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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenMon, 15 Apr 2024 14:56:28 +0000Thermal Ignition Using Moving Hot Particles
https://resolver.caltech.edu/CaltechTHESIS:06032016-210051818
Authors: {'items': [{'email': 'stephaniecoronel@gmail.com', 'id': 'Coronel-Stephanie-Alexandra', 'name': {'family': 'Coronel', 'given': 'Stephanie Alexandra'}, 'orcid': '0000-0002-7088-7976', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9W37T9X
<p>In this work, ignition of n-hexane-air mixtures was investigated using moving hot spheres of various diameters and surface temperatures. Alumina spheres of 1.8-6 mm diameter were heated using a high power CO2 laser and injected with an average velocity of 2.4 m/s into a premixed n-hexane-air mixture at a nominal initial temperature and pressure of 298 K and 100 kPa, respectively. The 90% probability of ignition using a 6 mm diameter sphere was 1224 K. High-speed experimental visualizations using interferometry indicated that ignition occurred in the vicinity of the separation point in the boundary layer of the sphere when the sphere surface temperature was near the ignition threshold. Additionally, the ignition threshold was found to be insensitive to the mixture composition and showed little variation with sphere diameter.</p>
<p>Numerical simulations of a transient one-dimensional boundary layer using detailed chemistry in a gas a layer adjacent to a hot wall indicated that ignition takes place away from the hot surface; the igniting gas that is a distance away from the surface can overcome diffusive heat losses back to the wall when there is heat release due to chemical activity. Finally, a simple approximation of the thermal and momentum boundary layer profiles indicated that the residence time within a boundary layer varies drastically, for example, a fluid parcel originating at very close to the wall has a residence time that is 65x longer than the residence time of a fluid parcel traveling along the edge of the momentum boundary layer.</p>
<p>A non-linear methodology was developed for the extraction of laminar flame properties from synthetic spherically expanding flames. The results indicated that for accurate measurements of the flame speed and Markstein length, a minimum of 50 points is needed in the data set (flame radius vs. time) and a minimum range of 48 mm in the flame radius. The non-linear methodology was applied to experimental n-hexane-air spherically expanding flames. The measured flame speed was insensitive to the mixture initial pressure from 50 to 100 kPa and increased with increasing mixture initial temperature. One-dimensional freely-propagating flame calculations showed excellent agreement with the experimental flame speeds using the JetSurF and CaltechMech chemical mechanisms.</p>https://thesis.library.caltech.edu/id/eprint/9844Simulation of Premixed Hydrocarbon Flames at High Turbulence Intensities
https://resolver.caltech.edu/CaltechTHESIS:05272016-105842881
Authors: {'items': [{'email': 'simon.lapointe.5@gmail.com', 'id': 'Lapointe-Simon', 'name': {'family': 'Lapointe', 'given': 'Simon'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z90V89SW
Turbulent premixed hydrocarbon flames in the thin and distributed reaction zones regimes are simulated using both Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES). A series of DNS is performed to study the transition from the thin reaction zones regime to the distributed reaction zones regime. Differential diffusion effects, distributed burning, and local extinctions are quantified. Different fuels, chemical mechanisms, and equivalence ratios are considered. The fuel Lewis number significantly influences the chemical source terms and turbulent flame speeds. More precisely, simulations with differential diffusion effects exhibit lower mean fuel consumption and heat release rates than their unity Lewis number counterparts. However, the differences are reduced as the reaction zone Karlovitz number is increased. The turbulent reaction zone surface areas increase with the turbulence intensity but aren't strongly affected by fuel, equivalence ratio, chemical mechanism, or differential diffusion. Unsurprisingly, changes in the integral length at a fixed Karlovitz number do not affect the chemical source terms but lead to an increase in flame surface area. Assumptions behind closure models for the filtered source term are then studied a priori using the DNS results. Using the concept of optimal estimators, it is shown that a tabulation approach using a progress variable and its variance can predict accurately the filtered progress variable source term. The filtered source terms are compared to predictions from two common presumed sub-filter Probability Density Functions (PDF) models. Both models show deviations from the filtered DNS source terms but predict accurately the mean turbulent flame speed. Finally, LES of experimentally-studied piloted premixed jet flames are performed using tabulated chemistry. Velocity and flame height measurements from simulations and experiments are compared. The LES are in good agreement with the experimental results for the four different hydrocarbon fuels and three different Reynolds numbers simulated. This corroborates that fuel and chemistry effects in turbulent flames are limited to effects present in laminar flames.https://thesis.library.caltech.edu/id/eprint/9784Numerical Simulations of Droplet Aerobreakup
https://resolver.caltech.edu/CaltechTHESIS:05262016-092840941
Authors: {'items': [{'email': 'jomela.meng@gmail.com', 'id': 'Meng-Jomela-Chen-Chen', 'name': {'family': 'Meng', 'given': 'Jomela Chen-Chen'}, 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z9KW5D09
The work presented in this thesis aims to bridge an existing gap in the state of droplet aerobreakup knowledge associated with the fundamental flow physics that govern the experimentally observable droplet morphologies. Using direct numerical simulations of the aerobreakup of water cylinders and droplets in the flow behind shock waves in air, we investigate the behavior of the surrounding gas flow to gain insight into the droplet’s deformation and evolution in the stripping breakup regime. The compressible multicomponent Navier-Stokes equations are solved using the Multicomponent Flow Code — a high-order accurate structured finite-volume flow solver with shock- and interface-capturing. Following qualitative descriptions of the aerobreakup process, comparisons are made with available experimental data. In 2D, accurate measurements of the cylinder’s center-of-mass acceleration across a range of incident shock Mach numbers allow characterization of the unsteady drag coefficient. Additionally, mass loss measurements from viscous simulations refute a well-known boundary layer stripping theory. The results of a 3D nonaxisymmetric aerobreakup simulation are presented with an emphasis on describing the intricate flow phenomena observable in the wake region. Subsequent analyses of the surface instabilities and a Fourier decomposition of the flow field reveal asymmetrical azimuthal modulations and broadband instability growth that result in the devolution of the wake region into chaotic flow.https://thesis.library.caltech.edu/id/eprint/9764Mixing, Chemical Reactions, and Combustion in Supersonic Flows
https://resolver.caltech.edu/CaltechTHESIS:05242016-143905617
Authors: {'items': [{'email': 'niccolo.cymbalist@gmail.com', 'id': 'Cymbalist-Niccolo', 'name': {'family': 'Cymbalist', 'given': 'Niccolo'}, 'show_email': 'YES'}]}
Year: 2016
DOI: 10.7907/Z9G73BNR
<p>Experiments were conducted at the GALCIT supersonic shear-layer facility to investigate
aspects of reacting transverse jets in supersonic crossflow using chemiluminescence and schlieren
image-correlation velocimetry. In particular, experiments were designed to examine mixing-delay
length dependencies on jet-fluid molar mass, jet diameter, and jet inclination.</p>
<p>The experimental results show that mixing-delay length depends on jet Reynolds number, when
appropriately normalized, up to a jet Reynolds number of 500,000. Jet inclination increases the
mixing-delay length, but causes less disturbance to the crossflow when compared to normal jet
injection. This can be explained, in part, in terms of a control-volume analysis that relates jet
inclination to flow conditions downstream of injection.</p>
<p>In the second part of this thesis, a combustion-modeling framework is proposed and developed
that is tailored to large-eddy simulations of turbulent combustion in high-speed flows. Scaling arguments place supersonic hydrocarbon combustion in a regime of autoignition-dominated distributed
reaction zones (DRZ). The proposed evolution-variable manifold (EVM) framework incorporates an
ignition-delay data-driven induction model with a post-ignition manifold that uses a Lagrangian
convected 'balloon' reactor model for chemistry tabulation. A large-eddy simulation incorporating
the EVM framework captures several important reacting-flow features of a transverse hydrogen jet
in heated-air crossflow experiment.</p>https://thesis.library.caltech.edu/id/eprint/9742On the Stability of Supersonic Boundary Layers with Injection
https://resolver.caltech.edu/CaltechTHESIS:05252016-141702166
Authors: {'items': [{'email': 'bryan.e.schmidt@gmail.com', 'id': 'Schmidt-Bryan-Eric', 'name': {'family': 'Schmidt', 'given': 'Bryan Eric'}, 'orcid': '0000-0001-9193-7760', 'show_email': 'NO'}]}
Year: 2016
DOI: 10.7907/Z93X84M6
The problem of supersonic flow over a 5 degree half-angle cone with injection of gas through a porous section on the body into the boundary layer is studied experimentally. Three injected gases are used: helium, nitrogen, and RC318 (octafluorocyclobutane). Experiments are performed in a Mach 4 Ludwieg tube with nitrogen as the free stream gas. Shaping of the injector section relative to the rest of the body is found to admit a "tuned" injection rate which minimizes the strength of shock waves formed by injection. A high-speed schlieren imaging system with a framing rate of 290 kHz is used to study the instability in the region of flow downstream of
injection, referred to as the injection layer. This work provides the first experimental data on the wavelength, convective speed, and frequency of the instability in such a flow. The stability characteristics of the injection layer are found to be very similar to those of a free shear layer. The findings of this work present a new paradigm for future stability analyses of supersonic flow with injection.https://thesis.library.caltech.edu/id/eprint/9755Maximum Entropy Reconstruction for Gas Dynamics
https://resolver.caltech.edu/CaltechTHESIS:05262017-215132894
Authors: {'items': [{'email': 'dustinsummy@gmail.com', 'id': 'Summy-Dustin-Phillip', 'name': {'family': 'Summy', 'given': 'Dustin Phillip'}, 'orcid': '0000-0002-6383-0621', 'show_email': 'YES'}]}
Year: 2017
DOI: 10.7907/Z9GT5K7W
<p>We present a method for selecting a unique and natural probability distribution function (PDF) which satisfies a given number of known moments and apply it for use in the closure of moment-based schemes for approximately solving the Boltzmann equation in gas dynamics.</p>
<p>The method used for determining the PDF is the Maximum Entropy Reconstruction (MER) procedure, which determines the PDF with maximum entropy which satisfies a given set of constraining moments. For the five-moment truncated Hamburger moment problem in one dimension, the MER takes the form of the exponential of a quartic polynomial. This implies a bimodal structure which gives rise to a small-amplitude packet of PDF-density sitting quite far from the mean. This is referred to as the Itinerant Moment Packet (IMP). It is shown by asymptotic analysis that the IMP gives rise to a solution that, in the space of constraining moments, is singular along a line emanating from, but not including, the point representing thermodynamic equilibrium. We use this analysis of the IMP to develop a numerical regularization of the MER, creating a procedure we call the Hybrid MER (HMER). Compared with the MER, the HMER is a significant improvement in terms of robustness and efficiency while preserving accuracy in its prediction of other important distribution features, such as higher order moments.</p>
<p>We apply the one-dimensional HMER to close a fourth order moment system derived from the Boltzmann equation by using a specific set of moment constraints which allow the full, three-dimensional velocity PDF to be treated as a product of three independent, one-dimensional PDFs. From this system, we extract solutions to the problem of spatially homogeneous relaxation and find excellent agreement with a standard method of solution. We further apply this method to the problem of computing the profile within a normal shock wave, and find that solutions exist only within a finite shock Mach number interval. We examine the structure of this solution and find that it has interesting behavior connected to the singularity of the MER and the IMP. Comparison is made to standard solution methods. It is determined that the use of the MER in gas dynamics remains uncertain and possible avenues for further progress are discussed.</p>https://thesis.library.caltech.edu/id/eprint/10214Plasma Surface Interactions in LaB₆ Hollow Cathodes with Internal Xe Gas Discharge
https://resolver.caltech.edu/CaltechTHESIS:06032019-100503451
Authors: {'items': [{'email': 'p.guerrero.eng@gmail.com', 'id': 'Guerrero-Vela-Pedro-Pablo', 'name': {'family': 'Guerrero Vela', 'given': 'Pedro Pablo'}, 'orcid': '0000-0001-5766-2038', 'show_email': 'NO'}]}
Year: 2019
DOI: 10.7907/4CW7-2K35
<p>The ultimate goals of space vehicles are to move faster, further, and more reliably in the space environment. Electric propulsion (EP) has proven to be a necessary technology in the exploration of our solar system ever since its working principle was empirically tested in space in 1964. Thanks to the high exhaust velocities of ionized propellant gases, EP enables efficient utilization of the limited supply of propellant aboard spacecrafts. This technology has opened the possibility of long distance autonomous space missions.</p>
<p>EP devices require electron sources to ionize the propellant gas and to neutralize charges that are leaving the spacecraft. In modern EP thrusters, this is achieved by the use of hollow cathodes -- complex devices that employ low work function materials to emit electrons. Hollow cathodes using polycrystalline LaB<sub>6</sub> inserts are attractive candidates for long duration EP based space missions. However, the physics behind LaB<sub>6</sub> hollow cathode operation has not been studied in detail, which limits the possibility of their optimization. This work presents an integrated experimental and computational approach to investigate LaB<sub>6</sub> hollow cathode thermal behaviour and the interplay between LaB<sub>6</sub> insert surface chemistry and xenon plasma.</p>
<p>Our investigation of the thermal behaviour of LaB<sub>6</sub> cathodes led to the unexpected discovery of a thermal transient when a new insert is first used. Specifically, we observed that the cathode temperature decreases by approximately 300 degrees over 50 hours before reaching steady state. This finding suggests a beneficial dynamic evolution of the cathode's chemical state when it interacts with its own plasma. This evolution is intrinsic to cathode operation and can only be precisely understood when the multiphysic nature of the cathode is self-consistently simulated. Thus, we built a numerical platform capable of combining the plasma, thermal and chemical behavior of a discharging hollow cathode. Simulations incorporating different neutralization models, inelastic ion-surface interaction and heterogeneous chemical evolution led to two major conclusions. First, simulations predicted a significant reduction of the LaB<sub>6</sub> work function (0.42~eV) compared to previously reported baseline values, which is of paramount importance for EP thruster efficiency and operational lifetimes. Second, simulations suggested that the interaction between xenon low energy ions (< 50 eV) and the LaB<sub>6</sub> surface occurs following a two step neutralization mechanism. The predicted work function reduction was experimentally confirmed by photoemission spectroscopy. Furthermore, using a combination of crystallographic analysis, scanning electron microscopy and profilometry, we demonstrated that work function reduction is caused by the creation of a crystallographic texture at the LaB<sub>6</sub> surface upon interaction with Xe plasma. In addition, we postulated the existence of a work function enhancing mechanism of secondary importance, which can be explained by forced cationic termination of plasma exposed crystals.</p>
<p>Our results revealed the unexpected phenomenon of work function reduction upon plasma exposure of LaB<sub>6</sub>. These findings suggest that LaB<sub>6</sub> hollow cathodes may outperform current technologies and become the component of choice in EP thrusters for future space missions.</p>https://thesis.library.caltech.edu/id/eprint/11673Hypervelocity Shock Tunnel Studies of Blunt Body Aerothermodynamics in Carbon Dioxide for Mars Entry
https://resolver.caltech.edu/CaltechTHESIS:05272020-173051776
Authors: {'items': [{'email': 'mgleibow14@gmail.com', 'id': 'Leibowitz-Matthew-Gregory', 'name': {'family': 'Leibowitz', 'given': 'Matthew Gregory'}, 'orcid': '0000-0002-7297-2592', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/chyn-ea06
<p>A low mass and reliable thermal protection system for Martian atmospheric entry requires an accurate prediction of the aerothermal environment encountered by the spacecraft. In order to move forward with predictive models for larger vehicles needed for manned and sample return missions, anomalous data needs to be resolved.
This work aims to address two critical problems relevant for Mars missions.</p>
<p>I) We investigate significant discrepancies between experimental and simulated blunt body bow shock standoff distance in ground test facilities. Experiments using high-speed and high-resolution schlieren imaging are conducted in the T5 reflected shock tunnel and the Hypervelocity Expansion Tube (HET) to examine facility
independence of the measurements. A recently-developed model for sphere and sphere-cone behavior is in good agreement with experiments, and with predictions from Navier-Stokes simulations with thermal and chemical nonequilibrium. The need to account for the divergence of the streamlines in conical nozzles is highlighted.
The contributions of vibrational and chemical nonequilibrium to the stagnation-line density profile are quantified using the simulation results in order to compare different reaction rate models.</p>
<p>II) We measure and characterize carbon dioxide mid-wave infrared radiation in hypervelocity flow. Initially assumed negligible in the design of the Mars Science Laboratory (MSL) mission heat shield, this mechanism of heating must be considered for accurate predictions of the heating environment. Specifically, carbon dioxide radiation can be a dominant source of heating in the afterbody, particularly later in the trajectory at lower velocities. Presented are spectral measurements of the 4.3 μm fundamental band of carbon dioxide radiation measured using fiber optics embedded on the surface of an MSL scaled heat shield model. When comparing experiments and simulations, good agreement is found when running the HET in shock tube mode where the shock layer is optically thick, while discrepancies are observed in expansion tube mode where the shock layer is optically thin. A thorough analysis of flow features in the line-of-sight including freestream uncertainties is performed to explore possible reasons for this discrepancy. After developing the spectroscopic calibration technique and obtaining forebody measurements in the expansion tube, an experimental campaign is completed in the T5 Reflected Shock Tunnel to measure spectral radiation in the forebody and afterbody. The accompanying T5 simulations needed for radiation predictions are being carried out by NASA Ames.</p>https://thesis.library.caltech.edu/id/eprint/13726Ultraviolet Radiation of Hypervelocity Stagnation Flows and Shock/Boundary-Layer Interactions
https://resolver.caltech.edu/CaltechTHESIS:02112020-170613058
Authors: {'items': [{'email': 'nelsonyanes135@gmail.com', 'id': 'Yanes-Nelson-Javier', 'name': {'family': 'Yanes', 'given': 'Nelson Javier'}, 'orcid': '0000-0001-8423-6958', 'show_email': 'NO'}]}
Year: 2020
DOI: 10.7907/WYHM-1218
<p>Shock/boundary-layer interactions can induce flow distortion, create flow separation with loss of control authority, and result in high thermal loads. Correct prediction of the flow structure and heating loads is vital for vehicle survival. However, a recent NATO workshop revealed severe underprediction of thermal loads and discrepancies in the location of separation by simulations of high enthalpy air flows. Due to the coupling between thermochemistry and fluid mechanics, a substantial effort has been placed on the development and validation of thermochemical models. As a result, there is a need for experimental data that are more than mean flow surface measurements.</p>
<p>Spatially resolved emission spectra are collected in the post-shock regime of hypervelocity flow over a circular cylinder and a 30-55 degree double wedge. The Hypervelocity Expansion Tube (HET) is used to generate high Mach number, high enthalpy flow (Mach numbers 5 - 7, h₀ = 4 - 8 MJ/kg) with minimal freestream dissociation. The NO γ band (A²Σ⁺ - X²Π) emission is measured in the ultraviolet range of 210-250 nm at downstream locations behind shock waves. Excitation temperatures are extracted from the NO γ emission from spectrum fitting. The result is a temperature relaxation profile that quantifies the state thermal non-equilibrium. Profiles of vibrational band intensity as a function of streamwise distance are used as direct measurements of chemical non-equilibrium in the flow.</p>
<p>Cylinder experiments are performed with varying freestream total enthalpy, Mach number, and test gas O₂ mole fraction to examine changes in relaxation profile. Schlieren images are used to accurately measure standoff distance. Temperature measurements are compared against a zero-dimensional state-to-state model. Strategies for spectrum fitting are presented for cases where the gas is not optically thin and for radiation containing multiple electronic states. For freestream mixtures with reduced oxygen mole fraction, an electronic excitation temperature is required to describe the radiation of the NO γ, β (B²Π - X²Π), and δ (C²Π - X²Π) transitions. The creation of electronically excited NO is discussed in the context of measured vibrational band intensities and computed NO(A) number density profiles using a two-temperature reactive Landau-Teller model.</p>
<p>Emission spectra are collected in the post bow shock and reattachment shock region of hypervelocity flow over a double wedge. High speed schlieren imaging is performed to investigate facility startup effects and for tracking features in a shock/boundary-layer interaction. Detector exposures occur at select times throughout the flow development process to study temporal changes in thermal and chemical non-equilibrium. Time evolution of temperatures at strategic locations of the flow is obtained from spectrum fitting. Two-temperature calculations of the oblique shock system are compared against the emission results. Radiation data are discussed in the context of recent simulation efforts.</p>https://thesis.library.caltech.edu/id/eprint/13637Integrating Quantum Optical and Superconducting Circuits with Quantum Acoustics for Scalable Quantum Network and Computation
https://resolver.caltech.edu/CaltechTHESIS:08282019-141610693
Authors: {'items': [{'email': 'jie.roger.luo@gmail.com', 'id': 'Luo-Jie', 'name': {'family': 'Luo', 'given': 'Jie'}, 'orcid': '0000-0002-6464-2761', 'show_email': 'YES'}]}
Year: 2020
DOI: 10.7907/P0YC-CQ43
<p>Due to its high coherence in transmission over a large distance in the ambient environment, the quantum optical system has been a prevailing platform for long-distance quantum communication, which was recently realized over a continental distance with a low earth orbit satellite and ground stations [102, 70]. However, the pure quantum optical system has so far shown weak interactions between photon and matter, which makes it inefficient in carrying out deterministic quantum gates for quantum repeater based scalable quantum network and quantum computing. On the other hand, superconducting quantum systems operating in the microwave domain with Josephson junction transmon qubits have proven to be capable of efficient deterministic quantum operations on quantum states [86, 87, 66]. Nevertheless, such architecture is prone to errors and decoherence due to cross-talk between microwave elements in a large-scale superconducting quantum circuit. Furthermore, superconducting systems, in general, also have large footprint (100s um) elements (resonators and superconducting quantum bits) [92, 60] that limit the ability to scale up a superconducting quantum system. Moreover, microwave quantum circuits require cooling to around 10 mK, making it unsuitable for communicating quantum information outside a dilution refrigerator (DF). Micro- and nano- acoustic elements have been extensively used in conventional integrated information processing systems due to their compactness and high coherence [97]. Acoustic systems in quantum engineering also have the advantage of being a platform for universal couplings between various quantum systems including spins, optical photons, and superconducting circuits. As it will be discussed in this thesis, elements critical to scalable optical quantum network and superconducting quantum circuit can be constructed relying on the cavity optomechanics and piezoelectric interactions.</p>
<p>Optomechanical interaction is concerned with the light pressure coupling of cavity mechanical deformation to a strong optical ﬁeld. This interaction has allowed the close to mechanical ground state cooling of mechanical resonators using laser and the ultra-sensitive displacement measurement that led to the detection of gravitational waves in the LIGO collaboration [125, 25]. Optomechanical crystals (OMCs) are lithographically patterned devices which contain a periodic structure that host bandgaps for both optical band electromagnetic waves and microwave band acoustic waves. A properly engineered defect in the crystal can conﬁne and localize acoustic and electromagnetic modes of similar wavelengths into a small mode volume [17, 20, 21]. A strong optomechanical coupling, which can be achieved between such strongly conﬁned co-localized optical and acoustic modes, can be used in engineering the quantum state of mechanical motion to realize useful quantum devices such as a high-coherence quantum memory [74] and an optomechanical high efficiency optical isolator for unidirectionally connecting distant optical cavities via an acoustic bus [37].</p>
<p>To strongly couple the mechanical degree of freedom with a superconducting quantum circuit, various methods can be used, ranging from electromechanic coupling (electric coupling to a mechanically compliant capacitor), magnetomechanical coupling (magnetic coupling to a vibrating SQUID loop), and piezoelectric coupling. The recent advent of quantum acoustics [23, 8, 9] was realized with the strong piezoelectric coupling between a superconducting transmon qubit and a high-coherence mechanical resonator. The engineered strong piezoacoustic coupling provides the possibility to carry out deterministic ultra-high ﬁdelity two-qubit quantum gates on non-classical mechanical quantum states [52]. This ability together with the recent demonstration of ultra-long phonon lifetime mechanical resonators show the possibility of integrating the ultra-high quality mechanical resonator as a compact quantum memory element and even a new ultra-compact (10s um) quantum bit architecture for scalable superconducting quantum circuits. Furthermore, the strong piezoelectric coupling that can transduce quantum state in a superconducting circuit into mechanical wave also makes it possible to efficiently transduce a quantum state between a superconducting quantum circuit and a telecommunication band optical channel via a mechanical waveguide connected to an optomechanical crystal cavity.</p>
https://thesis.library.caltech.edu/id/eprint/11768Thermal Ignition by Vertical Cylinders
https://resolver.caltech.edu/CaltechTHESIS:12182020-055522985
Authors: {'items': [{'email': 'silkenmjones@gmail.com', 'id': 'Jones-Silken-Michelle', 'name': {'family': 'Jones', 'given': 'Silken Michelle'}, 'orcid': '0000-0003-3496-7191', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/9g5j-2b97
<p>Accidental thermal ignition events present a significant hazard to the aviation industry. There is scarcity of experimental data on ignition by external natural convection flows for surface areas larger than 10 cm². In this work, thermal ignition of external natural convection flows by vertical cylinders is investigated. The effect of geometry is studied by resistively heating stainless steel cylinders of various sizes in a stoichiometric n-hexane and air mixture at 298 K and 1 bar. Cylinder lengths range from 12.7 to 25.4 cm, and cylinder surface areas vary from 25 to 200 cm². Logistic regression is used to provide statistical information about the ignition threshold (50% probability of ignition). The maximum ignition threshold found is 1117 K for a cylinder 12.7 cm long and 50 cm² in surface area. The minimum ignition threshold found is 1019 K for a cylinder 25.4 cm long and 200 cm² in surface area. The maximum uncertainty on these ignition thresholds is ±29 K, which comes from the maximum uncertainty on the pyrometer measurement used to record cylinder surface temperatures.
The dependence of ignition threshold on both surface area and length of a cylinder is found to be minor. High speed visualizations of ignition indicated that ignition occurs near the top edge of all cylinders.</p>
<p>The entire experimental setup is heated to allow for ignition tests with multi-component, heavy-hydrocarbon fuels including Jet A and two surrogate fuels, Aachen and JI. The cylinder used for all testing of heavier fuels is 25.4 cm long and 200 cm² in surface area. Hexane is also tested with the heated vessel to investigate the effect of ambient temperature on ignition. At an ambient temperature of 393 K, the ignition threshold of hexane is 933 K. Aachen has an ignition threshold of 947 K at an ambient temperature of 373 K. JI has an ignition temperature of 984 K at an ambient temperature of 393 K. Jet A has an ignition temperature of 971 K at an ambient temperature of 333 K. The maximum uncertainty on these thresholds is ±29 K. JI is found to be the most appropriate surrogate for Jet A.</p>
<p>From the experiments, two main conclusions are reached. Ignition threshold temperatures in external natural convection flows are very weakly correlated with surface area. The observed ignition thresholds do not show the drastic transition of ignition temperature with surface area that is observed in internal natural convection situations. Observed ignition thresholds for comparable surface areas (100 to 200 cm²) are 500 to 600 K higher for external natural convection than internal natural convection. Hexane was found to be a reasonable surrogate for Jet A (38 K difference in ignition threshold) in external natural convection ignition testing. The more complex multi-component JI surrogate, while having an ignition threshold more comparable to Jet A (13 K difference in ignition threshold), requires heating the experimental apparatus and associated difficulties of fuel handling as well as the soot generation by combustion.</p>
<p>Two simplified models of ignition are explored. The first is an investigation of ignition chemistry using a zero-dimensional reactor and a detailed kinetic mechanism for hexane. The temperature history of the reactor is prescribed by an artificial streamline whose rate of temperature increase is parametrically varied. The results from the zero-dimensional reactor computation reveal that a gradually heated streamline exhibits two-stage ignition behavior, while a rapidly heated streamline only experiences one ignition event. The second model of ignition is a one-dimensional simulation of ignition adjacent to a cylinder at a prescribed temperature. The formulation included diffusion of species and thermal energy as well as chemical reaction and employed Lagrangian coordinates. The chemistry is modeled with a reaction mechanism for hydrogen to reduce numerical demand. Heat flux and energy balance are analysed to gain insight into the ignition dynamics. Initially, heat transfer is from the wall into the gas, and a mostly nonreactive thermal boundary layer develops around the cylinder. As reaction in the gas near the surface begins to release energy, the heat transfer decreases, and, near the critical temperature for ignition, the direction of heat flux reverses and is from the gas into the wall. In a case where ignition takes place, there is rapid rise in temperature in the gas within the thermal layer, and a propagating flame is observed to emerge into surrounding cold gas. The heat transfer from the hot combustion products results in a continuous heat flux from the gas into the wall. In a case where ignition does not take place, no flame is observed and the heat flux at the wall is slightly positive. For the critical condition just below the ignition threshold, a balance between energy release and diffusion in the adjacent gas results in a small temperature rise in the thermal layer, but a propagating flame is not created. The Van't Hoff ignition criterion of vanishing heat flux at the ignition threshold is approximately but not exactly satisfied. Contrasting the two modeling ideas, we observe that modeling adiabatic flows along computed nonreactive streamlines is useful in examining the role of detailed chemistry but lacks important diffusion effects. Including mass and thermal transport provides more insight into important ignition dynamics but comes at the expense of increased computational complexity.</p>https://thesis.library.caltech.edu/id/eprint/14034Focused Laser Differential Interferometry
https://resolver.caltech.edu/CaltechTHESIS:05132021-180953405
Authors: {'items': [{'email': 'joel.m.lawson@gmail.com', 'id': 'Lawson-Joel-Michael', 'name': {'family': 'Lawson', 'given': 'Joel Michael'}, 'orcid': '0000-0002-3042-0909', 'show_email': 'YES'}]}
Year: 2021
DOI: 10.7907/5thh-f652
<p>The focused laser differential interferometer (FLDI) is a non-imaging optical diagnostic that is sensitive to density disturbances. A distinguishing feature is reduced sensitivity away from the focal plane of its beams. The spatial resolution is sub-mm, and the temporal resolution is restricted only by photodetector bandwidth, typically >10 MHz. These traits make FLDI particularly suited to measurements in hypervelocity ground-testing facilities, where the low densities, short time-scales, and harsh environments preclude the use of intrusive diagnostics. Line of sight integration issues associated with other optical techniques are therefore minimized, a distinct advantage for measurements in impulse facilities, where the core flow of interest is often surrounded by highly-turbulent shear layers.</p>
<p>The systematic design principles for single and double FLDI systems are discussed, based on ray transfer matrix analysis combined with Gaussian optics. A detailed guide is presented for the practicalities of aligning, calibrating, and operating an FLDI.</p>
<p>A modular numerical implementation of Schmidt and Shepherd's FLDI ray-tracing model is developed, capable of accepting arbitrary flow-fields defined via analytical expressions, simulation coupling, or experimental datasets. This numerical implementation is used to perform the first comprehensive experimental validation of the model, using known static and dynamic phase objects. Quantitatively-accurate predictions of the response of real FLDI systems are obtained. Importantly, the spatial sensitivity of the instrument is found to be dependent on disturbance wavelength, with scaling matching that predicted analytically from the model. Propagating shock waves are used as another highly-dynamic test phase object, and it is shown that FLDI maintains its theoretical performance at sub-μs time-scales.</p>
<p>The validated ray-tracing model is used to develop analytical expressions for the response of FLDI to propagating plane waves, extending on the results of Schmidt and Shepherd, and Settles and Fulghum. For the first time, the inverse problem is solved for this class of flow-field, allowing the density fluctuation spectrum to be recovered quantitatively from FLDI phase shift data. This approach is validated using synthetic flow-fields with the numerical ray-tracing scheme, and is also compared with the approximate approach introduced by Parziale et al.</p>
<p>FLDI is used to make freestream density fluctuation measurements on two facilities: a conventional blowdown tunnel, and an expansion tube. On the conventional tunnel, a comparison is made between pitot-probe and FLDI measurements after converting both to freestream pressure fluctuation spectra. A modification of Stainback and Wagner's theory, incorporating recent numerical results from Chaudhry et al., is used to interpret the pitot data, while the new inversion algorithm is applied to the FLDI data. Close agreement is found between the two sets of spectra, showing that accurate quantitative data can be obtained with FLDI, and used to extend spectra beyond the pitot bandwidth.</p>
<p>On the expansion tube, the theory of Paull and Stalker for freestream noise originating in the driver gas is investigated. Their proposed relationship between freestream density fluctuations and the primary interface sound speed ratio is not observed. Spectral banding is also absent, however this is expected due to the relatively low secondary expansion strengths. The envelope of accessible conditions is somewhat restricted due to the low mean freestream densities that lead to signal-to-noise issues.</p>
<p>Significant performance improvements can still be made to FLDI, in terms of its noise and bandwidth limitations, and to the spatial localization of its sensitive region; suggestions are given for possible approaches. With the ray-tracing model now validated, it can be used to optimize FLDI, or even to suggest derivative instruments based on similar principles.</p>https://thesis.library.caltech.edu/id/eprint/14146Characterization and Optimization of a Fully Passive Flapping Foil in an Unsteady Environment for Power Production and Propulsion
https://resolver.caltech.edu/CaltechTHESIS:05312022-024822211
Authors: {'items': [{'email': 'morglhooper@gmail.com', 'id': 'Hooper-Morgan-Louise', 'name': {'family': 'Hooper', 'given': 'Morgan Louise'}, 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/tymc-1985
<p>This thesis provides an experimental window into the duality between thrust production and energy harvesting by a flapping foil subject to unsteadiness in an oncoming flow. In particular, an airfoil is placed downstream of a circular cylinder, and allowed to interact with the vorticity shed in its wake to produce motions in both the transverse and streamwise directions. It is confirmed that under the right conditions, passive fluid-structure interactions arising from such a configuration give rise to simultaneous extraction of energy from the flow, coupled with net thrust larger than net drag experienced by the airfoil.</p>
<p>Measurements of forces acting on the airfoil and the motion that arises are presented, for cases where the flapping motion is both active (the foil is driven through a pre-planned trajectory) and fully passive (the foil is allowed to react to the fluid forcing it experiences). These are coupled with simultaneous Particle Image Velocimetry (PIV) measurements of the flow field in the region of the airfoil. These measurements allow for direct observation of fluid-structure interactions which give rise to both thrust production and power extraction potential, illuminating the mechanisms driving each. The dynamics of a fully passive flapping foil are largely determined by the mounting system used to facilitate its motion. It is shown that by leveraging Cyber-Physical Fluid Dynamics (CPFD) capabilities to tune these mounting parameters, the behaviour of a fully passive flapping foil can be made similar to that of a representative driven system. A framework based on a simplified linear model for mounting system dynamics is presented, to allow for the optimization of such a system for power extraction potential subject to relevant engineering constraints. The effects of nonlinearity on airfoil behaviour, particularly those due to friction in the mechanism(s) permitting passive flapping, are also explored. Finally, two-dimensional motion of a fully passive flapping foil is demonstrated, allowing for the foil to travel upstream against the oncoming flow solely due to forces induced by interactions with oncoming unsteadiness.</p>https://thesis.library.caltech.edu/id/eprint/14653Optimal Receptivity and the Generalization of the One-Way Navier-Stokes (OWNS) Equations to Complex High-Speed Boundary Layers and Jets
https://resolver.caltech.edu/CaltechTHESIS:01162023-041217909
Authors: {'items': [{'email': 'okamal2018@gmail.com', 'id': 'Kamal-Omar', 'name': {'family': 'Kamal', 'given': 'Omar'}, 'orcid': '0000-0002-3431-2964', 'show_email': 'YES'}]}
Year: 2023
DOI: 10.7907/haet-h558
<p>Prediction of the linear amplification of disturbances in hypersonic boundary layers is challenging due to the presence and interactions of discrete modes (e.g. Tollmien-Schlichting and Mack) and continuous modes (entropic, vortical, and acoustic). While direct numerical simulations (DNS) and global analysis can be used, the large grids required make the stability calculations expensive, particularly when a large parameter space is required. At the same time, parabolized stability equations are non-convergent and unreliable for problems involving multi-modal and non-modal interactions. We therefore apply the One-Way Navier-Stokes (OWNS) Equations to study transitional hypersonic boundary layers. OWNS is based on a rigorous, approximate parabolization of the equations of motion that removes disturbances with upstream group velocity using a higher-order recursive filter. We extend the original algorithm by considering non-orthogonal curvilinear coordinates and incorporate full compressibility with temperature-dependent fluid properties. The generalized OWNS methodology is validated by comparing to DNS data for flat plates and a sharp cone, and to linear stability theory results for local disturbances on the centerline of the Mach 6 HIFiRE-5 elliptic cone. OWNS provides DNS-quality results for the former flows at a small fraction of the computational expense. We further demonstrate the capability of OWNS to track fully 3D instabilities by applying the algorithm to a complex Mach 6 finned-cone geometry as well as a 3D Mach 1.5 turbulent jet. </p>
<p>It is often desirable, especially for design purposes, to compute worst-case disturbances, i.e. solving the inverse problem, otherwise known as resolvent or input-output analysis. While DNS and global analysis can be used to compute optimal forced responses, their large computational expense render these tools less practical for large design parameter spaces. We address this issue by modifying the original OWNS framework to find the optimal forcing and responses using Lagrangian multipliers via an iterative, adjoint-based, space-marching technique that appreciably reduces the computational burden compared to the global approach that uses singular value decomposition without sacrificing accuracy. The input-output OWNS model is validated against optimal forcings and responses of a Mach 4.5 flat-plate boundary layer from literature and a Mach 1.5 turbulent jet. We then apply these equations to study worst-case disturbances on the centerline of the Mach 6 HIFiRE-5 elliptic cone and on a highly cooled Mach 6 flat-plate boundary layer.</p>
<p>Although the worst-case forcings are theoretically informative, they are not physically realizable. In natural receptivity analysis, disturbances are forced by matching local solutions within the boundary layer to outer solutions consisting of free-stream vortical, entropic, and acoustic disturbances. We pose a scattering formalism to restrict the input forcing to a set of realizable disturbances associated with plane-wave solutions of the outer problem. The formulation is validated by comparing with DNS of a Mach 4.5 flat-plate boundary layer. We show that the method provides insight into transition mechanisms by identifying those linear combinations of plane-wave disturbances that maximize energy amplification over a range of frequencies. We also discuss how the framework can be extended to accommodate scattering from shocks and in shock layers for supersonic flow.</p>https://thesis.library.caltech.edu/id/eprint/15083Experiments in Thermal Ignition: Influence of Natural Convection on Properties of Gaseous Explosions
https://resolver.caltech.edu/CaltechTHESIS:06022023-192522565
Authors: {'items': [{'email': 'cmarty716@gmail.com', 'id': 'Martin-Conor-Daniel', 'name': {'family': 'Martin', 'given': 'Conor Daniel'}, 'orcid': '0000-0003-2332-7383', 'show_email': 'NO'}]}
Year: 2023
DOI: 10.7907/twcf-m219
<p>Explosion hazards exist in many industrial sectors including chemical processing, mining, nuclear power, and aviation. Thermal ignition is the name given to the particular case where the initiation energy is supplied via thermal heating of a gas. The critical conditions leading to thermal ignition are in general highly configuration dependent and require a broad set of experimentation to investigate the influence of wide ranging physical processes on ignition. To aid this effort the present work comprises three main experiments covering a range of thermal ignition hazards. First, a heated atmosphere test with fuel injection (ASTM-E659) was implemented to enable the study of heavy hydrocarbon fuels such as Jet A and multicomponent surrogates. This approach showed the existence of cool flame ignition modes near the ignition thresholds for most fuels. The autoignition temperature (AIT) of commodity Jet A was found to be reasonably reproducible by most alkane fuels including n-hexane. Multicomponent surrogates were also able to match the cool flame ignition regimes reasonably well.</p>
<p>Next, ignition using a vertical heated surface in a cold reactive atmosphere was studied in the laminar flow regime. The effects of dilution with nitrogen and reduced pressure were explored for n-hexane/oxygen/nitrogen mixtures. Results found a modest dependence of minimum ignition temperatures on pressure and nitrogen fraction however, with a significant reduction in explosion severity as measured by the maximum overpressure and transient duration. At sufficiently reduced oxygen concentrations, localized weakly propagating flames were found to form in the thermal layer near the surface and produce sustained puffing flame instabilities. One-dimensional flame simulations with detailed kinetics were conducted to supplement and aid in interpretation of the experimental measurements for diluted mixtures. Correlation of ignition thresholds were found to be possible using simplified flame properties and laminar natural convection boundary layer theory. </p>
<p>Finally, a novel experiment was designed to explore the effects of turbulent transition and confinement of large heated surfaces on ignition thresholds. Modeling of the energy balance for resistive heating showed that cylinders up to 36 in. long could be heated using modest power supplies. Six cylinder sizes of varying length were chosen based on this analysis to explore laminar, transitional, and turbulent flow regimes. A large scale flow visualization system was created to study these flow regimes and found that turbulent transition occurred for cylinders as small as 10 in. long for wall temperatures of 1000 K. A study of the transitional dependence on temperatures for large temperature difference (T = 555--1140 K), highly non-Boussinesq conditions found that the transitional Rayleigh number decreased by two orders of magnitude in this regime. The thermal layer thickness at the transition height was estimated in order to obtain a relevant length scale to the boundary layer transition problem. Using this a more consistent transition criteria was obtained (Ra using the thermal thickness length scale) and found to vary by only a factor of two in the high temperature cases studied.</p>
<p>The implementation of these cylinders in ignition testing revealed that there was a strong influence of heating rate due to confinement. The use of absorption spectroscopy showed that for low heating rates the fuel was mostly consumed in low temperature reactions prior to or in place of rapid ignition. This resulted in larger ignition temperatures and weak flames which propagate only in the thermal boundary layer. This effect was explained as a consequence of reduced flow recirculation times due to confinement. A strong influence of turbulence was also found for ignition thresholds when compared with other data for ignition by vertical hot surfaces in the laminar regime. Turbulence was also found to strongly influence the explosion properties due to turbulent flame acceleration. This resulted in larger explosion pressures, shorter transients, and faster flames.</p>https://thesis.library.caltech.edu/id/eprint/16060Numerical Simulations of Cavitating Bubbles in Elastic and Viscoelastic Materials for Biomedical Applications
https://resolver.caltech.edu/CaltechTHESIS:10162023-141935060
Authors: {'items': [{'email': 'jeansebastien.spratt@gmail.com', 'id': 'Spratt-Jean-Sébastien-Alexandre', 'name': {'family': 'Spratt', 'given': 'Jean-Sébastien Alexandre'}, 'orcid': '0000-0002-1962-4214', 'show_email': 'NO'}]}
Year: 2024
DOI: 10.7907/g34e-6p65
<p>The interactions of cavitating bubbles with elastic and viscoelastic materials play a central role in many biomedical applications. This thesis makes use of numerical modeling and data-driven approaches to characterize soft biomaterials at high strain rates via observation of bubble dynamics, and to model burst-wave lithotripsy, a focused ultrasound therapy to break kidney stones.</p>
<p>In the first part of the thesis, a data assimilation framework is developed for cavitation rheometry, a technique that uses bubble dynamics to characterize soft, viscoelastic materials at high strain-rates. This framework aims to determine material properties that best fit observed cavitating bubble dynamics. We propose ensemble-based data assimilation methods to solve this inverse problem. This approach is validated with surrogate data generated by adding random noise to simulated bubble radius time histories, and we show that we can confidently and efficiently estimate parameters of interest within 5% given an iterative Kalman smoother approach and an ensemble- based 4D-Var hybrid technique. The developed framework is applied to experimental data in three distinct settings, with varying bubble nucleation methods, cavitation media, and using different material constitutive models. We demonstrate that the mechanical properties of gels used in each experiment can be estimated quickly and accurately despite experimental inconsistencies, model error, and noisy data. The framework is used to further our understanding of the underlying physics and identify limitations of our bubble dynamics model for violent bubble collapse.</p>
<p>In the second part of the thesis, we simulate burst-wave lithotripsy (BWL), a non- invasive treatment for kidney stones that relies on repeated short bursts of focused ultrasound. Numerical approaches to study BWL require simulation of acoustic waves interacting with solid stones as well as bubble clouds which can nucleate ahead of the stone. We implement and validate a hypoelastic material model, which, with the addition of a continuum damage model and calibration of a spherically- focused transducer array, enables us to determine how effective various treatment strategies are with arbitrary stones. We present a preliminary investigation of the bubble dynamics occurring during treatment, and their impact on damage to the stone. Finally, we propose a strategy to reduce shielding by collapsing bubbles ahead of the stone via introduction of a secondary, low-frequency ultrasound pulse during treatment.</p>https://thesis.library.caltech.edu/id/eprint/16208