Numerical simulations of fluid-structure interaction (FSI) problems were performed in an effort to verify and validate a commercially available FSI tool. This tool uses an iterative partitioned coupling scheme between CD-adapco's STAR-CCM+ finite volume fluid solver and Simulia's Abaqus finite element structural solver to simulate the FSI response of a system. Preliminary verification and validation work (VandV) was carried out to understand the numerical behavior of the codes individually and together as a FSI tool. Verification and Validation work that was completed included code order verification of the respective fluid and structural solvers with Couette-Pouiselle flow and Euler-Bernoulli beam theory. These results confirmed the 2nd order accuracy of the spatial discretizations used. Following that, a mixture of solution verifications and model calibrations was performed with the inclusion of the physics models implemented in the solution of the FSI problems. Solution verifications were completed for fluid and structural stand-alone models as well as for the coupled FSI solutions. These results re-confirmed the spatial order of accuracy but for more complex flows and physics models as well as the order of accuracy of the temporal discretizations. In lieu of a good material definition, model calibration is performed to reproduce the experimental results. This work used model calibration for both instances of hyperelastic materials which were presented in the literature as validation cases because these materials were defined as linear elastic. Calibrated, three dimensional models of the bow seal on the University of Michigan bow seal test platform showed the ability to reproduce the experimental results qualitatively through averaging of the forces and seal displacements. These simulations represent the only current 3D results for this case. One significant result of this study is the ability to visualize the flow around the seal and to directly measure the seal resistances at varying cushion pressures, seal immersions, forward speeds, and different seal materials. SES design analysis could greatly benefit from the inclusion of flexible seals in simulations, and this work is a positive step in that direction. In future work, the inclusion of more complex seal geometries and contact will further enhance the capability of this tool. / Ph. D.

This work extends a previously developed finite-volume overset-grid fluid flow solver to enable the characterisation of rigid-body-fluid interaction problems. To this end, several essential components have been developed and blended together. The inherent time-dependent nature of fluid-solid interaction problems is captured through the laminar transient incompressible Navier-Stokes equations for the fluid, and the Euler-Newton equations for rigid-body motion. First and second order accurate time discretisation schemes have been implemented for the former, whereas second and third order accurate time discretisation schemes have been made available for the latter. Without doubt the main advantage the overset-grid method offers regarding moving entities is the avoidance of the time consuming grid regeneration step, and the resulting grid distortion that can often cause numerical stability problems in the solution of the flow equations. Instead, body movement is achieved by the relative motion of a body fitted grid over a suitable background mesh. In this case, the governing equations of fluid flow are formulated using a Lagrangian, Eulerian, or hybrid flow description via the Arbitrary Lagrangian-Eulerian method. This entails the need to guarantee that mesh motion shall not disturb the flow field. With this in mind, the space conservation law has been hard-coded. The compliance of the space conservation law has the added benefit of preventing spurious mass sources from appearing due to mesh deformation. In this work, two-way fluid-solid interaction problems are solved via a partitioned approach. Coupling is achieved by implementing a Picard iteration algorithm. This allows for flexible degree of coupling specificationby the user. Furthermore, if strong coupling is desired, three variants of interface under-relaxation can be chosen to mitigate stability issues and to accelerate convergence. These include fixed, or two variants of Aitken’s adaptive under-relaxation factors. The software also allows to solve for one-way fluid-solid interaction problems in which the motion of the solid is prescribed. Verification of the core individual components of the software is carried out through the powerful method of manufactured solutions (MMS). This purely mathematically based exercise provides a picture of the order of accuracy of the implementation, and serves as a filter for coding errors which can be virtually impossible to detect by other means. Three instances of one-way fluid-solid interaction cases are compared with simulation results either from the literature, or from the OpenFOAM package. These include: flow within a piston cylinder assembly, flow induced by two oscillating cylinders, and flow induced by two rectangular plates exhibiting general planar motion. Three cases pertaining to the class of two-way fluid-interaction problems are presented. The flow generated by the free fall of a cylinder under the action of gravity is computed with the aid of an intermediate ‘motion tracking’ grid. The solution is compared with the one obtained using a vorticity based particle solver for validation purposes. Transverse vortex induced vibrations (VIV) of a circular cylinder immersed in a fluid, and subject to a stream are compared with experimental data. Finally, the fluttering motion of a rectangular plate under different scenarios is analysed.

A interação entre fluidos e estruturas caracteriza um problema multi-físico não linear e está presente numa grande variedade de áreas da engenharia. Este trabalho apresenta o desenvolvi mento de ferramentas computacionais com base no Método dos Elementos Finitos (MEF) para a análise de interação fluido-estrutura (IFE) considerando escoamentos com baixas velocidades. Dada a interdisciplinaridade do tema, se faz necessário o estudo em três diferentes assuntos: a dinâmica das estruturas computacional, a dinâmica dos fluidos computacional, e o problema de acoplamento. No caso da dinâmica das estruturas empregar-se um elemento finito que seja adequado para a simulação de problemas de IFE, que claramente demandam uma análise não linear geométrica, optando-se pelo emprego de uma formulação descrita em posições, a qual evita problemas relativos à aproximação de rotações finitas. Quanto à dinâmica dos fluidos computacional, é empregado um método estável e ao mesmo tempo sensível à movimentação da estrutura, utilizando a descrição Lagrangeana-Euleriana Arbitrária (ALE). Os casos considerados neste trabalho, assim como muitos dos problemas de engenharia, ocorrem com escoamentos em baixas velocidades, implicando na incompressibilidade do fluido, o que demanda, para um método estável, a utilização de elementos que atendam à condição de Ladyzhenskaya-Babuska-Brezzi (LBB). Além disso, é necessário também o emprego de métodos que consigam neutralizar as variações espúrias decorrentes da não-linearidade de possíveis escoamentos com convecção dominante e que surgem com a aplicação do processo clássico de Galerkin. Para superar esse problema, é aplicado o método Streamline-Upwind/Petrov-Galerkin (SUPG), que adiciona difusividade artificial na direção do escoamento, controlando a amplitude dos termos convectivos. No que se refere ao acoplamento fluido-casca, buscam-se modularidade e versatilidade adotando-se o modelo particionado. O modelo de acoplamento implementado garante ainda a utilização de malhas do fluido e da estrutura sem a necessidade de coincidência de nós. / Interaction between fluids and structures characterizes a nonlinear multi-physics problem presente in a wide range of engineering fields. This works presets the development of computational tools based on finite element method (FEM) for fluid-structure interaction (FSI) analysis considering low speed flows (incompressible), as a great part of the engineering problems. Given the topic multidisciplinary nature, it is necessary to study three different subjects: the computational structural dynamics, the computational fluid mechanics and the coupling problem. Regarding structural mechanics, we seek to employ a finite element adequate to FSI simulation, what clearly demands a geometric nonlinear analysis. We chose to employ shell elements with formulation in terms of positions, which avoids problems related to finite rotations approximations. Concerning computational fluid dynamics, we employ a stable method, at same time sensible o structural movements, which is written in the arbitrary Lagrangian-Eulerian (ALE) description. The flow incompressibility demands, for a stable method, the use of elements according to the Ladyzhenskaya-Bbuska-Brezzi (LBB) condition. It is also necessary to employ methods able to neutralize the spurious variations that appears from convection dominated flows when applying the standard Galerking method. In order to overcome this problem, we apply the Streamline-Upwind/Petrov-Galerkin (SUPG) method, which adds artificial diffusivity to the streamline direction, controlling spurious variations. Considering the fluid-shell coupling, we seek modularity and versatility, adopting the partitioned model. The developed coupling model ensure the use of fluid and structure meshes with no need for matching nodes.

A interação entre fluidos e estruturas caracteriza um problema multi-físico não linear e está presente numa grande variedade de áreas da engenharia. Este trabalho apresenta o desenvolvi mento de ferramentas computacionais com base no Método dos Elementos Finitos (MEF) para a análise de interação fluido-estrutura (IFE) considerando escoamentos com baixas velocidades. Dada a interdisciplinaridade do tema, se faz necessário o estudo em três diferentes assuntos: a dinâmica das estruturas computacional, a dinâmica dos fluidos computacional, e o problema de acoplamento. No caso da dinâmica das estruturas empregar-se um elemento finito que seja adequado para a simulação de problemas de IFE, que claramente demandam uma análise não linear geométrica, optando-se pelo emprego de uma formulação descrita em posições, a qual evita problemas relativos à aproximação de rotações finitas. Quanto à dinâmica dos fluidos computacional, é empregado um método estável e ao mesmo tempo sensível à movimentação da estrutura, utilizando a descrição Lagrangeana-Euleriana Arbitrária (ALE). Os casos considerados neste trabalho, assim como muitos dos problemas de engenharia, ocorrem com escoamentos em baixas velocidades, implicando na incompressibilidade do fluido, o que demanda, para um método estável, a utilização de elementos que atendam à condição de Ladyzhenskaya-Babuska-Brezzi (LBB). Além disso, é necessário também o emprego de métodos que consigam neutralizar as variações espúrias decorrentes da não-linearidade de possíveis escoamentos com convecção dominante e que surgem com a aplicação do processo clássico de Galerkin. Para superar esse problema, é aplicado o método Streamline-Upwind/Petrov-Galerkin (SUPG), que adiciona difusividade artificial na direção do escoamento, controlando a amplitude dos termos convectivos. No que se refere ao acoplamento fluido-casca, buscam-se modularidade e versatilidade adotando-se o modelo particionado. O modelo de acoplamento implementado garante ainda a utilização de malhas do fluido e da estrutura sem a necessidade de coincidência de nós. / Interaction between fluids and structures characterizes a nonlinear multi-physics problem presente in a wide range of engineering fields. This works presets the development of computational tools based on finite element method (FEM) for fluid-structure interaction (FSI) analysis considering low speed flows (incompressible), as a great part of the engineering problems. Given the topic multidisciplinary nature, it is necessary to study three different subjects: the computational structural dynamics, the computational fluid mechanics and the coupling problem. Regarding structural mechanics, we seek to employ a finite element adequate to FSI simulation, what clearly demands a geometric nonlinear analysis. We chose to employ shell elements with formulation in terms of positions, which avoids problems related to finite rotations approximations. Concerning computational fluid dynamics, we employ a stable method, at same time sensible o structural movements, which is written in the arbitrary Lagrangian-Eulerian (ALE) description. The flow incompressibility demands, for a stable method, the use of elements according to the Ladyzhenskaya-Bbuska-Brezzi (LBB) condition. It is also necessary to employ methods able to neutralize the spurious variations that appears from convection dominated flows when applying the standard Galerking method. In order to overcome this problem, we apply the Streamline-Upwind/Petrov-Galerkin (SUPG) method, which adds artificial diffusivity to the streamline direction, controlling spurious variations. Considering the fluid-shell coupling, we seek modularity and versatility, adopting the partitioned model. The developed coupling model ensure the use of fluid and structure meshes with no need for matching nodes.

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.