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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.

<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>

A coupled fluid-structure interaction solver capability is developed and validated. A high fidelity fluids solver, Loci-Chem, is coupled with a finite-element structural dynamics toolkit, MAST. The coupled solver is validated for the prediction of several panel instability cases in uniform flows and in the presence of an impinging shock for a range of subsonic and supersonic Mach numbers, dynamic pressures, and pressure ratios. The panel deflections and limit-cycle oscillation amplitudes, frequencies, and bifurcation point predictions compare very well with benchmark results for 2D simulations. The same procedures outlined in the validation study have been applied to simulations of varying dynamic pressures at M = 2 for an impinging oblique shockwave. The influence of inviscid, laminar and turbulent boundary layer profiles on the development of flow field characteristics has been analyzed, and laminar predictions characterized by a large flow separation results in vastly different behavior than that of traditional flutter.

The thesis develops singly-periodic mathematical models for complex laminar wakes which are formed behind vortex-shedding bluff bodies. These wake structures exhibit a variety of patterns as the bodies oscillate or are in close proximity of one another. The most well-known formation comprises two counter-rotating vortices in each shedding cycle and is popularly known as the vk vortex street. Of the more complex configurations, as a specific example, this thesis investigates one of the most commonly occurring wake arrangements, which consists of two pairs of vortices in each shedding period. The paired vortices are, in general, counter-rotating and belong to a more general definition of the 2P mode, which involves periodic release of four vortices into the flow. The 2P arrangement can, primarily, be sub-classed into two types: one with a symmetric orientation of the two vortex pairs about the streamwise direction in a periodic domain and the other in which the two vortex pairs per period are placed in a staggered geometry about the wake centerline. The thesis explores the governing dynamics of such wakes and characterizes the corresponding relative vortex motion. In general, for both the symmetric as well as the staggered four vortex periodic arrangements, the thesis develops two-dimensional potential flow models (consisting of an integrable Hamiltonian system of point vortices) that consider spatially periodic arrays of four vortices with their strengths being +/-1 and +/-2. Vortex formations observed in the experiments inspire the assumed spatial symmetry. The models demonstrate a number of dynamic modes that are classified using a bifurcation analysis of the phase space topology, consisting of level curves of the Hamiltonian. Despite the vortex strengths in each pair being unequal in magnitude, some initial conditions lead to relative equilibrium when the vortex configuration moves with invariant size and shape. The scaled comparisons of the model results with experiments conducted in a flowing soap film with an airfoil, which was imparted with forced oscillations, are satisfactory and validate the reduced order modeling framework. The experiments have been performed by a collaborator group at the Department of Physics and Fluid Dynamics at the Technical University of Denmark (DTU), led by Dr. Anders Andersen. Similar experiments have also been run at Virginia Tech as part of this dissertation and the preliminary results are included in this treatise. The thesis also employs the same dynamical systems techniques, which have been applied to study the 2P regime dynamics, to develop a mathematical model for the P+S mode vortex wakes, with three vortices present in each shedding cycle. The model results have also been compared favorably with an experiment and the predictions regarding the vortex circulation data match well with the previous results from literature. Finally, the thesis introduces a novel concept of clean and renewable energy extraction from vortex-induced vibrations of bluff bodies. The slow-moving currents in the off-shore marine environments and riverine flows are beyond the operational capabilities of the more established hydrokinetic energy converters and the discussed technology promises to be a significant tool to generate useful power from these copiously available but previously untapped sources. / Ph. D.

A high fidelity approach coupling the computational fluid dynamics method (CFD) and multi-body dynamics method (MBD) is presented for aero-servo-elastic wind turbine simulations. The approach uses the incompressible CFD dynamic overset code CFDShip-Iowa v4.5 to compute the aerodynamics, coupled with the MBD code Virtual.Lab Motion to predict the motion responses to the aerodynamic loads. The IEC 61400-1 ed. 3 recommended Mann wind turbulence model was implemented in this thesis into the code CFDShip-Iowa v4.5 as boundary and initial conditions, and used as the explicit wind turbulence for CFD simulations. A drivetrain model with control systems was implemented in the CFD/MBD framework for investigation of drivetrain dynamics. The tool and methodology developed in this thesis are unique, being the first time with complete wind turbine simulations including CFD of the rotor/tower aerodynamics, elastic blades, gearbox dynamics and feedback control systems in turbulent winds. Dynamic overset CFD simulations were performed with the benchmark experiment UAE phase VI to demonstrate capabilities of the code for wind turbine aerodynamics. The complete turbine geometry was modeled, including blades and approximate geometries for hub, nacelle and tower. Unsteady Reynolds-Averaged Navier-Stokes (URANS) and Detached Eddy Simulation (DES) turbulence models were used in the simulations. Results for both variable wind speed at constant blade pitch angle and variable blade pitch angle at fixed wind speed show that the CFD predictions match the experimental data consistently well, including the general trends for power and thrust, sectional normal force coefficients and pressure coefficients at different sections along the blade. The implemented Mann wind turbulence model was validated both theoretically and statistically by comparing the generated stationary wind turbulent field with the theoretical one-point spectrum for the three components of the velocity fluctuations, and by comparing the expected statistics from the simulated turbulent field by CFD with the explicit wind turbulence inlet boundary from the Mann model. The proposed coupled CFD/MBD approach was applied to the conceptual NREL 5MW offshore wind turbine. Extensive simulations were performed in an increasing level of complexity to investigate the aerodynamic predictions, turbine performance, elastic blades, wind shear and atmospheric wind turbulence. Comparisons against the publicly available OC3 simulation results show good agreements between the CFD/MBD approach and the OC3 participants in time and frequency domains. Wind turbulence/turbine interaction was examined for the wake flow to analyze the influence of turbulent wind on wake diffusion. The Gearbox Reliability Collaborative project gearbox was up-scaled in size and added to the NREL 5MW turbine with the purpose of demonstrating drivetrain dynamics. Generator torque and blade pitch controllers were implemented to simulate realistic operational conditions of commercial wind turbines. Interactions between wind turbulence, rotor aerodynamics, elastic blades, drivetrain dynamics at the gear-level and servo-control dynamics were studied, showing the potential of the methodology to study complex aerodynamic/mechanic systems.

Cette thèse vise à analyser expérimentalement une structure flexible et légère dans différents régimes d’écoulement, dont le régime cavitant. Un protocole expérimental a donc été mis en place afin de caractériser le comportement hydroélastique d’un profil NACA 0015 en polyoxyméthylène (POM) et de le comparer à un profil en acier inoxydable considéré comme « rigide ». Des mesures en écoulement subcavitant ont été réalisées : chargement hydrodynamique, contraintes, déformées statiques, réponse vibratoire et champ de vitesse ont été mesurés pour les deux matériaux. Enfin, une analyse vibratoire a été menée en écoulement cavitant. Ces mesures nous ont permis de constater que les déformées statiques du profil flexible sont similaires aux déformations observées sur une poutre encastrée : la flexion est la déformation principale et la torsion est faible. Toutefois les performances du profil flexible sont moins bonnes que pour un profil rigide : la portance diminue tandis que la traînée augmente. D’autre part, il apparaît que la dynamique du profil est contrôlée par l’écoulement. En effet, lorsque l’incidence du profil est proche de l’angle de décrochage, une fréquence liée au détachement tourbillonnaire apparaît sur les spectres de vibration des profils. Elle conduit à une réduction des fréquences propres liées à la flexion : si l’influence de cette fréquence sur le profil rigide reste faible à basse vitesse, sa proximité avec la fréquence propre du profil flexible conduit à un lock-in. Celui-ci se produit également en écoulement cavitant : lorsque la poche de cavitation devient instable, sa fréquence d’oscillation devient très énergétique et prend le contrôle de la dynamique du profil flexible. Le lock-in prend fin quand une supercavitation se développe autour du profil. Il conduit à une augmentation de la masse ajoutée au profil alors qu’elle devrait diminuer en présence de vapeur d’eau. / This work deals with an experimental analysis of a flexible and light lifting profile for various flow conditions, including cavitation. An experimental protocol was set up to study a flexible NACA 0015 made of polyoxymethylene (POM) and compare its behaviour with a foil made of steel, which is considered as rigid. The forces, strains, stresses and vibrations of the foils were measured, as well as the velocity field. Moreover, a vibratory analysis was performed in cavitating flow. The flexible foil behaves like a built-in beam : the deformations corresponds to predictions from the beam theory, with high bending and low twisting. These deformations imply lower lift and higher drag compared to the rigid foil. The vortex shedding frequency appears on the vibration spectra near stall. It increases with flow velocity and leads to a decrease of the natural bending frequency. But flexibility involves lower natural frequencies : the first bending frequency of the flexible foil is 3.5 times lower than the rigid one. This allows lock-in between the first bending frequency of the flexible foil and the vortex shedding frequency. Lock-in occurs in cavitating flows too : when cavitation becomes unstable, it oscillates with a frequency close to the bending natural frequency of the flexible foil. This lock-in ends when the cavitation number is low enough, what leads to a decrease of the cavitation oscillation frequency. In those conditions, the added mass of the flexible foil does not decrease with the cavitation number as the added mass of the rigid foil.