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<p>  </p> <p>Three key challenges in developing advanced image-based computational models of cerebral aneurysms are: (i) disentangling the effect of biomechanics and confounding clinical risk factors on aneurysmal progression, (ii) accounting for arterial wall mechanics, and (iii) incorporating the effect of surrounding tissue support on vessel motion and deformation. This thesis addresses these knowledge gaps by developing fluid-structure interaction (FSI) models of subject-specific geometries of cerebral aneurysms to elucidate the effect of coupled hemodynamics and biomechanics. A consistent methodology for obtaining physiologically realistic computational FSI models from standard-of-care imaging data is developed. In this process, a novel technique to estimate heterogeneous arterial wall thickness in the absence of subject-specific arterial wall imaging data is proposed. To address a limitation in the mesh generation workflow of the state-of-the-art cardiovascular flow modeling tool SimVascular, generation of meshes with boundary-layer mesh refinement near the blood-vessel wall interface is proposed for computational geometries with nonuniform wall thickness. Computational murine models of thoracic aortic aneurysms were developed using the proposed methodology. These models were used to inform external tissue support boundary conditions for human cerebral aneurysm subjects via a scaling analysis. Then, the methodology was applied to subjects with multiple unruptured cerebral aneurysms. A comparative computational FSI analysis of aneurysmal biomechanics was performed for each subject-specific pair of computational models for the stable and growing aneurysms, which act as self-controls for confounding clinical risk factors. A higher percentage of area exposed to low shear and high median-peak-systolic arterial wall deformation, each by factors of 1.5 to 2, was observed in growing aneurysms, compared to stable ones. Furthermore, a novel metric – the oscillatory stress index (OStI) – was defined and proposed to indicate locations of oscillating arterial wall stresses. Growing aneurysms demonstrated significant areas with a combination of low wall shear and low OStI, which were hypothesized to be associated with regions of collagen degradation and remodeling. On the other hand, such regions were either absent (or were a small percentage of the total aneurysmal area) in the stable cases. This thesis, therefore, provides a groundwork for future studies, with larger patient cohorts, which will evaluate the role of these biomechanical parameters in cerebral aneurysm growth.</p>

Le problème de l’hydroplannage a fait l’objet de peu de travaux de simulation jusqu’à présent du fait de sa complexité : couplage fluide-structure, complexité de la structure du pneu du fait des matériaux en présence, contact avec l’asphalte, complexité de l’écoulement fluide résultant (interface extrêmement complexe,assèchement de la route, ventilation, développement éventuel de la turbulence et de cavitation). Dans ce contexte, Michelin, Centrale Nantes et NextFlowSoftware ont cherché récemment à évaluer la capacité du solveur SPH développé par ces deux derniers pour classifier des pneumatiques en fonction de la géométrie de leurs structures surfaciques, sans prendre en compte la phase gazeuse. Cela a permis de démontrer la faisabilité de telles simulations par méthode SPH, et même d’obtenir de bons résultats avec pour avantages de s’absoudre des difficultés liées au maillage. L’autre avantage conséquent d’utiliser la méthode SPH pour modéliser le fluide dans ce contexte est apparu dans sa capacité à se coupler relativement aisément à des solveurs classiques de type Eléments Finis pour le problème structurel. L’objectif du doctorat est triple, poursuivre la qualification du couplage SPH–Eléments Finis, en particulier en termes énergétiques, développer des schémas permettant d’assurer un bon compromis stabilité / précision / temps de calcul. Deuxièmement quantifier l’influence des différents phénomènes physiques en jeu pour déterminer lesquels doivent être modélisés. Enfin adapter des modélisations SPH permettant de prendre en compte simultanément les différents phénomènes influant pour réaliser des simulations du problème complet. / The aquaplaning problem has been the topic of simulation works emphasizing its complexity: fluid structure interactions, structures modelling, materials involved, contact with asphalt and the complexity of the resulting fluid flow (extremely complex interface, drying up the road, ventilation, possible development of turbulence and cavitation). As additional difficulty, the tire is a highly deformable body and fluid-structure interaction effects should be considered, leading to a challenging problem for the numerical modelling. Then Michelin, Ecole Centrale Nantes and NextFlow Software have recently tested the ability of the SPH solver developed by the two latter to classify tires based on their surface structure geometries, without considering the gas phase. In this context, the interest of SPH for modelling efficiently the aquaplaning flow has been underlined. The meshless and Lagrangian feature of SPH naturally avoid the problem of fluid/solid grid compatibility. The other significant advantage of the SPH method, in this context, appears in its ability to be relatively easily coupled to with conventional Finite Element solvers. The aim of this workis three fold. First, qualify the SPH-FE coupling strategy, especially in terms of energy and then develop schemes to ensure a good compromise among stability, accuracy and computation time. Second, quantify the influence of different involved physical phenomena to determine which should be modelled. Finally, adapt SPH models to simultaneously consider different phenomena and to performe simulations of the complete problem.

Une modélisation efficace et une connaissance précise du comportement mécanique du cœur du réacteur sont nécessaires pour estimer les effets de l'excitation sismique sur une centrale nucléaire. La présence d'un écoulement d'eau (dans les REP) engendre des phénomènes d'interaction fluide structure. La modélisation des interactions fluide structure sur les assemblages combustible revêt donc une importance fondamentale pour la sécurité des réacteurs nucléaires. L’objectif principal du projet de thèse présenté dans ce document est d’étudier les interactions fluide structure afin de mieux comprendre les phénomènes impliqués. La modélisation et l'approche expérimentale sont considérées. Un nouveau modèle linéaire simplifié pour les interactions fluide structure est développé en utilisant la théorie de l'écoulement potentiel pour la modélisation des forces fluide, tandis que le modèle de poutre d'Euler-Bernoulli est utilisé pour la partie structurelle. Le modèle est d'abord développé pour un seul cylindre et il est validé avec des ouvrages de référence dans la littérature. Les effets de la taille de confinement et du nombre d'onde sont examinés. Le modèle d'écoulement potentiel développé pour un seul cylindre est ainsi étendu à une géométrie multicylindre. La démarche expérimentale est donc nécessaire pour valider le modèle développé. Une nouvelle installation expérimentale, ICARE, a été conçue pour étudier les phénomènes d’interaction fluide structure sur des assemblages combustible à demi-échelle. Dans ce document, les résultats fournis par les mesures de déplacement et de LDV sont largement analysés. Le comportement dynamique de l'assemblage combustible et les effets de couplage sont étudiés. Les calculs sont comparés aux résultats expérimentaux afin de valider le modèle et d’en analyser ses limites. Le modèle est en accord avec les résultats expérimentaux concernant l'effet de masse ajouté. De plus, le modèle prédit qualitativement les effets des couplages dans différentes directions. Par contre, le modèle d'écoulement potentiel ne permet pas de prédire des effets d'amortissement ajouté, principalement dus aux forces visqueuses. Enfin, dans ce document, une autre application du modèle développé est décrite. Le modèle est utilisé afin de simuler des expériences réalisées sur une maquette d'assemblage combustible dans l'installation expérimentale installée à l'Université George Washington (GWU). Le modèle est capable de prédire et de fournir une interprétation valide de la perturbation du débit d'eau due au mouvement de l'ensemble excité. La thèse se termine par des perspectives d'amélioration du modèle, en intégrant des termes visqueux dans les équations. L'analyse des données de vélocimétrie par image de particules (PIV) recueillies au cours des campagnes expérimentales ICARE doit être poursuivie. / Efficient modelling and accurate knowledge of the mechanical behaviour of the reactorcore are needed to estimate the effects of seismic excitation on a nuclear power plant. Thepresence of cooling water flow (in PWRs) gives rise to fluid structure interaction phenomena.Modelling of fluid structure interactions on fuel assemblies is thus of fundamentalimportance in order to assure the safety of nuclear reactors. The main objective of thePhD project which is presented in this document is to investigate fluid structure interactionsin order to have a better understanding of the involved phenomena. Both modellingand experimental approach are considered. A new simplified linear model for fluid structureinteractions is developed by using the potential flow theory for fluid force modellingwhile the Euler-Bernoulli beam model is used for the structural part. The model, is firstdeveloped for a single cylinder and it is validated with reference works in literature. Theeffects of the confinement size and of the wavenumber are investigated. The potential flowmodel developed for a single cylinder, is thus extended to a multi cylinders geometry. Theexperimental approach is thus needed in order to validate the developed model. A newexperimental facility, ICARE, is designed in order to investigate fluid structure interactionphenomena on half scale fuel assemblies. In this document, the results provided bydisplacement and LDV measurements are widely analysed. The dynamical behaviour ofthe fuel assembly and coupling effects are investigated. Calculations are compared to theexperimental results in order to validate the model and to analyse its limits. The model isin agreement with experimental results regarding the added mass effect. In addition, themodel qualitatively predicts couplings effects on different directions. As a drawback, thepotential flow model cannot predict added damping effects, which are mainly due to viscousforces. Finally in this document another application of the developed model is described.The model is used in order to simulate experiments performed on a surrogate fuel assemblyin the experimental facility installed at George Washington University (GWU). The modelis able to predict and to provide a valid interpretation for the water flow perturbation dueto the motion of the excited assembly. The thesis concludes with perspectives for furtherimprovements of the model, by integrating viscous terms in the equations. Work needs tobe carried out on the analysis of Particle Image Velocimetry (PIV) data collected duringICARE experimental campaigns.

High-fidelity CFD-MBD FSI (Computational Fluid Dynamics - Multi Body Dynamics Fluid-Structure Interaction) code development and validation by full-scale experiments is presented, for a novel hull form, WAM-V (Wave Adaptive Modular Vessel). FSI validation experiments include cylinder drop with suspended mass and 33 ft WAM-V sea-trials. Calm water and single-wave sea-trails were with the original suspension, while the rough-water testing was with a second generation suspension. CFDShip-Iowa is used as CFD solver, and is coupled to Matlab Simulink MBD models for cylinder drop and second generation WAM-V suspension. For 1DOF cylinder drop, CFD verification and validation (V&V) studies are carried out including grid and time-step convergence. CFD-MBD results for 2DOF cylinder drop show that 2-way coupling is required to capture coupled physics. Overall, 2-way results are validated with an overall average error value of E=5.6%DR for 2DOF cylinder drop. For WAM-V in calm water, CFD-MBD 2-way results for relative pod angle are validated with E=14.2%DR. For single-wave, CFD-MBD results show that 2-way coupling significantly improves the prediction of the peak amplitude in pontoon motions, while the trough amplitudes in suspension motions are under-predicted. The current CFD-MBD 2-way results for single-wave are validated with E=17%DR. For rough-water, simulations are carried out in regular head waves representative of the irregular seas. CFD-MBD 2-way results are validation with E=23%D for statistical values and the Fourier analysis results, which is reasonable given the differences between simulation waves and experiments.

Radio-frequency microelectromechanical systems (RF MEMS) are widely used for contact actuators and capacitive switches. These devices typically consist of a metallic membrane which is activated by a time-periodic electrostatic force and makes periodic contact with a contact pad. The increase in switch capacitance at contact causes the RF signal to be deflected and the switch thus closes. Membrane motion is damped by the surrounding gas, typically air or nitrogen. As the switch opens and closes, the flow transitions between the continuum and rarefied regimes. Furthermore, creep is a critical physical mechanism responsible for the failure in these devices, especially those operating at high RF power. Simultaneous and accurate modeling of all these different physics is required to understand the dynamical membrane response in these devices and to estimate device lifetime and to improve MEMS reliability. It is advantageous to model fluid and structural mechanics and electrostatics within a single comprehensive numerical framework to facilitate coupling between them. In this work, we develop a single unified finite volume method based numerical framework to study this multi-physics problem in RF MEMS. Our objective required us to develop structural solvers, fluid flow solvers, and electrostatic solvers using the finite volume method, and efficient mechanisms to couple these different solvers. A particular focus is the development of flow solvers which work efficiently across continuum and rarefied regimes. A number of novel contributions have been made in this process. Structural solvers based on a fully implicit finite volume method have been developed for the first time. Furthermore, strongly implicit fluid flow solvers have also been developed that are valid for both continuum and rarefied flow regimes and which show an order of magnitude speed-up over conventional algorithms on serial platforms. On parallel platforms, the solution techniques developed in this thesis are shown to be significantly more scalable than existing algorithms. The numerical methods developed are used to compute the static and dynamic response of MEMS. Our results indicate that our numerical framework can become a computationally efficient tool to model the dynamics of RF MEMS switches under electrostatic actuation and gas damping. / text

Large structures exposed to the environment such as the collinear omni and large panel communication antennas in this research suffer damage from cyclic wind, rain, hail, ice load and impacts from birds and stones. Stresses from self-weight, ice loading and wind gusts will produce deformations of the structure that will lead to performance deterioration of the antenna. In order to avoid such a case, it is important to understand the static, dynamic and aerodynamic behavior of these structures and thus optimization can be achieved. In this research the current fluid-structure interaction methods are used to model, simulate and analyze these communication antennas in order to assess whether failure would occur under service loads. The FEA models developed are verified against analytical models and/or experiments. Different antenna configurations are compared based on their capacity to minimize vibration effects, stress-induced deformations and aerodynamic loading effects.

Large structures exposed to the environment such as the collinear omni and large panel communication antennas in this research suffer damage from cyclic wind, rain, hail, ice load and impacts from birds and stones. Stresses from self-weight, ice loading and wind gusts will produce deformations of the structure that will lead to performance deterioration of the antenna. In order to avoid such a case, it is important to understand the static, dynamic and aerodynamic behavior of these structures and thus optimization can be achieved. In this research the current fluid-structure interaction methods are used to model, simulate and analyze these communication antennas in order to assess whether failure would occur under service loads. The FEA models developed are verified against analytical models and/or experiments. Different antenna configurations are compared based on their capacity to minimize vibration effects, stress-induced deformations and aerodynamic loading effects.

<div>This thesis examines the problem of low Reynolds number viscous fluid–structure interactions (FSIs) at the microscale. A myriad of examples of such phenomena exist, both in nature (blood flow in arteries, air flow in lungs), as well as in the laboratory (microfluidics devices, soft robotics). For this thesis, we restrict to internal flows in conduits with deformable walls. Specifically, two types of conduits of different cross-sectional shapes are considered: microchannels and microtubes. Both of these geometries are slender and thin.</div><div>Different types of material behavior are considered, via constitutive laws, in the solid domain, namely linearly elastic, hyperelastic and viscoelastic; and in the fluid domain, namely Newtonian and power-law fluids with shear-dependent viscosity. Similarly, the geometry and dimensions of the structures allow us to use shell and plate theories in the solid domain, and the lubrication approximation of low Reynolds number flow in the fluid domain.</div><div>First, we study a rectangular microchannel with a deformable top wall of moderate thickness, conveying a power-law fluid at steady conditions. We obtain a nonlinear differential equation for pressure as a function of imposed steady flow rate, consisting of infinite expansions of hypergeometric functions. We also conduct simulations of FSI using the commercial computer-aided engineering (CAE) software ANSYS, to both benchmark our perturbative theory and to establish the limits of its applicability.</div><div>Next, we study fluid–structure interactions in a thin microtube constituted of a linearly elastic material conveying a generalized Newtonian fluid. Here, we employ the Donnell shell theory to model the deformation field in the structure of the tube. As a novel contribution, we formulate an analytical expression for the (radial) deformation of the tube using the method of matched asymptotic expansions, taking into account the bending boundary layers near the clamped ends. Using our perturbative theory, we also improve certain classical but partial results, like Fung’s model and the law of Laplace, to match with direct numerical simulations in ANSYS.</div><div>Subsequently, we explore FSI in hyperelastic tubes via the Mooney–Rivlin model. In a thin-walled vessel, we formulate a novel nonlinear relationship between (local) deformation and (local) pressure A similar approach for the thick-walled tube, yields a nonlinear ODE to be solved numerically. Due to strain hardening, the hyperelastic tube appears stiffer and supports higher pressure drops than a linearly elastic tube.</div><div>Finally, we study transient compressible flow being conveyed in a linearly viscoelastic tube. By employing a double perturbation expansion (for weak compressibility and weak FSI), a predictive relationship between the deformed radius, the flow rate and the (local) pressure is obtained. We find that, due to FSI, the Stokes flow takes a finite time to adjust to any changes emanating from the boundary motion. In the case of oscillatory pressure imposed at the inlet, acoustic streaming is shown to arise due to FSI in this compressible flow. Fundamentally, the goal of the research in this thesis is to generate a catalog of flow rate–pressure drop relationships for different types of fluid–structure interactions, depending on the combinations of fluid mechanics and structural mechanics models (behaviors). These relationships can then be used to solve practical problems. We formulate a theory of hydrodynamic bulge testing, through which the elastic modulus is estimated from the pressure drop and flow rate measurements in a microchannel with a (thick and pre-stressed) compliant top wall, without measuring the deformation. A sensitivity analysis, via Monte Carlo simulation, shows that the hydrodynamic bulge test is only a slightly less accurate</div><div>than the traditional bulge test, but is less susceptible to uncertainty emanating from the noise in measurements.</div>

Includes bibliographical references / In this thesis, the development of a strongly coupled, partitioned fluid-structure interactions (FSI) solver is outlined. Well established methods are analysed and new methods are proposed to provide robust, accurate and efficient FSI solutions. All the methods introduced and analysed are primarily geared towards the solution of incompressible, transient FSI problems, which facilitate the use of black-box sub-domain field solvers. In the first part of the thesis, radial basis function (RBF) interpolation is introduced for interface information transfer. RBF interpolation requires no grid connectivity information, and therefore presents an elegant means by which to transfer information across a non-matching and non-conforming interface to couple finite element to finite volume based discretisation schemes. The transfer scheme is analysed, with particular emphasis on a comparison between consistent and conservative formulations. The primary aim is to demonstrate that the widely used conservative formulation is a zero order method. Furthermore, while the consistent formulation is not provably conservative, it yields errors well within acceptable levels and converges within the limit of mesh refinement. A newly developed multi-vector update quasi-Newton (MVQN) method for implicit coupling of black-box partitioned solvers is proposed. The new coupling scheme, under certain conditions, can be demonstrated to provide near Newton-like convergence behaviour. The superior convergence properties and robust nature of the MVQN method are shown in comparison to other well-known quasi-Newton coupling schemes, including the least squares reduced order modelling (IBQN-LS) scheme, the classical rank-1 update Broyden's method, and fixed point iterations with dynamic relaxation. Partitioned, incompressible FSI, based on Dirichlet-Neumann domain decomposition solution schemes, cannot be applied to problems where the fluid domain is fully enclosed. A simple example often provided in the literature is that of balloon inflation with a prescribed inflow velocity. In this context, artificial compressibility (AC) will be shown to be a useful method to relax the incompressibility constraint, by including a source term within the fluid continuity equation. The attractiveness of AC stems from the fact that this source term can readily be added to almost any fluid field solver, including most commercial solvers. AC/FSI is however limited in the range of problems it can effectively be applied to. To this end, the combination of the newly developed MVQN method with AC/FSI is proposed. In so doing, the AC modified fluid field solver can continue to be treated as a black-box solver, while the overall robustness and performance are significantly improved. The study concludes with a demonstration of the modularity offered by partitioned FSI solvers. The analysis of the coupled environment is extended to include steady state FSI, FSI with free surfaces and an FSI problem with solid-body contact.