltechETD:etd-01052007-125557
Authors: {'items': [{'email': 'cm@k-a-p.com', 'id': 'Mouton-Christopher-Andre', 'name': {'family': 'Mouton', 'given': 'Christopher Andre'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/TEA0-Q468
<p>A study of the shock-reflection domain for steady flow is presented. Conditions defining boundaries between different possible shock-reflection solutions are given, and where possible, simple analytic expressions for these conditions are presented. A new, more accurate estimate of the steady-state Mach stem height is derived based on geometric considerations of the flow. In particular, the location of the sonic throat through which the subsonic convergent flow behind the Mach stem is accelerated to divergent supersonic flow is considered. Comparisons with previous computational and experimental work show that the theory presented in this thesis more accurately predicts the Mach stem height than previous theories. The Mach stem height theory is generalized to allow for a moving triple point. Based on this moving triple point theory, a Mach stem growth rate theory is developed. This theory agrees well with computational and experimental results. Numerical computations of the effects of water vapor disturbances are also presented. These disturbances are shown to be sufficient to cause transition from regular reflection to Mach reflection in the dual-solution domain. These disturbances are also modeled as a simple energy deposition on one of the wedges, and an estimate for the minimum energy required to cause transition is derived.</p>
<p>Experimental results using an asymmetric wedge configuration in the Ludwieg tube facility at the California institute of Technology are presented. A Mach 4.0 nozzle was designed and built for the Ludwieg tube facility. This Mach number is sufficient to provide a large dual-solution domain, while being small enough not to require preheating of the test gas. The test time of the facility is 100ms, which requires the use of high-speed cinematography and a fast motor to rotate one of the two wedges. Hysteresis in the transition between regular to Mach reflection was successfully demonstrated in the Ludwieg tube facility. The experiments show that regular reflection could be maintained up to a shock angle approximately halfway between the von Neumann condition and the detachment condition.</p>
<p>Energy deposition studies were performed using an Nd:YAG laser. Triggering transition in this manner is found to depend on the location of the energy deposition. This finding is consistent with the numerical work presented in this thesis. Experiments were also performed to measure the Mach stem height and its growth rate. These results are compared with the theoretical estimates presented in this thesis. Excellent agreement between the steady-state Mach stem height and the theoretical estimates is seen. Comparisons of Mach stem growth rate with theoretical estimates show significant differences, but do show good agreement regarding the time required to reach the steady-state height.</p>https://thesis.library.caltech.edu/id/eprint/36On the Non-Local Geometry of Turbulence
https://resolver.caltech.edu/CaltechETD:etd-05092008-173614
Authors: {'items': [{'email': 'ivan.bermejo.moreno@gmail.com', 'id': 'Bermejo-Moreno-Ivan', 'name': {'family': 'Bermejo-Moreno', 'given': 'Ivan'}, 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/DH9W-Y894
<p>A multi-scale methodology for the study of the non-local geometry of eddy structures in turbulence is developed. Starting from a given three-dimensional field, this consists of three main steps: extraction, characterization and classification of structures. The extraction step is done in two stages. First, a multi-scale decomposition based on the curvelet transform is applied to the full three-dimensional field, resulting in a finite set of component fields, one per scale. Second, by iso-contouring each component field at one or more iso-contour levels, a set of closed iso-surfaces is obtained that represents the structures at that scale. For periodic domains, those structures intersecting boundaries are reconnected with their continuation in the opposite boundaries. The characterization stage is based on the joint probability density function (jpdf), in terms of area coverage on each individual iso-surface, of two differential-geometry properties, the shape index and curvedness, plus the stretching parameter, a dimensionless global invariant of the surface. Taken together, this defines the geometrical signature of the iso-surface. The classification step is based on the construction of a finite set of parameters, obtained from algebraic functions of moments of the jpdf of each structure, that specify its location as a point in a multi-dimensional 'feature space'. At each scale the set of points in feature space represents all structures at that scale, for the specified iso-contour value. This allows the application, to the set, of clustering techniques that search for groups of structures with a common geometry.</p>
<p>Results are presented of a first application of this technique to a passive scalar field obtained from 512³ direct numerical simulation of scalar mixing by forced, isotropic turbulence (Re<sub>λ</sub>=265). These show transition, with decreasing scale, from blob-like structures in the larger scales to blob- and tube-like structures with small or moderate stretching in the inertial range of scales, and then toward tube and, predominantly, sheet-like structures with high level of stretching in the dissipation range of scales. Implications of these results for the dynamical behavior of passive scalar stirring and mixing by turbulence are discussed.</p>
<p>We apply the same methodology to the enstrophy and kinetic energy dissipation rate instantaneous fields of a second numerical database of incompressible homogeneous isotropic turbulence decaying in time obtained by DNS in a periodic box. Three different resolutions are considered: 256³, 512³ and 1024³ grid points, with k<sub>max</sub>η̅ approximately 1, 2, and 4, respectively, the same initial conditions and Re<sub>λ</sub> ≈ 77. This allows a comparison of the geometry of the structures obtained for different resolutions. For the highest resolution, structures of enstrophy and dissipation evolve in a continuous distribution from blob-like and moderately stretched tube-like shapes at the large scales to highly stretched sheet-like structures at the small scales. The intermediate scales show a predominance of tube-like structures for both fields, much more pronounced for the enstrophy field. The dissipation field shows a tendency toward structures with lower curvedness than those of the enstrophy, for intermediate and small scales. The 256³ grid resolution case (k<sub>max</sub>η̅ ≈ 1) was unable to detect the predominance of highly stretched sheet-like structures at the smaller scales.</p>
<p>The same methodology, but without the multi-scale decomposition, is then applied to two scalar fields used by existing local criteria for the eduction of tube- and sheet-like structures in turbulence, Q and [A<sub>ij</sub>]<sub>+</sub> respectively, obtained from invariants of the velocity gradient tensor and alike in the 1024³ case. This adds the non-local geometrical characterization and classification to those local criteria, assessing their validity in educing particular geometries.</p>
<p>Finally we introduce a new methodology for the study of proximity issues among different sets of structures, based also on geometrical and non-local analyses. We apply it to four of the fields previously studied. Tube-like structures of Q are mainly surrounded by sheets of [A<sub>ij</sub>]<sub>+</sub>, which appear at close distances. For the enstrophy, tube-like structures at an intermediate scale are primarily surrounded by sheets of smaller scales of the enstrophy and structures of dissipation at the same and smaller scales. A secondary contribution results from tubes of enstrophy at smaller scales appearing at farther distances. Different configurations of composite structures are presented.</p>
https://thesis.library.caltech.edu/id/eprint/1713Numerical Simulation and Subgrid-Scale Modeling of Mixing and Wall-Bounded Turbulent Flows
https://resolver.caltech.edu/CaltechETD:etd-05292009-123828
Authors: {'items': [{'email': 'daniel.chung@unimelb.edu.au', 'id': 'Chung-Daniel', 'name': {'family': 'Chung', 'given': 'Daniel'}, 'orcid': '0000-0003-3732-364X', 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/NE1Y-M812
<p>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.</p>
<p>First, 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.</p>
<p>We 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.</p>
<p>We 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 $Rey_ au=2, extrm{k}$--$20, extrm{M}$. Results, including SGS-extended spectra, compare favorably with DNS at Rey_ au=2, extrm{k}$, and maintain an $O(1)$ grid dependence on $Rey_ au$.</p>
<p>Finally, 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.</p>https://thesis.library.caltech.edu/id/eprint/2274Optimized Feedback Control of Vortex Shedding on an Inclined Flat Plate
https://resolver.caltech.edu/CaltechTHESIS:06072010-131025711
Authors: {'items': [{'email': 'won@caltech.edu', 'id': 'Joe-Won-Tae', 'name': {'family': 'Joe', 'given': 'Won Tae'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/6D11-2Y92
<p>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.</p>
<p>First, 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.</p>
<p>With 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.</p>
<p>Optimal 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.</p>
<p>Finally, 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.</p>https://thesis.library.caltech.edu/id/eprint/5928Characteristics of Three-dimensional Vortex Formation and Propulsive Performance in Flapping Locomotion
https://resolver.caltech.edu/CaltechTHESIS:06072010-114858790
Authors: {'items': [{'email': 'daegyoum@caltech.edu', 'id': 'Kim-Daegyoum', 'name': {'family': 'Kim', 'given': 'Daegyoum'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/8VZJ-7Z78
<p>Three-dimensional vortex formation and propulsive performance were studied experimentally to identify some of the main characteristic mechanisms of flapping locomotion. Mechanical models with thin plates were used to simulate flapping and translating motions of animal propulsors. Three-dimensional flow fields were mapped quantitatively using defocusing digital particle image velocimetry.</p>
<p>First, vortex structures made by impulsively translating low aspect-ratio plates were studied. The investigation of translating plates with a 90 degree angle of attack is important since it is a fundamental model for a better understanding of drag-based propulsion systems. Rectangular flat-rigid, flexible, and curved-rigid thin plastic plates with the same aspect ratio were used to compare their vortex structures and hydrodynamic forces. The interaction of the tip flow and the nearby vortex is a critical flow phenomenon to distinguish vortex patterns among these three cases. In the flexible plate case, slow development of the vortex structure causes a small initial peak in hydrodynamic force during the acceleration phase. However, after the initial peak, the flexible plate generates large force magnitude comparable to that of the flat-rigid plate case.</p>
<p>Drag-based paddling propulsion was also studied to explain some of the fundamental differences in vortex formation of lift-based and drag-based propulsions. While the temporal change of the inner area enclosed by the vortex loop is an important factor in thrust generation of lift-based propulsion, the temporal change of the vortex strength becomes more important in drag-based propulsion. Spanwise flow behind the paddling plate plays an important role in tip vortex motion and thrust generation. The distribution of spanwise flow depends on the propulsor shape and the Reynolds number. A delta-shaped propulsor generates strong spanwise flow compared to a rectangular propulsor. For the low Reynolds number case, the spanwise flow is not as strong as that of the high Reynolds number case. The flexible propulsor can smooth out force peaks during impulsive motions without sacrificing total impulse, which is advantageous in avoiding structural failures and stabilizing body motion. The role of the stopping vortex was addressed in optimizing a stroke angle of paddling animals.</p>
<p>In addition, vortex formation of clapping propulsion was investigated by varying aspect ratio and stroke angle. A low aspect-ratio propulsor produces larger total impulse than a high aspect-ratio propulsor. As the aspect ratio increases, circulation of the vortex is strengthened, and the inner area enclosed by the vortex structure tends to enlarge. Moreover, in terms of thrust, the advantage of a single plate over double clapping plates is larger for the lower aspect-ratio case. These results offer information to better understand the benefit of low aspect-ratio wings in force generation under specific locomotion modes. When a pair of plates claps, a vortex loop forms from two counter-rotating tip vortices by a reconnection process. The dynamics of wake structures are dependent on the aspect ratio and the stroke angle.</p>
<p>Vortex formation and vorticity transport processes of translating and rotating plates with a 45 degree angle of attack were investigated as well. In both translating and rotating cases, the spanwise flow over the plate and the vorticity tilting process inside the leading-edge vortex were observed. The distribution of spanwise flow is a prominent distinction between the vortex structures of these two cases. While spanwise flow is confined inside the leading-edge vortex for the translating case, it is widely present over the plate and the wake region of the rotating case. As the Reynolds number decreases, due to the increase in viscosity, leading-edge and tip vortices tend to spread inside the area swept by the rotating plate, which results in lower lift force generation.</p>
<p>Lastly, for translating motion, the dynamics of the vortex in corner regions was compared among three different corner shapes. For a large corner angle, the forward movement of the vortex tends to be uniform along the plate edges. However, for a small corner angle, the vortex close to the corner moves forward following the plate while the vortex away from the corner retards its forward movement.</p>https://thesis.library.caltech.edu/id/eprint/5927Effect of Surface Morphological Changes on Flow Over a Sphere
https://resolver.caltech.edu/CaltechTHESIS:05212010-124044645
Authors: {'items': [{'email': 'norman@caltech.edu', 'id': 'Norman-Adam-Keith', 'name': {'family': 'Norman', 'given': 'Adam Keith'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/SED1-5250
An experimental investigation was undertaken to examine the effect of a morphing surface on the flow over a sphere in the Reynolds number range of 5x10⁴ to 5x10⁵. Here, a morphing surface is defined as a continuous surface that undergoes small amplitude changes in order to excite flow instabilities, rather than utilizing large mechanical changes to the overall shape as with traditional aerodynamic control surfaces. The sphere was chosen as an ideal geometry for testing morphing surfaces, because of the well-known sensitivity of the flow to small asymmetries on the surface. In this study, an approximation of a morphing surface was made by dynamically moving a small isolated roughness element along the sphere, thus producing small amplitude time-dependent changes to the surface shape. An experimental apparatus was designed that produced the actuation with an internal motor, which moved the roughness element via magnetic interaction. A three-component piezoelectric force sensor placed inside the sphere allowed for accurate, instantaneous measurements of the global effect of the actuator on the flow. It was found that the moving roughness could produce an instantaneous lateral force as large as the drag. Simultaneous force and particle image velocimetry measurements in the subcritical regime were used to show that there is a relatively long timescale associated with the instability growth, entrainment of fluid, and local change of the position of separation. This allowed the roughness to trip an extended region of the flow at once. It is shown that the three-dimensionality of the disturbance leads to the production of two helical counter-rotating vortices in the wake. In addition, it is demonstrated that a mean side force can be obtained by oscillating the roughness element about a point, producing a lateral force an order of magnitude larger than the force caused by a stationary roughness element. Finally, the results from the dynamic roughness were used to help interpret the underlying physical mechanisms that govern the forcing on a smooth sphere.https://thesis.library.caltech.edu/id/eprint/5821Spark Ignition: Experimental and Numerical Investigation With Application to Aviation Safety
https://resolver.caltech.edu/CaltechTHESIS:05272010-173243262
Authors: {'items': [{'email': 'sbane@purdue.edu', 'id': 'Bane-Sally-Page-Moffett', 'name': {'family': 'Bane', 'given': 'Sally Page Moffett'}, 'orcid': '0000-0002-4764-3228', 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/W1NB-5W06
<p>Determining the risk of accidental ignition of flammable mixtures is a topic of tremendous importance in industry and aviation safety. The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, in recent years, particularly in the aviation safety industry, the viewpoint has changed to one where ignition is statistical in nature. Approaching ignition as statistical rather than a threshold phenomenon appears to be more consistent with the inherent variability in the engineering test data.</p>
<p>Ignition tests were performed in lean hydrogen-based aviation test mixtures and in two hexane-air mixtures using low-energy capacitive spark ignition systems. Tests were carried out using both short, fixed sparks (1 to 2 mm) and variable length sparks up to 10 mm. The results were analyzed using statistical tools to obtain probability distributions for ignition versus spark energy and spark energy density (energy per unit spark length). Results show that a single threshold MIE value does not exist, and that the energy per unit length may be a more appropriate parameter for quantifying the risk of ignition than only the energy. The probability of ignition versus spark charge was also investigated, and the statistical results for the spark charge and spark energy density were compared. It was found that the test results were less variable with respect to the spark charge than the energy density. However, variability was still present due to phenomena such as plasma instabilities and cathode effects that are caused by the electrodynamics.</p>
<p>Work was also done to develop a two-dimensional numerical model of spark ignition that accurately simulates all physical scales of the fluid mechanics and chemistry. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. One-step chemistry models were used to allow for highly resolved simulations, and methods for calculating effective one-step parameters were developed using constant pressure explosion theory. The one-step model was tuned to accurately simulate the flame speed, temperature, and straining behavior using one-dimensional flame computations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process.</p>
https://thesis.library.caltech.edu/id/eprint/5868A Robust Control Approach to Understanding Nonlinear Mechanisms in Shear Flow Turbulence
https://resolver.caltech.edu/CaltechTHESIS:05272010-195149679
Authors: {'items': [{'email': 'dennice@jhu.edu', 'id': 'Gayme-Dennice-F-Maynard', 'name': {'family': 'Gayme', 'given': 'Dennice F. Maynard'}, 'orcid': '0000-0003-0330-415X', 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/P1DS-Q379
<p>A robust control framework is used to investigate a streamwise constant projection of the Navier Stokes equations for plane Couette flow. Study of this streamwise constant model is motivated by both numerical and experimental observations that suggest the prevalence and importance of streamwise and quasi-streamwise elongated structures. Small-amplitude Gaussian noise forcing is applied to a two-dimensional, three-velocity component (2D/3C) model to describe its response in the presence of disturbances, uncertainty and modeling errors. A comparison of the results with Direct Numerical Simulation (DNS) data demonstrates that the simulations capture salient features of fully developed turbulence. In particular, the change in mean velocity profile from the nominal laminar to the characteristic “S” shaped turbulent profile. The application of Taylor’s hypothesis shows that the model can also reproduce downstream information in the form of large-scale coherence resembling numerically and experimentally observed flow features. The 2D/3C model is able to generate “turbulent-like” behavior under small-amplitude stochastic noise. The laminar flow solution is globally stable, therefore transition to turbulence in this model is likely a consequence of the laminar flow solution’s lack of robustness in the presence of disturbances and uncertainty. In fact, large disturbance amplification is common in both this model and the linearized Navier Stokes equations.</p>
<p>Periodic spanwise/wall-normal (z–y) plane stream functions are used as input to develop a forced 2D/3C streamwise velocity equation. The resulting steady-state solution is qualitatively similar to a fully turbulent spatial field of DNS data. Both numerical methods and a perturbation analysis confirm that the momentum transfer that produces a “turbulent-like” mean profile requires a nonlinear streamwise velocity equation.</p>
<p>A system theoretic approach is used to study the amplification mechanisms that develop through the 2D/3C nonlinear coupling in the streamwise velocity equation. The spanwise/wall-normal plane forcing required to produce each stream function is computedand used to define an induced norm from this forcing input to the streamwise velocity. This input-output response is used to determine the energy optimal spanwise wavelength (i.e.,the preferential spacing) over a range of Reynolds numbers and forcing amplitudes.</p>https://thesis.library.caltech.edu/id/eprint/5871A Novel Experimental Study of a Valveless Impedance Pump for Applications at Lab-On-Chip, Microfluidic, and Biomedical Device Size Scales
https://resolver.caltech.edu/CaltechTHESIS:05262011-111659863
Authors: {'items': [{'email': 'jameier@gmail.com', 'id': 'Meier-John-Allen', 'name': {'family': 'Meier', 'given': 'John Allen'}, 'show_email': 'YES'}]}
Year: 2011
DOI: 10.7907/DBKG-EJ21
<p>In 1954, Gerhart Liebau demonstrated a simple valveless pumping phenomenon utilizing the periodic compression of a compliant tube and some systematic asymmetry to pump water out of a bucket. Liebau's goal was to explain peculiarities seen in the human circulatory system. In the years that have followed, the Liebau phenomenon has been studied in a variety of open and closed loop configurations, through experimental, computational, and analytical studies.</p>
<p>Recent advances in microfluidic and microelectromechanical systems (MEMS) technology have enabled a wide range of small scale engineering systems. The further development of many important systems is limited by the absence of an appropriate means of fluid transport. Valveless pumps based on the Liebau phenomenon show great promise, particularly in lab-on-chip (LOC), biological, and medical applications in which biocompatibility and the ability to move sensitive molecules without damage are key design requirements.</p>
<p>The purpose of this thesis is to synthesize previous studies of the Liebau phenomenon and produce the first extensive experimental study of a novel valveless pump at size scales and geometries that are relevant to lab-on-chip, microfluidic, and biomedical device applications. For the first time, detailed, dynamic pressure and flow data have been recorded during the operation of these valveless pumps for a large range of operating parameters. This dynamic data allowed us to identify new flow regimes and observe previously undocumented pump behaviors and performance. Parameters investigated include pump material properties and geometry, working fluid density and viscosity, pump excitation properties (amplitude, offset, location, and frequency), and flow loop/system properties. A critical relationship between the relative volumetric compliance of the valveless pump to the system it acts upon is identified, and the implications for practical implementation of valveless pumps at small size scales are discussed.</p>https://thesis.library.caltech.edu/id/eprint/6450The Development of Low-Order Models for the Study of Fluid-Structure Interactions
https://resolver.caltech.edu/CaltechTHESIS:09242010-133354529
Authors: {'items': [{'email': 'atchieu@gmail.com', 'id': 'Tchieu-Andrew-Allen', 'name': {'family': 'Tchieu', 'given': 'Andrew Allen'}, 'show_email': 'NO'}]}
Year: 2011
DOI: 10.7907/SYHX-8A77
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.
https://thesis.library.caltech.edu/id/eprint/6053Thermal Ignition
https://resolver.caltech.edu/CaltechTHESIS:05162012-131336010
Authors: {'items': [{'email': 'philipp.boettcher@gmail.com', 'id': 'Boettcher-Philipp-Andreas', 'name': {'family': 'Boettcher', 'given': 'Philipp Andreas'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/H2W9-ZK95
<p>Accidental ignition of flammable gases is a critical safety concern in many industrial applications. Particularly in the aviation industry, the main areas of concern on an aircraft are the fuel tank and adjoining regions, where spilled fuel has a high likelihood of creating a flammable mixture. To this end, a fundamental understanding of the ignition phenomenon is necessary in order to develop more accurate test methods and standards as a means of designing safer air vehicles. The focus of this work is thermal ignition, particularly auto-ignition with emphasis on the effect of heating rate, hot surface ignition and flame propagation, and puffing flames.</p>
<p>Combustion of hydrocarbon fuels is traditionally separated into slow reaction, cool flame, and ignition regimes based on pressure and temperature. Standard tests, such as the ASTM E659, are used to determine the lowest temperature required to ignite a specific fuel mixed with air at atmospheric pressure. It is expected that the initial pressure and the rate at which the mixture is heated also influences the limiting temperature and the type of combustion. This study investigates the effect of heating rate, between 4 and 15 K/min, and initial pressure, in the range of 25 to 100 kPa, on ignition of n-hexane air mixtures. Mixtures with equivalence ratio ranging from 0.6 to = 1.2 were investigated. The problem is also modeled computationally using an extension of Semenov's classical auto-ignition theory with a detailed chemical mechanism. Experiments and simulations both show that in the same reactor either a slow reaction or an ignition event can take place depending on the heating rate. Analysis of the detailed chemistry demonstrates that a mixture which approaches the ignition region slowly undergoes a significant modification of its composition. This change in composition induces a progressive shift of the explosion limit until the mixture is no longer flammable. A mixture that approaches the ignition region sufficiently rapidly undergoes only a moderate amount of thermal decomposition and explodes quite violently. This behavior can also be captured and analyzed using a one-step reaction model, where the heat release is in competition with the depletion of reactants.</p>
<p>Hot surface ignition is examined using a glow plug or heated nickel element in a series of premixed n-hexane air mixtures. High-speed schlieren photography, a thermocouple, and a fast response pressure transducer are used to record flame characteristics such as ignition temperature, flame speed, pressure rises, and combustion mode. The ignition event is captured by considering the dominant balance of diffusion and chemical reaction that occurs near a hot surface. Experiments and models show a dependence of ignition temperature on mixture composition, initial pressure, and hot surface size. The mixtures exhibit the known lower flammability limit where the maximum temperature of the hot surface was insufficient at igniting the mixture. Away from the lower flammability limit, the ignition temperature drops to an almost constant value over a wide range of equivalence ratios (0.7 to 2.8) with large variations as the upper flammability limit is approached. Variations in the initial pressure and equivalence ratio also give rise to different modes of combustion: single flame, re-ignition, and puffing flames. These results are successfully compared to computational results obtained using a flamelet model and a detailed chemical mechanism for n-heptane. These different regimes can be delineated by considering the competition between inertia, i.e., flame propagation, and buoyancy, which can be expressed in the Richardson number.</p>
<p>In experiments of hot surface ignition and subsequent flame propagation a 10 Hz puffing flame instability is visible in mixtures that are stagnant and premixed prior to the ignition sequence. By varying the size of the hot surface, power input, and combustion vessel volume, we determined that the instability is a function of the interaction of the flame with the fluid flow induced by the combustion products rather than the initial plume established by the hot surface. The phenomenon is accurately reproduced in numerical simulations and a detailed flow field analysis revealed a competition between the inflow velocity at the base of the flame and the flame propagation speed. The increasing inflow velocity, which exceeds the flame propagation speed, is ultimately responsible for creating a puff. The puff is then accelerated upward, allowing for the creation of the subsequent instabilities. The frequency of the puffing is proportional to the gravitational acceleration and inversely proportional to the flame speed. We propose a relation describing the dependence of the frequency on gravitational acceleration, hot surface diameter, and flame speed. This relation shows good agreement for lean and rich n-hexane-air as well as lean hydrogen-air flames.</p>https://thesis.library.caltech.edu/id/eprint/7037Large-Eddy Simulation of the Flat-Plate Turbulent Boundary Layer at High Reynolds Numbers
https://resolver.caltech.edu/CaltechTHESIS:05222012-183141047
Authors: {'items': [{'email': 'michio.i@gmail.com', 'id': 'Inoue-Michio', 'name': {'family': 'Inoue', 'given': 'Michio'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/PXTM-W616
<p>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.</p>
<p>First, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Re<sub>θ</sub> based on the free-stream velocity and the momentum thickness in the range Re<sub>θ</sub> = 10<sup>3</sup> - 10<sup>12</sup>. Results include the inverse skin friction coefficient, √2/C<sub>f</sub>, velocity profiles, the shape factor H, the Karman "constant", and the Coles wake factor as functions of Re<sub>θ</sub>. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Re<sub>θ</sub> = O(10<sup>6</sup>. At extremely large Re<sub>θ</sub>, 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<sub>θ</sub> of the present simulations appears to be limited by machine precision and it is speculated, but not demonstrated, that even larger Re<sub>θ</sub> could be achieved with quad- or higher-precision arithmetic.</p>
<p>Second, 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<sub>τ</sub> based on the friction velocity and the boundary layer thickness, Re<sub>τ</sub> =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<sub>τ</sub> =62,000, 100,000 and 200,000 that lie within a gap in log(Re<sub>τ</sub>) 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<sub>τ</sub> and also provide a means of extending LES results at large Reynolds numbers to the near-wall region of wall-bounded turbulent flows.</p>
<p>Finally, 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.</p>https://thesis.library.caltech.edu/id/eprint/7065Spatio-Temporal Analysis of the Turbulent Boundary Layer and An Investigation of the Effects of Periodic Disturbances
https://resolver.caltech.edu/CaltechTHESIS:05232012-142127799
Authors: {'items': [{'email': 'lehewj@gmail.com', 'id': 'LeHew-Jeffrey-Allen', 'name': {'family': 'LeHew', 'given': 'Jeffrey Allen'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/20CM-EV70
<p>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.</p>
<p>A 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.</p>https://thesis.library.caltech.edu/id/eprint/7067Planar Reflection of Gaseous Detonation
https://resolver.caltech.edu/CaltechTHESIS:06112013-153305610
Authors: {'items': [{'email': 'jason.damazo@gmail.com', 'id': 'Damazo-Jason-Scott', 'name': {'family': 'Damazo', 'given': 'Jason Scott'}, 'orcid': '0000-0002-4155-7177', 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/4QW7-TK55
<p>Pipes containing flammable gaseous mixtures may be subjected to internal detonation. When the detonation normally impinges on a closed end, a reflected shock wave is created to bring the flow back to rest. This study built on the work of Karnesky (2010) and examined deformation of thin-walled stainless steel tubes subjected to internal reflected gaseous detonations. A ripple pattern was observed in the tube wall for certain fill pressures, and a criterion was developed that predicted when the ripple pattern would form. A two-dimensional finite element analysis was performed using Johnson-Cook material properties; the pressure loading created by reflected gaseous detonations was accounted for with a previously developed pressure model. The residual plastic strain between experiments and computations was in good agreement.</p>
<p>During the examination of detonation-driven deformation, discrepancies were discovered in our understanding of reflected gaseous detonation behavior. Previous models did not accurately describe the nature of the reflected shock wave, which motivated further experiments in a detonation tube with optical access. Pressure sensors and schlieren images were used to examine reflected shock behavior, and it was determined that the discrepancies were related to the reaction zone thickness extant behind the detonation front. During these experiments reflected shock bifurcation did not appear to occur, but the unfocused visualization system made certainty impossible. This prompted construction of a focused schlieren system that investigated possible shock wave-boundary layer interaction, and heat-flux gauges analyzed the boundary layer behind the detonation front. Using these data with an analytical boundary layer solution, it was determined that the strong thermal boundary layer present behind the detonation front inhibits the development of reflected shock wave bifurcation.</p>https://thesis.library.caltech.edu/id/eprint/7890Identification of Thermally-Tagged Coherent Structures in the Zero Pressure Gradient Turbulent Boundary Layer
https://resolver.caltech.edu/CaltechTHESIS:06172013-144232973
Authors: {'items': [{'email': 'rlrought@caltech.edu', 'id': 'Rought-Rebecca-Lynn', 'name': {'family': 'Rought', 'given': 'Rebecca Lynn'}, 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/Z2V0-9V82
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∞. 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.https://thesis.library.caltech.edu/id/eprint/7900Models of Turbulent Pipe Flow
https://resolver.caltech.edu/CaltechTHESIS:11272012-130849053
Authors: {'items': [{'email': 'jeanloup.bourguignon@gmail.com', 'id': 'Bourguignon-Jean-Loup', 'name': {'family': 'Bourguignon', 'given': 'Jean-Loup'}, 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/BPEZ-VM28
<p>The physics of turbulent pipe flow was investigated via the use of two models based on simplified versions of the Navier-Stokes equations. The first model was a streamwise-constant projection of these equations, and was used to study the change in mean flow that occurs during transition to turbulence. The second model was based on the analysis of the turbulent pipe flow resolvent, and provided a radial basis for the modal decomposition of turbulent pipe flow. The two models were tested numerically and validated against experimental and numerical data.</p>
<p>Analysis of the streamwise-constant model showed that both non-normal and nonlinear effects are required to capture the blunting of the velocity profile, which occurs during pipe flow transition. The model generated flow fields characterized by the presence of high- and low-speed streaks, whose distribution over the cross-section of the pipe was remarkably similar to the one observed in the velocity field near the trailing edge of the puff structures present in pipe flow transition.</p>
<p>A modal decomposition of turbulent pipe flow, in the three spatial directions and in time, was performed, and made possible by the significant reduction in data requirements achieved via the use of compressive sampling and model-based radial basis functions. The application and efficiency of compressive sampling in wall-bounded turbulence was demonstrated.</p>
<p>Approximately sparse representations of turbulent pipe flow by propagating waves with model-based radial basis functions, were derived. The basis functions, obtained by singular value decomposition of the resolvent, captured the wall-normal coherence of the flow; and provided a link between the propagating waves and the governing equations, allowing for the identification of the dominant mechanims sustaining the waves, as a function of their streamwise wavenumber.</p>
<p>Analysis of the resolvent showed that the long streamwise waves are amplified mainly via non-normality effects, and are also constrained to be tall in the wall-normal direction, which decreases the influence of viscous dissipation. The short streamwise waves were shown to be localized near the critical-layer (defined as the wall-normal location where the convection velocity of the wave equals the local mean velocity), and thus exhibit amplification with a large contribution from criticality. The work in this thesis allows the reconciliation of the well-known results concerning optimal disturbance amplification due to non-normal effects with recent resolvent analyses, which highlighted the importance of criticality effects.</p>
https://thesis.library.caltech.edu/id/eprint/7287Structure of the Turbulent Boundary Layer under Static and Dynamic Impulsive Roughness Perturbation
https://resolver.caltech.edu/CaltechTHESIS:07102012-152431583
Authors: {'items': [{'email': 'jacobi@alum.mit.edu', 'id': 'Jacobi-Ian', 'name': {'family': 'Jacobi', 'given': 'Ian'}, 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/H5WJ-RK31
<p>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.</p>
<p>The 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.</p>
<p>The 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.</p> https://thesis.library.caltech.edu/id/eprint/7175Dynamics and Scaling of Self-Excited Passive Vortex Generators for Underwater Propulsion
https://resolver.caltech.edu/CaltechTHESIS:05282013-114822808
Authors: {'items': [{'email': 'rwhittlesey@gmail.com', 'id': 'Whittlesey-Robert-Wells', 'name': {'family': 'Whittlesey', 'given': 'Robert Wells'}, 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/SC4M-8896
<p>A series of experiments was conducted on the use of a device to passively generate vortex rings, henceforth a passive vortex generator (PVG). The device is intended as a means of propulsion for underwater vehicles, as the use of vortex rings has been shown to decrease the fuel consumption of a vehicle by up to 40% Ruiz (2010).</p>
<p>The PVG was constructed out of a collapsible tube encased in a rigid, airtight box. By adjusting the pressure within the airtight box while fluid was flowing through the tube, it was possible to create a pulsed jet with vortex rings via self-excited oscillations of the collapsible tube.</p>
<p>A study of PVG integration into an existing autonomous underwater vehicle (AUV) system was conducted. A small AUV was used to retrofit a PVG with limited alterations to the original vehicle. The PVG-integrated AUV was used for self-propelled testing to measure the hydrodynamic (Froude) efficiency of the system. The results show that the PVG-integrated AUV had a 22% increase in the Froude efficiency using a pulsed jet over a steady jet. The maximum increase in the Froude efficiency was realized when the formation time of the pulsed jet, a nondimensional time to characterize vortex ring formation, was coincident with vortex ring pinch-off. This is consistent with previous studies that indicate that the maximization of efficiency for a pulsed jet vehicle is realized when the formation of vortex rings maximizes the vortex ring energy and size.</p>
<p>The other study was a parameter study of the physical dimensions of a PVG. This study was conducted to determine the effect of the tube diameter and length on the oscillation characteristics such as the frequency. By changing the tube diameter and length by factors of 3, the frequency of self-excited oscillations was found to scale as f~D_0^{-1/2} L_0^0, where D_0 is the tube diameter and L_0 the tube length. The mechanism of operation is suggested to rely on traveling waves between the tube throat and the end of the tube. A model based on this mechanism yields oscillation frequencies that are within the range observed by the experiment.</p>https://thesis.library.caltech.edu/id/eprint/7757Slender-Body Hypervelocity Boundary-Layer Instability
https://resolver.caltech.edu/CaltechTHESIS:05312013-164534236
Authors: {'items': [{'email': 'nick.parziale@gmail.com', 'id': 'Parziale-Nicholaus-J', 'name': {'family': 'Parziale', 'given': 'Nicholaus J.'}, 'orcid': '0000-0001-9880-1727', 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/KZJ1-Y009
<p>With novel application of optical techniques, the slender-body hypervelocity boundary-layer instability is characterized in the previously unexplored regime where thermo-chemical effects are important. Narrowband disturbances (500-3000 kHz) are measured in boundary layers with edge velocities of up to 5~km/s at two points along the generator of a 5 degree half angle cone. Experimental amplification factor spectra are presented. Linear stability and PSE analysis is performed, with fair prediction of the frequency content of the disturbances; however, the analysis over-predicts the amplification of disturbances. The results of this work have two key implications: 1) the acoustic instability is present and may be studied in a large-scale hypervelocity reflected-shock tunnel, and 2) the new data set provides a new basis on which the instability can be studied.</p>https://thesis.library.caltech.edu/id/eprint/7808Sustainable Energy Solutions for Irrigation and Harvesting in Developing Countries
https://resolver.caltech.edu/CaltechTHESIS:06112013-114509037
Authors: {'items': [{'email': 'prakharmehrotra@gmail.com', 'id': 'Mehrotra-Prakhar', 'name': {'family': 'Mehrotra', 'given': 'Prakhar'}, 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/KV5Y-4960
One of the critical problems currently being faced by agriculture industry in developing nations is the alarming rate of groundwater depletion. Irrigation accounts for over 70% of the total groundwater withdrawn everyday. Compounding this issue is the use of polluting diesel generators to pump groundwater for irrigation. This has made irrigation not only the biggest consumer of groundwater but also one of the major contributors to green house gases. The aim of this thesis is to present a solution to the energy-water nexus. To make agriculture less dependent on fossil fuels, the use of a solar-powered Stirling engine as the power generator for on-farm energy needs is discussed. The Stirling cycle is revisited and practical and ideal Stirling cycles are compared. Based on agricultural needs and financial constraints faced by farmers in developing countries, the use of a Fresnel lens as a solar-concentrator and a Beta-type Stirling engine unit is suggested for sustainable power generation on the farms. To reduce the groundwater consumption and to make irrigation more sustainable, the conceptual idea of using a Stirling engine in drip irrigation is presented. To tackle the shortage of over 37 million tonnes of cold-storage in India, the idea of cost-effective solar-powered on-farm cold storage unit is discussed.https://thesis.library.caltech.edu/id/eprint/7889Large-Eddy Simulations of Fully Developed Turbulent Channel and Pipe Flows with Smooth and Rough Walls
https://resolver.caltech.edu/CaltechTHESIS:02142014-112419793
Authors: {'items': [{'email': 'namiko12@icloud.com', 'id': 'Saito-Namiko', 'name': {'family': 'Saito', 'given': 'Namiko'}, 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/WKNJ-ET18
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 Re<sub>τ</sub> = O(10<sup>2</sup>)-O(10<sup>8</sup>) 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 Re<sub>τ</sub> ≥ O(10<sup>6</sup>) 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.https://thesis.library.caltech.edu/id/eprint/8077Velocity Resolved - Scalar Modeled Simulations of High Schmidt Number Turbulent Transport
https://resolver.caltech.edu/CaltechTHESIS:06042014-163735743
Authors: {'items': [{'email': 'verma_siddhu@yahoo.com', 'id': 'Verma-Siddhartha', 'name': {'family': 'Verma', 'given': 'Siddhartha'}, 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/PTD9-W004
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.https://thesis.library.caltech.edu/id/eprint/8481A Multi-Scale Approach to Shaping Carbon Nanotube Structures for Hollow Microneedles
https://resolver.caltech.edu/CaltechTHESIS:05302014-012121120
Authors: {'items': [{'email': 'bradley.lyon@gmail.com', 'id': 'Lyon-Bradley-Joseph', 'name': {'family': 'Lyon', 'given': 'Bradley Joseph'}, 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/BJGT-TB74
<p>The concept of a carbon nanotube microneedle array is explored in this thesis from multiple perspectives including microneedle fabrication, physical aspects of transdermal delivery, and in vivo transdermal drug delivery experiments. Starting with standard techniques in carbon nanotube (CNT) fabrication, including catalyst patterning and chemical vapor deposition, vertically-aligned carbon nanotubes are utilized as a scaffold to define the shape of the hollow microneedle. Passive, scalable techniques based on capillary action and unique photolithographic methods are utilized to produce a CNT-polymer composite microneedle. Specific examples of CNT-polyimide and CNT-epoxy microneedles are investigated. Further analysis of the transport properties of polymer resins reveals general requirements for applying arbitrary polymers to the fabrication process. </p>
<p>The bottom-up fabrication approach embodied by vertically-aligned carbon nanotubes allows for more direct construction of complex high-aspect ratio features than standard top-down fabrication approaches, making microneedles an ideal application for CNTs. However, current vertically-aligned CNT fabrication techniques only allow for the production of extruded geometries with a constant cross-sectional area, such as cylinders. To rectify this limitation, isotropic oxygen etching is introduced as a novel fabrication technique to create true 3D CNT geometry. Oxygen etching is utilized to create a conical geometry from a cylindrical CNT structure as well as create complex shape transformations in other CNT geometries.</p>
<p>CNT-polymer composite microneedles are anchored onto a common polymer base less than 50 µm thick, which allows for the microneedles to be incorporated into multiple drug delivery platforms, including modified hypodermic syringes and silicone skin patches. Cylindrical microneedles are fabricated with 100 µm outer diameter and height of 200-250 µm with a central cavity, or lumen, diameter of 30 µm to facilitate liquid drug flow. In vitro delivery experiments in swine skin demonstrate the ability of the microneedles to successfully penetrate the skin and deliver aqueous solutions. </p>
<p>An in vivo study was performed to assess the ability of the CNT-polymer microneedles to deliver drugs transdermally. CNT-polymer microneedles are attached to a hand actuated silicone skin patch that holds a liquid reservoir of drugs. Fentanyl, a potent analgesic, was administered to New Zealand White Rabbits through 3 routes of delivery: topical patch, CNT-polymer microneedles, and subcutaneous hypodermic injection. Results demonstrate that the CNT-polymer microneedles have a similar onset of action as the topical patch. CNT-polymer microneedles were also vetted as a painless delivery approach compared to hypodermic injection. Comparative analysis with contemporary microneedle designs demonstrates that the delivery achieved through CNT-polymer microneedles is akin to current hollow microneedle architectures. The inherent advantage of applying a bottom-up fabrication approach alongside similar delivery performance to contemporary microneedle designs demonstrates that the CNT-polymer composite microneedle is a viable architecture in the emerging field of painless transdermal delivery.</p>
https://thesis.library.caltech.edu/id/eprint/8436Advancing EDL Technologies for Future Space Missions: From Ground Testing Facilities to Ablative Heatshields
https://resolver.caltech.edu/CaltechTHESIS:05302014-140011538
Authors: {'items': [{'email': 'jason.rabinovitch@gmail.com', 'id': 'Rabinovitch-Jason', 'name': {'family': 'Rabinovitch', 'given': 'Jason'}, 'orcid': '0000-0002-1914-7964', 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/XKM7-7368
<p>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.</p>
<p>Due 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.</p>
<p>The 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. </p>
<p>Finally, 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.</p>
<p>Through 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.</p>https://thesis.library.caltech.edu/id/eprint/8445On the Behavior of Pliable Plate Dynamics in Wind: Application to Vertical Axis Wind Turbines
https://resolver.caltech.edu/CaltechTHESIS:05272014-160129404
Authors: {'items': [{'email': 'jcosse@gmail.com', 'id': 'Cossé-Julia-Theresa', 'name': {'family': 'Cossé', 'given': 'Julia Theresa'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/X7S3-CS74
<p>Numerous studies have shown that flexible materials improve resilience and durability of a structure. Several studies have investigated the behavior of elastic plates under the influence of a free stream, such as studies of the fluttering flag and others of shape reconfiguration, due to a free stream.</p>
<p>The principle engineering contribution of this thesis is the design and development of a vertical axis wind turbine that features pliable blades which undergo various modes of behavior, ultimately leading to rotational propulsion of the turbine. The wind turbine design was tested in a wind tunnel and at the Caltech Laboratory for Optimized Wind Energy. Ultimately, the flexible blade vertical axis wind turbine proved to be an effective way of harnessing the power of the wind.</p>
<p>In addition, this body of work builds on the current knowledge of elastic cantilever plates in a free stream flow by investigating the inverted flag. While previous studies have focused on the fluid structure interaction of a free stream on elastic cantilever plates, none had studied the plate configuration where the trailing edge was clamped, leaving the leading edge free to move. Furthermore, the studies presented in this thesis establish the geometric boundaries of where the large-amplitude flapping occurs.</p>https://thesis.library.caltech.edu/id/eprint/8401Towards Understanding the Mixing Characteristics of Turbulent Buoyant Flows
https://resolver.caltech.edu/CaltechTHESIS:05212014-101703985
Authors: {'items': [{'email': 'plcarroll2@gmail.com', 'id': 'Carroll-Phares-Lynn', 'name': {'family': 'Carroll', 'given': 'Phares Lynn'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/RDHJ-3X60
<p>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.</p>
<p>The 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.</p>
<p>This 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).</p>
https://thesis.library.caltech.edu/id/eprint/8253Characterization and Modeling of Premixed Turbulent n-Heptane Flames in the Thin Reaction Zone Regime
https://resolver.caltech.edu/CaltechTHESIS:05282015-154709861
Authors: {'items': [{'email': 'bruno.savard@gmail.com', 'id': 'Savard-Bruno', 'name': {'family': 'Savard', 'given': 'Bruno'}, 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9GM858F
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.https://thesis.library.caltech.edu/id/eprint/8904Storm Track Response to Perturbations in Climate
https://resolver.caltech.edu/CaltechTHESIS:05112015-075223217
Authors: {'items': [{'email': 'C.Mbengue@wolfson.oxon.org', 'id': 'Mbengue-Cheikh-Oumar', 'name': {'family': 'Mbengue', 'given': 'Cheikh Oumar'}, 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9FT8J05
<p>This thesis advances our understanding of midlatitude storm tracks and how they respond to perturbations in the climate system. The midlatitude storm tracks are regions of maximal turbulent kinetic energy in the atmosphere. Through them, the bulk of the atmospheric transport of energy, water vapor, and angular momentum occurs in midlatitudes. Therefore, they are important regulators of climate, controlling basic features such as the distribution of surface temperatures, precipitation, and winds in midlatitudes. Storm tracks are robustly projected to shift poleward in global-warming simulations with current climate models. Yet the reasons for this shift have remained unclear. Here we show that this shift occurs even in extremely idealized (but still three-dimensional) simulations of dry atmospheres. We use these simulations to develop an understanding of the processes responsible for the shift and develop a conceptual model that accounts for it.</p>
<p>We demonstrate that changes in the convective static stability in the deep tropics alone can drive remote shifts in the midlatitude storm tracks. Through simulations with a dry idealized general circulation model (GCM), midlatitude storm tracks are shown to be located where the mean available potential energy (MAPE, a measure of the potential energy available to be converted into kinetic energy) is maximal. As the climate varies, even if only driven by tropical static stability changes, the MAPE maximum shifts primarily because of shifts of the maximum of near-surface meridional temperature gradients. The temperature gradients shift in response to changes in the width of the tropical Hadley circulation, whose width is affected by the tropical static stability. Storm tracks generally shift in tandem with shifts of the subtropical terminus of the Hadley circulation.</p>
<p>We develop a one-dimensional diffusive energy-balance model that links changes in the Hadley circulation to midlatitude temperature gradients and so to the storm tracks. It is the first conceptual model to incorporate a dynamical coupling between the tropical Hadley circulation and midlatitude turbulent energy transport. Numerical and analytical solutions of the model elucidate the circumstances of when and how the storm tracks shift in tandem with the terminus of the Hadley circulation. They illustrate how an increase of only the convective static stability in the deep tropics can lead to an expansion of the Hadley circulation and a poleward shift of storm tracks.</p>
<p>The simulations with the idealized GCM and the conceptual energy-balance model demonstrate a clear link between Hadley circulation dynamics and midlatitude storm track position. With the help of the hierarchy of models presented in this thesis, we obtain a closed theory of storm track shifts in dry climates. The relevance of this theory for more realistic moist climates is discussed.</p>https://thesis.library.caltech.edu/id/eprint/8854Stability of Hypervelocity Boundary Layers
https://resolver.caltech.edu/CaltechTHESIS:06052015-111128842
Authors: {'items': [{'email': 'bitternp@gmail.com', 'id': 'Bitter-Neal-Phillip', 'name': {'family': 'Bitter', 'given': 'Neal Phillip'}, 'show_email': 'NO'}]}
Year: 2015
DOI: 10.7907/Z9Q23X5Z
<p>The early stage of laminar-turbulent transition in a hypervelocity boundary layer is studied using a combination of modal linear stability analysis, transient growth analysis, and direct numerical simulation. Modal stability analysis is used to clarify the behavior of first and second mode instabilities on flat plates and sharp cones for a wide range of high enthalpy flow conditions relevant to experiments in impulse facilities. Vibrational nonequilibrium is included in this analysis, its influence on the stability properties is investigated, and simple models for predicting when it is important are described.</p>
<p>Transient growth analysis is used to determine the optimal initial conditions that lead to the largest possible energy amplification within the flow. Such analysis is performed for both spatially and temporally evolving disturbances. The analysis again targets flows that have large stagnation enthalpy, such as those found in shock tunnels, expansion tubes, and atmospheric flight at high Mach numbers, and clarifies the effects of Mach number and wall temperature on the amplification achieved. Direct comparisons between modal and non-modal growth are made to determine the relative importance of these mechanisms under different flow regimes. </p>
<p>Conventional stability analysis employs the assumption that disturbances evolve with either a fixed frequency (spatial analysis) or a fixed wavenumber (temporal analysis). Direct numerical simulations are employed to relax these assumptions and investigate the downstream propagation of wave packets that are localized in space and time, and hence contain a distribution of frequencies and wavenumbers. Such wave packets are commonly observed in experiments and hence their amplification is highly relevant to boundary layer transition prediction. It is demonstrated that such localized wave packets experience much less growth than is predicted by spatial stability analysis, and therefore it is essential that the bandwidth of localized noise sources that excite the instability be taken into account in making transition estimates. A simple model based on linear stability theory is also developed which yields comparable results with an enormous reduction in computational expense. This enables the amplification of finite-width wave packets to be taken into account in transition prediction. </p>https://thesis.library.caltech.edu/id/eprint/8995Aerodynamics of Vertical-Axis Wind Turbines in Full-Scale and Laboratory-Scale Experiments
https://resolver.caltech.edu/CaltechTHESIS:12022015-023535926
Authors: {'items': [{'email': 'dbaraya@uh.edu', 'id': 'Araya-Daniel-Borsodi', 'name': {'family': 'Araya', 'given': 'Daniel Borsodi'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9VD6WC2
<p>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.</p>
<p>One 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.</p>https://thesis.library.caltech.edu/id/eprint/9303Non-Linear Scale Interactions in a Forced Turbulent Boundary Layer
https://resolver.caltech.edu/CaltechTHESIS:02292016-143116051
Authors: {'items': [{'email': 'mangaloreman@gmail.com', 'id': 'Duvvuri-Subrahmanyam', 'name': {'family': 'Duvvuri', 'given': 'Subrahmanyam'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9Z31WKP
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.https://thesis.library.caltech.edu/id/eprint/9595Numerical Investigation of Vertical-Axis Wind Turbines at Low Reynolds Number
https://resolver.caltech.edu/CaltechTHESIS:05272016-150613633
Authors: {'items': [{'email': 'else731@gmail.com', 'id': 'Tsai-Hsieh-Chen', 'name': {'family': 'Tsai', 'given': 'Hsieh-Chen'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9SF2T5R
<p>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.</p>
<p>Analysis 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.</p>
<p>A 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.</p>https://thesis.library.caltech.edu/id/eprint/9796Unsteady Aerodynamics and Optimal Control of an Airfoil at Low Reynolds Number
https://resolver.caltech.edu/CaltechTHESIS:05272016-220450949
Authors: {'items': [{'email': 'jeesoonchoi@gmail.com', 'id': 'Choi-Jeesoon', 'name': {'family': 'Choi', 'given': 'Jeesoon'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9J1014Q
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.https://thesis.library.caltech.edu/id/eprint/9808Mixing, Chemical Reactions, and Combustion in Supersonic Flows
https://resolver.caltech.edu/CaltechTHESIS:05242016-143905617
Authors: {'items': [{'email': 'niccolo.cymbalist@gmail.com', 'id': 'Cymbalist-Niccolo', 'name': {'family': 'Cymbalist', 'given': 'Niccolo'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9G73BNR
<p>Experiments were conducted at the GALCIT supersonic shear-layer facility to investigate
aspects of reacting transverse jets in supersonic crossflow using chemiluminescence and schlieren
image-correlation velocimetry. In particular, experiments were designed to examine mixing-delay
length dependencies on jet-fluid molar mass, jet diameter, and jet inclination.</p>
<p>The experimental results show that mixing-delay length depends on jet Reynolds number, when
appropriately normalized, up to a jet Reynolds number of 500,000. Jet inclination increases the
mixing-delay length, but causes less disturbance to the crossflow when compared to normal jet
injection. This can be explained, in part, in terms of a control-volume analysis that relates jet
inclination to flow conditions downstream of injection.</p>
<p>In the second part of this thesis, a combustion-modeling framework is proposed and developed
that is tailored to large-eddy simulations of turbulent combustion in high-speed flows. Scaling arguments place supersonic hydrocarbon combustion in a regime of autoignition-dominated distributed
reaction zones (DRZ). The proposed evolution-variable manifold (EVM) framework incorporates an
ignition-delay data-driven induction model with a post-ignition manifold that uses a Lagrangian
convected 'balloon' reactor model for chemistry tabulation. A large-eddy simulation incorporating
the EVM framework captures several important reacting-flow features of a transverse hydrogen jet
in heated-air crossflow experiment.</p>https://thesis.library.caltech.edu/id/eprint/9742Dynamic Stall on Vertical Axis Wind Turbines
https://resolver.caltech.edu/CaltechTHESIS:09042015-152813860
Authors: {'items': [{'email': 'reeve.dunne@gmail.com', 'id': 'Dunne-Reeve', 'name': {'family': 'Dunne', 'given': 'Reeve'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z92Z13FX
<p>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.</p>
<p>Detailed 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. </p>
<p>The 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.</p>
<p>Finally, 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.</p>https://thesis.library.caltech.edu/id/eprint/9140Advancements in Jet Turbulence and Noise Modeling: Accurate One-Way Solutions and Empirical Evaluation of the Nonlinear Forcing of Wavepackets
https://resolver.caltech.edu/CaltechTHESIS:01062016-163653523
Authors: {'items': [{'email': 'atowne15@gmail.com', 'id': 'Towne-Aaron-S', 'name': {'family': 'Towne', 'given': 'Aaron S.'}, 'orcid': '0000-0002-7315-5375', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z99884XJ
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.https://thesis.library.caltech.edu/id/eprint/9361Thermal Ignition Using Moving Hot Particles
https://resolver.caltech.edu/CaltechTHESIS:06032016-210051818
Authors: {'items': [{'email': 'stephaniecoronel@gmail.com', 'id': 'Coronel-Stephanie-Alexandra', 'name': {'family': 'Coronel', 'given': 'Stephanie Alexandra'}, 'orcid': '0000-0002-7088-7976', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9W37T9X
<p>In this work, ignition of n-hexane-air mixtures was investigated using moving hot spheres of various diameters and surface temperatures. Alumina spheres of 1.8-6 mm diameter were heated using a high power CO2 laser and injected with an average velocity of 2.4 m/s into a premixed n-hexane-air mixture at a nominal initial temperature and pressure of 298 K and 100 kPa, respectively. The 90% probability of ignition using a 6 mm diameter sphere was 1224 K. High-speed experimental visualizations using interferometry indicated that ignition occurred in the vicinity of the separation point in the boundary layer of the sphere when the sphere surface temperature was near the ignition threshold. Additionally, the ignition threshold was found to be insensitive to the mixture composition and showed little variation with sphere diameter.</p>
<p>Numerical simulations of a transient one-dimensional boundary layer using detailed chemistry in a gas a layer adjacent to a hot wall indicated that ignition takes place away from the hot surface; the igniting gas that is a distance away from the surface can overcome diffusive heat losses back to the wall when there is heat release due to chemical activity. Finally, a simple approximation of the thermal and momentum boundary layer profiles indicated that the residence time within a boundary layer varies drastically, for example, a fluid parcel originating at very close to the wall has a residence time that is 65x longer than the residence time of a fluid parcel traveling along the edge of the momentum boundary layer.</p>
<p>A non-linear methodology was developed for the extraction of laminar flame properties from synthetic spherically expanding flames. The results indicated that for accurate measurements of the flame speed and Markstein length, a minimum of 50 points is needed in the data set (flame radius vs. time) and a minimum range of 48 mm in the flame radius. The non-linear methodology was applied to experimental n-hexane-air spherically expanding flames. The measured flame speed was insensitive to the mixture initial pressure from 50 to 100 kPa and increased with increasing mixture initial temperature. One-dimensional freely-propagating flame calculations showed excellent agreement with the experimental flame speeds using the JetSurF and CaltechMech chemical mechanisms.</p>https://thesis.library.caltech.edu/id/eprint/9844Experimental Generation and Modeling of Vortical Gusts and Their Interactions with an Airfoil
https://resolver.caltech.edu/CaltechTHESIS:05232017-111820349
Authors: {'items': [{'email': 'estebanhufstedler@gmail.com', 'id': 'Hufstedler-Esteban-Antonio-Lemus', 'name': {'family': 'Hufstedler', 'given': 'Esteban Antonio Lemus'}, 'orcid': '0000-0001-7162-920X', 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9Q52MN5
<p>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.</p>
<p>The 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.</p>
<p>A 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.</p>
<p>Experiments 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.</p>
<p>The 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.</p>
<p>With 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 α=0°, this was 5-10 convective time units, which is characteristic of attached flows. The airfoil at α=10° 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.</p>https://thesis.library.caltech.edu/id/eprint/10194Coherent Structures, their Interactions, and their Effects on Passive Scalar Transport and Aero-Optic Distortion in a Turbulent Boundary Layer
https://resolver.caltech.edu/CaltechTHESIS:11272017-152918929
Authors: {'items': [{'email': 'tsaxtonf@gmail.com', 'id': 'Saxton-Fox-Theresa-Ann', 'name': {'family': 'Saxton-Fox', 'given': 'Theresa Ann'}, 'orcid': '0000-0003-1328-4148', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Z9W0943P
<p>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.</p>https://thesis.library.caltech.edu/id/eprint/10570Egospace Motion Planning Representations for Micro Air Vehicles
https://resolver.caltech.edu/CaltechTHESIS:10242017-193520989
Authors: {'items': [{'email': 'fragoso.at@gmail.com', 'id': 'Fragoso-Anthony-Thomas', 'name': {'family': 'Fragoso', 'given': 'Anthony Thomas'}, 'orcid': '0000-0002-5805-9668', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Z9GX48RJ
Navigation of micro air vehicles (MAVs) in unknown environments is a complex sensing and trajectory generation task, particularly at high velocities. In this work, we introduce an efficient sense-and-avoid pipeline that compactly represents range measurements from multiple sensors, trajectory generation, and motion planning in a 2.5–dimensional projective data structure called an egospace representation. Egospace coordinates generalize depth image obstacle representations and are a particularly convenient choice for configuration flat mobile robots, which are differentially flat in their configuration variables and include a number of commonly used MAV plant models. After characterizing egospace obstacle avoidance for robots with trivial dynamics and establishing limits on applicability and performance, we generalize to motion planning over full configuration flat dynamics using motion primitives expressed directly in egospace coordinates. In comparison to approaches based on world coordinates, egospace uses the natural sensor geometry to combine the benefits of a multi-resolution and multi-sensor representation architecture into a single simple and efficient layer.
We also present an experimental implementation, based on perception with stereo vision and an egocylinder obstacle representation, that demonstrates the specialization of our theoretical results to particular mission scenarios. The natural pixel parameterization of the egocylinder is used to quickly identify dynamically feasible maneuvers onto radial paths, expressed directly in egocylinder coordinates, that enable finely detailed planning at extreme ranges within milliseconds. We have implemented our obstacle avoidance pipeline with an Asctec Pelican quadcopter, and demonstrate the efficiency of our approach experimentally with a set of challenging field scenarios. The scalability potential of our system is discussed in terms of sensor horizon, actuation, and computational limitations and the speed limits that each imposes, and its generality to more challenging environments with multiple moving obstacles is developed as an immediate extension to the static framework.https://thesis.library.caltech.edu/id/eprint/10543Analysis of Flapping Propulsion: Comparison, Characterization, and Optimization
https://resolver.caltech.edu/CaltechTHESIS:06072018-133402239
Authors: {'items': [{'email': 'nathanmartin13@yahoo.com', 'id': 'Martin-Nathan-Koon-Hung', 'name': {'family': 'Martin', 'given': 'Nathan Koon-Hung'}, 'orcid': '0000-0001-6038-6177', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Q6CG-QY57
<p>In recent decades, the development of autonomous underwater vehicles (AUVs) has rapidly increased and inspiration for novel designs has recently come from nature, primarily based on the fast, efficient, and maneuverable flapping motion of fish. Due to its potential, flapping propulsion is investigated through three studies.</p>
<p>The first study involves the comparison between swimming by flapping and by periodic contractions. A direct comparison is made between the two propulsion mechanisms by simplifying the motions, utilizing a machine that can operate in either mode of propulsion, and evaluating the average thrust generated and the average input power required per cycle between the two mechanisms when the overall kinematics are identical. The two propulsion mechanisms are tested using a variety of overall kinematics, flexible plates, and modified duty cycles, all of which suggest that flapping propulsion is the more efficient; however, periodic contractions with a modified duty cycle are shown to generate more thrust per cycle.</p>
<p>The second study involves the characterization of the impact of chord-wise curvature on the hydrodynamic forces and torques, motivated by the dorso-ventral bending of a fish's caudal fin during locomotion. The impact of curvature is shown to depend on the planform area of the flapping plate. Plates with a smaller or an identical planform area compared with a baseline rigid flat rectangular plate either decrease or increase the generated thrust, respectively. These phenomena are utilized to develop an actuated plate for velocity modulation and a snap-buckling plate to provide a greater thrust and efficiency compared with a rigid plate propulsor.</p>
<p>The third study involves the development and demonstration of a method to experimentally optimize an arbitrary three-dimensional trajectory for a flapping propulsor. The trajectory is parameterized by variables inspired by birds and fish, executed by a mechanism that can actuate an arbitrary motion in a hemisphere, and optimized using an adaptive evolutionary strategy. The trajectories are scored based upon their difference from a desired force set-point and their efficiency. All trajectory searches demonstrate good convergence properties and match the desired force set-point almost immediately. Additional generations primarily improve the efficiency. This novel approach finds optimal trajectories for generating side-forces, similar to how a fish's pectoral fin or a bird's wing functions, and for generating thrust, similar to how a fish's caudal fin operates.</p>https://thesis.library.caltech.edu/id/eprint/11036On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics
https://resolver.caltech.edu/CaltechTHESIS:08182017-103752052
Authors: {'items': [{'email': 'morgane.grivel@gmail.com', 'id': 'Grivel-Morgane-Anne-Marie', 'name': {'family': 'Grivel', 'given': 'Morgane Anne Marie'}, 'orcid': '0000-0002-4391-799X', 'show_email': 'YES'}]}
Year: 2018
DOI: 10.7907/Z9736P2V
<p>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.</p>
<p>This 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 <i>Re</i> ~ 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.</p>
<p>These 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 <i>Re</i> on the order of 10<sup>4</sup> to 10<sup>5</sup>. 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.</p>https://thesis.library.caltech.edu/id/eprint/10381Resolvent-Based Modeling of Flows in a Channel
https://resolver.caltech.edu/CaltechTHESIS:06012018-114927289
Authors: {'items': [{'email': 'kevin.t.rosenberg@gmail.com', 'id': 'Rosenberg-Kevin-Thomas', 'name': {'family': 'Rosenberg', 'given': 'Kevin Thomas'}, 'orcid': '0000-0001-6101-3823', 'show_email': 'YES'}]}
Year: 2018
DOI: 10.7907/PHDW-Z389
<p>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.</p>https://thesis.library.caltech.edu/id/eprint/10998Reconstruction and Estimation of Flows Using Resolvent Analysis and Data-Assimilation
https://resolver.caltech.edu/CaltechTHESIS:05302018-181049042
Authors: {'items': [{'email': 'seansymon@gmail.com', 'id': 'Symon-Sean-Pearson', 'name': {'family': 'Symon', 'given': 'Sean Pearson'}, 'orcid': '0000-0001-9085-0778', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/B4K7-K876
<p>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.</p>
<p>The 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.</p>
<p>The 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.</p>
<p>The 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 <i>α</i> = 0° and <i>Re</i> = 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.</p>https://thesis.library.caltech.edu/id/eprint/10976Investigations of Incompressible Variable-Density Turbulence in an External Acceleration Field
https://resolver.caltech.edu/CaltechTHESIS:12052017-154614667
Authors: {'items': [{'email': 'ilana.gat@gmail.com', 'id': 'Gat-Ilana-Batya', 'name': {'family': 'Gat', 'given': 'Ilana Batya'}, 'orcid': '0000-0003-0223-0507', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/Z9JM27V7
<p>Of interest to turbulence modeling is the behavior of variable-density flow at high Reynolds numbers - a flow difficult to model. This thesis provides insight into variable-density flow behavior by examining the dynamics and mixing of variable-density turbulence subject to an externally imposed acceleration field. The flow is studied in the zero-Mach-number limit with a series of direct numerical simulations. The flow configuration consists of alternating slabs of high- and low-density fluid in a triply periodic domain. Density ratios in the range of 1.005 to 10 are investigated. The flow produces temporally evolving shear layers.</p>
<p>A perpendicular mean density–pressure gradient is maintained as the flow evolves, with multi-scale baroclinic torques generated in the turbulent flow that ensues. For all density ratios studied, the simulations attain Reynolds numbers at the beginning of the fully developed turbulence regime.</p>
<p>An empirical relation for the convection velocity predicts the observed entrainment-ratio and dominant mixed-fluid composition statistics. Two mixing-layer temporal evolution regimes are identified: an initial diffusion-dominated regime with a growth rate with the square-root of time followed by a turbulence-dominated regime with a cubic growth rate in time. In the turbulent regime, composition probability density functions within the shear layers exhibit a slightly tilted ('non-marching') hump, corresponding to the most probable mole fraction. The shear layers preferentially entrain low-density fluid by volume at all density ratios, which is reflected in the mixed-fluid composition.</p>
<p>The mixed-fluid orientations of vorticity, baroclinic torques, density gradients, and pressure gradients are presented. Baroclinic torques, the cross product of the density and pressure gradients, tend to be aligned with positive or negative vorticity direction, with vorticity preferentially aligning with the intermediate eigenvector of the local strain-rate tensor, with some variance.</p>https://thesis.library.caltech.edu/id/eprint/10586On the Dynamics of Flat Plates in a Fluid Environment: A Study of Inverted Flag Flapping and Caudal Fin Maneuvering
https://resolver.caltech.edu/CaltechTHESIS:06072019-103225366
Authors: {'items': [{'email': 'cecilia.huertas@gmail.com', 'id': 'Huertas-Cerdeira-Cecilia', 'name': {'family': 'Huertas-Cerdeira', 'given': 'Cecilia'}, 'orcid': '0000-0003-4553-0470', 'show_email': 'NO'}]}
Year: 2019
DOI: 10.7907/326X-M576
<p>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.</p>
<p>The 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.</p>
<p>The 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.</p>https://thesis.library.caltech.edu/id/eprint/11707An EnKF-Based Flow State Estimator for Aerodynamic Problems
https://resolver.caltech.edu/CaltechTHESIS:09072018-105527896
Authors: {'items': [{'email': 'andre.fernando.t10@gmail.com', 'id': 'da-Silva-Andre-Fernando-de-Castro', 'name': {'family': 'da Silva', 'given': 'Andre Fernando de Castro'}, 'orcid': '0000-0002-8125-6010', 'show_email': 'YES'}]}
Year: 2019
DOI: 10.7907/W327-VF41
<p>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.</p>
<p>However, 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.</p>https://thesis.library.caltech.edu/id/eprint/11176Linear and Non-linear Interactions in a Rough-Wall Turbulent Boundary Layer
https://resolver.caltech.edu/CaltechTHESIS:04112019-234812867
Authors: {'items': [{'email': 'jpmorgan89@gmail.com', 'id': 'Morgan-Jonathan-Philip', 'name': {'family': 'Morgan', 'given': 'Jonathan Philip'}, 'orcid': '0000-0003-2898-4868', 'show_email': 'NO'}]}
Year: 2019
DOI: 10.7907/7RSR-3277
<p>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.</p>https://thesis.library.caltech.edu/id/eprint/11453Spatio-Temporal Response of a Compliant-Wall, Turbulent Boundary Layer System to Dynamic Roughness Forcing
https://resolver.caltech.edu/CaltechTHESIS:04232019-162807004
Authors: {'items': [{'email': 'david.huynh.58@gmail.com', 'id': 'Huynh-David-Pham', 'name': {'family': 'Huynh', 'given': 'David Pham'}, 'orcid': '0000-0002-8430-6255', 'show_email': 'YES'}]}
Year: 2019
DOI: 10.7907/A5PS-GT54
<p>This thesis investigates the interaction between an elastic compliant surface and a turbulent boundary layer exposed to dynamic roughness forcing. The goals are to explore a unique perspective of this fluid-structural problem through narrow-band forcing, and to further develop the understanding of dynamic roughness. Water tunnel experiments are designed with flow and surface measurements, both phase-locked to the roughness actuation. This enables a phase-averaged analysis, which leverages the deterministic input to isolate the temporally correlated components of the flow and surface response. Identifying the directly interacting velocity and deformation modes allows the complex, fluid-structural system to be studied in a more tractable, input-output manner.</p>
<p>The first experiment is conducted with a smooth-wall turbulent boundary layer forced by dynamic roughness, and contributes to the knowledge of this type of forcing through structure-resolved particle image velocimetry. This allows for the streamwise-spatial nature and the wall-normal velocity component (v) of the roughness-forced flow to be explored, which had not been previously studied. A spatial amplitude modulation is observed in the synthetic structure and investigated directly through the spatial spectra. Through a parametric study and an empirical fit, the forcing frequency may now be selected to target a particular streamwise length scale.</p>
<p>The second experiment implements a gelatin sample subject to an unforced turbulent boundary layer. The surface response is characterized and serves as a base case with which to identify the roughness-forced component of the deformations. This naturally leads to the third experiment, where the full compliant-wall, dynamic-roughness-forced turbulent boundary layer system is considered. The surface response to the synthetic flow structure is confirmed, which sets the stage for a comparison between the smooth-wall and compliant-wall data to study the effect of the compliant surface.</p>
<p>The smooth/compliant comparison is guided by a resolvent analysis, which predicts a virtual wall feature in the v velocity mode for the elastic material under consideration. Using this prediction to inform a conditional average, the virtual wall is revealed in the experimental data. Thus, the action of the elastic surface is interpreted as opposing the v velocity near the wall, in a manner similar to wall-jet opposition control.
Previous experimental studies of viscoelastic compliant surfaces have demonstrated the potential for turbulent drag reduction, though either indirectly via the turbulence intensities or with relatively high skin friction measurement error. A common observation in these studies was the importance of the interaction between the surface and the coherent structures in the flow. To that end, this study has isolated and modeled the behavior of the fluid-structural system with a single spatio-temporal scale generated by dynamic roughness forcing. The results provide a physical interpretation of the effect of an elastic surface on turbulent boundary layer flow structures and informs the ongoing development of a reduced-order modeling tool in the resolvent analysis.</p>https://thesis.library.caltech.edu/id/eprint/11484Large-Eddy Simulation of Turbulent Boundary Layers with Spatially Varying Roughness
https://resolver.caltech.edu/CaltechTHESIS:09082018-212920204
Authors: {'items': [{'email': 'akshay.sridhar.nz@gmail.com', 'id': 'Sridhar-Akshay', 'name': {'family': 'Sridhar', 'given': 'Akshay'}, 'orcid': '0000-0002-2642-8246', 'show_email': 'NO'}]}
Year: 2019
DOI: 10.7907/8YWS-B862
<p>This dissertation addresses high Reynolds number turbulent boundary layers flows with different inhomogeneous surface roughness distributions using large eddy simulations. The stretched vortex subgrid scale model for the outer flow LES is coupled with a virtual-wall model for the friction velocity with a correction accounting for local roughness effects.</p>
<p>A semi-empirical model that describes a fully developed rough-walled turbulent boundary layer with sand-grain roughness length-scale <i>k<sub>s</sub></i> = <i>αx</i> that varies linearly with streamwise distance is first developed, with <i>α</i> a dimensionless constant. For large <i>Re<sub>x</sub></i> and a free-stream velocity <i>U<sub>∞</sub> ~ x<sup>m</sup></i>, a simple log-wake model of the local turbulent mean-velocity profile is used that contains a standard mean-velocity correction for the asymptotic, fully rough regime. A two parameter <i>(α; m)</i> family of solutions is obtained for which <i>U<sub>∞</sub><sup>+</sup></i> (or equivalently <i>C<sub>f</sub></i>) and boundary-layer measures can be calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable <i>z/k<sub>s</sub></i> where z is the wall-normal co-ordinate. Results over a range of <i>α</i> are discussed for cases including the zero-pressure gradient (<i>m = 0</i>) and sink-flow (<i>m = -1</i>) boundary layers. Model trends are supported by high Re wall-modeled LES. Linear streamwise growth of boundary layer measures is confirmed, while for each <i>α</i>, mean-velocity profiles and streamwise turbulent stresses are shown to collapse against <i>z/(αx)</i>. Inner scaled velocity defects are shown to collapse against <i>z/Δ</i>, where <i>Δ</i> is the Rotta-Clauser parameter. The present results suggest that these flows may be interpreted as the fully-rough limit for boundary layers in the presence of small-scale, linear roughness.</p>
<p>Next, an LES study of a flat-plate turbulent boundary layer at high Re under nonequilibrium flow conditions due to the presence of abrupt changes in surface roughness is presented. Two specific cases, smooth-rough (SR) and rough-smooth (RS) transition are examined in detail. Streamwise developing velocity and turbulent stress profiles are considered and sharp departures from equilibrium flow properties with subsequent relaxation are shown downstream. Relaxation trends are studied using integral parameters and higher-order mean flow statistics with emphasis on <i>Re<sub>τ</sub></i> and <i>k<sub>s</sub><sup>+</sup></i> dependence. Results are compared with RS experiments at matched <i>Re<sub>τ</sub></i>, and show good agreement in terms of recovery rates.</p>
<p>Finally, the case of static, impulsive wall-roughness in flows at high <i>Re</i> is addressed using the same LES framework. The initial perturbation from smooth-to-rough appears to dominate the flow behaviour with the length of the impulsive patch showing little effect on recovery rates at matched <i>Re<sub>τ</sub></i> and <i>k<sub>s</sub><sup>+</sup></i>. The resulting trends show good agreement with low Re experiments and support the wall-modeled LES framework as a suitable method for analysing high <i>Re</i> flows in practical applications.</p>https://thesis.library.caltech.edu/id/eprint/11179Oceanic Bottom Boundary Layers and Abyssal Overturning Circulation
https://resolver.caltech.edu/CaltechTHESIS:05302019-121137858
Authors: {'items': [{'email': 'saberruan@hotmail.com', 'id': 'Ruan-Xiaozhou', 'name': {'family': 'Ruan', 'given': 'Xiaozhou'}, 'orcid': '0000-0003-1240-1584', 'show_email': 'NO'}]}
Year: 2019
DOI: 10.7907/6EZA-R251
<p>The vast amount of carbon and heat exchange between the abyssal and upper ocean and subsequently the atmosphere is paced by the abyssal overturning circulation. A key component of the abyssal overturning circulation is the formation and consumption of the densest water mass on Earth, Antarctic Bottom Water (AABW), namely the conversion of North Atlantic Deep Water (NADW) to AABW and the consumption of AABW via small-scale diapycnal mixing. Yet, this pathway of AABW spanning thousands of kilometers has not been successfully reproduced in large-scale general circulation models (GCM). What is missing is essentially the understanding and resolution of small-scale physics involved in converting deep and bottom waters from one density class to another, the water mass transformation (WMT). In this thesis, we focus on small-scale (both in the horizontal and vertical directions) dynamics near the BBL, where enhanced shear, mixing and turbulence exist to facilitate effective WMT above the seafloor.</p>
<p>From high-resolution ocean glider observations around Antarctica, we find that a portion of Lower Circumpolar Deep Water, a branch of NADW, becomes lighter via mixing with light shelf water over the continental slope and shelf, instead of being converted into dense AABW under sea ice. This mixing is likely induced by submesoscale symmetric instability coming from a strong boundary current interacting with the sloping topography in the BBL. We then consider how to sustain the consumption of AABW in the global mid-ocean ridge system. Using numerical models, we show that submesoscale baroclinic eddies are crucial to maintaining strong stratification over the flanks of the mid-ocean ridges and thus permitting effective WMT. Lastly, we consider the interaction between external mean flows and stratified BBL over sloping topography. With the large-scale turbulence resolved in a large-eddy simulation model, we propose a new theoretical framework to describe the evolution of the BBL and the Eulerian advection of its associated stratification when external barotropic flows are present. This new framework can be used to parameterize bottom friction, important for closing the kinetic energy budget of the global ocean. We further extend this interaction to a horizontally-sheared and temporally-oscillating external mean flow and explore the response of the BBL and the BBL-interior mass exchange with simple turbulent parameterizations.</p>
<p>Using a combination of different approaches, we confirm that the long-overlooked oceanic BBL is the key location for closing the abyssal overturning circulation. More importantly, without appropriate techniques to tackle the currently unresolved small-scale processes, they will likely remain a narrow bottleneck in understanding the abyssal overturning circulation.</p>https://thesis.library.caltech.edu/id/eprint/11568Aspects of Reduced-Order Modeling of Turbulent Channel Flows: From Linear Mechanisms to Data-Driven Approaches
https://resolver.caltech.edu/CaltechTHESIS:05282020-161209039
Authors: {'items': [{'email': 'rmcmullen54@gmail.com', 'id': 'McMullen-Ryan-Michael', 'name': {'family': 'McMullen', 'given': 'Ryan Michael'}, 'orcid': '0000-0003-1371-7150', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/wayx-eh75
<p>This thesis concerns three key aspects of reduced-order modeling for turbulent shear flows. They are linear mechanisms, nonlinear interactions, and data-driven techniques. Each aspect is explored by way of example through analysis of three different problems relevant to the broad area of turbulent channel flow.</p>
<p>First, linear analyses are used to both describe and better understand the dominant flow structures in elastoinertial turbulence of dilute polymer solutions. It is demonstrated that the most-amplified mode predicted by resolvent analysis (McKeon and Sharma, 2010) strongly resembles these features. Then, the origin of these
structures is investigated, and it is shown that they are likely linked to the classical Tollmien-Schichting waves.</p>
<p>Second, resolvent analysis is again utilized to investigate nonlinear interactions in Newtonian turbulence. An alternative decomposition of the resolvent operator into Orr-Sommerfeld and Squire families (Rosenberg and McKeon, 2019b) enables a highly accurate low-order representation of the second-order turbulence statistics. The reason for its excellent performance is argued to result from the fact that the decomposition enables a competition mechanism between the Orr-Sommerfeld and Squire vorticity responses. This insight is then leveraged to make predictions about how resolvent mode weights belonging to several special classes scale with increasing Reynolds number.</p>
<p>The final application concerns special solutions of the Navier-Stokes equations known as exact coherent states. Specifically, we detail a proof of concept for a data-driven method centered around a neural network to generate good initial guesses for upper-branch equilibria in Couette flow. It is demonstrated that the neural network is capable of producing upper-branch solution predictions that successfully converge to numerical solutions of the governing equations over a limited range of Reynolds numbers. These converged solutions are then analyzed, with a particular emphasis on symmetries. Interestingly, they do not share any symmetries with the known equilibria used to train the network. The implications of this finding, as well as broader outlook for the scope of the proposed method, are discussed.</p>https://thesis.library.caltech.edu/id/eprint/13730Ultraviolet Radiation of Hypervelocity Stagnation Flows and Shock/Boundary-Layer Interactions
https://resolver.caltech.edu/CaltechTHESIS:02112020-170613058
Authors: {'items': [{'email': 'nelsonyanes135@gmail.com', 'id': 'Yanes-Nelson-Javier', 'name': {'family': 'Yanes', 'given': 'Nelson Javier'}, 'orcid': '0000-0001-8423-6958', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/WYHM-1218
<p>Shock/boundary-layer interactions can induce flow distortion, create flow separation with loss of control authority, and result in high thermal loads. Correct prediction of the flow structure and heating loads is vital for vehicle survival. However, a recent NATO workshop revealed severe underprediction of thermal loads and discrepancies in the location of separation by simulations of high enthalpy air flows. Due to the coupling between thermochemistry and fluid mechanics, a substantial effort has been placed on the development and validation of thermochemical models. As a result, there is a need for experimental data that are more than mean flow surface measurements.</p>
<p>Spatially resolved emission spectra are collected in the post-shock regime of hypervelocity flow over a circular cylinder and a 30-55 degree double wedge. The Hypervelocity Expansion Tube (HET) is used to generate high Mach number, high enthalpy flow (Mach numbers 5 - 7, h₀ = 4 - 8 MJ/kg) with minimal freestream dissociation. The NO γ band (A²Σ⁺ - X²Π) emission is measured in the ultraviolet range of 210-250 nm at downstream locations behind shock waves. Excitation temperatures are extracted from the NO γ emission from spectrum fitting. The result is a temperature relaxation profile that quantifies the state thermal non-equilibrium. Profiles of vibrational band intensity as a function of streamwise distance are used as direct measurements of chemical non-equilibrium in the flow.</p>
<p>Cylinder experiments are performed with varying freestream total enthalpy, Mach number, and test gas O₂ mole fraction to examine changes in relaxation profile. Schlieren images are used to accurately measure standoff distance. Temperature measurements are compared against a zero-dimensional state-to-state model. Strategies for spectrum fitting are presented for cases where the gas is not optically thin and for radiation containing multiple electronic states. For freestream mixtures with reduced oxygen mole fraction, an electronic excitation temperature is required to describe the radiation of the NO γ, β (B²Π - X²Π), and δ (C²Π - X²Π) transitions. The creation of electronically excited NO is discussed in the context of measured vibrational band intensities and computed NO(A) number density profiles using a two-temperature reactive Landau-Teller model.</p>
<p>Emission spectra are collected in the post bow shock and reattachment shock region of hypervelocity flow over a double wedge. High speed schlieren imaging is performed to investigate facility startup effects and for tracking features in a shock/boundary-layer interaction. Detector exposures occur at select times throughout the flow development process to study temporal changes in thermal and chemical non-equilibrium. Time evolution of temperatures at strategic locations of the flow is obtained from spectrum fitting. Two-temperature calculations of the oblique shock system are compared against the emission results. Radiation data are discussed in the context of recent simulation efforts.</p>https://thesis.library.caltech.edu/id/eprint/13637Streamwise Homogeneous Turbulent Boundary Layers
https://resolver.caltech.edu/CaltechTHESIS:06062021-094519451
Authors: {'items': [{'email': 'josephyruan@gmail.com', 'id': 'Ruan-Joseph-Y', 'name': {'family': 'Ruan', 'given': 'Joseph Y.'}, 'orcid': '0000-0002-9110-0458', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/qjfk-5q05
<p>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.</p>
<p>The 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<sub>δ<sup>*</sup></sub>=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 14δ<sub>99</sub> and width 5δ<sub>99</sub>, 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.</p>
<p>Thanks 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.</p>
<p>This methodology has natural extensions to slowly growing boundary layer flows, including thermal boundary layers, rough wall boundary layers and mild pressure gradient flows.</p>https://thesis.library.caltech.edu/id/eprint/14249Thermal Ignition by Vertical Cylinders
https://resolver.caltech.edu/CaltechTHESIS:12182020-055522985
Authors: {'items': [{'email': 'silkenmjones@gmail.com', 'id': 'Jones-Silken-Michelle', 'name': {'family': 'Jones', 'given': 'Silken Michelle'}, 'orcid': '0000-0003-3496-7191', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/9g5j-2b97
<p>Accidental thermal ignition events present a significant hazard to the aviation industry. There is scarcity of experimental data on ignition by external natural convection flows for surface areas larger than 10 cm². In this work, thermal ignition of external natural convection flows by vertical cylinders is investigated. The effect of geometry is studied by resistively heating stainless steel cylinders of various sizes in a stoichiometric n-hexane and air mixture at 298 K and 1 bar. Cylinder lengths range from 12.7 to 25.4 cm, and cylinder surface areas vary from 25 to 200 cm². Logistic regression is used to provide statistical information about the ignition threshold (50% probability of ignition). The maximum ignition threshold found is 1117 K for a cylinder 12.7 cm long and 50 cm² in surface area. The minimum ignition threshold found is 1019 K for a cylinder 25.4 cm long and 200 cm² in surface area. The maximum uncertainty on these ignition thresholds is ±29 K, which comes from the maximum uncertainty on the pyrometer measurement used to record cylinder surface temperatures.
The dependence of ignition threshold on both surface area and length of a cylinder is found to be minor. High speed visualizations of ignition indicated that ignition occurs near the top edge of all cylinders.</p>
<p>The entire experimental setup is heated to allow for ignition tests with multi-component, heavy-hydrocarbon fuels including Jet A and two surrogate fuels, Aachen and JI. The cylinder used for all testing of heavier fuels is 25.4 cm long and 200 cm² in surface area. Hexane is also tested with the heated vessel to investigate the effect of ambient temperature on ignition. At an ambient temperature of 393 K, the ignition threshold of hexane is 933 K. Aachen has an ignition threshold of 947 K at an ambient temperature of 373 K. JI has an ignition temperature of 984 K at an ambient temperature of 393 K. Jet A has an ignition temperature of 971 K at an ambient temperature of 333 K. The maximum uncertainty on these thresholds is ±29 K. JI is found to be the most appropriate surrogate for Jet A.</p>
<p>From the experiments, two main conclusions are reached. Ignition threshold temperatures in external natural convection flows are very weakly correlated with surface area. The observed ignition thresholds do not show the drastic transition of ignition temperature with surface area that is observed in internal natural convection situations. Observed ignition thresholds for comparable surface areas (100 to 200 cm²) are 500 to 600 K higher for external natural convection than internal natural convection. Hexane was found to be a reasonable surrogate for Jet A (38 K difference in ignition threshold) in external natural convection ignition testing. The more complex multi-component JI surrogate, while having an ignition threshold more comparable to Jet A (13 K difference in ignition threshold), requires heating the experimental apparatus and associated difficulties of fuel handling as well as the soot generation by combustion.</p>
<p>Two simplified models of ignition are explored. The first is an investigation of ignition chemistry using a zero-dimensional reactor and a detailed kinetic mechanism for hexane. The temperature history of the reactor is prescribed by an artificial streamline whose rate of temperature increase is parametrically varied. The results from the zero-dimensional reactor computation reveal that a gradually heated streamline exhibits two-stage ignition behavior, while a rapidly heated streamline only experiences one ignition event. The second model of ignition is a one-dimensional simulation of ignition adjacent to a cylinder at a prescribed temperature. The formulation included diffusion of species and thermal energy as well as chemical reaction and employed Lagrangian coordinates. The chemistry is modeled with a reaction mechanism for hydrogen to reduce numerical demand. Heat flux and energy balance are analysed to gain insight into the ignition dynamics. Initially, heat transfer is from the wall into the gas, and a mostly nonreactive thermal boundary layer develops around the cylinder. As reaction in the gas near the surface begins to release energy, the heat transfer decreases, and, near the critical temperature for ignition, the direction of heat flux reverses and is from the gas into the wall. In a case where ignition takes place, there is rapid rise in temperature in the gas within the thermal layer, and a propagating flame is observed to emerge into surrounding cold gas. The heat transfer from the hot combustion products results in a continuous heat flux from the gas into the wall. In a case where ignition does not take place, no flame is observed and the heat flux at the wall is slightly positive. For the critical condition just below the ignition threshold, a balance between energy release and diffusion in the adjacent gas results in a small temperature rise in the thermal layer, but a propagating flame is not created. The Van't Hoff ignition criterion of vanishing heat flux at the ignition threshold is approximately but not exactly satisfied. Contrasting the two modeling ideas, we observe that modeling adiabatic flows along computed nonreactive streamlines is useful in examining the role of detailed chemistry but lacks important diffusion effects. Including mass and thermal transport provides more insight into important ignition dynamics but comes at the expense of increased computational complexity.</p>https://thesis.library.caltech.edu/id/eprint/14034Control of Wall-Bounded Turbulence Through Closed-Loop Wall Transpiration
https://resolver.caltech.edu/CaltechTHESIS:05272021-055610816
Authors: {'items': [{'email': 's.todtli@posteo.de', 'id': 'Toedtli-Simon-Silvio', 'name': {'family': 'Toedtli', 'given': 'Simon Silvio'}, 'orcid': '0000-0001-9371-9572', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/me3y-te05
<p>Many wall-bounded flows of practical relevance are turbulent, including the flows past airplanes and ships. The turbulent motions enhance momentum mixing and, as a result, the drag force on the engineering surface increases, for transportation vessels typically by at least a factor of two compared to laminar flow. Turbulent flow control aimed at drag reduction therefore has the potential to deliver enormous energetic and economic savings, but many challenges remain despite active research for well over a century. The present thesis aims to contribute towards two open questions of the field: first, what are suitable controller design tools for high Reynolds number flows? And second, how does actuation through closed-loop wall transpiration change the flow physics? We investigate aspects of these questions through direct numerical simulation (DNS) and modal analyses of an example control scheme, which is applied to a low Reynolds number turbulent channel flow. The controller is a generalization of the opposition control scheme, and introduces a phase shift between the Fourier transformed sensor measurement and actuator response.</p>
<p>The first part of the thesis demonstrates that a low-order model based on the resolvent framework is able to approximate the drag reduction results of DNS over the entire parameter space considered. The model is about two orders of magnitude cheaper to evaluate than DNS at low Reynolds numbers, and we present a strategy based on subsampling of the wave number space and analytical scaling laws that enables model-based flow control design at technologically relevant Reynolds numbers. The second part of the thesis shows that the physics of the controlled flow can be understood from two distinct families of spatial scales, termed streamwise-elongated and spanwise-elongated scales, respectively. Wall transpiration with streamwise-elongated scales attenuates or amplifies the near-wall cycle and therefore leads to drag reduction or increase, depending on the phase shift. In contrast, wall transpiration with spanwise-elongated scales only leads to drag increase, which occurs at positive phase shifts and is due to the appearance of spanwise rollers which largely enhance momentum mixing. Both patterns are robust features of flows with closed-loop wall transpiration, and the present study offers a simple explanation of their origin in terms of phase relations at distinct spatial scales. The findings of this study may set the stage for a unifying framework for various forms of wall transpiration, and implications for future flow control design are discussed.</p>https://thesis.library.caltech.edu/id/eprint/14179Complexity Reduction of Fluid-Structure Systems at Low Forcing Frequencies
https://resolver.caltech.edu/CaltechTHESIS:05282021-215050622
Authors: {'items': [{'email': 'maysamshamai@gmail.com', 'id': 'Shamai-Maysam', 'name': {'family': 'Shamai', 'given': 'Maysam'}, 'orcid': '0000-0002-1099-1456', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/rhs5-yq49
<p>This thesis addresses complexity reduction in periodic fluid-structure systems at low forcing frequencies. A novel quasi-steady time scaling framework is developed to relate the dynamics of a forced system to a corresponding unforced system. </p>
<p>Particle Image Velocimetry and dye flow visualization are used to study the streamwise-oscillating cylinder's wake at a mean Reynolds number of 900. Forcing frequencies both one and two orders of magnitude below the stationary shedding frequency are considered. Forcing amplitudes are such that the instantaneous Reynolds number remains above the critical value at all times. It is shown that this forcing regime is synonymous with the development of both frequency and amplitude modulation in the wake. While frequency modulation is linked to vortex shedding, amplitude modulation arises due to symmetric reorganization of the wake at certain phases in the forcing cycle. Furthermore, Dynamic Mode Decomposition is used to extract underlying flow structures and quasi-steady time scaling is employed to relate dynamics to the corresponding unforced system. Specifically, forcing regimes where quasi-steady shedding can develop are identified and time is scaled to transform the system to resemble the stationary cylinder at the same mean Reynolds number.</p>
<p>Experimental flowfields are also used to analyze the wake of a surface mounted hemisphere subject to a highly pulsatile freestream, characterized by a forcing amplitude equal to the mean. Although this flow sees regular shedding of hairpin vortices in the unforced case, pulsatile forcing leads to significant deviations. For a nominal mean Reynolds number of 1000, analysis of the wake shows that forcing at a frequency much smaller than that associated with hairpin shedding can lead to frequency modulated shedding. Consequently, time scaling is employed to reduce system complexity associated with hairpin shedding and to relate wake dynamics to the analogous unforced system.</p>https://thesis.library.caltech.edu/id/eprint/14196Part I: The Equations of Plasma Physics and the Richtmyer-Meshkov Instability in Magnetohydrodynamics. Part II: Evolution of Perturbed Planar Shockwaves.
https://resolver.caltech.edu/CaltechTHESIS:10042020-174941725
Authors: {'items': [{'email': 'shennaijian@gmail.com', 'id': 'Shen-Naijian', 'name': {'family': 'Shen', 'given': 'Naijian'}, 'orcid': '0000-0002-0533-8081', 'show_email': 'NO'}]}
Year: 2021
DOI: 10.7907/e9b8-y318
<p>Part I: Mitigating the Richtmyer-Meshkov instability (RMI) is critical for energy production in inertial confinement fusion. Suitable plasma models are required to study the hydrodynamic and electromagnetic interactions associated with the RMI in a conducting medium. First, a sequence of asymptotic expansions in several small parameters, as formal limits of the non-dissipative and non-resistive two-fluid plasma equations, leads to five simplified plasma/magnetohydrodynamics (MHD) systems. Each system is characterized by its own physical range of validity and dispersion relations, and includes the widely used magnetohydrodynamic (MHD) and Hall-MHD equations. Next we focus on the RMI in MHD. Using ideal MHD, it has been shown that the RMI is suppressed by the presence of an external magnetic field. We utilize the incompressible, Hall-MHD model to investigate the stabilization mechanism when the plasma ion skin depth and Larmor radius are nonzero. The evolution of an impulsively accelerated, sinusoidally perturbed density interface between two conducting fluids is solved as a linearized initial-value problem. An initially uniform background magnetic field of arbitrary orientation is applied. The incipient RMI is found suppressed through oscillatory motions of the interface due to the ion cyclotron effect. This suppression is most effective for near tangential magnetic fields but becomes less effective with increasing plasma length scales. The vorticity dynamics that facilitates the stabilization is discussed.</p>
<p>Part II: We consider the evolution of a planar gas-dynamic shock wave subject to smooth initial perturbations in both Mach number and shock shape profile. A complex variable formulation for the general shock motion is developed based on an expansion of the Euler equations proposed by Best [<i>Shock Waves</i>, {1}: 251-273, (1991)]. The zeroth-order truncation of Best's system is related to the well-known geometrical shock dynamics (GSD) equations while higher-order corrections provide a hierarchy of closed systems, as detailed initial flow conditions immediately behind the shock are prescribed. Solutions to Best's generalized GSD system for the evolution of two-dimensional perturbations are explored numerically up to second order in the weak and strong shock limits. Two specific problems are investigated: a shock generated by an impulsively accelerated piston with a corrugated surface, and a shock traversing a density gradient. For the piston-driven flow, it is shown that this approach allows full determination of derivative jump conditions across the shock required to specify initial conditions for the retained, higher-order correction equations. In both cases, spontaneous development of curvature singularity in the shock shape is detected. The critical time at which a singularity occurs follows a scaling inversely proportional to the initial perturbation size. This result agrees with the weakly nonlinear GSD analysis of Mostert <i>et al.</i> [<i>J. Fluid Mech.</i>, {846}: 536-562, (2018)].</p>https://thesis.library.caltech.edu/id/eprint/13974Resolvent Modeling of Turbulent Jets
https://resolver.caltech.edu/CaltechTHESIS:03022021-005902351
Authors: {'items': [{'email': 'ethan.pickering@case.edu', 'id': 'Pickering-Ethan-Marcus', 'name': {'family': 'Pickering', 'given': 'Ethan Marcus'}, 'orcid': '0000-0002-4485-6359', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/szxb-f168
<p>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 <i>resolvent analysis</i> 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.</p>
<p>In 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.</p>
<p>We 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.</p>
<p>Finally, 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.</p>https://thesis.library.caltech.edu/id/eprint/14097On the Experimental Simulation of Atmospheric-Like Disturbances Near the Surface
https://resolver.caltech.edu/CaltechTHESIS:05272022-085410375
Authors: {'items': [{'email': 'cjdougherty211@gmail.com', 'id': 'Dougherty-Christopher-John', 'name': {'family': 'Dougherty', 'given': 'Christopher John'}, 'orcid': '0000-0002-0974-5696', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/293w-ev66
<p>Any and every 'decision-maker'’ gravity-bound to the planetary surface (or very nearly so) must contend with the frictional complexities confined to its relatively small surface layer. From the perspective of the near-surface bound small autonomous flyer, it is the microclimatic local set of atmospheric conditions (i.e. the weather), characterized by moisture, temperature, and the parameters describing wind, that determines the baseline flowfields within which these flyers must navigate and negotiate. Unlike their human-on-board counterparts, mission parameters relegate small (nearly) massless autonomous flyers to the lower regions of the atmospheric boundary layer, where they may not be fortuned enough to soar above the effects of friction or wait for clearer skies. Relatively little focus has been placed on the experimental strategies of how these machines might learn to function in challenging scenarios well-before encountering them in the real-world. To address such shortcomings, this work focuses on the experimental simulation of flight-relevant environments through the development of multi-source wind generating apparatuses (i.e. fan arrays) that can initialize velocity distributions discretely-individually or in-concert to produce appropriate mean and fluctuating velocities through an ample open-air test envelope that enables full-scale conventional statically-mounted aerodynamic-characterizations up through free-flight autonomous vehicle testing. Though outside the scope of current experimental work, as full of an environmental description (i.e. moisture, temperature, and wind) is given as possible, prior to ultimately reducing the scope to a neutrally stable atmosphere devoid of any major weather events other than a reasonably strong prevailing wind. Nearly always set amongst the backdrop of a high Reynolds number turbulent flowfield, two primary prototypical flowfields (continuous-gust and discrete-gust) are identified as meriting consideration for mainstay experimental simulation. The core features within the spectral overlap of these windy disturbance environments with the response characteristics of flyers of interest ensure that the turbulence of consideration is nearly always of the mechanical-type. Unlike air motions far above local effects in the inertial sublayer (ISL), the dominant flow mechanism within regions of interest near canopied surfaces is augmented by the presence of coherent structures due to the prevalence of locally initiated mixing layers and wakes such that the task becomes one of simulation of suitable forcing spectra in the physical domain for the regions of interest during anticipated times-of-flight.</p>
<p>Likely to prove challenging to the small autonomous flyer are encounters of a change in wind state that occur upon piercing the dividing streamline of air masses of two different velocities. From the view of the flyer navigating the built-up environment, intermittent free shear layers due to wind-interactions with surface roughness elements are unavoidable and are experienced discretely when the flyer and shear layer dynamics are decoupled. Fan array techniques for the generation of mixing layers, the basic building block of any such free shear layer, is explored as a candidate flowfield for the experimental simulation of a discrete gust forcing input for the flyer near the surface. Both initialized dual-stream and triple-stream mixing layers at flight-relevant freestream velocity differences are explored and found to principally behave like the mixing layers developed in a more conventional splitterplate experiment. The Reynolds number Re<sub>δ<sub>ω</sub></sub> based on the velocity difference ΔU and vorticity thickness δ<sub>ω</sub> (both outer scale parameters) is shown to linearly increase with downstream development as the vorticity thickness increases commensurately. The spectral analysis along the centerline confirms local isotropy for every tested case.</p>
<p>The continuous-gust flowfield (simply referred to as 'turbulence) is prevalent throughout the atmospheric boundary layer as are quasi-coherent flowfields of superimposed wakes within canopied environments. Because velocity fluctuations manifest as (predominantly) random deviations at any given instant, these flowfields are good candidates for statistical analysis. Generation techniques to produce such turbulent flowfields are introduced and compared against the uniform flow modality (i.e. all fan units set to produce nominally the same initial velocity condition to develop a well-mixed turbulent flowfield beyond x/L ∼ 0.5 with Re<sub>λ<sub>T</sub></sub> = 135). The random-phase (R-P) perturbation technique proves useful in increasing Re<sub>λ<sub>T</sub></sub> upwards of nearly sevenfold with only a slight further-loss-of-uniformity (to within 3.7% of the mean). The uniform flow modality with the (R-P) perturbation activated is shown, through the presence of a -5/3 slope power law region, to be locally isotropic at relevant freestream velocities. Significant increases in Re<sub>λ<sub>T</sub></sub> are made through a static-reconfiguring of the discrete source fan units into a so called quasi-grid (Q-G) configuration. The highest recorded Taylor microscale Reynolds number was found to be Re<sub>λ<sub>T</sub></sub> = 2700, likely accompanied by a non-negligible loss of uniformity at the fixed measurement location, though traverses were not undertaken during this campaign so no direct statement of homogeneity is put forth.</p>
<p>For all the flow modalities presented (i.e uniform, pseudo-random, quasi-coherent, and mixing layer), the high-Re number criteria (Re<sub>δ<sub>ω</sub></sub> ≈ 10<sup>4</sup> , Re<sub>λ<sub>T</sub></sub> ≈ 10<sup>2</sup>) has been met. This serves, then, as a necessary minimum benchmark in the development of multi-source wind tunnels with intended use as environmental simulators for flyers near the surface and also provides the basis for a spectral framework of comparison to enable systematic development of flowfields in future work. Characteristics of the evolving flowfields can further be tuned through the introduction of perturbation techniques applied as initial conditions to both increase the standard deviation of the fluctuating velocities about a desired mean as well as to initiate, evolve, and combine flowfields in representative ways. A preliminary example of one such combination of flow modalities (pseudo-random and mixing layer) indicates significant alteration of flow development compared to a nominal mixing layer case.</p>https://thesis.library.caltech.edu/id/eprint/14636Nonlinear Dynamics and Stability of Viscous Free-Surface Microcapillary Flows in V-Shaped Channels and on Curved Surfaces
https://resolver.caltech.edu/CaltechTHESIS:05292022-001428228
Authors: {'items': [{'email': 'nwhite@posteo.net', 'id': 'White-Nicholas-Conlan', 'name': {'family': 'White', 'given': 'Nicholas Conlan'}, 'orcid': '0000-0002-7603-9329', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/yd3w-ck87
<p>The last two decades have brought a revolution in miniaturization of space technology. Thanks to improved microelectronic sensors and MEMS devices, nanosatellites can perform communication and scientific studies previously limited to large satellites, significantly reducing the financial barriers to space access. But development of a reliable, long-running, small-scale propulsion system for orbital maneuvers remains a key challenge. One solution is the microfluidic electrospray propulsion (MEP) thruster under development at NASA's Jet Propulsion Laboratory (JPL).</p>
<p>This thesis analytically addresses aspects of the MEP system's propellant management, specifically, capillary flow in the groove network delivering fluid propellant from the reservoir to the emitters. Building upon the reduced-order model of viscous capillary flow in straight V-shaped channels ("V-grooves") of Weislogel (1996) and Romero and Yost (1996), we prove stability of steady-state and self-similar flows. Because the MEP design requires an electric field above the grooves, and further calls for grooves which curve and bend in three dimensions, we extend earlier V-groove models to include these effects, and also perform stability analyses of the new models. The results not only validate the use of V-grooves as a robust propellant delivery system, but also provide a theoretical basis for the design of future microfluidic devices with compact, three-dimensional designs and electric fields.</p>
<p>In order to lay the groundwork for future studies of early-time behavior of propellant on emitter tips before the Taylor cone necessary for ion emission is formed, we develop the technique of generalized linear stability analysis (Farrell and Ioannou, 1996) of capillary flow of thin viscous films coating curved surfaces (governed by the equation first developed by Roy and Schwartz, 1997). This methodology was first applied to films coating cylinders and spheres by Balestra et al. (2016, 2018); we instead apply the technique and analyze for the first time a viscous-capillary instability arising on a torus coated with a uniform thin film.</p>
<p>Besides the capillary fluid dynamics results, two additional pieces of work are included in the thesis. First, in an unorthodox application of Noether's Theorem to non-Lagrangian gradient flow equations, we show that each variational symmetry of the governing functional induces a constraint on the evolution of the system. Second, to support JPL's efforts to directly detect a "fifth force," we introduce and implement numerical methods for computation of the scalar Cubic Galileon Gravity (CGG) field at solar system scales.</p>https://thesis.library.caltech.edu/id/eprint/14648Characterization and Optimization of a Fully Passive Flapping Foil in an Unsteady Environment for Power Production and Propulsion
https://resolver.caltech.edu/CaltechTHESIS:05312022-024822211
Authors: {'items': [{'email': 'morglhooper@gmail.com', 'id': 'Hooper-Morgan-Louise', 'name': {'family': 'Hooper', 'given': 'Morgan Louise'}, 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/tymc-1985
<p>This thesis provides an experimental window into the duality between thrust production and energy harvesting by a flapping foil subject to unsteadiness in an oncoming flow. In particular, an airfoil is placed downstream of a circular cylinder, and allowed to interact with the vorticity shed in its wake to produce motions in both the transverse and streamwise directions. It is confirmed that under the right conditions, passive fluid-structure interactions arising from such a configuration give rise to simultaneous extraction of energy from the flow, coupled with net thrust larger than net drag experienced by the airfoil.</p>
<p>Measurements of forces acting on the airfoil and the motion that arises are presented, for cases where the flapping motion is both active (the foil is driven through a pre-planned trajectory) and fully passive (the foil is allowed to react to the fluid forcing it experiences). These are coupled with simultaneous Particle Image Velocimetry (PIV) measurements of the flow field in the region of the airfoil. These measurements allow for direct observation of fluid-structure interactions which give rise to both thrust production and power extraction potential, illuminating the mechanisms driving each. The dynamics of a fully passive flapping foil are largely determined by the mounting system used to facilitate its motion. It is shown that by leveraging Cyber-Physical Fluid Dynamics (CPFD) capabilities to tune these mounting parameters, the behaviour of a fully passive flapping foil can be made similar to that of a representative driven system. A framework based on a simplified linear model for mounting system dynamics is presented, to allow for the optimization of such a system for power extraction potential subject to relevant engineering constraints. The effects of nonlinearity on airfoil behaviour, particularly those due to friction in the mechanism(s) permitting passive flapping, are also explored. Finally, two-dimensional motion of a fully passive flapping foil is demonstrated, allowing for the foil to travel upstream against the oncoming flow solely due to forces induced by interactions with oncoming unsteadiness.</p>https://thesis.library.caltech.edu/id/eprint/14653Rheological Measurements in Moderate Reynolds Number Liquid-Solid Flows
https://resolver.caltech.edu/CaltechTHESIS:06062022-033735914
Authors: {'items': [{'email': 'jqsongyichuan@gmail.com', 'id': 'Yichuan-Song', 'name': {'family': 'Song', 'given': 'Yichuan'}, 'orcid': '0000-0001-7276-2029', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/5ey8-v324
<p>Liquid-solid flows with inertial and viscous effects are critical for many engineering and geophysical applications, such as the processing of biomass slurry and the control of debris flows. However, modeling the rheological behaviors of these complex flows remains a challenge. Prior investigations on the liquid-solid flows typically cover suspensions in which the particle Reynolds numbers (<i>Re</i>) based on the particle diameter and shear rate are less than 1. Limited prior study at Caltech focuses on particle Reynolds numbers above 10. This thesis focuses on rheological experiments for the moderate Reynolds number regime where both inertial and viscous effects are important, with particle Reynolds numbers from 0.5 to 800. The rheological experiments include torque measurements of <i>mm</i> scale-sized polystyrene and SAN particles with a range of solid fractions from 10% to 50%, considering both neutrally-buoyant and settling suspensions with density ratios of 1 and 1.05. This thesis discusses rheological measurements of three different fields: pure fluids, neutrally-buoyant suspensions, and non-neutrally-buoyant suspensions.</p>
<p>The pure fluids measurements determine the flow starts to transition to turbulent flow for gap Reynolds numbers above 6500 in the Caltech Couette flow device. For suspensions with matched particle and fluid densities and solid fractions less than 40%, we find that the effective viscosity only depends on the particle solid fraction until we observe the shear-thickening behaviors for <i>Re</i> of approximately 10. For the intermediate <i>Re</i> from 10 to 100 and lower solid fractions, the effective viscosity not only depends on the particle solid fraction, but also shows increased dependence on <i>Re</i>. For <i>Re</i> greater than 100, the liquid-solid flows transition to the turbulent regime, similar to what we see for the pure fluids. At the maximum solid fraction of 50%, the magnitude of the effective viscosity has increased by a factor of 20 as compared to the results of the 10% solid fraction, but the effective viscosity is nearly independent of <i>Re</i>. A particle Reynolds number (<i>Re'</i>) based on the maximum shear flow velocity and the particle diameter is introduced to examine the effective viscosity of the suspensions. Since the present studies use particles with different sizes, <i>Re'</i> is found to be a better way to correlate the effective viscosity than the traditional <i>Re</i>. For the analysis of liquid-solid flows with a density ratio of 1.05, the effective viscosity of the particulate flow increases with the Stokes number for loading fractions of 10% and 20%, while the dependence is reversed for higher solid fractions.</p>https://thesis.library.caltech.edu/id/eprint/14948On the Variational Principles of Linear and Nonlinear Resolvent Analysis
https://resolver.caltech.edu/CaltechTHESIS:03222022-135834919
Authors: {'items': [{'email': 'bhhbarthel@gmail.com', 'id': 'Barthel-Benedikt', 'name': {'family': 'Barthel', 'given': 'Benedikt'}, 'orcid': '0000-0002-6890-5047', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/sy44-d841
<p>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.</p>https://thesis.library.caltech.edu/id/eprint/14520Rheological Characterization of Polymer Additives for Mist Control and Drag Reduction
https://resolver.caltech.edu/CaltechTHESIS:05262022-231652129
Authors: {'items': [{'email': 'rlhota13@gmail.com', 'id': 'Lhota-Red-C', 'name': {'family': 'Lhota', 'given': 'Red C.'}, 'orcid': '0000-0002-8481-3716', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/wav1-4t47
<p>Long flexible polymers in solution at low concentrations strongly change the extensional properties of fluids due to chain stretching that resists flow, while their compact conformation in shear has weak effects. This dramatic difference between their effects on extension and shear is desirable in a variety of applications--controlling drop size in sprayed mists, reducing drag in turbulent flow, and preventing rebound in drop impact. Traditional long covalent polymers, however, are not practical in many applications because they undergo mechanical degradation, i.e. chain scission, under strong flow conditions. Megasupramolecular polymer systems, consisting of long end-associative telechelic polymers that assemble in solutions into multi-million molecular weight supramolecules, meet this practical need. Through association, they act like traditional covalently-bonded polymers in extension, while reversibly dissociating under the strong flows that cause scission for those long polymers.</p>
<p>This thesis examines the interplay of flow and degradation that imposes an upper-bound on useful lengths of invididual end-associative chains (how long is too long) (Chapters 2 and 3); the quiescent coil size that affects the onset of stretching in fluids of interest (water and polyalphaolefin lubricant) (Chapters 2 and 4);
rheological approachs to detect variations in the degree of end-functionalization that affect formation of ultra-long supramolecules (Chapter 5); and the changes to turbulent flow when long polymers are present at low concentration (Chapter 2). Ultimately, the audience who might enjoy this thesis is limited by barriers of rheological jargon. In the pursuit of broader rheological and overall scientific understanding, I describe evidence-based pedagogical techniques and my approach to implementing them in chemical engineering and polymer physics classrooms (Chapter 6).</p>https://thesis.library.caltech.edu/id/eprint/14630Instabilities in the Flow Over a Spinning Disk at Angle of Attack
https://resolver.caltech.edu/CaltechTHESIS:09282021-234035965
Authors: {'items': [{'email': 'marcusklee93@gmail.com', 'id': 'Lee-Marcus-Kuok-Kuan', 'name': {'family': 'Lee', 'given': 'Marcus Kuok Kuan'}, 'orcid': '0000-0003-3972-843X', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/kmhn-7e49
<p>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(10<sup>2</sup>) and tip-speed ratios (non-dimensional spin rate) up to 3. </p>
<p>At 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.</p>https://thesis.library.caltech.edu/id/eprint/14377Dynamics and Performance of Wind-Energy Systems in Unsteady Flow Conditions
https://resolver.caltech.edu/CaltechTHESIS:06012023-233342281
Authors: {'items': [{'email': 'nwei@alumni.princeton.edu', 'id': 'Wei-Nathaniel-James', 'name': {'family': 'Wei', 'given': 'Nathaniel James'}, 'orcid': '0000-0001-5846-6485', 'show_email': 'NO'}]}
Year: 2023
DOI: 10.7907/d9wh-pj98
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.https://thesis.library.caltech.edu/id/eprint/15270Linear Amplification in Nonequilibrium Turbulent Boundary Layers
https://resolver.caltech.edu/CaltechTHESIS:08312023-005517217
Authors: {'items': [{'email': 'rey.gomez@berkeley.edu', 'id': 'Gomez-De-La-Cruz-Salvador-Rey', 'name': {'family': 'Gomez De La Cruz', 'given': 'Salvador Rey'}, 'orcid': '0000-0002-7568-721X', 'show_email': 'YES'}]}
Year: 2024
DOI: 10.7907/hn98-c285
<p>Resolvent analysis is applied to nonequilibrium incompressible adverse pressure gradient (APG) turbulent boundary layers (TBL) and hypersonic boundary layers with high temperature real gas effects, including chemical nonequilibrium. Resolvent analysis is an equation-based, scale-dependent decomposition of the Navier Stokes equations, linearized about a known mean flow field. The decomposition identifies the optimal response and forcing modes, ranked by their linear amplification. To treat the nonequilibrium APG TBL, a biglobal resolvent analysis approach is used to account for the streamwise and wall-normal inhomogeneities in the streamwise developing flow. For the hypersonic boundary layer in chemical nonequilibrium, the resolvent analysis is constructed using a parallel flow assumption, incorporating N₂, O₂, NO, N, and O as a mixture of chemically reacting gases.</p>
<p>Biglobal resolvent analysis is first applied to the zero pressure gradient (ZPG) TBL. Scaling relationships are determined for the spanwise wavenumber and temporal frequency that admit self-similar resolvent modes in the inner layer, mesolayer, and outer layer regions of the ZPG TBL. The APG effects on the inner scaling of the biglobal modes are shown to diminish as their self-similarity improves with increased Reynolds number. An increase in APG strength is shown to increase the linear amplification of the large-scale biglobal modes in the outer region, similar to the energization of large scale modes observed in simulation. The linear amplification of these modes grows linearly with the APG history, measured as the streamwise averaged APG strength, and relates to a novel pressure-based velocity scale.</p>
<p>Resolvent analysis is then used to identify the length scales most affected by the high-temperature gas effects in hypersonic TBLs. It is shown that the high-temperature gas effects primarily affect modes localized near the peak mean temperature. Due to the chemical nonequilibrium effects, the modes can be linearly amplified through changes in chemical concentration, which have non-negligible effects on the higher order modes. Correlations in the components of the small-scale resolvent modes agree qualitatively with similar correlations in simulation data.</p>
<p>Finally, efficient strategies for resolvent analysis are presented. These include an algorithm to autonomously sample the large amplification regions using a Bayesian Optimization-like approach and a projection-based method to approximate resolvent analysis through a reduced eigenvalue problem, derived from calculus of variations.</p>https://thesis.library.caltech.edu/id/eprint/16170