No description available.

Development of improved turbulence closure models for compressible fluid flow simulations requires better understanding of the effects of compressibility on various underlying processes of turbulence. Fundamental studies of turbulent velocity gradients hold the key to understanding several non-linear processes like material element deformation, energy cascading, intermittency and mixing. Experiments, direct numerical simulation (DNS) and simple mathematical models are three approaches to study velocity gradients. With the goal of furthering our understanding of the effects of compressibility on turbulent velocity gradients, this dissertation (i) employs DNS results to characterize some of the effects of compressibility on turbulent velocity gradients, and (ii) develops simple mathematical models for velocity gradient dynamics in compressible turbulence. In the first part of the dissertation, effects of compressibility on velocity gradient invariants and the local topology of compressible turbulence are characterized employing DNS results of compressible decaying isotropic turbulence. Joint statistics of second and third invariants of velocity gradient tensor and the exact probability of occurrence of associated topologies conditioned upon dilatation (degree of compression/expansion of fluid) are computed. These statistics are found to be (i) highly dependent on dilatation and (ii) substantially different from the statistics observed in incompressible turbulence. These dilatation-conditioned statistics of compressible turbulence, however, are found to be fairly independent of Mach number and Reynolds number. In the second part of the dissertation, two mathematical models for compressible velocity gradient dynamics are developed. To take into account the significant aero-thermodynamic coupling that exists in compressible flows, the models are derived explicitly using the continuity, energy and state equations, along with the momentum equation. The modeling challenge involved in the development of these models lies in capturing the inherently non-local nature of pressure and viscous effects as a function of local terms to derive a closed set of ordinary differential equations. The models developed in this dissertation are evaluated in a variety of flow regimes - incompressible limit (low Mach number); pressure-released limit (extremely high Mach number); and intermediate (sub-sonic Mach numbers) - and are shown to recover a range of known compressibility effects.

Thesis (Ph. D.)--University of North Texas, August, 2008. / Title from title page display. Includes bibliographical references.

Le marché des matériaux composites à matrice organique (CMO) pour la réalisation de pièces structurales "froides" (-55°C < T < Tamb) arrive à saturation et l’industrie aéronautique vise à utiliser les CMO tissés 3D dans les pièces structurales dites "chaudes" (50°C < T < 300°C) des avions (nacelles,turbomoteurs). Ces conditions environnementales peuvent entrainer des phénomènes de dégradation à long terme. L'action de l'environnement et la complexité de la microstructure peuvent conduire au développement de gradients de propriétés dans les matériaux. À haute température, la matrice polymère peut présenter un comportement complexe dépendant du temps. Il est donc nécessaire de mettre au point une technique expérimentale capable de caractériser le comportement du matériau en fonction du temps à l'échelle locale, pour saisir les gradients des propriétés. Ce travail propose la mise en place d'un essai cyclique d'indentation instrumentée pour répondre à ces questions.L'indentation instrumentée est une technique expérimentale qui a rencontrée un grand succès au cours des dernières années. Dans sa forme classique, développée pour les matériaux à comportement élasto-plastique, elle consiste à réaliser un seul cycle de charge/décharge pour en déduire la dureté du matériau et le module élastique d’indentation. Cette analyse n'est pas appropriée pour les matériaux présentant un comportement dépendant du temps, ce qui nécessite de développer une nouvelle méthode optimisée pour les matériaux polymères. Les méthodes d'indentation pour les matériaux polymères proposées dans la littérature (fluage, dynamique), sont limitées à certaines charges. L'essai cyclique d'indentation proposé dans ce travail vise à mettre en évidence la complexité du comportement du polymère. Il est dérivé des essais cycliques macroscopiques et emploi une méthode d’analyse similaire. La technique est développée sur un polymère thermoplastique, le PEHD, pour lequel la réponse cyclique macroscopique en traction et cisaillement est connue. En suivant l’évolution au cours du temps des principaux indicateurs du comportement cyclique (le module d'indentation,l’aire de la boucle d'hystérésis et l'accumulation du déplacement) pour différentes fréquences, il est possible de mettre en évidence la réponse du matériau et d'effectuer une comparaison qualitative avec le comportement macroscopique. Le protocole d'indentation cyclique est ensuite utilisé pour étudier les gradients de propriétés dans la résine époxy thermodurcissable PR520 soumise à un vieillissement thermique à 150°C sous air à pression atmosphérique (jusqu'à 1000h), sous 2 bar d’O2et de N2 (pour 400h). Il est montré que la cinétique d’évolution du module d'indentation et du déplacement n'est pas affectée par le vieillissement. Cependant, leurs valeurs absolues varient de la surface au cœur du polymère, ce qui indique la présence de gradients. L'hystérésis du premier cycle est différente à travers le gradient, mais à partir du deuxième cycle, l'hystérésis est similaire pour toutes les conditions de vieillissement et les distances de la surface exposée. Les résultats obtenus sur des échantillons vieillis sous 2 bar d’O2 et de N2 permettent de conclure que la cinétique de vieillissement n'est pas de la pure thermo-oxydation. La méthode d'indentation cyclique est ensuite appliquée pour caractériser le comportement de la matrice époxy PR520 dans un composite tissé 3D,à l'état vierge et vieilli à 150°C sous air à pression atmosphérique. L'étude du composite à l'état vierge révèle que le comportement de la matrice polymère à proximité de la surface externe est différent de celui situé dans les zones internes du composite et du polymère pur. La comparaison entre le polymère pur et la matrice à l’état vieilli montre que les gradients de propriétés induits par l'environnement sont similaires. / The market of organic matrix composite (OMC) materials for the realisation of “cold” (-55°C < T < RT) structural parts is going towards saturation and aircraft manufacturers foresee the employment of 3D reinforced OMC in “warm” (50°C < T < 300°C) aircraft structural parts (nacelles, turbo-engines). These environmental conditions may lead to degradation phenomena over long time. The action of environment and the complexity of the material microstructure may lead to the development of material property gradients. At high temperature, the organic polymer matrix may exhibit complex time-dependent behaviour. Therefore, there is a need to develop an experimental technique able to characterise the material behaviour at local scale and to capture material gradients and time-dependent behaviour. The present work proposes the development of a cyclic instrumented indentation test to tackle all these issues. Instrumented indentation is a popular testing technique: its basic version, appropriate for elasto-plastic materials, includes the realisation of a single loading/unloading test, the measurement of the material hardness through the analysis of the indentation print, and the analysis of the unloading curve based on the assumption of elastic unloading behaviour to obtain the indentation modulus. This analysis is inappropriate for materials exhibiting time-dependent behaviour, which leads to the need of a new method optimized for polymer materials.Several indentation methods are available for polymer materials (indentation creep, nanoDMA), but are limited to some specific loadings. The instrumented indentation cyclic test proposed in this work tries to emphasize the whole complexity of the polymer behaviour, is inspired by macroscopic cyclic tests and is analysed similarly. The technique is first set up and developed by testing a HDPE thermoplastic polymer, for which the response to macroscopic cyclic tension and shear loading isknown. By following, at different frequencies, the evolution with time (with cycles) of the principal indicators of the cyclic behaviour – that is, the indentation modulus, the hysteresis loop area and the indentation depth accumulation - it is possible to highlight the time-dependent response of the material and to perform a proper – though qualitative - comparison with the macroscopic behaviour. The cyclic indentation protocol is then employed to study the material gradients in a thermoset PR520epoxy resin subjected to thermal aging at 150°C under air at atmospheric pressure (up to 1000h), 2 barO2 (for 400h) and N2. It is shown that the evolution with cycles of the indentation modulus and the cyclic creep is not affected by thermal aging. However, their absolute values vary from the surface to the core of polymer indicating the presence of gradients. The hysteresis of the first cycle is different through the gradient: from the second cycle, however, the hysteresis is similar for all aging conditions and distances from the exposed surface. Moreover, the time-dependent behaviour stays unchanged. Results from samples aged under 2 bar O2 and 2 bar N2 allow to conclude that the aging kinetics is not related to pure thermo-oxidation phenomena. The cyclic indentation method is finally applied to characterise the behaviour of PR520 epoxy matrix within a 3D interlock textile composite, in its virgin state and thermally aged at 150°C under air at atmospheric pressure (up to 1000h). The polymer inlarge matrix pockets between the fibrous reinforcements is studied in this work, so the constrainingeffect coming from the reinforcement is negligible. The study of the composite in virgin state revealsthat the behaviour of polymer matrix close to the external surface is different from that located ininternal zones of the composite and from the neat polymer. The comparison between the thermallyaged neat polymer and matrix in internal zones of the composite shows that the environment-inducedproperty gradients are similar.

No description available.

Cell migration is a complex physiological process that is important from embryogenesis to senescence. In vivo, the migration of cells is guided by a complex combination of signals and cues. Directed migration is typically observed when one of these cues is presented to cells as a gradient. Several studies have been conducted into directed migration on gradients that are purely mechanical or chemical. Our goal was to investigate cellular migratory behavior when cells are presented with a choice and have to choose between increasing substrate rigidity or higher protein concentration. We chose to focus on this unique environment since it recapitulates several interfacial regions in vivo. We have designed novel hydrogels that exhibit dual and opposing chemical and mechanical profiles using photo-polymerization. Our studies demonstrate that durotaxis, a well-known phenomenon, can be reversed when cells sense a steep protein profile in the opposite direction. Fibroblasts were co-cultured with macrophages to obtain an understanding on how migration occurs when two different cell types are present in the same microenvironment. First, we investigated the migratory behavior of macrophages. These cell types exhibited a statistically significant preference to move towards the rigid/low collagen region of the interface. Interestingly, fibroblasts when co-cultured with macrophages, exhibited a preference for the low modulus-high collagen region of the interface. However, with the current sample size, these trends are statistically insignificant. On the contrary, the presence of fibroblasts in the cellular microenvironment did not result in the reversal of durotaxis exhibited by macrophages. Macrophages secreted significantly higher levels of secreted tumor necrosis factor (TNF-alpha) in mono-cultures in contrast to fibroblast-macrophage co-cultures. This trend could be an indication of macrophage plasticity between mono- and co-cultures. In summary, we have designed dual and opposing rigidity-protein gradients on a hydrogel substrate that can provide new insights into cellular locomotion. These results can be used to design biomimetic interfaces, biomaterial implants and for tissue engineering applications. / Ph. D.

The legacy of historical biogeography impacts many organisms and results in a wide range of character variation over a latitudinal gradient. The pitcher-plant mosquito Wyeomyia smithii is one such organism that demonstrates a wide range of phenotypic and genotypic variation over the entirety of its range from the Gulf Coast to Canada. A geographic cline established by the presence and recession of the Laurentide Ice Sheet is manifest in the narrow range of thermal tolerance exhibited by different populations and also in the differing propensity of bloodfeeding by these mosquitoes. These contemporary clines were analyzed by a variety of experimental methods ranging from year-long fitness assays, scanning electron microscopy, and RNA-sequencing to determine the patterns underlying the resulting evolutionary differences among established populations. This dissertation includes both unpublished and co-authored material.

Organic petrology and RockEval-TOC pyrolysis studies were carried out on over 600 samples of shales, siltstones, marls and coals from forty three sections in the Upper Devonian and Carboniferous strata of the northern part of the Liard Basin and adjacent areas of northern Canada between latitudes 59° 30' N and 60° 30' N and longitudes 121° 30' W and 125° W, to determine the hydrocarbon source potential, thermal maturity and thermal history. The focus of the study is on the optical characteristics and genesis of bitumens and their viability as indicators of thermal maturity. Total organic carbon (TOC) contents indicate that potential oil and gas source rocks occur in the Muskwa, Kotcho, Besa River, Exshaw, Lower Banff, Yohin, Clausen, Golata and Mattson formations; the latter includes thin algal laminites and sapropelic coals. Hydrogen versus oxygen indices indicate that the organic matter comprises Type II kerogen, except for the Golata and Mattson formations which contain mixed Type II and Type III kerogen. Petrographic analysis indicates that the organic components consist of primary and secondary liptinites of marine origin and indigenous microscopic bitumens. In addition, shales in the Golata and Mattson contain structured liptinite macerals, predominantly sporinite derived from Carboniferous mega- and microspores. ... Organic petrology and RockEval-TOC pyrolysis studies were carried out on over 600 samples of shales, siltstones, marls and coals from forty three sections in the Upper Devonian and Carboniferous strata of the northern part of the Liard Basin and adjacent areas of northern Canada between latitudes 59° 30' N and 60° 30' N and longitudes 121° 30' W and 125° W, to determine the hydrocarbon source potential, thermal maturity and thermal history. The focus of the study is on the optical characteristics and genesis of bitumens and their viability as indicators of thermal maturity. Total organic carbon (TOC) contents indicate that potential oil and gas source rocks occur in the Muskwa, Kotcho, Besa River, Exshaw, Lower Banff, Yohin, Clausen, Golata and Mattson formations; the latter includes thin algal laminites and sapropelic coals. Hydrogen versus oxygen indices indicate that the organic matter comprises Type II kerogen, except for the Golata and Mattson formations which contain mixed Type II and Type III kerogen. Petrographic analysis indicates that the organic components consist of primary and secondary liptinites of marine origin and indigenous microscopic bitumens. In addition, shales in the Golata and Mattson contain structured liptinite macerals, predominantly sporinite derived from Carboniferous mega- and microspores.

No description available.

We consider the problem of structural optimization which has many important applications in the engineering sciences. The goal is to find an optimal distribution of the material within a certain volume that will minimize the mechanical and/or thermal compliance of the structure. The physical system is governed by the standard models of elasticity and heat transfer expressed in terms of boundary-value problems for elliptic systems of partial differential equations (PDEs). The structural optimization problem is then posed as a suitably constrained PDE optimization problem, which can be solved numerically using a gradient approach. As a main contribution to the thesis, we derive expressions for gradients (sensitivities) of different objective functionals. This is done in both the continuous and discrete setting using the Riesz representation theorem and adjoint analysis. The sensitivities derived in this way are then tested computationally using simple minimization algorithms and some standard two-dimensional test problems. / Thesis / Master of Science (MSc)