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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 01:55:09 +0000Studies of mixing and combustion in hypervelocity flows with hot hydrogen injection
https://resolver.caltech.edu/CaltechETD:etd-08222007-092852
Authors: {'items': [{'email': 'jacquesbelanger@comcast.net', 'id': 'Belanger-Jacques', 'name': {'family': 'Belanger', 'given': 'Jacques'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/012K-GE91
<p>The ability to build an air-breathing single-stage-to-orbit propulsion system requires examination of key elements such as turbulent mixing rates, especially at the "zero shear" fuel-air mixing condition, and combustion efficiency. The required data can only be obtained in experiments which simultaneously match the flight total pressure and total enthalpy as well as the fuel conditions. GALCIT, with its new free piston shock tunnel T5, has the capability to do some of these combustion experiments. But prior to these tests, it was felt that there was a need to simulate the gas dynamical processes in the free piston shock tunnel and also in a new combustion driven shock tunnel built for these experiments so that both systems could be used as efficiently as possible. The numerical code helped explain the piston motion in the free piston shock tunnel. The code was also very useful for the design of the combustion driven shock tunnel.</p>
<p>Because hydrogen has to be injected into the combustion chamber of the propulsion system after being used as a cooling fluid, a combustion driven shock tunnel was built to reproduce this "hot" hydrogen fuel. The system has been used successfully to supply hydrogen at up to 1500 K for the experiments. To reduce the complexity of the problem, a very basic configuration for the hydrogen injection system was tested. This was first done with an injection system mounted flush with the surface of a flat plate in the test section of T5. Different test conditions as well as Mach 2 and 5 nozzle injectors at angles of 15° or 30° were tested to determine criteria for significant combustion. Lower limits in pressure and enthalpy were found where hydrogen combustion becomes very limited using this "hot" hydrogen fuel. The second set of experiments still used an injection system mounted flush with the surface but involved a small combustor model previously tested in the hypervelocity HYPULSE facility. Low pressure experiments were performed to reproduce some of the HYPULSE tests and excellent agreement was found. Experiments at high pressure were also performed to better match the real flight total pressure and some hydrogen combustion was detected in these tests.</p>https://thesis.library.caltech.edu/id/eprint/3197Laser-Induced Thermal Ccoustics
https://resolver.caltech.edu/CaltechETD:etd-09182007-085047
Authors: {'items': [{'id': 'Cummings-Eric-Bryant', 'name': {'family': 'Cummings', 'given': 'Eric Bryant'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/p7mb-d967
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Laser-induced thermal acoustics (LITA) is a new technique for remote nonintrusive measurement of thermophysical gas properties. LITA involves forming, via opto-acoustic effects, grating-shaped perturbations of gas properties using intersecting beams from a short-pulse laser. A third beam scatters coherently into a signal beam off the perturbation grating via acousto-optical effects. The evolution of the gas perturbations modulates the scattered signal beam. Accurate values of the sound speed, transport properties, and composition of the gas can be extracted by analyzing the signal beam.
An analytical expression for the spectrum, absolute magnitude, and time history of the LITA signal is derived. The optoacoustic effects of thermalization and electrostriction are treated. Finite beam-diameter, beam-duration, and thermalization-rate effects are included in the analysis. The expression accurately models experimental signals over a wide range of gas conditions.
Experimental tests using LITA have been conducted on pure and [...]-seeded air and helium at pressures ranging from ~0.1 kPa-14 MPa. Carbon dioxide has been explored near its liquid-vapor critical point. Accuracies of 0.1% in sound speed measurements have been achieved in these tests. Accuracies of ~1% have been achieved in measurements of thermal diffusivity, although beam misalignment effects have typically degraded this accuracy by a factor of ~10-20. Using LITA, susceptibility spectra have been taken of approximately a femtogram of [...]. The effects of fluid motion and turbulence have been explored. LITA velocimetry has been demonstrated, in which the Doppler shift of light scattered from a flowing fluid is measured. LITA velocimetry requires no particle seeding, has a coherent signal beam, and can be applied to pulsed flows. LITA has also been applied to measure single-shot [...] or "Rayleigh scattering" spectra of a gas using a technique of wavelength-division multiplexing, called multiplex LITA. The LITA apparatus used in these tests costs about one-tenth that of many conventional laser diagnostics. Narrowband LITA measurements of the sound speed and transport properties and multiplex LITA measurements of the spectral properties of gases may be taken in a single laser shot.
https://thesis.library.caltech.edu/id/eprint/3627Part I. Mechanisms of injury associated with extracorporeal shock wave lithotripsy; Part II. Exsolution of volatiles
https://resolver.caltech.edu/CaltechETD:etd-10242005-083544
Authors: {'items': [{'email': 'danny.d.howard@boeing.com', 'id': 'Howard-D-D', 'name': {'family': 'Howard', 'given': 'Danny Dwayne'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/995X-8517
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Part I - Shock waves are focused in extracorporeal shock wave lithotripsy (ESWL) machines to strengths sufficient to fracture kidney stones. Substantial side effects-most of them acute-have resulted from this procedure, including injury to soft tissue. The focusing of shock waves through various layers of tissue is a complex process which stimulates many bio-mechano-chemical responses.
This thesis presents results of an in vitro study of the initial mechanical stimulus. Planar nitrocellulose membranes of order 10 um thick were used as models of thin tissue structures. Two modes of failure were recorded: Failure due to cavitation collapsing on or near the membranes, and failure induced by altering the structure of shock waves. Tests were done in water at and around F2 to characterize the extent of cavitation damage, and was found to be confined within the focal region, 1.2 cm along the axis of focus.
Scattering media were used to simulate the effects of acoustic nonuniformity of tissue and to alter the structure of focusing shock waves. 40 um diameter (average) hollow glass spheres were added to ethylene glycol, glycerine and castor oil to vary the properties of the scattering media. Multiple layer samples of various types of phantom tissue were tested in degassed castor oil to gauge the validity of the scattering media. The scattering media and tissue samples increased the rise time decreased strain rate in a similar fashion. Membranes were damaged by the decreased strain rate and accumulated effects of the altered structure: After about 20 or so shocks immersed in the scattering media and after about 100 shocks behind the tissue samples. The mode of failure was tearing with multiple tears in some cases from about .1 cm to about 3 cm depending of the number of shocks and membrane thickness.
Part II - This work examines the exsolution of volatiles-carbon dioxide from water-in a cylindrical test cell under different pressure conditions. Water was supersaturated with carbon dioxide under various pressures (620 to 1062 kPa), and depressurized rapidly to investigate how carbon dioxide is undissolved, exsolution, and its effects on the surrounding environment. Cavities grow as a result of convective diffusion: They move before depleting carbon dioxide in a given region. The radius of a cavity in this environment grows at a faster rate [...] than that of a cavity at rest [...]. Bubble growth rates were inferred by measuring the bulk liquid using high speed motion pictures. Water in the test-cell is accelerated as a result of buoyancy induced by cavity growth. Cavities are elliptical in shape and grow until mutual interaction causes them to fragment. Accelerations range from 10 to 100 g were measured with velocities ranging from 7 to 13 m/s.
https://thesis.library.caltech.edu/id/eprint/4228Enthalpy Effects on Hypervelocity Boundary Layers
https://resolver.caltech.edu/CaltechETD:etd-01072008-111636
Authors: {'items': [{'email': 'padam@alumni.caltech.edu', 'id': 'Adam-Philippe-H', 'name': {'family': 'Adam', 'given': 'Philippe H.'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/YF0K-HK91
More than 50 shots with air and 35 shots with carbon dioxide were carried out in the T5 shock tunnel at GALCIT to study enthalpy effects on hypervelocity boundary layers. The model tested was a 5° half-angle cone measuring approximately 1 meter in length. It was instrumented with 51 chromel-constantan coaxial thermocouples and the surface heat transfer rate was computed to deduce the state of the boundary layer and, when applicable, the transition location.
Transitional boundary layers obtained confirm the stabilizing effect of enthalpy. As the reservoir enthalpy is increased, the transition Reynolds number evaluated at the reference conditions increases as well. The stabilizing effect is more rapid in gases with lower dissociation energy and it seems to level off when no further dissociation can be achieved. These effects do not appear when the transition location is normalized with the edge conditions. Further normalizing the reservoir enthalpy with the edge enthalpy appears to collapse the data for all gases onto a single curve. A similar collapse is obtained when normalizing both the transition location and the reservoir enthalpy with maximum temperature conditions obtained with BLIMPK, a nonequilibrium boundary layer code.
The observation that the reference conditions seem more appropriate to normalize high enthalpy transition data was taken a step further by comparing the tunnel data with results from a reentry experiment. When the edge conditions are used, the tunnel data are around an order of magnitude below the flight data. This is commonly attributed to the fact that disturbance levels in tunnels are high, causing the boundary layer to transition prematurely. However, when the conditions at the reference temperature are used instead, the data come within striking distance of one another although the trend with enthalpy seems to be a destabilizing one for the flight data. This difference could be due to the cone bending and blunting observed during the reentry.
Experimental laminar heat transfer levels were compared to numerical results obtained with BLIMPK. Results for air indicate that the reactions are probably in nonequilibrium and that the wall is catalytic. The catalycity is seen to yield higher surface heat transfer rates than the noncatalytic and frozen chemistry models. The results for carbon dioxide, however, are inconclusive. This is, perhaps, because of inadequate modeling of the actual reactions. Experimentally, an anomalous yet repeatable, rise in the laminar heat transfer level can be seen at medium enthalpies in carbon dioxide boundary layers.
https://thesis.library.caltech.edu/id/eprint/57Unsplit Numerical Schemes for Hyperbolic Systems of Conservation Laws with Source Terms
https://resolver.caltech.edu/CaltechETD:etd-06032005-161139
Authors: {'items': [{'email': 'miltos@term.ucl.ac.be', 'id': 'Papalexandris-Miltiadis-Vassilios', 'name': {'family': 'Papalexandris', 'given': 'Miltiadis Vassilios'}, 'show_email': 'YES'}]}
Year: 1997
DOI: 10.7907/HW7S-AR36
In this thesis, a new method for the design of unsplit numerical schemes for hyperbolic systems of conservation laws with source terms is developed. Appropriate curves in space-time are introduced, along which the conservation equations decouple to the characteristic equations of the corresponding one-dimensional homogeneous system. The local geometry of these curves depends on the source terms and the spatial derivatives of the solution vector. Numerical integration of the characteristic equations is performed on these curves.
In the first chapter, a scalar conservation law with a stiff, nonlinear source term is studied using the proposed unsplit scheme. Various tests are made, and the results are compared with the ones obtained by conventional schemes. The effect of the stiffness of the source term is also examined.
In the second chapter, the scheme is extended to the one-dimensional, unsteady Euler equations for compressible, chemically-reacting flows. A numerical study of unstable detonations is performed. Detonations in the regime of low overdrive factors are also studied. The numerical simulations verify that the dynamics of the flow-field exhibit chaotic behavior in this regime.
The third chapter deals with the development and implementation of the unsplit scheme, for the two-dimensional, reactive Euler equations. In systems with more than two independent variables there are one-parameter families of curves, forming manifolds in space-time, along which the one-dimensional characteristic equations hold. The local geometry of these manifolds and their position relative to the classical characteristic rays are studied. These manifolds might be space-like or time-like, depending on the local flow gradients and the source terms.
In the fourth chapter a numerical study of two-dimensional detonations in performed. These flows are intrinsically unstable and produce very complicated patterns, such as cellular structures and vortex sheets. The proposed scheme appears to be capable of capturing many of the the important details of the flow-fields. Unlike traditional schemes, no explicit artificial-viscosity mechanisms need to be used with the proposed scheme.https://thesis.library.caltech.edu/id/eprint/2427Full Field Study of Strain Distribution Near the Crack Tip in the Fracture of Solid Propellants Via Large Strain Digital Image Correlation and Optical Microscopy
https://resolver.caltech.edu/CaltechETD:etd-12212004-164817
Authors: {'items': [{'email': 'javier@namascar.net', 'id': 'Gonzalez-Javier-Gonzalez', 'name': {'family': 'Gonzalez', 'given': 'Javier Gonzalez'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/HRM1-RJ74
A full field method for visualizing deformation around the crack tip in a fracture process with large strains is developed. A digital image correlation program (DIC) is used to incrementally compute strains and displacements between two consecutive images of a deformation process. Values of strain and displacements for consecutive deformations are added, this way solving convergence problems in the DIC algorithm when large deformations are investigated. The method developed is used to investigate the strain distribution within 1 mm of the crack tip in a particulate composite solid (propellant) using microscopic visualization of the deformation process.https://thesis.library.caltech.edu/id/eprint/5096A 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/1370High-enthalpy shock/boundary-layer interaction on a double wedge
https://resolver.caltech.edu/CaltechETD:etd-02272008-125333
Authors: {'items': [{'email': 'jpdavis@sandia.gov', 'id': 'Davis-J', 'name': {'family': 'Davis', 'given': 'Jean-Paul'}, 'show_email': 'NO'}]}
Year: 1999
DOI: 10.7907/4C98-MN23
Interaction between a shock wave and a boundary layer at a compression corner can produce a region of separated flow. The length of separation is important in determining aerodynamic forces, and the heat transfer at reattachment is important for the design of thermal protection systems. The effects of high-enthalpy flow on these phenomenon, particularly separation length, are not well known. Experiments to measure separation length and reattachment heating are performed in the T5 Hypervelocity Shock Tunnel using nitrogen test gas and a double-wedge geometry which allows greater control over local flow conditions at separation and, at high incidence angle, may produce real-gas effects due to dissociation behind the leading shock. Local external flow conditions were found by computational reconstruction of the inviscid nonequilibrium flow field.
Application of results from asymptotic theory to a simple model for separation leads to a new scaling parameter which approximately accounts for wall temperature effects on separation length for a laminar nonreacting boundary layer and extends previous results to arbitrary viscosity law. A. classification is introduced which divides mechanisms for real-gas effects into those acting internal and external to viscous regions of the flow, with internal mechanisms further subdivided into those arising upstream and downstream of separation. Application of the ideal dissociating gas model to a scaling law based on local external flow parameters and a nonreacting boundary layer shows that external mechanisms due to dissociation decrease separation length at low incidence but depend on the free-stream dissociation at high incidence, and have only a small effect on peak heating. A limited numerical study of reacting boundary layers shows that internal mechanisms due to recombination in the upstream boundary layer cause a slight decrease in separation length and a large increase in heat flux relative to a nonreacting boundary layer with the same external conditions.
Correlations are presented of experimentally measured separation length using local external flow parameters computed for reacting flow, which scales out external mechanisms but not internal mechanisms. These show the importance of the new scaling parameter in high-enthalpy flows, a linear relationship between separation length and reattachment pressure ratio as found previously for supersonic interactions, and a Reynolds-number effect for transitional interactions. A significant increase in scaled separation length is observed for high-enthalpy data in the laminar regime, and this is attributed to an internal recombination mechanism occurring in the separated shear layer. Experimental data for reattachment heat flux are found to agree roughly with existing correlations and to exhibit an increase due to an internal recombination mechanism, but cannot provide further insight due to large scatter.https://thesis.library.caltech.edu/id/eprint/788The instability of shear layers produced by curved shocks
https://resolver.caltech.edu/CaltechETD:etd-02212008-090557
Authors: {'items': [{'email': 'plemieux@calpoly.edu', 'id': 'Lemieux-P', 'name': {'family': 'Lemieux', 'given': 'Patrick'}, 'show_email': 'YES'}]}
Year: 1999
DOI: 10.7907/V9R0-JD15
A curved shock of general shape in hypersonic flow generates vorticity, so that a shear layer is formed in the flow downstream of the shock. The parameters affecting the distribution of vorticity in the shear layer are identified. Experiments aimed at determining the preferred wavelength of structures that develop in these flows are carried out in the T5 Hypervelocity Shock Tunnel. To visualize these structures, a new technique using streaklines is developed. The results are compared with numerical simulations of perfect-gas flows.
The numerical study also points to a flow regime, as the Newtonian limit is approached, where the instability of the shear layer is such that the shock becomes distorted. A series of experiments aimed at investigating flows approaching this limit is performed, using the T5 Light Gas Gun facility, and confirms the existence of this new regime.
https://thesis.library.caltech.edu/id/eprint/701Shock detachment process on cones in hypervelocity flows
https://resolver.caltech.edu/CaltechETD:etd-02082008-162753
Authors: {'items': [{'email': 'ialr2003@gmail.com', 'id': 'Leyva-I-A', 'name': {'family': 'Leyva', 'given': 'Ivett A.'}, 'show_email': 'YES'}]}
Year: 1999
DOI: 10.7907/62TN-GA26
<p>The shock detachment process on cones in hypervelocity flows is one of the most sensitive flows to relaxation effects. The critical angle for shock detachment under frozen conditions can be very different from the critical angle under chemical and thermal equilibrium. The rate of increase of the detachment distance with cone angle is also affected by the relaxation rate.</p>
<p>The purpose of this study is to explain the effects of nonequilibrium on the shock detachment distance and its growth rate on cones in hypervelocity flows. The study consists of an experimental and a computational program. The experimental part has been carried out at Caltech's hypervelocity reflected shock tunnel (T5). Six different free-stream conditions have been chosen, four using N<sub>2</sub> as the test gas and two using CO<sub>2</sub>. About 170 shots were performed on 24 cones. The cones range in diameter from 2 cm to 16 cm with half-angles varying from 55° to 75°. The experimental data obtained are holographic interferograms of every shot, and surface temperature and pressure measurements for the bigger cones. Extensive numerical simulations were made for the N<sub>2</sub> flows and some were also made for the CO<sub>2</sub> flows. The code employed is a Navier-Stokes solver that can account for thermal and chemical nonequilibrium in axisymmetric flows.</p>
<p>The experimental and computational data obtained for the shock detachment distance confirms a previous theoretical model that predicts the detachment distance will grow more slowly for relaxing flows than for frozen or equilibrium flows. This difference is explained in terms of the behavior of the sonic line inside the shock layer. Different growth rates result when the detachment distance is controlled by the diameter of the cone (frozen and equilibrium cases) than when it is controlled by the extent of the relaxation zone inside the shock layer (nonequilibrium flows). The experimental data are also complemented with computational data to observe the behavior of the detachment distance from the frozen to equilibrium limits for a given cone half-angle and free-stream condition. As deduced by a previous simple scaling argument, the ratio of the detachment distance to the diameter of the cone is constant in the two extremes and rapidly switches from one value to the other for cone diameters of about 2 cm to 16 cm. The experimental interferograms are also compared with numerical ones in terms of the detachment distance, the number of fringes in the shock layer, and the shape of the fringes.</p>
<p>The heat flux traces obtained from the temperature measurements show different behaviors for the attached and detached cases, but these effects can be related to the conditions at the edge of and inside the boundary layer and to the Reynolds number of the flow rather than to nonequilibrium effects. The pressure measurements were insensitive to the degree of nonequilibrium.</p>
https://thesis.library.caltech.edu/id/eprint/564Detonation Diffraction Through an Abrupt Area Expansion
https://resolver.caltech.edu/CaltechETD:etd-11122003-180459
Authors: {'items': [{'id': 'Schultz-Eric', 'name': {'family': 'Schultz', 'given': 'Eric'}, 'show_email': 'NO'}]}
Year: 2000
DOI: 10.7907/96F1-QR61
The problem of a self-sustaining detonation wave diffracting from confinement into an unconfined space through an abrupt area change is characterized by the geometric scale of the confinement and the reaction scale of the detonation. Previous investigations have shown that this expansion associated with a detonation transitioning from planar to spherical geometry can result in two possible outcomes depending upon the combustible mixture composition, initial thermodynamic state, and confining geometry. Competition between the energy release rate and expansion rate behind the diffracting wave is crucial. The sub-critical case is characterized by the rate of expansion exceeding the energy release rate. As the chemical reactions are quenched, the shock wave decouples from the reaction zone and rapidly decays. The energy release rate dominates the expansion rate in the super-critical case, maintaining the coupling between the shock and reaction zone which permits successful transition across the area change. A critical diffraction model has been developed in the present research effort from which the initial conditions separating the sub-critical and super-critical cases can be analytically determined. Chemical equilibrium calculations and detonation simulations with validated detailed reaction mechanisms provide the model input parameters. Experiments over a wide range of initial conditions with single- and multi-sequence shadowgraphy and digital chemiluminescence imaging support the model derivation and numerical calculations. Good agreement has been obtained between the critical diffraction model and experimental results.https://thesis.library.caltech.edu/id/eprint/4528The Significance of Vortex Ring Formation and Nozzle Exit Over-Pressure to Pulsatile Jet Propulsion
https://resolver.caltech.edu/CaltechETD:etd-09142005-111030
Authors: {'items': [{'email': 'pkrueger@engr.smu.edu', 'id': 'Krueger-Paul-Samuel', 'name': {'family': 'Krueger', 'given': 'Paul Samuel'}, 'show_email': 'YES'}]}
Year: 2001
DOI: 10.7907/3QWF-8G05
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Pulsatile jet propulsion can be accomplished using a fully-pulsed jet (i.e., a periodic series of starting jets or pulses), the unsteady nature of which engenders vortex ring formation. The significance of vortex ring formation for this type of propulsion is studied experimentally using a piston-cylinder mechanism to generate starting and fully-pulsed, round jets of water into water at a maximum jet Reynolds number of 13,000. Starting jets are considered separately since they are the limiting case of a fully-pulsed jet at zero pulsing frequency. Direct measurements of the total impulse per pulse (starting jets) and time-averaged thrust (fully-pulsed jets) are made using a force balance. Hotfilm anemometry is used to measure the jet velocity and Digital Particle Image Velocimetry (DPIV) is used to measure vortex ring position, vorticity, energy, circulation, and impulse.
The pulses for both types of jets are generated using piston stroke to diameter ratios (L/D) in the range 2 to 8 for piston velocity programs in a generally positive-sloping (PS) or negative-sloping (NS) family. The range of L/D considered brackets the transition between the case where an individual vortex ring is produced with each pulse (small L/D) and the case where the vortex ring stops growing and pinches off from its generating jet, producing a trailing jet (large L/D). This transition occurs at a higher L/D for the PS ramps, allowing the effects of vortex ring formation and pinch off to be illuminated by comparison of the results for the NS and PS ramps.
The significance of vortex ring formation is first analyzed for starting jets. Measurements of the total impulse per pulse as a function of L/D show that a leading vortex ring adds more impulse per unit L/D than a trailing jet. This leads to a maximum in the average thrust during a pulse at the L/Ds just before vortex ring pinch off is observed for both the PS and NS ramps. The propulsive benefit provided by a leading vortex ring over a trailing jet is connected to over-pressure at the nozzle exit plane during vortex ring formation. DPIV measurements demonstrate that nozzle exit over-pressure also makes an important contribution to energy and circulation. It is shown that this over-pressure can be related to the momentum that must be supplied by the forming vortex ring to ambient fluid in the form of added and entrained mass. A model is proposed for nozzle exit over-pressure near the initiation of an impulsive velocity program where entrainment can be ignored. The model readily accounts for the pressure contribution to circulation in the NS ramps, but modeling of entrainment is required to properly determine impulse and energy.
For the fully-pulsed jet experiments, a normalized thrust, [...], is introduced to characterize the pressure effects associated with vortex ring formation. The pulsing frequency is expressed in dimensionless form as [...], which is between 0 and 1 for all fully-pulsed jets. A propulsive benefit from pressure ([...]) is observed for all L/D and [...] considered. At low [...], the results are similar to those for the starting jets. At higher [...], [...] decreases with L/D as with the starting jets, which is related to the existence of vortex ring pinch off for all observed [...]. At a fixed L/D, two dominant decreasing trends in [...] with [...] appear and seem to be related to the effects of previously ejected pulses on forming vortex rings. No dramatic increase in [...] with [...] (associated with the increased convective velocity of multiple coaxial vortex rings over that of individual vortex rings) is observed since (a) the ring separation is never reduced low enough to see an increase in the ring velocity (even for [...]), and (b) the vortex rings don't remain coaxial or coherent as [...].https://thesis.library.caltech.edu/id/eprint/3530Application of Diamond Films to Electric Propulsion: Low Energy Sputter Yield Measurement and MPD Plasma Assisted Chemical Vapor Deposition
https://resolver.caltech.edu/CaltechETD:etd-03252005-143838
Authors: {'items': [{'email': 'blandino@wpi.edu', 'id': 'Blandino-John-Joseph', 'name': {'family': 'Blandino', 'given': 'John Joseph'}, 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/270K-TR61
<p>Progress made in the area of chemically vapor deposited (CVD) diamond films has led to its consideration for a number of novel applications. One potential application under evaluation at the Jet Propulsion Laboratory involves the use of diamond films as coatings for ion thruster electrodes which are subject to sputter erosion. In order to assess their benefit in mitigating sputter erosion as a failure mode it was necessary to first measure the erosion rate compared with molybdenum and carbon-carbon when subjected to ion bombardment.</p>
<p>Sputter yields were measured for polycrystalline diamond, single crystal diamond, a carbon-carbon composite, and molybdenum subject to xenon ion bombardment. The tests were performed using a 3 cm Kaufman ion source to produce incident ions with energy in the range of 150 - 750 eV and a profilometry-based technique to measure the amount of sputtered material. The yields increased monotonically with energy with values ranging from 0.16 atoms/ion at 150 eV to 0.80 at 750 eV for the molybdenum and 0.06 to 0.14 for the carbon-carbon. At 150 eV the yield for both diamond samples was 0.07 and at 750 eV, 0.19 and 0.17 for the CVD and single crystal diamond respectively. In terms of erosion rate, this translates into a factor of 7 - 12 lower erosion rate for diamond compared to molybdenum and at least a factor of 1.5 compared to carbon-carbon.</p>
<p>With the erosion rates established, the remaining effort concentrated on the experimental and analytical investigation of an electromagnetic (magnetoplasmadynamic or NIPD) plasma source for diamond CVD using a mixture of methane and hydrogen. Specific questions to be addressed included identifying the implications of higher velocity, dissociation, and ionization levels than those expected in an electrothermal (arcjet) plasma source. An experimental facility was used in which the process conditions produced were representative of the high temperature, ionization, and dissociation levels one would expect from an MPD in an actual reactor although not the high velocity. Numerous trials were conducted using methane to hydrogen mixture ratios of 1.5 - 3.5 percent by volume, four different methane injector configurations, and substrate biasing at potentials of 25 - 75 V positive with respect to facility ground. These tests were performed at discharge currents of 700 - 950 A at approximately 18 V (12 - 17 kW).</p>
<p>Crystalline films were produced with predominantly (110) oriented facets. X-ray diffraction spectroscopy was used to identify at least one unambiguous diamond peak in each sample with numerous films containing metal or metallic carbide impurities. Growth rates of 0.8 to 6.3 [micrometers]/hr were measured. The films all exhibited poor Raman spectra with no well defined peak at 1332 cm⁻¹ and a broad background possibly due to high background levels of nitrogen, defects, and contamination.</p>
<p>The question of high velocity and high ionization level was investigated analytically using estimated hydrogen MPD plume data from the literature. For conditions expected with an MPD source, Knudsen numbers in the plume are calculated to be approximately 0.1 in the free stream and less than 0.01 (i.e., transition-continuum boundary) in the stagnation boundary layer. The heavy particle static temperature in the plume is expected to be on the order of 10000 K in the core. For Mach numbers in the range 1.0 - 2.0, the stagnation temperature can be expected to reach peak values of over 20000 K. This temperature far exceeds the range of available thermodynamic and transport databases for hydrocarbon mixtures needed to model stagnation boundary layer chemistry and growth rates, so a scaling relation was used to obtain a relative comparison of the atomic hydrogen mole fraction at the substrate for an electrothermal and electromagnetic accelerated plasma source. Because of its higher jet velocity and lower operating pressure, the electromagnetic: source operates in more of a convection dominated regime resulting in a calculated hydrogen mole fraction at the substrate approximately 40 percent higher than that predicted for an electrothermal accelerator.</p>
<p>An energy balance was used to determine an upper bound on the level of electron heating obtainable for a given bias current density. Results show that even for pressures of a few Torr and ionization fractions of 5 - 25 percent, the required current for a few thousand degree increase in electron temperature over the heavy particle gas temperature is on the order of several tens of A/cm². From this analysis it is concluded that high plasma conductivity in an MPD plume and electron energy losses through inelastic molecular collisions will preclude the effective use of ohmic heating of the electrons as a means of enhancing electron catalyzed dissociation in the boundary layer.</p>
<p>Estimates were made of the optimal residence time for methane decomposition in the plume in order to maximize the flux of methyl radicals and atomic hydrogen to the substrate; two species which have been identified as having a significant role in high rate, high quality diamond synthesis. At pressures of 100 Pa and 333 Pa, which approximate the pressure in the plume and stagnation regions, there is little recombination of the hydrogen even at a temperature of 5000 K which one could expect well into the thermal boundary layer for an MPD stagnation flow. The methyl mole fraction reaches a maximum at a residence time of 1 - 3 [millisec]. Achieving adequate entrainment and mixing of the methane in the hydrogen jet oil such a short time scale is a very difficult challenge.</p>
<p>Two major conclusions of this thesis are: 1) The lower erosion rate measured for CVD diamond as compared with molybdenum makes coating of grid electrodes with CVD diamond a possible option for extending the lifetime of ion thrusters worth further investigation. 2) Based on both the experimental and analytical investigation of the MPD source for diamond deposition, the potential for higher growth rates than those obtainable with more conventional plasma sources is not significant enough to offset the disadvantages associated with contamination of the film due to metal vapor from the high current electrodes and poor entrainment of the carbon precursor gas due to the short, residence time in the plume.</p>https://thesis.library.caltech.edu/id/eprint/1115Damage Mechanisms in Shock Wave Lithotripsy (SWL)
https://resolver.caltech.edu/CaltechETD:etd-03162005-130412
Authors: {'items': [{'email': 'murtuzalok@hotmail.com', 'id': 'Lokhandwalla-Murtuza', 'name': {'family': 'Lokhandwalla', 'given': 'Murtuza'}, 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/VZWS-7Z85
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Shock wave lithotripsy is a 'non-invasive' therapy for treating kidney stones. Focused shock waves fragment stones to a size that can be passed naturally. There is, however, considerable tissue injury associated with this treatment, and the mechanisms of stone fragmentation and tissue injury are not well understood. This work investigates potential tissue damage mechanisms, with an aim towards modifying the wave-field parameters, so as to enhance stone fragmentation and minimize tissue damage.
Lysis of red blood cells (RBC's) due to in vitro exposure to shock waves was considered as a model of cellular level damage. Fluid flow-fields induced by a non-uniform shock wave, as well as radial expansion/implosion of a bubble was hypothesized to cause lysis of cells. Both the above flow-fields constitute an unsteady, extensional flow exerting inertial as well as viscous forces on the RBC membrane. The resultant membrane tension and the membrane areal strain ([Delta]A/A) due to the above flow-fields were estimated. Both were found to exert a significantly higher inertial force (50 - 100 mN/m) than the critical membrane tension (10 mN/m). Bubble-induced flow-field was estimated to last for a longer duration ([...]) compared to the shock-induced flow ([...]) and hence, was predicted to be lytically more effective, in typical in vitro experimental conditions. However, in vivo conditions severely constrain bubble growth, and cell lysis due to shock-induced shear could be dominant.
Hemolysis due to shock-induced shear, in absence of cavitation, was experimentally investigated. The lithotripter-generated shock wave was refocused by a parabolic reflector. This refocused wave-field had a tighter focus (smaller beam-width and a higher amplitude) than the lithotripter wave-field. Cavitation was eliminated by applying overpressure to the fluid. A novel passive cavitation detector (HP-PCD) operating at high overpressure (upto 7 MPa) was used to measure acoustic emission due to bubble activity. Aluminum foils were also used to differentiate cavitational from non-cavitational mode of damage. RBC's suspended in phosphate-buffered saline PBS) were exposed to the reflected wave-field from the parabolic reflector and also from a flat reflector, the latter serving as a control experiment. Exposure to the wave-field from the parabolic reflector increased hemolysis four-fold compared to untreated controls and was twice that of cell lysis with the flat reflector. This result corroborated the hypothesis of shock-induced shear as a cell damage mechanism in the absence of cavitation.
https://thesis.library.caltech.edu/id/eprint/966An investigation of ion engine erosion by low energy sputtering
https://resolver.caltech.edu/CaltechETD:etd-02242002-122344
Authors: {'items': [{'email': 'olivier@alumni.caltech.edu', 'id': 'Duchemin-O-B', 'name': {'family': 'Duchemin', 'given': 'Olivier Bernard'}, 'show_email': 'YES'}]}
Year: 2001
DOI: 10.7907/K408-J123
Unlike chemical propulsion systems, which are fundamentally limited in performance by propellant energy density, electric propulsion devices, such as ion engines, are limited in iotal deliverable impulse by maximum propellant throughput due to engine wear.
In order to perform realistic modeling of engine lifetime, the erosion mechanisms involved must be understood. In particular, the damage---or sputtering---caused by slow ions on solid surfaces is extremely difficult to quantify. We first review the engine failure modes in which sputtering of molybdenum by slow xenon ions plays a critical role. We then present the relevant physical mechanisms, and describe a model for estimating the minimum kinetic energy necessary to dislodge a surface atom.
Over seventeen analytical approaches to the energy dependence of sputtering have been published in the literature. We implement the four that are most relevant to ion engine erosion processes. In addition, we use the Monte-Carlo simulation program TRIM to calculate sputtering yields. We find, in particular, that the relative sensitivity of sputtering yield to surface binding energy increases dramatically near the sputtering threshold energy. Although the surface binding energy is a (weak) function of temperature, we show that the sputtering yield should not increase significantly at temperatures typical of ion engine operation.
An experimental approach to the measurement of low energy sputtering yields is implemented and validated. Based on the Quartz Crystal Microbalance (QCM) technique, this method takes advantage of the differential mass sensitivity exhibited by the piezoelectric quartz resonator used in this study. Because of the importance of surface contamination in low energy sputtering, a surface kinetics model is presented to describe a surface under the simultaneous cleaning effect of ion bombardment, and background gas flow contamination.
A special case of simultaneous surface contamination and erosion occurs during engine ground testing, where carbon is backsputtered on the accelerator grid from the facility. We describe experiments to measure ion-induced desorption cross-sections for carbon on molybdenum, before concluding that the protective effect of the carbon contamination is unlikely to significantly affect engine erosion, so that ground testing results are applicable to space operationshttps://thesis.library.caltech.edu/id/eprint/725Formation and Near-Field Dynamics of a Wing Tip Vortex
https://resolver.caltech.edu/CaltechTHESIS:02072013-122723580
Authors: {'items': [{'id': 'Zuhal-Lavi-Rizki', 'name': {'family': 'Zuhal', 'given': 'Lavi Rizki'}, 'show_email': 'NO'}]}
Year: 2001
DOI: 10.7907/VNJW-6592
<p>The search for a more efficient method to destroy aircraft trailing vortices requires
a good understanding of the early development of the vortices. For that purpose, an
experimental investigation has been conducted to study the formation and near-field
dynamics of a wing tip vortex.</p>
<p>Two versions of the Digital Particle Image Velocimetry (DPIV) technique were
used in the studies. Planar DPIV was used to obtain velocity fields adjacent to the wing
surface. Stereoscopic DPIV, which allows instantaneous measurements of all three
components of velocity within a planar slice, was used to measure velocity fields behind
the wing. The trailing vortex was produced by a rectangular half-wing model with an
NACA 0012 profile. All measurements were made at Reynolds number, based on chord
length, of 9040.</p>
<p>The present study has found that the wing sheds multiple vortices. A structure
that closely resembles a wing tip vortex is first observed on the suction side of the wing
near the tip at the mid-chord section of the wing. At the trailing edge of the wing, a
smaller vortex with an opposite sense of rotation is observed next to the tip vortex. In
addition to the two vortices, two vortex layers with opposite sense of rotation, one on the
pressure side and one on the suction side, are apparent at the trailing edge. Farther
downstream, most of the vorticity in the vortex layer, with the same sense of rotation as
the tip vortex, rolls up into the wing tip vortex. The vortices, with opposite sense of
rotation, break up into smaller vortices which orbit around the tip vortex. At least one
relatively strong satellite vortex appears in some of the instantaneous fields. The studies
found that the interaction of the tip vortex and satellite vortices give rise to the unsteady
motion of the wing tip vortex. In addition, the studies also examined the effects of the
boundary layer and the tip geometry to the strength and motion of the trailing vortex.</p>https://thesis.library.caltech.edu/id/eprint/7469Passive Hypervelocity Boundary Layer Control Using an Ultrasonically Absorptive Surface
https://resolver.caltech.edu/CaltechETD:etd-08192001-143746
Authors: {'items': [{'email': 'arasheed@galcit.caltech.edu', 'id': 'Rasheed-Adam', 'name': {'family': 'Rasheed', 'given': 'Adam'}, 'show_email': 'YES'}]}
Year: 2001
DOI: 10.7907/EFZZ-X764
A series of exploratory boundary layer transition experiments was performed on a sharp 5.06 degree half-angle round cone at zero angle-of-attack in the T5 Hypervelocity Shock Tunnel in order to test a novel hypersonic boundary layer control scheme. Recently performed linear stability analyses suggested that transition could be delayed in hypersonic boundary layers by using an ultrasonically absorptive surface that would damp the second mode (Mack mode). The cone used in the experiments was constructed with a smooth surface on half the cone (to serve as a control) and an acoustically absorptive porous surface on the other half. It was instrumented with flush-mounted thermocouples to detect the transition location. Test gases investigated included nitrogen and carbon dioxide at M = 5 with specific reservoir enthalpy ranging from 1.3 MJ/kg to 13.0 MJ/kg and reservoir pressure ranging from 9.0 MPa to 50.0 MPa. Detailed comparisons were performed to insure that previous results obtained in similar boundary layer transition experiments (on a regular smooth surface) were reproduced and the results were extended to examine the effects of the porous surface. These experiments indicated that the porous surface was highly effective in delaying transition provided that the hole size was significantly smaller than the viscous length scale.https://thesis.library.caltech.edu/id/eprint/3168Shock Wave Propagation in Periodically Layered Composites
https://resolver.caltech.edu/CaltechTHESIS:05102011-141326530
Authors: {'items': [{'id': 'Zhuang-Shiming', 'name': {'family': 'Zhuang', 'given': 'Shiming'}, 'show_email': 'NO'}]}
Year: 2002
DOI: 10.7907/988X-1V27
Mathematically, a shock wave is treated as a discontinuity in a medium. In reality, however, a shock wave is always structured, i.e., its front takes a finite time to rise from an initial material state to the final shocked state. The structuring of a shock front is due to the competition between the nonlinearity of material behavior and the dissipation processes occurring during the wave propagation. There are many mechanisms which may be responsible for the dissipation and/or dispersion of shock wave energy. In homogeneous media, such as metals, one common interpretation for the structuring of a shock wave is that the viscoplasticity processes (dislocation, twinning, etc.) are responsible for the dissipation of energy. While in heterogeneous composites, besides the viscous dissipative processes existing in each of its constituents, due to the existence of internal interfaces, the scattering induced by the interface during shock compression could be another important mechanism.
In this study, the interface scattering effects on shock wave propagation in heterogeneous media were investigated by subjecting periodically layered composites to planar impact loading with a flyer plate. The flyer plate was accelerated to a desired velocity using a powder gun loading system. In order to measure shock particle velocity time history at an internal or the free surface of the specimen, the so-called VISAR (Velocity Interferometry System for Any Reflector) diagnostic system was constructed and used during shock compression experiments. Manganin stress gages were embedded inside the specimen at selected internal interfaces to measure shock stress time history. To study the scattering mechanisms of the interface to waves, two-component composite specimens with different interface mechanical properties and heterogeneity were prepared and tested. Different types of composites were prepared with differing mechanical impedance. Specimens with different heterogeneity were obtained by changing the geometrical configuration (length scale) of the layered stack. Two-dimensional numerical simulations were also carried out to understand the process of shock wave evolution in the layered composites.
Experimental and numerical studies show that periodically layered composites support steady structured shock waves. The influence of internal interfaces on the shock wave propagation is through the scattering mechanism, i.e., multiple reflection of waves in the layers and their interaction with the shock wave. The interface scattering affects both the bulk and the deviatoric response of the composite to shock compression. The influence of scattering on the bulk behavior is to slow down the velocity of the shock wave in the composites, while its influence on the deviatoric response is to structure the shock wave profile. If all the dissipative and dispersive effects are collectively termed as viscosity, which causes the shock front structuring, i.e., the shock front rise-time increasing, then the effective shock viscosity increases with the increase of interface impedance mismatch and decreases with the increase of interface density (interface area per unit volume) and shock loading strength. The existing mixture model for constructing the constitutive relation for composites based on the known properties of its component materials can only, at best, reasonably predict the response of the composites under strong shock loading conditions. In order to fully describe the response of a heterogeneous composite to shock compression loading, accurate physics-based constitutive relations need to be formulated to take into account the scattering effects induced by the heterogeneous microstructure.
https://thesis.library.caltech.edu/id/eprint/6380The Role of Instability in Gaseous Detonation
https://resolver.caltech.edu/CaltechETD:etd-05292003-150534
Authors: {'items': [{'email': 'joanna@caltech.edu', 'id': 'Austin-Joanna-Maria-Karol', 'name': {'family': 'Austin', 'given': 'Joanna Maria Karol'}, 'orcid': '0000-0003-3129-5035', 'show_email': 'YES'}]}
Year: 2003
DOI: 10.7907/X7YH-T687
<p>In detonation, the coupling between fluid dynamics and chemical energy release is critical. The reaction rate behind the shock front is extremely sensitive to temperature perturbations and, as a result, detonation waves in gases are always unstable. A broad spectrum of behavior has been reported for which no comprehensive theory has been developed. The problem is extremely challenging due to the nonlinearity of the chemistry-fluid mechanics coupling and extraordinary range of length and time scales exhibited in these flows. Past work has shown that the strength of the leading shock front oscillates and secondary shock waves propagate transversely to the main front. A key unresolved issue has emerged from the past 50 years of research on this problem: What is the precise nature of the flow within the reaction zone and how do the instabilities of the shock front influence the combustion mechanism?</p>
<p>This issue has been examined through dynamic experimentation in two facilities. Key diagnostic tools include unique visualizations of superimposed shock and reaction fronts, as well as short but informative high-speed movies. We study a range of fuel-oxidizer systems, including hydrocarbons, and broadly categorize these mixtures by considering the hydrodynamic stability of the reaction zone. From these observations and calculations, we show that transverse shock waves do not essentially alter the classic detonation structure of Zeldovich-von Neumann-Doring (ZND) in weakly unstable detonations, there is one length scale in the instability, and the combustion mechanism is simply shock-induced chemical-thermal explosion behind a piecewise-smooth leading shock front. In contrast, we observe that highly unstable detonations have substantially different behavior involving large excursions in the lead shock strength, a rough leading shock front, and localized explosions within the reaction zone. The critical decay rate model of Eckett et al. (JFM 2000) is combined with experimental observations to show that one essential difference in highly unstable waves is that the shock and reaction front may decouple locally. It is not clear how the ZND model can be effectively applied in highly unstable waves. There is a spectrum of length scales and it may be possible that a type of "turbulent" combustion occurs. We consider how the coupling between chemistry and fluid dynamics can produce a large range of length scales and how possible combustion regimes within the front may be bounded.</p>https://thesis.library.caltech.edu/id/eprint/2234An Experimental Investigation of Richtmyer-Meshkov Instability
https://resolver.caltech.edu/CaltechETD:etd-02212003-140109
Authors: {'items': [{'id': 'Kumar-Sanjay', 'name': {'family': 'Kumar', 'given': 'Sanjay'}, 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/NKDR-TT39
<p>In this study, the interaction of a shock wave with an interface between two gases is studied experimentally. The basic mechanism for the initial growth of perturbations on the interface is the baroclinic generation of vorticity which results from the misalignment of the pressure gradient in the shock and the density gradient at the interface. The growth of perturbations soon enters into a nonlinear regime with the appearance of bubbles of light fluid rising into heavy fluid and spikes of heavy fluid falling into light fluid. In the nonlinear regime, interaction between various scales and the appearance of other instabilities, such as Kelvin-Helmholtz instability, along the boundaries of the spikes occur, which results in the breakup of the interface. These processes lead to a turbulent mixing zone (TMZ) which grows with time. The main focus of this study is to understand the growth of TMZ with time in a cylindrical geometry with square cross section and for the the first time study the effect of area convergence in a conical geometry on its growth rate. The present set of experiments is done in the GALCIT 17 in. shock tube with air and sulfur hexafluoride as light and heavy gases. The growth of the TMZ is studied in a straight test section for single-mode initial perturbation consisting of two different wavelength and amplitude combinations at incident shock Mach number of 1.55. The multimode initial perturbation growth at late times is studied in a conical geometry to study the effect of area convergence at incident Mach numbers of 1.55 and 1.39. The results are compared with the experiments of Vetter which were done in the same shock tube with a straight test section with no area convergence and at the same Mach number.</p>
<p>In the study of the Richtmyer-Meshkov (RM) instability of single-scale perturbations on air/sulfur-hexafluoride interface in a straight test section, the initially sinusoidal interface is formed by a polymeric membrane of thickness 1.5 micron and the flow visualization is done using schlieren imaging technique. The interface thickness is measured visually from the photographs. It is found that the growth rate decreases rapidly with time with a small dependence on the initial wavelength persisting until late times.</p>
<p>In the case of the RM instability, growth of multimode initial perturbations in a conical geometry, it is found from the schlieren flow visualization images that the interface thickness grows about 40-50 % more rapidly than in Vetter's experiments. Experimental results for laser-induced scattering at late times are presented for air/He gas combinations at the interface. In situations when the rear of the interface is not clearly demarcated, the thickness is determined by an image processing technique. This technique is also used to determine the possible dominant eddy/blob size in the TMZ from the schlieren images. Some inviscid computational studies, with a planar or spherical shock interacting with a planar or spherical initial interface in light-heavy (air/sulfur-hexafluoride) and heavy-light (air/He) configurations, are also presented. In the conical geometry there is a reflected shock originating from the triple point. This reflection is a consequence of the transition from the cylindrical shock tube to the converging cone. Due to the vorticity created by the interaction of reflected shock from the cone wall with the interface in initial stage, it is found that the interface curves toward or away from the apex of the cone, depending on the sign of density gradient. This curving of the interface could have a role to play in the diffuse rear boundary of the interface in schlieren flow visualization images but the laser-induced scattering image shows that the mixing zone indeed does not have a well-defined rear boundary. Rather, small blobs of fluids on the right are scattered in the mixing zone. An inviscid computational study is also done on cylindrical and conical test section geometries to study the effect of transverse reflected waves on the growth of small sinusoidal initial perturbations. It is found by comparison with cylindrical geometry (where reflected waves do not exist) that the transverse reflected waves do not affect the growth of perturbations on the interface.</p>https://thesis.library.caltech.edu/id/eprint/690Algorithms 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/2292A Numerical and Analytical Study of Detonation Diffraction
https://resolver.caltech.edu/CaltechETD:etd-02122003-152525
Authors: {'items': [{'id': 'Arienti-Marco', 'name': {'family': 'Arienti', 'given': 'Marco'}, 'orcid': '0000-0001-8166-0016', 'show_email': 'NO'}]}
Year: 2003
DOI: 10.7907/MAGN-R628
An investigation of detonation diffraction through an abrupt area change has been carried out via two-dimensional, parallel simulations. The existence of critical conditions for successful diffraction is closely related to the occurrence of localized re-initiation mechanisms, and is relevant to propulsion and safety concepts concerning detonation transmission. Our analysis is specialized to a reactive mixture with perfect gas equation of state and a single-step reaction in the Arrhenius form. The concept of shock decoupling from the reaction zone is the simplest idea used to explain the behavior of a diffracting detonation front. Lagrangian particles are injected into the flow in order to identify the dominant terms in the equation that describes the temperature rate of change of a fluid element, expressed in a shock-based reference system. Conveniently simplified, this equation provides an insight into the competition between the energy release rate and the expansion rate behind the diffracting front. We also examine the mechanism of spontaneous generation of transverse waves along the front. This mechanism is related to the sensitivity of the reaction rate to temperature, and it is investigated in the form of a parametric study for the activation energy. We study in detail three highly resolved cases of detonation diffraction that illustrate different types of behavior, super-, sub-, and near-critical diffraction. We review the applicability of existing shock dynamics models to the corner-turning problem. Numerical results from the parametric study are compared with predictions from these theories in the attempt to find a formula for shock decay in a quenching detonation. This estimate is then used in the simplified temperature rate of change equation to provide a relation between critical channel width and activation energy. We conclude this study by examining the spontaneous formation of transverse waves along the wavefront of a successfully transmitted detonation. The problem is simplified to a planar CJ detonation moving in a channel over a small obstacle to investigate how acoustic waves propagate within the reaction zone. Depending on the reaction kinetics, we show that such waves may be amplified due to feedback between the chemical reaction and fluid motion. The amplification can lead to shock steepening and formation of transverse detonation waves.
https://thesis.library.caltech.edu/id/eprint/610The 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/1886Impulse 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/1451Gaseous Detonation-Driven Fracture of Tubes
https://resolver.caltech.edu/CaltechETD:etd-04062004-165940
Authors: {'items': [{'id': 'Chao-Tong-Wa', 'name': {'family': 'Chao', 'given': 'Tong Wa'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/TEZP-YC46
<p>An experimental investigation of fracture response of aluminum 6061-T6 tubes under internal gaseous detonation loading has been carried out. The pressure load, with speeds exceeding 2 km/s, can be characterized as a pressure peak (ranging from 2 to 6 MPa) followed by an expansion wave. The unique combination of this particular traveling load and tube geometry produced fracture data not available before in the open literature. Experimental data of this type are useful for studying the fluid-structure-fracture interaction and various crack curving and branching phenomena, and also for validation for multi-physics and multi-scale modeling.</p>
<p>Axial surface flaws were introduced to control the crack initiation site. Fracture threshold models were developed by combining a static fracture model and an extensively studied dynamic amplification factor for tubes under internal traveling loads. Experiments were also performed on hydrostatically loaded preflawed aluminum 6061-T6 tubes for comparison. Significantly different fracture behavior was observed and the difference was explained by fluid dynamics and energy considerations. The experiments yielded comparison on crack speeds, strain, and pressure histories.</p>
<p>In other experiments, the specimens were also pre-torqued to control the propagation direction of the cracks. Measurements were made on the detonation velocity, strain history, blast pressure from the crack opening, and crack speeds. The curved crack paths were digitized. The Chapman-Jouguet pressure, initial axial flaw length, and torsion level were varied to obtain different crack patterns. The incipient crack kinking angle was found to be consistent with fracture under mixed-mode loading. High-speed movies of the fracture events and blast wave were taken and these were used in interpreting the quantitative data.</p>
<p>Numerical simulations were performed using the commercial explicit finite-element software LS-Dyna. The detonation wave was modeled as a traveling boundary load. Both non-fracturing linear elastic simulations and elastoplastic simulations with fracture were conducted on three-dimensional models. The simulated fracture was compared directly with an experiment with the same conditions. The overall qualitative fracture behavior was captured by the simulation. The forward and backward cracks were observed to branch in both the experiment and simulation.</p>https://thesis.library.caltech.edu/id/eprint/1276Compressible Vortices and Shock-Vortex Interactions
https://resolver.caltech.edu/CaltechETD:etd-05262004-145030
Authors: {'items': [{'id': "O'Reilly-Gerard-Kieran", 'name': {'family': "O'Reilly", 'given': 'Gerard Kieran'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/FGJD-0Z31
Secondary instabilities on the organized, spanwise, vortical structures in incompressible shear layers, play an important role in generating the onset of three-dimensional turbulence in such flows. The effect of increasing compressibility on these instabilities is examined by using the compressible Stuart vortex as a model for a compressible shear layer. It is found that both two- and three-dimensional subharmonic instabilities cease to promote pairing events even at moderate free stream Mach numbers. The fundamental mode becomes dominant as the free stream Mach number is increased, and a new instability corresponding to an instability on a parallel shear layer is observed. The interaction of a shock with a compressible vortex may be viewed as a simplified model of the general interaction of a shock with the coherent structures in a turbulent flow field. An approximate theory for computing shock-compressible-vortex interactions is developed, based on Ribner (1954). The problem of convection of a frozen patern of vorticity, dilatation, temperature and entropy through a planar shock wave is considered. The refraction and modification of the upstream disturbances into the three basis modes permitted by the linear Euler equations is derived, as well as the perturbation to the shock wave. This theory is used to compute approximate post-shock states corresponding to shock-CSV interactions, a model for shock shear layer interactions. The method is verified by comparing its approximate post-shock fields with those computed explicitly using AMROC, a finite difference, AMR-WENO code. Finally, numerical solutions corresponding to a compressible analogue of the Mallier and Maslowe vortex (a periodic array of counter-rotating vortices) are presented. These solutions admit the existence of large regions of smooth supersonic flow, and could potentially be used to model the counter-rotating vortices arising from the single- and multi-mode Richtmyer-Meshkov instability.https://thesis.library.caltech.edu/id/eprint/2082Part I: 3DPTV: Advances and Error Analysis. Part II: Extension of Guderley's Solution for Converging Shock Waves
https://resolver.caltech.edu/CaltechETD:etd-05312005-111401
Authors: {'items': [{'email': 'nponchaut@hotmail.com', 'id': 'Ponchaut-Nicolas-Frederic', 'name': {'family': 'Ponchaut', 'given': 'Nicolas Frederic'}, 'show_email': 'YES'}]}
Year: 2005
DOI: 10.7907/09ZH-9M66
This work is divided into two unrelated parts. In the first part, a full three-dimensional particle tracking system was developed and tested. Three images, from three separate CCDs placed at the vertices of an equilateral triangle, permit the three-dimensional location of particles to be determined by triangulation. Particle locations measured at two different times can then be used to create a three-component, three-dimensional velocity field. Key developments are the ability to accurately process overlapping particle images, offset CCDs to significantly improve effective resolution, treatment of dim particle images, and a hybrid particle tracking technique ideal for three-dimensional flows when only two sets of images exist. An in-depth theoretical error analysis was performed, which gives the important sources of error and their effect on the overall system. This error analysis was verified through a series of experiments, and a vortex flow measurement was performed.
In the second part, the problem of a cylindrically or spherically imploding and reflecting shock wave in a flow initially at rest was examined. Guderley's strong shock solution around the origin was improved by adding two more terms in the series expansion solution for both the incoming and the reflected shock waves. A series expansion was also constructed for the case where the shock is still very far from the origin. In addition, a program based on the characteristics method was written. Thanks to an appropriate change of variables, the shock motion could be computed from virtually infinity to very close to the reflection point. Comparisons were made between the series expansions, the characteristics program, and the results obtained using an Euler solver. These comparisons showed that the addition of two terms to the Guderley solution significantly increases the accuracy of the series expansion.https://thesis.library.caltech.edu/id/eprint/2326Detonation Diffraction in Mixtures with Various Degrees of Instability
https://resolver.caltech.edu/CaltechETD:etd-02072005-173741
Authors: {'items': [{'id': 'Pintgen-Florian-Peter', 'name': {'family': 'Pintgen', 'given': 'Florian Peter'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/YSG0-TH85
<p>Planar laser induced fluorescence (PLIF) is widely used in combustion diagnostics but has only recently been successfully applied to detonation. The strong spatial variations in temperature, pressure, and background composition under these conditions influence the quantitative link between OH-number density and fluorescence intensity seen on images. Up to now, this has lead to uncertainties in interpreting the features seen on PLIF images obtained in detonations. A one-dimensional fluorescence model has been developed, which takes into account light sheet attenuation by absorption, collisional quenching, and changing absorption line shape. The model predicts the fluorescence profile based on a one-dimensional distribution in pressure, temperature, and mixture composition. The fluorescence profiles based on a calculated ZND detonation profile were found to be in good agreement with experiments.</p>
<p>The PLIF technique is used to study the diffraction process of a self-sustained detonation wave into an unconfined space through an abrupt area change. Simultaneous schlieren images enable direct comparison of shock and reaction fronts. Two mixture types of different effective activation energy [theta] are studied in detail, these represent extreme cases in the classification of detonation front instability and cellular regularity. Striking differences are seen in the failure mechanisms for the very regular H2-O2-Ar mixture ([theta] ~ 4.5) and the highly irregular H2-N2O mixture ([theta] ~ 9.4). Detailed image analysis quantifies the observed differences. Stereoscopic imaging reveals the complex three-dimensional structure of the transverse detonation and its location with respect to the shock front. The study is concluded by using the experimentally-obtained shock and reaction front profiles in a simplified model to examine the decoupling of the shock from the chemical reaction. The rapid increase in activation energy for the H2-O2-Ar mixtures with decreasing shock velocity is proposed as an important new element in the analysis of diffraction for these mixture.</p>https://thesis.library.caltech.edu/id/eprint/534Experiments and Modeling of Impinging Jets and Premixed Hydrocarbon Stagnation Flames
https://resolver.caltech.edu/CaltechETD:etd-05242005-165713
Authors: {'items': [{'email': 'jeffrey.bergthorson@mcgill.ca', 'id': 'Bergthorson-Jeffrey-Myles', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey Myles'}, 'orcid': '0000-0003-2924-7317', 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/7FQZ-EY88
<p>To model the combustion of long-chain hydrocarbon fuels, an accurate kinetics mechanism must first be developed for the oxidation of small hydrocarbons, such as methane, ethane, and ethylene. Even for methane, a generally accepted mechanism is still elusive due to a lack of kinetically independent experimental data. In this work, a combined experimental and modeling technique is developed to validate and further optimize these mechanisms. This technique relies on detailed measurements of strained flames in a jet-wall stagnation flow using simultaneous Particle Streak Velocimetry (PSV) and CH Planar Laser Induced Fluorescence (PLIF). Stagnation flames are simulated using an axisymmetric, one-dimensional model with accurate specification of the requisite boundary conditions. Direct comparisons between experiment and simulation allow for an assessment of the various models employed, with an emphasis on the chemistry model performance.</p>
<p>The flow field for a cold impinging laminar jet is found to be independent of the nozzle-to-plate separation distance if velocities are scaled by the Bernoulli velocity. The one-dimensional formulation is found to accurately model the stagnation flow if the velocity boundary conditions are appropriately specified. The boundary-layer-displacement-thickness corrected diameter is found to be an appropriate scale for axial distances and allows the identification of an empirical, analytical expression for the flow field of the impinging laminar jet.</p>
<p>Strained methane-air flame experiments confirm that the reacting flow is also independent of the nozzle-to-plate separation distance. Methane, ethane, and ethylene flames are studied as functions of the applied strain rate, mixture dilution, and mixture fraction. Mechanism performance is found to be relatively insensitive to both the mixture dilution and the imposed strain rate, while exhibiting a stronger dependence on the fuel type and flame stoichiometry. The approach and diagnostics presented here permit an assessment of the predictions of strained-hydrocarbon flames for several combustion chemistry mechanisms. The data presented in this thesis are made available to kineticists looking for optimization targets, with the goal of developing a predictive kinetics model for hydrocarbon fuels. The methodology described here can allow new optimization targets to be rapidly measured, reducing the experimental burden required to fully constrain the chemistry models.</p>https://thesis.library.caltech.edu/id/eprint/2004Gaseous Detonation Initiation Via Wave Implosion
https://resolver.caltech.edu/CaltechETD:etd-05242005-151253
Authors: {'items': [{'email': 'sjackson@lanl.gov', 'id': 'Jackson-Scott-Irving', 'name': {'family': 'Jackson', 'given': 'Scott Irving'}, 'orcid': '0000-0002-6814-3468', 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/MKP3-VC84
<p>Efficient detonation initiation is a topic of intense interest to designers of pulse detonation engines. This experimental work is the first to detonate propane-air mixtures with an imploding detonation wave and to detonate a gas mixture with a non-reflected, imploding shock. In order to do this, a unique device has been developed that is capable of generating an imploding toroidal detonation wave inside of a tube from a single ignition point without any obstruction to the tube flow path. As part of this study, an initiator that creates a large-aspect-ratio planar detonation wave in gas-phase explosive from a single ignition point has also been developed.</p>
<p>The effectiveness of our initiation devices has been evaluated. The minimum energy required by the imploding shock for initiation was determined to scale linearly with the induction zone length, indicating the presence of a planar initiation mode. The imploding toroidal detonation initiator was found to be more effective at detonation initiation than the imploding shock initiator, using a comparable energy input to that of current initiator tubes.</p>https://thesis.library.caltech.edu/id/eprint/2001On the Richtmyer-Meshkov Instability in Magnetohydrodynamics
https://resolver.caltech.edu/CaltechETD:etd-05272005-145538
Authors: {'items': [{'email': 'vincent.wheatley@gmail.com', 'id': 'Wheatley-Vincent', 'name': {'family': 'Wheatley', 'given': 'Vincent'}, 'orcid': '0000-0002-7287-7659', 'show_email': 'YES'}]}
Year: 2005
DOI: 10.7907/N407-2B54
<p>The Richtmyer-Meshkov instability is important in a wide variety of applications including inertial confinement fusion and astrophysical phenomena. In some of these applications, the fluids involved may be plasmas and hence be affected by magnetic fields. For one configuration, it has been numerically demonstrated that the growth of the instability in magnetohydrodynamics is suppressed in the presence of a magnetic field. Here, the nature of this suppression is theoretically and numerically investigated.</p>
<p>In the framework of ideal incompressible magnetohydrodynamics, we examine the stability of an impulsively accelerated, sinusoidally perturbed density interface in the presence of a magnetic field that is parallel to the acceleration. This is accomplished by analytically solving the linearized initial value problem, which is a model for the Richtmyer-Meshkov instability. We find that the initial growth rate of the interface is unaffected by the presence of a magnetic field, but for a finite magnetic field the interface amplitude asymptotes to a constant value. Thus the instability of the interface is suppressed. The interface behavior from the analytical solution is compared to the results of both linearized and non-linear compressible numerical simulations for a wide variety of conditions.</p>
<p>We then consider the problem of the regular refraction of a shock at an oblique, planar contact discontinuity separating conducting fluids of different densities in the presence of a magnetic field aligned with the incident shock velocity. Planar ideal MHD simulations indicate that the presence of a magnetic field inhibits the deposition of vorticity on the shocked contact, which leads to the suppression of the Richtmyer-Meshkov instability. We show that the shock refraction process produces a system of five to seven plane waves that may include fast, intermediate, and slow MHD shocks, slow compound waves, 180° rotational discontinuities, and slow-mode expansion fans that intersect at a point. In all solutions, the shocked contact is vorticity free and hence stable. These solutions are not unique, but differ in the type of waves that participate. The set of equations governing the structure of these multiple-wave solutions is obtained in which fluid property variation is allowed only in the azimuthal direction about the wave-intersection point. Corresponding solutions are referred to as either quintuple-points, sextuple-points, or septuple-points, depending on the number of participating waves. A numerical method of solution is described and examples are compared to the results of numerical simulations for moderate magnetic field strengths. The limit of vanishing magnetic field at fixed permeability and pressure is studied for two solution types. The relevant solutions correspond to the hydrodynamic triple-point with the shocked contact replaced by a singular structure consisting of a wedge, whose angle scales with the applied field magnitude, bounded by either two slow compound waves or two 180° rotational discontinuities, each followed by a slow-mode expansion fan. These bracket the MHD contact which itself cannot support a tangential velocity jump in the presence of a non-parallel magnetic field. The magnetic field within the singular wedge is finite and the shock-induced change in tangential velocity across the wedge is supported by the expansion fans that form part of the compound waves or follow the rotational discontinuities. To verify these findings, an approximate leading order asymptotic solution appropriate for both flow structures was computed. The full and asymptotic solutions are compared quantitatively and there is shown to be excellent agreement between the two.</p>https://thesis.library.caltech.edu/id/eprint/2156Aerodynamic Control and Mixing with Ramp Injection
https://resolver.caltech.edu/CaltechETD:etd-05262005-112117
Authors: {'items': [{'email': 'johns3c@uwindsor.ca', 'id': 'Johnson-Michael-Bernard', 'name': {'family': 'Johnson', 'given': 'Michael Bernard'}, 'show_email': 'NO'}]}
Year: 2005
DOI: 10.7907/8EVK-FK75
<p>Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility (S3L) to investigate the behaviour of a flow and geometry with many features that are potentially useful for a Supersonic Combustion Ramjet (SCRAMJET) engine - a recirculation zone for flameholding, enhanced mixing between fuel and air, and low total-pressure losses. In a subsonic diffuser configuration with no mass injection, the exit velocity and guidewall static-pressure profiles collapse over a large range of inlet Reynolds numbers. Significant control of exit velocity and guidewall pressure profiles is possible via injection through a perforated ramp into the freestream. The control authority on the overall pressure coefficient increases with increasing inlet Reynolds number. Simple control volume models put bounds on the overall pressure coefficient for the device.</p>
<p>In low-supersonic flow, the area ratio calculated from measured pressures agrees well with the visual shear-layer thickness, illustrating the low total-pressure losses present.</p>
<p>Further control is possible through variable heat release from a fast-chemical reaction between reactants carried in the two streams. At the highest heat release studied, mass injection requirements are lowered by, roughly, a factor of two. Measurements of mixing inferred from the temperature rise from such a reaction indicate a high level of mixing vs. classical free shear layers. As in free shear layers, however, the level of mixing begins to decrease with increasing heat release.</p>https://thesis.library.caltech.edu/id/eprint/2091Stereo Digital Particle Image Velocimetry Investigation of a Free Surface Mixing Layer
https://resolver.caltech.edu/CaltechETD:etd-06022005-180557
Authors: {'items': [{'email': 'bdooley@alumni.rice.edu', 'id': 'Dooley-Bradley-Scott', 'name': {'family': 'Dooley', 'given': 'Bradley Scott'}, 'show_email': 'YES'}]}
Year: 2005
DOI: 10.7907/EH41-N436
<p>Shear flows in the vicinity of a free surface are a problem with numerous applications, perhaps the most obvious being the wakes of seagoing surface vessels. The flow behind a full-scale ship is extremely complex – so much so that it is frequently more instructive to consider simpler cases highlighting particular elements of the larger problem. To that end, an experimental investigation has been conducted to study the behavior of a turbulent plane mixing layer intersecting a free surface at low Froude number. The local Reynolds number, based on the velocity differential across the layer and the momentum thickness, was approximately 10,000.</p>
<p>The technique of Stereoscopic Digital Particle Image Velocimetry (SDPIV) was implemented to obtain instantaneous three-component velocity measurements within planar slices of the steady-state, spatially developing mixing layer flow. Guided by previous studies of the same flow conditions, specific depths were chosen at a single downstream station for investigation – specifically those in and around counter-rotating streamwise vortices known to exist in the mean flow very near the free surface. 3,000 consecutive SDPIV image pairs were recorded at a rate of 15 per second at each location, giving ample data for Reynolds decomposition and spectral analysis of the velocity fields.</p>
<p>The present study has found that the anisotropy known to exist in some other free surface flows, such as surface-parallel submerged jets, is also present in the case of the mixing layer. Power spectra of all three velocity components are shown to capture part of the inertial subrange; the isotropic energy cascade seen to be present away from the free surface is also seen to disappear near the surface, as surface-normal velocity fluctuations are severely attenuated.</p>
<p>Additionally, a low-frequency spanwise oscillation is deduced from the velocity power spectra and cospectra in the immediate vicinity of the mean streamwise vortices. Not present at all at significant depth, the motions at this frequency are also observed to markedly decrease – in all components – at locations closer to the surface. These observations appear to have both parallels and key differences compared to previously observed meandering of model boat wakes, and the possibility that the oscillation stems from the vortex-pair instability is discussed.</p>https://thesis.library.caltech.edu/id/eprint/2394Detonation Interaction with Sharp and Diffuse Interfaces
https://resolver.caltech.edu/CaltechETD:etd-11172005-092205
Authors: {'items': [{'id': 'Lieberman-Daniel-Howard', 'name': {'family': 'Lieberman', 'given': 'Daniel Howard'}, 'show_email': 'NO'}]}
Year: 2006
DOI: 10.7907/9JZE-X524
<p>Detonation interaction with an interface was investigated, where the interface separated a combustible from an oxidizing mixture. The ethylene-oxygen combustible mixture had a fuel-rich composition to promote secondary combustion with the oxidizer in the turbulent mixing zone that resulted from the interaction. Both sharp and diffuse interfaces were studied.</p>
<p>Diffuse interfaces were created by the formation of a gravity current using a sliding valve that initially separated the test gas and combustible mixture. Opening the valve allowed a gravity current to develop before the detonation was initiated. By varying the delay between opening the valve and initiating the detonation it was possible to achieve a wide range of interface conditions. Sharp interfaces were created by using a nitro-cellulose membrane to separate the two mixtures. The membrane was destroyed by the detonation wave.</p>
<p>The interface orientation and thickness with respect to the detonation wave have a profound effect on the outcome of the interaction. Diffuse interfaces result in curved detonation waves with a transmitted shock and following turbulent mixing zone. Sharp interfaces result in an interaction occurring at a node point similar to regular shock refraction (Henderson, 1989). The impulse was measured to quantify the degree of secondary combustion accounting for 5-6% of the total impulse. A model was developed that estimated the volume expansion of a fluid element due to combustion in the turbulent mixing zone (Dimotakis, 1991) to predict the impulse in the limit of infinite Damkohler number.</p>https://thesis.library.caltech.edu/id/eprint/4592Proximal Bodies in Hypersonic Flow
https://resolver.caltech.edu/CaltechETD:etd-04242006-172719
Authors: {'items': [{'email': 'stulaurence@gmail.com', 'id': 'Laurence-Stuart-Jon', 'name': {'family': 'Laurence', 'given': 'Stuart Jon'}, 'orcid': '0000-0001-8760-8366', 'show_email': 'YES'}]}
Year: 2006
DOI: 10.7907/VZJV-KJ48
<p>The problem of proximal bodies in hypersonic flow is encountered in several important situations, both natural and man-made. The present work seeks to investigate one aspect of this problem by exploring the forces experienced by a secondary body when some part of it is within the shocked region created by a primary body travelling at hypersonic speeds.</p>
<p>An analytical methodology based on the blast wave analogy is developed and used to predict the secondary force coefficients for simple geometries in both two and three dimensions. When the secondary body is entirely inside the primary shocked region, the nature of the lateral coefficient is found to depend strongly on the relative size of the two bodies. For two spheres, the methodology predicts that the secondary body will experience an exclusively attractive lateral force if the secondary diameter is larger then one-sixth the primary diameter. The analytical results are compared with numerical simulations carried out using the AMROC software and good agreement is obtained if an appropriate normalization for the lateral displacement is used.</p>
<p>Results from a series of experiments in the T5 hypervelocity shock tunnel are also presented and compared with perfect-gas numerical simulations, again with good agreement. In order to model this situation experimentally, a new force-measurement technique for short-duration hypersonic facilities has been developed, and results from the validation experiments are included.</p>
<p>Finally, the analytical methodology is used to model two physical situations. First, the entry of a binary asteroid system into the Earth's atmosphere is simulated. Second, a model for a fragmenting meteoroid in a planetary atmosphere is developed, and simulations are carried out to determine whether the secondary scatter patterns in the Sikhote-Alin crater field may be attributed to aerodynamic interactions between fragments rather than to secondary fragmentation. It is found that while aerodynamic interactions lead to increased secondary crater grouping, these groups do not exhibit the typically elliptical shape that we would expect secondary fragmentation to produce.</p>https://thesis.library.caltech.edu/id/eprint/1490The 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/2405Mechanical Characterization of Damage and Failure in Polymeric Foams and Glass/Epoxy Composites
https://resolver.caltech.edu/CaltechETD:etd-11102006-182329
Authors: {'items': [{'id': 'Kidd-Theresa-Hiromi', 'name': {'family': 'Kidd', 'given': 'Theresa Hiromi'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/G25Y-KE07
<p>The mechanical characterization including evolution of damage and failure of foams and composites are becoming increasingly important, as they form the basic components of sandwich structures. Sandwich structures consist of two faceplates that surround a core material. In many modern applications, faceplates and cores are typically comprised of composite materials and polymeric foam, respectively. Knowledge of the failure behavior of these individual components is necessary for understanding the failure behavior and design of sandwich structures. A systematic investigation of the damage evolution and failure behavior of foams and composites was conducted using a variety of experimental techniques.</p>
<p>In-situ ultrasonic measurements were used to track the damage behavior in PVC polymeric foams with densities ranging from 130 to 250 kg/m³. The wave speeds were measured by two quartz piezoelectric shear transducers with a resonant frequency of 5 MHz in the transmission mode. A fixture was developed and constructed to protect the transducers during compression, while allowing them to take sound speed measurements of the sample along the axis of the load train. This fixture was placed in a servo-hydraulic MTS (Materials Testing System) machine, where the load-displacement response of the foam was recorded. A digital image correlation (DIC) method was used to capture the progression of failure under compression. Two dominant failure modes, elastic buckling and plastic collapse, were identified – and their onsets corresponded to the change in elastic wave speeds in the material, measured by the in-situ ultrasonic technique.</p>
<p>The transverse response of S-Glass/Epoxy unidirectional composites was investigated under varying degrees of confinement and strain rates. The experimental setup utilizes a fixture that allowed for independent measurement of the three principal stresses in a confined specimen. A servo-hydraulic materials testing system and a Kolsky (split Hopkinson) pressure bar generated strain rates between 10⁻³ to 10⁴ s⁻¹. Post-test scanning electron microscopy (SEM) observations suggest that under transverse loading at low-strain rates, confinement contributes to localized band formation. In addition, micrographs indicated that macroscopic transverse failure is dominated by shear stress, and occurs within these localized bands. These shear dominated failure bands were found inclined in a direction approximately 35° to the direction of loading. Implications of this orientation deviation of failure bands from maximum shear trajectories at 45° are discussed in reference to the state of confinement.</p>https://thesis.library.caltech.edu/id/eprint/4500Transition Between Regular Reflection and Mach Reflection in the Dual-Solution Domain
https://resolver.caltech.edu/CaltechETD:etd-01052007-125557
Authors: {'items': [{'email': 'cm@k-a-p.com', 'id': 'Mouton-Christopher-Andre', 'name': {'family': 'Mouton', 'given': 'Christopher Andre'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/TEA0-Q468
<p>A study of the shock-reflection domain for steady flow is presented. Conditions defining boundaries between different possible shock-reflection solutions are given, and where possible, simple analytic expressions for these conditions are presented. A new, more accurate estimate of the steady-state Mach stem height is derived based on geometric considerations of the flow. In particular, the location of the sonic throat through which the subsonic convergent flow behind the Mach stem is accelerated to divergent supersonic flow is considered. Comparisons with previous computational and experimental work show that the theory presented in this thesis more accurately predicts the Mach stem height than previous theories. The Mach stem height theory is generalized to allow for a moving triple point. Based on this moving triple point theory, a Mach stem growth rate theory is developed. This theory agrees well with computational and experimental results. Numerical computations of the effects of water vapor disturbances are also presented. These disturbances are shown to be sufficient to cause transition from regular reflection to Mach reflection in the dual-solution domain. These disturbances are also modeled as a simple energy deposition on one of the wedges, and an estimate for the minimum energy required to cause transition is derived.</p>
<p>Experimental results using an asymmetric wedge configuration in the Ludwieg tube facility at the California institute of Technology are presented. A Mach 4.0 nozzle was designed and built for the Ludwieg tube facility. This Mach number is sufficient to provide a large dual-solution domain, while being small enough not to require preheating of the test gas. The test time of the facility is 100ms, which requires the use of high-speed cinematography and a fast motor to rotate one of the two wedges. Hysteresis in the transition between regular to Mach reflection was successfully demonstrated in the Ludwieg tube facility. The experiments show that regular reflection could be maintained up to a shock angle approximately halfway between the von Neumann condition and the detachment condition.</p>
<p>Energy deposition studies were performed using an Nd:YAG laser. Triggering transition in this manner is found to depend on the location of the energy deposition. This finding is consistent with the numerical work presented in this thesis. Experiments were also performed to measure the Mach stem height and its growth rate. These results are compared with the theoretical estimates presented in this thesis. Excellent agreement between the steady-state Mach stem height and the theoretical estimates is seen. Comparisons of Mach stem growth rate with theoretical estimates show significant differences, but do show good agreement regarding the time required to reach the steady-state height.</p>https://thesis.library.caltech.edu/id/eprint/36A Theory of Stationarity and Asymptotic Approach in Dissipative Systems
https://resolver.caltech.edu/CaltechETD:etd-01122007-114557
Authors: {'items': [{'id': 'Rubel-Michael-Thomas', 'name': {'family': 'Rubel', 'given': 'Michael Thomas'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/VWDE-GB16
<p>The approximate dynamics of many physical phenomena, including turbulence, can be represented by dissipative systems of ordinary differential equations. One often turns to numerical integration to solve them. There is an incompatibility, however, between the answers it can produce (i.e., specific solution trajectories) and the questions one might wish to ask (e.g., what behavior would be typical in the laboratory?) To determine its outcome, numerical integration requires more detailed initial conditions than a laboratory could normally provide. In place of initial conditions, experiments stipulate how tests should be carried out: only under statistically stationary conditions, for example, or only during asymptotic approach to a final state. Stipulations such as these, rather than initial conditions, are what determine outcomes in the laboratory.</p>
<p>This theoretical study examines whether the points of view can be reconciled: What is the relationship between one's statistical stipulations for how an experiment should be carried out--stationarity or asymptotic approach--and the expected results? How might those results be determined without invoking initial conditions explicitly?</p>
<p>To answer these questions, stationarity and asymptotic approach conditions are analyzed in detail. Each condition is treated as a statistical constraint on the system--a restriction on the probability density of states that might be occupied when measurements take place. For stationarity, this reasoning leads to a singular, invariant probability density which is already familiar from dynamical systems theory. For asymptotic approach, it leads to a new, more regular probability density field. A conjecture regarding what appears to be a limit relationship between the two densities is presented.</p>
<p>By making use of the new probability densities, one can derive output statistics directly, avoiding the need to create or manipulate initial data, and thereby avoiding the conceptual incompatibility mentioned above. This approach also provides a clean way to derive reduced-order models, complete with local and global error estimates, as well as a way to compare existing reduced-order models objectively.</p>
<p>The new approach is explored in the context of five separate test problems: a trivial one-dimensional linear system, a damped unforced linear oscillator in two dimensions, the isothermal Rayleigh-Plesset equation, Lorenz's equations, and the Stokes limit of Burgers' equation in one space dimension. In each case, various output statistics are deduced without recourse to initial conditions. Further, reduced-order models are constructed for asymptotic approach of the damped unforced linear oscillator, the isothermal Rayleigh-Plesset system, and Lorenz's equations, and for stationarity of Lorenz's equations.</p>https://thesis.library.caltech.edu/id/eprint/141On the Development of Defocusing Digital Particle Image Velocimetry with Full Characterization
https://resolver.caltech.edu/CaltechETD:etd-05252007-140239
Authors: {'items': [{'id': 'Graff-Emilio-Castaño', 'name': {'family': 'Graff', 'given': 'Emilio Castaño'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/Z9HM56F8
Defocusing Digital Particle Image Velocimetry is the first volumetric, three-dimensional PIV method ever put into practice. This manuscript contains the details of its development, a detailed analysis of its performance (both through simulation and real measurements), and a series of experimental demonstrations of the capability of the technique. The system is capable of resolving upwards of 7,000 vectors per pair with an absolute error on the order of 0.03% of the volume size.https://thesis.library.caltech.edu/id/eprint/5200Numerical Simulations of Three-Dimensional Instabilities in Cavity Flows
https://resolver.caltech.edu/CaltechETD:etd-04262007-095945
Authors: {'items': [{'email': 'guillaume@alumni.caltech.edu', 'id': 'Brès-Guillaume-Alain', 'name': {'family': 'Brès', 'given': 'Guillaume Alain'}, 'orcid': '0000-0003-2507-8659', 'show_email': 'YES'}]}
Year: 2007
DOI: 10.7907/Z96W988B
<p>Direct numerical simulations are performed to investigate the stability of compressible flow over three-dimensional open cavities for future control applications.</p>
<p>First, the typical self-sustained oscillations, commonly referred as shear-layer (Rossiter) modes, are characterized for two-dimensional cavities over a range of flow conditions. A linear stability analysis is then conducted to search for three-dimensional global instabilities of the 2D mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of cavity configurations. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately 1 cavity depth and oscillates with a frequency about an order-of-magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities reported in flows over backward-facing steps, lid-driven cavity flows, and Couette flows.</p>
<p>Results from three-dimensional simulations of the nonlinear compressible Navier-Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. These results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows.</p>https://thesis.library.caltech.edu/id/eprint/1515Modeling and Simulation of Axisymmetric Stagnation Flames
https://resolver.caltech.edu/CaltechETD:etd-04252007-170838
Authors: {'items': [{'id': 'Sone-Kazuo', 'name': {'family': 'Sone', 'given': 'Kazuo'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/DYGA-YJ20
<p>Laminar flame modeling is an important element in turbulent combustion research. The accuracy of a turbulent combustion model is highly dependent upon our understanding of laminar flames and their behavior in many situations. How much we understand combustion can only be measured by how well the model describes and predicts combustion phenomena. One of the most commonly used methane combustion models is GRI-Mech 3.0. However, how well the model describes the reacting flow phenomena is still uncertain even after many attempts to validate the model or quantify uncertainties.</p>
<p>In the present study, the behavior of laminar flames under different aerodynamic and thermodynamic conditions is studied numerically in a stagnation-flow configuration. In order to make such a numerical study possible, the spectral element method is reformulated to accommodate the large density variations in methane reacting flows. In addition, a new axisymmetric basis function set for the spectral element method that satisfies the correct behavior near the axis is developed, and efficient integration techniques are developed to accurately model axisymmetric reacting flow within a reasonable amount of computational time. The numerical method is implemented using an object-oriented programming technique, and the resulting computer program is verified with several different verification methods.</p>
<p>The present study then shows variances with the commonly used GRI-Mech 3.0 chemical kinetics model through a direct simulation of laboratory flames that allows direct comparison to experimental data. It is shown that the methane combustion model based on GRI-Mech 3.0 works well for methane-air mixtures near stoichiometry. However, GRI-Mech 3.0 leads to an overprediction of laminar flame speed for lean mixtures and an underprediction for rich mixtures. This result is slightly different from conclusion drawn in previous work, in which experimental data are compared with a one-dimensional numerical solutions. Detailed analysis reveals that flame speed is sensitive to even slight flame front curvature as well as its finite extension in the radial direction. Neither of these can be incorporated in one-dimensional flow modeling.</p>https://thesis.library.caltech.edu/id/eprint/1500Charge-Exchange Collision Dynamics and Ion Engine Grid Geometry Optimization
https://resolver.caltech.edu/CaltechETD:etd-02282007-154751
Authors: {'items': [{'id': 'Morris-Bradford-S', 'name': {'family': 'Morris', 'given': 'Bradford S.'}, 'show_email': 'NO'}]}
Year: 2007
DOI: 10.7907/W996-M220
<p>The development of a new three-dimensional model for determining the absolute energy distribution of ions at points corresponding to spacecraft surfaces to the side of an ion engine is presented. The ions resulting from elastic collisions, both charge-exchange (CEX) and direct, between energetic primary ions and thermal neutral xenon atoms are accounted for. Highly resolved energy distributions of CEX ions are found by integration over contributions from all points in space within the main beam formed by the primary ions.</p>
<p>The sputtering rate due to impingement of these ions on a surface is calculated. The CEX ions that obtain significant energy (10 eV or more) in the collision are responsible for the majority of the sputtering, though this can depend on the specific material being sputtered. In the case of a molybdenum surface located 60 cm to the side of a 30 cm diameter grid, nearly 90% of the sputtering is due to the 5% of ions with the highest collision exit energies. Previous models that do not model collision energetics cannot predict this. The present results agree with other models and predict that the majority of the ion density is due to collisions where little to no energy is transferred.</p>
<p>The sputtering model is combined with a grid-structure model in an optimization procedure where the sputtering rate at specified locations is minimized by adjustment of parameters defining the physical shape of the engine grids. Constraints are imposed that require that the deflection of the grid under a specified load does not exceed a maximum value, in order to ensure survivability of the grids during launch. To faciliate faster execution of the calculations, simplifications based on the predicted behavior of the CEX ions are implemented. For diametrically opposed sputtering locations, a rounded barrel-vault shape reduces the expected sputtering rate by up to 30% in comparison to an NSTAR-shaped grid.</p>https://thesis.library.caltech.edu/id/eprint/807Premixed Hydrocarbon Stagnation Flames: Experiments and Simulations to Validate Combustion Chemical-Kinetic Models
https://resolver.caltech.edu/CaltechETD:etd-05302008-113043
Authors: {'items': [{'email': 'benezech.laurent@gmail.com', 'id': 'Benezech-Laurent-Jean-Michel', 'name': {'family': 'Benezech', 'given': 'Laurent Jean-Michel'}, 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/TVB9-4266
<p>A methodology based on the comparison of flame simulations relying on reacting flow models with experiment is applied to C<sub>1</sub>–C<sub>3</sub> stagnation flames. The work reported targets the assessment and validation of the modeled reactions and reaction rates relevant to (C<sub>1</sub>–C<sub>3</sub>)-flame propagation in several detailed combustion kinetic models. A concensus does not, as yet, exist on the modeling of the reasonably well-understood oxidation of C<sub>1</sub>–C<sub>2</sub> flames, and a better knowledge of C<sub>3</sub> hydrocarbon combustion chemistry is required before attempting to bridge the gap between the oxidation of C<sub>1</sub>–C<sub>2</sub> hydrocarbons and the more complex chemistry of heavier hydrocarbons in a single kinetic model.</p>
<p>Simultaneous measurements of velocity and CH-radical profiles were performed in atmospheric propane(C<sub>3</sub>H<sub>8</sub>)- and propylene(C<sub>3</sub>H<sub>6</sub>)-air laminar premixed stagnation flames stabilized in a jet-wall configuration. These nearly-flat flames can be modeled by one-dimensional simulations, providing a means to validate kinetic models. Experimental data for these C<sub>3</sub> flames and similar experimental data for atmospheric methane(CH<sub>4</sub>)-, ethane(C<sub>2</sub>H<sub>6</sub>)-, and ethylene(C<sub>2</sub>H<sub>4</sub>)-air flames are compared to numerical simulations performed with a one-dimensional hydrodynamic model, a multi-component transport formulation including thermal diffusion, and different detailed-chemistry models, in order to assess the adequacy of the models employed. A novel continuation technique between kinetic models was developed and applied successfully to obtain solutions with the less-robust models. The 2005/12 and 2005/10 releases of the San Diego mechanism are found to have the best overall performance in C<sub>3</sub>H<sub>8</sub> and C<sub>3</sub>H<sub>6</sub> flames, and in CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, and C<sub>2</sub>H<sub>4</sub> flames, respectively.</p>
<p>Flame position provides a good surrogate for flame speed in stagnation-flow stabilized flames. The logarithmic sensitivities of the simulated flame locations to variations in the kinetic rates are calculated via the "brute-force" method for fifteen representative flames covering the five fuels under study and the very lean, stoichiometric, and very rich burning regimes, in order to identify the most-important reactions for each flame investigated. The rates of reactions identified in this manner are compared between the different kinetic models. Several reaction-rate differences are thus identified that are likely responsible for the variance in flame-position (or flame-speed) predictions in C<sub>1</sub>–C<sub>2</sub> flames.</p>https://thesis.library.caltech.edu/id/eprint/2316Large-Eddy Simulations of Molecular Mixing in a Recirculating Shear Flow
https://resolver.caltech.edu/CaltechETD:etd-05262008-152803
Authors: {'items': [{'email': 'georgios.matheou@uconn.edu', 'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571', 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/VFKF-SC30
<p>The flow field and mixing in an expansion-ramp geometry is studied using large-eddy simulation (LES) with subgrid scale (SGS) modeling based on the stretched-vortex model. The expansionramp geometry was developed to provide enhanced mixing and flameholding characteristics while maintaining low total-pressure losses, elements that are important in the design and performance of combustors for hypersonic air-breathing propulsion applications. The mixing was studied by tracking a passive scalar without taking into account the effects of chemical reactions and heat release.</p>
<p>In order to verify the solver and the boundary closure implementation, a method utilizing results from linear stability analysis (LSA) theory is developed. LSA can be used to compute unstable perturbations to a flow, subject to certain approximations. The perturbations computed from LSA are used as an inflow condition to the flow computed by the solver been assessed. A projection based metric is constructed that only assumes the shape of the solution and not the growth rate of the perturbations, thus also allowing the latter to be determined as part of the verification. The growth rate of the perturbations for an unbounded (effectively) incompressible shear layer and a confined compressible shear layer is found to be in agreement with the prediction of the LSA.</p>
<p>The flow and mixing predictions of the LES are in good agreement with experimental measurements. Total (resolved and subgrid) probability density functions (PDFs) of the passive scalar are estimated using an assumed beta-distribution model for the subgrid scalar field. The improved mixing characteristics of the expansion-ramp geometry compared to free shear layers are illustrated by the shapes of the PDFs. Moreover, the temperature rise and the probability of mixed fluid profiles are in good agreement with the experimental measurements, indicating that the mixing on a molecular scale is correctly predicted by the LES–SGS model. Finally, the predictions of the LES are shown to be resolution-independent. The mean fields and passive scalar PDFs have essentially converged at the two finer grid-resolutions used.</p>https://thesis.library.caltech.edu/id/eprint/2117Numerical Simulations of Non-Spherical Bubble Collapse with Applications to Shockwave Lithotripsy
https://resolver.caltech.edu/CaltechETD:etd-05092008-171346
Authors: {'items': [{'email': 'ejohnsen@umich.edu', 'id': 'Johnsen-Eric', 'name': {'family': 'Johnsen', 'given': 'Eric'}, 'orcid': '0000-0001-9530-408X', 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/WPQB-2W24
<p>Shockwave lithotripsy (SWL) is a non-invasive medical procedure in which shockwaves are focused on kidney stones in an attempt to break them. Because the stones are usually immersed in liquid, cavitation occurs during the process. However, the stone comminution mechanisms and the bubble dynamics of SWL are not fully understood. In the present thesis, numerical simulations are employed to study axisymmetric Rayleigh collapse and shock-induced collapse of a single gas bubble in a free field and near a wall. A high-order accurate, quasi-conservative, shock- and interface-capturing scheme is developed to solve the multicomponent Euler equations.</p>
<p>The primary contributions of the present work are the development of a new numerical framework to study compressible multicomponent flows, the characterization of the dynamics of non-spherical bubble collapse, and quantitative measurements of wall pressures generated by bubble collapse. Because of asymmetries in the flow field, a re-entrant jet develops and generates a large water-hammer pressure upon impact onto the distal side. Jet properties are calculated and, as an indication of potential damage, wall pressures are measured; pressures on the order of 1 GPa are achieved locally. In shock-induced collapse, the wall pressure is amplified by the presence of bubbles within several initial radii from the wall. Thus, the pressure generated by the bubble collapse is larger than the incoming shock. The results extended to SWL show that shock-induced collapse has tremendous potential for damage along the stone surface. Furthermore, the simulations are coupled to an elastic wave propagation code to show that bubble collapse may cause damage within kidney stones as well.</p>
https://thesis.library.caltech.edu/id/eprint/1712Detonation Stability with Reversible Kinetics
https://resolver.caltech.edu/CaltechETD:etd-06022008-170629
Authors: {'items': [{'id': 'Kao-Shannon-Theresa', 'name': {'family': 'Kao', 'given': 'Shannon Theresa'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/H8JN-VS03
<p>Detonation propagation is unsteady due to the innate instability of the reaction zone structure. Up until the present, investigations of detonation stability have been exclusively concerned with model systems using the perfect gas equation of state and primarily single-step irreversible reaction mechanisms.</p>
<p>This study investigates detonation stability characteristics with reversible chemical kinetics models. To allow for more general kinetics models, we generalize the perfect gas, one-step irreversible kinetics, linear stability equations to a set of equations using the ideal gas equation of state and a general reaction scheme. We linearly perturb the reactive Euler equations following the method of Lee and Stewart (1990) and Short and Stewart (1998). Our implementation uses Cantera (Goodwin, 2005) to evaluate all thermodynamic quantities and evaluate generalized analytic derivatives of quantities dependent on the kinetics model.</p>
<p>The computational domain is the reaction zone in the shock-fixed frame such that the left boundary conditions are the perturbed shock jump conditions which we have derived for a general equation of state and implemented for an ideal gas equation of state. At the right boundary, the system must satisfy a radiation condition requiring that all waves travel out of the domain. Unlike the case of a single reversible reaction, in a truly multistep kinetics model, the radiation boundary condition cannot be solved analytically. In this work, we provide a general methodology for satisfying the appropriate boundary condition.</p>
<p>We then investigate the effects of reversibility on the characteristics of the instability in one and two dimensions. These characteristics are quantified by the unstable eigenvalues as well as the shape of the base flow and eigenfunctions. We show that there is an exchange of stability as a function of reversibility. To confirm the results our work, we have performed unsteady calculations. We show that we can match the frequencies predicted by our linear stability calculations near the stability threshold.</p>https://thesis.library.caltech.edu/id/eprint/2409Richtmyer-Meshkov Instability in Converging Geometries
https://resolver.caltech.edu/CaltechETD:etd-05302008-140331
Authors: {'items': [{'email': 'manuel.lombardini@polytechnique.org', 'id': 'Lombardini-Manuel', 'name': {'family': 'Lombardini', 'given': 'Manuel'}, 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/5SNE-4003
<p>We investigate the Richtmyer-Meshkov instability (RMI) in converging geometries analytically and computationally. The linear, or small amplitude, regime is first covered as it is the onset to subsequent non-linear stages of the perturbation growth. While the plane interaction of a shock with a slightly perturbed density interface is classically viewed as a single interface evolving as baroclinic vorticity have been initially deposited on it, we propose a simple but more complete model characterizing the early interaction between the interface and the receding waves produced by the shock-interface interaction, in the case of a reflected shock. A universal time scale representing the time needed by the RMI to reach its asymptotic growth rate is found analytically and confirmed by ideal gas computations for various incident shock Mach numbers MI and Atwood ratios A, and could be useful especially for experimentalists in non-dimensionalizing their data.</p>
<p>Considering again linear perturbations, we then obtain a general analytical model for the asymptotic growth rate reached by the instability during the concentric interaction of an imploding/exploding cylindrical shock with a cylindrical interface containing three-dimensional orthogonal perturbations, in the azimuthal and axial directions. Stable perturbations, typical of the converging geometry, are discovered. Comparisons are made with simulations where the effects of compressibility, wave reverberations, and flow convergence are isolated. Azimuthal and axial perturbation evolution are compared with results obtained for the plane RMI at comparable initial wavelengths.</p>
<p>A second interaction occurs when the transmitted shock, produced by the incident converging shock impacting the interface, converges to the axis and reflects to reshock the initially accelerated interface. This leads to highly non-linear perturbation growth. To isolate the complex wave interaction process, the interface is considered initially unperturbed so that the flow is radially symmetric. An accurate visualization procedure is performed to characterize the underlying physics behind the reshock event. We study extensively the cylindrical and spherical geometry, for various MI and for the air → SF6 (A=0.67) and SF6 → (A=-0.67) interactions, and draw important differences with the equivalent plane configuration.</p>
<p>A hybrid, low-numerical dissipation/shock-capturing method, embedded into an adaptive mesh refinement framework is optimized in order to achieve large-eddy simulations of the self-similar cylindrical converging shock-driven RMI and the turbulent mixing generated by the reshock. Computations are produced for MI=1.3 and 2.0, and for air -> SF6 SF6 -> air interfaces. We develop statistics tools to study extensively the growth of the turbulent mixing zone using cylindrical averages as well as various measures such as probability density functions of the mixing and turbulent power spectra, with the objectives of understanding the turbulent mixing in this particular geometry.</p>https://thesis.library.caltech.edu/id/eprint/2319Dynamics of Plasma Structures Interacting with External and Self-Generated Magnetic Fields
https://resolver.caltech.edu/CaltechETD:etd-07242007-162442
Authors: {'items': [{'email': 'gunsu@caltech.edu', 'id': 'Yun-Gunsu-Soonshin', 'name': {'family': 'Yun', 'given': 'Gunsu Soonshin'}, 'orcid': '0000-0002-1880-5865', 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/9CMD-C377
Plasmas interacting with external and self-generated magnetic fields often develop a long tubular structure of nearly uniform cross section. Such long collimated plasma tubes have been observed in a variety of contexts ranging from astrophysical plasma jets (10<sup>15</sup>–10<sup>22</sup> m) to solar coronal loops (10<sup>7</sup>–10<sup>8</sup> m). Remarkably, much smaller-sized plasmas (0.1–1 m) produced by the Caltech planar spheromak gun develop collimated structures bearing a striking resemblance to these natural plasma tubes. This thesis presents experimental observations of gun-produced plasma tubes that support a recently-proposed magnetohydrodynamic (MHD) pumping model as a universal collimation mechanism. For any flared flux tube carrying a finite axial current, the model predicts (i) magnetic pumping of plasma particles from a constricted region into a bulged region and (ii) tube collimation if the flow slows down at the bulged region, leading to accumulation of mass and thus concentrating the azimuthal magnetic flux frozen in the mass flow (i.e., increasing the pinch force). Time- and space-resolved spectroscopic measurements of gun-produced plasmas show (i) suprathermal Alfvenic flow (30–50 km/s), (ii) large density amplification from ~10<sup>17</sup> to ~10<sup>22</sup> m<sup>-3</sup> in an Alfvenic time scale (5–10 µs), and (iii) flow slowing down and mass accumulation at the flow front, the place where the tube collimation occurs according to high-speed camera imaging. These observations are consistent with the predictions of the MHD pumping model, and thus the model offers valuable insight into the formation mechanism of laboratory, solar, and astrophysical plasma structures.https://thesis.library.caltech.edu/id/eprint/2979Nonreflecting Boundary Conditions Obtained from Equivalent Sources for Time-Dependent Scattering Problems
https://resolver.caltech.edu/CaltechETD:etd-05202008-111349
Authors: {'items': [{'email': 'dhoch@acm.caltech.edu', 'id': 'Hoch-David', 'name': {'family': 'Hoch', 'given': 'David'}, 'show_email': 'NO'}]}
Year: 2008
DOI: 10.7907/5M0P-NR33
<p>In many engineering applications, scattering of acoustic or electromagnetic waves from a body of arbitrary shape is considered in an infinite medium. Solving the underlying partial differential equations with a standard numerical method such as finite elements or finite differences requires truncating the unbounded domain of definition into a finite computational region. As a consequence, an appropriate boundary condition must be prescribed at the artificial boundary. Many approaches have been proposed for this fundamental problem in the field of wave scattering. All of them fall into one of three main categories.</p>
<p>The first class of methods is based on mathematical approximations or physical heuristics. These boundary conditions are easy to implement and run in short computing times. However, these approaches give rise to spurious reflections at the artificial boundary, which travel back into the computational domain and corrupt the solution.</p>
<p>A second group consists of accurate and convergent methods. However, these formulations are usually harder to implement and often more expensive than the computation of the interior scheme itself.</p>
<p>Finally, there are methods which are accurate and fast. The drawback of these approaches lies in the fact that the outer boundary must be taken to be either a sphere, a plane, or a cylinder. For many applications of interest, this may require use of a computational domain much larger than actually needed, which leads to an expensive overall numerical scheme.</p>
<p>This work introduces a new methodology in order to compute the fields at the artificial boundary. Like the second class of methods described above, the proposed algorithm is accurate and numerically convergent, yet its computational cost is less than the underlying portion of the volumetric calculation. And, unlike the third category, this new approach allows us to choose the artificial boundary to be arbitrarily close to the scatterer. This method is based on a novel concept of "equivalent source' representations which allows a highly accurate and fast evaluation of the boundary condition.</p>https://thesis.library.caltech.edu/id/eprint/1899On the Non-Local Geometry of Turbulence
https://resolver.caltech.edu/CaltechETD:etd-05092008-173614
Authors: {'items': [{'email': 'ivan.bermejo.moreno@gmail.com', 'id': 'Bermejo-Moreno-Ivan', 'name': {'family': 'Bermejo-Moreno', 'given': 'Ivan'}, 'show_email': 'YES'}]}
Year: 2008
DOI: 10.7907/DH9W-Y894
<p>A multi-scale methodology for the study of the non-local geometry of eddy structures in turbulence is developed. Starting from a given three-dimensional field, this consists of three main steps: extraction, characterization and classification of structures. The extraction step is done in two stages. First, a multi-scale decomposition based on the curvelet transform is applied to the full three-dimensional field, resulting in a finite set of component fields, one per scale. Second, by iso-contouring each component field at one or more iso-contour levels, a set of closed iso-surfaces is obtained that represents the structures at that scale. For periodic domains, those structures intersecting boundaries are reconnected with their continuation in the opposite boundaries. The characterization stage is based on the joint probability density function (jpdf), in terms of area coverage on each individual iso-surface, of two differential-geometry properties, the shape index and curvedness, plus the stretching parameter, a dimensionless global invariant of the surface. Taken together, this defines the geometrical signature of the iso-surface. The classification step is based on the construction of a finite set of parameters, obtained from algebraic functions of moments of the jpdf of each structure, that specify its location as a point in a multi-dimensional 'feature space'. At each scale the set of points in feature space represents all structures at that scale, for the specified iso-contour value. This allows the application, to the set, of clustering techniques that search for groups of structures with a common geometry.</p>
<p>Results are presented of a first application of this technique to a passive scalar field obtained from 512³ direct numerical simulation of scalar mixing by forced, isotropic turbulence (Re<sub>λ</sub>=265). These show transition, with decreasing scale, from blob-like structures in the larger scales to blob- and tube-like structures with small or moderate stretching in the inertial range of scales, and then toward tube and, predominantly, sheet-like structures with high level of stretching in the dissipation range of scales. Implications of these results for the dynamical behavior of passive scalar stirring and mixing by turbulence are discussed.</p>
<p>We apply the same methodology to the enstrophy and kinetic energy dissipation rate instantaneous fields of a second numerical database of incompressible homogeneous isotropic turbulence decaying in time obtained by DNS in a periodic box. Three different resolutions are considered: 256³, 512³ and 1024³ grid points, with k<sub>max</sub>η̅ approximately 1, 2, and 4, respectively, the same initial conditions and Re<sub>λ</sub> ≈ 77. This allows a comparison of the geometry of the structures obtained for different resolutions. For the highest resolution, structures of enstrophy and dissipation evolve in a continuous distribution from blob-like and moderately stretched tube-like shapes at the large scales to highly stretched sheet-like structures at the small scales. The intermediate scales show a predominance of tube-like structures for both fields, much more pronounced for the enstrophy field. The dissipation field shows a tendency toward structures with lower curvedness than those of the enstrophy, for intermediate and small scales. The 256³ grid resolution case (k<sub>max</sub>η̅ ≈ 1) was unable to detect the predominance of highly stretched sheet-like structures at the smaller scales.</p>
<p>The same methodology, but without the multi-scale decomposition, is then applied to two scalar fields used by existing local criteria for the eduction of tube- and sheet-like structures in turbulence, Q and [A<sub>ij</sub>]<sub>+</sub> respectively, obtained from invariants of the velocity gradient tensor and alike in the 1024³ case. This adds the non-local geometrical characterization and classification to those local criteria, assessing their validity in educing particular geometries.</p>
<p>Finally we introduce a new methodology for the study of proximity issues among different sets of structures, based also on geometrical and non-local analyses. We apply it to four of the fields previously studied. Tube-like structures of Q are mainly surrounded by sheets of [A<sub>ij</sub>]<sub>+</sub>, which appear at close distances. For the enstrophy, tube-like structures at an intermediate scale are primarily surrounded by sheets of smaller scales of the enstrophy and structures of dissipation at the same and smaller scales. A secondary contribution results from tubes of enstrophy at smaller scales appearing at farther distances. Different configurations of composite structures are presented.</p>
https://thesis.library.caltech.edu/id/eprint/1713Stable High-Order Finite-Difference Interface Schemes with Application to the Richtmyer-Meshkov Instability
https://resolver.caltech.edu/CaltechETD:etd-03132009-095507
Authors: {'items': [{'email': 'rmjkramer@gmail.com', 'id': 'Kramer-Richard-Michael-Jack', 'name': {'family': 'Kramer', 'given': 'Richard Michael Jack'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/HXGM-DC92
<p>High-order adaptive mesh refinement offers the potential for accurate and efficient resolution of problems in fluid dynamics and other fields where a wide range of length scales is present. A critical requirement for the interface closures used with these methods is stability in the context of hyperbolic systems of partial differential equations.</p>
<p>In this study, a class of energy-stable high-order finite-difference interface closures is presented for grids with step resolution changes in one dimension. Asymptotic stability in time for these schemes is achieved by imposing a summation-by-parts condition on the interface closure, which is thus also nondissipative. Interface closures compatible with interior fourth- and sixth-order explicit, and fourth-order implicit centered schemes are presented. Validation tests include linear and nonlinear problems in one and in two dimensions with tensor-product grid refinement.</p>
<p>A second class of stable high-order interface closures is presented for two-dimensional cell-centered grids with patch-refinement and step-changes in resolution. For these grids, coarse and fine nodes are not aligned at the mesh interfaces, resulting in hanging nodes. Stability is achieved by again imposing a summation-by-parts condition, resulting in nondissipative closures, at the cost of accuracy at corner interfaces. Interface stencils for an explicit fourth-order finite-difference scheme are presented for each geometry. Validation tests confirm the stability and accuracy of these closures for linear and nonlinear problems.</p>
<p>The Richtmyer-Meshkov instability is investigated using a novel first-order perturbation of the two-dimensional Navier-Stokes equations about a shock-resolved base flow. The computational domain is efficiently resolved using the one-dimensional fourth-order interface scheme. Results are compared to analytic models of the instability, showing agreement with predicted asymptotic growth rates in the inviscid range, while significant discrepancies are noted in the transient growth phase. Viscous effects are found to be poorly predicted by existing models.</p>
https://thesis.library.caltech.edu/id/eprint/947Experimental Investigations of Magnetohydrodynamic Plasma Jets
https://resolver.caltech.edu/CaltechETD:etd-04092009-163047
Authors: {'items': [{'email': 'deepak.kumar@eli-beams.eu', 'id': 'Kumar-Deepak', 'name': {'family': 'Kumar', 'given': 'Deepak'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/ENZ7-QV92
<p>This thesis primarily focuses on understanding the plasma behavior during the helicity injection stage of a pulsed spheromak experiment. Spheromak formation consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves toward a Taylor state equilibrium. The dynamical evolution stage has been modeled as a series of equilibrium states in the past. However, the experiments at the Caltech spheromak facility have revealed that unbalanced J x B forces drive non equilibrium Alfvénic flows during these preliminary stages.</p>
<p>The Caltech spheromak experiment uses coplanar electrodes to produce a collimated plasma jet flowing away from the electrodes. The jet formation stage precedes the spheromak formation and serves as a mechanism for feeding particles, magnetic helicity, energy, and toroidal flux into the system. Detailed density and flow velocity measurements of hydrogen and deuterium plasma jets have revealed that the jets are extremely dense with β [subscript thermal] ~1. Furthermore, the flow velocity was found to be Alfvénic with respect to the the toroidal magnetic field produced by the axial current within the plasma. An existing magnetohydrodynamics (MHD) model has been generalized to successfully predict the effect of plasma current on the jet's density and flow velocity. The behavior of these laboratory jets is in stark contrast to the often considered model for astrophysical jets describing them as equilibrium configurations with hollow density profiles.</p>
<p>Other contributions of this thesis include the following.</p>
<p>1. The thesis presents an analytical proof that resistive MHD equilibrium with closed flux tubes is not feasible. This implies that sustained spheromak experiments cannot maintain helicity while being in a strict equilibrium.</p>
<p>2. The thesis describes measurements to characterize the circuit parameters of the high voltage discharge circuit used in the Caltech spheromak experiment.</p>
<p>3. The thesis also describes the setup of novel He-Ne laser interferometers used to measure the density of plasma jets. The ease of alignment of these interferometers was greatly enhanced by having unequal path lengths of the scene and reference beams.</p>
<p>4. Finally, the thesis details the setup for a soft X-ray (SXR)/Vacuum ultra violet (VUV) imaging system. Some preliminary images of reconnecting flux tubes captured by the imaging setup are also presented.</p>
https://thesis.library.caltech.edu/id/eprint/1321Rheological 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/858High-Order Solution of Elliptic Partial Differential Equations in Domains Containing Conical Singularities
https://resolver.caltech.edu/CaltechETD:etd-08042008-005339
Authors: {'items': [{'email': 'zhiyi@acm.caltech.edu', 'id': 'Li-Zhiyi', 'name': {'family': 'Li', 'given': 'Zhiyi'}, 'show_email': 'NO'}]}
Year: 2009
DOI: 10.7907/VEEB-AV75
In this thesis we introduce an algorithm, based on the boundary integral equation method, for the numerical evaluation of singular solutions of the Laplace equation in three dimensional space, with singularities induced by a conical point on an otherwise smooth boundary surface. This is a model version of a fundamental problem in science and engineering: accurate evaluation of solutions of Partial Differential Equations in domains whose boundaries contain geometric singularities. For simplicity we assume a small region near the conical point coincides with a straight cone of given cross section; otherwise the boundary surface is not restricted in any way. Our numerical results demonstrate excellent convergence as discretizations are refined, even at the singular point where the solutions tend to infinity.https://thesis.library.caltech.edu/id/eprint/3013Continuum 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/5273The Role of Unsteady Hydrodynamics in the Propulsive Performance of a Self-Propelled Bioinspired Vehicle
https://resolver.caltech.edu/CaltechTHESIS:05272010-122004587
Authors: {'items': [{'email': 'ucladolphin@hotmail.com', 'id': 'Ruiz-Lydia-Ann', 'name': {'family': 'Ruiz', 'given': 'Lydia Ann'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/7JWD-TB88
<p>Aquatic animals differ from typical engineering systems in their method of locomotion. In general, aquatic animals propel using unsteady dynamics producing vortex rings. Researchers have long shown interest in designing devices that resemble their shape and propulsive behavior. Traditional definitions of propulsive efficiency used to model these behaviors have not taken unsteady effects into account and are typically based on steady flow through propellers or rocket motors. Measurements of aquatic animals based on these quasi-steady metrics have suggested propulsive efficiencies over 80% when utilizing certain swimming kinematics. However, the mechanical efficiency of muscle-actuated biological propulsion has been found to be much lower, typically less than 20%. It is important to take into account the total efficiency of the system, the product of the mechanical and propulsive efficiency, when designing and implementing a biologically inspired propulsive device.</p>
<p>The purpose of my research is to make a direct, experimental comparison between biological and engineering propulsion systems. For this study, I designed an underwater vehicle that has the capability of producing either a steady or unsteady jet for propulsion, akin to a squid and jellyfish, while utilizing the same mechanical efficiency. I show that it is unnecessary to take an approach that mimics animal shape and kinematics to achieve the associated propulsive performance. A bioinspired, propeller-based platform that mimics animal wake dynamics can be similarly effective.</p>
<p>A study on how vortex dynamics plays a key role in improving the propulsive efficiency of pulsed jet propulsion was conducted. Measurements of propulsive performance resulted in superior performance for the pulsed-jet configuration in comparison to the steady jet configuration particularly at higher motor speeds. The analysis demonstrated that vortex ring formation led to the acceleration of two classes of ambient fluid, entrained and added mass, and this consequently led to an increased total fluid impulse of the jet and propulsive performance. The first source of ambient fluid acceleration investigated was entrained mass. The magnitude of the entrainment ratio was measured and found to be smaller for the steady jet mode of propulsion in comparison to the pulsed jet mode of propulsion given comparable motor speeds. The role of the added mass effect was also investigated in increasing propulsive performance. A model developed by Krueger is used to determine the fraction of the total impulse imparted to the flow that was contributed by the added mass effect. Results demonstrated that the added mass effect associated with the acceleration of ambient fluid at the initiation of a starting jet provides an increase in the total impulse and is thus a source for increased propulsive performance. Last, a model was developed to investigate how an increase in the total fluid impulse due to vortex ring formation is related to the propulsive efficiency. Results obtained using the model are in agreement, within uncertainty, with previous experimental results for the measurement of propulsive efficiency. The results support that the additional force generated from the acceleration of two classes of ambient fluid are the source of increased propulsive efficiency for the pulsed jet configuration in comparison to the steady jet configuration. This model serves as an additional metric for determining the propulsive efficiency of a system utilizing pulsed jet propulsion.</p>
https://thesis.library.caltech.edu/id/eprint/5857Solid-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/5923The Physics of High-Velocity Ions in the Hall Thruster Near-Field
https://resolver.caltech.edu/CaltechTHESIS:04022010-134100215
Authors: {'items': [{'email': 'rsulli@alum.mit.edu', 'id': 'Sullivan-Regina-Mariko', 'name': {'family': 'Sullivan', 'given': 'Regina Mariko'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/8F2Y-NM32
<p>A study of the physics underlying high velocity ion trajectories within the near-field region of a Hall thruster plume is presented. In this context, "high velocity" ions are ions that have been accelerated through the full potential drop of the thruster (sometimes referred to as "primary energy" or "primary beam energy" ions). Results from an experimental survey of an SPT-70 thruster plume are shown, along with simulated data from a Hall thruster code and from a plasma sheath model. Two main features are examined: the central jet along the Hall thruster centerline, and the population of high velocity ions at high angles.</p>
<p>In the experimental portion of the investigation, three diagnostic instruments were employed: (1) a Faraday probe for measuring ion current density, (2) an ExB velocity filter for mapping ions with the primary beam energy, and (3) a Retarding Potential Analyzer (RPA) for determining ion energy distributions. In the numerical portion, two codes were employed: (1) a hybrid-PIC Hall thruster code known as HPHall, and (2) a model of the plasma sheath near the exit plane of the thruster, which was developed by the author.</p>
<p>A comparison between the measured and simulated data sets is made, to analyze the degree to which different mechanisms are responsible for the evolution of the thruster
plume in the near-field region. This analysis shows that the central jet is both a function of symmetric expansion of the ion beam as well as asymmetry in the internal potential field of the thruster. Additionally, it is suggested that high energy, high angle ions could be generated given a specific internal electric field configuration, while oscillations are ruled out as the cause of these ions. The results from the sheath model show that while the sheath can change trajectory angles by 10 to 20 degrees, it can not fully explain the presence of
high angle ions with high energies.</p>https://thesis.library.caltech.edu/id/eprint/5695Catalytic Modification of Flammable Atmosphere in Aircraft Fuel Tanks
https://resolver.caltech.edu/CaltechTHESIS:06092010-102349721
Authors: {'items': [{'email': 'inkichoi@caltech.edu', 'id': 'Choi-Inki', 'name': {'family': 'Choi', 'given': 'Inki'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/69YQ-XZ33
A facility for investigating catalytic combustion and measurement of fuel molecule concentration was built to examine catalyst candidates for inerting systems in aircraft. The facility consists of fuel and oxygen supplies, a catalytic-bed reactor, heating system, and laser-based diagnostics. Two supplementary systems consisting of a calibration test cell and a nitrogen-purged glove box were also constructed. The catalyst under investigation was platinum, and it was mixed with silica particles to increase the surface area available to react. The catalyst/silica mixture was placed in a narrow channel section of the reactor and supported from both sides by glass wool. The fuels investigated were n-octane and n-nonane because their vapor pressure is sufficiently high to create ammable gaseous mixtures with atmospheric air at room temperature. Calibration experiments were performed to determine the absorption cross-section of the two fuels as a function of temperature. The cross-section values were then used to determine the fuel concentration before the flow entered the reactor and after exposure to the heated catalyst. An initial set of experiments was performed with the catalytic-bed reactor at two temperatures, 255 and 500°C, to investigate pyrolysis and oxidation of the fuel. The presence of the catalyst increased the degree of pyrolysis and oxidation at both temperatures. The results show that catalytic modification of ammable atmospheres may yield a viable alternative for inerting aircraft fuel tanks. However, further tests are required to produce oxidation at sufficiently low temperature to comply with aircraft safety regulations.
https://thesis.library.caltech.edu/id/eprint/5942Spark Ignition: Experimental and Numerical Investigation With Application to Aviation Safety
https://resolver.caltech.edu/CaltechTHESIS:05272010-173243262
Authors: {'items': [{'email': 'sbane@purdue.edu', 'id': 'Bane-Sally-Page-Moffett', 'name': {'family': 'Bane', 'given': 'Sally Page Moffett'}, 'orcid': '0000-0002-4764-3228', 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/W1NB-5W06
<p>Determining the risk of accidental ignition of flammable mixtures is a topic of tremendous importance in industry and aviation safety. The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, in recent years, particularly in the aviation safety industry, the viewpoint has changed to one where ignition is statistical in nature. Approaching ignition as statistical rather than a threshold phenomenon appears to be more consistent with the inherent variability in the engineering test data.</p>
<p>Ignition tests were performed in lean hydrogen-based aviation test mixtures and in two hexane-air mixtures using low-energy capacitive spark ignition systems. Tests were carried out using both short, fixed sparks (1 to 2 mm) and variable length sparks up to 10 mm. The results were analyzed using statistical tools to obtain probability distributions for ignition versus spark energy and spark energy density (energy per unit spark length). Results show that a single threshold MIE value does not exist, and that the energy per unit length may be a more appropriate parameter for quantifying the risk of ignition than only the energy. The probability of ignition versus spark charge was also investigated, and the statistical results for the spark charge and spark energy density were compared. It was found that the test results were less variable with respect to the spark charge than the energy density. However, variability was still present due to phenomena such as plasma instabilities and cathode effects that are caused by the electrodynamics.</p>
<p>Work was also done to develop a two-dimensional numerical model of spark ignition that accurately simulates all physical scales of the fluid mechanics and chemistry. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. One-step chemistry models were used to allow for highly resolved simulations, and methods for calculating effective one-step parameters were developed using constant pressure explosion theory. The one-step model was tuned to accurately simulate the flame speed, temperature, and straining behavior using one-dimensional flame computations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process.</p>
https://thesis.library.caltech.edu/id/eprint/5868Instability Wave Models of Turbulent Jets from Round and Serrated Nozzles
https://resolver.caltech.edu/CaltechTHESIS:01242010-111852941
Authors: {'items': [{'email': 'kristjan@caltech.edu', 'id': 'Guðmundsson-Kristján', 'name': {'family': 'Guðmundsson', 'given': 'Kristján'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/BQH9-G487
In this thesis we study pressure fluctuations associated with large-scale coherent structures in turbulent round and serrated jets. Linear disturbances to the turbulent mean flow of the round jet are modeled via linear stability analysis and the Parabolized Stability Equations (PSE). We show that PSE provides better agreement with near-field microphone-array data at low frequencies than previous models based on linear stability theory. We examine the extent to which microphone data is contaminated by fluctuations uncorrelated with large-scale structures. By filtering out the uncorrelated fluctuations, via the proper orthogonal decomposition (POD), better agreement between data and theory is obtained. We next extend the linear stability analysis of round jets to include the effects of azimuthal inhomogeneities of serrated jets. We solve the resulting system of equations and find new modes, associated with the streamwise vorticity of the serrated-jet mean flow. All unstable modes of the serrated jet are stabilized, potentially explaining the noise reduction achieved by such jets. We also compare these predictions to POD-filtered microphone measurements, generally finding good agreement.
https://thesis.library.caltech.edu/id/eprint/5539Effects of Polydispersity in Bubbly Flows
https://resolver.caltech.edu/CaltechTHESIS:05272010-133830557
Authors: {'items': [{'email': 'kando@caltech.edu', 'id': 'Ando-Keita', 'name': {'family': 'Ando', 'given': 'Keita'}, 'orcid': '0000-0002-9572-8242', 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/SW8K-Y135
<p>This thesis concerns the dynamics of bubbly flows with a distribution of equilibrium bubble sizes. The main goal is to formulate the physical and numerical models of continuum bubbly flows that enable us to efficiently compute the average mixture dynamics. Numerical simulations are conducted to quantify the effects of bubble size distributions on the averaged dynamics for several model flows.</p>
<p>First, the ensemble-averaged conservation laws for polydisperse bubbly flows are derived. One-way-coupled flow computations are conducted to illustrate that the different-sized bubbles can oscillate with different frequencies. The resulting phase cancellations can be regarded as an apparent damping of the averaged dynamics of polydisperse flows. A high-order-accurate finite-volume method is then developed to compute the flow, paying special attention to issues of wave dispersion and stiffness.</p>
<p>Next, computations of one-dimensional shock propagation through bubbly liquids are performed. The numerical experiments reveal that the bubble size distribution has a profound impact on the averaged shock structure. If the distribution is sufficiently broad, the apparent damping due to the phase cancellations can dominate over the single-bubble-dynamic dissipation (due to thermal, viscous, and compressibility effects) and the averaged shock dynamics become insensitive to the individual bubble dynamics. One-dimensional cloud cavitation caused by fluid-structure interaction is also solved to investigate the collapse of cavitation clouds with both monodisperse and polydisperse nuclei. The phase cancellations among the cavitation bubbles with broad nuclei size distributions are found to eliminate violent cloud collapse in the averaged dynamics.</p>
<p>Finally, shock propagation through a bubbly liquid-filled, deformable tube is considered. The quasi-one-dimensional conservation law that takes into account structural deformation is formulated and steady shock relations are derived. The results are compared to water-hammer experiments; the present shock theory gives better agreement with the measured wave speeds than linear theory. This indicates that the gas-phase nonlinearity needs to be included to accurately predict the propagation speeds of finite-amplitude waves in a deformable tube filled with a bubbly liquid.</p>https://thesis.library.caltech.edu/id/eprint/5859Numerical Simulation of Wave Focusing and Scattering in Shock Wave Lithotripsy
https://resolver.caltech.edu/CaltechTHESIS:05282010-150032204
Authors: {'items': [{'email': 'jeff.krimmel@gmail.com', 'id': 'Krimmel-Jeffrey-James', 'name': {'family': 'Krimmel', 'given': 'Jeffrey James'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/XWED-RZ95
<p>In this work we simulate shock wave focusing and scattering that occurs during shock wave lithotripsy, a noninvasive medical treatment for kidney stone disease. Shock waves are generated outside the body of the patient and are focused at the kidney stone with the intention of pulverizing the stone while it remains inside the patient. The patient can then ostensibly pass the debris naturally. We use a multidimensional second-order method of the Godunov type with slope limiters and shock capturing capability to solve the inviscid Euler equations. Because we begin with the fundamental statements of conservation of mass, momentum, and energy, we simulate all the relevant acoustics occurring during a typical treatment.</p>
<p>Lithotripters, the machines that generate and focus shock waves, can be classified according to the mechanism of shock generation. In this work, we simulate three different types of lithotripters: electrohydraulic, piezoelectric, and electromagnetic. We choose one representative of each lithotripter type: the Dornier HM3, a research piezoelectric lithotripter array, and the XX-Es, respectively. We first study a model of the in vitro setting for each lithotripter, where shock waves are generated and focus in a bath of pure water. Next, we introduce different heterogeneous materials near the focus of the lithotripter to model the effect of the body of an actual patient, i.e., the in vivo condition. We use two approaches in this modeling effort. One approach is to use simple geometrical models for the body cavity and kidney that we created ourselves. The other approach is to import real anatomical data made available from the VOXEL-MAN Group.</p>
<p>In studying the focal region acoustics, we specifically examine the maximum calculated pressures. These pressures represent the forces that will ultimately cause the kidney stone to break. We also study the pulse intensity integral, i.e., the energy density carried by the focusing shock wave. In addition to these pressures and energy densities, we are interested in investigating how soft tissue in the focal region may potentially be damaged by the resulting wavefields. We isolate two mechanisms that are thought to be important in soft tissue injury: shearing and cavitation. We calculate estimates for the maximum principal normal and shear strains in the focal region in addition to the corresponding strain rates and use these as metrics for the potential for damage via shearing. We study the calculated negative pressure fields in this region as a surrogate for potential damage caused by cavitation.</p>
<p>We find that our simple geometrical anatomical models cause little deviation from the acoustics observed in a water bath. However, when the real anatomical data of the VOXEL-MAN Group is used, the fields of the various relevant flow quantities become more highly oscillatory and produce secondary extrema that could produce damage not predicted from the water bath case. In addition to the conclusions from our own work, we discuss how our results motivate future studies that will hopefully help elucidate specific mechanisms by which kidney stones break and soft tissue becomes damaged.</p>https://thesis.library.caltech.edu/id/eprint/5886Detonation Induced Strain in Tubes
https://resolver.caltech.edu/CaltechTHESIS:05142010-174001426
Authors: {'items': [{'email': 'jim.karnesky@gmail.com', 'id': 'Karnesky-James-Alan', 'name': {'family': 'Karnesky', 'given': 'James Alan'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/GTKC-FY91
<p>When a detonation wave propagates through a piping system, it acts as a traveling pressure load to the pipe wall. The detonation wave must be followed by an expansion wave in order to bring the combustion products to zero velocity at the ignition end. When it reaches a closed end-wall, a reflected shock is formed which propagates back into the tube with a decaying pressure. The present study aims to develop predictive models for the stresses and strains produced in such a situation. To this end, two series of experiments are discussed. The first series used strain gauges and a laser vibrometer to measure the elastic response of the tube to the incident detonation in thin aluminum tubes. The second series used strain gauges and high speed video to measure the plastic response of steel tubes to incident detonations and reflected shocks. In these experiments a novel mode of plastic deformation was discovered in which the residual plastic deformation in the tube wall had a periodic sinusoidal pattern.</p>
<p>A semi-empirical model of the pressure history was developed for use as a boundary condition in models of the mechanical response of the tube. This model was tested against experiment, and it was found that the pressure and arrival time could not be simultaneously predicted from the simple model. This and the general form of the pressure traces in the experiment seem to suggest an interaction between the reflected shock and the boundary layer behind the detonation resulting in a possible bifurcation in the reflected shock wave.</p>
<p>With these considerations in mind, the model was applied to single degree of freedom and finite element models of the tube wall. The ripples observed in the experiment were present in the 1-D single degree of freedom models, indicating that they are a result of the interaction of the reflected shock wave with the elastic oscillations set in motion by the detonation wave. Strain-rate hardening was found to be an important consideration under detonation loading conditions. With proper consideration of rate hardening, a single material model may be used to arrive at reasonable predictions the plastic strains resulting from detonations and reflections at initial pressures of 2 and 3 bar initial pressures.</p>https://thesis.library.caltech.edu/id/eprint/5809Low Temperature Catalytic Ethanol Conversion Over Ceria-Supported Platinum, Rhodium, and Tin-Based Nanoparticle Systems
https://resolver.caltech.edu/CaltechTHESIS:06102010-171305208
Authors: {'items': [{'email': 'eugeneldmahmoud@yahoo.com', 'id': 'Mahmoud-Eugene-Leo-Draine', 'name': {'family': 'Mahmoud', 'given': 'Eugene Leo Draine'}, 'show_email': 'NO'}]}
Year: 2010
DOI: 10.7907/Q9W4-0356
<p>Due to the feasibility of ethanol production in the United States, ethanol has become more attractive as a fuel source and a possible energy carrier within the hydrogen economy. Ethanol can be stored easily in liquid form, and can be internally pre-formed prior to usage in low temperature (200C – 400C) solid acid and polymer electrolyte membrane fuel cells. However, complete electrochemical oxidation of ethanol remains a challenge. Prior research of ethanol reforming at high temperatures (> 400C) has identified several metallic and oxide-based catalyst systems that improve ethanol conversion, hydrogen production, and catalyst stability. In this study, ceria-supported platinum, rhodium, and tin-based nanoparticle catalyst systems will be developed and analyzed in their performance as low-temperature ethanol reforming catalysts for fuel cell applications.</p>
<p>Metallic nanoparticle alloys were synthesized with ceria supports to produce the catalyst systems studied. Gas phase byproducts of catalytic ethanol reforming were analyzed for temperature-dependent trends and chemical reaction kinetic parameters. Results of catalytic data indicate that catalyst composition plays a significant role in low-temperature ethanol conversion. Analysis of byproduct yields demonstrate how ethanol steam reforming over bimetallic catalyst systems (platinum-tin and rhodium-tin) results in higher hydrogen selectivity than was yielded over single-metal catalysts. Additionally, oxidative steam reforming results reveal a correlation between catalyst composition, byproduct yield, and ethanol conversion. By analyzing the role of temperature and reactant composition on byproduct yields from ethanol reforming, this study also proposes how these parameters may contribute to optimal catalytic ethanol reforming.</p>
https://thesis.library.caltech.edu/id/eprint/5947The Simulation of Shock- and Impact-Driven Flows with Mie-Grüneisen Equations of State
https://resolver.caltech.edu/CaltechTHESIS:12162010-115725941
Authors: {'items': [{'email': 'geoff@caltech.edu', 'id': 'Ward-Geoffrey-M', 'name': {'family': 'Ward', 'given': 'Geoffrey M.'}, 'show_email': 'NO'}]}
Year: 2011
DOI: 10.7907/8Q2Q-GT29
<p>An investigation of shock- and impact-driven flows with Mie-Grüneisen equation of state derived from a linear shock-particle speed Hugoniot relationship is presented. Cartesian mesh methods using structured adaptive refinement are applied to simulate several flows of interest in an Eulerian frame of reference. The flows central to the investigation include planar Richtmyer-Meshkov instability, the impact of a sphere with a plate, and an impact-driven Mach stem.</p>
<p>First, for multicomponent shock-driven flows, a dimensionally unsplit, spatially high-order, hybrid, center-difference, limiter methodology is developed. Effective switching between center-difference and upwinding schemes is achieved by a set of robust tolerance and Lax-entropy-based criteria [49]. Oscillations that result from such a mixed stencil scheme are minimized by requiring that the upwinding method approaches the center-difference method in smooth regions. To attain this property a blending limiter is introduced based on the norm of the deviation of WENO reconstruction weights from ideal. The scheme is first demonstrated successfully for the linear advection equation in spatially fourth- and sixth-order forms. A spatially fourth-order version of the method that combines a skew-symmetric kinetic-energy preserving center-difference scheme with a Roe-Riemann solver is then developed and implemented in Caltech's Adaptive Mesh Refinement, Object-oriented C++ (AMROC) [16,17] framework for Euler flows.</p>
<p>The solver is then applied to investigate planar Richtmyer-Meshkov instability in the context of an equation of state comparison. Comparisons of simulations with materials modeled by isotropic stress Mie-Grüneisen equations of state derived from a linear shock-particle speed Hugoniot relationship [36,52] to those of perfect gases are made with the intention of exposing the role of the equation of state. First, results for single- and triple-mode planar Richtmyer-Meshkov instability between mid-ocean ridge basalt (MORB) and molybdenum modeled by Mie-Grüneisen equations of state are presented for the case of a reflected shock. The single-mode case is explored for incident shock Mach numbers of 1.5 and 2.5. For the planar triple-mode case a single incident Mach number of 2.5 is examined with the initial corrugation wave numbers related by k₁=k₂+k₃. A comparison is drawn to Richtmyer-Meshkov instability in fluids with perfect gas equations of state utilizing matching of a nondimensional pressure jump across the incident shock, the post-shock Atwood ratio, post-shock amplitude-to-wavelength ratio, and time nondimensionalized by the Rcithmyer linear-growth rate time constant prediction. Result comparison demonstrates difference in start-up time and growth rate oscillations. Growth rate oscillation frequency is seen to correlate directly to the expected oscillation frequency of the transmitted and reflected shocks. For the single-mode cases, further comparison is given for vorticity distribution and corrugation centerline shortly after shock interaction that demonstrates only minor differences.</p>
<p>Additionally, examined is single-mode Richtmyer-Meshkov instability when a reflected expansion wave is present for incident Mach numbers of 1.5 and 2.5. Comparison to perfect gas solutions in such cases yields a higher degree of similarity in start-up time and growth rate oscillations. Vorticity distribution and corrugation centerline shortly after shock interaction is also examined. The formation of incipient weak shock waves in the heavy fluid driven by waves emanating from the perturbed transmitted shock is observed when an expansion wave is reflected.</p>
<p>Next, the ghost fluid method [83] is explored for application to impact-driven flows with Mie-Grüneisen equations of state in a vacuum. Free surfaces are defined utilizing a level-set approach. The level-set is reinitialized to the signed distance function periodically by solution to a Hamilton-Jacobi differential equation in artificial time. Flux reconstruction along each Cartesian direction of the domain is performed by subdividing in a way that allows for robust treatment of grid-scale sized voids. Ghost cells in voided regions near the material-vacuum interface are determined from surface-normal Riemann problem solution. The method is then applied to several impact problems of interest. First, a one-dimensional impact problem is examined in Mie-Grüneisen aluminum with simple point erosion used to model separation by spallation under high tension. A similar three-dimensional axisymmetric simulation of two rods impacting is then performed without a model for spallation. Further results for three-dimensional axisymmetric simulation of a sphere hitting a plate are then presented.</p>
<p>Finally, a brief investigation of the assumptions utilized in modeling solids as isotropic fluids is undertaken. An Eulerian solver approach to handling elastic and elastic-plastic solids is utilized for comparison to the simple fluid model assumption. First, in one dimension an impact problem is examined for elastic, elastic-plastic, and fluid equations of state for aluminum. The results demonstrate that in one dimension the fluid models the plastic shock structure of the flow well. Further investigation is made using a three-dimensional axisymmetric simulation of an impact problem involving a copper cylinder surrounded by aluminum. An aluminum slab impact drives a faster shock in the outer aluminum region yielding a Mach reflection in the copper. The results demonstrate similar plastic shock structures. Several differences are also notable that include a lack of roll-up instability at the material interface and slip-line emanating from the Mach stem's triple point.</p>https://thesis.library.caltech.edu/id/eprint/6211Damage Evolution in Composite Materials and Sandwich Structures Under Impulse Loading
https://resolver.caltech.edu/CaltechTHESIS:05122011-154526450
Authors: {'items': [{'email': 'mlsilva@gmail.com', 'id': 'Silva-Michael-Lee', 'name': {'family': 'Silva', 'given': 'Michael Lee'}, 'show_email': 'YES'}]}
Year: 2011
DOI: 10.7907/CRX1-7D43
<p>Damage evolution in composite materials is a rather complex phenomenon. There are numerous failure modes in composite materials stemming from the interaction of the various constituent materials and the particular loading conditions. This thesis is concerned with investigating damage evolution in sandwich structures under repeated transient loading conditions associated with impulse loading due to hull slamming of high-speed marine craft. To fully understand the complex stress interactions, a full field technique to reveal stress or strain is required. Several full field techniques exist but are limited to materials with particular optical properties. A full field technique applicable to most materials is known as thermoelastic stress analysis (TSA) and reveals the variation in sum of principal stresses of a cyclically loaded sample by correlating the stresses to a small temperature change occurring at the loading frequency. Digital image correlation (DIC) is another noncontact full field technique that reveals the deformation field by tracking the motion of subsets of a random speckle pattern during the loading cycles.</p>
<p>A novel experimental technique to aid in the study of damage progression that combines TSA and DIC simultaneously utilizing a single infrared camera is presented in this thesis. A technique to reliably perform DIC with an infrared (IR) camera is developed utilizing variable emissivity paint. The thermal data can then be corrected for rigid-body motion and deformation such that each pixel represents the same material point in all frames. TSA is then performed on this corrected data, reducing motion blur and increasing accuracy. This combined method with a single infrared camera has several advantages, including a straightforward experimental setup without the need to correct for geometric effects of two spatially separate cameras. Additionally, there is no need for external lighting in TSA as the measured electromagnetic radiation is emitted by the sample’s thermal fields.</p>
<p>The particular stress resolution of TSA will depend on properties of the material of interest but the noise floor for the temperature variation is universal to the camera utilized. For the camera system in this thesis, the noise floor was found to be fairly frequency independent with a magnitude of 0.01 oC, giving the minimum measurable stress for 2024 aluminum alloy of 3.6 MPa and for Nylon of 0.84 MPa. The average displacement range found during a static DIC test with IR images was 0.1 pixels. The maximum displacement variation at 1 Hz was 0.018 pixels. The average variation in strain at 1 Hz was 25 microstrain comparable to traditional DIC measurements in the visible optical regime.</p>
<p>The combined TSA-DIC method in IR was validated with several benchmark example problems including plate structures with holes, cracks, and bimaterials. The validated technique was applied to foam-core sandwich composite beams under repeated simulated wave slamming loading. There are numerous failure modes in sandwich composite materials and the full field stress and strain from TSA and DIC, respectively, allow for improved failure analysis and prediction. Understanding damage in sandwich structures under impulse loading is a complex open area of research and the combined TSA-DIC method provides further insight into the failure process.</p>
https://thesis.library.caltech.edu/id/eprint/6389Fields, Forces, and Flows: What Laboratory Experiments Reveal About the Dynamics of Arched Plasma Structures
https://resolver.caltech.edu/CaltechTHESIS:06102012-025123301
Authors: {'items': [{'email': 'eveofdiscovery@gmail.com', 'id': 'Stenson-Eve-Virginia', 'name': {'family': 'Stenson', 'given': 'Eve Virginia'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/24HR-J675
<p>Magnetic flux tubes and, more generally, magnetic field structures that link a plasma volume to its boundary are prominent features in plasma systems of significant interest, such as the solar atmosphere and the interiors of magnetic fusion devices.</p>
<p>In order to study the fundamental physics of these systems, experiments were conducted in the laboratory using a magnetized plasma gun to produce individual arched, plasma-filled magnetic flux tubes. More complex plasma topologies were also explored. The absence of confining walls allowed plasmas to evolve freely — which they did, very dynamically, over the course of several microseconds. The experiment setup featured excellent reproducibility, extensive diagnostic accessibility, and several tunable parameters. In particular, a plasma "color coding" technique and magnetic measurements provided new and interesting results.</p>
<p>The single arches or "loops" of plasma exhibited sustained axial collimation, even during a dramatic evolution from a small, semicircular arch into a kinked structure up to seven times larger. The loops' magnetic structure was verified as consistent with that of a flux tube, and their evolution was found to be in quantitative agreement with two interrelated magnetohydrodynamic (MHD) theories: a simplified hoop force model for the axis expansion and a recently proposed MHD flow model for the collimation. More complex plasma structures were found to be similarly dominated by the effects of the magnetic field, exhibiting behavior that was highly repeatable but varied significantly from one magnetic structure to the next.</p>
<p>These findings suggest that MHD-driven flows are an important mechanism for the transport of plasma in arched flux tubes and other magnetic plasma structures. Because MHD has no inherent length scale, the forces driving the evolution of these experiments are expected to similarly affect other systems with low plasma beta and a high Lundquist number.</p>https://thesis.library.caltech.edu/id/eprint/7155Simulations of Compressible, Diffusive, Reactive Flows with Detailed Chemistry Using a High-Order Hybrid WENO-CD Scheme
https://resolver.caltech.edu/CaltechTHESIS:12302011-185742249
Authors: {'items': [{'email': 'jackalak@gmail.com', 'id': 'Ziegler-John-Lewis-Jack', 'name': {'family': 'Ziegler', 'given': 'John Lewis (Jack)'}, 'orcid': '0000-0001-6127-5567', 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/ZKW8-ES97
<p>A hybrid weighted essentially non-oscillatory (WENO)/centered-difference (CD) numerical method, with low numerical dissipation, high-order shock-capturing, and structured adaptive mesh refinement (SAMR), has been developed for the direct numerical simulation (DNS) of the multicomponent, compressive, reactive Navier-Stokes equations. The method enables accurate resolution of diffusive processes within reaction zones. This numerical method is verified with a series of one- and two-dimensional test problems, including a convergence test of a two-dimensional unsteady reactive double Mach reflection problem. Validation of the method is conducted with experimental comparisons of three applications all of which model multi-dimensional, unsteady reactive flow: an irregular propane detonation, shock and detonation bifurcations, and spark ignition deflagrations.</p>
<p>The numerical approach combines time-split reactive source terms with a high-order, shock-capturing scheme specifically designed for diffusive flows. A description of the order-optimized, symmetric, finite difference, flux-based, hybrid WENO / centered-difference scheme is given, along with its implementation in a high-order SAMR framework. The implementation of new techniques for discontinuity flagging, scheme-switching, and high-order prolongation and restriction is described. In particular, the refined methodology does not require upwinded WENO at grid refinement interfaces for stability, allowing high-order prolongation and thereby eliminating a significant source of numerical diffusion within the overall code performance.</p>
<p>A minimally reduced irregular detonation mixture mechanism (22 species and 53 reversible reactions) is developed and combined with the WENO-CD numerical method to accurately model two-dimensional hydrocarbon (propane) detonations with detailed chemistry and transport. First of its kind, resolved double Mach reflection (DMR) detonation simulations with a large hyrdocarbon mixture are presented. Detailed discussions and comparisons of the influence of grid resolution, lower-order numerical methods, and inviscid approximations are made in addition to the detailed presentation of fluid dynamics found in an unsteady, highly unstable, reactive DMR simulation. Also conducted are direct experimental comparisons to soot foils and schlieren images with an unresolved large-scale propane detonation channel simulation.</p>
<p>The numerical method is also applied to the DNS of two other problems, detonation/shock bifurcations and spark ignited deflagrations. Through the resolution of viscous/diffusive scales, new insights into how a bifurcated foot develops after a detonation end wall reflection, and how geometry can influence the development of a flame kernel after spark ignition are found.</p>
https://thesis.library.caltech.edu/id/eprint/6759Thermal Ignition
https://resolver.caltech.edu/CaltechTHESIS:05162012-131336010
Authors: {'items': [{'email': 'philipp.boettcher@gmail.com', 'id': 'Boettcher-Philipp-Andreas', 'name': {'family': 'Boettcher', 'given': 'Philipp Andreas'}, 'show_email': 'YES'}]}
Year: 2012
DOI: 10.7907/H2W9-ZK95
<p>Accidental ignition of flammable gases is a critical safety concern in many industrial applications. Particularly in the aviation industry, the main areas of concern on an aircraft are the fuel tank and adjoining regions, where spilled fuel has a high likelihood of creating a flammable mixture. To this end, a fundamental understanding of the ignition phenomenon is necessary in order to develop more accurate test methods and standards as a means of designing safer air vehicles. The focus of this work is thermal ignition, particularly auto-ignition with emphasis on the effect of heating rate, hot surface ignition and flame propagation, and puffing flames.</p>
<p>Combustion of hydrocarbon fuels is traditionally separated into slow reaction, cool flame, and ignition regimes based on pressure and temperature. Standard tests, such as the ASTM E659, are used to determine the lowest temperature required to ignite a specific fuel mixed with air at atmospheric pressure. It is expected that the initial pressure and the rate at which the mixture is heated also influences the limiting temperature and the type of combustion. This study investigates the effect of heating rate, between 4 and 15 K/min, and initial pressure, in the range of 25 to 100 kPa, on ignition of n-hexane air mixtures. Mixtures with equivalence ratio ranging from 0.6 to = 1.2 were investigated. The problem is also modeled computationally using an extension of Semenov's classical auto-ignition theory with a detailed chemical mechanism. Experiments and simulations both show that in the same reactor either a slow reaction or an ignition event can take place depending on the heating rate. Analysis of the detailed chemistry demonstrates that a mixture which approaches the ignition region slowly undergoes a significant modification of its composition. This change in composition induces a progressive shift of the explosion limit until the mixture is no longer flammable. A mixture that approaches the ignition region sufficiently rapidly undergoes only a moderate amount of thermal decomposition and explodes quite violently. This behavior can also be captured and analyzed using a one-step reaction model, where the heat release is in competition with the depletion of reactants.</p>
<p>Hot surface ignition is examined using a glow plug or heated nickel element in a series of premixed n-hexane air mixtures. High-speed schlieren photography, a thermocouple, and a fast response pressure transducer are used to record flame characteristics such as ignition temperature, flame speed, pressure rises, and combustion mode. The ignition event is captured by considering the dominant balance of diffusion and chemical reaction that occurs near a hot surface. Experiments and models show a dependence of ignition temperature on mixture composition, initial pressure, and hot surface size. The mixtures exhibit the known lower flammability limit where the maximum temperature of the hot surface was insufficient at igniting the mixture. Away from the lower flammability limit, the ignition temperature drops to an almost constant value over a wide range of equivalence ratios (0.7 to 2.8) with large variations as the upper flammability limit is approached. Variations in the initial pressure and equivalence ratio also give rise to different modes of combustion: single flame, re-ignition, and puffing flames. These results are successfully compared to computational results obtained using a flamelet model and a detailed chemical mechanism for n-heptane. These different regimes can be delineated by considering the competition between inertia, i.e., flame propagation, and buoyancy, which can be expressed in the Richardson number.</p>
<p>In experiments of hot surface ignition and subsequent flame propagation a 10 Hz puffing flame instability is visible in mixtures that are stagnant and premixed prior to the ignition sequence. By varying the size of the hot surface, power input, and combustion vessel volume, we determined that the instability is a function of the interaction of the flame with the fluid flow induced by the combustion products rather than the initial plume established by the hot surface. The phenomenon is accurately reproduced in numerical simulations and a detailed flow field analysis revealed a competition between the inflow velocity at the base of the flame and the flame propagation speed. The increasing inflow velocity, which exceeds the flame propagation speed, is ultimately responsible for creating a puff. The puff is then accelerated upward, allowing for the creation of the subsequent instabilities. The frequency of the puffing is proportional to the gravitational acceleration and inversely proportional to the flame speed. We propose a relation describing the dependence of the frequency on gravitational acceleration, hot surface diameter, and flame speed. This relation shows good agreement for lean and rich n-hexane-air as well as lean hydrogen-air flames.</p>https://thesis.library.caltech.edu/id/eprint/7037Dynamics of Magnetically Driven Plasma Jets: An Instability of an Instability, Gas Cloud Impacts, Shocks, and Other Deformations
https://resolver.caltech.edu/CaltechTHESIS:04132012-150652134
Authors: {'items': [{'email': 'auna.moser@gmail.com', 'id': 'Moser-Auna-Louise', 'name': {'family': 'Moser', 'given': 'Auna Louise'}, 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/V7P0-AW84
The Caltech experiment described here produces plasmas relevant to both astrophysical and fusion energy studies. A disk-shaped set of electrodes mounted on the inside of a meter-scale vacuum chamber provides the energy to break a neutral gas, provided at the electrodes only, down into a plasma. The plasma starts as eight loops, and then self-organizes into a single magnetically driven collimated plasma jet. This thesis explores the dynamic evolution of that plasma jet and how changes made to the jet power source or environment alter the evolution. The most significant finding is the discovery that a series of instabilities can lead to magnetic reconnection. The jet undergoes a first, primary instability, called the kink instability. The exponential growth of kink amplitude provides an acceleration that drives a smaller scale, secondary instability, called the Rayleigh–Taylor instability. The Rayleigh–Taylor instability can drop the diameter of the plasma to a small enough scale to allow magnetic reconnection. A second set of experiments explores the range of collision interactions between the plasma jet and a cloud of neutral gas in its path. A final set of experiments shows the dependence of jet radius on a time-changing current. A new power supply that led to the observation of some of these new dynamics is also described.
https://thesis.library.caltech.edu/id/eprint/6934Particle-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/6881Plasma-Surface Interactions in Hollow Cathode Discharges for Electric Propulsion
https://resolver.caltech.edu/CaltechTHESIS:05312012-113856351
Authors: {'items': [{'email': 'angie.capece@gmail.com', 'id': 'Capece-Angela-Maria', 'name': {'family': 'Capece', 'given': 'Angela Maria'}, 'orcid': '0000-0003-4147-7174', 'show_email': 'NO'}]}
Year: 2012
DOI: 10.7907/7TDQ-DR81
<p>Electric thrusters generate high exhaust velocities and can achieve specific impulses in excess of 1000 s. The low thrust generation and high specific impulse make electric propulsion ideal for interplanetary missions, spacecraft station keeping, and orbit raising maneuvers. Consequently, these devices have been used on a variety of space missions including Deep Space 1, Dawn, and hundreds of commercial spacecraft in Earth orbit. In order to provide the required total impulses, thruster burn time can often exceed 10,000 hours, making thruster lifetime essential.</p>
<p>One of the main life-limiting components on ion engines is the hollow cathode, which serves as the electron source for ionization of the xenon propellant gas. Reactive contaminants such as oxygen can modify the cathode surface morphology and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase evaporation of the emission materials and reduce cathode life. A deeper understanding of the mechanisms initiating cathode failure will improve thruster operation, increase lifetime, and ultimately reduce cost.</p>
<p>A significant amount of work has been done previously to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity, and its role during cathode poisoning is not completely understood. The work presented here represents the first attempt at understanding how oxygen impurities in the xenon discharge plasma alter the emitter surface and affect operation of a 4:1:1 BaO-CaO-Al<sub>2</sub>O<sub>3</sub> hollow cathode.</p>
<p>A combination of experimentation and modeling was used to investigate how oxygen impurities in the discharge plasma alter the emitter surface and reduce the electron emission capability. The experimental effort involved operating a 4:1:1 hollow cathode at various conditions with oxygen impurities in the xenon flow. Since direct measurements of the emitter surface state cannot be obtained because of the cathode geometry and high particles fluxes, measurements of the emitter temperature using a two-color pyrometer were used to determine the oxygen surface coverage and characterize the rate processes that occur during poisoning.</p>
<p>A model describing the material transport in the plasma discharge was developed and is used to predict the barium and oxygen fluxes to the emitter surface during cathode operation by solving the species continuity and momentum equations. The dominant ionization process for molecular oxygen in the plasma gas is resonant charge exchange with xenon ions. Barium is effectively recycled in the plasma; however, BaO and O<sub>2</sub> are not. The model shows that the oxygen flux to the surface is not diffusion limited. </p>
<p>Experimental results indicate that the oxygen poisoning rate is slow and that the oxygen poisoning coverage on the emitter surface is less than 3%. A time-dependent model of the reaction kinetics of oxygen and barium at the tungsten surface was developed using the experimental results.</p>
<p>The experiments and kinetics model indicate that the dominant processes at the emitter surface are dissociative adsorption of O<sub>2</sub>, sputtering of the O<sub>2</sub> precursor, and desorption of O. Ion sputtering of the weakly bound O<sub>2</sub> precursor state limits the poisoning rate and yields low oxygen coverage. Removal of chemisorbed atomic oxygen is dominated by thermal processes. Based on the low oxygen coverage and long poisoning transients, plasma cathodes appear to be able to withstand higher oxygen concentrations than vacuum cathodes.</p>https://thesis.library.caltech.edu/id/eprint/7108Planar Reflection of Gaseous Detonation
https://resolver.caltech.edu/CaltechTHESIS:06112013-153305610
Authors: {'items': [{'email': 'jason.damazo@gmail.com', 'id': 'Damazo-Jason-Scott', 'name': {'family': 'Damazo', 'given': 'Jason Scott'}, 'orcid': '0000-0002-4155-7177', 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/4QW7-TK55
<p>Pipes containing flammable gaseous mixtures may be subjected to internal detonation. When the detonation normally impinges on a closed end, a reflected shock wave is created to bring the flow back to rest. This study built on the work of Karnesky (2010) and examined deformation of thin-walled stainless steel tubes subjected to internal reflected gaseous detonations. A ripple pattern was observed in the tube wall for certain fill pressures, and a criterion was developed that predicted when the ripple pattern would form. A two-dimensional finite element analysis was performed using Johnson-Cook material properties; the pressure loading created by reflected gaseous detonations was accounted for with a previously developed pressure model. The residual plastic strain between experiments and computations was in good agreement.</p>
<p>During the examination of detonation-driven deformation, discrepancies were discovered in our understanding of reflected gaseous detonation behavior. Previous models did not accurately describe the nature of the reflected shock wave, which motivated further experiments in a detonation tube with optical access. Pressure sensors and schlieren images were used to examine reflected shock behavior, and it was determined that the discrepancies were related to the reaction zone thickness extant behind the detonation front. During these experiments reflected shock bifurcation did not appear to occur, but the unfocused visualization system made certainty impossible. This prompted construction of a focused schlieren system that investigated possible shock wave-boundary layer interaction, and heat-flux gauges analyzed the boundary layer behind the detonation front. Using these data with an analytical boundary layer solution, it was determined that the strong thermal boundary layer present behind the detonation front inhibits the development of reflected shock wave bifurcation.</p>https://thesis.library.caltech.edu/id/eprint/7890Current 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/7442Structure of the Turbulent Boundary Layer under Static and Dynamic Impulsive Roughness Perturbation
https://resolver.caltech.edu/CaltechTHESIS:07102012-152431583
Authors: {'items': [{'email': 'jacobi@alum.mit.edu', 'id': 'Jacobi-Ian', 'name': {'family': 'Jacobi', 'given': 'Ian'}, 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/H5WJ-RK31
<p>The zero-pressure gradient turbulent boundary layer at Reynolds numbers (based on momentum thickness) ranging from 2700--4100 was perturbed using an impulsively short patch of two-dimensional, spanwise roughness elements. A spatial perturbation was considered in which the roughness patch was held statically on the flat-plate, and the flow downstream of the perturbation was measured by hotwire and particle-image velocimetry. A dynamic perturbation, in which the roughness patch was actuated periodically in time, was also studied, and additional measurements were taken by phase-locking to the dynamic actuation itself.</p>
<p>The static perturbation distorted the boundary layer through the generation of a `stress bore' which modified the mean streamwise velocity gradient. The effect of this stress bore was observed in a modification of statistical and spectral measures of the turbulence, as well as a redistribution of coherent structures in the boundary layer. The characterization of the statically perturbed boundary layer provided a base flow from which to consider the dynamically perturbed flow. The dynamically perturbed flow manifested both effects analogous to the static perturbation, as well as a coherent, periodic, large-scale velocity fluctuation. The extent to which these two features could be treated as linearly independent was studied by a variety of statistical and spectral means. Moreover, the very large scale motion synthesized by the dynamic perturbation was isolated by phase-locked measurement, and its behavior was predicted with reasonable success by employing a resolvent operator approach to a forced version of the Orr-Sommerfeld equation.</p>
<p>The relationship between large-scale motions and an envelope of small-scale motions in the turbulent boundary layer was studied in both the unperturbed and perturbed flows. A variety of correlation techniques were used to interpret the interaction between the different scale motions in the context of a phase-relationship between large and small scales. This phase relationship was shown to provide a physically-grounded perspective on the relationship between the synthetic very large scale motion produced by the dynamic perturbation and the smaller scales in the flow, and was able to provide a foundation for thinking about new approaches to controlling turbulence through large-scale forcing.</p> https://thesis.library.caltech.edu/id/eprint/7175Slender-Body Hypervelocity Boundary-Layer Instability
https://resolver.caltech.edu/CaltechTHESIS:05312013-164534236
Authors: {'items': [{'email': 'nick.parziale@gmail.com', 'id': 'Parziale-Nicholaus-J', 'name': {'family': 'Parziale', 'given': 'Nicholaus J.'}, 'orcid': '0000-0001-9880-1727', 'show_email': 'YES'}]}
Year: 2013
DOI: 10.7907/KZJ1-Y009
<p>With novel application of optical techniques, the slender-body hypervelocity boundary-layer instability is characterized in the previously unexplored regime where thermo-chemical effects are important. Narrowband disturbances (500-3000 kHz) are measured in boundary layers with edge velocities of up to 5~km/s at two points along the generator of a 5 degree half angle cone. Experimental amplification factor spectra are presented. Linear stability and PSE analysis is performed, with fair prediction of the frequency content of the disturbances; however, the analysis over-predicts the amplification of disturbances. The results of this work have two key implications: 1) the acoustic instability is present and may be studied in a large-scale hypervelocity reflected-shock tunnel, and 2) the new data set provides a new basis on which the instability can be studied.</p>https://thesis.library.caltech.edu/id/eprint/7808Dynamics and Scaling of Self-Excited Passive Vortex Generators for Underwater Propulsion
https://resolver.caltech.edu/CaltechTHESIS:05282013-114822808
Authors: {'items': [{'email': 'rwhittlesey@gmail.com', 'id': 'Whittlesey-Robert-Wells', 'name': {'family': 'Whittlesey', 'given': 'Robert Wells'}, 'show_email': 'NO'}]}
Year: 2013
DOI: 10.7907/SC4M-8896
<p>A series of experiments was conducted on the use of a device to passively generate vortex rings, henceforth a passive vortex generator (PVG). The device is intended as a means of propulsion for underwater vehicles, as the use of vortex rings has been shown to decrease the fuel consumption of a vehicle by up to 40% Ruiz (2010).</p>
<p>The PVG was constructed out of a collapsible tube encased in a rigid, airtight box. By adjusting the pressure within the airtight box while fluid was flowing through the tube, it was possible to create a pulsed jet with vortex rings via self-excited oscillations of the collapsible tube.</p>
<p>A study of PVG integration into an existing autonomous underwater vehicle (AUV) system was conducted. A small AUV was used to retrofit a PVG with limited alterations to the original vehicle. The PVG-integrated AUV was used for self-propelled testing to measure the hydrodynamic (Froude) efficiency of the system. The results show that the PVG-integrated AUV had a 22% increase in the Froude efficiency using a pulsed jet over a steady jet. The maximum increase in the Froude efficiency was realized when the formation time of the pulsed jet, a nondimensional time to characterize vortex ring formation, was coincident with vortex ring pinch-off. This is consistent with previous studies that indicate that the maximization of efficiency for a pulsed jet vehicle is realized when the formation of vortex rings maximizes the vortex ring energy and size.</p>
<p>The other study was a parameter study of the physical dimensions of a PVG. This study was conducted to determine the effect of the tube diameter and length on the oscillation characteristics such as the frequency. By changing the tube diameter and length by factors of 3, the frequency of self-excited oscillations was found to scale as f~D_0^{-1/2} L_0^0, where D_0 is the tube diameter and L_0 the tube length. The mechanism of operation is suggested to rely on traveling waves between the tube throat and the end of the tube. A model based on this mechanism yields oscillation frequencies that are within the range observed by the experiment.</p>https://thesis.library.caltech.edu/id/eprint/7757Progress in Numerical Modeling of Non-Premixed Combustion
https://resolver.caltech.edu/CaltechTHESIS:05292014-112456783
Authors: {'items': [{'email': 'yxuan.caltech@gmail.com', 'id': 'Xuan-Yuan', 'name': {'family': 'Xuan', 'given': 'Yuan'}, 'orcid': '0000-0001-9326-2197', 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/ZED4-A872
<p>Progress is made on the numerical modeling of both laminar and turbulent non-premixed flames. Instead of solving the transport equations for the numerous species involved in the combustion process, the present study proposes reduced-order combustion models based on local flame structures.</p>
<p>For laminar non-premixed flames, curvature and multi-dimensional diffusion effects are found critical for the accurate prediction of sooting tendencies. A new numerical model based on modified flamelet equations is proposed. Sooting tendencies are calculated numerically using the proposed model for a wide range of species. These first numerically-computed sooting tendencies are in good agreement with experimental data. To further quantify curvature and multi-dimensional effects, a general flamelet formulation is derived mathematically. A budget analysis of the general flamelet equations is performed on an axisymmetric laminar diffusion flame. A new chemistry tabulation method based on the general flamelet formulation is proposed. This new tabulation method is applied to the same flame and demonstrates significant improvement compared to previous techniques.</p>
<p>For turbulent non-premixed flames, a new model to account for chemistry-turbulence interactions is proposed. %It is found that these interactions are not important for radicals and small species, but substantial for aromatic species. The validity of various existing flamelet-based chemistry tabulation methods is examined, and a new linear relaxation model is proposed for aromatic species. The proposed relaxation model is validated against full chemistry calculations. To further quantify the importance of aromatic chemistry-turbulence interactions, Large-Eddy Simulations (LES) have been performed on a turbulent sooting jet flame. %The aforementioned relaxation model is used to provide closure for the chemical source terms of transported aromatic species. The effects of turbulent unsteadiness on soot are highlighted by comparing the LES results with a separate LES using fully-tabulated chemistry. It is shown that turbulent unsteady effects are of critical importance for the accurate prediction of not only the inception locations, but also the magnitude and fluctuations of soot.</p>https://thesis.library.caltech.edu/id/eprint/8421Advancing EDL Technologies for Future Space Missions: From Ground Testing Facilities to Ablative Heatshields
https://resolver.caltech.edu/CaltechTHESIS:05302014-140011538
Authors: {'items': [{'email': 'jason.rabinovitch@gmail.com', 'id': 'Rabinovitch-Jason', 'name': {'family': 'Rabinovitch', 'given': 'Jason'}, 'orcid': '0000-0002-1914-7964', 'show_email': 'YES'}]}
Year: 2014
DOI: 10.7907/XKM7-7368
<p>Motivated by recent MSL results where the ablation rate of the PICA heatshield was over-predicted, and staying true to the objectives outlined in the NASA Space Technology Roadmaps and Priorities report, this work focuses on advancing EDL technologies for future space missions.</p>
<p>Due to the difficulties in performing flight tests in the hypervelocity regime, a new ground testing facility called the vertical expansion tunnel is proposed. The adverse effects from secondary diaphragm rupture in an expansion tunnel may be reduced or eliminated by orienting the tunnel vertically, matching the test gas pressure and the accelerator gas pressure, and initially separating the test gas from the accelerator gas by density stratification. If some sacrifice of the reservoir conditions can be made, the VET can be utilized in hypervelocity ground testing, without the problems associated with secondary diaphragm rupture.</p>
<p>The performance of different constraints for the Rate-Controlled Constrained-Equilibrium (RCCE) method is investigated in the context of modeling reacting flows characteristic to ground testing facilities, and re-entry conditions. The effectiveness of different constraints are isolated, and new constraints previously unmentioned in the literature are introduced. Three main benefits from the RCCE method were determined: 1) the reduction in number of equations that need to be solved to model a reacting flow; 2) the reduction in stiffness of the system of equations needed to be solved; and 3) the ability to tabulate chemical properties as a function of a constraint once, prior to running a simulation, along with the ability to use the same table for multiple simulations. </p>
<p>Finally, published physical properties of PICA are compiled, and the composition of the pyrolysis gases that form at high temperatures internal to a heatshield is investigated. A necessary link between the composition of the solid resin, and the composition of the pyrolysis gases created is provided. This link, combined with a detailed investigation into a reacting pyrolysis gas mixture, allows a much needed consistent, and thorough description of many of the physical phenomena occurring in a PICA heatshield, and their implications, to be presented.</p>
<p>Through the use of computational fluid mechanics and computational chemistry methods, significant contributions have been made to advancing ground testing facilities, computational methods for reacting flows, and ablation modeling.</p>https://thesis.library.caltech.edu/id/eprint/8445Boundary-Layer Transition on a Slender Cone in Hypervelocity Flow with Real Gas Effects
https://resolver.caltech.edu/CaltechTHESIS:05292014-220110640
Authors: {'items': [{'email': 'jjewell@gmail.com', 'id': 'Jewell-Joseph-Stephen', 'name': {'family': 'Jewell', 'given': 'Joseph Stephen'}, 'orcid': '0000-0002-4047-9998', 'show_email': 'NO'}]}
Year: 2014
DOI: 10.7907/Z9H9935V
<p>The laminar to turbulent transition process in boundary layer flows in thermochemical nonequilibrium at high enthalpy is measured and characterized. Experiments are performed in the T5 Hypervelocity Reflected Shock Tunnel at Caltech, using a 1 m length 5-degree half angle axisymmetric cone instrumented with 80 fast-response annular thermocouples, complemented by boundary layer stability computations using the STABL software suite. A new mixing tank is added to the shock tube fill apparatus for premixed freestream gas experiments, and a new cleaning procedure results in more consistent transition measurements. Transition location is nondimensionalized using a scaling with the boundary layer thickness, which is correlated with the acoustic properties of the boundary layer, and compared with parabolized stability equation (PSE) analysis. In these nondimensionalized terms, transition delay with increasing CO<sub>2</sub> concentration is observed: tests in 100% and 50% CO<sub>2</sub>, by mass, transition up to 25% and 15% later, respectively, than air experiments. These results are consistent with previous work indicating that CO<sub>2</sub> molecules at elevated temperatures absorb acoustic instabilities in the MHz range, which is the expected frequency of the Mack second-mode instability at these conditions, and also consistent with predictions from PSE analysis. A strong unit Reynolds number effect is observed, which is believed to arise from tunnel noise. N<sub>Tr</sub> for air from 5.4 to 13.2 is computed, substantially higher than previously reported for noisy facilities. Time- and spatially-resolved heat transfer traces are used to track the propagation of turbulent spots, and convection rates at 90%, 76%, and 63% of the boundary layer edge velocity, respectively, are observed for the leading edge, centroid, and trailing edge of the spots. A model constructed with these spot propagation parameters is used to infer spot generation rates from measured transition onset to completion distance. Finally, a novel method to control transition location with boundary layer gas injection is investigated. An appropriate porous-metal injector section for the cone is designed and fabricated, and the efficacy of injected CO<sub>2</sub> for delaying transition is gauged at various mass flow rates, and compared with both no injection and chemically inert argon injection cases. While CO<sub>2</sub> injection seems to delay transition, and argon injection seems to promote it, the experimental results are inconclusive and matching computations do not predict a reduction in N factor from any CO<sub>2</sub> injection condition computed.</p>https://thesis.library.caltech.edu/id/eprint/8433Battery-Powered RF Pre-Ionization System for the Caltech Magnetohydrodynamically-Driven Jet Experiment: RF Discharge Properties and MHD-Driven Jet Dynamics
https://resolver.caltech.edu/CaltechTHESIS:05012015-172120954
Authors: {'items': [{'email': 'vernon.chaplin@gmail.com', 'id': 'Chaplin-Vernon-Hampden', 'name': {'family': 'Chaplin', 'given': 'Vernon Hampden'}, 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9T43R08
<p>This thesis describes investigations of two classes of laboratory plasmas with rather different properties: partially ionized low pressure radiofrequency (RF) discharges, and fully ionized high density magnetohydrodynamically (MHD)-driven jets. An RF pre-ionization system was developed to enable neutral gas breakdown at lower pressures and create hotter, faster jets in the Caltech MHD-Driven Jet Experiment. The RF plasma source used a custom pulsed 3 kW 13.56 MHz RF power amplifier that was powered by AA batteries, allowing it to safely float at 4-6 kV with the cathode of the jet experiment. The argon RF discharge equilibrium and transport properties were analyzed, and novel jet dynamics were observed.</p>
<p>Although the RF plasma source was conceived as a wave-heated helicon source, scaling measurements and numerical modeling showed that inductive coupling was the dominant energy input mechanism. A one-dimensional time-dependent fluid model was developed to quantitatively explain the expansion of the pre-ionized plasma into the jet experiment chamber. The plasma transitioned from an ionizing phase with depressed neutral emission to a recombining phase with enhanced emission during the course of the experiment, causing fast camera images to be a poor indicator of the density distribution. Under certain conditions, the total visible and infrared brightness and the downstream ion density both increased after the RF power was turned off. The time-dependent emission patterns were used for an indirect measurement of the neutral gas pressure.</p>
<p>The low-mass jets formed with the aid of the pre-ionization system were extremely narrow and collimated near the electrodes, with peak density exceeding that of jets created without pre-ionization. The initial neutral gas distribution prior to plasma breakdown was found to be critical in determining the ultimate jet structure. The visible radius of the dense central jet column was several times narrower than the axial current channel radius, suggesting that the outer portion of the jet must have been force free, with the current parallel to the magnetic field. The studies of non-equilibrium flows and plasma self-organization being carried out at Caltech are relevant to astrophysical jets and fusion energy research.</p>https://thesis.library.caltech.edu/id/eprint/8844Simulation of Shock-Induced Bubble Collapse with Application to Vascular Injury in Shockwave Lithotripsy
https://resolver.caltech.edu/CaltechTHESIS:01222015-234921548
Authors: {'items': [{'email': 'vcoralic@gmail.com', 'id': 'Coralic-Vedran', 'name': {'family': 'Coralic', 'given': 'Vedran'}, 'show_email': 'NO'}]}
Year: 2015
DOI: 10.7907/Z91N7Z26
Shockwave lithotripsy is a noninvasive medical procedure wherein shockwaves are repeatedly focused at the location of kidney stones in order to pulverize them. Stone comminution is thought to be the product of two mechanisms: the propagation of stress waves within the stone and cavitation erosion. However, the latter mechanism has also been implicated in vascular injury. In the present work, shock-induced bubble collapse is studied in order to understand the role that it might play in inducing vascular injury. A high-order accurate, shock- and interface-capturing numerical scheme is developed to simulate the three-dimensional collapse of the bubble in both the free-field and inside a vessel phantom. The primary contributions of the numerical study are the characterization of the shock-bubble and shock-bubble-vessel interactions across a large parameter space that includes clinical shockwave lithotripsy pressure amplitudes, problem geometry and tissue viscoelasticity, and the subsequent correlation of these interactions to vascular injury. Specifically, measurements of the vessel wall pressures and displacements, as well as the finite strains in the fluid surrounding the bubble, are utilized with available experiments in tissue to evaluate damage potential. Estimates are made of the smallest injurious bubbles in the microvasculature during both the collapse and jetting phases of the bubble's life cycle. The present results suggest that bubbles larger than 1 <em>μ</em>m in diameter could rupture blood vessels under clinical SWL conditions.https://thesis.library.caltech.edu/id/eprint/8758Stability of Hypervelocity Boundary Layers
https://resolver.caltech.edu/CaltechTHESIS:06052015-111128842
Authors: {'items': [{'email': 'bitternp@gmail.com', 'id': 'Bitter-Neal-Phillip', 'name': {'family': 'Bitter', 'given': 'Neal Phillip'}, 'show_email': 'NO'}]}
Year: 2015
DOI: 10.7907/Z9Q23X5Z
<p>The early stage of laminar-turbulent transition in a hypervelocity boundary layer is studied using a combination of modal linear stability analysis, transient growth analysis, and direct numerical simulation. Modal stability analysis is used to clarify the behavior of first and second mode instabilities on flat plates and sharp cones for a wide range of high enthalpy flow conditions relevant to experiments in impulse facilities. Vibrational nonequilibrium is included in this analysis, its influence on the stability properties is investigated, and simple models for predicting when it is important are described.</p>
<p>Transient growth analysis is used to determine the optimal initial conditions that lead to the largest possible energy amplification within the flow. Such analysis is performed for both spatially and temporally evolving disturbances. The analysis again targets flows that have large stagnation enthalpy, such as those found in shock tunnels, expansion tubes, and atmospheric flight at high Mach numbers, and clarifies the effects of Mach number and wall temperature on the amplification achieved. Direct comparisons between modal and non-modal growth are made to determine the relative importance of these mechanisms under different flow regimes. </p>
<p>Conventional stability analysis employs the assumption that disturbances evolve with either a fixed frequency (spatial analysis) or a fixed wavenumber (temporal analysis). Direct numerical simulations are employed to relax these assumptions and investigate the downstream propagation of wave packets that are localized in space and time, and hence contain a distribution of frequencies and wavenumbers. Such wave packets are commonly observed in experiments and hence their amplification is highly relevant to boundary layer transition prediction. It is demonstrated that such localized wave packets experience much less growth than is predicted by spatial stability analysis, and therefore it is essential that the bandwidth of localized noise sources that excite the instability be taken into account in making transition estimates. A simple model based on linear stability theory is also developed which yields comparable results with an enormous reduction in computational expense. This enables the amplification of finite-width wave packets to be taken into account in transition prediction. </p>https://thesis.library.caltech.edu/id/eprint/8995Characterization and Modeling of Premixed Turbulent n-Heptane Flames in the Thin Reaction Zone Regime
https://resolver.caltech.edu/CaltechTHESIS:05282015-154709861
Authors: {'items': [{'email': 'bruno.savard@gmail.com', 'id': 'Savard-Bruno', 'name': {'family': 'Savard', 'given': 'Bruno'}, 'show_email': 'YES'}]}
Year: 2015
DOI: 10.7907/Z9GM858F
n-heptane/air premixed turbulent flames in the high-Karlovitz portion of the thin reaction zone regime are characterized and modeled in this thesis using Direct Numerical Simulations (DNS) with detailed chemistry. In order to perform these simulations, a time-integration scheme that can efficiently handle the stiffness of the equations solved is developed first. A first simulation with unity Lewis number is considered in order to assess the effect of turbulence on the flame in the absence of differential diffusion. A second simulation with non-unity Lewis numbers is considered to study how turbulence affects differential diffusion. In the absence of differential diffusion, minimal departure from the 1D unstretched flame structure (species vs. temperature profiles) is observed. In the non-unity Lewis number case, the flame structure lies between that of 1D unstretched flames with "laminar" non-unity Lewis numbers and unity Lewis number. This is attributed to effective Lewis numbers resulting from intense turbulent mixing and a first model is proposed. The reaction zone is shown to be thin for both flames, yet large chemical source term fluctuations are observed. The fuel consumption rate is found to be only weakly correlated with stretch, although local extinctions in the non-unity Lewis number case are well correlated with high curvature. These results explain the apparent turbulent flame speeds. Other variables that better correlate with this fuel burning rate are identified through a coordinate transformation. It is shown that the unity Lewis number turbulent flames can be accurately described by a set of 1D (in progress variable space) flamelet equations parameterized by the dissipation rate of the progress variable. In the non-unity Lewis number flames, the flamelet equations suggest a dependence on a second parameter, the diffusion of the progress variable. A new tabulation approach is proposed for the simulation of such flames with these dimensionally-reduced manifolds.https://thesis.library.caltech.edu/id/eprint/8904On 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/9755Small 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/9611Thermal 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/9784Mixing, 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/9742Numerical 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/9764An Experimental and Theoretical Study of Active Flow Control
https://resolver.caltech.edu/CaltechTHESIS:06092017-112408552
Authors: {'items': [{'email': 'd_hirsch@gmx.ch', 'id': 'Hirsch-Damian-George', 'name': {'family': 'Hirsch', 'given': 'Damian George'}, 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9N014KR
<p>The accelerating growth of environmental awareness has not stopped at the aerospace industry. The need for greener and more efficient airplanes threatens to outpace the flow of new technology. This has ignited development in several fields, one of which is active flow control (AFC). Active flow control has quickly proven its tremendous potential for real applications. Even though the roots of this technology date back a century, we still lack fundamental understanding. This thesis combines both modern and traditional approaches to lay out a new foundation for future research.</p>
<p>The thesis first focuses on the rising stars of active flow control: the so-called fluidic oscillators or sweeping jet actuators. These devices consist of simple, rigid internal geometries that create a sweeping output jet motion. The fluid dynamic interactions with the internal geometry are studied in detail using high-speed Schlieren imaging. Additionally, the influence of adjacent sweeping jets is investigated. It is revealed that the internal driving mechanism is far stronger than the fluid dynamic interactions at the outlet, resulting in a completely independent jet behavior.</p>
<p>Next, a high-lift airfoil design is combined with active flow control, and an extensive wind tunnel study is carried out. It is shown that for the given wing design active flow control leads to much higher lift benefits when applied to the trailing edge. Applied to the leading edge active flow control disrupts the vortex lift of the high-lift airfoil, resulting in a deleterious lift effect; however, it shows potential for pitch moment control. This project also underlines the advantages of jet-like active flow control over steady blowing actuation at limited available mass flow rates.</p>
<p>The momentum input coefficient as an important parameter in active flow control is discussed in detail, identifying common misconceptions and difficulties that hinder its proper calculation. An innovative, much simpler approach is introduced. This allows a detailed study of the underlying physics, unveiling unknown limitations of active flow control. The approach is then used as a model to derive the novel concept of thermal active flow control. Experimental studies, including a wind tunnel test campaign, are performed to confirm the viability of the concept for practical applications.</p>
<p>The new calculation method of the input momentum coefficient emphasizes its weakness as a similarity parameter in active flow control studies. The extended mass flow coefficient is introduced as a new parameter. It is shown that it can overcome the deficiencies of the input momentum coefficient without suffering other disadvantages. Its further investigation leads to a deeper understanding of active flow control, which is supported by PIV experiments. The main findings of this investigation divide active flow control into three different "states": boundary layer thickening, separation control, and supercirculation.</p>https://thesis.library.caltech.edu/id/eprint/10332Thermo-Acoustic Coupling and Dynamic Response of a Premixed Methane-Air Flame
https://resolver.caltech.edu/CaltechTHESIS:11302017-214955280
Authors: {'items': [{'email': 'slpalm@cox.net', 'id': 'Palm-Steven-Leslie', 'name': {'family': 'Palm', 'given': 'Steven Leslie'}, 'orcid': '0000-0003-3095-0368', 'show_email': 'NO'}]}
Year: 2017
DOI: 10.7907/Z9V12309
<p>The work herein generally applies to the problem of combustion instability. Combustion instabilities first arose in engineering practice in the 1940s when they were experienced during the development of solid and liquid propellant rocket engines. Later, similar problems arose in gas turbine combustors and afterburners. However, the earliest technical case of the phenomenon dates back to Rijke in 1859 with his "singing" tube.</p>
<p>The presented work focuses on the study of a simple, stagnation plane stabilized, laminar, flat-flame burner. In particular the dynamic response of the burner is examined under excitation by a driven acoustic field. After characterization of the burner’s operational range, the response of the system is measured from 20 Hz to nearly 2000 Hz over the span of operating parameters using an optically filtered PMT and lens combination. A library of the collected and reduced data is generated.</p>
<p>A deeper investigation of the burner dynamics at a given reference operating condition is performed using phase-resolved PLIF. Fluctuations in the spatial distributions of the LIF signals for several target species (OH, CH, CH<sub>2</sub>O) under acoustic forcing are measured. In addition, visualization of the unsteady reactant flow using precision acetone seeding and PLIF at 277 nm is performed. Subsequent cinematographic sequences are produced along with spatially resolved plots of the combustion response function and the forced Rayleigh index for numerous drive frequencies. A library of the collected and reduced data is assembled.</p>
<p>Analysis of the collected data reveals two principal mechanisms contributing to the unsteady response of the flame. Structure development in (and subsequent convention along) the unsteady shear layer of the laminar jet dominates the response at the outer reaches of the flame. The inner region of the flame is driven largely by the Helmholtz response of the burner nozzle cavity. These two operations mutually contribute to produce the general shape of the combustion response curve. Ultimately, the data is used to construct a simplified model for the combustion response function. The model is enhanced with two additional revisions guided by the improved understanding of the mechanisms involved.</p>
<p>The document ends with numerous appendices describing, in detail, the equipment used, much of which was fabricated specifically for this work. These appendices, in combination with information presented in the chapters, provide substantial detail regarding the experimental configuration and operating conditions. Great effort was made to provide the necessary information to allow replication of the experiments as well as to support future modeling endeavors as a validation dataset.</p>
https://thesis.library.caltech.edu/id/eprint/10574Numerical Investigations of Transport and Chemistry Modeling for Lean Premixed Hydrogen Combustion
https://resolver.caltech.edu/CaltechTHESIS:05312018-170312588
Authors: {'items': [{'email': 'jason.schlup@gmail.com', 'id': 'Schlup-Jason-Robert', 'name': {'family': 'Schlup', 'given': 'Jason Robert'}, 'orcid': '0000-0002-3121-3477', 'show_email': 'NO'}]}
Year: 2018
DOI: 10.7907/KFN6-7K54
<p>The use of hydrogen as a fuel for power generation applications has been suggested as an additive to, or replacement of, hydrocarbon fuels. The safety of hydrogen combustion has also received recent attention due to nuclear power plant disasters and the rise of hydrogen refuelling stations. In these uses and scenarios, lean hydrogen--air flames are prone to thermo-diffusive instabilities which can be dangerous to equipment and personnel. These instabilities are heavily influenced by two mechanisms: transport properties (e.g., diffusion) and chemical species production rates. This thesis investigates lean premixed hydrogen combustion using direct numerical simulations. A wide range of flame configurations are considered, spanning one-dimensional steady configurations to three-dimensional unsteady laminar and turbulent flames with high curvature. In particular, the two controlling mechanisms of thermo-diffusive instabilities are carefully investigated. </p>
<p>The effects of transport properties, in particular the importance of thermal diffusion in these mixtures, are quantified through global and local evaluations. Thermal diffusion is found to change flame speeds in one-dimensional flat flames, and also modify species profiles due to the increased diffusivity of light reactants. The impact of thermal diffusion is greatly enhanced in the presence of flame curvature, resulting in higher flame speeds (20% to 30% for two- and three-dimensional laminar and turbulent flames), fuel consumption, and flame surface area relative to simulations neglecting thermal diffusion. The mixture-averaged thermal diffusion model proposed by Chapman and Cowling (1970) is found to accurately reproduce global and local flame statistics (including enhanced burning and local extinction) computed using multicomponent transport at significantly reduced costs. Further cost reductions of the mixture-averaged thermal diffusion method are undertaken, and a new model is developed with constant computational requirements for large (~100 species) chemical models. The resulting reduced thermal diffusion model additionally improves upon the accuracy of the mixture-averaged thermal diffusion technique.</p>
<p>The effects of fluctuating chemical source terms on flame instabilities are then investigated using tabulated chemistry. One-dimensional unstretched flames including non-equal diffusion and thermal diffusion are incorporated into a chemistry table. This table successfully captures the interaction of differential diffusion and flame curvature. The chemistry tabulation approach is applied to a similar set of flame configurations, and accurate predictions of global and local statistics are found. The tabulated chemistry method reproduces flame curvature, local enhanced burning, and local extinction of unstable flames using one-dimensional, flat, burning flames in its construction. The proposed reduced-order thermal diffusion and chemistry tabulation models significantly reduce computational costs while simultaneously including physical properties necessary to predict lean premixed hydrogen--air flame instabilities.</p> https://thesis.library.caltech.edu/id/eprint/10991Pressure and Stress Transients in Autoinjector Devices
https://resolver.caltech.edu/CaltechTHESIS:02172019-174051312
Authors: {'items': [{'email': 'jc@alumni.caltech.edu', 'id': 'Veilleux-Jean-Christophe', 'name': {'family': 'Veilleux', 'given': 'Jean-Christophe'}, 'orcid': '0000-0002-5420-9411', 'show_email': 'YES'}]}
Year: 2019
DOI: 10.7907/VJSH-TF65
<p>The viscosity of drug solutions delivered parenterally has been increasing over the years. Injecting viscous drug solutions using spring-actuated autoinjector devices is challenging due to a number of technical and human factor constraints. Some of the related challenges are investigated in this thesis.</p>
<p>Actuation of autoinjector devices powered using stiff springs can create deleterious pressure and stress transients which are not needed to achieve the normal functions of the device. Experimental measurements have shown that peak pressures and stresses substantially larger than what is needed to achieve the normal device function can occur during the actuation phase, creating unnecessary potential for device failure.</p>
<p>The acceleration of the syringe during actuation can be very large, often creating transient cavitation in the cone region. The occurrence or absence of cavitation is determined by the relative timing of syringe pressurization and syringe acceleration, which is affected by several factors such as the presence, location, and size of an air gap inside the syringe, and the friction between the plunger-stopper and the syringe.</p>
<p>Experiments and numerical simulations have shown that sharp pressure waves traveling inside the syringe can be amplified within the cone terminating the syringe. Despite the potential for shock focusing, the impulsive pressurization and the rapid deceleration of pre-filled syringes create a potential for failure which is localized in the syringe shoulder and at the junction between the flange and the barrel, not inside the cone. The cavitation events, on the other hand, create a potential for failure which is limited to a region in close proximity of the bubble upon collapse. The collapse of cavitation bubbles located within the syringe cone can be enhanced due to geometrical effects, and the resulting stresses can be large enough to cause syringe failure.</p>
<p>This thesis demonstrates that static and quasi-static analyses do not provide accurate estimates of the peak pressures and stresses occurring within the device. The pressure and stresses created by the highly dynamic events occurring during actuation need to be accounted for during device design in order to improve device reliability, the user's experience, and patient's adherence to prescribed treatments. The findings discussed in this work provide insights and guidance as to how the transient events can be mitigated.</p>https://thesis.library.caltech.edu/id/eprint/11395Plasma 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/11673Ultraviolet 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/13637Hypervelocity 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/13726Thermal 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/14146Axial Descent of Multirotor Configurations -- Experimental Studies for Terrestrial and Extraterrestrial Applications
https://resolver.caltech.edu/CaltechTHESIS:01252022-055518852
Authors: {'items': [{'email': 'marcel_veismann@web.de', 'id': 'Veismann-Marcel', 'name': {'family': 'Veismann', 'given': 'Marcel'}, 'orcid': '0000-0001-8106-6738', 'show_email': 'NO'}]}
Year: 2022
DOI: 10.7907/w49w-qy54
<p>Axial descent, specifically the vortex ring state (VRS), poses great challenges for rotorcraft operation as this flight stage is typically accompanied by severe aerodynamic losses and excessive vibrational loads due to the re-ingestion of rotor downwash. Given the hazardous nature of this flight stage, its fluid dynamic properties in regards to single, large-scale rotors have been extensively investigated since the early stages of manned helicopter flight. In light of the rapidly expanding use of small-scale multirotor systems, the field of VRS research has recently received increased interest, with a shifted focus towards small-scale rotors, as the thrust generation and stability of these aerial systems have also been shown to be adversely affected by complex descent aerodynamics. While experimental studies have started examining low Reynolds number rotor aerodynamics in steep or vertical descent, the influence of small-scale rotor geometry and aerodynamic coupling between neighboring rotors have not yet been sufficiently explored.</p>
<p>The objective of this work is, therefore, to extend the current understanding of rotorcraft vortex ring state aerodynamics to low Reynolds number multirotor systems. A series of experimental studies employing various wind tunnel setups and flow visualization techniques is presented with the aim of identifying the underlying ﬂuid-structure interactions, and quantifying rotor performance losses during multirotor axial descent. The work is divided into two fundamental experimental approaches, one utilizing statically mounted rotor systems and one utilizing free-flight testing.</p>
<p>The first part of this work (Chapters 4 and 5) presents the results of wind-tunnel tested statically-mounted rotors for precise aerodynamic identification of rotor performance under simulated descent conditions. Chapter 4 covers a parametric analysis to comprehensively assess the extent to which relevant geometric parameters of a small-scale rotor influence its descent characteristic. Chapter 5 then explores the influence of separation between rotors and identifies potential rotor-rotor interactions in the VRS. The studies in this part of the thesis also make use of PIV setups for visualizing the flow field around small-scale rotors in the axial descent regime, subject to changing geometric parameters and rotor separation.</p>
<p>In the second part (Chapters 6 and 7), a series of free-flight investigations is described for realistically simulated axial descent scenarios. Chapter 6 introduces the methodology for quantifying thrust generation of a multirotor in free-flight without rigid attachment to a load cell, and presents the results of exploratory axial flight studies. Chapter 7 discusses a study on axial descent of variable-pitch multirotor configurations, which was carried out to evaluate the feasibility of deploying a future Mars helicopter in mid air. Findings from this study helped to inform the entry descent and landing (EDL) strategy for JPL's future Martian rotorcraft missions.</p>https://thesis.library.caltech.edu/id/eprint/14485Experiments 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/16060