The first part of this Thesis deals with the analysis of piecewise-smooth mechanical systems and the definition of special stability criteria in presence of non-conservative follower forces. To illustrate the peculiar stability properties of this kind of dynamical system, a reference 2 d.o.f. structure has been considered, composed of a rigid bar, with one and constrained to slide, without friction, along a curved profile, whereas the other and is subject to a follower force. In particular, the curved constraint is assumed to be composed of two circular profiles, with different and opposite curvatures, defining two separated subsystems. Due to this jump in the curvature, located at the junction point between the curved profiles, the entire mechanical structure can be modelled by discontinuous equations of motion, the differential equations valid in each subsystem can be combined, leading to the definition of a piecewise-smooth dynamical system. When a follower force acts on the structure, an unexpected and counterintuitive behaviour may occur: although the two subsystems are stable when analysed separately, the composed structure is unstable and exhibits flutter-like exponentially-growing oscillations. This special form of instability, previously known only from a mathematical point of view, has been analysed in depth from an engineering perspective, thus finding a mechanical interpretation based on the concept of non-conservative follower load. Moreover, the goal of this work is also the definition of some stability criteria that may help the design of these mechanical piecewise-smooth systems, since classical theorems cannot be used for the investigation of equilibrium configurations located at the discontinuity. In the literature, this unusual behaviour has been explained, from a mathematical perspective, through the existence of a discontinuous invariant cone in the phase space. For this reason, starting from the mechanical system described above, the existence of invariant cones in 2 d.o.f. mechanical systems is investigated through Poincaré maps. A complete theoretical analysis on piecewise-smooth dynamical systems is presented and special mathematical properties have been discovered, valid for generic 2~d.o.f. piecewise-smooth mechanical systems, which are useful for the characterisation of the stability of the equilibrium configurations. Numerical tools are implemented for the analysis of a 2~d.o.f. piecewise-smooth mechanical system, valid for piecewise-linear cases and extendible to the nonlinear ones. A numerical code has been developed, with the aim of predicting the stability of a piecewise-linear dynamical system a priori, varying the mechanical parameters. Moreover, “design maps” are produced for a given subset of the parameters space, so that a system with a desired stable or unstable behaviour can easily be designed. The aforementioned results can find applications in soft actuation or energy harvesting. In particular, in systems devoted to exploiting the flutter-like instability, the range of design parameters can be extended by using piecewise-smooth instead of smooth structures, since unstable flutter-like behaviour is possible also when each subsystem is actually stable. The second part of this Thesis deals with the numerical analysis of an elastic cloak for transient flexural waves in Kirchhoff-Love plates and the design of special metamaterials for this goal. In the literature, relevant applications of transformation elastodynamics have revealed that flexural waves in thin elastic plates can be diverted and channelled, with the aim of shielding a given region of the ambient space. However, the theoretical transformations which define the elastic properties of this “invisibility cloak” lead to the presence of a strong compressive prestress, which may be unfeasible for real applications. Moreover, this theoretical cloak must present, at the same time, high bending stiffness and a null twisting rigidity. In this Thesis, an orthotropic meta-structural plate is proposed as an approximated elastic cloak and the presence of the prestress has been neglected in order to be closer to a realistic design. With the aim of estimating the performance of this approximated cloak, a Finite Element code is implemented, based on a sub-parametric technique. The tool allows the investigation of the sensitivity of specific stiffness parameters that may be difficult to match in a real cloak design. Moreover, the Finite Element code is extended to investigate a meta-plate interacting with a Winkler foundation, to analyse how the substrate modulus transforms in the cloak region. This second topic of the Thesis may find applications in the realization of approximated invisibility cloaks, which can be employed to reduce the destructive effects of earthquakes on civil structures or to shield mechanical components from unwanted vibrations.

Dans cette thèse nous étudions l'impact de perturbations magnétiques résonnantes sur la dynamique de la barrière de transport dans un tokamak. Pour cela nous avons réalisé des simulations numériques tridimensionnelles de turbulence dans le plasma de bord du tokamak. Nos simulations numériques ont reproduit les résultats expérimentaux observés dans différents tokamaks. Dans le régime de confinement amélioré (mode H), la barrière de transport n'est pas stable : elle effectue des oscillations de relaxation, qui partagent des caractéristiques communes avec les "modes localisés au bord'' (Edge Localized Modes, ELMs). Ces ELMs ont à la fois des avantages et des inconvénients. D'un côté, ils permettent d'évacuer les impuretés présentes dans le coe ur du plasma. Mais d'un autre côté, la charge thermique induite sur la paroi pendant un ELM peut endommager les matériaux de première paroi. Pour cette raison, ils doivent être contrôlés. Cette thèse s'inscrit dans le contexte du projet ITER actuellement en construction en France. Sur ITER, le contrôle des ELMs sera indispensable en raison de la quantité d'énergie évacuée. Parmi les différentes façons de contrôler les ELMs, les perturbations magnétiques résonnantes (Resonant Magnetic Perturbations, RMPs) semblent prometteuses. Ces perturbations magnétiques résonnantes sont créées par des bobines externes. Nous nous plaçons dans le cas du tokamak TEXTOR et nous considérons deux configurations pour les bobines externes : dans un premier temps, une perturbation magnétique résonnante comprenant plusieurs harmoniques, qui permet d'avoir une zone stochastique au bord du plasma lorsque les chaînes d'îlots magnétiques se superposent. / In this PhD thesis we study the impact of resonant magnetic perturbations on the transport barrier dynamics in a tokamak. In this goal we have performed turbulence tridimensional numerical simulations in the edge plasma of a tokamak, which reproduced the experimental results observed in different tokamaks. In the improved confinement regime (H mode), the transport barrier is not stable : it does relaxation oscillations, which share common features with the ``Edge Localized Modes'' (ELMs). These ELMs both have advantages and drawbacks. On the one hand, they enable to push away the impurities present in the plasma core. But on the other hand, the thermal load induced on the wall during an ELM can damage the first wall materials. For this reason, they must be controlled. This PhD thesis belongs to the frame of the ITER project, which is today in construction in France. On ITER the ELMs control will be compulsory due to the quantity of energy released. Among the different ways of controlling the ELMs, the resonant magnetic perturbations (RMPs) seem promising. These resonant magnetic perturbations are created by external coils. We consider the TEXTOR tokamak case and we consider two configurations for the external coils : first, a resonant magnetic perturbation with several harmonics, which enables to have a stochastic zone at the plasma edge when the magnetic island chains overlap ; then, a resonant magnetic perturbation with a single harmonic, which therefore creates a single magnetic island chain. In this PhD thesis, we focus on the non-axisymmetric equilibrium created in the plasma by the resonant magnetic perturbation.

[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.

ith one controlled extraction and one uncontrolled extraction, calculation of the flow channel at all stage, design and calculation of the regulation valve and create connection diagram of steam turbine and air cooled condenser. At the beginning of this work is an overview of manufacturers of steam turbines and their unified products. Master thesis was developer with G-Team, a.s. as using calculations and the instructions given in the recommended literature with supporting CFD simulations to determine the loss coefficients and FEA simulations to determine the eigenfrequencies blades.

<p align="justify">This work deals with the development of numerical methods for compressible flow simulation with application to the interaction of fluid flows and structural bodies.</p><p><p><p align="justify">First, we develop numerical methods based on multidimensional upwind residual distribution (RD) schemes. Theoretical results for the stability and accuracy of the methods are given. Then, the RD schemes for unsteady problems are extended for computations on moving meshes. As a second approach, cell centered and vertex centered finite volume (FV) schemes are considered. The RD schemes are compared to FV schemes by means of the 1D modified equation and by the comparison of the numerical results for scalar problems and system of Euler equations. We present a number of two and three dimensional steady and unsteady test cases, illustrating properties of the numerical methods. The results are compared with the theoretical solution and experimental data.</p><p><p><p align="justify">In the second part, a numerical method for fluid-structure interaction problems is developed. The problem is divided into three distinct sub-problems: Computational Fluid Dynamics, Computational Solid Mechanics and the problem of fluid mesh movement. The problem of Computational Solid Mechanics is formulated as a system of partial differential equations for an anisotropic elastic continuum and solved by the finite element method. The mesh movement is determined using the pseudo-elastic continuum approach and solved again by the finite element method. The coupling of the problems is achieved by a simple sub-iterative approach. Capabilities of the methods are demonstrated on computations of 2D supersonic panel flutter and 3D transonic flutter of the AGARD 445.6 wing. In the first case, the results are compared with the theoretical solution and the numerical computations given in the references. In the second case the comparison with experimental data is presented.</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Due to the increase of rotor size in horizontal axis wind turbine (HAWT) during the past 25 years in order to achieve higher power output, all wind turbine components and blades in particular, have to withstand higher structural loads. This upscalingproblem could be solved by applying technologies capable of reducing aerodynamic loads the rotor has to withstand, either with passive or active control solutions. These control devices and techniques can reduce the fatigue load upon the blades up to 40% and therefore less maintenance is needed, resulting in an important money savings for the wind farm manager. This project consists in a study of load control techniques for offshore wind turbines from an aerodynamic and aeroelastic point ofview, with the aim to assess a cost effective, robust and reliable solution which could operate maintenance free in quite hostile environments. The first part of this study involves 2D and 3D aerodynamic and aeroelastic simulations to validate the computational model with experimental data and to analyze the interaction between the fluid and the structure. The second part of this study is an assessment of the unsteady aerodynamic loads produced by a wind gust over the blades and to verify how a trailing edge flap would influence the aerodynamic control parameters for the selected wind turbine blade. / På grund av ökningen av rotorstorleken hos horisontella vindturbiner (HAWT) under de senaste 25 åren, en design som har uppstod för att uppnå högre effekt, måste alla vindkraftkomponenter och blad stå emot högre strukturella belastningar. Detta uppskalningsproblem kan lösas genom att använda metoder som kan minska aerodynamiska belastningar som rotorn måste tåla, antingen med passiva eller aktiva styrlösningar. Dessa kontrollanordningar och tekniker kan minska utmattningsbelastningen på bladen med upp till 40 % och därför behövs mindre underhåll, vilket resulterar i viktiga besparingar för vindkraftsägaren. Detta projekt består av en studie av lastkontrolltekniker för havsbaserade vindkraftverk ur en aerodynamisk och aeroelastisk synvinkel, i syfte att bedöma en kostnadseffektiv, robust och pålitlig lösning som kan fungera underhållsfri i tuffa miljöer. Den första delen av denna studie involverar 2D- och 3D-aerodynamiska och aeroelastiska simuleringar för att validera beräkningsmodellen med experimentella data och för att analysera interaktionen mellan fluiden och strukturen. Den andra delen av denna studie är en bedömning av de ojämna aerodynamiska belastningarna som produceras av ett vindkast över bladen och för att verifiera hur en bakkantklaff skulle påverka de aerodynamiska styrparametrarna för det valda vindturbinbladet.