Railway tracks experience elevated rail deflections when the supporting soil is soft and/or the train speed is greater than approximately 50% of the wave propagation velocity in the track-soil system (i.e. the critical velocity). Such vibrations are undesirable, so soil replacement or soil improvement of the natural soil (or alternatively mini-piles or lime-cement treatment) is often used to increase track-ground stiffness prior to line construction. Although areas of existing soft subgrade might be easily identified on a potential new rail route, it is challenging to determine the type and depth of ground remediation required. Therefore, major cost savings can be made by optimising ground replacement/improvement strategies. This paper presents a numerical railway model, designed for the dynamic analysis of track-ground vibrations induced by high speed rail lines. The model simulates the ground using a thin-layer finite element formulation capable of calculating 3D stresses and strains within the soil during train vehicle passage. The railroad track is modelled using a multi-layered formulation which permits wave propagation in the longitudinal direction, and is coupled with the soil model in the frequency-wavenumber domain. The model is validated using a combination of experimental railway field data, published numerical data and a commercial finite element package. It is shown to predict track and ground behaviour accurately for a range of train speeds. The railway simulation model is computationally efficient and able to quickly assess dynamic, multi-layered soil response in the presence of ballast and slab track structures. Therefore it is well-suited to analysing the effect of different soil replacement strategies on dynamic track behaviour, which is particularly important when close to critical speed. To show this, three soil-embankment examples are used to compare the effect of different combinations of stiffness improvement (stiffness magnitude and remediation depths up to 5 m) on track behaviour. It is found that improvement strategies must be carefully chosen depending upon the track type and existing subgrade layering configuration. Under certain circumstances, soil improvement can have a negligible effect, or possibly even result in elevated track vibration, which may increase long-term settlement. However, large benefits are possible, and if detailed analysis is performed, it is possible to minimise soil improvement depth with respect to construction cost.

[pt] Pontes com vãos superiores a 2.000 m tornam-se muito sensíveis à ação do vento, particularmente ao drapejamento. Nesta tese é estudado um método para a supressão do drapejamento em pontes de grandes vãos através de um controle aerodinâmico ativo. Apresentam-se técnicas analíticas de projeto para o controle ativo do sistema aero elástico constituído pelo tabuleiro e por duas superfícies de controle. Estas técnicas são baseadas em aproximações racionais das cargas aerodinâmicas não permanentes (ou auto-excitadas) no domínio Laplaciano, no qual as equações de movimento são representadas por equações matriciais de coeficientes constantes. A primeira parte da tese é dedicada à formulação matricial das funções racionais conhecida como Minimum State, assim como a aplicações a dados aerodinâmicos obtidos experimentalmente para vários tipos de seções transversais de pontes. A precisão das aproximações é calculada. Desenhos dos derivativos aerodinâmicos, dados sob forma de tabelas, e das respectivas aproximações, são elaborados para fins de comparação. Em seguida, são apresentadas as equações em espaço de estado descrevendo o comportamento aeroelástico de uma seção transversal de ponte. A partir dos dados geométricos e características dinâmicas de uma determinada ponte, (massa, momento de inertia polar, frequências naturais e fatores de amortecimento), e assumindo a semelhança geométrica entre as seções transversais da ponte em verdadeira grandeza e do modelo em escala do qual os derivativos aerodinâmicos foram extraídos, é possível calcular a velocidade crítica desta ponte, utilizando os programas em linguagem MATLAB apresentados no corpo deste trabalho. Esta parte da tese mostra ser possível construir um catálogo com vários perfis de pontes, caracterizados por derivativos aerodinâmicos variáveis em função de frequências reduzidas adimensionais, e das funções racionais correspondentes. A segunda parte é dedicada à fomulação das equações de movimento em espaço de estado, descrevendo o comportamento aeroelástico do sistema tabuleiro - superfícies de controle. As equações resultantes são ampliadas com novos estados aerodinâmicos responsáveis pela modelagem da influência do fluxo de ar sobre o tabuleiro e sobre as superfícies de controle em movimento. As equações de movimento são função da velocidade média do vento incidente. A dependência da equação de movimento à velocidade do vento motivou a aplicação dos conceitos de realimentação de ganhos, constante e variável, ao problema da supressão do drapejamento, os quais são apresentados separadamente em dois capítulos.O enfoque de ganho variável de saída é formulado em termos de minimização de um índice de desempenho dimensionalmente proporcional à soma do trabalho realizado pelas superfícies de controle e da energia cinética proporcional à velocidade vertical do tabuleiro. Apresenta-se também em detalhe um método sistemático para determinar a matriz de controle de ganhos variável, aplicada ao caso hipotético da ponte de Gibraltar. Neste caso, o conceito de realimentação de ganhos variável mostrou-se muito efetivo em suprimir o drapejamento do tabuleiro da ponte. Diferentes características geométricas e dinâmicas de outras pontes podem ser introduzidas nos programas MATLAB apresentados no Apêndice, para obtenção da velocidade crítica nos casos de tabuleiros isolados, tabuleiros com asas estacionárias e tabuleiros com asas giratórias ativamente controladas, para supressão do drapejamento do tabuleiro. / [en] Long span bridges, with main spans beyond 2.000 m become highly sensitive to wind action, particularly to flutter. An active aerodynamic control method of suppressing flutter of very long span bridges is studied in this thesis. Analytical design techniques for active control of the aeroelastic system consisting of the bridge deck and two control surfaces are presented. These techniques are based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yieds matrix equations of motion with constant coefficientes. The first part of this thesis is dedicated to the matrix formulation of the rational functions known as Minimum State and to applications to aerodynamic data obtained experimentally for various types of bridge profiles. The precision of the approximations iscalculated, and plots of the approximation functions compared to the available tabular data are drawn. Next, the state-space equations of motion describing the aeroelastic behaviour of a section of a bridge deck is presented. Given the dynamic data of a bridge structure (mass, rotational mass moment of inertia, natural frequencies, stiffness and damping ratios), and assuming that a geometric similitude exists between the profiles of the full-scale bridge deck and the sectional model from which the frequency dependent aerodynamic data was extracted, it is possible to calculate the critical velocity of that particular bridge. This part of the thesis shows that it is possible to build up a catalog of several profiles, characterized by frequency dependent aerodynamic data and the corresponding rational functions. The second part is dedicated to the formulation of the state-space equations of motion describing the aeroelastic behaviour of the entire system consisting of the bridge deck and control surfaces. The resulting equation includes new aerodynamic states which model the air flow influence on the moving deck. The equation of motion is a function of the mean velocity of the incoming wind. The dependence of the equation of motion on the wind velocity motivated the application of a constant and a variable-gain feedback concept to the problem of flutter suppressing, which are presented separatelly. The output variable-gain approach is formulated in terms of minimizing a performance index dimensionally proportional to the sum of the work done by the rotating control surfaces and the kinetic energy of the heaving velocity. A sistematic method to determine the matrix of variable control gains is shown in detail, as applied to the hypothethical case of Gibraltar bridge. Application of the variablegain feedback concept was found to be very effective in suppressing flutter of the bridge deck. Different geometric and dynamic characteristics can be introduced in the MATLAB programs included in this work, in order to obtain the critical velocities of a bridge deck alone, a bridge deck with stationary wings and a bridge with moving wings activelly controled.

La compréhension des effets des interactions dans un ensemble de particules quantiques représente un enjeu majeur de la physique moderne. Les atomes ultra-froids sont rapidement devenus un outil incomparable pour étudier ces systèmes quantiques fortement corrélés. Dans cette thèse, nous présentons plusieurs travaux portant sur les propriétés d’un mélange de superfluides de Bose et de Fermi créé à l’aide de vapeurs ultra-froides de ⁷Li et de ⁶Li. Nous étudions tout d'abord les propriétés hydrodynamiques du mélange en créant un contre-courant entre les superfluides. L'écoulement est dissipatif uniquement au dessus d'une vitesse critique que nous mesurons dans le crossover BEC-BCS. Une simulation numérique d’un contre-courant de deux condensats permet de mieux comprendre les mécanismes sous-jacents mis en jeu dans la dynamique. En particulier, l'étude numérique fournit des preuves supplémentaires que l'origine de la dissipation dans nos expériences est liée à l'émission d'excitation élémentaires dans chaque superfluide. Finalement, nous nous intéressons aux pertes inélastiques par recombinaison à trois corps qui peuvent limiter la stabilité de nos nuages. Ces pertes sont intimement liées aux corrélations à courte distance présentes dans le système et sont ainsi connectées aux propriétés universelles du gaz quantique. Cela se manifeste notamment par l’apparition de dépendances en densité ou en température inusuelles du taux de perte lorsque le système devient fortement corrélé. Nous démontrons cet effet dans deux exemples où les interactions sont résonantes, le cas du gaz de Bose unitaire et celui de notre mélange de superfluides Bose-Fermi. Plus généralement, nos travaux montrent que ces pertes inélastiques peuvent être utilisées pour sonder les corrélations quantiques dans un système en fortes interactions. / Understanding the effect of interactions in quantum many-body systems presents some of the most compelling challenges in modern physics. Ultracold atoms have emerged as a versatile platform to engineer and investigate these strongly correlated systems. In this thesis, we study the properties of a mixture of Bose and Fermi superfluids with tunable interactions produced using ultracold vapors of ⁷Li and ⁶Li. We first study the hydrodynamic properties of the mixture by creating a counterflow between the superfluids. The relative motion only exhibit dissipation above a critical velocity that we measure in the BEC-BCS crossover. A numerical simulation of counterflowing condensates allows for a better understanding of the underlying mechanisms at play in the dynamics. In particular, this numerical study provides additional evidence that the onset of friction in our experiment is due to the simultaneous generation of elementary excitations in both superfluids. Finally, we consider the inelastic losses that occur via three-body recombination in our cold gases. This few-body process is intimately related to short-distance correlations and is thereby connected to the universal properties of the many-body system. This manifests as the apparition of an unusual dependence on density or temperature in the loss rate when increasing the interactions. We demonstrate this effect in two examples where interactions are resonant: the case of a dilute unitary Bose gas and the one of impurities weakly coupled to a unitary Fermi gas. More generally, our work shows that inelastic losses can be used to probe quantum correlations in a many-body system.

The diploma thesis deals with analytical description of vehicle - track dynamic interface. There are described basic analytical models which are subsequently extended in order to get a more precise description of dynamic phenomena. The aim is to compile a model that faithfully describes the dynamic phenomena in the track. These new compiled models are qualitatively compared with data obtained by measuring in the track.