Cette thèse, intitulée NSIBM : un solveur parallèle de Navier-Stokes avec raffinement automatique basé sur la méthode des frontières immergées, a été effectuée au sein du laboratoire iCube, département de mécanique, à Strasbourg, dans le quartier de l'Orangerie, sous la direction du professeur Yannick Hoarau. L'essentiel du travail effectué consiste en le développement d'un programme capable de résoudre numériquement l'équation de Navier-Stokes qui régit des fluides en mouvement. Une attention particulière a été portée à la production de maillages conformes aux géométries proposées et à leur génération. Les moyens mis en œuvre ici pour gérer l'éternel problème de la finesse du maillage opposée au trop grand nombre de cellules sont multiples : le raffinement, la parallélisation et les frontières immergées. Dans un premier temps, j'ai conçu un générateur de maillage en deux et trois dimensions en y intégrant la possibilité de diviser des cellules, et cela de manière automatique, par des critères géométriques, numériques ou physiques. Il permet également de supprimer des cellules, de manière à ne pas mailler le vide ou les parties solides de la géométrie.Dans un deuxième temps, j'ai rendu ce code parallèle en lui donnant la capacité d'utiliser plusieurs processeurs, afin de calculer plus vite et donc d'utiliser davantage de mailles. Cette étape fait appel à deux technologies : Metis, qui partage équitablement les mailles sur le nombre choisi de processeurs et OpenMPI, qui est l'interface de communication entre ces processeurs. Enfin, la méthode des frontières immergées a été introduite au code pour gérer les bords non verticaux ou horizontaux dans un maillage cartésien, c'est-à-dire formé de rectangles ou de pavés droits. Elle consiste à donner un caractère hybride à une cellule traversée par une frontière par l'introduction d'un terme numérique de forçage simulant la présence de la paroi.Ce travail de développement a ensuite été mis à l'épreuve et validé dans une série de cas tests en deux comme en trois dimensions. Des exemples de maillages complexes générés facilement sont donnés. / This thesis, entitled NSIBM: a parallel Navier-Stokes solver with automatic mesh refinement based on immersed boundary method, has been conducted within the iCube laboratory dedicated to mechanics and located in Strasbourg. It has been supervised by Professor Yannick Hoarau. This work mainly deals with coding a program able to solve the Navier-Stokes equations that governs moving fluids, in a numerical way. Particular attention was paid to the production of meshes that suit given geometries and their generation.The means used here to handle the eternal problem of the fineness of the mesh opposed to too many cells are several~:refinement, parallelization and the immersed boundary method.Initially, I designed a two and three-dimensional mesh generator that includes the possibility of dividing cells,in an automatic way, by geometrical, numerical or physical criteria. It also allows to remove cells, where there is no point keeping it. Secondly, I parallelized the program by giving him the ability to use multiple processors to calculate faster and therefore use bigger meshes.This step uses two available libraries~: \textit{Metis}, which gives a optimal mesh partition, and \textit{openMPI}, which deals with communication between nodes. Finally, the immersed boundary method has been implemented to handle non-vertical or non-horizontal edges in a cartesian grid. Its principle is to confer a hybrid status to a cell which is crossed by an edge by adding a numerical force term simulating the presence of the boundary. This development work was then tested and validated in a serie of test cases in two and three dimensions. Examples of complex meshes easily generated are given.

The goal of this thesis is to design and implement a framework allowing password recovery in a distributed environment. The research is therefore focused on analyzing the security of passwords, techniques used for attacks on them and also presents methods preventing attacks on passwords. Described is the Wrathion tool which is allowing password recovery using acceleration on graphic cards through the integration of OpenCL framework. Conducted is also an analysis of available environments providing means to run computing tasks on multiple devices, based on which the OpenMPI platform is chosen for extending Wrathion. Disclosed are various modifications and added components, and the entire system is also subjected to experiments aiming at the measuring of scalability and network traffic performance. The financial side of the use of Wrathion tool is also discussed in terms of its usability in cloud based distributed environment.

This work is part of the k-Wave project, which is a toolbox designed for time ultrasound simulations in complex and heterogeneous media. The simulation functions are based on the k-space pseudospectral method. The goal of this work is to compute these simulations on graphics cards using local domain decompostion. Thanks to decomposition we could compute these simulations faster, and on larger data grids. The main goal of this work is to achieve efficiency and scalability.

This thesis is a part of the k-Wave project, which is a toolbox for the simulation and reconstruction of acoustic wave felds and one of its main contributions is the planning of focused ultrasound surgeries (HIFU). One simulation can take tens of hours and about 60% of the simulation time is taken by the calculation of the 3D Fast Fourier transforms. Up until now the 3D FFT has been calculated purely by the FFTW library and its 1D decomposition, whose major limitation is the maximum number of employable cores. Therefore we introduce a new approach, called the 2D hybrid decomposition of the 3D FFT (HybridFFT), where we combine both MPI processes and OpenMP threads to reach as best performance as possible. On a low number of cores, on the order of a few hundreds, we are about as fast or slightly faster than FFTW and pure MPI 2D decomposition libraries (PFFT and P3DFFT). One of the best results was achieved on a 512^3FFT using 512 cores, where our hybrid version run 31ms, FFTW run 39ms and PFFT run 44ms. The most significant performance advantage should be seen when employing around 8-16 thousand cores, however we haven't had an access to a machine with such resources. Almost a linear scalability has been proven for up to 2048 employed cores.