Elias Farah

Méthode de frontière immergée avec forçage direct implicite amélioré pour les problèmes d'interaction fluide-structure: Cette recherche vise à développer une nouvelle formulation pour la résolution des problèmes d’interaction fluide-structure (IFS), en mettant l’accent sur les écoulements incompressibles interagissant avec des corps solides complexes en mouvement libre et de grande amplitude. L’objectif est de surmonter les défis associés aux simulations IFS en proposant des solutions plus efficaces et pratiques, adaptées aux applications académiques et industrielles. Un aspect clé de ce travail est l’adoption de la méthode des frontières immergées (IBM). L’IBM permet aux structures rigides de se déplacer librement dans un fluide incompressible sans nécessiter de maillages complexes adaptés au corps, simplifiant ainsi la génération du maillage et éliminant les techniques coûteuses de remaillage. Pour répondre à ces défis, une méthode améliorée avec forçage direct implicite (DF-IBM) est proposée pour les problèmes d’interaction fluide-corps rigide. Cette approche exploite l’algorithme PISO afin de gérer efficacement les doubles contraintes du système fluide-solide de manière ségréguée. Ainsi, la condition de divergence nulle est maintenue dans le domaine eulérien, tandis que la condition cinématique d’adhérence des vitesses est appliquée exactement à la frontière immergée. Les équations de Navier-Stokes sont couplées à la dynamique du corps rigide dans le cadre de la DF-IBM améliorée. Un algorithme de couplage implicite est développé pour traiter les problèmes fortement couplés selon une approche partitionnée. Une stratégie de point fixe est employée, complétée par une technique de relaxation fixe pour l’effet de masse ajoutée, garantissant ainsi la robustesse de la méthode.

In this paper, we study the stability of breakwaters made of discrete ar-mour units, which requires specialized numerical methods to account foreach block’s shape, motion, and inter-element contact. Furthermore, theeffects of the porosity and thickness of the porous layer on stability are as-sessed. Therefore, a fluid-porous-solid coupling model is developed to investi-gate the stability of discrete composite breakwater. The fluid-porous modelis formulated from the Volume-Averaged Reynolds-Averaged Navier-Stokes(VARANS) equations, in which nonlinear Forchheimer terms have been incor-porated to describe flow through the porous layer. The solid model is basedon the Nodal-based Discontinuous Deformation Analysis (NDDA) method tocompute the displacements of individual blocks. This coupled model enablesexamination of how the thickness and porosity of porous layer influence themaximum impacting wave height (IWHmax) and the generation of turbulentkinetic energy (TKE). Then to investigate the stability of breakwater withdifferent kinds of armour units.

An improved implicit direct-forcing immersed boundary method (DF-IBM) is presented for simulating incompressible flows around complex rigid structures undergoing arbitrary motion. The current approach harnesses the pressure implicit with splitting of operators algorithm to handle the fluid–solid system's dual constraints in a segregated manner. As a result, the divergence-free condition is preserved throughout the Eulerian domain, and the no-slip velocity boundary condition is exactly enforced on the immersed boundary. A new pressure Poisson equation (PPE) is derived, incorporating the boundary force where the no-slip condition is already met, enabling the use of fast iterative PPE solvers without modifications. The improvement involves integrating Lagrangian weight methods having better reciprocity over the IBM-related linear operators with the implicit formulation. An additional force initialization scheme is introduced to further boost the algorithm's performance. The method's accuracy, efficiency, and capability are verified through various stationary and moving immersed boundary cases. The results are validated against experimental and numerical data from the literature. The proposed improvements seamlessly integrate into existing incompressible fluid solvers with minimal adjustments to the original system equations, highlighting their ease of implementation.

In inland waterways, waves generated by passing boats cause the movement of riprap used for bank protection and stabilization. Previous studies have mainly concerned the natural channel erosion problem caused by wind-generated waves and neglected the research and findings on shipping behavior. This work presents a coupling of the computational fluid dynamics model and the discrete element method. The aim is to first study the interactions between ship waves in the confined channel and the movements of the riprap of the banks and shorelines and second to analyze the stability of these armourstones under the actions of various ship velocities and draught depths. These varying conditions exert influence on the ship-induced waves, the crucial point being the drawdown amplitude, and consequently create more intense flow behavior in the near-shore region, which results in instability and destruction of the overall structure of the armor protection layer. The higher ship velocity and the larger draught depth lead to the aggravation of instability processes of the blocs composing the protective layer. There is a critical phenomenon in the influence mechanism of vessel velocity, whereas the variation process induced by draught depth is relatively linear. In addition, the stability of the blocs also depends on their shapes, sizes, and the initial positions within the protective layer. This study could contribute to the high-quality development of inland navigation.