Two-Way Fluid-Structure Coupling Methodology for Modeling 3D Flexible Hydrofoils in Viscous Flow

RESUME Les surfaces portantes. telles que des pâles, ailes, et hydrofoil sont sujets a des instabilites comme la divergence, le battement et la resonnance qui peuvent provoquer la fatigue de la structure et reduire sa tenue en service. Par consequent, il est important de comprendre et de predire avec precision la reponse et la stabilite de telles structures afin d’en assurer la securite, et de faciliter la conception et optimisation de concepts nouveaux et existants. L’interaction entre un ecoulement et une structure, nommee interaction fluide-structure (IFS), doit etre prise en compte lors de l’etude la reponse elastique et des instabilites de surfaces portantes. Pour de telles applications, l’ecoulement et la structure sont couples au travers de la charge qui s’exerce sur la structure par le fluide, et la deformation qui en decoule. Pour certaines applications IFS, le fluide et le solide peuvent etre couples par un transfert unidirectionnel de la charge. Dans ce cas, un champ donne peut fortement affecter l’autre sans l’etre lui-meme. Cependant, pour certaines applications en ingenierie, ou il y a une relation forte et potentiellement non-lineaire entre ces champs, un couplage unidirectionnel n’est pas adequat. Alors, les deplacements de la structure causes par l’ecoulement accentuent les forces du fluide de telle sorte que le fluide et la structure interagissent en boucle de facon complexe. Donc, une analyse bi-directionnelle doit etre entreprise. Les structure legeres et flexibles sont couramment utilisees grâce aux avancees recentes dans les technologies des materiaux afin d’ameliorer les caracteristiques hydrodynamiques et structurelles par rapport aux materiaux lourds et rigides. Dans cette these, on cherche une meilleure comprehension de la phenomenologie hydroelastique d’hydrofoils hautement flexibles, qui subissent de grandes deformations sous de fortes charges. Ceci milite en faveur d’une methodologie IFS bidirectionnelle fortement couplee, en plus de l’incorporation de techniques numeriques avancees pour la modelisation de la deformation de maillages adaptes. Pour des nombres de Reynolds moyens a eleves, le developpement d’un ecoulement turbulent autour de l’hydrofoil provoque un transfert du mouvement perpendiculaire a la paroi et permet a l’ecoulement de se rattacher, et ainsi former une bulle laminaire de separation (Laminar Separation Bubble, LSB). Le decrochage de l’ecoulement, la formation de tourbillons dans le sillage, la localisation et le mouvement de la bulle laminaire sont tous des phenomenes qui affectent les charges hydrodynamiques et les vibrations de la structure. Par consequent, une methodologie numerique avancee, avec une precision spatiale suffisamment elevee a ete incorporee dans ce travail pour capturer finement ces phenomenes a l’interface fluide-structure, tels que l’apparition et l’etendue de la zone de separation. ----------ABSTRACT Lifting bodies, such as blades, wings, and hydrofoils, may be subject to instabilities, such as divergence, flutter, and resonance, which can fatigue the structure and reduce its operational longevity. Therefore, it is important to understand and accurately predict the response and stability of such structures to ensure their structural safety and facilitate the design and optimization of new and existing concepts. The interaction between fluid and structure, known as Fluid-Structure Interaction (FSI), should be taken into account in the study of elastic response and instabilities of flexible lifting bodies. In such applications, the fluid flow and structure are coupled through the loads exerted on the structure by the fluid, which results in the the structural deformation. In some FSI applications, fluid and solid can be coupled by one-way (unidirectional) load transfer. In this case, a given field may strongly affect, but not be affected significantly by the other field. However, for some practical engineering applications, in which there is a strong and potentially nonlinear relationship between the fields, one-way coupling is not adequate. In such cases, the structural displacement caused by the flow further enhances the fluid forces in such a way that both fluid and structure are interacting in a complex feedback fashion. Hence, two-way FSI analysis needs to be undertaken. Light-weight, flexible structures are widely used through recent advances in material technologies to improve hydrodynamic and structural performance compared to heavy and stiff materials. This project seeks to gain greater insight into the hydroelastic response of highly flexible hydrofoils, which undergo large deformation when subjected to high hydrodynamic loadings. This increases the necessity of incorporating strongly-coupled two-way FSI methodology in addition to the numerical challenges in mesh deformation modelling. At moderate to high Reynolds numbers, the development of turbulent flow around a hydrofoil causes a momentum transfer normal to the wall and allows the flow to re-attach, and form a Laminar Separation Bubble (LSB). Flow separation, formation of trailing edge vortices, location and movement of the LSB affect the hydrodynamic loading and structural vibration. Hence, an advanced numerical technique with sufficiently high spatial accuracy is incorporated in the present study to precisely capture these local interface phenomena, such as the onset and the amount of flow separation. The interaction between the foil and surrounding flow involve significant three-dimensional features that will not be neglected in the present study.