Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 254-260). Also available on the Internet.

Thesis (M.S.)--University of California, Berkeley, 1961. / "Engineering and Equipment, UC-38" -t.p. "TID-4500 (16th Ed.)" -t.p. Includes bibliographical references (p. 62-63).

xi, 172 p. : ill. (some col.) / Accretion shock waves are present in many important astrophysical systems and have been a focus of research for decades. These investigations provide a large body of understanding as to the nature, characteristics, and evolutionary behaviors of accretion shock waves over a wide range of conditions. However, largely absent are investigations into the properties of accretion shock waves in the presence of strong magnetic fields. In such cases these strong magnetic fields can significantly alter the stability behaviors and evolution of the accretion shock wave through the production and propagation of magnetic waves as well as magnetically constrained advection. With strong magnetic fields likely found in a number of accretion shock systems, such as compact binary and protostellar systems, a better understanding of the behaviors of magnetic accretion shock waves is needed. A new magnetohydrodynamics simulation tool, IMOGEN, was developed to carry out an investigation of instabilities in strong, slow magnetic accretion shocks by modelling their long-term, nonlinear evolution. IMOGEN implements a relaxed, second-order, total variation diminishing, monotonic upwind scheme for conservation laws and incorporates a staggered-grid constrained transport scheme for magnetic advection. Through the simulated evolution of magnetic accretion shocks over a wide range of initial conditions, it has been shown, for sufficiently high magnetic field strengths, that magnetic accretion shocks are generally susceptible to corrugation instabilities, which arise in the presence of perturbations of the initial shock front. As these corrugation instabilities grow, they manifest as magnetic wave propagation in the upstream region of the accretion column, which propagate away from the accretion shock front, and as density columns, or fingers, that grow into the higher density downstream flow, defined and constrained by current loops created during the early evolution of the instability. / Committee in charge: Dr. James Schombert, Chair; Dr. James Imamura, Advisor; Dr. Alan Rempel, Member; Dr. John Toner, Member; Dr. Kent Stevens, Outside Member

We develop a simple one-zone model of the steady-state Crab nebula spectrum encompassing both the radio/soft X-ray and the GeV/multi-TeV observations. By solving the transport equation for GeV-TeV electrons injected at the wind termination shock as a log-parabola momentum distribution and evolved via energy losses, we determine analytically the resulting differential energy spectrum of photons. We find an impressive agreement with the observed spectrum of synchrotron emission, and the synchrotron self-Compton component reproduces the previously unexplained broad 200-GeV peak that matches the Fermi/Large Area Telescope (LAT) data beyond 1 GeV with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) data. We determine the parameters of the single log-parabola electron injection distribution, in contrast with multiple broken power-law electron spectra proposed in the literature. The resulting photon differential spectrum provides a natural interpretation of the deviation from power law customarily fitted with empirical multiple broken power laws. Our model can be applied to the radio-to-multi-TeV spectrum of a variety of astrophysical outflows, including pulsar wind nebulae and supernova remnants, as well as to interplanetary shocks.

The present work investigates the dynamics and stability of shock waves in granular gases. The problem was modelled for a piston propagating into a system of disks that can undergo inelastic collisions if an impact threshold is exceeded. The model was addressed numerically at the microscopic and macroscopic levels. The molecular dynamics methodology employed the Event-Driven Molecular Dynamics method, and the continuum model was formulated using the Navier-Stokes equations for granular gases with the transport terms of Jenkins and Richman and a modified cooling rate term. The inviscid steady state shock structure was derived and analyzed. The results indicated that a relaxing shock structure is expected for sufficiently strong shock waves. Beyond this limit the structure was shown to be independent of the initial energy, a finding similar to the strong shock approximation in molecular gases. One-dimensional simulations demonstrated that the molecular dynamics and continuum models yield similar evolutions and structures of the shock wave, validating the continuum description of this study. Two-dimensional results showed that sufficiently strong shock waves can exhibit multi-dimensional instability with high density non-uniformities and convective rolls within the structure, with the size of instabilities shown to scale with the relaxation length of the shock structure. Instabilities were observed with the continuum description only with the inclusion of statistical fluctuations to density mimicking the molecular model. The cases that were unstable were shown to be in a regime whereby statistical fluctuations can become important, following the description for this regime by Bird. Based on these findings, it is proposed that unstable shock behaviour can be observed for highly dissipative shock waves that yield short relaxation length scales, where fluctuations become important. The current work may shed light on unstable shock behaviour observed in dissipative gases, having implications for both granular media and molecular gases.

Les étoiles Miras sont des étoiles froides et évoluées (étoiles AGB), caractérisées par une variation de luminosité régulière et de longue période liée à une pulsation radiale. Cela se traduit par une atmosphère froide, étendue et faiblement liée gravitationnellement. Dans le spectre de ces étoiles, la détection variable d'émissions intenses dans les raies de Balmer de l'hydrogène est usuellement reliée à la propagation périodique d'une onde de choc radiative hypersonique dans l'atmosphère. Mon travail de thèse a eu d'abord pour objectif de confirmer l'existence d'une forte signature de polarisation linéaire accompagnant ces émissions, d'origine inconnue et déjà observée auparavant, et de caractériser son évolution temporelle. Je me base sur un suivi spectropolarimétrique inédit et effectué dans le cadre de ma thèse avec l'instrument NARVAL pour plusieurs Miras afin d'établir cette caractérisation. Ces suivis ont été réalisé sur tous les paramètres de Stokes (polarisation linéaire et circulaire) et surtout à des phases autour du maximum de luminosité.Un résultat majeur de ces observations est la détection systématique de signatures polarisées (polarisation surtout linéaire mais aussi parfois circulaire) liées aux émissions en intensité dans les raies de Balmer. L'utilisation d'un code de simulation d'atmosphère dynamique m'a permis de confirmer le lien entre une onde de choc et la présence d'émission dans ces raies, chose qui n'avait jamais été montrée par un modèle jusque là. Ces éléments me font affirmer que le mécanisme de polarisation est intrinsèque à l'onde de choc. La discussion sur l'origine de cette polarisation comporte deux grands axes : l'asymétrie globale de l'onde de choc amenant à une polarisation nette du rayonnement et la production locale dans le front du choc d'un champ magnétique responsable d'une polarisation par impact dans la zone de production des photons Balmer, c'est-à-dire le sillage du choc. D'autre part, j'invoque la possibilité d'une instabilité de Parker comme deuxième facteur de polarisation par impact et je discute le rôle potentiel de l'effet Hanle. / Mira stars are cool and evolved stars (AGB stars), characterized by a regular luminosity variation and a long period linked to a radial pulsation. All this imply a cool and extended atmosphere which is weakly linked by gravity. In the spectra of those stars, the variable detection of intense emissions in the Balmer lines of hydrogen are usually linked to the periodical propagation of a hypersonic radiative shock wave in the atmosphere. My thesis work's first objective was to confirm the existence of a strong polarimetric signature accompanying those emissions, whose origin is still unknown and already observed before, and to characterize its evolution through time. I rely on a novel spectropolarimetric survey, done in the framework of my thesis with the NARVAL instrument for several Mira stars in order to establish this characterization. Those surveys were done on all Stokes parameters (linear and circular polarization) and mainly during phases around the maximum of luminosity.A major result of these observations is the systematic detection of polarized signatures (mainly linear polarization but also circular sometimes) linked to the emissions in intensity in the Balmer lines. The use of a dynamical atmosphere simulation code allowed me to confirm the link between a shock wave and the presence of emissions in those lines. Such a result has never been been produced by a model until now. These elements make me state that the polarization mechanism is intrinsic to the shock wave. The discussion on the origin of such polarization consists of two main axes : the global asymmetry of the shock wave leading to a net polarization of the radiation and the local production in the shock's front of a magnetic field responsible for an impact polarization in the area of production of Balmer photons, namely the shock's wake. Besides that, I mention the possibility of a Parker instability as a second factor of impact polarization and I discuss the potential role of the Hanle effect.

The atomization of a liquid by an air stream has been an item of much concern in the past, and there have been numerous experimental studies of the process made in an effort to correlate liquid and air- stream physical properties into a general model which will predict the degree of and the time required for atomization for a given physical situation. The breakup of liquid drops, a major portion of the atomization process, finds application in the field of meteorology, where the formation, breakup, and shape of raindrops is an item of much concern; in the field of internal combustion engines, where the carburization of a fuel is of major importance; in the industrial field, where the atomization of paint and plant insecticides and spray drying processes are oft-mentioned objects of concern; in the chemical industries, where the emulsification of liquid- liquid systems, the formation of froths, the production of aerosols, and dispersion processes in general are most important considerations; and in the science of rheology, wherein the motion and dispersion of liquids, gases, and solids must take into consideration various breakup mechanisms. Despite the oft-mentioned importance of such liquid atomization processes, there has been little theoretical work done in .an effort to mathematically correlate the important variables and parameters common to all breakup and dispersion processes in general.

This thesis describes the simulation of ERDC’s Blast Load Simulator (BLS) using MSU’s Loci/BLAST. The BLS was created to replicate waveforms found in blast scenarios. Loci/BLAST is an explicit, unstructured CFD code that specializes in moving waveforms. ERDC conducted various tests, and a grid for each scenario was created using the SolidMesh tool. Each grid was simulated, and the results were displayed as time history plots and spatial plots. Simulations were also performed that compared 2D and 3D grids and determined the effect of the grate and striker components. There was a strong correlation between the experimental and simulation results for each case, demonstrating that Loci/BLAST is fully capable of modeling the BLS waveforms. A 2D grid produced results comparable to those on a full 3D grid. The grate and striker were critical in the simulation because they significantly affected the waveform.

No description available.

In the study of blast waves from a finite sphere, it has been observed that in addition to the primary strong shock wave ahead of the driving gas, a second shock also forms inside of this driving gas. This formation of the second shock will invalidate all the existing theories of blast waves in that an additional non-isentropic region actually exists inside of the interface between the driven and the driving gas. This thesis investigates the behavior of this second shock and the resultant flow patterns. / Ph. D.