<p> The compressible Rayleigh-Taylor instability (RTI) occurs when a fluid of low molar mass supports a fluid of higher molar mass against a gravity-like body force or in the presence of an accelerating front. Intrinsic to the problem are highly stratified background states, acoustic waves, and a wide range of physical scales. The objective of this thesis is to develop a specialized computational framework that addresses these challenges and to apply the advanced methodologies for direct numerical simulations of compressible RTI. Simulations are performed using the Parallel Adaptive Wavelet Collocation Method (PAWCM). Due to the physics-based adaptivity and direct error control of the method, PAWCM is ideal for resolving the wide range of scales present in RTI growth. Characteristics-based non-reflecting boundary conditions are developed for highly stratified systems to be used in conjunction with PAWCM. This combination allows for extremely long domains, which is necessary for observing the late time growth of compressible RTI. Initial conditions that minimize acoustic disturbances are also developed. The initialization is consistent with linear stability theory, where the background state consists of two diffusively mixed stratified fluids of differing molar masses. The compressibility effects on the departure from the linear growth, the onset of strong non-linear interactions, and the late-time behavior of the fluid structures are investigated. It is discovered that, for the thermal equilibrium case, the background stratification acts to suppress the instability growth when the molar mass difference is small. A reversal in this monotonic behavior is observed for large molar mass differences, where stratification enhances the bubble growth. Stratification also affects the vortex creation and the associated induced velocities. The enhancement and suppression of the RTI growth has important consequences for a detailed understanding of supernovae flame front acceleration and fuel capsule designs for inertial confinement fusion.</p>

<p> Aero-optical distortions are detrimental to airborne optical systems. To study distortion mechanisms, compressible large-eddy simulations are performed for a Mach 0.5 turbulent boundary layer and a separated shear layer over a cylindrical turret with and without passive control in the upstream boundary layer. Optical analysis is carried out using ray tracing based on the computed density field and Gladstone-Dale relation.</p><p> In the flat-plate boundary layer, the effects of aperture size, Reynolds number, small-scale turbulence, different flow regions and beam elevation angle are examined, and the underlying flow physics is analyzed. Three momemtum-thickness Reynolds numbers, <i>Re</i>_&thetas; = 875, 1770 and 3550, are considered. It is found that the level of optical distortions decreases with increasing Reynolds number within the Reynolds number range considered. The contributions from the viscous sublayer and buffer layer are small, while the wake region plays a dominant role followed by the logarithmic layer. By low-pass filtering the fluctuating density field, it is shown that small-scale turbulence is optically inactive. Consistent with previous experimental findings, the distortion magnitude is dependent on the propagation direction due to anisotropy of the boundary-layer vortical structures. Density correlations and length scales are analyzed to understand the elevation-angle dependence and its relation to turbulence structures. The applicability of Sutton's linking equation to boundary-layer flows is examined, and excellent agreement between linking equation predictions and directly integrated distortions is obtained when the density length scale is appropriately defined.</p><p> The second case studied involves a separated shear layer over a cylindrical turret with a flat window, with inflow from a flat-plate boundary layer with and without passive control devices. The flow and optical results show reasonable agreement with experimental data for the baseline case without control. Aperture size effect, frequency spectra of OPD and two-point spatial correlations of OPD are investigated. The similarities and differences of distortion characteristics compared to those induced by turbulent boundary layers are discussed. The distortions by a separated shear layer are much larger in magnitude and spatially less homogeneous than those induced by an attached boundary layer. It is found that pressure fluctuations are significant and play a dominant role in inducing density fluctuations and associated optical distortions in a separated shear layer, in contrast to the dominant role of temperature fluctuations in a turbulent boundary layer. When passive control is applied using a row of thin and tall pins in the upstream boundary layer, the numerical results confirm key experimental findings. The flow above the optical window is characterized by two distinct shear layers, whose combined effect leads to a significant reduction of density fluctuation magnitude in the main shear layer and associated optical distortions compared to the uncontrolled flow with a single strong shear layer.</p>

<p> Numerical investigations of flow past a partially rotating, circular cross-section, stepped cylinder in uniform flow at varying velocity ratios, diameter ratios and Reynolds Numbers are performed. The resulting flow fields are analyzed visually for the effect of rotation on the wake and numerically for the effect on lift/drag forces as well as vortex shedding rates. Simulations are run where either the large or small-diameter cylinder was rotating while the other cylinder was held stationary. Simulations were performed with end plates to minimize end effects. Results show with increasing the velocity ratio when the large cylinder rotates, vortex shedding is suppressed across the entire wake span. Comparatively, an increase in velocity ratio for the small-diameter cylinder during rotation results in increased wake activity and a larger drag force.</p>

Thesis (Ph.D.)--Lehigh University, 2008. / Adviser: P.A. Blythe.

Thèse (M.Sc.A.)--Université de Sherbrooke (Canada), 2007. / Titre de l'écran-titre (visionné le 1 février 2007). In ProQuest dissertations and theses. Publié aussi en version papier.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 7098. Adviser: Narayana R. Aluru. Includes bibliographical references (leaves 130-140) Available on microfilm from Pro Quest Information and Learning.

<p>Microelectromechanical (MEMS) devices are not capable of withstanding harsh operating environments, which may include high temperatures, pressures and corrosive agents. Ni-base superalloys have been used successfully in the hot stages of jet turbine engines despite the presence of these conditions. In my thesis work, I developed two techniques compatible with micro-processing methods to produce Ni-base superalloy micro-components for MEMS applications. The mechanical properties of these materials were accessed at room and elevated temperatures. Microstructural studies were performed, linking microstructural features to mechanical properties. </p><p> The first technique modified LIGA Ni (LIGA is a German acronym for lithography, electroplating and molding) microtensile specimens using a vapor phase aluminization process. A subsequent homogenization heat treatment produced a two phase Ni-Ni3A1 microstructure characteristic of modern Ni-base superalloys. Al composition was used to tailor both the precipitate size and volume fraction. Aluminized LIGA Ni micro-components exhibited room temperature yield and ultimate strengths 3 to 4 times LIGA Ni micro-components subject to the same heat treatment. </p><p> The second technique involved sputtering a commercial Ni-base superalloy, Haynes 718, to produce thick sputtered foils (up to 20 gam) on silicon and brass substrates. The as-deposited foils were nanocrystalline solid solutions with chemical compositions similar to the bulk material. Foils subject to ageing heat treatments exhibited unique precipitation mechanisms and good thermal stability. Strengths as high as 750 MPa at 700&deg;C were observed with several percent ductility. This is a significant improvement over state of the art metallic MEMS materials. </p><p> Furthermore, a new high temperature microtensile testing technique was developed. The technique embeds a displacement based force sensor into the hot zone of a furnace. This arrangement ensures temperature uniformity during testing, while permitting friction along the loading axis by measuring force locally. Load and displacement are measured simultaneously with a single camera using digital image correlation techniques. Initial results, using Al 6061-T6 as a reference material, were promising. </p>

Neste trabalho, estudamos os efeitos de ordem de curto alcance resentes na suscetibilidade paramagnética, para temperaturas acima da temperatura de transição antiferro-paramagnética, nos seguintes materiais: CoCl2.6H2O, CoBr2.6H2O, NiCl2.4H2O, MnCl2.4H2O e MnBr2.4H2O. As caracterísitcas apresentadas por estes materiais permitiram a investigação da dependência deste efeito com o valor do spin, com a dimensionalidade e com o tipo de anisotropia do sistema magnético. Desenvolvemos, também, um tratamento teórico simples, baseado no princípio variacional da energia livre, o qual é suficiente para explicar os comportamentos observados em nossos estudos experimentais. / In this work we have studied the effect of short-range ordering on the paramagnetic susceptibility, for temperatures above the antiferro-paramagnetic transition in the following materials: CoCl2.6H2O, CoBr2.6H2O, NiCl2.4H2O, MnCl2.4H2O and MnBr2.4H2O. The properties of by these materials permit the investigation of the dependence of this effect with the value of the spin, with the dimensionality and the anisotropy of the magnetic system. A simple theoretical treatment, based on variational principle of free energy, is developed, which accounts for most of the features observed in our experimental studies.

Neste trabalho, estudamos os efeitos de ordem de curto alcance resentes na suscetibilidade paramagnética, para temperaturas acima da temperatura de transição antiferro-paramagnética, nos seguintes materiais: CoCl2.6H2O, CoBr2.6H2O, NiCl2.4H2O, MnCl2.4H2O e MnBr2.4H2O. As caracterísitcas apresentadas por estes materiais permitiram a investigação da dependência deste efeito com o valor do spin, com a dimensionalidade e com o tipo de anisotropia do sistema magnético. Desenvolvemos, também, um tratamento teórico simples, baseado no princípio variacional da energia livre, o qual é suficiente para explicar os comportamentos observados em nossos estudos experimentais. / In this work we have studied the effect of short-range ordering on the paramagnetic susceptibility, for temperatures above the antiferro-paramagnetic transition in the following materials: CoCl2.6H2O, CoBr2.6H2O, NiCl2.4H2O, MnCl2.4H2O and MnBr2.4H2O. The properties of by these materials permit the investigation of the dependence of this effect with the value of the spin, with the dimensionality and the anisotropy of the magnetic system. A simple theoretical treatment, based on variational principle of free energy, is developed, which accounts for most of the features observed in our experimental studies.