La structure de l’oeil permet d’observer la microcirculation, grâce aux caméras de fond d’oeil. Ces appareils sont bon marché et couramment utilisés dans la pratique clinique, permettant le dépistage de maladies oculaires. La capacité des vaisseaux à adapter leur diamètre (autorégulation) afin de réguler le débit sanguin est importante dans la microcirculation. L’hémodynamique de l’oeil est impactée par la pression à l’intérieur du globe oculaire (IOP), qui est à son tour influencée par le flux sanguin oculaire. Les altérations de l’autorégulation et l’IOP jouent un rôle dans les maladies oculaires. La modélisation mathématique peut aider à interpréter l’interaction entre ces phénomènes et à mieux exploiter les données médicales disponibles. Dans la première partie, nous présentons un modèle simplifié d’interaction fluidestructure qui inclut l’autorégulation, appliqué à un reseau 3D obtenu par imagerie médicale. Les cellules musculaires lisses regulant le diamètre du vaisseau sont modélisés dans la structure. Ensuite, nous utilisons des équations de poroélasticité pour décrire le flux sanguin dans la choroïde, dans un modèle multi-compartiments de l’oeil. Cette approche permet de rendre compte de la transmission de la pulsatilité de la choroïde à la chambre antérieure, où l’IOP est mesurée. Nous présentons des résultats préliminaires sur la choroïde, l’humeur aqueuse et sur la choroïde couplée avec la vitrée. Enfin, nous présentons un modèle d’ordre réduit pour accélérer des simulations multi-physique. Des modèles de haute précision sont utilisés pour les compartiments d’intérêt et une représentation réduite de l’opérateur de Steklov-Poincaré est utilisée pour les autres compartiments. / The structure of the eye offers a unique opportunity to directly observe the microcirculation, by means, for instance, of fundus camera, which are cheap devices commonly used in the clinical practice. This can facilitate the screening of systemic deseases such as diabetes and hypertension, or eye diseases such as glaucoma. A key phenomenon in the microcirculation is the autoregulation, which is the ability of certain vessels to adapt their diameter to regulate the blood flow rate in response to changes in the systemic pressure or metabolic needs. Impairments in autoregulation are strongly correlated with pathological states. The hemodynamics in the eye is influenced by the intraocular pressure (IOP), the pressure inside the eye globe, which is in turn influenced by the ocular blood flow. The interest in the IOP stems from the fact that it plays a role in several eye-diseases, such as glaucoma. Mathematical modelling can help in interpreting the interplay between these phenomena and better exploit the available data. In the first part of the thesis we present a simplified fluid-structure interaction model that includes autoregulation. A layer of fibers in the vessel wall models the smooth muscle cells that regulate the diameter of the vessel. The model is applied to a 3D image-based network of retinal arterioles. In the second part, we propose a multi-compartments model of the eye. We use the equations of poroelasticity to model the blood flow in the choroid. The model includes other compartments that transmit the pulsatility from the choroid to the anterior chamber, where the measurements of the IOP are actually performed. We present some preliminary results on the choroid, the aqueous humor and on the choroid coupled with the vitreous. Finally, we present a reduced order modelling technique to speed up multiphysics simulations. We use high fidelity models for the compartments of particular interest from the modelling point of view. The other compartments are instead replaced by a reduced representation of the corresponding Steklov-Poincaré operator.

This thesis deals with dynamic behavior of rotor dynamics system vibrating in a liquid. Work is factually oriented on influence of the liquid to natural frequences of rotor of vortex turbine. There is described the creation of geometric and computational model of the system and the results of natural frequences and damping in dependence on environment are presen-ted. There are compared variations in natural frequences of the rotor system, which are caused of the interaction of the various level of the water environment. The step of integration are tested and compared for choise solving method. Problem is solved by computational simulation in commercial software ANSYS 11.0 There is used software tools Multiphysics/FSI.

L’aorte est une artère majeure et la rupture de l’aorte (RA) est la lésion la plus commune parmi les larges vaisseaux. Une RA est détectée dans 10 à 15% des cas mortels liés aux accidents de voiture et constitue la cause secondaire des morts consécutives aux chocs traumatiques associés à ces accidents. Les mécanismes variés de RA (éclatement soudain de l’aorte, la contrainte des structures osseuses, le « coup de bélier » et la combinaison de ces mécanismes) peuvent être considérés comme une combinaison de deux types de contributions : la distraction aortique et la pression aortique.L’objectif de ce travail de doctorat est d’étudier les mécanismes liés aux RAs dans les accidents de voitures en se focalisant sur les mécanismes associés à la pression intra-aortique. Le travail est organisé en quatre parties : 1) quantifier la réponse aortique sous des conditions de chargement physiologiques, 2) identifier la nécessité de considérer la pression intra-aortique dans les RA associés aux accidents, 3) développer un modèle d’éléments finis incluant la contribution des mécanisme de lésions et 4) et finalement étudier le mécanisme de RA avec le modèle nouvellement développé. / Blunt aortic rupture (BAR) is the second leading cause of death following blunt trauma in motor vehicle crash accidents (MVCAs). Aortic distraction was postulated to be a primary BAR mechanism, but intra-aortic pressure effect on BAR is controversial. Previous finite element (FE) simulations did not simultaneously study the BAR mechanisms of the two contribution sources. Therefore, the BAR mechanisms remain to be ascertained under the effects of physiological intra-aortic pressure and intra-thoracic interactions during MVCAs.Our objective is to investigate BAR mechanisms in MVCAs with a focus on intra-aortic pressure. The work is organized: 1) to quantify aortic responses under cardiac function, 2) to identify the necessity of considering intra-aortic pressure, 3) to develop a FE model including both injury contribution sources and 4) to investigate BAR mechanism during MVCA with the new model. The aortic responses under cardiac loadings should be considered for BAR. Intra-aortic pressure could induce a significant BAR risk. BAR in MVCA resulted from a combined mechanism with aortic stretch and intra-aortic pressure as the primary and secondary cause.

The aim of this work is to find a stable scheme which would solve the Stokes problem of the fluid flow, in which an elastic structure is immersed. Unlike most of the schemes solving fluid-structure interaction problems, in our scheme meshes of fluid and structure do not have to coincide. We have restricted ourselves to two-dimensional domain occupied by fluid with one-dimensional im- mersed structure. To describe a fluid-structure interaction, we have used an Immersed boundary method. At first we consider the strucure to be massless. We have modified an existing scheme and made it unconditionally stable, which was mathematically proven and numerically tested. Then we have proposed a modification where the structure is not massless and also proved the uncondi- tional stability in this case. The proposed schemes were implemented using the Freefem++ software and tested on aneurysm-like geometry. We have tested the behavior of our scheme in case when the qrowing aneurysm touches an obstacle, for example a bone (with no-slip condition on the bone boundary). Powered by TCPDF (www.tcpdf.org)

This report presents the work done within the framework of my master thesis in the program Infrastructure Engineering at KTH Royal Institute of Technology, Stockholm. This project has been proposed and sponsored by the French company Setec TPI, part of the Setec group, located in Paris. The overall goal of this study is to investigate fluid-structure interaction and particularly sloshing in liquid-containing structures subjected to seismic or other dynamic action. After a brief introduction, the report is composed of three main chapters. The first one presents and explains fluid-structure interaction equations. Fluid-structure interaction problems obey a general flow equation and several boundary conditions, given some basic assumptions. The purpose of the two following chapters is to solve the corresponding system of equations. The first approach proposes an analytical solution: the problem is solved for 2D rectangular tanks. Different models are considered and compared in order to analyze and describe sloshing phenomenon. Liquid can be decomposed in two parts: the lower part that moves in unison with the structure is modeled as an impulsive added mass; the upper part that sloshes is modeled as a convective added mass. Each of these two added mass creates hydrodynamic pressures and simple formulas are given in order to compute them. The second approach proposes a numerical solution: the goal is to be able to solve the problem for any kind of geometry. The differential problem is resolved using a singularity method and Gauss functions. It is stated as a boundary integral equation and solved by means of the Boundary Element Method. The linear system obtained is then implemented on Matlab. Scripts and results are presented. Matlab programs are run to solve fluid-structure interaction problems in the case of rectangular tanks: the results concur with the analytical solution which justifies the numerical solution. This report gives a good introduction to sloshing phenomenon and gathers several analytical solutions found in the literature. Besides, it provides a Matlab program able to model effects of sloshing in any liquid-containing structures.

New approach for computational modeling of the dynamic behavior of elastic body immersed in incompressible viscous stagnant fluid is described in this work. It is based on determination of added effects (added mass and added damping). This effects are inserted into computational model and it replace influence of the fluid. Commonly used commercial computational software may be used. Approach is based on assumption appropriate for the linear flow. Two pressure field are determined. One for movement of the unite acceleration of the fluid boundary and the second for unite velocity. Nonlinear model (Navier-Stokes equation in ALE form) had to be used for determination of the added damping, hence results are valid only for pre-selected amplitude of vibration.

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The work in this thesis focuses on the use of explicit finite element analysis (FEA) in the modelling of fluid-structure interaction of panel-water impacts. Paper A, considers modelling of a two-dimensional rigid wedge impacting a calm water surface. From analytical methods and results of a systematic parameter study a generalised approach for determination of fluid discretization and contact parameters in the modelling of arbitrary hull-water impact situations is developed and presented. In paper B the finite element modelling methodology suggested in paper A is evaluated for elastic structures by a convergence study of structural response and hydrodynamic load. The structural hydroelastic response is systematically studied by a number of FE-simulations of different impact situations concerning panel deadrise, impact velocity and boundary conditions. In paper B a tentative method for dynamic characterization is also derived. The results are compared with other published results concerning hydroelasticity in panel water impacts. The long-term goal of this work is to develop design criteria, by which it can be determined whether the loading situation of a certain vessel type should be regarded as quasi-static or dynamic, and which consequence on the design a dynamic loading has. / QC 20101126

Plaque rupture with superimposed thrombosis is the primary cause of acute coronary syndromes of unstable angina, myocardial infarction and sudden death. Although intensive studies in the past decade have shed light on the mechanism that causes unstable atheroma, none has directly addressed the clinical observation that most myocardial infarction (MI) patients have moderate stenoses (less than 50%). Considering the important role the arterial wall compliance and pulsitile blood flow play in atheroma rupture, fluid-structure interaction (FSI) phenomenon has been of interest in recent studies. In this thesis, the impact is investigated numerically of coupled blood flow and structural dynamics on coronary plaque rupture. The objective is to determine a unique index that can be used to characterize plaque rupture potential. The FSI index, developed in this study for the first time derives from the theory of buckling of thin-walled cylinder subjected to radial pressure. Several FSI indices are first defined by normalizing the predicted hemodynamic endothelial shear stress by the structural stresses, specifically, by the maximum principal stress (giving the ratio ), and the Von Mises stress (giving the ratio ). The predicted at the location of maximum (i.e { }) denoted , is then chosen to characterize plaque rupture through systematic investigation of a variety of plaque characteristics and simulated patient conditions. The conditions investigated include varying stenosis levels ranging from 20% to 70%, blood pressure drop ranging from 3125 Pa/m to 9375 Pa/m, fibrous cap thickness ranging from to , lipid pool location ranging from the leading to the trailing edge of plaque, lipid pool volume relative to stenosis volume ranging from 24% to 80%, Calcium volume relative to stenosis volume ranging from 24% to 80% and arterial remodeling. The predicted varies with the stenosis severity and indicates that the plaques investigated are prone to rupture at approximately 40-45% stenosis levels. It predicts that high pressure significantly lowers the threshold stenosis rate for plaque rupture. In addition, the plaque potential to rupture increases for relatively thin fibrous cap, lipid core located near the leading plaque shoulder, and dramatically for relative lipid pool volume above 60%. However, calcium deposit has marginal effect on plaque rupture. Overall, the predicted results are consistent with clinical observations, indicating that the has the potential to characterize plaque rupture when properly established. In the appendix, the unsteady flow in a collapsible tube model of a diseased artery is solved analytically. The novelty of our approach is that the set of governing equations is reduced to a single integro-differential equation in the transient state. The equation was solved using the finite difference method to obtain the pressure and compliant wall behavior. The analytical approach is less computer-intensive than solving the full set of governing equations. The predicted membrane deflection is quite large at low inlet velocity, suggesting possible approach to breakdown in equilibrium. As the transmural pressure increases with wall deflection, bulges appear at the ends of the membrane indicating critical stage of stability, consistent with previous studies. An increase in wall thickness reduces the wall deflection and ultimately results in its collapse. The collapse is due to breakdown in the balance of wall governing equation. An increase in internal pressure is required to maintain membrane stability.