<p>Positive displacement machines (PDMs) operate based on the principle of positive displacement, which necessitates a periodic alteration of volume. This volume variation is accomplished through relative motion between machine components. PDMs find extensive applications in diverse domains, encompassing fluid power systems, lubrication systems, fluid transport systems, fuel injection systems, and more. The primary distinction among PDMs lies in the geometric mechanisms employed for fluid displacement, as well as the flow distribution mechanisms they employ. PDMs can be broadly classified into piston machines, vane machines, screw machines, and gear machines. In fluid power systems, the most commonly used PDMs are the piston and gear machines. Piston machines can be further classified into radial piston machines, in-line piston machines, and axial piston machines. The most commonly used piston machines are the axial piston machine owing to their superior efficiency and compactness. Gear machines can be further classified into external gear machines, internal gear machines, and annular gear machines. The most commonly used gear machine is the external gear machine owing to its price.</p> <p><br></p> <p>PDMs typically involve multiple solid bodies in relative motion, with micron-level gaps between them. These gaps, known as lubricating interfaces, present a significant design challenge during the machine development process. They are a primary source of power losses and play a crucial role in determining the efficiency and durability of the machine. The lubricating interfaces must effectively balance loads and maintain a high-pressure fluid seal. Achieving this delicate balance necessitates a comprehensive understanding of the underlying physical phenomena. Lubricating interfaces generate substantial heat due to viscous dissipation, which directly impacts the operation of the entire machine. The viscosity of the working fluid rapidly decays with temperature, causing the warmer fluid within the lubricating interface to possess lower viscosity. Consequently, it can support lesser loads and is more prone to leakage. Moreover, as the solid bodies enclosing the warmer fluid heat up, they undergo thermal expansion, further changing the clearance and leading to a decline in performance. Additionally, the elevated temperature of the fluid within the lubricating interface affects the compressibility of the displacement chamber fluid, thereby influencing the pressurization characteristics of the entire unit. Thus, thermal effects play a critical role in the performance of PDMs.</p> <p><br></p> <p>  The ever-increasing market demand for more compact, efficient, and reliable designs requires a continuous process of design improvements over previous designs, and sometimes completely new designs. Sophisticated simulation tools are a necessity for such a design process. Additionally, these simulation tools also prove to be valuable in formulating design modifications in case of underperforming designs. Due to the complexity associated with the operation of such units, the simulation tools need to capture a wide variety of physical phenomena. Over the past few decades, owing to the increasing computing power of the desktop computer, several simulation tools have been proposed across the literature to aid the design process of such machines with each having limitations of their own.</p> <p><br></p> <p>  The objective of the present thesis is to propose a modeling approach that assists in the design process of positive displacement machines, addressing various limitations identified in the existing literature. The approach is intentionally designed to be generic, enabling its application across a diverse range of positive displacement machines. The modeling approach encompasses three distinct domains: the displacement chamber fluid domain, the lubricating interface fluid domain, and the solid domain. A novel thermal model that integrates all three domains is introduced. </p> <p><br></p> <p>  To validate the effectiveness of the proposed modeling approach, two separate validation studies are conducted. The first study focuses on a model for an isolated piston/cylinder interface of an axial piston machine, operating under the mixed lubrication regime. The model demonstrates a strong agreement with the measured data. The second study involves steady-state measurements of an entire axial piston machine. The model is validated by comparing the steady-state flow characteristics and temperature distribution on the valveplate, both of which are accurately captured by a single fully coupled model. The modeling approach developed in this study, specifically, the energy conservation in the lubricating interface, heat transfer in the solid bodies, and thermal deformation in the solid bodies are all generalized for applicability in different types of PDMs. However, the results presented in this thesis pertain to an axial piston machine.</p>

<p>This thesis is concerned with a theoretical and experimental investigation of a duckbill valve (DBV). Duckbill valves are non-return valves made of a composite material, which deforms to open the valve as the upstream pressure increases. The head-discharge behavior is a fluid-structure interaction (FSI) problem since the discharge depends on the valve opening that in turn depends on the pressure distribution along the valve produced by the discharge. To design a duckbill valve, a theoretical model is required, which will predict the head-discharge characteristics as a function of the fluid flow through the valve and the valve material and geometry.</p> <p>The particular valves of concern in this study, which can be very large, are made from laminated, fiber-reinforced rubber. Thus, the structural problem has strong material as well as geometric nonlinearities due to large deflections. Clearly, a fully coupled FSI analysis using three-dimensional viscous flow would be very challenging and therefore, a simplified approach was sought that treats the essential aspects of the problem in a tractable way. For this purpose, the DBV was modeled using thick shell finite elements, which included the laminates of hyperelastic rubber and orthotropic fabric reinforcement. The finite element method (FEM) was simplified by assuming that the arch side edges of the valve were clamped. The unsteady 1D flow equation was used to model the ideal fluid dynamics that enabled a full FSI analysis. Moreover, verification for the ideal flow was carried out using a transient, Reynolds-averaged Navier-Stokes finite volume solver for the viscous flow corresponding to the deformed valve predicted by the simplified FSI model.</p> <p>In order to validate the predictions of the FSI simulations, an experimental study was performed at several mass flow rates. Pressure drops along the water tunnel, valve inlet and outlet velocity profiles were measured, as well as valve opening deformations as functions of upstream pressures.</p> <p>Additionally, the valve deformations under various back pressures were analyzed when the downstream pressure exceeded the upstream pressure using the layered shell model without coupling and with simplified boundary constraints to avoid solving the contact problem for the inward-deformed duckbill valve. Flow-induced vibration (FIV) of the valve at small openings was also examined to improve our understanding of the valve stability behaviour. Some interesting valve oscillation phenomena were observed.</p> <p>Conclusions are drawn regarding the FSI model on the predictions and comparisons with the experimental results. The transient 1D flow equation has been demonstrated to adequately model the fluid dynamics of a duckbill valve, largely due to the fact that viscous effects are negligible except when the valve is operating at very small openings. Fiber reinforcement of the layered composite rubber was found to play an important role in controlling duckbill valve material stretch, especially at large openings. The model predicts oscillations at small openings but more research is required to better understand this behaviour.</p> / Doctor of Philosophy (PhD)

<p dir="ltr">Positive displacement machines are used in various industries spanning the power spectrum, from industrial robotics to heavy construction equipment to aviation. These machines should be highly efficient, compact, and reliable. It is very advantageous for designers to use virtual simulations to design and improve the performance of these units as they significantly reduce cost and downtime. The recent trends of electrification and the goal to increase power density force these units to work at higher pressures and higher rotational speeds while maintaining their efficiencies and reliability. This push means that the simulation models need to advance to account for various aspects during the operation of these machines. </p><p dir="ltr">These machines typically have several bodies in relative motion with each other. Quantifying these motions and solving for their effect on the fluid enclosed are vital as they influence the machine's performance. The push towards higher rotational speeds introduces unwanted cavitation and aeration in these units. To model these effects, keeping the design evaluation time low is key for a designer. The lumped parameter approach offers the benefit of computational speed, but a major drawback that comes along with it is that it typically assumes fluid inertia to be negligible. These effects cannot be ignored, as quantifying and making design considerations to negate these effects can be beneficial. Therefore, this thesis addresses these key challenges of cavitation dynamics, body dynamics, and accounting for fluid inertia effects using a lumped parameter formulation.</p><p dir="ltr">To account for dynamics features related to cavitation, this thesis proposes a novel approach combining the two types of cavitation, i.e., gaseous and vaporous, by considering that both vapor and undissolved gas co-occupy a spherical bubble. The size of the spherical bubble is solved using the Rayleigh-Plesset equation, and the transfer of gas through the bubble interface is solved using Henry's Law and diffusion of the dissolved gas in the liquid. These equations are coupled with a novel pressure derivative equation. To account for body dynamics, this thesis introduces a novel approach for solving the positions of the bodies of a hydraulic machine while introducing new methods to solve contact dynamics and the application of Elasto Hydrodynamic Lubrication (EHL) friction at those contact locations. This thesis also proposes strategies to account for fluid inertia effects in a lumped parameter-based approach, taking as a reference an External Gear Machine. This thesis proposes a method to study the effects of fluid inertia on the pressurization and depressurization of the tooth space volumes of these units. The approach is based on considering the fluid inertia in the pressurization grooves and inside the control volumes with a peculiar sub-division. Further, frequency-dependent friction is also modeled to provide realistic damping of the fluid inside these channels.</p><p dir="ltr">To show the validity of the proposed dynamic cavitation model, the instantaneous pressure of a closed fluid volume undergoing expansion/compression is compared with multiple experimental sources, showing an improvement in accuracy compared to existing models. This modeling is then further applied to a gerotor machine and validated with experiments. Integrating this modeling technique with current displacement chamber simulation can further improve the understanding of cavitation in hydraulic systems. Formulations for body dynamics are tested on a prototype Gerotor and Vane unit. For both gerotor and vane units, comparisons of simulation results to experimental results for various dynamic quantities, such as pressure ripple, volumetric, and hydromechanical efficiency for multiple operating conditions, have been done. Extensive validation is performed for the case of gerotors where shaft torque ripple and the motion of the outer gear is experimentally validated. The thesis also comments on the distribution of the different torque loss contributions. The model for fluid inertia effects has been validated by comparing the lumped parameter model with a full three-dimensional Navier Stokes solver. The quantities compared, such as tooth space volume pressures and outlet volumetric flow rate, show a good match between the two approaches for varying operating speeds. A comparison with the experiments supports the modeling approach as well. The thesis also discusses which operating conditions and geometries play a significant role that governs the necessity to model such fluid inertia effects in the first place.</p>

In this thesis, a miniaturized energy harvester system is developed. The energy harvester converts fluid kinetic energy into electrical energy without using any rotating components. The working principle of the energy harvester is based on the so called vortex-induced vibration. Such systems have the potential to provide energy for wireless sensor networks in the field of inline measurements for gas, oil or water transportation systems. The theoretical background of the vortex-induced vibration (VIV) is studied. Based on the studies, a fluid-structure interaction simulation is carried out to optimize the structure of the energy harvester. As result, the conversion efficiency is significantly improved, which is experimentally confirmed. A series of demonstrators are manufactured according to the simulation and optimization results. It is tested on a self-constructed test bench. To further improve the performance, an electromagnetic generator is proposed, and therefore, a multimethod demonstrator realized. The demonstrators are working in air flow already at a velocity of 2 m/s, and reach the maximum efficiency at 3.6 m/s. This performance ranks among the best published results and is discussed in detail. / In der vorliegenden Arbeit wird ein miniaturisiertes Energiegewinnungssystem entwickelt, das unter Verzicht auf rotierende Komponenten kinetische Strömungsenergie in elektrische Energie umwandelt. Die Funktion dieses Wandlers basiert auf der sogenannten wirbelinduzierten Vibration. Derartige Systeme besitzen unter anderem das Potenzial, drahtlose Sensornetzwerke zur Erfassung von Messdaten in Gas-, Öl- oder Wassertransportsystemen mit Energie zu versorgen zu können. In der Arbeit wird der theoretische Hintergrund der wirbelinduzierten Vibration untersucht und darauf basierend werden Fluid-Struktur-Wechselwirkungssimulationen zur Strukturoptimierung durchgeführt in deren Ergebnis eine theoretische Verbesserung der Effizienz des Wandlers um ein Mehrfaches erreicht wird, die auch praktisch bestätigt wird. Unter Berücksichtigung der Simulations- und Optimierungsergebnisse wurden eine Reihe von Demonstratoren gefertigt, die auf einem selbst konstruierten Prüfstand getestet wurden. Zur weiteren Erhöhung der Leistungsfähigkeit des Wandlers wird ein zusätzlicher elektromagnetischer Generator vorgeschlagen und damit ein Multi-Methoden-Demonstrator technisch realisiert. Die Demonstratoren arbeiten in strömender Luft bereits bei Geschwindigkeiten von 2 m/s und erreichen bei 3,6 m/s ihre maximale Effizienz. Die erreichten Ergebnisse ordnen sich im Vergleich mit denen aus entsprechenden Publikationen vorn ein und werden ausführlich diskutiert.

Cette thèse est consacrée à la modélisation de la ventilation mécanique chez l'humain et à l'analyse numérique des systèmes en découlant. Des simulations directes d'écoulement d'air dans l'ensemble des voies aériennes étant impossibles (maillages indisponibles et géométrie trop complexe), il est nécessaire de considérer un domaine d'intérêt réduit, qui implique de travailler dans une géométrie tronquée, comportant des frontières artificielles ou encore de considérer des modèles réduits simples mais représentatifs. Si on cherche à effectuer des simulations numériques 3D où l'écoulement du fluide est décrit par les équations de Navier-Stokes, différentes problématiques sont soulevées :- Si on considère que la ventilation est la conséquence de différences de pression, les conditions aux limites associées sont des conditions de type Neumann. Cela aboutit à des questions théoriques en terme d'existence et d'unicité de solution et à des questions numériques en terme de choix de schémas et de méthodes adaptées.- Lorsque l'on travaille dans un domaine tronqué, il peut être nécessaire de prendre en compte les phénomènes non décrits grâce à des modèles réduits appropriés. Ici nous considérons des modèles 0D. Ces couplages 3D/0D sont à l'origine d'instabilités numériques qu'on étudie mathématiquement et numériquement dans ce manuscrit. Par ailleurs, lorsqu'on s'intéresse à des régimes de respiration forcée, les modèles usuels linéaires sont invalidés par les expériences. Afin d'observer les différences entre les résultats expérimentaux et numériques, il est nécessaire de prendre en compte plusieurs types de non linéarités, comme la déformation du domaine ou les phénomènes de type Bernoulli. Une approche par modèles réduits est adoptée dans ce travail.Pour finir, on a cherché à valider les modèles obtenus en comparant des résultats numériques et des résultats expérimentaux dans le cadre d'un travail interdisciplinaire.Parvenir à modéliser et simuler ces écoulements permet de mieux comprendre les phénomènes et paramètres qui entrent en jeu lors de pathologies (asthme, emphysème...). Un des objectifs à moyen terme est d'étudier l'influence du mélange hélium-oxygène sur le dépôt d'aérosol, toujours dans le cadre du travail interdisciplinaire. A plus long terme, l'application de ces modèles à des situations pathologiques pourrait permettre de construire des outils d'aide à la décision dans le domaine médical (compréhension de la pathologie, optimisation de thérapie...). / In this thesis, we study the modelling of the human mecanical ventilation and the numerical analysis of linked systems. Direct simulations of air flow in the whole airways are impossible (complex geometry, unavailable meshes). Then a reduced area of interest can be considered, working with reduced geometries and artificial boundaries. One can also use reduced models, simple but realistic. If one try to make 3D numerical simulations where the fluid flow is described by the Navier-Stokes equations, various issues are raised:- If we consider that ventilation is the result of pressure drops, the associated boundary conditions are Neumann conditions. It leads to theoretical questions in terms of existence and uniqueness of solution and numerical issues in terms of scheme choice and appropriate numerical methods.- When working in a truncated domain, it may be necessary to take into account non-described phenomena with appropriate models. Here we consider 0D models. These 3D/0D couplings imply numerical instabilities that we mathematically and numerically study in this thesis.Furthermore, when we focus on forced breathing, linear usual models are invalidated by experiments. In order to observe the differences between the experimental and numerical results, it is necessary to take into account several types of non-linearities, such as deformation of the domain or the Bernoulli phenomenon. A reduced model approach is adopted in this work. Finally, we sought to validate the obtained models by comparing numerical and experimental results in the context of interdisciplinary work.Achieving model and simulate these flows allow to better understand phenomena and parameters that come into play in diseases (asthma, emphysema ...). A medium-term objective is to study the influence of helium-oxygen mixture in the aerosol deposition. In the longer term, the application of these models to pathological situations could afford to build decision support tools in the medical field (understanding of pathology, therapy optimization ...).

O presente trabalho consiste no uso do Método dos Elementos Finitos (MEF) para a análise de interação fluido-estruturas de barras com foco em problemas transientes envolvendo explosões ou outras ações com propagação de ondas de choque. Para isso é necessário o estudo de três diferentes aspectos: a dinâmica das estruturas computacional, a dinâmica dos fluidos computacional e o problema do acoplamento. No caso da dinâmica das estruturas computacional deve-se identificar em função da cinemática de deformações, quais são os requisitos para que um elemento seja adequado para analisar tais problemas, tendo em vista que a formulação deve admitir grandes deslocamentos. Para evitar problemas relacionados com aproximações de rotações finitas, opta-se por empregar uma formulação descrita em termos de posições e que leva em consideração os efeitos de empenamento da seção transversal. No caso da dinâmica dos fluidos computacional, busca-se uma formulação para escoamentos compressíveis que seja estável e ao mesmo tempo sensível ao movimento da estrutura, sendo empregado um algoritmo de integração temporal explícito baseado em características com as equações governantes descritas na forma Lagrangeana-Euleriana Arbitrária (ALE). No que se refere ao acoplamento, busca-se modularidade e versatilidade, empregando-se um modelo particionado fraco (explícito) de acoplamento e técnicas de transferência das condições de contorno (Dirichlet-Neummann), sendo estudados os efeitos de utilizar transferência bidirecional ou unidirecional dessas condições de contorno. / This work consists in the use of the Finite Element Method (FEM) for numerical analysis of fluid-bar structures, focusing on transient problems involving explosions or other actions with shock waves propagation. For this purpose, one needs to study three different aspects: the computational structural dynamics, the computational fluid dynamics and the coupling problem. Regarding computational structural dynamics, one need firstly to identify the requirements for an element to be adequate to analyze such problems, taking into account the fact that such element should admit large displacements. In order to avoid problems related to finite rotation approximations and to give a realist representation of a 3D bar structure, we chose a formulation defined in terms of positions and that considers the cross-section warping effects. Regarding computational fluid dynamics, we seek for a stable formulation for compressible flows, and at same time, sensitive to the movement of the structure, leading to an explicit time integration algorithm based on characteristics with governing equations described in the Arbitrary Lagrangian-Eulerian (ALE) form. Regarding to coupling, we chose to use a weak (explicit) partitioning coupling model in order to ensure modularity and versatility. The developed coupling scheme is bases on boundary conditions transfer techniques (Dirichlet-Neummann), and we study the effects of using bidirectional or unidirectional boundary conditions transfers.

Le but de cette thèse pluridisciplinaire est d’étudier le problème de l’interaction fluide-structure à partir du point de vue mathématique et physique. Des problèmes d’interaction d’un fluide visqueux avec une structure élastique décrivent, par exemple, des interactions entre le manteau terrestre et de la croûte terrestre, le sang et la paroi vasculaire dans un vaisseau sanguin, etc. En génie l’interaction fluide visqueux-structure apparaît lors de la formation de solution colloïdale quand un laser passe à travers le fluide influençant le substrat (ablation laser dans un liquide). Fusion sélective au laser (FSL) est utilisée pour étudier le comportement des contraintes résiduelles en dépendance des propriétés thermoélastiques et mécaniques du matériau et des formes variées des cordons rechargés. A partir du point de vue mathématique le système couplé “flux fluide visqueux – plaque mince élastique” en 3D lorsque l’épaisseur de la plaque, E, tend vers zéro, tandis que la densité et le module de Young du matériau élastique sont d’ordre 1 et E-3, respectivement, est considéré. Le solide est couché par le fluide qui occupe un domaine épais. La modélisation multi-échelle est effectuée pour la partie élastique. Le développement asymptotique complet est construit lorsque E tend vers zéro. L’existence, la régularité et l’unicité de la solution pour le problème initial sont étudiées au moyen de techniques variationnelles. La méthode de décomposition asymptotique partielle du domaine est appliquée pour le système couplé. L’erreur de la méthode est évaluée / The goal of this multi-disciplinary thesis is to study the fluid-structure interaction problem from mathematical and physical viewpoints. Viscous fluid-structure interaction problems describe, for example, interactions between the Earth mantle and the Earth crust, the blood and the vascular wall in a blood vessels, etc. In engineering viscous fluid-structure interaction appears during colloidal solution formation when a laser pierce through the fluid influencing the substrate (laser ablation in a liquid). Selective laser melting (SLM) is used to study the behavior of residual stresses depending on the thermoelastic and mechanical properties of the material and on various forms of reloaded beads. From mathematical point of view the coupled system “viscous fluid flow-thin elastic plate” in 3D when the thickness of the plate, E, tends to zero, while the density and the Young’s modulus of the plate material are of order 1 and E-3, respectively, is considered. The plate lies on the fluid which occupies a thick domain. The multi-scale modeling is performed for the elastic part. The complete asymptotic expansion is constructed when E tends to zero. The existence, the regularity and the uniqueness of the solution for the original problem are studied by means of variational techniques. The method of asymptotic partial domain decomposition is applied for the coupled system. The error of the method is evaluated

A prevalent morphology in the microscopic world of artificial microswimmers, bacteria and viruses is that of a helix. The intriguingly different physics at play at the small scale level make it necessary for bacteria to employ swimming strategies different from our everyday experience, such as the rotation of a helical filament. Bio-inspired microswimmers that mimic bacterial locomotion achieve propulsion at the microscale level using magnetically actuated, rotating helical filaments. A promising application of these artificial microswimmers is in non-invasive medicine, for drug delivery to tumours or microsurgery. Two crucial features need to be addressed in the design of microswimmers. First, the ability to selectively control large ensembles and second, the adaptivity to move through complex conduit geometries, such as the constrictions and curves of the tortuous tumour microvasculature. In this dissertation, a mechanics-based selective control mechanism for magnetic microswimmers is proposed, and a model and simulation of an elastic helix passing through a constricted microchannel are developed. Thereafter, a theoretical framework is developed for the propulsion by stiff elastic filaments in viscous fluids. In order to address this fluid-structure problem, a pertubative, asymptotic, elastohydrodynamic approach is used to characterise the deformation that arises from and in turn affects the motion. This framework is applied to the helical filaments of bacteria and magnetically actuated microswimmers. The dissertation then turns to the sub-bacterial scale of bacteriophage viruses, 'phages' for short, that infect bacteria by ejecting their genetic material and replicating inside their host. The valuable insight that phages can offer in our fight against pathogenic bacteria and the possibility of phage therapy as an alternative to antibiotics, are of paramount importance to tackle antibiotics resistance. In contrast to typical phages, flagellotropic phages first attach to bacterial flagella, and have the striking ability to reach the cell body for infection, despite their lack of independent motion. The last part of the dissertation develops the first theoretical model for the nut-and-bolt mechanism (proposed by Berg and Anderson in 1973). A nut being rotated will move along a bolt. Similarly, a phage wraps itself around a flagellum possessing helical grooves, and exploits the rotation of the flagellum in order to passively travel along and towards the cell body, according to this mechanism. The predictions from the model agree with experimental observations with respect to directionality, speed and the requirements for succesful translocation.

L’objectif de ce travail porte sur le développement d’un modèle d’interaction fluide-structure adapté à la dynamique des éoliennes de grandes tailles avec des pales flexibles qui se déforment de manière significative sous l’effet de la pression exercée par le vent. Le modèle développé est basé sur une approche efficace d’IFS partitionnée pour un fluide incompressible et non visqueux en interaction avec une structure flexible soumise a des grandes transformations. Il permet de fournir une meilleure estimation de la charge aérodynamique et de la réponse dynamique associée du système (pales, mat, attachements, câbles) avec un temps de calcul raisonnable et pour des simulations sur des longues périodes. Pour la modélisation structurale, un élément fini de type solide 3D est développé pour l’étude dynamique des pales d’éolienne soumises à des grands déplacements et des grandes rotations. Une amélioration du comportement en flexion est proposée par l’introduction des degrés de liberté en rotation et l’enrichissement du champ de déplacements afin de décrire plus précisément la flexibilité des pales. Cet élément solide est apte de capter des modes de hautes fréquences qui peuvent s’avérer néfastes pour la stabilité du calcul. Deux techniques sont donc proposées pour les contrôler : la régularisation de la matrice masse et le développement des schémas d’intégration robustes de conservation et de dissipation d’énergie. Les chargements aérodynamiques sont modélisés en utilisant la Panel Method. Il s’agit d’une méthode aux frontières, relativement rapide par rapport à la CFD mais suffisamment précise pour calculer la distribution de la pression exercée sur la pale. Les modèles fluide et structure interagissent via un algorithme de couplage partitionné itératif dans lequel des considérations particulières sont prises en compte dans le contexte des grandes transformations. Dans un effort visant à instaurer un indicateur de fatigue dans la méthodologie proposée, des câbles précontraints sont introduits reliant le mat de l’éolienne au support. Une nouvelle formulation complémentaire en termes de contraintes est ainsi développée pour l’analyse dynamique des câbles 3D en comportement élasto-visco-plastique. Chaque méthode proposée a été d’abord validée sur des cas tests pertinents. Par la suite, des simulations numériques d’éoliennes avec des pales flexibles sont effectuées en vue d’affiner la compréhension de leur comportement dynamique et l’intérêt que la flexibilité des pales peut apporter à leur fonctionnement. / In this work, a numerical model of fluid-structure interaction is developed for dynamic analysis of giant wind turbines with flexible blades that can deflect significantly under wind loading. The model is based on an efficient partitioned FSI approach for incompressible and inviscid flow interacting with a flexible structure undergoing large transformations. It seeks to provide the best estimate of true design aerodynamic load and the associated dynamic response of such system (blades, tower, attachments, cables). To model the structure, we developed a 3D solid element to analyze geometrically nonlinear statics and dynamics of wind turbine blades undergoing large displacements and rotations. The 3D solid bending behavior is improved by introducing rotational degrees of freedom and enriching the approximation of displacement field in order to describe the flexibility of the blades more accurately. This solid iscapable of representing high frequencies modes which should be taken under control. Thus, we proposed a regularized form of the mass matrix and robust time-stepping schemes based on energy conservation and dissipation. Aerodynamic loads are modeled by using the 3D Vortex Panel Method. Such boundary method is relatively fast to calculate pressure distribution compared to CFD and provides enough precision. The aerodynamic and structural parts interact with each other via a partitioned coupling scheme with iterative procedure where special considerations are taken into account for large overall motion. In an effort to introduce a fatigue indicator within the proposed framework, pre-stressed cables are added to the wind turbine, connecting the tower to the support and providing more stability. Therefore, a novel complementary force-based finite element formulation is constructed for dynamic analysis of elasto-viscoplastic cables. Each of theproposed methods is first validated with differents estexamples.Then,several numerical simulations of full-scale wind turbines are performed in order to better understand its dynamic behavior and to eventually optimize its operation.

Cette thèse de doctorat porte sur le calcul par la méthode des éléments finis du comportement dynamique de réservoirs élastiques précontraints contenant un liquide interne à surface libre. Nous considérons que la pression hydrostatique exercée par le fluide interne incompressible sur les parois flexibles du réservoir est à l’origine de grands déplacements, conduisant ainsi à un état d’équilibre non-linéaire géométrique. Le changement de raideur lié à cet état précontraint induit un décalage des fréquences de résonances du problème de vibrations linéaires couplées.L’objectif principal du travail est donc d’estimer, par des approches numériques précises et efficaces, l’influence des non-linéarités géométriques sur le comportement hydroélastique du système réservoir/liquide interne autour de différentes configurations d’équilibre. La méthodologie développée s’effectue en deux étapes. La première consiste à calculer l’état statique non-linéaire par une approche éléments finis lagrangienne totale. L’action du fluide sur la structure est ici modélisée par des forces suiveuses hydrostatiques. La deuxième étape porte sur le calcul des vibrations couplées linéarisées. Un modèle d’ordre réduit original est notamment proposé pour limiter les coûts de calcul associés à l’estimation de l’effet de masse ajoutée. Enfin, divers exemples sont proposés et comparés à des résultats de la littérature (issus de simulations numériques ou d’essais expérimentaux) pour montrer l’efficacité et la validité des différentes approches numériques développées dans ce travail. / This doctoral thesis focuses on the calculation by the finite element method of the dynamic behavior of prestressed elastic tanks containing an internal liquid with a free surface. We consider that the hydrostatic pressure exerted by the incompressible internal fluid on the flexible walls of the tank causes large displacements, thus leading to a geometric non-linear equilibrium state. The change of stiffness related to this prestressed state induces a shift in the resonance frequencies of the coupled linear vibration problem. The main objective of the work is therefore to estimate, through precise and efficient numerical approaches, the influence of geometric nonlinearities on the hydroelastic behavior of the reservoir/internal liquid system around different equilibrium configurations. The methodology developed is carried out in two stages. The first one consists in calculating the non-linear static state by a total Lagrangian finite element approach.The action of the fluid on the structure is modelled here by hydrostatic following forces. The second step is the calculation of linearized coupled vibrations. In particular, an original reduced order model is proposed to limit the calculation costs associated with the estimation of the added mass effect. Finally, various examples are proposed and compared with results from the literature (from numerical simulations or experimental tests) to show the effectiveness and validity of the different numerical approaches developed in this work.