Dans cette thèse nous étudions les propriétés statistique des courants dans des systèmes à l'échelle mésoscopique. Nous utilisons le formalisme de la statistique de comptage afin de caractériser les fluctuations de courant importantes à cette échelle. Celle-ci est obtenue en partant du Hamiltonien microscopique décrivant la dynamique des électrons sur le circuit considéré dans le régime quantique.<p>Nous considérons deux modèles particuliers de circuits à deux canaux, chacun comportant deux électrodes. Le premier modèle étudié est constitué de deux plots quantiques en couplage capacitif, et chacun échangeant des électrons avec deux électrodes. Le deuxième modèle est quant à lui constitué d'un double plot quantique connecté à deux électrodes et modulant le courant dans un point quantique formé lui-même par la jonction de deux électrodes. Pour ces deux modèles, chaque canal est soumis à une différence de potentiel, ou force thermodynamique, générant des courants stationnaires fluctuants.<p>La statistique des courants pour ces deux modèles est obtenue en utilisant une équation maîtresse pour les probabilités d'occupation dans les plots quantiques et le nombre d'électrons transférés entre ceux-ci et les électrodes. Nous vérifions que la distribution de probabilité jointes des courants dans chaque canal ainsi obtenue vérifie un théorème de fluctuation dans la limite des temps long faisant intervenir les forces thermodynamique des deux canaux.<p>La question de l'émergence d'un théorème de fluctuation effectif pour la distribution de probabilité marginale du courant dans un des deux canaux est également investiguée. Nous montrons que dans la limite ou le rapport des courants est grande, un tel théorème de fluctuation effectif est satisfait individuellement pour le canal de plus faible courant comme observé expérimentalement. Ce théorème fait intervenir une affinité effective dépendante des forces thermodynamiques des deux canaux et des spécificités du modèle considéré. Son étude détaillée est faite pour les deux modèles mentionnés.<p>Par ailleurs, nous posons également la question de l'existence d'un théorème de fluctuation pour des temps de mesure finis. Nous montrons qu'en présence d'un théorème de fluctuation dans la limite de temps longs, un critère peut être énoncé sur la condition initiale des plots quantiques menant à un théorème de fluctuations à temps fini. Ce critère est également étendu au cas des théorèmes de fluctuations effectifs.<p>Finalement, nous faisons une étude thermodynamique du modèle composé d'un double plot quantique en présence de différences de potentiel électrique et de température entre les électrodes du circuit. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In this dissertation, we studied the generation of squeezed states induced by a timedependent interaction and the in uence of temperature on the strength of the squeezing in a condensed matter model. The model studied comprised two quantum harmonic oscillators, with a time-dependent, non-linear coupling between them. The in uence of the thermal bath on the non-equilibrium dynamics of the model was represented in terms of non-Hamiltonian thermostats and a collection of independent harmonic oscillators with Ohmic spectral density. The equations of motion were studied in the Wigner representation, which introduces a phase space description for the model. The representation of the system in quantum phase space allowed us to investigate the di erence between purely classical evolution and the relative importance of quantum corrections with respect to the dynamics. The dynamics was studied by means of computer simulation techniques. The numerical simulation of the non-equilibrium statistical mechanics of both time-dependent and non-linear interactions allowed us to investigate conditions beyond those in recent literature [1, 2]. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 209-216). Also available via the Internet.

Thesis (Ph. D. in Interdisciplinary Materials Science)--Vanderbilt University, Aug. 2007. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 96-107).

Here, we propose a method for generating a hierarchy of mean-field approximations to study the properties of the driven diffusive Ising model at nonequilibrium steady state. In addition, the present study offers a demonstration of the practical application of the information theoretic methods to a simple interacting nonequilibrium system. The application of maximum entropy principle to the system, which is in contact with a heat reservoir, leads to a minimization principle for the generalized Helmholtz free energy. At every level of approximation the latter is expressed in terms of the corresponding mean—field variables. These play the role of variational parameters. The rate equations for the mean-field variables, which incorporate the dynamics of the system, serve as constraints to the minimization procedure. The method is applicable to high temperatures as well to the low temperature phase coexistence regime and also has the potential for dealing with first-order phase transitions. At low temperatures the free energy is nonconvex and we use a Maxwell construction to find the relevant information for the system. To test the method we carry out numerical calculations at the pair level of approximation for the 2-dimensional driven diffusive Ising model on a square lattice with attractive interactions. The results reproduce quite well all the basic properties of the system as reported from Monte Carlo simulations. / Ph. D.

This work uses computational fluid dynamics to study the flowfield around a hypersonic missile with two lateral jets to provide control in place of control surfaces. The jets exhaust an H2-O2 mixture at Mach number of 2.9 with a jet pressure ratio of roughly 10,500. The jets are staggered axially and circumferentially in such a way to produce pitch and yaw. The flowfield of such a jet configuration is characterized at several angles of attack and the corresponding force coefficients and amplification factors are provided. The freestream air and H2-O2 plume is treated as inert for the majority of the calculations. Special cases are treated with finite rate chemical kinetics and compared to the inert flowfield to ascertain the effects that chemical reactions have on the force coefficients. It was found that the flowfield was only slightly altered from the familiar one jet flowfield when the second jet is active. The flow topology and vortex structures tend to shift towards the second jet but the overall structure remains the same. The normal force amplification factors are close to unity over the range of angle of attack due to the thrust being so high with the two jet configuration having a lower amplification factor compared to firing a single jet. Treating the flowfield as chemically reacting did not affect the force values much: the difference being 0.3% for an angle of attack of 0°. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Title from first page of PDF file (viewed March 12, 2010). Available via ProQuest Digital Dissertations. Includes bibliographical references (p. 117-136). Also issued in print.

Using equilibrium molecular dynamics simulations, an analysis of the key thermophysical properties critical to heat transfer processes is performed. Replication of thermal conductivity and shear viscosity observations found in experimental investigations were performed using a theoretical nanopthesis-fluid system and a novel colloid-fluid interaction potential to investigate the key nanofluid parameters. Analysis of both the heat current (thermal conductivity) and stress (shear viscosity) autocorrelation functions have suggested that the dominant physical mechanisms for thermal and momentum transport arises from enhancements to the longitudinal and transverse acoustic modes energy transfer brought about by the increased mass ratio of the nanopthesis to the fluid. This conclusion was further supported by analysis of the local density fluctuations surrounding increasing nanopthesis diameters where the longitudinal acoustic mode characteristics for density fluxes were seen to be enhanced by the presence of the heavier platinum nanopthesiss. It is then concluded that the key macroscopic characteristic in obtaining the largest thermal energy transfer enhancement is through the mass of the nanopthesis relative to the base fluid. Also, the small local density effects in the nanofluid are greatly affects the viscosity calculations. These conclusions provide the theoretical framework for many of the experimental results obtained.

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