L'écoulement autour des véhicules produit une couche limite turbulente très proche de la paroi. Le caractère turbulent induit des fluctuations de pression pariétale qui font vibrer les panneaux du véhicule. Ces vibrations sont alors transmises à travers la structure et rayonnent du bruit dans l’habitacle. Les niveaux sonores dus à l'écoulement augmentent avec la vitesse du véhicule. Pour cette raison, cette problématique connaît un intérêt croissant dans le secteur aéronautique.Le but de cette thèse est double : comprendre les mécanismes à l’origine de ces fluctuations de pression à la paroi et prédire l’excitation de la structure avion due à l’écoulement turbulent.Pour ce faire, des calculs Large Eddy Simulation (LES) sont disponibles. Il s’agit de bases de données numériques d’écoulements de couches limites turbulentes en présence de gradients de pression favorable, adverse et nul. Ceci permet de caractériser l’écoulement sur des géométries courbes telles que la pointe avant d’un avion. L’effet du gradient de pression sur des structures cohérentes de type « hairpins » et paquets de « hairpins » a pu être identifié et quantifié à travers des méthodes de visualisation et d’analyse statistique. Une méthode d’estimation stochastique du champ de vitesse a révélé la présence de tourbillons contra-rotatifs au-dessus des paquets de hairpins. Ces tourbillons ont une vorticité opposée à celle des hairpins et un modèle de « hairpins inversé » a été proposé.En s’inspirant du travail de Ahn et al. (2010), un modèle stochastique de spectre de la pression pariétale a été développé. Il s’agit de reconstruire un champ stochastique de vitesse instantanée et d’en déduire le champ de pression à la paroi en résolvant une équation de Poisson sur la pression. Le champ de vitesse est obtenu en soumettant des structures de type paquets de hairpins à un écoulement moyen. Les caractéristiques des paquets de hairpins en fonction du gradient de pression sont basées sur l’analyse des bases LES. Les résultats de ce modèle sont comparés à ceux issues de bases de données numériques et expérimentales. Enfin, ce modèle est utilisé pour caractériser l’écoulement de couche limite turbulente dans une simulation de Statistical Energy Analysis (SEA) afin de prédire les niveaux de vibration des panneaux d’une portion de fuselage d’avion. / The flow around vehicles creates a turbulent boundary layer in the vicinity of the wall. The turbulent behavior induces pressure fluctuations that make the panels vibrate. These vibrations are then transmitted though the structure of the vehicle and radiate noise inside the cabin. The flow-induced noise levels increase with the speed of the vehicle. For this reason, aircraft manufacturers show a great interest in this topic.There are two objectives for this thesis: understand the mechanisms responsible for the wall-pressure fluctuations and predict this source of aircraft panel excitation.A study of available Large Eddy Simulation (LES) computations was performed. The database consists in simulations of turbulent boundary layer flows submitted to favorable, adverse and zero pressure gradients. This is necessary to understand the nature of the flow over curved geometries such as the aircraft flight deck. The effect of pressure gradients on coherent hairpin structures and hairpin packets could be identified and quantified based on visualization and statistical analysis methods. Linear stochastic estimation of the velocity fields revealed a pair of counter-rotating streamwise vortices above hairpin packets. These vortices have a vorticity opposite to that of the hairpins and an “inverse hairpin” model was proposed.Following the work of Ahn et al. (2010), a stochastic model for wall-pressure spectrum was developed. The idea is to build a stochastic turbulent velocity field using hairpin packets which are subjected to a mean flow. The characteristics of the packets depending on the pressure gradient are based on the analyses of the LES database. The pressure field at the wall is obtained by solving a Poisson equation. The results of the hairpin packet model are compared to numerical and experimental data. Finally, the model is used as input for a Statistical Energy Analysis (SEA) simulation in order to predict the levels of vibrations of panels submitted to a turbulent boundary layer flow over a portion of an aircraft cabin.

In this article a Novikov engine with fluctuating hot heat bath temperature is presented. Based on this model, the performance measure maximum expected power as well as the corresponding efficiency and entropy production rate is investigated for four different stationary distributions: continuous uniform, normal, triangle, quadratic, and Pareto. It is found that the performance measures increase monotonously with increasing expectation value and increasing standard deviation of the distributions. Additionally, we show that the distribution has only little influence on the performance measures for small standard deviations. For larger values of the standard deviation, the performance measures in the case of the Pareto distribution are significantly different compared to the other distributions. These observations are explained by a comparison of the Taylor expansions in terms of the distributions’ standard deviations. For the considered symmetric distributions, an extension of the well known Curzon–Ahlborn efficiency to a stochastic Novikov engine is given.

The experimental discovery of graphene in 2004 has opened up a new research field in the direction of atomically thin two-dimensional layered materials for exploration of many fundamental research problems and technological applications. The charge carriers in graphene are massless Dirac fermions due to which they exhibit absence of localization, thereby giving rise to huge intrinsic mobilities and ballistic transport even at room temperatures. But it was observed that the extrinsic disorder and intrinsic structural disorder can significantly influence the transport in graphene films. This thesis focuses on three different aspects of graphene -disorder, magnetism and proximity-induced superconductivity. We have reported conductance fluctuations-based transport studies to investigate these aspects as they provide more detailed information than what can be obtained from the standard transport measurements. Even though these conductivity fluctuations pose a serious bottleneck for various applications, they can also yield useful insights into the various scattering mechanisms and the symmetry properties of graphene. In the first half of the thesis, we describe the measurement of low frequency 1/f noise in large area polycrystalline graphene films to understand the role of grain boundaries in charge carrier transmission in graphene. TEM studies on the low and high angled GBs formed in these graphene samples showed that they form distinct disordered regions of varying widths depending on the tilt angle of the GBs. At low temperatures, the 1/f noise measurements indicated spontaneous breaking of time reversal symmetry across graphene grain boundaries which suggests the magnetic nature of these grain boundaries. In the second half of the thesis, we will concentrate on the universal conductance fluctuations (UCF) in graphene which is the manifestation of quantum interference phenomena at low temperatures. We find that the absolute magnitude of the UCF is directly related to various symmetry-breaking disorder present in graphene. We also discuss how the UCF can be used to study the nature of proximity-induced superconducting correlations in graphene. In the end, we have proposed new device schemes for the integration of ferromagnetic and superconducting materials with graphene.

Le but de ce travail de thèse est l’étude de la dynamique de sphères de grande taille dans un écoulement fortement turbulent. Pour ce faire, nous avons développé une nouvelle technique optique permettant de suivre la dynamique à 6 dimensions – position et orientation absolues – de plusieurs particules dans un écoulement complexe. Bien que la taille des particules soit comparable à l’échelle intégrale de l’écoulement, nous trouvons que sa dynamique de rotation et de translation est intermittente. De plus, nous observons que la translation et la rotation sont reliées par la force de Magnus. La répartition statistique de l’accélération n’est pas gaussienne et l’échange d’énergie avec le fluide est gouverné par la théorie mathématique des grandes déviations. Nous trouvons que le diamètre influence fortement la manière dont la particule explore l’écoulement. Nous avons ensuite appliqué le suivi de position et d’orientation à une particule instrumentée. Ce système mesure en permanence l’accélération lagrangienne qu’il subit via un accéléromètre embarqué et émet l’information à travers une électronique radio fréquence. L’orientation absolue est nécessaire pour exprimer les signaux de l’accéléromètre et ceux du suivi optique dans un repère commun; cela nous permet de comparer rigoureusement les mesures issues de ces deux techniques indépendantes. À partir de ces résultats nous avons développé des méthodes pour inférer des propriétés de l’écoulement à partir des signaux d’accélération de la particule instrumentée. / The aim of this dissertation is to gain a better understanding of the Lagrangian dynamics of solid large spheres in a complex turbulent environment. Therefore, a novel measurement technique to optically track the 6–dimensional dynamics – position and absolute orientation – of large spheres advected by a complex flow is developed. Although the sphere’s diameter is comparable to the integral length of the underlying flow, we find intermittency for both the translation and the rotation. Moreover, rotation and translation couple in agreement with a lift force. Apart from the fact that the acceleration statistics are not gaussian, and the exchange of energy between the particle and the carrier flow falls into the mathematical theory of large deviations. Additionally, we find that the particle diameter has a surprisingly strong influence on how a particle samples the flow. The 6D–tracking technique is then applied to an instrumented particle, which embarks a 3D–accelerometer and a radio-transmission system to constantly emit the felt Lagrangian acceleration as it is advected in the flow. Measuring the particle’s absolute orientation is a crucial step here to project the acceleration measured by the particle into the laboratory reference frame and enables us to compare the forces obtained by the two independent measurements. Based thereon methods for interpreting the acceleration signals of the instrumented particle are developed and tested.

Cette thèse est consacrée à l’étude mathématique des effets de désordre dans divers systèmes physiques. On commence par trois problèmes d’homogénéisation stochastique en lien avec des questions statiques de physique classique. Premièrement, en vue de la déduction rigoureuse de l’élasticité non-linéaire à partir de la physique statistique de réseaux de chaînes de polymères, on établit l’existence de propriétés effectives pour des matériaux hyperélastiques hétérogènes aléatoires sous des hypothèses générales de croissance. Deuxièmement, dans un cadre linéarisé simplifié, on étudie les formules de Clausius-Mossotti pour les propriétés effectives d’alliages binaires dilués: on donne la première preuve générale et rigoureuse de ces formules, ainsi qu’une extension aux ordres supérieurs. Troisièmement, encore pour des systèmes linéarisés, on propose d’étudier les déviations par rapport aux propriétés effectives et on établit la première théorie générale des fluctuations en homogénéisation stochastique. Dans la seconde partie de cette thèse, on se focalise sur la compétition entre désordre et interactions, et on étudie plus particulièrement la dynamique des vortex de Ginzburg-Landau dans des supraconducteurs 2D de type II en présence d’impuretés. Bien que la compréhension mathématique des propriétés vitreuses complexes de ces systèmes semble hors de portée, on établit rigoureusement la limite de champ moyen pour la dynamique d’un grand nombre de vortex, et on étudie l’homogénéisation de ces équations limites et leurs propriétés. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Conselho Nacional de Desenvolvimento Científico e Tecnológico / The specific heat and the condensation energy of a two-dimensional Hubbard model, suitable to discuss high Tc superconductors (HTSTC), is studied taking into account hopping to first (t) and second (t2) nearest neighbors. Results for the Hubbard model show that the specific heat as a function of the temperature C(T) presents a two peaks structure (DUFFY; MOREO, 1997). The low temperature peak has been associated with spin fluctuation while the high temperature peak is related to charge fluctuation. Experimental results for the specific heat of HTSC s (LORAM et al., 2001), for instance, the YBCO and LSCO, indicate a close relation between the pseudogap and the specific heat. In the present work, we investigate the specific heat by the Green s function method within the n-pole approximation proposed by L. Roth (ROTH, 1969). The specific heat is calculated on the pseudogap and on the superconducting regions. Superconductivity with dx2−y2- wave pairing is considered following the procedure proposed by Beenen and Edwards (BEENEN; EDWARDS, 1995). The analytical expressions for the specific heat and for the condensation energy have been obtained following the formalism presented in reference (KISHORE; JOSHI, 1971). In the present scenario, the pseudogap emerges when the antiferromagnetic (AF) fluctuations (present in the Roth s band shift) become strongly sufficient to push down the region of the nodal point (π,π) on the renormalized quasi-particle bands. We observed that above a given total occupation nT , the specific heat decreases signaling the pseudogap presence. The effects of the antiferromagnetic fluctuations on the condensation energy and on superconductivity are also investigated.x / No presente trabalho estuda-se o calor específico de um modelo de Hubbard considerado adequado para discutir supercondutores de altas temperaturas. Resultados para o modelo de Hubbard mostram que o calor específico em função da temperatura apresenta uma estrutura de dois picos (DUFFY; MOREO, 1997). O pico de baixa temperatura está associado às flutuações de spin, enquanto que o pico em alta temperatura está relacionado às flutuações de carga. Por outro lado, resultados experimentais do calor específico de supercondutores de altas temperaturas (LORAM et al., 2001), como por exemplo o Y BCO e o LSCO, indicam uma forte relação entre o calor específico e o pseudogap. Portanto, neste trabalho investiga-se a relação entre o pseudogap e o calor específico de um modelo de Hubbard usando a técnica das funções de Green em conjunto com a aproximação de n-pólos proposta por L. Roth (ROTH, 1969). O calor específico é calculado na região do pseudogap e da supercondutividade. Considera-se supercondutividade com simetria de onda dx2 − y2 e o parâmetro de ordem supercondutor é obtido seguindo-se o procedimento de fatorização proposto por Beenen e Edwards (BEENEN; EDWARDS, 1995). A expressão analítica do calor específico é obtida seguindo o formalismo proposto na referência (KISHORE; JOSHI, 1971). No cenário adotado, o pseudogap emerge quando flutuações antiferromagnéticas, as quais estão relacionadas a correlações antiferromagnéticas (presentes no deslocamento de banda da Roth), tornam-se fortes o suficiente para puxar as bandas renormalizadas para energias abaixo do potencial químico no ponto (π,π). Observou-se que acima de uma certa ocupação, o salto no calor específico decresce sinalizando a abertura do pseudogap. Os efeitos das flutuações antiferromagnéticas sobre a energia de condensação e sobre a supercondutividade também são investigados.

In this thesis first the behavior of induction generators under unbalanced voltage conditions is studied. In this context, first the existing schemes for characterizing the unbalance in simple ways have been studied. The two schemes IEC, NEMA which are in use to quantify the unbalance, as an unbalance factor require the use of complex algebra. A new scheme to characterize voltage unbalance, without using any complex algebra has been proposed and its effectiveness is compared with the NEMA scheme. The performance of the induction generators under unbalanced voltage conditions is first studied using the symmetrical component approach and a detailed simulation. The machine currents for the same unbalanced conditions are estimated using the proposed scheme and its effectiveness is highlighted. Existing approximate schemes based on unbalance factor do not consider the phase differences between positive and negative sequence voltages while estimating currents. A method of including this feature is proposed here. A simple way to find the maximum line current among the three phases as a function of the positive and negative voltages has been proposed. Knowing the positive, negative sequence voltages and the angle between them, a scheme to compute the generator currents under unbalanced conditions is proposed. Finally a few affecting the machine heating due to unbalanced currents have been studied. The nature of the induction generator fault currents is studied to understand their implication on the system performance. First the fault currents that have been recorded from experiments on relatively small induction motors(26kW and 122kW) are compared with those obtained through detailed dynamic simulation, in order to validate the usage of the detailed dynamic simulations to study the fault current behavior of the induction machines in the absence of experimental or field records. In order to assess the impact of error in the measured machine parameters on the estimated fault current values, the sensitivity of the estimated values of fault currents to machine parameter variations is investigated. The fault currents in large grid connected induction generators (3MW to 500kW) have been estimated through detailed simulation. The effect of the instant of fault (w.r.t voltage cycle), input load levels and shunt capacitors on the fault current values is investigated. An eigen value analysis of the generator model has been carried out to understand the reasons for the observed behavior using the linearized machine model. Fitting functions have been adopted to quantify the fault currents, in order to facilitate comparison of fault currents. A method of including induction generators into the system fault studies has been proposed. Methods of choosing proper sequence impedances for representing the induction generator have been developed and validated using simulated values of balanced and unbalanced fault currents. Simulations of self excited generators have been used for determining the unbalanced fault currents, so as to ensure that the induction generator fault currents correspond to only those contributed by the machine. The impact of the induction generators penetration on the short circuit levels is investigated considering a 14 bus test system by incorporating the proposed induction generator models in a system level fault study.

Equilibrium statistical mechanics has a remarkable property: the steady state probability distribution can be calculated by a procedure independent of the detailed dynamics of the system under consideration. The partition function contains the complete thermodynamics of the system. The calculation of the partition function itself might be a daunting task and one might need to resort to approximate methods in practice. But there is no problem in principle on how to do the statistical mechanics of a system that is at thermal equilibrium. Nonequilibrium statistical mechanics is a completely different story. There is no general formalism, even in principle, the application of which is guaranteed to yield the probability distribution, even for stationary states, without explicit consideration of the dynamics of the system. Instead, there are several methods of wide applicability drawn from experience which work for particular classes of systems. Frequently, one writes down phenomenological equations of motion based on general principles of conservation and symmetry and attempts to extract the dynamical response and correlations. The motivation for studying nonequilibrium systems is the very simple fact that they are ubiquitous in nature and exhibit very rich, diverse and often counter-intuitive phenomenon. We ourselves are an example of a very complex nonequilibrium system. This thesis examines three problems which illustrate the generic features of a typical driven system maintained out of thermal equilibrium. The first chapter provides a very brief discussion of nonequilibrium systems. We outline the tools that are commonly employed in the theoretical description of driven systems, and discuss the response of physical systems to applied perturbations. Chapter two considers a very simple model for a single self-propelled particle with an internal asymmetry, and nonequilibrium energy input in the form of Gaussianwhite noise. Our model connects three key nonequilibrium quantities – drift velocity, mean internal force and position-velocity correlations. We examine this model in detail and solve it using perturbative, numerical and exact methods. We begin chapter three with a brief introduction to the sedimentation of particle-fluid suspensions. Some peculiarities of low Reynolds number hydrodynamics are discussed with particular emphasis on the sedimentation of colloidal particles in a viscous fluid. We then introduce the problem of velocity fluctuations in steady sedi-mentation. The relevance of the current study to an earlier model and improvements made in the present work are then discussed. A physical understanding of our model and the conclusions that result from its analysis are an attempt to resolve the old problem of divergent velocity fluctuations in steadily sedimentating suspensions. The fourth chapter is a study to probe the nature of the fluctuations in a driven suspension of point-particles. Fluctuation relations that characterise large-deviations are a current topic of intense study. We show in this chapter that the random dynamics of suspended particles in a driven suspension occasionally move against the driving force, and that the probability of such rare events obeys a steady state fluctuation relation. In the final chapter, we summarise the models studied and point out the common features that they display. We conclude by pointing out some ways in which the problems discussed in this thesis can be extended upon in the future.

Since the adoption of the North American Free Trade Agreement (NAFTA), the Canadian dollar has come to be regarded as a petro-currency. Consequently, rising prices of oil and gas (as well as other natural resources) would increase capital inflows that would lead to a higher exchange rate and contribute to the decimation of the export-oriented Canadian manufacturing sector by making Canadian products less competitive internationally. Some have argued that the Canadian economy has started to show symptoms related to the Dutch Disease. One important symptom is the slow rate of productivity growth, which consequently leads to the theory that Canada’s productivity performance depends significantly on the foreign exchange value of the domestic currency. This dissertation attempts to address these issues and seeks to solve the question of whether the Canadian economy is suffering from the Dutch Disease, as well as whether or not movements of the Canadian dollar are responsible for the low Canadian productivity growth since the 1990s.

La turbulence diminue le confinement du plasma et dégrade les performances des réacteurs à fusion. Différentes instabilités peuvent induire la turbulence comme les Ion Temperature Gradient (ITG) et les Trapped Electron Modes (TEM). Etant donné que les ITG et les TEM sont déstabilisés par différent gradients, ils peuvent tous les deux être stables, instables, ou donner un seul mode dominant.Il est important de pouvoir discriminer expérimentalement les régimes dominés par les TEM ou les ITG car le transport et la rotation qu’ils induisent peuvent être sensiblement différents.Des méthodes expérimentales existent pour cela, mais elles sont complexes et pas toujours réalisables. Cette thèse montre qu’une analyse des spectres fréquentiels de fluctuation peut fournir une indication expérimentale du mode dominant.En fonction du scénario du plasma, les spectres peuvent exhiber différentes composantes. Dans cette thèse il est montré que l’un d’eux appelé mode "Quasi-Cohérent" (QC) est induit par les TEM dans le plasma de cœur.Cette découverte a été faite dans des plasmas Ohmiques par des mesures de réflectométrie et des simulations gyrocinétiques combinées à un réflectomètre synthétique. Elles montrent que les TEM induisent des spectres fréquentiels étroits responsables des modes QC observés expérimentalement dans le cœur du plasma. Ces modes QC ont été renommés QC-TEM en référence à leur lien avec les TEM.Les premières applications de ces résultats ont ensuite été faites dans des plasmas Ohmiques et dans ceux chauffés par ondes à la fréquence cyclotronique électronique. De plus, des transitions ont été rapportées entre les QC-TEM et des modes MHD. / Plasma Turbulence is responsible of the anomalous transport which degrades the performances of the fusion devices. Turbulence is trigger by different instabilities such as the Ion Temperature Gradient Modes (ITG) and the Trapped Electron Modes (TEM). As ITG and TEM are driven by different gradients, they can either both be stable, both coexist or give way to a single dominant mode. The transport and the toroidal velocity induced by ITG and TEM can be noticeably different thus it is important to be able to discriminate experimentally the dominant modes. Different experimental approaches exist to distinguish ITG from TEM but they are complex and not systematically feasible. This thesis shows that frequency fluctuation spectra can provide an additional experimental indication of the dominant mode.These spectra can show different components:-Broadband spectra (delta-f around hundreds of kHz) which are generally attributed to turbulence.-Coherent modes (delta-f around 1 kHz) which oscillate at a very well-defined frequency.-Quasi-Coherent (QC) modes (delta-f around tens of kHz) which oscillate at a rather well defined frequency but are reminiscent of the broadband fluctuations.Reflectometry measurements and gyrokinetic simulations combined with a synthetic reflectometer diagnostics show that TEM can induce QC modes in the core region of Ohmic plasmas. The QC signature of TEM is due to their narrow frequency spectrum. The QC modes observed in the plasma core were renamed QC-TEM due to their TEM origins. Then, the first applications of these results are made in Ohmic and ECRH plasmas to investigate the role of TEM, and transitions are reported between QC-TEM and MHD modes.