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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 15:27:04 +0000Experimental Investigation of the Effect of Cavitation on the Rotordynamic Forces on a Whirling Centrifugal Pump Impeller
https://resolver.caltech.edu/CaltechETD:etd-02022007-133417
Authors: {'items': [{'email': 'rfranz@energent.net', 'id': 'Franz-Ronald-John', 'name': {'family': 'Franz', 'given': 'Ronald John'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/5JXT-CZ84
The interaction of a rotating impeller and the working fluid introduce forces on the rotor. These fluid-induced forces can cause self-excited whirl, where the rotor moves away from and whirls along a trajectory eccentric to its undeflected position. When designing a turbomachine, particularly one which is to operate at high speed, it is important to be able to predict the fluid-induced forces, both steady and unsteady, acting on the various components of the machine. The fluid-induced rotordynamic forces acting upon the impeller and therefore on the bearings was investigated for a centrifugal impeller in a spiral volute in the presence of cavitation.
An experiment in forced vibration was made to study the fluid-induced rotordynamic force on an impeller whirling around its machine axis of rotation in water. The whirl trajectory of the rotor is prescribed to be a circular orbit of a fixed radius. A dynamometer mounted behind the rotor and rotating with it measures the force on the impeller. The force measured is a combination of a steady radial force due to volute asymmetries and an unsteady force due to the eccentric motion of the rotor. These measurements have been carried out over a full range of whirl/impeller speed ratios at different flow coefficients for various turbomachines. A destabilizing force was observed over a region of positive whirl ratio. Cavitation corresponding to a three percent head loss did not have a significant effect upon this destabilizing force. However, a lesser degree of cavitation at the design point for the impeller-volute combination tested increased this destabilizing force for a particular set of whirl ratioshttps://thesis.library.caltech.edu/id/eprint/467Experimental and Analytical Investigations of Granular Materials: Shear Flow and Convective Heat Transfer
https://resolver.caltech.edu/CaltechETD:etd-09152004-135555
Authors: {'items': [{'email': 'hojinahn@bigfoot.com', 'id': 'Ahn-Hojin', 'name': {'family': 'Ahn', 'given': 'Hojin'}, 'show_email': 'NO'}]}
Year: 1989
DOI: 10.7907/XFYT-7909
<p>Granular materials flowing down an inclined chute were studied experimentally and analytically. Characteristics of convective heat transfer to granular flows were also investigated experimentally and numerically.</p>
<p>Experiments on continuous, steady flows of granular materials in an inclined chute were conducted with the objectives of understanding the characteristics of chute flows and of acquiring information on the rheological behavior of granular material flow. Two neighboring fibre optic displacement probes were employed to measure mean velocity, one component of velocity fluctuations, and linear concentration at the wall and free surface boundaries. A shear gauge was also developed to make direct measurement of shear stress at the chute base. Measurements of solid fraction, velocity, shear rate, and velocity fluctuations were analyzed to understand the chute flow characteristics, and the rheological behavior of granular materials was studied with the present experimental data. The vertical profiles of mean velocity, velocity fluctuation, and solid fraction were also obtained at the sidewalls.</p>
<p>Existing constitutive equations and governing equations were used to solve for fully developed chute flows of granular materials, and thus the boundary value problem was formulated with two parameters (the coefficient of restitution between particles, and the chute inclination) and three boundary values at the chute base wall (the values of solid fraction, granular temperature, and mean velocity at the wall). The boundary value problem was numerically solved by the "shooting method." The boundary conditions at the free surface were satisfied by the proper choice of a gradient of granular temperature at the wall. The results show a significant role played by granular conduction in determining the profiles of granular temperature, solid fraction, and mean velocity in chute flows. These analytical results were also compared with the present experimental measurements and with the computer simulations by other investigators in the literature.</p>
<p>Experiments on heat transfer to granular flows over a flat heating plate were conducted with three sizes of glass beads, polystyrene beads, and mustard seeds. A modification on the existing model for the convective heat transfer was made using the effective Nusselt number and the effective Peclet number, which include the effects of solid fraction variations. The slightly modified model could describe the heat transfer characteristics of both fast and slow flows (supercritical and subcritical flows).</p>
<p>A numerical analysis of the convective heat transfer to granular flows was also performed. The results were compared with the present experimental data, and reasonable agreement was found in the comparison.</p>https://thesis.library.caltech.edu/id/eprint/3544Inorganic Components of Atmospheric Aerosols
https://resolver.caltech.edu/CaltechETD:etd-07172007-083859
Authors: {'items': [{'email': 'aswexler@ucdavis.edu', 'id': 'Wexler-Anthony-Stein', 'name': {'family': 'Wexler', 'given': 'Anthony Stein'}, 'show_email': 'YES'}]}
Year: 1991
DOI: 10.7907/EYF6-3M05
<p>The inorganic components comprise 15% to 50% of the mass of atmospheric aerosols and, these along with the relative humidity, control the aerosol water content. For about the past 10 years the mass of the inorganic components of atmospheric aerosol was predicted assuming thermodynamic equilibrium between the volatile aerosol-phase inorganic species, NH<sub>4</sub>NO<sub>3</sub> and NH<sub>4</sub>Cl, and their gas-phase counterparts, NH<sub>3</sub>, HNO<sub>3</sub>, and HCl. In this thesis I examine this assumption and prove that 1) the time scales for equilibration between the gas and aerosol phases are often too long for equilibrium to hold, and 2) even when equilibrium holds, transport considerations often govern the size distribution of these aerosol components.</p>
<p>Water can comprise a significant portion of atmospheric aerosols under conditions of high relative humidity, whereas under conditions of sufficiently low relative humidity atmospheric aerosols tend to be dry. The deliquescence point is the relative humidity where the aerosol goes from a solid dry phase to an aqueous or mixed solid-aqueous phase. Previous to this thesis little had been known about the temperature and composition dependence of the deliquescence point. In this thesis I first derive an expression for the temperature dependence of the deliquescence point and then prove that in multicomponent solutions the deliquescence point is lower than in the deliquescence point of the individual single component solutions.</p>
<p>These theories of the transport, thermodynamic, and deliquescent properties of atmospheric aerosols are integrated into an aerosol inorganics model, AIM. The equilibrium predictions of AIM compare well to fundamental thermodynamic measurements. Comparison of the prediction of AIM to those of other aerosol equilibrium models show substantial disagreement in the predicted water content at lower relative humidities. The difference is due to the improved treatment of the deliquescence properties of mixed solute aerosols that is contained in AIM.</p>
<p>In the summer and fall of 1987 the California Air Resources Board conducted the Southern California Air Quality Study, SCAQS. During this study the atmospheric aerosols were measured at nine sites in the Los Angeles air basin. The measurements determined the size and composition distributions of the components of the aerosol and the concentrations of their gas phase counterparts during a series of intensive study periods. The comparison of these SCAQS measurements to the predictions of AIM have so much scatter that a departure from equilibrium, that can be attributed to transport limitations, cannot be discerned. When the measured size distributions are compared as another indication of transport-limited departure from equilibrium, we find that different size aerosol particles are not in mutual equilibrium. Although the SCAQS data do not indicate a transport-limited departure from equilibrium, they do support our hypothesis that transport considerations are essential to predicting the size distribution of the volatile inorganic species.</p>https://thesis.library.caltech.edu/id/eprint/2910A computational model for low-pressure diamond synthesis
https://resolver.caltech.edu/CaltechTHESIS:04132011-093359207
Authors: {'items': [{'id': 'Gavillet-G-G', 'name': {'family': 'Gavillet', 'given': 'Guillaume G.'}, 'show_email': 'NO'}]}
Year: 1991
DOI: 10.7907/phqr-a780
No abstract.https://thesis.library.caltech.edu/id/eprint/6320Axisymmetric Buoyant Jets in a Cross Flow with Shear: Transition and Mixing
https://resolver.caltech.edu/CaltechETD:etd-08272007-090225
Authors: {'items': [{'email': 'dugan.regina.e@gmail.com', 'id': 'Dugan-Regina-E', 'name': {'family': 'Dugan', 'given': 'Regina E.'}, 'show_email': 'NO'}]}
Year: 1993
DOI: 10.7907/1jmz-kj71
<p>It has been proposed that axisymmetric buoyant jets discharged vertically into a horizontal turbulent boundary layer flow undergo a transition from self-induced mixing to an ultimate state where mixing is dominated by the shear-flow turbulence. Both plume mixing and ambient shear-flow mixing have been separately well characterized by many previous studies and can be thought of as asymptotic mixing regimes. This investigation focuses on the transition between the two asymptotic regimes that is not well understood and that is often of particular engineering interest.</p>
<p>In this work, we present the results of a detailed experimental analysis of buoyant jet mixing in a turbulent shear flow. Our purpose is to obtain a detailed picture of the turbulent velocity field and the concentration distributions throughout the various mixing regimes in order to discern the effects of changes in various flow parameters on the predominant mixing mechanisms. The experimental technique employs buoyant jets whose fluid is optically homogeneous with that of the ambient shear flow. This enables the combined use of laser-Doppler velocimetry and laser-induced fluorescence to measure the velocity and concentration profiles, respectively.</p>
<p>Dimensional analysis indicates that the cross-flow shear velocity and the plume specific buoyancy flux are the parameters controlling the transition from plume mixing to diffusion mixing. Quantitative analysis of the experimental results indicates that the mixing is dominated entirely by diffusion, or shear-flow mixing, even close to the point of discharge. Further, we observe that within the diffusive mixing regime, a transition occurs from a region where the turbulent mixing coefficient is proportional to the local elevation to a region where the turbulent mixing coefficient is proportional to the boundary layer thickness. Detailed instantaneous spatial concentration distributions indicate that regions of dilution far below mean values persist well into the mixing regime dominated by shear-flow turbulence. This indicates that both plume mixing and diffusion-type mixing models may provide a false sense of security with regard to the absolute minimum dilutions observed in actual flow situations since both methods focus on the minimum average dilution.</p>https://thesis.library.caltech.edu/id/eprint/3239Shear-induced transport properties of granular material flows
https://resolver.caltech.edu/CaltechETD:etd-08292007-090134
Authors: {'items': [{'id': 'Hsiau-S', 'name': {'family': 'Hsiau', 'given': 'Shu-San'}, 'show_email': 'NO'}]}
Year: 1993
DOI: 10.7907/yj3a-tm14
A granular flow is a two-component flow with an assembly of discrete solid particles dispersed in a fluid. Because of the similarity between the random motion of particles in a granular flow and the motion of molecules in a gas, the dense-gas kinetic theory has been broadly employed to analyze granular flows. However, most research only discusses aspects of momentum transport; three issues have received less attention: the diffusion process, the heat transfer problem, and the behavior of binary mixtures. The current research emphasizes these aspects.
A granular flow diffusion experiment was conducted in a vertical channel to investigate the effects that result in mixing of the material. The mean velocities, the longitudinal fluctuating velocities, and the mixing-layer thickness were measured. A simple analysis based on the diffusion equation shows that the thickness of the mixing layer increases with the square-root of downstream distance and depends on the magnitude of the velocity fluctuations relative to the mean velocity. The experimental velocity profiles were also compared with profiles calculated from theoretical analysis based on kinetic theory.
The analytical relations were developed for the flow-induced particle diffusivity and the thermal conductivity based on dense-gas kinetic theory. The two coefficients were found to increase with the square-root of the granular temperature, a term that quantifies the specific kinetic energy of the flow. The theoretical particle diffusivity was used to compare with the current experimental measurements involving the granular flow mixing layer. The analytical expression for the effective thermal conductivity was also compared with experimental measurements. The differences between the predictions and the measurements suggest limitations in some of the underlying kinetic-theory assumptions.
The constitutive relations were presented for a binary-mixture of granular materials as derived from the revised Enskog theory. The current research focuses on the process of granular thermal diffusion - a diffusion process resulting from the granular temperature gradient. A granular flow of binary-mixtures in an oscillatory no-flow system, in a sheared system, and in a vertical channel were examined, and indicated a complete segregation when granular thermal diffusion effect was sufficiently large.https://thesis.library.caltech.edu/id/eprint/3267Supersonic Film Cooling Including the Effect of Shock Wave Interaction
https://resolver.caltech.edu/CaltechETD:etd-12112007-084103
Authors: {'items': [{'email': 'khalid@alumni.caltech.edu', 'id': 'Al-Juhany-Khalid-Ahmed-Bin-Talal', 'name': {'family': 'Al Juhany', 'given': 'Khalid Ahmed Bin Talal'}, 'show_email': 'NO'}]}
Year: 1994
DOI: 10.7907/7ry6-fy87
<p>The current work is an investigation of supersonic film cooling effectiveness including interactions with a two-dimensional shock wave. Air and helium, which are either heated or cooled, are injected at Mach numbers between 1.2 and 2.2 into a Mach 2.4 air freestream. The adiabatic wall temperature is measured directly. The injection velocity and mass flux are varied by changing the total temperature and Mach number while maintaining matched pressure conditions.</p>
<p>Heated injection, with the injectant to freestream velocity ratios greater than 1, exhibit a rise in wall temperature downstream of the slot yielding effectiveness values greater than one. The temperature rise, which also occurs for cooled injection, is attributed to the merging of the injectant boundary layer and the lip-wake. As a result comparisons between heated and cooled injection may not be valid. With the exception of heated helium runs, larger injection Mach numbers slightly increase the effective cooling length per mass injection rate. The results for helium injection indicate an increase in effectiveness as compared to that for air injection. The experimental results are compared with studies in the literature.</p>
<p>Flow profiles at several axial locations, up to 90 slot heights, indicate that for the same Mach number the helium injections induce a larger wake and a thicker boundary layer than air injection.</p>
<p>The influence of the shock impingement on the recovery temperature is not large if the flow remains attached. Once separation occurs the temperature changes drastically with downstream distance. The shock strength for incipient separation is smaller when helium is injected than when no film coolant is present. However, the converse is true with air injection even though, for the same Mach number, the momentum flux for the air injection is less than that for the helium injection. The induced separation in the case of helium is attributed to the reduced fullness of its momentum flux profile prior to interaction. This research demonstrates how the performance of supersonic film cooling for thermal control is undermined by the susceptibility to shock induced separation, and raises concerns about hydrogen film cooling for N.A.S.P.</p>https://thesis.library.caltech.edu/id/eprint/4945The study of Taylor-Couette flows with superimposed isothermal and heated axial flows at high Taylor numbers
https://resolver.caltech.edu/CaltechETD:etd-12112007-104052
Authors: {'items': [{'id': 'Shih-A-C', 'name': {'family': 'Shih', 'given': 'Angela Chao-Hsuan'}, 'show_email': 'NO'}]}
Year: 1994
DOI: 10.7907/3r06-hm27
This experimental study investigates the effect of an isothermal or heated superimposed axial flow on a Taylor-Couette flow in an open, vertical annulus with the inner cylinder rotating. The tangential component of the velocity is measured using a hot-wire anemometer, and the velocity power spectra are calculated. The flows studied are for Taylor numbers ranging from 1.2 x 10[superscript 7] to 2.4 x 10 [superscript 7], and the axial Reynolds number from 0 to 2500. At a low axial Reynolds number, the power spectrum of the velocity measurements shows a single dominant frequency. The frequency is indicative of the uniformly-spaced vortices passing through the anemometer, and roughly corresponds with the axial velocity divided by the vortex spacing. As the rotational speed is increased at a fixed axial flow rate, the dominant frequency decreases, indicating a change in the size of the vortices. As the axial Reynolds number is increased at a fixed rotational speed, the power spectra first indicate a decrease in the dominant frequency, and then a subsequent increase in the other frequencies. For very large axial flow rates, the power spectra indicate a broad distribution in frequencies.
The experiment also include the measurements of the transient and the local fluid temperatures, and the corresponding temperature spectra are calculated. Heating of the axial flow also changes the characteristic of the velocity spectra, where peaks at higher frequencies emerge in the spectra. In heated flows, the peaks of the greatest spectral strength in the velocity and temperature spectra are different, possibly indicating that the largest temperature and velocity fluctuations occur in different directions. The average temperature measurements indicate that as the axial flow rate is increased, the mean temperature distribution curves shift upward. The temperature ratio, (T[subscript max] - T[subscript min])/(T[subscript in] - T[subscript out]), also increases with an increasing in the axial Reynolds numbers.
https://thesis.library.caltech.edu/id/eprint/4947An experimental and numerical investigation into reacting vortex structures associated with unstable combustion
https://resolver.caltech.edu/CaltechETD:etd-10122007-131523
Authors: {'items': [{'email': 'dkendrick@leanflame.com', 'id': 'Kendrick-D-W', 'name': {'family': 'Kendrick', 'given': 'Donald William'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/9QDG-AP23
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
An experimental and numerical investigation into reacting vortex structures shed from a rearward facing step flameholder was performed to gain insight into the fundamental reasons why certain acoustic modes of the laboratory dump combustor were excited for a given set of flow parameters. Cases examined used premixed [...] air mixtures [...], various duct heights (2.54, 5.08 and 7.62 cm) and dump plane speeds (21, 30 and 35 m/s). The above parameters permitted observing instabilities having either one or both of the longitudinal acoustic modes present (188 or 234 Hz) in their respective pressure and velocity spectra.
Ignition of the vortex structures was found to be heavily dependent on geometry (i.e., duct height) and invariant to stoichiometric variations. This fact indicated the dominance of turbulent exchange processes over chemical effects for the pulsating flowfield. The coherent structures which typically convected at the local dump plane speed and exhibited high initial strain rates, were found to exhibit shorter burning times and more intense combustion for decreasing duct heights. Use of high-speed shadowgraph and chemiluminescent (CCD) imagery permitted a complete description of the typically nonuniform, reacting flowfield. Time-resolved vortex and floor temperature measurements as well as time-averaged floor heat flux measurements completed the quantitative description of the vortex structures.
Culick's technique of expanding the acoustic filed into orthogonal modes was employed to confirm mode selection theories and suggest the importance of the shape of the average burner distribution. A nonlinear heat release model was formulated whereby the vortices were characterized as gaussian envelopes convecting at the local dump plane speed. The system of equations formulated was a set of two coupled oscillators with a nonlinear driving term. A final discussion was also undertaken to infer the geometrical implications into the mode selection process (what system acoustic mode was excited).
https://thesis.library.caltech.edu/id/eprint/4056A Study of Tip Vortices and Cavitation on a Propeller in a Non-Uniform Flow Field
https://resolver.caltech.edu/CaltechETD:etd-03262007-131335
Authors: {'items': [{'email': 'mckenn1@earthlink.net', 'id': 'McKenney-Elizabeth-Anne', 'name': {'family': 'McKenney', 'given': 'Elizabeth Anne'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/7THC-NG74
Unsteady lifting surface flows are important subjects for study, both for the purposes of improving propulsive or lifting efficiency and also for mitigating the destructive effects and noise caused by cavitation. Some progress may be made by selecting a simple type of unsteadiness for closer study. In the present work, this tactic was implemented in two ways: the operation of a propeller at an angle of yaw to the freestream and the pitching oscillation of a finite-span hydrofoil.
A new facility was designed and constructed to set a propeller at an angle of yaw to the freestream, creating a fairly simple non-uniformity in the propeller inflow. Tip vortex cavitation inception measurements were made for a range of yaw angles and freestream velocities, and photographs of the cavitation were taken to illustrate the effects of the yaw angle.
The unsteady tip vortex flow field was measured on an oscillating finite aspect ratio hydrofoil using Particle Image Velocimetry (PIV), revealing how the circulation varied during a typical oscillation cycle. The results were compared with unsteady infinite-span theory, and also with recent measurements using LDV techniques on the same foil.
The hydrofoil was also the focus of a study of surface cavitation. High-speed motion pictures of the cavitation cycle helped to separate the process into its component stages, and variations with cavitation number and reduced frequency of oscillation were observed. The acoustic signals generated by the cavity collapse were correlated with the motion pictures, providing insights into the correspondence between the flow structures involved in the cavity collapse process and the sound generated by them.
The results from these studies provide valuable insights into the effects of unsteadiness in lifting surface flows.
https://thesis.library.caltech.edu/id/eprint/1145Response Control of Structural Systems Using Semi-Actively Controlled Interactions
https://resolver.caltech.edu/CaltechTHESIS:01032013-113224489
Authors: {'items': [{'id': 'Hayen-Jeffrey-Clyde', 'name': {'family': 'Hayen', 'given': 'Jeffrey Clyde'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/2c70-e383
<p>The objective of the research described herein is to demonstrate conditions under which controlled interactions between two structures or structural components can be made effective in reducing the response of structures that are subjected to seismic excitation. It is shown that the effectiveness depends upon such factors as the control
strategy implementation, the interaction element mechanical properties, and the parameters which characterize the dynamic behavior of the structural systems.</p>
<p>A study is conducted to examine the performance of a structural response control approach referred to as Active Interface Damping (AID). This control approach utilizes
controlled interactions between two distinct structural systems or different components of a single structural system to reduce the resonance buildup that develops
during an external excitation. Control devices or elements may be employed to physically produce the interactions between the systems. The proposed control approach
differs from some other control approaches in that the sensors, processors, and switching components all operate actively, whereas the interaction elements function passively. The major advantage of this semi-active control technology is that relatively large control forces can be generated with minimal power requirements, which is of prime importance for the control of relatively massive systems, such as structures.</p>
<p>In the most simple form, the strategy of the control approach is to remove energy associated with vibration from only one system (the primary system). This process is
accomplished through the transfer of energy to another system (the auxiliary system) by means of interaction elements, the dissipation of energy directly in the interaction elements, or a combination of both these methods. In a more complex form, the control strategy may be to minimize some composite response measure of the combined primary-auxiliary system. Only the most simple form of the control strategy is considered in the present study.</p>
<p>Several physical interpretations of the control approach are possible: one is that the systems represent two adjacent multi-story buildings; another is that the primary system represents a single multi-story building, while the auxiliary system could represent either an externally- situated resilient frame or a relatively small, unrestrained mass - or even be completely absent (in this latter scenario, the interaction elements are internally-mounted control devices). The interactions consist of reaction forces that are developed within and transmitted through the elements which are located between the two systems (or different points of a single system). The mechanical properties of these elements can be altered in real time by control signals, so the reaction forces applied to the systems may be changed, and the response control objective is achieved by actively changing the
interactions at the interface of the two systems (or different points of a single system).</p>
<p>Initially, a preliminary study of the proposed control approach is conducted within the specialized setting of linear single-degree-of-freedom (SDOF) primary and auxiliary
systems. Numerical simulations are performed for a series of control cases using horizontal ground accelerations from an ensemble of earthquake time histories as excitation input. Subsequently, a follow-on study of the proposed control approach is conducted for linear multiple-degree-of-freedom (MDOF) primary and auxiliary systems intended to represent actual structural systems. Based upon the investigation and insight obtained from the preliminary study, a limited number of control cases are considered
which include those deemed most effective and implementable. Numerical simulations are again performed using the same excitation input as for the SDOF systems. The
control approach is targeted at reducing the response contribution from the fundamental or dominant mode of vibration associated with the primary system. Uniformly-discretized models of a 6-story primary structural system capable of only lateral deformations are considered in most cases. A few cases involving models of a 3-story primary
structural system are also examined.</p>
https://thesis.library.caltech.edu/id/eprint/7365Rotordynamic Forces Due to Annular Leakage Flows in Shrouded Centrifugal Pumps
https://resolver.caltech.edu/CaltechETD:etd-03262007-103727
Authors: {'items': [{'id': 'Sivo-Joseph-Michael', 'name': {'family': 'Sivo', 'given': 'Joseph Michael'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/bpfm-0a24
<p>Previous experimental and analytical results have shown that discharge-to-suction leakage flows in the annulus of a shrouded centrifugal pump contribute substantially to the fluid induced rotordynamic forces (Adkins, 1988). Experiments conducted in the Rotor Force Test Facility (RFTF) at Caltech on an impeller undergoing a prescribed circular whirl have indicated that the leakage flow contribution to the normal and tangential forces can be as much as 70% and 30% of the total, respectively (Jery, 1986). Recent experiments at Caltech have examined the rotordynamic consequences of leakage flows and have shown that the rotordynamic forces are functions not only of the whirl ratio but also of the leakage flow rate and the impeller shroud to pump housing clearance. The forces were found to be inversely proportional to the clearance and a region of forward subsynchronous whirl was found for which the average tangential force was destabilizing. This region decreased with flow coefficient (Guinzburg, 1992).</p>
<p>The present research is a continuation of the previous experimental work and has been motivated by the rotordynamic stability problems with the recently developed Alternate Turbopump Design (ATD) of the Space Shuttle High Pressure Oxygen Turbopump. The present study investigates the influence of swirl brakes, installed in the annular leakage path, as a means of reducing the undesirable rotordynamic forces over a range of flow rates. Also, the present study evaluates the effect on the rotordynamic forces of tip leakage restrictions at discharge used by the ATD for establishing axial thrust balance. As a first step to understanding the flow field in the leakage annulus, the region is probed with a laser velocimeter to provide basic information on these unsteady turbulent three-dimensional leakage flows and to serve as a standard of comparison for approximate theoretical models as well as applications of computational fluid dynamics.</p>https://thesis.library.caltech.edu/id/eprint/1138An Investigation of Velocity and Temperature Fields in Taylor-Couette Flows
https://resolver.caltech.edu/CaltechETD:etd-01102008-131126
Authors: {'items': [{'id': 'Kedia-Rajesh', 'name': {'family': 'Kedia', 'given': 'Rajesh'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/0AAS-AW20
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
In many experiments, especially those investigating aspects of fluid flow, it is common to observe time series data exhibiting chaos. Chaos lies in the realm of nonlinear dynamics, and specialized methods are available for the analysis of nonlinear time series. One particular method, called time delay analysis, is particularly useful for extracting information from time series representing measurements at a single point in space. In this thesis, hot-wire anemometry is used to obtain velocity time series from experiments on isothermal Taylor-Couette flow. For R/R[subscript c]=1.6, a simple limit cycle is observed, yielding an attractor of dimension of 1. For R/R[subscript c]=11.1, the attractor dimension increases, and the reconstructed attractor exhibits features characteristic of a transition to turbulence. In addition, various other states and transitions of the Taylor-Couette system are studied as well.
Direct numerical simulations (DNS) have also been performed to study the effects of the gravitational and the centrifugal potentials on the stability of heated, incompressible Taylor-Couette flow. The flow is confined between two differentially heated, concentric cylinders and the inner cylinder is allowed to rotate. The Navier-Stokes equations and the coupled energy equation are solved using a spectral method. To validate the code, comparisons are made with existing linear stability analysis and with experiments. The code is used to calculate the local and average heat transfer coefficients for a fixed Reynolds number (R=100) and a range of Grashof numbers. The variation of the local coefficients of heat transfer on the cylinder surface is investigated, and maps showing different stable states of the flow are presented. Calculations of the time and space averaged equivalent conductivity show that the heat transfer decreases with Grashof number in axisymmetric Taylor vortex flow regime and increases with Grashof number after the flow becomes non-axisymmetric.
The numerical simulations also demonstrate the existence of a hysteresis loop in heated Taylor-Couette flow, obtained by slowly varying the Grashof number. Two different stable states with same heat transfer are found to exist at the same Grashof number. The validity of Colburn's correlation is investigated as well; the Prandtl number dependence is found to be slightly different from Pr[...] for the range of Reynolds number investigated. Finally, a time delay analysis of the radial velocity and the local heat transfer coefficient time series obtained from the numerical simulation of the radially heated Taylor-Couette flow is performed. The two-dimensional projection of the reconstructed attractor shows a limit cycle for Gr[...]-1700. The limit cycle behavior disappears at Gr[...]-2100, and the reconstructed attractor becomes irregular. The attractor dimension increases to about 3.2 from a value of 1 for the limit cycle case.
https://thesis.library.caltech.edu/id/eprint/112Material and thermal transport in vertical granular flows
https://resolver.caltech.edu/CaltechETD:etd-01182008-091551
Authors: {'items': [{'id': 'Natarajan-V-V-R', 'name': {'family': 'Natarajan', 'given': 'Venkata V. R.'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/wze4-p323
The term "granular material flow" is applied in the literature to particulate flows such as the flow of coal down an inclined chute, the discharge of grains from a hopper or the motion of debris in a landslide. In these flows, the material has an overall bulk motion; however, individual particles may collide, roll or slide against each other, and may interact with the bounding surfaces. Hence, the individual particle motions are composed of a mean velocity component and a fluctuating, or random, velocity component. An analogy is drawn between this random motion and the random motion of molecules. As a result, much of the theoretical analysis of these flows has developed from concepts derived from dense-gas kinetic theory. Although this random velocity component is a key property in analytical studies, there have been few attempts to measure its magnitude in experimental studies. In the current work, measurements were made of two components of the average and fluctuating velocities in the flow of granular material in a vertical chute for flows with different particle and boundary properties. The fluctuation velocities were highly anisotropic, with the streamwise components being 2 to 2.5 times the magnitude of the transverse components. Increasing the surface roughness of the particles reduced the fluctuation velocities significantly.
Another area of considerable industrial interest is particle mixing in monodisperse and polydisperse particle flows. Because of the random component of particle motion, the particles can exhibit a diffusive motion similar to that found in gases and liquids. In the second part of this work, local self diffusion coefficients were measured in the granular flow using image processing techniques to track individual particles. The influence of flow shear rates and fluctuation velocities on the self diffusion coefficients was investigated. The self-diffusion coeffecients were found to increase with the shear rate and the fluctuation velocity, with the coefficients in the streamwise direction being an order-of-magnitude higher than those for the transverse direction. The surface roughness of the particles led to a decrease in the self-diffusion coefficients.
The effect of shearing on the convective heat transfer from a heater immersed in a granular flow was investigated experimentally. Comparisons were made with previous experiments and with results obtained for unsheared plug flows. The results indicated that the medium density close to the wall played a critical role in determining the overall heat transfer.
Finally, theoretical solutions, based on a combination of the dense-gas kinetic theory and an empirical friction model, were generated to study and compare experimental and theoretical results for velocity profiles and heat transfer characteristics in vertical, fully developed granular flows. The results indicated good agreement between theoretical and experimentally measured mean velocity proflies but the fluctuation velocity magnitudes were usually underpredicted by the theoretical solutions. There was qualitative agreement between experimental and theoretical results for convective heat transfer.
https://thesis.library.caltech.edu/id/eprint/221I. Statistical mechanics of bubbly liquids. II. Behavior of sheared suspensions of non-Brownian particles
https://resolver.caltech.edu/CaltechETD:etd-06222005-110302
Authors: {'items': [{'email': 'yyurkovetsky@ccny.cuny.edu', 'id': 'Yurkovetsky-Y', 'name': {'family': 'Yurkovetsky', 'given': 'Yevgeny'}, 'show_email': 'YES'}]}
Year: 1998
DOI: 10.7907/NMJQ-2X32
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
I. Statistical mechanics of bubbly liquids. The dynamics of bubbles at high Reynolds numbers is studied from the viewpoint of statistical mechanics. Individual bubbles are treated as dipoles in potential flow. A virtual mass matrix of the system of bubbles is introduced, which depends on the instantaneous positions of the bubbles, and is used to calculate the energy of the bubbly flow as a quadratic form of the bubbles' velocities. The energy is shown to be the system's Hamiltonian and is used to construct a canonical ensemble partition function, which explicitly includes the total impulse of the suspension along with its energy. The Hamiltonian is decomposed into an effective potential due to the bubbles' collective motion and a kinetic term due to the random motion about the mean. An effective bubble temperature - a measure of the relative importance of the bubbles' relative to collective motion--is derived with the help of the impulse-dependent partition function. Two effective potentials are shown to operate: one, due to the mean motion of the bubbles, dominates at low bubble temperatures where it leads to their grouping in flat clusters normal to the direction of the collective motion, while the other, temperature invariant, is due to the bubbles' position-dependent virtual mass and results in their mutual repulsion. Numerical evidence is presented for the existence of the effective potentials, the condensed and dispersed phases and a phase transition.
II. Behavior of sheared suspensions of non-Brownian particles. Suspensions of non-Brownian particles in simple shear flow of a Newtonian solvent in the range of particle phase concentration, [...], from 0.05 to 0.52, are studied numerically by Stokesian Dynamics. The simulations are a function of [...] and the dimensionless shear rate, [...], which measures the relative importance of the shear and short-ranged interparticle forces. The pair-distribution functions, shear viscosity, normal stress differences, suspension pressure, long-time self-diffusion coefficients, and mean square of the particle velocity fluctuations in the velocity-gradient and vorticity directions are computed, tabulated and plotted. In concentrated suspensions ([...] > 0.45), two distinct microstructural patterns are shown to exist at the highest and lowest shear rates. At [...] = 0.1 the particles form hexagonally packed strings in the flow direction. As [...] increases, the strings are gradually being replaced by non-compact clusters of particles kept together by strong lubrication forces while the particle pair-distribution displays a broken fore-aft symmetry. These changes in the microstructure are accompanied by increases in the shear viscosity, normal stress differences, suspension pressure, longtime self-diffusion coefficients, and fluctuational motion. Agreement is found between the simulation results and the theoretical predictions of Brady and Morris (1997).
https://thesis.library.caltech.edu/id/eprint/2684A Study of Heat Transport Processes in the Wake of a Stationary and Oscillating Circular Cylinder Using Digital Particle Image Velocimetry/Thermometry
https://resolver.caltech.edu/CaltechETD:etd-04132004-150955
Authors: {'items': [{'email': 'Han.G.Park@jpl.nasa.gov', 'id': 'Park-Han-G', 'name': {'family': 'Park', 'given': 'Han G.'}, 'show_email': 'YES'}]}
Year: 1998
DOI: 10.7907/C9KN-RQ12
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
An experimental investigation is carried out on the processes of heat transfer associated with a heated circular cylinder in crossflow. Two studies are made. First, a study of the transport of heat in the near wake (x/D<5) of a stationary and transversely oscillated cylinder is made at Reynolds number of 610. Second, a study is made of the surface heat transfer from a cylinder which is undergoing forced oscillations in the transverse direction.
The studies are made using the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) which allows simultaneous measurements of both the velocity and temperature fields of the flow. The temperature is measured by seeding the flow with thermochromic liquid crystal (TLC) particles which change their reflected wavelength as function of temperature. By calibrating reflected wavelength versus temperature using a color multi-CCD camera, the local temperature of the flow may be deduced. The velocity is measured by using the same particles as Lagrangian flow tracers, and local velocity or displacement of the flow may be measured by cross-correlating two sequential images. A limitation of DPIV/T, which is the low level of precision (5% - 20% of the temperature span of TLC particles), may be overcome by a process in which the temperature at a given location is computed by averaging the temperatures of the particles within a specified sampling window. This process increases the precision to 2% - 10%.
In the study of the heat transport in the near wake, the velocity and temperature measurements obtained from DPIV/T are decomposed into their mean, coherent, and incoherent components using the triple decomposition. It is found that the heat from the cylinder is transported down the wake mostly by the mean heat flux and is laterally transported out of the wake by the coherent and the incoherent heat fluxes. In examining the direction of the turbulent heat flux vectors, the vectors are found not to be co-linear with the gradient of mean temperature. This misalignment implies that the gradient transport models are inappropriate for modeling the turbulent heat transport in the near wake of a circular cylinder. In examining the production of turbulence, it is found that that kinetic energy fluctuations are produced in the saddle regions (regions where the fluid is being stretched in one direction and compressed in another) while the temperature fluctuations are produced at the edges of center regions (regions where the fluid is rotating), i.e., the edges of the vortex cores.
From the study of the heat convection from a cylinder as function of forced oscillation frequency [...] and amplitudes (A/D=0.1, 0.2), it is found that besides the previously known increase near the natural vortex shedding frequency, there also exists a large increase in the heat transfer at approximately three times this frequency for A/D=0.1. For A/D=0.2, there exist large increases at roughly two and three times the natural vortex shedding frequency. From a DPIV/T study, it is found that the wake pattern becomes synchronized with the mechanical oscillation of the cylinder at these frequencies where the heat transfer increases significantly. At the frequencies corresponding to roughly two and three times the unforced vortex shedding frequency, the wake pattern may become synchronized by processes of period doubling and tripling with respect to the cylinder oscillation period, respectively. The increase in the heat transfer rate is found to correlate with the distance at which vortices roll-up behind the cylinder. The distance is observed to decrease sharply at the frequencies corresponding to a sharp increase in the heat transfer. Therefore, the near wake is found to play a critical role in the heat transfer from the surface of a circular cylinder, and the cause of the increase in heat transfer is believed to the removal of the stagnant and low heat convecting fluid at the base of the cylinder during the roll-up of the vortices.https://thesis.library.caltech.edu/id/eprint/1370Collisional mechanics in solid-liquid flows
https://resolver.caltech.edu/CaltechETD:etd-08122005-140811
Authors: {'items': [{'email': 'zenit@servidor.unam.mx', 'id': 'Zenit-Camacho-J-R', 'name': {'family': 'Zenit Camacho', 'given': 'Jose Roberto'}, 'show_email': 'YES'}]}
Year: 1998
DOI: 10.7907/E0D9-C117
Experimental measurements of the particle pressure were obtained for a liquid fluidized bed and for a vertical gravity driven liquid-solid flow. The particle, or granular, pressure is defined as the extra pressure generated by the action of particles in a particulate multi-phase flow. Using a high-frequency-response pressure transducer, individual collisions of particles were collected and measured to obtain a time-averaged particle pressure. Results were obtained for a number of different particles and for two different test section diameters. Results show that the particle pressure experiences a maximum at intermediate concentrations, and that its magnitude is scaled with the particle density and the square of the terminal velocity of the particles. The particle pressure was found to be composed of two main contributions: one from pressure pulses generated by direct collisions of particles against the containing walls (direct component), and a second one from pressure pulses due to collisions between individual particles that are transmitted through the liquid (radiated component).
The direct component of the particle pressure was studied by an analysis of particle collisions submerged in a liquid. A simple pendulum experiment provides controlled impacts in which measurements are made of the particle trajectories for different particles immersed in water. The velocity of the approaching particle is measured using a high speed digital camera; the magnitude of the collision is quantified using a high frequency response pressure transducer at the colliding surface. The measurements show that most of the particle deceleration occurs at less than half a particle diameter away from the wall. The measured collision pressure appears to increase with the impact velocity. Comparisons are drawn between the measured pressures and the predictions by Hertzian theory. A simple control-volume model is proposed to account for the effects of fluid inertia and viscosity. The pressure profile is estimated, and then integrated over the surface of the particle to obtain a force. The model predicts a critical Reynolds number at which the particle reaches the wall with zero velocity. Comparisons between the proposed model and the experimental measurements show qualitative agreement.
Experiments involving binary collisions of particles were performed to investigate the radiated component of the particle pressure. This component results from the pressure front generated by the impulsive motion of a fluid resulting from a collision of particles in a liquid. When the two particles come into contact, the impulsive acceleration due to the elastic rebound produces a pressure pulse, which is transmitted through the fluid. A simple dual pendulum experiment was set up to generate controlled collisions. Measurements were obtained for a range of impact velocities, angles of incidence, and distances away from the wall for different pairs of particles. The magnitude of the impulse pressure appears to scale with the particle impact velocity and the density of the fluid. Based on the impulse pressure theory, a prediction for pressure generated due to the collision can be obtained. The model appears to agree well with the experimental measurements.
The fluctuating component of the solid fraction was studied, as one of the sources of the particle pressure. The instantaneous cross-sectional averaged solid fraction was measured using an impedance meter. The root-mean square fluctuation of the solid fraction signal was measured in a liquid fluidized bed and a vertical gravity-driven flow, for different particle sizes and densities. Two types of fluctuations were identified: low-frequency large-scale fluctuations which dominate at high concentrations, and high-frequency small-scale fluctuations which are dominant at intermediate solid fractions. The effect of each type was isolated by filtering. When the large-scale fluctuations were present, the magnitude of the rms fluctuation was found to scale with particle diameter, but when eliminated the mean fluctuation appeared to scale with the particle mass instead.
https://thesis.library.caltech.edu/id/eprint/3104Buoyant flows in vertical channels relating to smoke movement in high-rise building fires
https://resolver.caltech.edu/CaltechETD:etd-02072008-074758
Authors: {'items': [{'id': 'Benedict-N-L', 'name': {'family': 'Benedict', 'given': 'Noel Lakshman'}, 'show_email': 'NO'}]}
Year: 1999
DOI: 10.7907/ynbt-qn25
This experimental study is motivated by the widespread loss of life and property due to accidental fires in high rise buildings, and investigates the progress of hot, toxic, products during such fires. The Stack Effect and Turbulent Mixing, two of the primary factors responsible for smoke movement within tall buildings are the focus of this study. The results of this investigation could be used for the development of fire modeling codes that simulate high-rise building fires.
The experiments involve a 2.6 m tall square shaft with various cross sections and openings. The shaft was situated above a large temperature controlled hot air reservoir, and the two chambers were initially separated by a partition. At the start of the experiment the partition was removed in a rapid horizontal motion and the hot and cold gases were allowed to mix. For the shafts with openings, the gases were withdrawn from the apertures at different rates with the Reynolds number varying between 600 and 7200. The temperature of the gas and wall, and the heat transfer to the wall were measured as functions of time at various locations in the shaft. Additionally, hot wire anemometry techniques were used to obtain velocity data in the channel. Some experiments involved monitoring a tracer gas in the vertical channel. Simple one-dimensional analytical modeling was performed to validate the experimental results.
The experiments indicated an initial transient period followed by a "pseudo steady state." At each elevation measured the cross-section averaged gas temperature, reached and fluctuated about a steady state value soon after the initial front of hot gas arrived at that location. For the closed channel experiments, the front arrival time was a function of the initial density ratio, the shaft width, and the gravitational constant.
The tracer gas trials suggested that the molecular diffusion was insignificant in comparison to the turbulent mixing. For the closed channels the observed velocity fluctuations were the same order of magnitude as the mean velocities. The time averaged heat transfer coefficient was weakly dependent on the initial reservoir temperature.
The vented shaft experiments indicated that the front propagation times are significantly affected by openings in the shaft and that the effect is more pronounced the higher the vents are located. The venting caused a significant rise in the steady state temperatures, and a reduction in both the temperature and velocity fluctuations. The local Nusselt number was independent of the Reynolds number and a function only of the Rayleigh number, indicating that the heat transfer was dominated by free convection effects.
The predictions made by the one dimensional analytical model agreed reasonably well with the experiments, particularly in the case of the closed channel.https://thesis.library.caltech.edu/id/eprint/538Couette Flows of Granular Materials: Mixing, Rheology, and Energy Dissipation
https://resolver.caltech.edu/CaltechETD:etd-03272007-093837
Authors: {'items': [{'email': 'annakarion@gmail.com', 'id': 'Karion-Anna', 'name': {'family': 'Karion', 'given': 'Anna'}, 'show_email': 'NO'}]}
Year: 2000
DOI: 10.7907/HX15-PC72
<p>This thesis examines the behavior of a granular material sheared in a gap between two moving boundaries. In fluid mechanics, this type of flow is known as a Couette flow. Two different kinds of granular Couette flows were studied. First, gravity-free flow between two infinite plates moving in opposite directions was investigated using computer simulations. Second, flow between a stationary outer cylinder and an inner rotating cylinder was studied using both experiments and computer simulations.</p>
<p>Two-dimensional discrete element computer simulations of infinite planar Couette flows were used to study the rheology, energy dissipation, and other flow properties in flows of particles of uniform size for three different gap widths. The energy dissipation rate was measured and a thermal analysis was conducted to determine the thermodynamic temperature rise and heat flux of such flows. Given a constant wall velocity, all of the properties in flows of identical particles were found to depend on the value of the solid fraction at the walls, which in turn depended on both the average solid fraction and the gap width. When the average solid fraction reached a critical threshold, the amount of work done on the flow drastically increased, increasing the average strain rate, granular temperature, wall stresses, and energy dissipation in the flow. This solid fraction threshold occurred after the center region of the flow had reached a dense limit and any further increase in solid fraction necessarily occurred in the wall regions. Various results from computer simulations were found to compare reasonably well with past results derived using kinetic theory.</p>
<p>Mixing and other flow properties were also investigated in planar Couette flows of two different particle sizes, as functions of the size ratio and solid fraction ratio of the two species. Larger particles were found to migrate away from the regions of high fluctuation energy near the two moving boundaries in all cases. Mixture flows were found to behave very similarly to flows of mono-sized particles at high ratios of the solid fraction of small to large particles. As the solid fraction ratio decreased and the number of large particles increased, results deviated from the corresponding flow of identical particles. Flows with large size ratios of large to small particles deviated the most from the result of mono-sized particles, because stresses and energy dissipation rates are both mass-dependent.</p>
<p>The second type of Couette flow, between two concentric cylinders, was investigated in a horizontal orientation (with the axis of rotation perpendicular to the direction of gravity) and in a vertical orientation (with the axis parallel to the direction of gravity), using both experiments and computer simulations. In the horizontal geometry, high-speed imaging was used to calculate experimental mean and fluctuation velocity profiles that were compared to results from three-dimensional discrete element simulations. Segregation of binary particle mixtures was also investigated in this geometry. Segregation in this flow was driven by a percolation mechanism acting at the free surface, causing large particles to migrate to the top. Computer simulations compared well qualitatively with experiments, successfully predicting the velocity profiles and the segregation pattern at the surface. When compared quantitatively, however, fluctuation velocities in the simulations were considerably greater than those found in the experiment, and the radial segregation observed in experiments did not occur to the same extent in simulations.</p>
<p>The vertically-oriented cylindrical Couette flow experiment was used to measure the shear stress on the outer cylinder wall as a function of different variables. The shear stress was found to be independent of the inner cylinder rotation rate, because the material was unconfined and allowed to dilate. The measured stress showed a linear dependence on the height of material in the apparatus, indicating a hydrostatic variation of the normal stress. The shear stress also varied significantly with the ratio of the gap width to the particle diameter.</p>https://thesis.library.caltech.edu/id/eprint/1171In-Situ Diagnostics for Metalorganic Chemical Vapor Deposition of YBCO
https://resolver.caltech.edu/CaltechETD:etd-09262005-143545
Authors: {'items': [{'email': 'dj_lightzout@hotmail.com', 'id': 'Tripathi-Ashok-Burton', 'name': {'family': 'Tripathi', 'given': 'Ashok Burton'}, 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/3ZJS-BE38
<p>A new stagnation flow MOCVD research reactor is described that is designed to serve as a testbed to develop tools for "intelligent" thin film deposition, such as in-situ sensors and diagnostics, control algorithms, and thin film growth models. The reactor is designed in particular for the deposition of epitaxial YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> on MgO, although with minor modifications it would be suitable for deposition of any metal-oxide thin films.</p>
<p>The reactor is specifically designed to permit closed-loop thermal and stoichiometric control of the film growth process. Closed-loop control of precursor flow rates is accomplished by using ultraviolet absorption spectroscopy on each precursor line. Also integrated into the design is a Fourier Transform Infrared (FTIR) spectroscopy system which collects real-time, in-situ infrared polarized reflectance spectra of the film as it grows. Numerical simulation was used extensively to optimize the fluid dynamics and heat transfer to provide uniform fluxes to the substrate. As a result, thickness uniformity across the substrate is typically within 3% from the center to the edge of the substrate.</p>
<p>Experimental studies of thin films grown in the Y/Ba/Cu/O system have been carried out. The films have been characterized by Rutherford Backscattering Spectrometry and X-ray Diffraction. Results indicate c-axis oriented grains with pure 1:2:3 phase YBCO, good spatial uniformity, and a low degree of c-axis wobble. Experimental growth data is used in a gas phase and surface chemistry model to calculate sticking coefficients for yttrium oxide, barium oxide, and copper oxide on YBCO.</p>
<p>In-situ FTIR and Coherent Gradient Sensing (CGS) analysis of growing films has been performed, yielding accurate substrate temperature, film thickness monitoring, and full-field, real-time curvature maps of the films. In addition, we have implemented CGS to obtain full-field in-situ images of local curvature during oxygenation and deoxygenation of YBCO films. An analysis of the oxygen diffusion is performed, and diffusivity constants are presented for a variety of temperature and film conditions.</p>https://thesis.library.caltech.edu/id/eprint/3785Measurements of Thermo-Acoustic Coupling
https://resolver.caltech.edu/CaltechETD:etd-08222001-170336
Authors: {'items': [{'email': 'winston9t4@yahoo.com', 'id': 'Pun-Winston', 'name': {'family': 'Pun', 'given': 'Winston'}, 'show_email': 'YES'}]}
Year: 2001
DOI: 10.7907/SPSR-VD18
The problem of combustion instabilities has existed since the early 1940s, when they were observed during the development of solid and liquid rocket engines. While various engineering solutions have served well in these fields, the problem is revisited in modern gas-turbine engines. The purpose of this work is to provide experimental measurements of laboratory devices that exhibit thermo-acoustic coupling, similar to the interaction observed during combustion instabilities, which will aid in the design and development of stable systems.
Possibly the simplest device which exhibits these characteristics is a Rijke tube. An electrical, horizontally mounted, 1 m long version of the original Rijke tube is presented, with measurements taken during unstable and stable operation. An accurate stability boundary with uncertainty is determined for a heater position of x/L = ?, as a function of mass flow rate and heater power. Hysteresis, not previously reported, is observed at flow rates above 3 g/s. A one-dimensional model of the stability boundary with linear acoustics is shown to have qualitative agreement with experimental data.
A novel technique has also been devised which can provide insight into the local dynamic response of a flame to an acoustic field. In the experiments, a test chamber is acoustically excited by a pair of low-frequency drivers. The response of the flame is visualized by two techniques; chemiluminescence and planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) radical, both of which are well-known indicators for heat release in flames. The resulting images are phase-resolved and averaged to yield a qualitative picture of the fluctuation of the heat release. The images are correlated with a pressure transducer near the flame, which allows stability to be evaluated using Rayleigh?s criterion and a combustion response function. This is the first known measurement of the combustion dynamics of a flame over a range of frequencies. Results indicate that the drive frequency and burner configuration have a pronounced effect on the response of the flame. Drive frequencies ranging from 22 Hz to 55 Hz are applied to the jet mixed burner, supplied with a premixed 50/50 mixture of methane and carbon dioxide at a Reynolds number of 20,000. The burner is operated in two configurations; with an aerodynamically stabilized flame and with a flame stabilized by two protruding bluff-bodies. Results indicate that in general the bluff-body stabilized flame is less sensitive to chamber acoustic excitation.https://thesis.library.caltech.edu/id/eprint/3192Rotordynamic Forces Generated by Annular Leakage Flows in Centrifugal Pumps
https://resolver.caltech.edu/CaltechTHESIS:12022010-081219787
Authors: {'items': [{'id': 'Hsu-Yun', 'name': {'family': 'Hsu', 'given': 'Yun'}, 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/fr51-ft92
Fluid-induced rotordynamic forces in pumping machinery are well documented but poorly understood. The present research focuses on the rotordynamics due to fluid flow in annuli, in particular, the discharge-to-suction leakage flow in centrifugal pumps. There are indications that the contribution of the front shroud leakage flow can be of the same order of magnitude as contributions from the nonuniform pressure acting on the impeller discharge. Previous investigations have established some of
the basic traits of these flows. This work furthers the experimental and computational approach to quantify and predict the shroud contribution to the rotordynamic
stability of pumping machinery.
Childs' bulk flow model for leakage paths is carefully examined, and convective relations for vorticity and total pressure are deduced. This analysis leads to a new
solution procedure for the bulk flow equations which does not resort to linearization or assumed harmonic forms of the flow variables.
Experimental results presented show the contributions of the inlet swirl velocities
to the rotordynamic forces. Antiswirl devices are evaluated for their effectiveness in reducing instability. Additional tests measuring the pressure distributions and the inlet swirl velocities of the leakage flow confirm some of the predictions by numerical analysis.https://thesis.library.caltech.edu/id/eprint/6192Accelerated Stokesian Dynamics: Development and Application to Sheared Non-Brownian Suspensions
https://resolver.caltech.edu/CaltechETD:etd-01202009-160503
Authors: {'items': [{'email': 'asierou@gmail.com', 'id': 'Sierou-Asimina', 'name': {'family': 'Sierou', 'given': 'Asimina'}, 'show_email': 'NO'}]}
Year: 2002
DOI: 10.7907/SX3A-8M59
<p>A new implementation of the conventional Stokesian Dynamics (SD) algorithm, called Accelerated Stokesian Dynamics (ASD), is presented. The equations governing the motion of N particles suspended in a viscous fluid at low particle Reynolds number are solved accurately and efficiently, including all hydrodynamic interactions, but with a significantly lower computational cost of O(N ln N). The main differences from the conventional SD method lie in the calculation of the many-body long-range interactions, where the Ewald-summed wave-space contribution is calculated as a Fourier Transform sum, and in the iterative inversion of the now sparse resistance matrix. The ASD method opens up an entire new class of suspension problems that can be investigated, including particles of non-spherical shape and a distribution of sizes, and can be readily extended to other low-Reynolds-number flow problems. The new method is applied to the study of sheared non-Brownian suspensions.</p>
<p>The rheological behavior of a monodisperse suspension of non-Brownian particles in simple shear flow in the presence of a weak interparticle force is studied first. The availability of a faster numerical algorithm permits the investigation of larger systems (typically of N = 512 — 1000 particles), and accurate results for the suspension viscosity, first and second normal stress differences and the particle pressure are determined as a function of the volume fraction. The system microstructure, expressed through the pair-distribution function, is also studied and it is demonstrated how the resulting anisotropy in the microstructure is correlated with the suspension non-Newtonian behavior. The ratio of the normal to excess shear stress is found to be an increasing function of the volume fraction, suggesting different volume fraction scalings for different elements of the stress tensor. The relative strength and range of the interparticle force is varied and its effect on the shear and normal stresses is analyzed. Volume fractions above the equilibrium freezing volume fraction (ø ≈ 0.494) are also studied, and it is found that the system exhibits a strong tendency to order under flow for volume fractions below the hard-sphere glass transition; limited results for ø = 0.60, however, show that the system is again disordered under shear.</p>
<p>Self-diffusion is subsequently studied and accurate results for the complete tensor of the shear-induced self-diffusivities are determined. The finite, and oftentimes large, auto-correlation time requires the mean-square displacement curves to be followed for longer times than was previously thought necessary. Results determined from either the mean-square displacement or the velocity autocorrelation function are in excellent agreement. The longitudinal (in the flow direction) self-diffusion coefficient is also determined, and it is shown that the finite autocorrelation time introduces an additional coupled term to the longitudinal self-diffusivity, a term which previous theoretical and numerical results omitted. The longitudinal self-diffusivities for a system of non-Brownian particles are calculated for the first time as a function of the volume fraction.</p>
https://thesis.library.caltech.edu/id/eprint/257A Dilating Vortex Particle Method for Compressible Flow with Applications to Aeroacoustics
https://resolver.caltech.edu/CaltechETD:etd-12282004-113953
Authors: {'items': [{'email': 'eldredge@seas.ucla.edu', 'id': 'Eldredge-Jeffrey-D', 'name': {'family': 'Eldredge', 'given': 'Jeffrey D.'}, 'orcid': '0000-0002-2672-706X', 'show_email': 'YES'}]}
Year: 2002
DOI: 10.7907/7EYY-0S65
<p>Vortex methods have become useful tools for the computation of incompressible fluid flow. In the present work, a vortex particle method for the simulation of unsteady two-dimensional compressible flow is developed and applied to several problems. The method is the first Langrangian simulation method for the full compressible Navier-Stokes equations. By decomposing the velocity into irrotational and solenoidal parts, and using particles that are able to change volume and that carry vorticity, dilation, enthalpy, entropy, and density, the equations of motion are satisfied. A general deterministic treatment of spatial derivatives in particle methods is developed by extending the method of particle strength exchange through the construction of higher-order-accurate, non-dissipative kernels for use in approximating arbitrary differential operators. The application of this technique to wave propagation problems is thoroughly explored. A one-sided operator is developed for approximating derivatives near the periphery of particle coverage; the operator is used to enforce a non-reflecting boundary condition for the absorption of acoustic waves at this periphery. Remeshing of the particles and the smooth interpolation of their strengths are addressed, and a criterion for the frequency of remeshing is developed on the principle axes of the rate-of-strain tensor. The fast multipole method for the fast summation of the velocity field is adapted for use with compressible particles. The new vortex method is applied to co-rotating and leapfrogging vortices in compressible flow, with the acoustic field computed using a two-dimensional Kirchoff surface, and the results agree will with those of previous work or analytical prediction. The method is also applied to the baroclinic generation of vorticity, and to the steepening of waves in the one-dimensional Burgers’ equation, with favorable results in both cases.</p>https://thesis.library.caltech.edu/id/eprint/5147Inertial Effects in Suspension Dynamics
https://resolver.caltech.edu/CaltechETD:etd-07042002-114141
Authors: {'items': [{'id': 'Subramanian-Ganesh', 'name': {'family': 'Subramanian', 'given': 'Ganesh'}, 'show_email': 'NO'}]}
Year: 2002
DOI: 10.7907/DSMP-HV88
<p>This work analyses the role of small but finite particle inertia on the microstructure of suspensions of heavy particles subjected to an external flow. The magnitude of particle inertia is characterized by the Stokes number, St, defined as the ratio of the inertial relaxation time of a particle to the flow time scale. Fluid inertia is neglected so that the fluid motion satisfies the quasi-steady Stokes equations. The statistics of the particles is governed by a Fokker-Planck equation in position and velocity space. For small St, a multiple scales formalism is developed to solve for the phase-space probability density of a single spherical Brownian particle in a linear flow. Though valid for an arbitrary flow field, the method fails for a spatially varying mass and drag coefficient. In all cases, however, a Chapman-Enskog-like formulation provides a valid multi-scale description of the dynamics both for a single Brownian particle and a suspension of interacting particles. For long times, the leading order solution simplifies to the product of a local Maxwellian in velocity space and a spatial density satisfying the Smoluchowski equation. The higher order corrections capture both short-time momentum relaxations and long-time deviations from the Maxwellian. The inertially corrected Smoluchowski equation includes a non-Fickian term at O(St).</p>
<p>The pair problem is solved to O(St) for non-Brownian spherical particles in simple shear flow. In contrast to the zero inertia case, the relative trajectories of two particles are asymmetric. Open trajectories in 'repels' nearby trajectories that spiral out onto a new stable limit cycle in the shearing plane. This limit cycle acts as a local attractor; in-plane trajectories from an initial offset of O(St¹/²) or less approach the limit cycle. The topology of the off-plane trajectories is more complicated because the gradient displacement changes sign away from the plane of shear. The 'neutral' off-plane trajectory with zero net gradient displacement acts to separate trajectories spiralling onto contact from those that go off to infinity. The aforementioned asymmetry leads to a non-Newtonian rheology and self-diffusivities in the gradient and voriticity directions that scale as St² ln St and St2, respectively.</p>https://thesis.library.caltech.edu/id/eprint/2808Algorithms for Reaction Mechanism Reduction and Numerical Simulation of Detonations Initiated by Projectiles
https://resolver.caltech.edu/CaltechETD:etd-05302003-142744
Authors: {'items': [{'id': 'Hung-Patrick-Hin-Fun', 'name': {'family': 'Hung', 'given': 'Patrick Hin Fun'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/H5GV-PV33
<p>The evolution of a homogeneous, chemically reactive system with n species forms a dynamical system in chemical state-space. Under suitable constraints, unique and stable equilibrium exists and can be interpreted as zeroth-dimensional (point like) attractors in this n-dimensional space. At these equilibrium compositions, the rates of all reversible reactions vanish and can, in fact, be determined from thermodynamics independent of chemical kinetics.</p>
<p>Generalizing this concept, an m-dimensional Intrinsic Low Dimensional Manifold (ILDM) represents an m-dimensional subspace in chemical state-space where all but the m-slowest aggregate reactions are in equilibrium, and these aggregate reactions are determined by eigenvalue considerations of the chemical kinetics. In this context, a certain composition is said to be m-dimensional if it is on an m-, but not an (m-1)-, dimensional ILDM.</p>
<p>Two new algorithms are proposed that allow the dimensionality of chemical compositions be determined simply. The first method is based on recasting the Maas and Pope algorithm. The second, and more efficient, method is inspired by the mathematical structure of the Maas and Pope algorithm and makes use of the technique known as arc-length reparameterization. In addition, a new algorithm for the construction of ILDM, and the application of these ideas to detonation simulations, is discussed.</p>
<p>In the second part of the thesis, numerical simulations of detonation waves initiated by hypervelocity projectiles are presented. Using detailed kinetics, only the shock-induced combustion regime is realized as simulating the conditions required for a stabilized detonation is beyond the reach of our current computational resources. Resorting to a one-step irreversible reaction model, the transition from shock-induced combustion to stabilized oblique detonation is observed, and an analysis of this transition based on the critical decay-rate model of Kaneshige (1999) is presented.</p>https://thesis.library.caltech.edu/id/eprint/2292Modeling and Control of Epitaxial Thin Film Growth
https://resolver.caltech.edu/CaltechETD:etd-10222002-115711
Authors: {'items': [{'email': 'martha.grover@chbe.gatech.edu', 'id': 'Gallivan-Martha-Anne', 'name': {'family': 'Gallivan', 'given': 'Martha Anne'}, 'orcid': '0000-0002-7036-776X', 'show_email': 'YES'}]}
Year: 2003
DOI: 10.7907/FXZB-XT91
<p>Thin film deposition is a manufacturing process in which tolerances may approach the size of individual atoms. The final film is highly sensitive to the processing conditions, which can be intentionally manipulated to control film properties. A lattice model of surface evolution during thin film growth captures many important features, including the nucleation and growth of clusters of atoms and the propagation of atomic-height steps. The dimension of this probabilistic master equation is too large to directly simulate for any physically realistic domain, and instead stochastic realizations of the lattice model are obtained with kinetic Monte Carlo simulations.</p>
<p>In this thesis simpler representations of the master equation are developed for use in analysis and control. The static map between macroscopic process conditions and microscopic transition rates is first analyzed. In the limit of fast periodic process parameters, the surface responds only to the mean transition rates, and, since the map between process parameters and transition rates is nonlinear, new effective combinations of transition rates may be generated. These effective rates are the convex hull of the set of instantaneous rates.</p>
<p>The map between transition rates and expected film properties is also studied. The dimension of a master equation can be reduced by eliminating or grouping configurations, yielding a reduced-order master equation that approximates the original one. A linear method for identifying the coefficients in a master equation is then developed, using only simulation data. These concepts are extended to generate low-order master equations that approximate the dynamic behavior seen in large Monte Carlo simulations. The models are then used to compute optimal time-varying process parameters.</p>
<p>The thesis concludes with an experimental and modeling study of germanium film growth, using molecular beam epitaxy and reflection high-energy electron diffraction. Growth under continuous and pulsed flux is compared in experiment, and physical parameters for the lattice model are extracted. The pulsing accessible in the experiment does not trigger a change in growth mode, which is consistent with the Monte Carlo simulations. The simulations are then used to suggest other growth strategies to produce rougher or smoother surfaces.</p>https://thesis.library.caltech.edu/id/eprint/4207The Relationship Between Near-Wake Structure and Heat Transfer for an Oscillating Circular Cylinder in Cross-Flow
https://resolver.caltech.edu/CaltechETD:etd-05202003-145011
Authors: {'items': [{'id': 'Pottebaum-Tait-Sherman', 'name': {'family': 'Pottebaum', 'given': 'Tait Sherman'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/C9NQ-N832
<p>A series of experiments were carried out in order to understand the relationship between wake structure and heat transfer for a transversely oscillating circular cylinder in cross-flow and to explore the dynamics of the vortex formation process in the wake. The cylinder's heat transfer coefficient was determined over a range of oscillation amplitudes up to 1.5 cylinder diameters and oscillation frequencies up to 5 times the stationary cylinder natural shedding frequency. The results were compared to established relationships between oscillation conditions and wake structure. Digital particle image thermometry/velocimetry (DPIT/V) was used to measure the temperature and velocity fields in the near-wake for a set of cases chosen to be representative of the variety of wake structures that exist for this type of flow. The experiments were carried out in a water tunnel at a Reynolds number of 690.</p>
<p>It was found that wake structure and heat transfer both significantly affect one another. The wake mode, a label indicating the number and type of vortices shed in each oscillation period, is directly related to the observed heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, explain this relationship. The streamwise spacing between shed vortices was also shown to affect heat transfer coefficient for the 2S mode, which consists of two single vortices shed per cycle. The streamwise spacing is believed to influence entrainment of freestream temperature fluid by the forming vortices, thereby affecting the temperature gradient at the cylinder base. This effect may exist for other wake modes, as well.</p>
<p>The cylinder's transverse velocity was shown to influence the heat transfer by affecting the circulation of the wake vortices. For a fixed wake structure, the effectiveness of the wake vortices at enhancing heat transfer depends on their circulation. Also, the cylinder's transverse velocity continually changes the orientation of the wake with respect to the freestream flow, thereby spreading the main source of heat transfer enhancement--the vortices near the cylinder base--over a larger portion of the cylinder surface.</p>
<p>Previously observed heat transfer enhancement associated with oscillations at frequencies near the natural shedding frequency and its harmonics were shown to be limited to amplitudes of less than about 0.5 cylinder diameters.</p>
<p>A new phenomenon was discovered in which the wake structure switches back and forth between distinct wake modes. Temperature induced variations in the fluid viscosity are believed to be the cause of this mode-switching. It is hypothesized that the viscosity variations change the vorticity and kinetic energy fluxes into the wake, thereby changing the wake mode and the heat transfer coefficient. This discovery underscores the role of viscosity and shear layer fluxes in determining wake mode, potentially leading to improved understanding of wake vortex formation and pinch-off processes in general.</p>
<p>Aspect ratio appears to play a role in determining the heat transfer coefficient mainly for non-oscillating cylinders. The heat transfer is also affected by aspect ratio for oscillation conditions characterized by weak synchronization of the wake to the oscillation frequency.</p>https://thesis.library.caltech.edu/id/eprint/1886Collisional Dynamics of Macroscopic Particles in a Viscous Fluid
https://resolver.caltech.edu/CaltechETD:etd-05302003-171943
Authors: {'items': [{'id': 'Joseph-Gonzalez-Gustavo', 'name': {'family': 'Joseph Gonzalez', 'given': 'Gustavo'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/FFFC-X753
<p>This thesis presents experimental measurements of the approach and rebound of a particle colliding with a wall in a viscous fluid. Steel, glass, nylon, and Delrin particles were used, with diameters ranging from 3 to 12 mm. The experiments were performed using a thick Zerodur or Lucite wall with various mixtures of glycerol and water. Normal and tangential coefficients of restitution were defined from the ratios of the respective velocity components at the point of contact just prior to and after impact. These coefficients account for losses due to lubrication effects and inelasticity.</p>
<p>The experiments clearly show that the rebound velocity depends strongly on the impact Stokes number and weakly on the elastic properties of the materials. Below a Stokes number of approximately 10, no rebound of the particle occurs. Above a Stokes number of approximately 500, the normal coefficient of restitution asymptotically approaches the value for a dry collision. The data collapse onto a single curve of restitution coefficient as a function of Stokes number when normalized by the dry coefficient of restitution.</p>
<p>Oblique collisions in a fluid are qualitatively similar to oblique collisions in a dry system, with a lowered friction coefficient dependent on surface roughness. For smooth surfaces the friction coefficient is drastically reduced due to lubrication effects. Values for the friction coefficient are predicted based on elastohydrodynamic lubrication theory. The particle surface roughness was found to affect the repeatability of some measurements, especially for low impact velocities.</p>
<p>A significant retardation of a particle approaching a target at a low Stokes number was observed and quantified. The distance at which the particle's trajectory varies due to the presence of the wall is dependent on the impact Stokes number. The observed slowdown can be predicted from hydrodynamic theory to a good approximation.</p>
<p>An analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements.</p>https://thesis.library.caltech.edu/id/eprint/2298Computation of Bubbly Cavitating Flow in Shock Wave Lithotripsy
https://resolver.caltech.edu/CaltechETD:etd-05282004-130028
Authors: {'items': [{'email': 'michel@dynaflow-inc.com', 'id': 'Tanguay-Michel', 'name': {'family': 'Tanguay', 'given': 'Michel'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/VQXV-Y948
<p>Lithotripsy is at the forefront of treatment of kidney stones. By firing shock waves at the stone, it can be broken down into small fragments. Although the treatment is non-invasive, both short- and long-term side effects occur. In order to understand and rectify these shortcomings, lithotripsy has been the subject of ongoing research. Based on in vitro experiments, it has been ascertained that the cloud of cavitating bubble produced in the wake of the shock wave is a crucial element in the stone comminution process.</p>
<p>Various solutions designed to maximize stone comminution and/or decrease tissue damage have been proposed over the years. However, the particulars of the comminution mechanism(s) are still undetermined. In this work, a numerical model of the two-phase flow inside an electrohydraulic lithotripter was used to provide additional insight in the behavior of the bubble cloud. The numerical model is based on an ensemble averaged two-phase flow model for a compressible liquid. The differential equations were discretized following the WENO shock capturing scheme in prolate spheroidal and cylindrical coordinate systems. The initial conditions for the flow field are estimated based on empirical observations and then validated by comparing the predicted pressure measurements and bubble cloud behavior against experimental values.</p>
<p>In order to gain additional insight in the mechanism for stone comminution, a variety of relevant initial conditions were modeled. The following lithotripter configurations were analyzed: free-field, dual-pulse and single-pulse with an artificial stone at the focus. The impact of parameters such as the intensity of the initial shock wave and the pulse rate frequency (PRF) has been investigated. Based on an energy argument, conclusions regarding the efficiency of stone comminution are presented. In addition, based on these conclusions, avenues for improvement of the numerical model are highlighted.</p>https://thesis.library.caltech.edu/id/eprint/2188Numerical Study of the Dynamics and Sound Generation of a Turbulent Vortex Ring
https://resolver.caltech.edu/CaltechETD:etd-06082004-151101
Authors: {'items': [{'id': 'Ran-Hongyu', 'name': {'family': 'Ran', 'given': 'Hongyu'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/ASHK-JV07
<p>In the present study, Direct Numerical Simulations (DNS) of the fully compressible, three-dimensional Navier-Stokes equations are used to generate an axisymmetric vortex ring to which three-dimensional stochastic disturbances are added. The radiated acoustic field is computed directly in the near field, and by solving the wave equation in a spherical coordinate system in the far field.</p>
<p>At high Reynolds number, a vortex ring will undergo an instability to azimuthal waves. The instability produces higher azimuthal modes and induces nonlinear interaction between the modes, and will cause the vortex ring to break down and transition to turbulence. The early stages of the simulation agree well with the linear instability theory. Nonlinear stage of instability, transition, formation of axial flow and streamwise vorticity are analyzed and compared with experimental results. After turbulent transition, the evolution of statistical quantities becomes independent of viscosity and the initial geometry, and the flow become self-similar. The temporal evolution of quantities including total circulation, axial velocity profile, vortex ring displacement and vorticity profile agrees well with the self-similarity law. Turbulent energy spectrum, Reynolds stresses and turbulence production are also presented.</p>
<p>The unsteady vorticity field generates acoustic waves with higher azimuthal modes, each mode with a distinctive spectrum and directivity. The ensemble averaged peak frequency, bandwidth, and the sound pressure level agrees qualitatively with reported experimental results. The directivity of each azimuthal mode is compared with predictions of vortex sound theory. The sound generation consists of three stages. The first is a deterministic stage when linear instability waves emerge and grow and generate relatively weak sound. The second stage is nonlinear interaction and vortex breakdown; at this stage the sound pressure level reaches a peak value. The third stage is the turbulent asymptotic decay of the acoustic field. Based on the self-similar decay of the turbulent near field, the self-similar decay of the sound field is investigated. Connection between the acoustic field and the vortex ring oscillations is also studied with vortex sound theory. Finally, we note some similarities between the sound radiated by a train of de-correlated vortex rings and turbulent jet noise. The sound pressure level, spectrum, and directivity of the train of vortex rings is similar to the sound field from a jet with similar Reynolds number and Mach number.</p>https://thesis.library.caltech.edu/id/eprint/2513Spike Train Characterization and Decoding for Neural Prosthetic Devices
https://resolver.caltech.edu/CaltechETD:etd-07232003-012018
Authors: {'items': [{'id': 'Cao-Shiyan', 'name': {'family': 'Cao', 'given': 'Shiyan'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/GK20-5W75
<p>Neural prosthetic device has the potential of benefiting millions of lock-in and spinal cord injury survivors. One branch of the ongoing research is to construct reach movement based prosthetic devices. This thesis proposes statistical methods based on applying the Haar wavelet packets to spike trains in order to answer some of the questions in this field.</p>
<p>Although spike train is the most frequently used data in the neural science community, its stochastic properties are not fully understood or characterized. This thesis suggests a formal spike train characterization method using the Haar wavelet packet. Because of the multi-scale property of the wavelet packet, Poisson characteristics at different scales can be assessed. Moreover, Poisson Scale-gram is proposed to help visualize the characteristics of the spike train at different scales.</p>
<p>Because some neurons display non-Poisson characteristics, it is necessary to extract the relevant features from spike trains in the context of decoding. The thesis suggests a feature extraction method that searches all the wavelet packet coefficients for the ones with the largest discriminability, quantified by mutual information. This technique returns the most informative feature(s) in the context of the Bayesian classifier. Decoding performance of this proposed method is compared against the one using mean firing rate only on both surrogate data and the actual data from PRR.</p>
<p>It is also crucial to decode cognitive states because they provide the extra control signals necessary for practical implementation of the prosthetic devices. This thesis proposes a simple finite state machine approach along with an interpreter that interprets the decoding results and to regulate when the transition should occur. It demonstrates that the finite state machine framework, when coupled with the interpreter, offers a simple autonomous control scheme for the neuron prosthetic system envisioned.</p>
<p>While the neural prosthetic system is in its infancy, many theoretical and experimental works lay the foundation for a bright future in this field. This thesis answers the spike train characterization and decoding questions in a theoretical manner while offering several novel techniques that bring new ideas and insights into the research field.</p>https://thesis.library.caltech.edu/id/eprint/2967Impulse Generation by Detonation Tubes
https://resolver.caltech.edu/CaltechETD:etd-05252004-164627
Authors: {'items': [{'email': 'macoope@sandia.gov', 'id': 'Cooper-Marcia-Ann', 'name': {'family': 'Cooper', 'given': 'Marcia Ann'}, 'orcid': '0000-0003-0831-6109', 'show_email': 'YES'}]}
Year: 2004
DOI: 10.7907/97GS-4N79
<p>Impulse generation with gaseous detonation requires conversion of chemical energy into mechanical energy. This conversion process is well understood in rocket engines where the high pressure combustion products expand through a nozzle generating high velocity exhaust gases. The propulsion community is now focusing on advanced concepts that utilize non-traditional forms of combustion like detonation. Such a device is called a pulse detonation engine in which laboratory tests have proven that thrust can be achieved through continuous cyclic operation. Because of poor performance of straight detonation tubes compared to conventional propulsion systems and the success of using nozzles on rocket engines, the effect of nozzles on detonation tubes is being investigated. Although previous studies of detonation tube nozzles have suggested substantial benefits, up to now there has been no systematic investigations over a range of operating conditions and nozzle configurations. As a result, no models predicting the impulse when nozzles are used exist. This lack of data has severely limited the development and evaluation of models and simulations of nozzles on pulse detonation engines.</p>
<p>The first experimental investigation measuring impulse by gaseous detonation in plain tubes and tubes with nozzles operating in varying environment pressures is presented. Converging, diverging, and converging-diverging nozzles were tested to determine the effect of divergence angle, nozzle length, and volumetric fill fraction on impulse. The largest increases in specific impulse, 72% at an environment pressure of 100 kPa and 43% at an environment pressure of 1.4 kPa, were measured with the largest diverging nozzle tested that had a 12 degree half angle and was 0.6 m long. Two regimes of nozzle operation that depend on the environment pressure are responsible for these increases and were first observed from these data. To augment this experimental investigation, all data in the literature regarding partially filled detonation tubes was compiled and analyzed with models investigating concepts of energy conservation and unsteady gas dynamics. A model to predict the specific impulse was developed partially filled tubes. The role of finite chemical kinetics in detonation products was examined through numerical simulations of the flow in nonsteady expansion waves.</p>https://thesis.library.caltech.edu/id/eprint/2047Application of Steady and Unsteady Detonation Waves to Propulsion
https://resolver.caltech.edu/CaltechETD:etd-04222004-121013
Authors: {'items': [{'id': 'Wintenberger-Eric', 'name': {'family': 'Wintenberger', 'given': 'Eric'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/2NXT-SE76
The present work investigates the applications of steady and unsteady detonation waves to air-breathing propulsion systems. The efficiency of ideal detonation-based propulsion systems is first investigated based on thermodynamics. We reformulate the Hugoniot analysis of steady combustion waves for a fixed initial stagnation state to conclude that steady detonation waves are less desirable than deflagrations for propulsion. However, a thermostatic approach shows that unsteady detonations have the potential for generating more work than constant-pressure combustion. The subsequent work focuses on specific engine concepts. A flow path analysis of ideal steady detonation engines is conducted and shows that their performance is limited and poorer than that of the ideal ramjet or turbojet engines. The limitations associated with the use of a steady detonation in the combustor are drastic and such engines do not appear to be practical. This leads us to focus on unsteady detonation engines, i.e., pulse detonation engnes. The unsteady generation of thrust in the simple configuration of a detonation tube is first analyzed using gas dynamics. We develop one of the first models to quickly and reliably estimate the impulse of a pulse detonation tube. The impulse is found to scale directly with the mass of explosive in the tube and the square root of the energy release per unit mass of the mixture. Impulse values for typical fuel-oxidizer mixtures are found to be on the order of 160 s for hydrocarbon-oxygen mixtures and 120 s for fuel-air mixtures at standard conditions. These results are then used as a basis to develop the first complete system-level performance analysis of a supersonic, single-tube, air-breathing pulse detonation engine. We show that hydrogen- and JP10-fueled pulse detonation engines generate thrust up to a Mach number of 4, and that the specific impulse decreases quasi-linearly with increasing flight Mach number. Finally, we find that the performance of our pulse detonation engine exceeds that of the ramjet below a Mach number of 1.35.https://thesis.library.caltech.edu/id/eprint/1451Investigations of Spheromak Plasma Dynamics: High-Speed Imaging at the Sustained Spheromak Physics Experiment and Magnetic Diagnostics at the Caltech Spheromak Experiment
https://resolver.caltech.edu/CaltechETD:etd-02042005-150634
Authors: {'items': [{'email': 'romero@umbc.edu', 'id': 'Romero-Talamás-Carlos-Alejandro', 'name': {'family': 'Romero-Talamás', 'given': 'Carlos Alejandro'}, 'orcid': '0000-0002-6830-3126', 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/Z21H-6A46
<p>This thesis consists of two parts. The first part describes a specially designed high-speed imaging system installed at the Sustained Spheromak Physics Experiment (SSPX). Thousands of images have been obtained at SSPX using a high-speed, 1280 x 1024 pixel, cooled and intensified CCD camera with double frame capability, and show unprecedented details of the SSPX plasma. From these images, three different stages were identified according to distinct plasma features. These stages are breakdown and ejection, sustainment, and decay.</p>
<p>During the breakdown and ejection stage, JxB forces push the plasma and stretches the initial vacuum field into the flux conserver. As the plasma enters the field of view of the camera, undulations in the expansion front are visible. These undulations are caused by filaments formed in the gun region, and merge as they travel towards the flux conserver and rotate around the chamber axis. In less than 100 microseconds after breakdown, a transient plasma column is formed. Just microseconds after this, the column bends impulsively and seemingly merges in the toroidal direction (around the axis of the chamber). It is conjectured that the bending precedes a reconnection event that leads to magnetic flux amplification.</p>
<p>Images taken during the sustainment stage show the presence of a central column which is very stable. Some images suggest nested current channels in this column. Comparisons of column diameter measurements versus numerical modeling (using the CORSICA code) are presented here. Bright and distinct patterns were observed on the surface of the source cathode, and appear to be related to the sustainment column and open flux surfaces. These patterns elongate toroidally in a constant direction which depends on the bias field polarity. It is conjectured that the pattern motion is caused by E x B drifts, or J x B effects near the cathode surface.</p>
<p>Most of the hardware was specially designed for the high-speed imaging system, including a double-branch fiber bundle that was used to produce rough tomography (at midplane) of the transient central column. The algorithm used for tomographical reconstruction is based on a maximum entropy restoration method that was also used to improve noisy and blurry images.</p>
<p>The second part of this thesis describes a 60-element magnetic probe array that was constructed using miniature commercial chip inductors. The coils are oriented in orthogonal directions to yield three-dimensional information. The probe has been used to investigate magnetic evolution at the Caltech Spheromak Experiment.</p>https://thesis.library.caltech.edu/id/eprint/491Wave Propagation in Granular Materials
https://resolver.caltech.edu/CaltechETD:etd-02072005-120514
Authors: {'items': [{'email': 'hostler@case.edu', 'id': 'Hostler-Stephen-Richard', 'name': {'family': 'Hostler', 'given': 'Stephen Richard'}, 'orcid': '0000-0002-3160-1269', 'show_email': 'YES'}]}
Year: 2005
DOI: 10.7907/NQ1S-5E45
<p>Wave propagation is a fundamental property of all physical systems. The wave speed is directly related to the compressibility of the system and determines the rate at which local disturbances are propagated into the bulk of the material. The wave propagation characteristics of conventional forms of matter are well understood and well documented. In contrast, waves in granular materials are more complex due to the heterogeneous nature of these systems. The key element of the mechanics of a granular system is the force chain. It is along these preferentially stressed chains of particles that waves are transmitted. These nonlinear chains are heavily dependent on the geometry of the bed and are prone to rearrangement even by the slightest of forces.</p>
<p>Results from both experiments and simulations on wave propagation in granular materials are presented in the current study. The experiments measure the pressures at two points within the granular bed that result from the motion of a piston at one end of the bed. The simulations are a two-dimensional version of the experiments and use a discrete, soft-particle method to detect the wave at both the output of the simulated bed and at any point within it. In addition to examining wave propagation in a granular bed at rest, simulations and experiments are also performed for a granular bed undergoing agitation perpendicular to the direction of the wave input. Imposed agitation increases the granular temperature of the bed and allows for the exploration of the effect of granular state changes on the wave propagation characteristics. Such information may provide a means to diagnose the state of a flowing granular material.</p>
<p>Measurements of the wave speed and attenuation in the bed reveal the unique properties of waves in granular systems that result from the nonlinearity of the bed and the heterogeneity of the force chains. Sinusoidal waves demonstrate the nondispersive nature of a granular bed and show the transient effects of force chain rearrangement. Pulsed waves display a semi-permanent shape qualitatively similar to predictions from nonlinear wave theory. In an agitated granular bed, measurements of the wave characteristics were found to be possible even in the presence of significant agitation. The prevailing confining pressure, which changes throughout the agitation cycle, was determined to be the system parameter that correlates best with changes to the wave speed.</p>https://thesis.library.caltech.edu/id/eprint/533Micro Electret Power Generators
https://resolver.caltech.edu/CaltechETD:etd-06092005-112430
Authors: {'items': [{'email': 'justin.s.boland@jpl.nasa.gov', 'id': 'Boland-Justin-Scott', 'name': {'family': 'Boland', 'given': 'Justin Scott'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/B16C-NT21
The taming of electricity and its widespread use allows people to see in the dark, to speak to one another instantaneously across the earth, and it allows retrieval of data from instruments sent out of the solar system. It is right to expect that the uses and demand for electricity will continue to grow, and to extend the ability to generate electricity; here two new micromachined devices for converting mechanical energy into electrical energy are presented. Aided by the wealth of micromachining process technology, generators that use an oscillatory motion to modify the physical structure of a capacitor with a built-in electric field provided by a permanent electret have been designed, built, and tested. The electret creates an electric field inside the capacitor structure, which induces mirror charge at some potential. The modification of the capacitor then generates an alternating displacement current through an external circuit, which provides useful electrical power. The electret microphone is a similar well known device for converting pressure waves into electrical signals by varying the distance between two charged capacitive plates. This work explores and proves feasible the ability to use mechanical forces to change the overlapping area of a charged capacitor structure and using mechanical forces to move a liquid into the gap of a charged capacitor structure, changing its permittivity to produce electricity. This work demonstrates 2.5mW of power from a 2cm diameter rotary generator at 12kRPM and 10[micro]w for a 0.1cm3 linear shaking generator at 60Hz.
https://thesis.library.caltech.edu/id/eprint/5228Tools and Algorithms for Mobile Robot Navigation with Uncertain Localization
https://resolver.caltech.edu/CaltechETD:etd-06012006-150109
Authors: {'items': [{'email': 'kristopher.l.kriechbaum@jpl.nasa.gov', 'id': 'Kriechbaum-Kristopher-Lars', 'name': {'family': 'Kriechbaum', 'given': 'Kristopher Lars'}, 'show_email': 'YES'}]}
Year: 2006
DOI: 10.7907/R6YB-NQ21
The ability for a mobile robot to localize itself is a basic requirement for reliable long range autonomous navigation. This thesis introduces new tools and algorithms to aid in robot localization and navigation. I introduce a new range scan matching method that incorporates realistic sensor noise models. This method can be thought of as an improved form of odometry. Results show an order of magnitude of improvement over typical mobile robot odometry. In addition, I have created a new sensor-based planning algorithm where the robot follows the locally optimal path to the goal without exception, regardless of whether or not the path moves towards or temporarily away from the goal. The cost of a path is defined as the path length. This new algorithm, which I call "Optim-Bug," is complete and correct. Finally, I developed a new on-line motion planning procedure that determines a path to a goal that optimally allows the robot to localize itself at the goal. This algorithm is called "Uncertain Bug." In particular, the covariance of the robot's pose estimate at the goal is minimized. This characteristic increases the likelihood that the robot will actually be able to reach the desired goal, even when uncertainty corrupts its localization during movement along the path. The robot's path is chosen so that it can use known features in the environment to improve its localization. This thesis is a first step towards bringing the tools of mobile robot localization and mapping together with ideas from sensor-based motion planning.https://thesis.library.caltech.edu/id/eprint/2363The Microwave Thermal Thruster and Its Application to the Launch Problem
https://resolver.caltech.edu/CaltechETD:etd-06022006-160023
Authors: {'items': [{'email': 'Kevin.Parkin@physics.org', 'id': 'Parkin-Kevin-L-G', 'name': {'family': 'Parkin', 'given': 'Kevin L.G.'}, 'orcid': '0000-0003-4521-8559', 'show_email': 'NO'}]}
Year: 2006
DOI: 10.7907/T337-T709
<p>Nuclear thermal thrusters long ago bypassed the 50-year-old specific impulse (Isp) limitation of conventional thrusters, using nuclear powered heat exchangers in place of conventional combustion to heat a hydrogen propellant. These heat exchanger thrusters experimentally achieved an Isp of 825 seconds, but with a thrust-to-weight ratio (T/W) of less than ten they have thus far been too heavy to propel rockets into orbit.</p>
<p>This thesis proposes a new idea to achieve both high Isp and high T/W: The Microwave Thermal Thruster. This thruster covers the underside of a rocket aeroshell with a lightweight microwave absorbent heat exchange layer that may double as a re-entry heat shield. By illuminating the layer with microwaves directed from a ground-based phased array, an Isp of 700–900 seconds and T/W of 50–150 is possible using a hydrogen propellant. The single propellant simplifies vehicle design, and the high Isp increases payload fraction and structural margins. These factors combined could have a profound effect on the economics of building and reusing rockets.</p>
<p>A laboratory-scale microwave thermal heat exchanger is constructed using a single channel in a cylindrical microwave resonant cavity, and new type of coupled electromagnetic-conduction-convection model is developed to simulate it. The resonant cavity approach to small-scale testing reveals several drawbacks, including an unexpected oscillatory behavior. Stable operation of the laboratory-scale thruster is nevertheless successful, and the simulations are consistent with the experimental results.</p>
<p>In addition to proposing a new type of propulsion and demonstrating it, this thesis provides three other principal contributions: The first is a new perspective on the launch problem, placing it in a wider economic context. The second is a new type of ascent trajectory that significantly reduces the diameter, and hence cost, of the ground-based phased array. The third is an eclectic collection of data, techniques, and ideas that constitute a Microwave Thermal Rocket as it is presently conceived, in turn selecting and motivating the particular experimental and computational analyses undertaken.</p>https://thesis.library.caltech.edu/id/eprint/2405Interaction Law for a Collision Between Two Solid Particles in a Viscous Liquid
https://resolver.caltech.edu/CaltechETD:etd-05262006-120244
Authors: {'items': [{'email': 'fulingy@gmail.com', 'id': 'Yang-Fu-Ling', 'name': {'family': 'Yang', 'given': 'Fu-Ling'}, 'orcid': '0000-0002-6633-6311', 'show_email': 'YES'}]}
Year: 2006
DOI: 10.7907/VFD0-C413
<p>This thesis addresses the problem of inter-particle collisions in a viscous liquid. Experimental measurements were made on normal and oblique collisions between identical and dissimilar pairs of solid spheres. The experimental evidence supports the hypothesis that the normal and the tangential component of motions are decoupled during a rapid collision.</p>
<p>The relative particle motion in the normal direction is crucial to an immersed collision process and can be characterized by an effective coefficient of restitution and a binary Stokes number. The effective coefficient of restitution monotonically decreases with a diminishing binary Stokes number, indicating a particle motion with less inertia and higher hindering fluid forces. The correlation between the two parameters exhibits a similar trend to what is observed in a sphere-wall collision, which motivates a theoretical modeling.</p>
<p>The collision model developed in the current work includes a flow model and a revised rebound scheme. The flow model considers the steady viscous drag, the added mass force, and the history force. How the presence of a second nearby solid boundary affects these forces is investigated. A flow model is proposed with wall-correction terms and is used to predict an immersed pendulum motion toward a solid wall. General agreement with the available experimental data validates the model. The rebound scheme considers the magnitude of the surface roughness and the minimum distance of approach resuling from an elastohydrodynamic contact.</p>
<p>The performance of the collision model in predicting the effective coefficient of restitution is evaluated through comparisons with experimental measurements and an existing elastohydrodynamic collision model that the current work is based on.</p>
<p>Based on the current experimental findings, the tangential component of motion can be described by a dry collision model, provided that the material parameters are properly modified for the interstitial liquid. Two pertinent parameters are the normal effective coefficient of restitution and an effective friction coefficient.</p>https://thesis.library.caltech.edu/id/eprint/2108Measurements of Combustion Dynamics with Laser-Based Diagnostic Techniques
https://resolver.caltech.edu/CaltechETD:etd-12182005-164746
Authors: {'items': [{'email': 'balmoa1@gmail.com', 'id': 'Kang-Dal-Mo', 'name': {'family': 'Kang', 'given': 'Dal Mo'}, 'show_email': 'YES'}]}
Year: 2006
DOI: 10.7907/6F5N-9G88
<p>Since the early days of gas turbine engines, combustion/flow instability inside the combustor has been an issue in many engines, but little has been understood as to how the dynamics of the system involved contribute to the instability. The primary objective of this work is to provide general experimental procedures and to validate methods for examining the dynamic behaviors of combustion systems, and to provide accurate measurements of the combustion dynamics for use as a foundation for further theoretical and numerical research. Knowledge of the fundamental dynamics of combustion systems is crucial in understanding and modeling the flame behavior and enabling the use of insights in design process and for creating robust active control of combustors.</p>
<p>Since mixing plays significant roles in combustion processes, the dynamics of fuel/air mixing were studied. A non-premixed burner was examined with acoustic excitations at 22~55 Hz to assess the mixing and its relation to the thermo-acoustic coupling. Phase-resolved acetone-PLIF was used to image the mixing, and from this the unmixedness was calculated, which quantifies the degree of mixing. The results show that (1) the acoustic waves induce periodicity in the degree of mixing; (2) the way the unmixedness behaves coincides well with the behavior of the Rayleigh index, implying the degree of mixing is a major factor in determining the stability of the combustion system; (3) the two-dimensional measurements of temporal unmixedness effectively visualize the shear mixing zone.</p>
<p>A second low-swirl premixed burner was studied to examine the impact of acoustic waves on the combustion dynamics. Measurements were performed with OH-PLIF, with acoustic forcing up to 400 Hz. Swirl burners at higher pressure are industry standard, and this study examined the dynamics at elevated combustor pressure. The results show that (1) the thermo-acoustic coupling seems to be closely coupled to the vortices generated at the flame boundary; (2) high magnitude of flame response coincides with the high absolute value of Rayleigh index; (3) the way the thermo-acoustic coupling is distributed over the space is highly dependent on the excitation frequencies; (4) high pressure suppresses the sensitivity of combustions system to outside disturbances.</p>https://thesis.library.caltech.edu/id/eprint/5041Numerical Study of Single-Chamber Solid Oxide Fuel Cells
https://resolver.caltech.edu/CaltechETD:etd-05252007-110313
Authors: {'items': [{'id': 'Hao-Yong', 'name': {'family': 'Hao', 'given': 'Yong'}, 'orcid': '0000-0001-7487-9327', 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/H3E7-TY92
<p>Single-chamber solid oxide fuel cells (SCFC) are ones in which the fuel and oxidizer are premixed, and selective electrode catalysts are used to generate the oxygen partial pressure gradient that in a conventional dual-chamber design is produced by physical separation of the fuel and oxidizer streams. The SCFC concept is a novel simplification of a conventional solid oxide fuel cell (SOFC), and SCFCs have been shown capable of generating power densities high enough to make them potentially useful in many applications where the simplicity of a single gas chamber and absence of seals offsets the expected lower efficiency of SCFCs compared to dual-chamber SOFCs.</p>
<p>SCFC performance is found to depend sensitively on cell microstructure, geometry, and flow conditions, and optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. However, research activity in this area is far from sufficient and insights about SCFC systems are very limited. The understanding of many fundamental physical and chemical processes required for improving SCFC designs is often beyond the capability of modern experimental techniques, and efficient experimental studies are often held back by the lack of guidance from theoretical models due to the fact that modeling study about SCFC is very rare to date, and existing models about conventional SOFCs are not suitable for simulating SCFCs because of the inherent differences of single-chamber SOFCs from conventional ones. In order to systematically investigate these problems and optimize the electrical performance of SCFC systems, a 2D numerical model of a single-chamber solid oxide fuel cell (SCFC) operating on hydrocarbon fuels is developed and presented in this work.</p>
<p>The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming/shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including flow rates, flow geometry, temperature, optimal fuel to air ratio, anode thickness, YSZ vs. SDC electrolytes, and fuel cell efficiency and fuel utilization are explored. The reaction zones in the anode of an SOFC with hydrocarbon fuel and oxygen addition is also investigated and much deeper insights are obtained compared to the existing literature. Numerical techniques needed for such investigations are also introduced.</p>
<p>The model is also expanded to simulate fuel cells in the commonly seen dual chamber configuration, including ones with either oxygen-ion conducting electrolytes (SOFCs) or proton conducting electrolytes (solid acid fuel cells). Good agreement with literature results and experimental measurements is obtained.</p>https://thesis.library.caltech.edu/id/eprint/5199The Immersed Boundary Projection Method and Its Application to Simulation and Control of Flows Around Low-Aspect-Ratio Wings
https://resolver.caltech.edu/CaltechETD:etd-05232008-124342
Authors: {'items': [{'email': 'ktaira@eng.famu.fsu.edu', 'id': 'Taira-Kunihiko-Sam', 'name': {'family': 'Taira', 'given': 'Kunihiko (Sam)'}, 'orcid': '0000-0002-3762-8075', 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/VSDD-P465
<p>First, we present a new formulation of the immersed boundary method that is algebraically identical to the traditional fractional step algorithm. This method, called the immersed boundary projection method, allows for the simulations of incompressible flows over arbitrarily shaped bodies under motion and/or deformation in both two and three dimensions. The no-slip condition along the immersed boundary is enforced simultaneously with the incompressibility constraint through a single projection. The boundary force is determined implicitly without any constitutive relations for the rigid body formulation, which in turn allows the use of high CFL numbers in our simulations compared to past methods.</p>
<p>Next, the above immersed boundary projection method is used to analyze three-dimensional separated flows around low-aspect-ratio flat-plate wings. A number of simulations highlighting the unsteady nature of the separated flows are performed for Re = 300 and 500 with various aspect ratios, angles of attack, and planform geometries. The aspect ratio and angle of attack are found to have a large influence on the stability of the wake profile and the force experienced by the low-aspect-ratio wing. At early times, following an impulsive start, topologies of the wake vortices are found to be the same across different aspect ratios and angles of attack. Behind low-aspect-ratio rectangular plates, leading-edge vortices form and eventually separate as hairpin vortices following the start-up. This phenomenon is found to be similar to dynamic stall observed behind pitching plates. The detached structure would then interact with the tip vortices, reducing the downward velocity induced by the tip vortices acting upon the leading-edge vortex. At large time, depending on the aspect ratio and angles of attack, the wakes reach one of the three states: (i) a steady state, (ii) a periodic unsteady state, or (iii) an aperiodic unsteady state. We have observed that the tip effects in three-dimensional flows can stabilize the flow and also exhibit nonlinear interaction with the shedding vortices.</p>
<p>At last, we apply steady blowing to separated flows behind the low-aspect-ratio rectangular wings. The objective of the flow control is to enhance lift at post-stall angles of attack by changing the dynamics of the wake vortices. This controller strengthens the tip vortices by engulfing the trailing-edge vortex sheet to increase the downward thrust and the downward induced velocity onto the leading-edge vortices. The tip vortices that are traditionally considered as an aerodynamic nuisance, have been used favorably to increase lift in post-stall flows for the considered low-aspect-ratio wings.</p>https://thesis.library.caltech.edu/id/eprint/1990Adaptive Feature Selection in Pattern Recognition and Ultra-Wideband Radar Signal Analysis
https://resolver.caltech.edu/CaltechETD:etd-05302008-134607
Authors: {'items': [{'email': 'jianghao@caltech.edu', 'id': 'Jiang-Hao', 'name': {'family': 'Jiang', 'given': 'Hao'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/7NR6-AR24
<p>Feature selection from measured data aims to extract informative features to reveal the statistic or stochastic mechanism underlying the complicated or high dimensional original data. In this thesis, the feature selection problem is probed under two situations, one is pattern recognition and the other is ultra-wideband radar signal analysis.</p>
<p>Classical pattern recognition methods select features by their ability to separate the multiple classes with certain gauge measure. The deficiency in this general strategy is its lack of adaptation in specific situations. This deficiency may be overcome by viewing the selected features as a function of not only the training samples but also the unlabeled test data. From this perspective, this thesis proposes an adaptive sequential feature selection algorithm which utilizes an information-theoretic measure to reduce the classification task complexity sequentially, and finally outputs the probabilistic classification result and its variation level. To verify the potential advantage of this algorithm, this thesis applies it to one important problem of neural prosthesis, which concerns decoding a finite number of classes, intended reach directions, from recordings of neural activities in the Parietal Reach Region of one rhesus monkey. Experimental results show that the classification scheme of combining the adaptive sequential feature selection algorithm and the information fusion method outperforms some classical pattern recognition rules, such as the nearest neighbor rule and support vector machine, in decoding performance.</p>
<p>The second scenario in this thesis targets developing a human presence and motion pattern detector through ultra-wideband radar signal analysis. To augment the detection robustness, both static and dynamic features should be utilized. The static features reflect the information of target geometry and its variability, while the dynamic features extract the temporal structure among radar scans. The problem of static feature selection is explored in this thesis, which utilizes the Procrustes shape analysis to generate the representative template for the target images, and makes statistical inference in the tangent space through the Hotelling one sample test. After that, the waveform shape variation structure is decomposed in the tangent space through the principal component analysis. The selected principal components not only accentuate the prominent dynamics of the target motion, but also generate another informative classification feature.</p>
https://thesis.library.caltech.edu/id/eprint/2318Robotic Training for Motor Rehabilitation after Complete Spinal Cord Injury
https://resolver.caltech.edu/CaltechETD:etd-09202007-135027
Authors: {'items': [{'email': 'yliang@caltech.edu', 'id': 'Liang-Yongqiang', 'name': {'family': 'Liang', 'given': 'Yongqiang'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/T01R-P904
<p>The spinal cord circuits have a great degree of automaticity and plasticity. They are able to generate complex locomotor patterns such as stepping and scratching even without input from supraspinal nervous systems. When provided with ensembles of afferent sensory information input associated with a specific motor task, e.g., stepping, the spinal cord can "learn" to perform that task even if it is isolated from the supraspinal nervous systems.</p>
<p>The plasticity of the spinal cord led researchers to study the use of physical locomotor training, e.g., treadmill step training with body weight support, to rehabilitate locomotor function after spinal cord injury (SCI). With intensive training, the spinal-cord-injured subject can recover some level of stepping ability. Explorations were made in this thesis to find an optimal training paradigm. Novel assist-as-needed paradigms were developed to allow variability during training since it is an intrinsic feature of normal stepping. Comparative experiments were conducted against fixed-trajectory training. Results demonstrated that variability is an important factor to induce more improvement in step training.</p>
<p>Standing is another important function in one's daily life, though it received less research attention than stepping. A prototype stand platform with 6 degrees of freedom was developed as an experimental tool for stand and postural study. Analogous to step training, we tested the effect of daily training on extensor responses in the hind limbs of complete spinal rats. The results showed no significant effect of the training. This led to the conclusion that without tonic input, the spinal cord has very limited ability to generate enough extensor muscle tone and to respond to postural disturbance. Further studies in standing rehabilitation should combine other methods to provide tonic inputs to the spinal cord.</p>https://thesis.library.caltech.edu/id/eprint/3666Surface Deformation in a Liquid Environment Resulting from Single Particle Collisions
https://resolver.caltech.edu/CaltechETD:etd-05222008-090253
Authors: {'items': [{'email': 'angelruizangulo@gmail.com', 'id': 'Ruiz-Angulo-Angel', 'name': {'family': 'Ruiz-Angulo', 'given': 'Angel'}, 'orcid': '0000-0002-7292-3002', 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/WVJT-TG10
<p>Multiphase flows are fairly complex and they are usually studied as a bulk. In this thesis, these flows are approached by looking at single particle interactions (particle-particle and particle-wall). This work presents experimental measurements of the approach and rebound of a particle colliding with a ``deformable' surface in a viscous liquid. The complex interaction between the fluid and the solid phases is coupled through the dynamics of the flow as well as the deformation process. A simple pendulum experiment was used to produced single controlled collisions; steel particles were used to impact different aluminum alloy samples (Al-6061, Al-2024, and Al-7075) using different aqueous mixtures of glycerol and water as a viscous fluid. The velocity of the particle before and after the collision was estimated by post-processing the particle position recorded with a high speed camera. For the combination of materials proposed, the elastic limit is reached at relatively low velocities. The deformations produced by the collision were analyzed using an optical profilometer. The measurements showed that the size of the indentations is independent of the fluid media. It was found that the size of the indentations was the same for collisions in air than for the rest of the collisions using various viscous fluids. The results show that the plastic deformation is only a function of the impact velocity and the material properties. The normal coefficient of restitution and deformation parameters account for losses due to lubrication effect and inelasticity, identifying then, the dominant energy loss mechanism during the collision process. </p>
<p>According to the strain imposed in the samples due to the collision, the deformations were either elastic or elastic-plastic. The equivalent load due to the impact velocities used in this work did not reach the fully-plastic regime. For the collisions in air, different models were used to compare the experimental results showing that the elastic-plastic regime is not well characterized by only the material properties and the impact velocity. The time-resolved contact force was measured during the process of the indentation for the dry collision experiments using a quartz load transducer.</p>
<p>The experiments clearly show four different regimes depending on the impact Stokes number: lubrication effect and elastic deformation, lubrication effect and elastic-plastic deformation, elastic deformation with no hydrodynamic effects, and elastic-plastic deformation with negligible lubrication effect. An analysis of the erosion of ductile materials during immersed collisions is presented. The size of the crater formed by the impact of a single particle against a ductile target can be estimated from theory, and these estimates agree well with experimental measurements. </p>https://thesis.library.caltech.edu/id/eprint/1961Rheological Measurements in Liquid-Solid Flows
https://resolver.caltech.edu/CaltechETD:etd-03032009-092653
Authors: {'items': [{'email': 'erinkoos@gmail.com', 'id': 'Koos-Erin-Crystal', 'name': {'family': 'Koos', 'given': 'Erin Crystal'}, 'orcid': '0000-0002-2468-2312', 'show_email': 'YES'}]}
Year: 2009
DOI: 10.7907/KKTC-B990
<p>This thesis presents experimental measurements of the shear stresses of a fluid-particulate flow at high Reynolds numbers as a function of the volume fraction of solids. From the shear stress measurements an effective viscosity, where the fluid-particulate flow is treated as a single fluid, is determined. This viscosity varies from the fluid viscosity when no solids are present to several orders of magnitude greater than fluid viscosity when the particles near their maximum packing state. It is the primary goal of this thesis to determine how the effective viscosity varies with the volume fraction of solids.</p>
<p>A variety of particle sizes, shapes, and densities were obtained through the use of polystyrene, nylon, polyester, styrene acrylonitrile, and glass particles, used in configurations where the fluid density was matched and where the particles were non-neutrally buoyant. The particle sizes and shapes ranged from 3 mm round glass beads to 6.4 mm nylon to polystyrene elliptical cylinders. To properly characterize the effect of volume fraction on the effective viscosity, the random loose- and random close-packed volume fractions were experimentally determined using a counter-top container that mimicked the in situ (concentric cylinder Couette flow rheometer) conditions. These volume fractions depend on the shape of the particles and their size relative to the container.</p>
<p>The effective viscosity for neutrally buoyant particles increases exponentially with volume fraction at fractions less than the random loose-packing. Between the random loose- and random close-packed states, the effective viscosity increases more rapidly with volume fraction and asymptotes to very large values at the close-packed volume fraction. The effective viscosity does not depend on the size or shape of particles beyond the influence these parameters have on the random packing volume fractions.</p>
<p>For non-neutrally buoyant particles, the difference in particle buoyancy requires an additional correction. The volume fraction at the time of the force measurement was recorded for several different ratios of particle-to-fluid density. This volume fraction increases with the shear rate of the Couette flow and decreases with the Archimedes number in a way that when plotted against the Reynolds number over the Archimedes number, these curves collapse onto one master curve. When the local volume fraction is used, the effective viscosity for non-neutrally buoyant particles shows the same dependence on volume fraction as the neutrally buoyant cases.</p>
<p>Particle velocities were also measured for both neutrally buoyant and non-neutrally buoyant particles. These particle velocities near the stationary inner wall show evidence for a small region near the walls with few particles. This particle depletion layer was measured directly using the velocity data and indirectly using the difference between the measured effective viscosities for the smooth- and rough-wall configurations. The slip in the smooth wall experiments can significantly affect the measured viscosity, but this deficiency can be corrected using the thickness of the depletion layer to find the actual value for the effective viscosity.</p>
https://thesis.library.caltech.edu/id/eprint/858Large-Eddy Simulation of Flow Separation and Control on a Wall-Mounted Hump
https://resolver.caltech.edu/CaltechETD:etd-06022009-183247
Authors: {'items': [{'email': 'jennifer.franck@gmail.com', 'id': 'Franck-Jennifer-Ann', 'name': {'family': 'Franck', 'given': 'Jennifer Ann'}, 'show_email': 'YES'}]}
Year: 2009
DOI: 10.7907/DH38-D592
Active flow control techniques such as synthetic jets have been successful in increasing the performance of naturally separating flows on post-stall airfoils, bluff body shedding, and internal flows such as wide-angle diffusers. However, in order to implement robust control techniques there is a need for accurate computational tools capable of predicting unsteady separation and control at high Reynolds numbers. This thesis developed a compressible large-eddy simulation (LES) and validated it by simulating the turbulent flow over a wall-mounted hump. The flow is characterized by an unsteady, turbulent recirculation region along the trailing edge of the geometry, and is simulated at a Reynolds number of 500,000. Active flow control is applied just before the natural separation point via steady suction and zero-net mass flux oscillatory forcing. The addition of control is shown to be effective in decreasing the size of the separation bubble and pressure drag. LES baseline and controlled results are validated against previously performed experiments by Seifert and Pack and those performed for the NASA Langley Workshop on Turbulent Flow Separation and Control. Three test cases are explored to determine the effect of explicit filtering and the Smagorinsky subgrid scale model on the average flow and turbulent statistics. The flow physics and the control effectiveness are investigated at two Mach numbers, M=0.25 and M=0.6. Compressibility is shown to increase the separation bubble length in the baseline case, but does not significantly change the effectiveness of the control. In terms of decreasing drag on the wall-mounted hump model, steady suction is more effective than oscillatory control, but both control techniques are effective in reducing the separation bubble length. Two-dimensional direct numerical simulations (DNS) of the wall-mounted hump flow are also presented, and the results show different baseline flow features than the 3D LES. However the controlled 2D flow gives an indication of the most receptive actuation frequencies around twice that of the natural shedding frequency. Two regimes of reduced actuation frequency are also explored with the 3D LES. It is found that the low frequency actuation is successful in reducing the separation bubble length, but high frequency actuation produces an average flow comparable to the baseline case, and does not result in drag or separation bubble length reduction.
https://thesis.library.caltech.edu/id/eprint/5221Continuum Modeling of Mixed Conductors: A Study of Ceria
https://resolver.caltech.edu/CaltechETD:etd-07212009-142144
Authors: {'items': [{'email': 'francesco.ciucci@gmail.com', 'id': 'Ciucci-Francesco', 'name': {'family': 'Ciucci', 'given': 'Francesco'}, 'orcid': '0000-0003-0614-5537', 'show_email': 'YES'}]}
Year: 2009
DOI: 10.7907/3TWK-W923
<p>In this thesis we have derived a new way to analyze the impedance response of mixed conducting materials for use in solid oxide fuel cells (SOFCs), with the main focus on anodic materials, in particular cerium oxides.</p>
<p>First we have analyzed the impact of mixed conductivity coupled to electrocatalytic behavior in the linear time-independent domain for a thick ceria sample. We have derived that, for a promising fuel cell material, Samarium Doped Ceria, chemical reactions are the determining component of the polarization resistance.</p>
<p>As a second step we have extended the previous model to the time-dependent case, where we focused on single harmonic excitation, the impedance spectroscopy conditions. We extended the model to the case where some input diffusivities are spatially nonuniform. For instance we considered the case where diffusivities change significantly in the vicinity of the electrocatalytic region.</p>
<p>As a third and final step we use to model to capture the two dimensional behavior of mixed conducting thin films, where the electronic motion from one side of the sample to the other is impeded. Such conditions are similar to those encountered in fuel cells where an electrolyte conducting exclusively oxygen ions is placed between the anode and the cathode. The framework developed was also extended to study a popular cathodic material, Lanthanum Manganite.</p>
<p>The model is used to give unprecedented insight in SOFC polarization resistance analysis of mixed conductors. It helps elucidate rigorously rate determining steps and to address the interplay of diffusion with diffusion losses. Electrochemical surface losses dominate for most experimental conditions of Samarium Doped Ceria and they are shown to be strongly dependent on geometry.</p>
https://thesis.library.caltech.edu/id/eprint/5273An Experimental and Numerical Study of Normal Particle Collisions in a Viscous Liquid
https://resolver.caltech.edu/CaltechTHESIS:05272010-165618198
Authors: {'items': [{'email': 'xiaobai@caltech.edu', 'id': 'Li-Xiaobai', 'name': {'family': 'Li', 'given': 'Xiaobai'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/S5V0-3E25
When two solid bodies collide in a liquid environment, the collision process is influenced by viscous effects and the increased pressure in the interstitial liquid layer between the two solid boundaries. A normal collision process is investigated for a range of impact Stokes numbers using both experimental and numerical methods. Experiments of a steel sphere falling under gravity and colliding with a Zerodur wall with Stokes number ranging from 5 to 100 are performed, which complement previous investigations of immersed particle-wall collision processes. The incompressible Navier-Stokes equations are solved numerically to predict the coupled motion of the falling particle and the surrounding fluid as the particle impacts and rebounds from the planar wall. The numerical method is validated by comparing the numerical simulations of a settling sphere with experimental measurements of the sphere trajectory and the accompanying flow-field. A contact model of the liquid-solid and solid-solid interaction is developed that incorporates the elasticity of the solids to permit the rebound trajectory to be simulated accurately. The contact model is applied when the particle is sufficiently close to the wall that it becomes difficult to resolve the thin lubrication layer. The model is calibrated with measured particle trajectories and is found to represent well the observed coefficient of restitution over a range of impact Stokes numbers from 1 to 1000. In addition, the model is modified to simulate the normal collision of two spheres. The effective coefficient of restitution obtained from the simulation shows a strong dependence on the binary Stokes number accordant with other researcher’s experimental results. The unique behaviors of the two spheres at low binary Stokes number including target motion prior to contact and group motion after collision are simulated by the current work.https://thesis.library.caltech.edu/id/eprint/5866The Bulk Viscosity of Suspensions
https://resolver.caltech.edu/CaltechTHESIS:05282010-012507201
Authors: {'items': [{'email': 'manuj80@gmail.com', 'id': 'Swaroop-Manuj', 'name': {'family': 'Swaroop', 'given': 'Manuj'}, 'show_email': 'YES'}]}
Year: 2010
DOI: 10.7907/HQGZ-DV22
<p>Particles suspended in a fluid are known to undergo variations in the local concentration in many flow situations; essentially a compression or expansion of the particle phase. The modeling of this behavior on a macroscopic scale requires knowledge of the effective bulk viscosity of the suspension, which has not been studied before. The bulk viscosity of a pure compressible fluid is defined as the constant of proportionality that relates the difference between the mechanical pressure and the thermodynamic pressure to the rate of compression. The bulk viscosity of a suspension is defined analogous to that for a pure fluid as the constant of proportionality relating the deviation of the trace of the macroscopic stress from its equilibrium value to the average rate of compression. The compression flow drives the suspension microstructure out of equilibrium and the thermal motion of the particles tries to restore equilibrium. The Peclet number (Pe), defined as the expansion rate made dimensionless with the Brownian time-scale, governs the departure of the microstructure from equilibrium. The microstructural forcing in compression is monopolar for small Pe resulting in a significantly slower spatial and temporal response of the microstructure compared to shearing or diffusive motion.</p>
<p>We have determined the effective suspension bulk viscosity for all concentrations and all rates of compression, accounting for the full thermodynamic and hydrodynamic interactions that particles experience at the micro-scale. Current simulation techniques were enhanced to enable the dynamic simulation of compression flows in a suspension. A 'compression thinning' of the suspension is observed at small rates of compression and there is some 'compression thickening' at large compression rates. The bulk viscosity diverges as the volume fraction nears maximum packing and is in fact larger than the shear viscosity. Existing models for multiphase flows must therefore include the bulk viscosity term to properly simulate variations in particle concentration.</p>
<p>An understanding of bulk viscosity effects in suspensions will enable the modeling of certain aggregation and separation behavior and lead to more accurate models for multiphase flows where there are variations in the particle concentration, such as filtration or fluidization.</p>https://thesis.library.caltech.edu/id/eprint/5874Solid-Oxide Fuel Cell Electrode Microstructures: Making Sense of the Internal Framework Affecting Gas Transport
https://resolver.caltech.edu/CaltechTHESIS:06072010-090420975
Authors: {'items': [{'id': 'Hanna-Jeffrey', 'name': {'family': 'Hanna', 'given': 'Jeffrey'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/NRY2-2K63
Optimal electrodes for solid-oxide fuel cells will combine high porosity for gas diffusion, high phase connectivity for ion and electron conduction, and high surface area for chemical and electrochemical reactions. Tracer-diffusion simulations are used to gain a better understanding of the interplay between microstructure and transport in porous materials. Results indicate that the coefficient of diffusion through a porous medium is a function of the details of the internal geometry (microscopic) and porosity (macroscopic). I report that current solid-oxide fuel cell electrodes produced from high-temperature sintering of ceramic powders severely hinder gas transport because the resulting structures are highly tortuous, complex three-dimensional networks. In addition, poor phase connectivities will assuredly limit ion and electron transport, as well as the density of active sites for power-producing reactions. With new access to a wide range of technologies, micro- and nano-fabrication capabilities, and high-performance materials, there is a new ability to engineer the fuel cell electrode architecture, optimizing the physical processes within, increasing performance, and greatly reducing cost per kilowatt. Even simple packed-sphere and inverse-opal architectures will increase gas diffusion by an order of magnitude, and provide a higher level of connectivity than traditional powder-based structures.https://thesis.library.caltech.edu/id/eprint/5923Booming Sand Dunes
https://resolver.caltech.edu/CaltechTHESIS:05312010-051551703
Authors: {'items': [{'email': 'nmvriend@alumni.caltech.edu', 'id': 'Vriend-Nathalie-Maria', 'name': {'family': 'Vriend', 'given': 'Nathalie Maria'}, 'orcid': '0000-0002-1456-2317', 'show_email': 'YES'}]}
Year: 2010
DOI: 10.7907/BFHE-0969
<p>"Booming" sand dunes are able to produce low-frequency sound that resembles a pure note from a music instrument. The sound has a dominant audible frequency (70-105 Hz) and several higher harmonics and may be heard from far distances away. A natural or induced avalanche from a slip face of the booming dune triggers the emission that may last for several minutes. There are various references in travel literature to the phenomenon, but to date no scientific explanation covered all field observations.</p>
<p>This thesis introduces a new physical model that describes the phenomenon of booming dunes. The waveguide model explains the selection of the booming frequency and the amplification of the sound in terms of constructive interference in a confined geometry. The frequency of the booming is a direct function of the dimensions and velocities in the waveguide. The higher harmonics are related to the higher modes of propagation in the waveguide.</p>
<p>The experimental validation includes quantitative field research at the booming dunes of the Mojave Desert and Death Valley National Park. Microphone and geophone recordings of the acoustic and seismic emission show a variation of booming frequency in space and time. The analysis of the sensor data quantifies wave propagation characteristics such as speed, dispersion, and nonlinear effects and allows the distinction between the source mechanism of the booming and the booming itself.</p>
<p>The migration of sand dunes results from a complicated interplay between dune building, wind regime, and precipitation. The morphological and morphodynamical characteristics of two field locations are analyzed with various geophysical techniques. Ground-penetrating radar images the subsurface structure of the dunes and reveal a natural, internal layering that is directly related to the history of dune migration. The seismic velocity increases abruptly with depth and gradually increases with downhill position due to compaction. Sand sampling shows local cementation of sand grains within the discrete layers that explains the increase in velocity and decrease in porosity. The subsurface layering may influence the speed of dune migration and therefore have important consequences on desertification.</p>
<p>The positive qualitative and quantitative correlation between the subsurface layering in the dune and the manifestation of the booming sound implies a close relation between environmental factors and the booming emission. In this thesis, the frequency of booming is correlated with the depth of the waveguide and the seismic velocities. The variability on location and season suggests that the waveguide theory successfully unravels the phenomenon
of booming sand dunes.</p>https://thesis.library.caltech.edu/id/eprint/5891Colloids in Confined Geometries: Hydrodynamics, Simulation and Rheology
https://resolver.caltech.edu/CaltechTHESIS:05262010-140604509
Authors: {'items': [{'email': 'jswan@mit.edu', 'id': 'Swan-James-W', 'name': {'family': 'Swan', 'given': 'James W.'}, 'orcid': '0000-0002-4244-8204', 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/JJ28-7641
The hydrodynamics of colloids in confined geometries is studied hierarchically beginning with the exact solutions for a spherical particle translating, rotating and deforming in the presence of a plane wall at low Reynolds number. The many-bodied hydrodynamic interactions among a collection of spherical particles near a plane wall are computed and used to study the Brownian motion of confined suspensions. The method of reflections is used to describe the motion of a single spherical particle embedded in the fluid constrained by two, parallel plane walls. From this, tables which are independent of the channel width are generated describing the particle’s response to various force moments. This same approach is expanded to describe the hydrodynamic interactions among the particles comprising a colloidal dispersion confined in a channel. The simulations arising from this theory depict the short-time self-diffusivity, sedimentation rate and high frequency viscosity of suspensions of varying volume fractions in channels of varying widths. A theory for the scattering of evanescent waves by colloidal dispersions is developed and cast in the form of the diffusivity measured by classical light scattering. A series of simulations is conducted to predict the short- time self-diffusivity and the collective diffusivity measured by evanescent wave dynamic light scattering. The thesis concludes with a discussion of how the developed simulations and theories can be extended to make dynamic measurements as well as a brief consideration of some remaining, open questions.https://thesis.library.caltech.edu/id/eprint/5845Particle-Based Modeling of Ni-YSZ Anodes
https://resolver.caltech.edu/CaltechTHESIS:03302012-133547448
Authors: {'items': [{'email': 'vaughantel1@gmail.com', 'id': 'Thomas-Vaughan-Lamar', 'name': {'family': 'Thomas', 'given': 'Vaughan Lamar'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/JF8P-5495
<p>In this work we examine the performance of particle-based models with respect to the Ni-YSZ composite anode system. The conductivity and triple-phase boundary (tpb) of particle-based systems is estimated. The systems considered have mono-dispersed particle size distributions, bi-modal particle size distributions with a YSZ:Ni particle size ratio of 1:0.781, and particle size distributions based on experimental measurements. All three types of systems show qualitative behavioral agreement in terms of conductivity, with clear transition from non-conducting behavior to high conducting behavior over a small transition regime which varied from a nickel phase fraction of .22-.28 for the mono- dispersed cases, 0.19-.0.25 for the bimodal cases, and 0.19-0.30 for the experimentally based cases. Mono-dispersed and simple-polydispersed particle size distribution show very low variation from case to case, with σ/μ ≤ 0.04. Cases based on empirical particle size distribution data demonstrated significantly higher variances which varied over a very large range, 0.3 ≤ σ/μ ≤ 1.1. With respect to the calculations of the TPB length, we find that the same pattern of variance in the measure of the triple-phase boundary length. The TPB length for the mono-dispersed and simple poly-dispersed systems was in the range of 3 × 1012 –4 × 1013 m/m3 . For empirical particle size distribution data the TPB length density was in the range of 8×109–2×1011 m/m3. The variance of the TPB length density follows the same pattern as the conductivity measurements with very low variance for the mono-dispersed and simple poly-dispersed systems and much larger variance for the empirically-based systems. We also examine the association between the TPB length and the availability of conducting pathways for the participating particles xv of individual TPBs. The probability of a TPB having a conducting pathway in the gas phase is essentially 100% in all cases. The probability of an individual tpb section having conducting pathways in either of the solid phases is directly related to percolation condition of that phase.</p>
<p>We also considered a particle-based composite electrode realization based on a three- dimensional reconstruction of an actual Ni-YSZ composite electrode. For this model we used particles which vary in nominal size from 85–465 nm, with size increments of 42.5 nm. We paid particular attention to the coordination numbers between particles and the distribution of particle size interconnections. We found that homogeneous inter-particle connections were far more common than would occur using a random distribution of particles. In particular we found that for a random collection of particles of similar composition the likelihood Ni-Ni particle connections was between 0.18–0.30. For the reconstruction we found the likelihood of Ni-Ni particle connections to be greater than 0.56 in all cases. Similarly, the distribution of connections between particles, with respect to particle size of the participating particles, deviated from what would be expected using a random distribution of particles. Particles in the range of 85–169 nm showed the highest coordination with particles of the same size. Particles in the range of 211–338 nm have the highest coordination with particles of radius 169 nm with very similar distributions. Particles with radius greater than 338 nm represented only 7.2 × 10−3 % of the particles within the reconstruction, and showed the highest coordination with particles of radius of 211 nm, but the distributions vary widely.</p>
<p>In the final chapter, we build a model which can account for mass transfer, hetero- geneous chemistry, surface chemistry, and electrochemistry within a porous electrode. The electric potential is calculated on a particle basis using a network model; gas phase concentrations and surface coverages are calculated with a one-dimensional porous me- dia model. Properties of the porous media are calculated via a TPMC method. TPB electrochemistry is calculated at individual triple phase boundaries within the particle xvi model, based on local gas phase concentrations, surface coverages and particle poten- tials, and then added to the porous media model. Using this tool we are able to calculate the spatial distribution of the Faradaic current within the electrode, and variation in gas phase concentrations within the porous media.</p>https://thesis.library.caltech.edu/id/eprint/6881Axel Rover Tethered Dynamics and Motion Planning on Extreme Planetary Terrain
https://resolver.caltech.edu/CaltechTHESIS:08312011-003358925
Authors: {'items': [{'email': 'pabloam@gmail.com', 'id': 'Abad-Manterola-Pablo', 'name': {'family': 'Abad-Manterola', 'given': 'Pablo'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/MPHD-PC75
<p>Some of the most appealing science targets for future exploration missions in our solar system lie in terrains that are inaccessible to state-of-the-art robotic rovers such as NASA's Opportunity, thereby precluding in situ analysis of these rich opportunities. Examples of potential high-yield science areas on Mars include young gullies on sloped terrains, exposed layers of bedrock in the Victoria Crater, sources of methane gas near Martian volcanic ranges, and stepped delta formations in heavily cratered regions. In addition, a recently discovered cryovolcano on Titan and frozen water near the south pole of our own Moon could provide a wealth of knowledge to any robotic explorer capable of accessing these regions.</p>
<p>To address the challenge of extreme terrain exploration, this dissertation presents the Axel rover, a two-wheeled tethered robot capable of rappelling down steep slopes and traversing rocky terrain. Axel is part of a family of reconfigurable rovers, which, when docked, form a four-wheeled vehicle nicknamed DuAxel. DuAxel provides untethered mobility to regions of extreme terrain and serves as an anchor support for a single Axel when it undocks and rappels into low-ground.</p>
<p>Axel's performance on extreme terrain is primarily governed by three key system components: wheel design, tether control, and intelligent planning around obstacles. Investigations in wheel design and optimizing for extreme terrain resulted in the development of grouser wheels. Experiments demonstrated that these grouser wheels were very effective at surmounting obstacles, climbing rocks up to 90% of the wheel diameter. Terramechanics models supported by experiments showed that these wheels would not sink excessively or become trapped in deformable terrain.</p>
<p>Predicting tether forces in different configurations is also essential to the rover's mobility. Providing power, communication, and mobility forces, the tether is Axel's lifeline while it rappels steep slopes, and a cut, abraded, or ruptured tether would result in an untimely end to the rover's mission. Understanding tether forces are therefore paramount, and this thesis both models and measures tension forces to predict and avoid high-stress scenarios.</p>
<p>Finally, incorporating autonomy into Axel is a unique challenge due to the complications that arise during tether management. Without intelligent planning, rappelling systems can easily become entangled around obstacles and suffer catastrophic failures. This motivates the development of a novel tethered planning algorithm, presented in this thesis, which is unique for rappelling systems.</p>
<p>Recent field experiments in natural extreme terrains on Earth demonstrate the Axel rover's potential as a candidate for future space operations. Both DuAxel and its rappelling counterpart are rigorously tested on a 20 meter escarpment and in the Arizona desert. Through analysis and experiments, this thesis provides the framework for a new generation of robotic explorers capable of accessing extreme planetary regions and potentially providing clues for life beyond Earth.</p>https://thesis.library.caltech.edu/id/eprint/6636Current Transport and Onset-Related Phenomena in an MPD Thruster Modified by Applied Magnetic Fields
https://resolver.caltech.edu/CaltechTHESIS:01252013-171305685
Authors: {'items': [{'email': 'robertcmoeller@gmail.com', 'id': 'Moeller-Robert-Carlos', 'name': {'family': 'Moeller', 'given': 'Robert Carlos'}, 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/39VT-GB93
<p>This work investigated the effects of tailored, externally-applied magnetic fields on current transport and near-anode processes in the plasma discharge of a magnetoplasmadynamic thruster (MPDT). Electrical and plasma diagnostics were used to determine whether applied magnetic fields could mitigate the effects of the "onset" phenomena, including large-amplitude terminal voltage fluctuations and high anode fall voltages associated with unstable operation and anode erosion. A new MPDT was developed and operated with quasi-steady 1 ms pulses from 36 kW to 3.3 MW with argon propellant. Three magnetic configurations studied included self-field operation (without external electromagnets) and two applied poloidal magnetic fields. One configuration used magnetic field lines tangential to the anode lip (and intersecting the anode further upstream) and the other created a magnetic cusp intersecting the anode downstream.</p>
<p>The influence of the applied fields on the discharge current streamlines, current densities, and key plasma properties (electron temperature, number density, and plasma potential) was studied. Key findings included that the current pattern and current densities redistributed to follow the applied magnetic field lines. Also, the anode fall voltage was substantially reduced with both applied fields over a large range of currents (and eliminated at 8 kA). These results occurred because applied magnetic field lines intersecting the anode provided a high conductivity path and reduced the local electric field required to sustain the radial current densities. The applied fields reduced the amplitude and frequency of the terminal voltage fluctuations (up to 49%) over a broad range of currents and also decreased transients in the ion saturation current, which suggest reduction of current filamentation and surface-eroding anode spots. Additionally, the cusp field reduced mean terminal voltages over the entire range of discharge currents (up to 31%), and the tangential field lowered terminal voltages below 10.7 kA. These significant reductions in onset-related behaviors should lead to improved thruster lifetime and increased efficiency. These results suggest a distinctive and more effective approach to influencing the near-anode phenomena and mitigating the effects of onset with appropriately designed applied magnetic fields that differ from those used in the vast majority of conventional, so-called "applied-field MPD thrusters."</p>https://thesis.library.caltech.edu/id/eprint/7442A Novel Methodology for Simulating Contact-Line Behavior in Capillary-Driven Flows
https://resolver.caltech.edu/CaltechTHESIS:05262014-160059300
Authors: {'items': [{'email': 'gerry.della.rocca@gmail.com', 'id': 'Della-Rocca-Gerry-V', 'name': {'family': 'Della Rocca', 'given': 'Gerry V.'}, 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/Z9CN71WW
<p>Despite the wide swath of applications where multiphase fluid contact lines exist, there is still no consensus on an accurate and general simulation methodology. Most prior numerical work has imposed one of the many dynamic contact-angle theories at solid walls. Such approaches are inherently limited by the theory accuracy. In fact, when inertial effects are important, the contact angle may be history dependent and, thus, any single mathematical function is inappropriate. Given these limitations, the present work has two primary goals: 1) create a numerical framework that allows the contact angle to evolve naturally with appropriate contact-line physics and 2) develop equations and numerical methods such that contact-line simulations may be performed on coarse computational meshes.</p>
<p>Fluid flows affected by contact lines are dominated by capillary stresses and require accurate curvature calculations. The level set method was chosen to track the fluid interfaces because it is easy to calculate interface curvature accurately. Unfortunately, the level set reinitialization suffers from an ill-posed mathematical problem at contact lines: a ``blind spot'' exists. Standard techniques to handle this deficiency are shown to introduce parasitic velocity currents that artificially deform freely floating (non-prescribed) contact angles. As an alternative, a new relaxation equation reinitialization is proposed to remove these spurious velocity currents and its concept is further explored with level-set extension velocities. </p>
<p>To capture contact-line physics, two classical boundary conditions, the Navier-slip velocity boundary condition and a fixed contact angle, are implemented in direct numerical simulations (DNS). DNS are found to converge only if the slip length is well resolved by the computational mesh. Unfortunately, since the slip length is often very small compared to fluid structures, these simulations are not computationally feasible for large systems. To address the second goal, a new methodology is proposed which relies on the volumetric-filtered Navier-Stokes equations. Two unclosed terms, an average curvature and a viscous shear VS, are proposed to represent the missing microscale physics on a coarse mesh.</p>
<p>All of these components are then combined into a single framework and tested for a water droplet impacting a partially-wetting substrate. Very good agreement is found for the evolution of the contact diameter in time between the experimental measurements and the numerical simulation. Such comparison would not be possible with prior methods, since the Reynolds number Re and capillary number Ca are large. Furthermore, the experimentally approximated slip length ratio is well outside of the range currently achievable by DNS. This framework is a promising first step towards simulating complex physics in capillary-dominated flows at a reasonable computational expense.</p>https://thesis.library.caltech.edu/id/eprint/8394Experimental Study on Inertial Effects in Liquid-Solid Flows
https://resolver.caltech.edu/CaltechTHESIS:06042015-153430245
Authors: {'items': [{'email': 'esperanza.linares@gmail.com', 'id': 'Linares-Guerrero-Esperanza-Crystal', 'name': {'family': 'Linares-Guerrero', 'given': 'Esperanza Crystal'}, 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9GT5K4J
This thesis presents experimental measurements of the rheological behavior of liquid-solid mixtures at moderate Reynolds (defined by the shear rate and particle diameter) and Stokes numbers, ranging from 3 ≤ Re ≤ 1.6 × 10<sup>3</sup> and 0.4 ≤ St ≤ 195. The experiments use a specifically designed Couette cylindrical rheometer that allows for probing the transition from transporting a pure liquid to transporting a dense suspension of particles. Measurements of the shear stress are presented for a wide range of particle concentration (10 to 60% in volume) and for particle to fluid density ratio between 1 and 1.05. The effective relative viscosity exhibits a strong dependence on the solid fraction for all density ratios tested. For density ratio of 1 the effective viscosity increases with Stokes number (St) for volume fractions (φ) lower than 40% and becomes constant for higher φ. When the particles are denser than the liquid, the effective viscosity shows a stronger dependance on St. An analysis of the particle resuspension for the case with a density ratio of 1.05 is presented and used to predict the local volume fraction where the shear stress measurements take place. When the local volume fraction is considered, the effective viscosity for settling and no settling particles is consistent, indicating that the effective viscosity is independent of differences in density between the solid and liquid phase. Shear stress measurements of pure fluids (no particles) were performed using the same rheometer, and a deviation from laminar behavior is observed for gap Reynolds numbers above 4× 10<sup>3</sup>, indicating the presence of hydrodynamic instabilities associated with the rotation of the outer cylinder. The increase on the effective viscosity with Stokes numbers observed for mixtures with φ ≤ 30% appears to be affected by such hydrodynamic instabilities. The effective viscosity for the current experiments is considerably higher than the one reported in non-inertial suspensions. https://thesis.library.caltech.edu/id/eprint/8989Dynamic Characterization of Micro-Particle Systems
https://resolver.caltech.edu/CaltechTHESIS:07082015-183754265
Authors: {'items': [{'email': 'wei.xun.lin@gmail.com', 'id': 'Lin-Wei-Hsun', 'name': {'family': 'Lin', 'given': 'Wei-Hsun'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9D798BJ
<p>Ordered granular systems have been a subject of active research for decades. Due to their rich dynamic response and nonlinearity, ordered granular systems have been suggested for several applications, such as solitary wave focusing, acoustic signals manipulation, and vibration absorption. Most of the fundamental research performed on ordered granular systems has focused on macro-scale examples. However, most engineering applications require these systems to operate at much smaller scales. Very little is known about the response of micro-scale granular systems, primarily because of the difficulties in realizing reliable and quantitative experiments, which originate from the discrete nature of granular materials and their highly nonlinear inter-particle contact forces.</p>
<p>In this work, we investigate the physics of ordered micro-granular systems by designing an innovative experimental platform that allows us to assemble, excite, and characterize ordered micro-granular systems. This new experimental platform employs a laser system to deliver impulses with controlled momentum and incorporates non-contact measurement apparatuses to detect the particles’ displacement and velocity. We demonstrated the capability of the laser system to excite systems of dry (stainless steel particles of radius 150 micrometers) and wet (silica particles of radius 3.69 micrometers, immersed in fluid) micro-particles, after which we analyzed the stress propagation through these systems.</p>
<p>We derived the equations of motion governing the dynamic response of dry and wet particles on a substrate, which we then validated in experiments. We then measured the losses in these systems and characterized the collision and friction between two micro-particles. We studied wave propagation in one-dimensional dry chains of micro-particles as well as in two-dimensional colloidal systems immersed in fluid. We investigated the influence of defects to wave propagation in the one-dimensional systems. Finally, we characterized the wave-attenuation and its relation to the viscosity of the surrounding fluid and performed computer simulations to establish a model that captures the observed response.</p>
<p>The findings of the study offer the first systematic experimental and numerical analysis of wave propagation through ordered systems of micro-particles. The experimental system designed in this work provides the necessary tools for further fundamental studies of wave propagation in both granular and colloidal systems.</p>https://thesis.library.caltech.edu/id/eprint/9054Dynamics of Chemically Active Suspensions
https://resolver.caltech.edu/CaltechTHESIS:05242016-214836974
Authors: {'items': [{'email': 'wenyan4work@gmail.com', 'id': 'Yan-Wen', 'name': {'family': 'Yan', 'given': 'Wen'}, 'orcid': '0000-0002-9189-0840', 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9Z60M1V
Chemically active particles may swim by self-diffusiophoresis in a concentration gradient of chemical solutes they created themselves by patterned surface catalytic reactions. Those particles can also interact via normal diffusiophoresis in the same solute concentration field. The interaction can be attractive or repulsive. This 'field-driven' nature of the system makes its dynamics different from a thermodynamic system and is analyzed with a new simulation method. Simulations show that attractive active particles exhibit coexistence of dense and dilute regions, but it is different from a liquid-gas phase equilibrium. To explain the behavior, a continuum mechanics theory is developed based on the minimal Active Brownian Particles (ABP) model. In the continuum description, the surface force is found to be the swim stress, which can be anisotropic. The body force includes the average swim force as an internal contribution and an 'activity-gradient' force contribution. Further, behaviors of active matter at the sub-continuum scale are also analyzed. The continuum mechanics theory is shown to accurately describe the behaviors of chemically active particles. Particle clustering is explained with a linear stability analysis, and the steady state is explained with a sedimentation-like mechanical force balance.https://thesis.library.caltech.edu/id/eprint/9743Exploring Thermal Phonon Transport from Atomic to Macroscopic Scales for Energy Conversion and Management
https://resolver.caltech.edu/CaltechTHESIS:06032016-104118221
Authors: {'items': [{'email': 'hua.chengyun1989@gmail.com', 'id': 'Hua-Chengyun', 'name': {'family': 'Hua', 'given': 'Chengyun'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z96H4FDH
<p>Heat is one of the most fundamental forms of energy, and the ability to control heat plays a critical role in most current and future energy applications. Recently, interface engineering between heterogeneous solids has provided new approaches to manipulate heat transport at the scales of the energy carriers in solids, <i>i.e.</i> phonons which are quantized lattice vibrations. For example, nanocrystalline materials, which are polycrystalline materials with nanoscale grain sizes, are promising thermoelectric (TE) materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. On the other hand, inefficient heat dissipation across interfaces has been a long-standing problem that shortens the lifetime of electronics such as light-emitting diodes.</p>
<p>Despite the importance of interfaces, we still lack a comprehensive understanding of interfacial thermal phonon transport. For instance, the Fresnel coefficients enable the straightforward mathematical description of light as it moves between media of differing dielectric constants. Similarly, interfacial phonon transport can also be characterized by transmission coefficients that vary over the broad phonon spectrum in an analogous manner to Fresnel coefficients for light. However, despite decades of work, the spectral profile of these coefficients and how the profile is influenced by the atomic structure of actual interfaces remains unclear. As a result, the basic phenomenon of interfacial heat transport remains among the most poorly understood transport processes.</p>
<p>To elucidate this process, in this thesis we investigate interfacial thermal phonon transport using both modeling and experiment. The first portion of the thesis examines the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys using a novel computational method. We find that the grain boundary may not be as effective as commonly considered in scattering certain phonons, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. Our result will help guide the design of more efficient TEs.</p>
<p>The second part of the thesis focuses on studying heat conduction using the Boltzmann transport equation (BTE), which is the governing equation of energy transport at length scales comparable to phonon mean free paths. The BTE is an integro-differential equation of time, real space, and phase space. Due to its high dimensionality, it is extremely challenging to solve. Here, we develop analytical methods to solve the frequency-dependent BTE, which allow us to obtain simple, closed-form solutions to complex multidimensional problems that have previously been possible to solve only with computationally expensive numerical simulations. We demonstrate that the solution leads to a more accurate measurement of phonon MFP spectra in thermal transient grating experiments.</p>
<p>Finally, we report the first measurements of thermal phonon transmission coefficients at a metal-semiconductor interface using ab-initio phonon transport modeling based on the BTE we develop in the second part and a thermal characterization technique, time-domain thermoreflectance. With our approach, we are able to directly link the atomic structure of an interface to the spectral content of the heat crossing it for the first time. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial heat conduction. </p>
https://thesis.library.caltech.edu/id/eprint/9832Small Scale Turbulence in High Karlovitz Number Premixed Flames
https://resolver.caltech.edu/CaltechTHESIS:03102016-211538603
Authors: {'items': [{'email': 'bobbitt.brock@gmail.com', 'id': 'Bobbitt-Brock-Douglas', 'name': {'family': 'Bobbitt', 'given': 'Brock Douglas'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9Z03649
<p>The purpose of this thesis is to characterize the behavior of the smallest turbulent scales in high Karlovitz number (Ka) premixed flames. These scales are particularly important in the two-way coupling between turbulence and chemistry and better understanding of these scales will support future modeling efforts using large eddy simulations (LES). The smallest turbulent scales are studied by considering the vorticity vector, ω, and its transport equation.</p>
<p>Due to the complexity of turbulent combustion introduced by the wide range of length and time scales, the two-dimensional vortex-flame interaction is first studied as a simplified test case. Numerical and analytical techniques are used to discern the dominate transport terms and their effects on vorticity based on the initial size and strength of the vortex. This description of the effects of the flame on a vortex provides a foundation for investigating vorticity in turbulent combustion.</p>
<p>Subsequently, enstrophy, ω<sup>2</sup> = ω • ω, and its transport equation are investigated in premixed turbulent combustion. For this purpose, a series of direct numerical simulations (DNS) of premixed n-heptane/air flames are performed, the conditions of which span a wide range of unburnt Karlovitz numbers and turbulent Reynolds numbers. Theoretical scaling analysis along with the DNS results support that, at high Karlovitz number, enstrophy transport is controlled by the viscous dissipation and vortex stretching/production terms. As a result, vorticity scales throughout the flame with the inverse of the Kolmogorov time scale, τ<sub>η</sub>, just as in homogeneous isotropic turbulence. As τ<sub>η</sub> is only a function of the viscosity and dissipation rate, this supports the validity of Kolmogorov’s first similarity hypothesis for sufficiently high Ka numbers (Ka ≳ 100). These conclusions are in contrast to low Karlovitz number behavior, where dilatation and baroclinic torque have a significant impact on vorticity within the flame. Results are unaffected by the transport model, chemical model, turbulent Reynolds number, and lastly the physical configuration.</p>
<p>Next, the isotropy of vorticity is assessed. It is found that given a sufficiently large value of the Karlovitz number (Ka ≳ 100) the vorticity is isotropic. At lower Karlovitz numbers, anisotropy develops due to the effects of the flame on the vortex stretching/production term. In this case, the local dynamics of vorticity in the strain-rate tensor, S, eigenframe are altered by the flame. At sufficiently high Karlovitz numbers, the dynamics of vorticity in this eigenframe resemble that of homogeneous isotropic turbulence.</p>
<p>Combined, the results of this thesis support that both the magnitude and orientation of vorticity resemble the behavior of homogeneous isotropic turbulence, given a sufficiently high Karlovitz number (Ka ≳ 100). This supports the validity of Kolmogorov’s first similarity hypothesis and the hypothesis of local isotropy under these condition. However, dramatically different behavior is found at lower Karlovitz numbers. These conclusions provides/suggests directions for modeling high Karlovitz number premixed flames using LES. With more accurate models, the design of aircraft combustors and other combustion based devices may better mitigate the detrimental effects of combustion, from reducing CO<sub>2</sub> and soot production to increasing engine efficiency.</p>https://thesis.library.caltech.edu/id/eprint/9611Dynamic 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/9140Fluid Transport by Aggregations of Small Swimming Organisms
https://resolver.caltech.edu/CaltechTHESIS:12232015-091951061
Authors: {'items': [{'email': 'monicamo@engr.ucr.edu', 'id': 'Martinez-Ortiz-Monica-Paola', 'name': {'family': 'Martinez-Ortiz', 'given': 'Monica Paola'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9057CX7
Diel vertical migration of zooplankton has been proposed to affect global ocean circulation to a degree comparable to physical phenomena. Almost a decade after shipboard measurements showed high kinetic energy dissipation rates in the vicinity of migrating krill swarms, the hypothesis that biogenic mixing is relevant to ocean dynamics and local fluid transport has remained controversial due to the inability to directly measure the efficiency of this biological process. In situ field measurements of individual swimming jellyfish have demonstrated large-scale fluid transport via Darwinian drift, but it has remained an open question how this transport mechanism is manifested in smaller species of vertically-migrating zooplankton that are sufficient in number to be accountable in the dynamics. The goals of the present study are, first, to devise and implement experimental instruments and develop methodologies to investigate this biological process in a laboratory setting and, second, to determine whether efficient fluid transport mechanisms become available during vertical collective motion and, if so, analyze how energy is distributed within the flow. By leveraging the phototactic abilities of zooplankton, a multi-laser guidance system was developed to achieve controllable vertical migrations of A. salina concurrently with laser velocimetry of the surrounding flow. Measurements show that the hydrodynamic interactions between neighboring swimmers during vertical migration result in the development of a pronounced jet opposite to animal motion. In non-stratified fluid, this hydrodynamic feature is shown to trigger a Kelvin-Helmholtz instability that results in the generation of eddy-like structures with characteristic length scales much larger than the individual size of the organisms. Experiments in a thermally stratified water column also display the presence of a downward jet despite the strong stable stratification. Furthermore, overturning regions larger than the size of an individual organism are observed adjacent to the migrating aggregation, suggesting an alternate energy transfer route from the small scale of individual swimmers to significantly larger scales, at which mixing can be efficient via a Rayleigh-Taylor instability. The computed velocity spectrum is consistent with these findings and displays energy input at scales larger than the body length of a single swimmer. The mixing efficiency, inferred from the spectral energy distribution with and without stratification, matches experimentally achieved mixing efficiencies via a Rayleigh-Taylor instability within a stable stratification. According to our findings, biogenic mixing does have the potential to redistribute temperature, salinity and nutrients effectively. We propose the employment of laser control to examine additional species as well as alternative oceanic environments and interrogate its effect on the efficiency of biogenic mixing.
https://thesis.library.caltech.edu/id/eprint/9347Two and Three Finger Caging of Polygons and Polyhedra
https://resolver.caltech.edu/CaltechTHESIS:12062015-164238181
Authors: {'items': [{'email': 'tomfallen@yahoo.com', 'id': 'Allen-Thomas-F', 'name': {'family': 'Allen', 'given': 'Thomas F.'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z93X84KR
<p>Multi-finger caging offers a rigorous and robust approach to robot grasping. This thesis provides several novel algorithms for caging polygons and polyhedra in two and three dimensions. Caging refers to a robotic grasp that does not necessarily immobilize an object, but prevents it from escaping to infinity. The first algorithm considers caging a polygon in two dimensions using two point fingers. The second algorithm extends the first to three dimensions. The third algorithm considers caging a convex polygon in two dimensions using three point fingers, and considers robustness of this cage to variations in the relative positions of the fingers.</p>
<p>This thesis describes an algorithm for finding all two-finger cage formations of planar polygonal objects based on a contact-space formulation. It shows that two-finger cages have several useful properties in contact space. First, the critical points of the cage representation in the hand’s configuration space appear as critical points of the inter-finger distance function in contact space. Second, these critical points can be graphically characterized directly on the object’s boundary. Third, contact space admits a natural rectangular decomposition such that all critical points lie on the rectangle boundaries, and the sublevel sets of contact space and free space are topologically equivalent. These properties lead to a caging graph that can be readily constructed in contact space. Starting from a desired immobilizing grasp of a polygonal object, the caging graph is searched for the minimal, intermediate, and maximal caging regions surrounding the immobilizing grasp. An example constructed from real-world data illustrates and validates the method.</p>
<p>A second algorithm is developed for finding caging formations of a 3D polyhedron for two point fingers using a lower dimensional contact-space formulation. Results from the two-dimensional algorithm are extended to three dimension. Critical points of the inter-finger distance function are shown to be identical to the critical points of the cage. A decomposition of contact space into 4D regions having useful properties is demonstrated. A geometric analysis of the critical points of the inter-finger distance function results in a catalog of grasps in which the cages change topology, leading to a simple test to classify critical points. With these properties established, the search algorithm from the two-dimensional case may be applied to the three-dimensional problem. An implemented example demonstrates the method.</p>
<p>This thesis also presents a study of cages of convex polygonal objects using three point fingers. It considers a three-parameter model of the relative position of the fingers, which gives complete generality for three point fingers in the plane. It analyzes robustness of caging grasps to variations in the relative position of the fingers without breaking the cage. Using a simple decomposition of free space around the polygon, we present an algorithm which gives all caging placements of the fingers and a characterization of the robustness of these cages.</p>https://thesis.library.caltech.edu/id/eprint/9307Packaging and Deployment of Large Planar Spacecraft Structures
https://resolver.caltech.edu/CaltechTHESIS:05232016-115519723
Authors: {'items': [{'email': 'manan.arya@gmail.com', 'id': 'Arya-Manan', 'name': {'family': 'Arya', 'given': 'Manan'}, 'orcid': '0000-0003-3522-6010', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9Z60M0D
<p>This thesis presents a set of novel methods to biaxially package planar structures by folding and wrapping. The structure is divided into strips connected by folds that can slip during wrapping to accommodate material thickness. These packaging schemes are highly efficient, with theoretical packaging efficiencies approaching 100%. Packaging tests on meter-scale physical models have demonstrated packaging efficiencies of up to 83%. These methods avoid permanent deformation of the structure, allowing an initially flat structure to be deployed to a flat state.</p>
<p>Also presented are structural architectures and deployment schemes that are compatible with these packaging methods. These structural architectures use either in-plane pretension -- suitable for membrane structures -- or out-of-plane bending stiffness to resist loading. Physical models are constructed to realize these structural architectures. The deployment of these types of structures is shown to be controllable and repeatable by conducting experiments on lab-scale models.</p>
<p>These packaging methods, structural architectures, and deployment schemes are applicable to a variety of spacecraft structures such as solar power arrays, solar sails, antenna arrays, and drag sails; they have the potential to enable larger variants of these structures while reducing the packaging volume required. In this thesis, these methods are applied to the preliminary structural design of a space solar power satellite. This deployable spacecraft, measuring 60 m x 60 m, can be packaged into a cylinder measuring 1.5 m in height and 1 m in diameter. It can be deployed to a flat configuration, where it acts as a stiff lightweight support framework for multifunctional tiles that collect sunlight, generate electric power, and transmit it to a ground station on Earth.</p>https://thesis.library.caltech.edu/id/eprint/9734Sediment Mobility in Steep Channels and the Transition to Landsliding
https://resolver.caltech.edu/CaltechTHESIS:05272016-111304491
Authors: {'items': [{'email': 'jeff.prancevic@gmail.com', 'id': 'Prancevic-Jeffrey-Paul', 'name': {'family': 'Prancevic', 'given': 'Jeffrey Paul'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9W66HRP
The mobility of sediment in steep mountain rivers controls the denudation rate and height of mountain ranges worldwide. Sediment movement within the steepest terrain often occurs as catastrophic shallow landsliding, posing significant hazards to those living downstream. Despite the importance of steep channels, our observations of sediment transport are mostly limited to rivers with slopes of less than 2°. This prevents us from predicting the runoff required to transport sediment throughout most of the drainage network and from knowing the mode of transport that should dominate (dilute river transport vs. landsliding). I performed a series of laboratory experiments in an artificial river with an adjustable slope to test the flow depths required to transport sediment on slopes up to the dry angle of repose. Counterintuitively, sediment becomes harder to move on steeper slopes by dilute river processes. Laboratory observations of flow hydraulics and field observations of cobble stability reveal that this reduced mobility is a hydraulic effect resulting from the shallow flows that are inherent to steep channels. In experiments that were conducted at slopes steeper than half of the dry angle of repose, sediment was more easily transported by shallow landsliding than dilute river processes. Within this landsliding regime, sediment was again observed to be more stable than predicted by traditional theory. Documentation of these experimental failures with high-speed video revealed that failures occur with a characteristic length scale that is shorter than predicted, and that these short failures experience a strong buttressing force at their downstream margin. These results suggest that landslide length scales consistently with width, and also provides new expectations for the saturation level required to initiate failures. Ultimately, these experiments provide us with expectations of the flow depth required to transport sediment throughout the entire drainage network, and also allow us to partition the drainage network into river-dominated and landslide-dominated regimes.https://thesis.library.caltech.edu/id/eprint/9785Kinematics and Local Motion Planning for Quasi-static Whole-body Mobile Manipulation
https://resolver.caltech.edu/CaltechTHESIS:05222016-095145651
Authors: {'items': [{'email': 'krishnashankar+thesis@gmail.com', 'id': 'Shankar-Krishna', 'name': {'family': 'Shankar', 'given': 'Krishna'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9KK98RX
<p>This thesis studies mobile robotic manipulators, where one or more robot manipulator arms are
integrated with a mobile robotic base. The base could be a wheeled or tracked vehicle, or it might be a
multi-limbed locomotor. As robots are increasingly deployed in complex and unstructured environments,
the need for mobile manipulation increases. Mobile robotic assistants have the potential to revolutionize human
lives in a large variety of settings including home, industrial and outdoor environments.</p>
<p>Mobile Manipulation is the use or study of such mobile robots as they interact with physical
objects in their environment. As compared to fixed base manipulators, mobile manipulators can take
advantage of the base mechanism’s added degrees of freedom in the task planning and execution process.
But their use also poses new problems in the analysis and control of base system stability, and the
planning of coordinated base and arm motions. For mobile manipulators to be successfully and
efficiently used, a thorough understanding of their kinematics, stability, and capabilities is required.
Moreover, because mobile manipulators typically possess a large number of actuators, new and efficient
methods to coordinate their large numbers of degrees of freedom are needed to make them practically
deployable. This thesis develops new kinematic and stability analyses of mobile manipulation, and new
algorithms to efficiently plan their motions.</p>
<p>I first develop detailed and novel descriptions of the kinematics governing the operation of multi-
limbed legged robots working in the presence of gravity, and whose limbs may also be simultaneously
used for manipulation. The fundamental stance constraint that arises from simple assumptions about
friction and the ground contact and feasible motions is derived. Thereafter, a local relationship between
joint motions and motions of the robot abdomen and reaching limbs is developed. Baseeon these
relationships, one can define and analyze local kinematic qualities including limberness, wrench
resistance and local dexterity. While previous researchers have noted the similarity between multi-
fingered grasping and quasi-static manipulation, this thesis makes explicit connections between these two
problems.</p>
<p>The kinematic expressions form the basis for a local motion planning problem that that
determines the joint motions to achieve several simultaneous objectives while maintaining stance stability
in the presence of gravity. This problem is translated into a convex quadratic program entitled the
balanced priority solution, whose existence and uniqueness properties are developed. This problem is
related in spirit to the classical redundancy resoxlution and task-priority approaches. With some simple
modifications, this local planning and optimization problem can be extended to handle a large variety of
goals and constraints that arise in mobile-manipulation. This local planning problem applies readily to
other mobile bases including wheeled and articulated bases. This thesis describes the use of the local
planning techniques to generate global plans, as well as for use within a feedback loop. The work in this
thesis is motivated in part by many practical tasks involving the Surrogate and RoboSimian robots at
NASA/JPL, and a large number of examples involving the two robots, both real and simulated, are
provided.</p>
<p>Finally, this thesis provides an analysis of simultaneous force and motion control for multi-
limbed legged robots. Starting with a classical linear stiffness relationship, an analysis of this problem for
multiple point contacts is described. The local velocity planning problem is extended to include
generation of forces, as well as to maintain stability using force-feedback. This thesis also provides a
concise, novel definition of static stability, and proves some conditions under which it is satisfied.</p>https://thesis.library.caltech.edu/id/eprint/9731Theoretical and Experimental Investigation of Phonon Boundary Scattering in Thin Silicon Membranes
https://resolver.caltech.edu/CaltechTHESIS:01172017-145551495
Authors: {'items': [{'email': 'navaneethakrishnan.ravichandran@gmail.com', 'id': 'Ravichandran-Navaneetha-Krishnan', 'name': {'family': 'Ravichandran', 'given': 'Navaneetha Krishnan'}, 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9SJ1HK2
<p>The thermal transport properties of thin semiconductor membranes play an important role in the performance of many technologies like micro-electronics and solid-state energy conversion. The dominant resistance to heat flow in thin membranes is offered by the scattering of thermal phonons at the membrane boundaries. In this dissertation, we examine the nature of microscopic phonon boundary scattering processes and their effect on the thermal conductivity of the thin membranes using a pump-probe experimental technique and computationally efficient solutions of the phonon Boltzmann transport equation (BTE).</p>
<p>First, we investigate the boundary scattering-limited thermal transport in nanostructures using an efficient variance-reduced Monte Carlo (MC) solution of the BTE to elucidate the impact of specular and diffuse phonon boundary scattering events on the thermal conductivity of the nanostructures. To directly measure the relative frequency of these two boundary scattering events, called the phonon specularity parameter, we design, implement and characterize a non-contact laser-based pump-probe experiment called the transient grating (TG) to perform phonon mode-dependent measurements of the specularity parameter in suspended free-standing thin silicon membranes. We describe the phenomenon of quasiballistic heat conduction, which enables the phonon mode-dependent measurements of the specularity parameter, and derive a transfer function based on the BTE with ab-initio phonon properties as inputs, to connect the specularity parameter with the experimentally measured thermal conductivity of the thin membranes.</p>
<p>Finally, we present the methodology adopted to invert the BTE transfer function to extract the phonon specularity parameter from the thermal conductivity measurements in the TG experiment, while rigorously accounting for the experimental uncertainties. We find that the observed magnitudes and trends of the thermal conductivity of the thin membranes cannot be explained by the 50-year old Ziman's model for the phonon specularity parameter and the Fuchs-Sondheimer theory of phonon boundary scattering. We also find that the partially specular boundary scattering picture of phonon boundary interactions works well for one of the membranes, enabling a direct measurement of the mode-dependent phonon specularity parameter for the first time in an experiment. We discuss the possibility of phonon mode conversion at the boundaries of a few membranes for which the partially specular phonon boundary scattering picture fails to explain the observed thermal conductivity trends. Considering the importance of understanding phonon boundary scattering to engineer and improve nanoscale device performance, we expect that the new experimental and computational tools developed in this work will advance a variety of nanoscale energy applications and further our understanding of nanoscale heat transport.</p>https://thesis.library.caltech.edu/id/eprint/10014Constant Stress and Pressure Rheology of Dense Colloidal Suspensions
https://resolver.caltech.edu/CaltechTHESIS:11182016-183910155
Authors: {'items': [{'email': 'mu.wang@gmail.com', 'id': 'Wang-Mu', 'name': {'family': 'Wang', 'given': 'Mu'}, 'orcid': '0000-0001-6090-6187', 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9BC3WHQ
<p>This thesis is a computational investigation on several aspects of the constant stress and pressure rheology of dense polydisperse colloidal suspensions. Using bidisperse suspensions as a model, we first study the influences of size polydispersity on short-time transport properties. The hydrodynamic interactions are calculated using a polydisperse implementation of Stokesian Dynamics (SD) via a Monte-Carlo approach. We carefully compare the SD computations with existing theoretical and numerical results, and critically assess the strengths and weaknesses of the SD algorithm. For suspensions, we find that the Pairwise Additive (PA) approximations with the Percus-Yevick structural input is valid up to volume fraction φ=0.1. We also develop an semi-analytical approximation scheme to predict the wavenumber-dependent partial hydrodynamic functions based on the δγ-scheme of Beenakker & Mazur [Physica 120A (1983) 388 & 126A (1984) 349], which is shown to be valid up to φ=0.4.</p>
<p>To meet the computation requirements of dynamic simulations, we then developed the Spectral Ewald Accelerated Stokesian Dynamics (SEASD) based on the framework of SD with extension to compressible solvents. The SEASD uses the Spectral Ewald (SE) method [Lindbo & Tornberg, J. Comput. Phys. 229 (2010) 8994] for mobility computation with flexible error control, a novel block-diagonal preconditioner for the iterative solver, and the Graphic Processing Units (GPU) acceleration. For further speedup, we developed the SEASD-nf, a polydisperse extension of the mean-field Brownian approximation of Banchio & Brady [J. Chem. Phys. 118 (2003) 10323]. The SEASD and SEASD-nf are extensively validated with static and dynamic computations, and are found to scale as O(NlogN) with N the system size. The SEASD and SEASD-nf agree satisfactorily over a wide range of parameters for dynamic simulations.</p>
<p>Next, we investigate the colloidal film drying processes to understand the structural and mechanical implications when the constant pressure constraint is imposed by confining boundaries. The suspension is sandwiched between a stationary substrate and an interface moving either at a constant velocity or with constant imposed stress. Using Brownian Dynamics (BD) simulations without hydrodynamic interactions, we find that both fast and slow interface movement promote crystallization via distinct mechanisms. The most amorphous suspension structures occur when the interface moves at a rate comparable to particle Brownian motion. Imposing constant normal stresses leads to similar suspension behaviors, except that the interface stops moving when the suspension osmotic pressure matches the imposed stress. We also compare the simulation results with a continuum model. This work reveals the critical role of interface movement on the stress and structure of the suspension.</p>
<p>Finally, we study the constant shear stress and pressure rheology of dense colloidal suspensions using both BD and SEASD-nf to identify the role of hydrodynamic interactions. The constant pressure constraint is imposed by introducing a compressible solvent. We focus on the rheological, structural, and dynamical characteristics of flowing suspensions. Although hydrodynamic interactions profoundly affect the suspension structure and dynamics, they only quantitatively influence the behaviors of amorphous suspensions. The suspension becomes glassy, i.e., exhibits flow-arrest transitions, when the imposed pressure is high, and reveals the Shear Arrest Point (SAP) in the non-Brownian limit. From a granular perspective, we find that the suspensions move away from the arrested state in a universal fashion regardless of the imposed pressure, suggesting the critical role of the jamming physics. The hydrodynamic simulations quantitatively agree with the experiments of Boyer et al. [Phys. Rev. Lett. 107 (2011) 188301] with a volume fraction shift. The results at all imposed stresses and pressures reveal a generalized Stokes-Einstein-Sutherland relation with an effective temperature proportional to the pressure. We develop a model that accurately describes the rheology and diffusion of glassy suspensions. Our results show the critical role of pressure on the behaviors of dense colloidal suspensions.</p>https://thesis.library.caltech.edu/id/eprint/9982Towards a priori Models for Differential Diffusion in Turbulent Non-Premixed Flames
https://resolver.caltech.edu/CaltechTHESIS:06062018-163232775
Authors: {'items': [{'email': 'nicholas@burali.it', 'id': 'Burali-Nicholas', 'name': {'family': 'Burali', 'given': 'Nicholas'}, 'orcid': '0000-0002-0733-0577', 'show_email': 'YES'}]}
Year: 2018
DOI: 10.7907/N3VJ-BE39
<p>In this work, progress is made towards the correct modeling of differential diffusion, both for resolved simulations, and for reduced-order combustion models. For resolved simulations, the validity and the limitations of the constant non-unity Lewis number approach in the description of molecular mixing in laminar and turbulent flames is studied. Three test cases are selected, including a lean, highly unstable, premixed hydrogen/air flame, a lean turbulent premixed n-heptane/air flame, and a laminar ethylene/air coflow diffusion flame. For the hydrogen flame, both a laminar and a turbulent configuration are considered. The three flames are characterized by Lewis numbers which are less than unity, greater than unity, and close to unity, respectively. For each flame, mixture-averaged transport simulations are carried out and used as reference data. The analysis suggests that, for numerous combustion configurations, the constant non-unity Lewis number approximation leads to small errors when the set of Lewis numbers is chosen properly. For the selected test cases and our numerical framework, the reduction of computational cost is found to be minimal. Two different methods of evaluating the Lewis numbers are tested, with both performing well, and neither consistently better than the other.</p>
<p>The flamelet-based chemistry tabulation technique is a popular reduced-order chemical model for non-premixed turbulent flames. In this approach, the correct choice of the species Lewis numbers in the flamelet equations plays an important role. Experimental results have highlighted that, in turbulent non-premixed jet flames, turbulent transport becomes gradually dominant over molecular mixing with (i) increasing axial distance from the burner exit plane, and (ii) increasing jet Reynolds number. In the current work, this transition is characterized and a priori models for the effective species Lewis numbers in turbulent non-premixed flames are assessed.</p>
<p>First, a flamelet-based methodology is proposed to extract these effective Lewis numbers from data sets of turbulent non-premixed flames. This methodology is then applied to the Sandia non-premixed methane/air jet flames B, C, D, and E (R. Barlow, Int. Work. Meas. Comput. Turb. Non-Prem. Flames, 2003). The effective Lewis numbers are found to transition from their laminar values, close to the burner exit plane, to unity further downstream. Previously-suggested scalings for the effective Lewis numbers are then assessed.</p>
<p>To overcome the limitations associated with the experimental data, a campaign of Direct Numerical Simulations (DNS) of Sandia flame B is carried out. A baseline grid is carefully designed, and grid independence is assessed through simulations using refined grids in the axial, radial and azimuthal directions. Radiation and differential diffusion effects are systematically isolated by considering radiating and unity Lewis number cases, respectively. The DNS database is then validated using available measured statistics for flame B, and comparisons to the higher Reynolds number flames are carried out. Effective Lewis numbers extracted from the DNS data are found to transition to unity with increasing downstream distance. Finally, the scalings for the effective Lewis numbers are re-computed from the DNS data base, and compared to the higher Reynolds number flames.</p>https://thesis.library.caltech.edu/id/eprint/11030Thermal Transport in Three-Dimensional Nanoarchitected Materials
https://resolver.caltech.edu/CaltechTHESIS:06022018-070416991
Authors: {'items': [{'email': 'nickdou@gmail.com', 'id': 'Dou-Nicholas-Gang', 'name': {'family': 'Dou', 'given': 'Nicholas Gang'}, 'orcid': '0000-0001-8199-5588', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/TPC8-VH59
<p>Materials that simultaneously possess ultralow thermal conductivity, high stiffness, and damage tolerance are highly desirable for engineering applications. However, this combination of properties has never been demonstrated in a single material because thermal and mechanical properties are coupled in most fully dense and porous solids. A new class of lattice materials with nanoscale features, called nanolattices, can fill this void in the material property space by virtue of their architecture and nanoscale dimensions. Extensive work on nanolattice mechanical properties report their excellent stiffness-to-density ratio and recoverability from large compressive strains. In contrast, the framework for studying their thermal properties has not been established. Our work develops the computational and experimental tools necessary to study heat conduction in nanoarchitected materials and applies those tools to prove the viability of octet-truss nanolattices as multifunctional thermal insulators.</p>
<p>We implement significant improvements to a phonon Monte Carlo method to solve the Boltzmann transport equation (BTE) in highly complex geometries like the octet-truss. No prior works solve the BTE in a domain as intricate as a nanolattice, so we create a geometry representation scheme that can model any arbitrary 3-D body. Our enhanced variance-reduced Monte Carlo code incorporates this scheme, allowing us to predict the thermal conductivity of nanolattices and analyze the phonon transport behavior in them. Results suggest that hollow-beam silicon nanolattices indeed reach ultralow thermal conductivities. Based on Monte Carlo and finite element simulations, we develop a predictive thermal conductivity model that accounts for both diffusive and radiative phonon transport in nanolattices.</p>
<p>We also devise custom modifications to the 3ω method to experimentally measure the thermal conductivity of additively manufactured nanolattices. Since the serial fabrication process of nanolattices makes it costly to cover large areas, we design a specialized 3ω sample that minimizes the required structure size while maintaining good experimental sensitivity. We derive a new thermal model to account for conductive losses through the heater line in our novel sample geometry. 3ω measurements and compression tests of hollow-beam alumina nanolattices show that they combine ultralow thermal conductivity with excellent mechanical stiffness and resilience, which proves that nanolattices occupy a previously unreachable region in material property space. Our work provides motivation to further investigate and improve the thermal properties of architected materials.</p>
https://thesis.library.caltech.edu/id/eprint/11012Derivation of Realistic Forcing Schemes to Reproduce Turbulent Characteristics of Round Jets on Centerline
https://resolver.caltech.edu/CaltechTHESIS:08262019-191842947
Authors: {'items': [{'email': 'jeffrah89@gmail.com', 'id': 'Rah-Kyupaeck-Jeff', 'name': {'family': 'Rah', 'given': 'Kyupaeck Jeff'}, 'orcid': '0000-0003-1898-2930', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/7SXH-V275
<p>Turbulence forcing techniques are often required in the numerical simulation of statistically stationary turbulent flows. However, the existing forcing techniques are not based on physics, but rather arbitrary numerical methods that sustain the turbulent kinetic energy. In this work, a realistic forcing technique is devised to reproduce the centerline turbulent characteristics of round jets in a triply periodic box.</p>
<p>A velocity forcing term is derived from the Navier-Stokes equations by applying a Reynolds decomposition with the mean velocity of the axisymmetric jet. The result is an anisotropic linear forcing term. A series of direct numerical simulations (DNS) are performed over a range of Reynolds numbers by applying the derived velocity forcing term in a 3D cubic box. The budget of the terms in the kinetic energy equation is found to be very close to the experimental measurement on the centerline. The anisotropy ratio, kinetic energy, and dissipation rate of the simulations are also comparable to experimental values. Finally, the kinetic energy spectrum in the axial direction is presented. With appropriate normalizations, the spectrum agrees well with the round jet spectrum on its centerline.</p>
<p>A similar procedure is applied to passive scalars to derive a scalar forcing term to simulate the centerline mixing properties of round jets. The term is derived from the scalar transport equation using a Reynolds-like decomposition of the scalar field. The equation is closed by applying the known mean velocity and scalar profiles of axisymmetric jets. The result is a combination of a mean gradient term and a linear scalar term. DNS at different Reynolds numbers have been performed with these source terms for unity Schmidt number. Scalar flux values and scaling exponents of scalar energy spectra from simulations are comparable to experimental values. In addition, a dimensional analysis shows that the normalized scalar statistics, such as variance, flux, and dissipation rate, should only be a function of Reynolds number; indeed, such quantities computed from our simulations approach constant values as the Reynolds number increases. The effects of velocity forcing on scalar fields are also investigated; changing velocity forcing terms may result in unstable scalar fields even under the same scalar forcing.</p>
<p>More computations on higher Schmidt number scalars are performed with the same velocity and scalar forcing terms. It is found that the scalar flux values decrease with increasing Schmidt number for low Reynolds number flows, and reach plateaus as the Schmidt number increases. The flux values also increase with the Reynolds number for all non-unity Schmidt numbers. The scaling exponents of scalar energy spectra are found to decrease with increasing Schmidt number for all Reynolds numbers.</p>https://thesis.library.caltech.edu/id/eprint/11766Three-Dimensional Quantitative Visualization for Mechanics of Discontinuous Materials
https://resolver.caltech.edu/CaltechTHESIS:08092019-151803660
Authors: {'items': [{'email': 'kamacdonald3@gmail.com', 'id': 'Mac-Donald-Kimberley-Kimberley-Ann', 'name': {'family': 'Mac Donald', 'given': 'Kimberley Ann'}, 'orcid': '0000-0003-4512-9740', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/0M4F-FG13
<p>The complexity and multiscale nature of material microstructures introduces significant intricacies to many mechanics problems for which we do not have a full theoretical understanding. Under loading, these microstructures can introduce significant nonlinearities that cannot be described sufficiently by current theories and models. This leads us to consider experiments we could perform to improve our understanding of such effects. This thesis describes the design of experiments exploring two aspects of material microstructure effects: (i) crack propagation and renucleation in soft brittle polymers and (ii) interparticle forces in granular materials.</p>
<p>First, experimental and analysis methods are developed to study fracture mechanics in soft brittle polymers with the goal of developing a more detailed understanding of the effects of microstructural heterogeneities on crack propagation and renucleation in three-dimensions. To better understand these processes, experiments on crack propagation in thin soft polymers using confocal microscopy images are conducted. Traditional metrics associated with crack propagation including stress intensity factor (SIF, <i>K</i>) and energy release rate (ERR, <i>G</i>) are calculated by direct measurement of the crack tip opening displacement (CTOD, <i>δ<sub>t</sub></i>) on the sub-millimeter scale. Errors in these calculations are comparable to those reported in the literature for more traditional fracture experiment geometries. Fluorescent speckle images are captured using confocal microscopy imaging, a fast and low cost 3D optical imaging technique, to study crack geometry during propagation. Images of renucleation events are also captured allowing investigation of factors contributing to slow crack roughening observed by earlier researchers. The goal of this study is to provide an experimental method to enhance understanding of crack interactions with microstructural heterogeneities and of renucleation events, which can significantly improve our ability to design material toughness.</p>
<p>To begin to understand the effects of engineered microstructural heterogeneities such as inclusions in materials, we must be able to produce such engineered systems and understand the interparticle interactions. To this end, a method to manufacture volumetrically speckled spheres in-house with controlled diameters was developed. Additionally, an experimental method combining confocal microscopy with digital volume correlation (DVC) was also used to study interparticle force transmission in 3D. Analysis of an in-plane 2D projection of volumetric surface data shows that three-dimensional effects play a significant role in the deformation of granular assemblies. Study of a single grain in 3D demonstrates progress in experimental capabilities and highlights the need for more studies to validate existing numerical models and theories for granular matter. Analysis of particle scale deformations and strains with the Granular Element Method (GEM) allows us to determine interparticle forces and understand the development and evolution of force chains in a granular assembly under a wide variety of loading conditions. These experiments can also lead to development of new understanding of the effects of inclusions on material properties, processes, and damage evolution.</p>https://thesis.library.caltech.edu/id/eprint/11759Numerical Simulation of Performance and Solar-to-Fuel Conversion Efficiency for Photoelectrochemical Devices
https://resolver.caltech.edu/CaltechTHESIS:06082021-054649127
Authors: {'items': [{'email': 'yikai.chen@outlook.com', 'id': 'Chen-Yikai-Katie', 'name': {'family': 'Chen', 'given': 'Yikai (Katie)'}, 'orcid': '0000-0002-2955-9671', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/65pq-7d93
<p>The Industrial Revolution was energized by coal, petroleum, and natural gas. It is clear that fossil fuels, which drive steam and electrical engines, made possible a monumental increase in the amount of productive energy available to humans. But in the meantime, the constant burning of fossil fuels has changed the natural greenhouse, intensified global warming, deteriorated air quality, and eventually caused irreversible environmental damage on our planet. Renewable energy especially solar energy offers a desirable approach toward meeting our growing energy needs while largely reducing fossil fuel burning. The major problems in terms of harvesting energy directly from sunlight turn out to be low energy concentration and intermittency. Building solar-fuel generators, which stores solar energy in chemical bonds, similar to photosynthesis in nature, provides a possible solution to these two problems. Carbon-free chemicals, such as hydrogen gas, which are produced by solar-driven water-splitting, or carbon-neutral chemicals, such as methane and ethylene, which are produced by solar-driven CO₂ reduction, are all promising clean fuels for solar storage.</p>
<p>This thesis is focused on studying the performance and solar to fuel conversion efficiency of existing and hypothetical test-bed photoelectrochemical prototypes using multi-physics modeling and simulation to lay a foundation for future implementation and scale-up of the integrated, solar-driven systems. For water-splitting systems, a sensitivity analysis has been made to assess the relative importance of improvements in electrocatalysts, light absorbers, and system geometry on the efficiency of solar-to-hydrogen generators. Besides, an integrated photoelectrolysis system sustained by water vapor is designed and modeled. Under concentrated sunlight, the performance of the photoelectrochemical system with 10× solar concentrators was simulated and the impact of hydrogen bubbles that are generated inside the cathodic chamber on the performance of the photoelectrolysis system was evaluated. For CO₂ reduction systems, operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO₂ were investigated. The spatial and light-intensity dependence of product distributions in an integrated photoelectrochemical CO₂ reduction system was modeled and simulated. Finally, the performance a flow-through gas diffusion electrode for electrochemical reduction of CO or CO₂ was evaluated.</p>
<p>This thesis can be divided into three parts. The first part discusses the importance of solar energy. The second part includes Chapter II, Chapter III, Chapter IV, and Chapter V, which deals with solar-driven water-splitting cells, and the third part includes Chapter VI, Chapter VII, and Chapter VIII, which deals with solar-driven CO₂ reduction cells.</p>https://thesis.library.caltech.edu/id/eprint/14265Streamwise 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/14249New Method and Analysis of Proximity Trajectory-Only Learned Dynamics for Small Body Gravity Fields
https://resolver.caltech.edu/CaltechTHESIS:05272021-220554457
Authors: {'items': [{'email': 'danineamati@gmail.com', 'id': 'Neamati-Daniel-A', 'name': {'family': 'Neamati', 'given': 'Daniel A.'}, 'orcid': '0000-0002-1555-1433', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/4csx-4636
<p>Recent missions to small bodies in the past decade (e.g., <i>Rosetta</i>, <i>Hayabusa 2</i>, and <i>OSIRIS-REx</i>) have reshaped our understanding of small bodies and inspired new, more-capable future missions. Despite the high demand for more missions, large uncertainties in small body properties make missions challenging. Recent work in stochastic optimal control can ensure safety in the face of uncertainty in state, constraints, and dynamics. These stochastic optimal controllers require a model of the underlying dynamics, which is difficult for proximity maneuvers and landing around small bodies. Shape models and finite element-like models are the state-of-the-art for high-fidelity gravity models, but they are computationally expensive and do not readily incorporate onboard data. No gravity model yet exists that can use short-horizon position and acceleration data from recent trajectories onboard in safety-critical autonomous proximity maneuvers and landing. Therefore, we propose a new trajectory-only learning-based method to develop a gravity model. We consider three learning frameworks: Gaussian Process Models, Neural Networks, and Physics-Informed Neural Networks. For each framework, we assess the benefits, computational costs, and limitations of the framework. We found that the Gaussian Process Model generally outperforms the other frameworks in cases of moderate uncertainty. As the uncertainty declines or the data is sufficiently filtered, Neural Networks with spectral normalization provide more accurate gravity models and are computationally cheaper to evaluate. Lastly, we reflect on the methods in this thesis and recommend possible problem reformulations for future research.</p>https://thesis.library.caltech.edu/id/eprint/14182Numerical Investigation of Compressibility Effects in Reacting Subsonic Flows
https://resolver.caltech.edu/CaltechTHESIS:11102020-065104205
Authors: {'items': [{'email': 'guillaume.beardsell@gmail.com', 'id': 'Beardsell-Guillaume', 'name': {'family': 'Beardsell', 'given': 'Guillaume'}, 'orcid': '0000-0001-7138-488X', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/dtfx-gy14
<p>Direct numerical simulations (DNS) of reacting flows are routinely performed either by solving the fully compressible Navier-Stokes equations or using the low Mach number approximation. The latter is obtained by performing a Mach number expansion of the Navier-Stokes equations for small Mach numbers. These two frameworks differ by their ability to capture compressibility effects, which can be broadly defined as phenomena that are not captured by the low Mach number approximation. These phenomena include acoustics, compressible turbulence, and shocks. In this thesis, we systematically isolate compressibility effects in subsonic flows by performing two sets of DNS: one using the fully compressible framework, and one using the low Mach number approximation. We are specifically interested in the interactions between turbulence, acoustics, and flames.</p>
<p>The addition of detailed chemistry in the compressible flow solver required the development of a novel time integration scheme. This scheme combines an iterative semi-implicit method for the integration of the species transport equations, and the classical Runge-Kutta method for the integration of the other flow quantities. It is found to perform well, yielding time steps limited by the acoustic CFL only. Furthermore, the computational cost per iteration of this hybrid scheme is low, being comparable to the one for the classical Runge-Kutta method.</p>
<p>After extensive validation, the first application is the investigation of flame-acoustics interactions in laminar premixed flames. The thermodynamic fluctuations that accompany the acoustic wave are shown to significantly impact the flame response. Using the Rayleigh criterion, the flame-acoustics system is found to be thermo-acoustically unstable for various fuels, flow conditions, and acoustic frequencies. As expected, the low Mach number approximation and the fully compressible framework are in good agreement at low frequencies, since the flame is very thin compared to the acoustic wavelength. The two frameworks differ for very large acoustic frequencies only. In the high frequency limit, the gain reaches a plateau using the low Mach number approximation, while it goes to zero using the fully compressible framework. This is related to the spatial variations in the acoustic pressure field, which are not present in the low Mach number approximation. However, for practically relevant acoustic frequencies, the low Mach number framework is found to yield accurate results.</p>
<p>Next, a numerical methodology to simulate compressible flows in geometries that lack a natural turbulence generation mechanism is presented. It is found that, unlike in incompressible flows, special care must be taken regarding the energy equation and the presence of standing acoustic modes. When using periodic boundary conditions, forcing the dilatational velocity field promotes the growth of unstable modes. This is explained by extracting the eigenvalues of the linearized forced Navier-Stokes equations. Based on these observations, it is found necessary to force the solenoidal velocity field only. This methodology is applied first to simulations of subsonic homogeneous non-reacting turbulence. We present simulations results for turbulent Mach numbers varying from 0.02 to 0.65. The Mach number dependence of various quantities, such as the dilatational to solenoidal kinetic energy ratio, is extracted. The Mach number scaling of all quantities of interest is found to be readily explained by the low Mach number expansion, specifically the zeroth and first order sets of equations, for turbulent Mach numbers up to 0.1.</p>
<p>Finally, the interaction between subsonic compressible turbulence and premixed flames is investigated. Compressibility effects are isolated by comparing results obtained with the low Mach number approximation and the fully compressible framework, at the same flow conditions. Compressibility effects on chemistry are found to be limited for turbulent Mach numbers at least up to 0.4, especially when contrasted with the large impact of the Karlovitz number. Compressibility effects give rise to significant thermodynamic fluctuations away from the flame front, but these remain small compared to the large fluctuations due to the presence of the turbulent flame brush. The low Mach number approximation thus remains a valid framework for the Mach numbers considered, when the primary goal is to characterize the impact of turbulence on the chemical processes at play.</p>https://thesis.library.caltech.edu/id/eprint/13996Investigation of Electronic Fluctuations in Semiconductor Materials and Devices through First-Principles Simulations and Experiments in Transistor Amplifiers
https://resolver.caltech.edu/CaltechTHESIS:12172021-224454947
Authors: {'items': [{'email': 'alex.choi45@gmail.com', 'id': 'Choi-Alexander-Youngsoo', 'name': {'family': 'Choi', 'given': 'Alexander Youngsoo'}, 'orcid': '0000-0003-2006-168X', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/tba0-pd94
<p>Electronic noise, or stochasticity in the current, voltage, and frequency of a carrier signal is caused by microscopic fluctuations in the occupation of quantum electronic states. In the context of scientific instrumentation, understanding the physical origin of these fluctuations is of paramount importance since the associated stochasticity ultimately limits the fidelity of information transmitted through electronically processed-signals. The unifying theme of the work presented in this thesis is the study of electronic fluctuations in semiconductor materials and devices. Our interest in this topic is twofold. First, while the Nyquist law dictates the equivalence of noise and transport properties for systems in thermal equilibrium, this relationship breaks down for systems driven out of equilibrium by external forcing. Simulating non-equilibrium electronic fluctuations can therefore provide new insights into the microscopic processes that control energy and momentum relaxation which would not be available from conventional studies of transport alone. Furthermore, because noise properties are sensitive to the microscopic details of the bandstructure and scattering, <i>ab initio</i> simulations of noise observables provide a more rigorous test of the accepted theory of charge transport and carrier scattering in materials. Second, cryogenic low noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in electromagnetic detector chains in applications such as radio astronomy, deep space communications, and quantum computing. The design and optimization of HEMT devices have conventionally relied upon empirical circuit-level models of fluctuations in devices. As the noise performance of modern low-noise amplifiers has saturated to levels five to ten times above the standard quantum limit, these empirical models are unable to resolve the microscopic origin of the limiting excess noise. Identifying the microscopic mechanisms underpinning noise in modern amplifiers is therefore necessary to produce better devices for scientific instrumentation. In this work, we investigate electronic noise in semiconductor materials and devices with a combination of first-principles simulations and Schottky thermometry experiments in transistor amplifiers.</p>
<p>First, we present our work on the development of novel parameter-free simulations of non-equilibrium noise in semiconductor materials. While the <i>ab initio</i> theory of low-field electronic transport properties such as carrier mobility is well-established, an equivalent treatment of electronic fluctuations about a non-equilibrium steady state has remained less explored. Starting from the Boltzmann Transport Equation, we develop an <i>ab initio</i> method for hot electron noise in semiconductors. In contrast with the typical numerical methods used for electronic noise such as Monte Carlo techniques, no adjustable parameters are required in the present formalism with the electronic band structure and scattering rates calculated from first-principles. Our formalism enables a parameter-free approach to probe the microscopic transport processes that give rise to electronic noise in semiconductors. Next, we apply the developed method to compute the spectral noise power in two materials of technological interest, GaAs and Si. In our first study in GaAs, we show that despite the well-known dominance of optical phonon scattering, the spectral features in AC transport properties and noise originate from a surprising quasi-elasticity in the scattering of warm electrons with the lattice. In our second study, we apply the method to Si which possesses a more complicated multivalley conduction band. This study demonstrates that the widely-accepted one-phonon scattering approximation is insufficient to reproduce the warm electron tensor and that incorporating second-order mechanisms, such as two-phonon scattering, may be critical to obtain an accurate description of noise in such materials.</p>
<p>Finally, we discuss our work on developing deeper understandings of electronic noise in real devices with a focus on transistor amplifiers. While the first-principles work described above is appropriate for evaluating noise in ideal materials, in real semiconductor devices, charge carriers are influenced by mechanisms such as defect scattering, size effects, and reflections at interfaces. Owing to the complexity of these mechanisms, HEMT noise is typically treated with empirical models, where the physical noise sources are reduced to fitting parameters. Existing models of HEMT noise, such as the Pospieszalski model, are unable to resolve the mechanisms that set the noise floor of modern transistor amplifiers. In particular, the magnitude of the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. We report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using a Schottky thermometry method. Based on our findings, we develop a phonon radiation model of heat transfer in the device and estimate that the thermal noise from the gate is several times larger than previously assumed. Our work suggests that self-heating results in a practical lower limit for the microwave noise figure of HEMTs at cryogenic temperatures.</p>https://thesis.library.caltech.edu/id/eprint/14453Swimming in Potential Flow
https://resolver.caltech.edu/CaltechTHESIS:12052021-002539806
Authors: {'items': [{'email': 'alec.glisman@gmail.com', 'id': 'Glisman-Alec-Gregory', 'name': {'family': 'Glisman', 'given': 'Alec Gregory'}, 'orcid': '0000-0001-9677-1958', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/6xkb-rs66
<p>Active bodies undergo self-propulsive motion in a fluid medium and span a broad range of length and time scales. This report focuses specifically on the motion at high Reynolds number, where inertial forces dominate the fluid dynamics. Many active systems spontaneously self-organize into visually striking structures: fish schooling, birds flocking, and bacterial colonies growing. Current models of emergent behavior in the inertial regime are mainly phenomenological and do not account for the fluid-mediated interactions between bodies. We seek to advance physical models of swimmers in high inertia environments. To this end, we explicitly model the hydrodynamics to discern what role the fluid medium plays in active group dynamics and whether it can reproduce the observed emergent phenomenon without the imposition of phenomenologically based interaction rules.</p>
<p>A minimal swimmer model consisting of three linked spheres is constructed, and we find self-propulsion without external forces or momentum transfer via vortex shedding. The inertial swimmer is also compared to an identical swimmer in the Stokes regime---where fluid inertia is neglected. The Stokes hydrodynamics are longer-ranged at leading order, and we demonstrate that the stronger hydrodynamic interactions lead to a greater center of mass translation after a period of articulation.</p>https://thesis.library.caltech.edu/id/eprint/14441A Technical and Systems Analysis of Hydrogen Fuel in Renewable Energy Systems
https://resolver.caltech.edu/CaltechTHESIS:06222021-223726087
Authors: {'items': [{'email': 'kzrinaldi3@gmail.com', 'id': 'Rinaldi-Katherine-Zoe', 'name': {'family': 'Rinaldi', 'given': 'Katherine Zoe'}, 'orcid': '0000-0002-0746-2852', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/h8x1-xb98
<p>Within the next century, we must tackle the dual challenges of continuing to meet the increasing global demand for energy services while stabilizing global temperatures to mitigate the effects of anthropogenic climate change. Doing so will require a major restructuring of all energy services on a global scale. Here, we contribute to the understanding of the role of hydrogen fuel in net-zero emissions systems from both a technical and systems perspective.</p>
<p>From the technical perspective, we evaluate the activation mechanism of an electrodeposited cobalt selenide hydrogen evolution reaction (HER) catalyst using operando Raman spectroscopy. During this activation process these films, which originally show no catalytic activity toward HER, undergo a compositional change in which selenium in the form of loose, polymeric chains is electrochemically reduced from the material. This work provides a facile method towards investigating catalytic materials under operando conditions, elucidates the changes that occur in this cobalt selenide material during the activation step, and offers potential paths toward the improvement of the cobalt selenide catalyst.</p>
<p>At the systems level, we use hourly weather data over multiple decades and historical electricity demand data to analyze the gaps between wind and solar supply and electricity demand for California (CA) and the Western Interconnect (WECC). We quantify the occurrence of resource droughts when the daily power from each resource was less than half of the 39-year daily mean for that day of the year. Using a macro-scale electricity model, we then evaluate the potential for both long-term storage (in the form of power-to-gas-to-power) and more geographically diverse generation resources to minimize system costs. For wind-solar-battery electricity systems, meeting California demand with WECC generation resources reduces the cost by 9% compared to constraining resources entirely to California. Adding long-duration storage lowers system costs by 21% when treating California as an island. This data-driven analysis quantifies rare weather-related events and provides an understanding that can be used to inform stakeholders in future electricity systems.</p>https://thesis.library.caltech.edu/id/eprint/14282Rheological 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/14948Transient Behavior of Granular Material
https://resolver.caltech.edu/CaltechTHESIS:06012023-222631854
Authors: {'items': [{'email': 'ic64133@gmail.com', 'id': 'Lin-Han-Hsin', 'name': {'family': 'Lin', 'given': 'Han-Hsin'}, 'orcid': '0009-0003-8640-470X', 'show_email': 'NO'}]}
Year: 2023
DOI: 10.7907/jjrv-x616
<p>This PhD thesis focuses on the flows on granular materials, such as sand, glass beads, and powders, which are sheared at low speeds with gravity perpendicular to the flow direction. The study is conducted using a combination of experiments, simulations, and theory, with the goal of developing a unifying theory of granular materials that can be described by continuum models. The main objective is to understand how microscale physics propagate to macroscale phenomena and to address issues related to setting boundary conditions and predicting timescales from unsteady to steady states. This research primarily aims to investigate stress variations in granular materials as a function of shear rate, encompassing both steady and unsteady states. Additionally, the thesis examines the phenomena of wall force anomalies and vortex flows. In Couette cell experiments and vertical plane shear simulations, granular material demonstrates a downward flow near the vertical shearing wall and an upward flow adjacent to another static vertical wall. Interestingly, this vortex flow causes a change in the direction of vertical shear stress when wall shearing commences, contradicting the prevalent assumption that particles consistently apply a downward force on the vertical wall.</p>
<p>The study concludes with key findings, including the observation that normal and shear stresses on the shearing wall increase slowly after the initiation of shearing, and that steady-state values for these stresses are independent of the shearing speed within a certain range. The study also found that the height of particles near the shearing wall decreases gradually with the presence of vortex flow, and that the shear rate near the moving wall is initially high and decreases slowly to reach a steady state. Additionally, we used a non-local constitutive model and Boussinesq approximation to predict the downward flow that is driven by gravity and variations in the solid fraction near the shearing surface, as well as the decay profile of velocity in an infinitely wide box for the steady state.</p>
<p>Overall, this thesis contributes to our understanding of granular materials in the slow flow regime, providing insights into their behavior under shear. The non-local model accurately predicts the downward flow and velocity decay profile, indicating its potential as a valuable tool for future research.</p>https://thesis.library.caltech.edu/id/eprint/15267Long-Duration Energy Storage in Reliable Wind and Solar Electricity Systems
https://resolver.caltech.edu/CaltechTHESIS:06042023-203620894
Authors: {'items': [{'email': 'jacqueline.a.dowling@gmail.com', 'id': 'Dowling-Jacqueline-Anne', 'name': {'family': 'Dowling', 'given': 'Jacqueline Anne'}, 'orcid': '0000-0001-5642-8960', 'show_email': 'NO'}]}
Year: 2023
DOI: 10.7907/gyzn-4n98
Several U.S. states mandate zero-carbon electricity systems based primarily on renewable technologies, such as wind and solar. Reliable and affordable electricity systems based on these variable resources may depend on the ability to store large quantities of low-cost energy over long timescales. This thesis combines techno-economic analysis with materials chemistry to advance long-duration energy storage in reliable wind and solar electricity systems. Our macro-energy model incorporated multi-decadal weather datasets and revealed unique long-duration energy storage roles, such as seasonal and multi-year storage, that increase the affordability of wind- and solar-based electricity, informing technology investments and policy. We find that low-cost energy storage, such as underground hydrogen, is valuable even if the charge/discharge cost is expensive. In U.S. wind and solar systems, hydrogen energy storage and conversion capital cost improvements are more valuable than efficiency improvements. Low-cost earth-abundant catalysts may be acceptable replacements for precious metal catalysts in proton exchange membrane electrolyzers despite lower efficiency for storage applications in wind and solar systems. We synthesized earth-abundant manganese antimony oxide catalysts via a new chemical vapor deposition route and assessed their long-term electrochemical durability for oxygen evolution. Multi-day tests confirmed the activity-stability tradeoff across the Mn:Sb composition space.https://thesis.library.caltech.edu/id/eprint/16082Investigation of Transport Phenomena in Semiconductors and Semiconductor Devices: Drain Noise, Two-Phonon Scattering, and Phonon Drag
https://resolver.caltech.edu/CaltechTHESIS:01242024-044612209
Authors: {'items': [{'email': 'tomiwaesho@gmail.com', 'id': 'Esho-Iretomiwa', 'name': {'family': 'Esho', 'given': 'Iretomiwa'}, 'orcid': '0000-0002-3746-6571', 'show_email': 'NO'}]}
Year: 2024
DOI: 10.7907/b30w-cr73
<p>The dynamics of charge carriers in semiconductors set the foundation for semiconductor device performance. Devices crucial for fields like radio astronomy rely on transistor amplifiers where hot electron dynamics impact noise significantly. The overarching goal of this work is to contribute towards the development of better transistor amplifiers by investigating electron transport in existing devices and emerging materials.</p>
<p>The physical mechanisms governing noise in a class of semiconductor devices called high electron mobility transistors (HEMTs) are not completely understood. HEMTs are transistors that use a junction between two materials of different band gaps as the channel. HEMTs are used as amplifiers by translating a small signal applied at the gate terminal to a large current at the drain terminal or output. The noise added at the input is well-characterized by the device physical temperature, while the origin of the noise added at the output is still up for debate. We attempt to fill this knowledge gap by proposing a theory of noise occurring at the drain terminal of these devices as a type of partition noise arising from two possible electron paths. This theory emphasizes the critical role of the conduction band offset between epitaxial layers of the device: a larger offset maximizes the channel sheet density and minimizes electron transfer between layers, potentially improving noise performance. The theory accounts for the magnitude and dependencies of the drain temperature and suggests strategies to realize devices with lower noise.</p>
<p>We then investigate phonon-limited charge transport in the semiconductor boron arsenide. Boron arsenide has drawn significant interest due to reports of simultaneous high thermal conductivity and ambipolar charge mobility, desirable properties for integration in electronic devices. The theoretical prediction of high electron and hole mobility assumed the dominance of charge carrier scattering by one phonon. We consider the effects of two-phonon electron and hole scattering processes in boron arsenide, and find that inclusion of these higher-order processes reduces the computed room-temperature electron and hole mobility significantly from the one-phonon value. Despite its potential, our predictions of electron and hole mobility contradict recent experimental reports based on photoexcited charge carrier diffusion. Several factors may explain this discrepancy, including another type of two-phonon scattering not considered in this work, superdiffusion of hot carriers, induced carrier concentration, or a combination of all or some of the above elements.</p>
<p>At high carrier concentrations, the phonon system may interact with the electron system on the timescale of the phonon-phonon interaction. When this happens, the nonequilibrium state of phonons becomes important for electron transport, and vice versa as these systems interact in a coupled manner. This coupled interaction could lead to an inflated value of the experimentally reported mobility. We quantify this effect, known as phonon drag, with a coupled electron-phonon Boltzmann transport equation framework and demonstrate that the electron mobility is indeed enhanced significantly at the relevant carrier densities.</p>https://thesis.library.caltech.edu/id/eprint/16282