Category: Internal Seminar

Internal seminar of the SERENA team, Thursday 10 December, 4pm-5pm in building 13: Nabil Birgle: Composite Method on Polygonal Meshes Abstract: We develop a reliable numerical method to approximate a flow in a porous media, modeled by an elliptic equation. The simulation is made difficult because of the strong heterogeneities of the medium, the size together with complex geometry of the domain. A regular hexahedral mesh does not allow to describe accurately the geological layers of the domain.  Consequently, this leads us to work with a mesh made of deformed cubes.  There exists several methods of type finite volumes or finite elements which solve this issue.  For our method, we wish to have only one degree of freedom per element for the pressure and one degree of freedom per face for the Darcy velocity, to stay as close to the habits of industrial software.  Since standard mixed finite element methods does not converge, our method is based on composite mixed finite element. In two dimensions, a polygonal mesh is split into triangles by adding a node to the vertices’s barycenter, and explicit formulation of the basis functions was obtained.  In dimension 3, the method extend naturally to the case of pyramidal mesh.  In the case of a hexahedron or a deformed cube, the element is divided into 24 tetrahedra by adding a node to the vertices’s barycenter and splitting the faces into 4 triangles.  The basis functions are then built by solving a discrete problem.  The proposed methods have been theoretically analyzed and completed by a posteriori estimators.  They have been tested on academical and realistic examples by using parallel computation.

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Internal seminar of the SERENA team, Thursday 03 December, 4pm-5pm in building 13: Francois Clément: Safe and Correct Programming for Scientific Computing Abstract: The increasing complexity of algorithms for modern scientific computing  makes it a major challenge to implement them in the traditional imperative languages that are popular in the community. The idea is to  explore the usage of formal tools from computing science, and in particular from the functional programming school, to design and  implement generic tools that may ease the development of scientific computing software. In this lecture, we will focus on: Sklml, an easy coarse grain parallelization compiler system; Ref-indic, a generic inversion platform for adaptive parameter  estimation; a comprehensive mechanical proof of correctness of a C program as a  PDE solver. Basic examples of the use of OCaml, Sklml, and Coq will be given.

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Internal seminar of the SERENA team, Thursday 19 November, 4pm-5pm in building 13: Jérôme Jaffré: Discrete reduced models for flow in porous media with fractures  and barriers Abstract: Flow in porous media is strongly influenced by the presence of fractures which can have higher or lower permeabilities (barriers). Depending on the goal of the study there are many models which take into account this influence. In this lecture we will focus on discrete fracture models, that are models where each fracture can be described individually, and reduced fracture models, where a fracture is reduced to an $(n-1)$ surface in an $n$-dimensional model. We will begin with one-phase flow models and continue with two-phase flow taking into account the change of rock types between the matrix rock and the fractures.

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Internal seminar of the SERENA team, Thursday 19 November, 4pm-5pm in building 13: Géraldine Pichot: Generation algorithms of stationary Gaussian random fields Abstract: Flow and transport equation in geological media involve physical coefficients like the permeability and the porosity. Those coefficients are classically modeled by Gaussian random fields. In order to study the impact of the variability of those coefficients on flow and transport phenomenon, a large number of those fields are simulated which requires an efficient simulation method. I will present different algorithms to simulate stationary Gaussian random fields over a regular grid and based on the classical circulant embedding approach. I will also discuss some parallel issues and present numerical results with different covariance functions. This is a joint work with Jocelyne Erhel and Mestapha Oumouni (INRIA, Rennes).

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Internal seminar of the SERENA team, Thursday 12 November, 3pm-4pm in building 13: Elyes Ahmed: Space-time domain decomposition method for two-phase flow equations Abstract: We consider a simplified model of two-phase flow model through a heterogeneous meduim. Focusing on the capillary forces motion, we consider the Optimized Schwarz method with non-linear Robin conditions in the context of non-linear degenerate parabolic problem which is approximated in a domain shared in two homogeneous parts, each of them being a different rock type, each rock is characterized by its relative permeability and capillary curves functions of the phase saturations. We then propose a hybridized finite volume scheme for the approximation of the multi-domain solution. It relies on the Optimized Robin-Schwarz algorithm with a finite volume discretization of the subdomain problems. The existence of a weak solution for the Robin subdomain problems involved in the OSWR method is proved using the convergence of a finite volume approximation. Numerical results for three-dimensional problems are presented to illustrate the performance of the method.

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Internal seminar of the SERENA team, Thursday 5 November, 4pm-5pm in building 13: Iain Smears: Robust and efficient preconditioners for the discontinuous Galerkin time-stepping method Abstract: The discontinuous Galerkin time-stepping method has many advantageous properties for solving parabolic equations. However, its practical use has been limited by the large and challenging nonsymmetric systems to be solved at each time-step. This work develops a fully robust and efficient preconditioning strategy for solving these systems. We first construct a left preconditioner, based on inf-sup theory, that transforms the linear system to a symmetric positive definite problem that can be solved by the preconditioned conjugate gradient (PCG) algorithm. We then prove that the transformed system can be further preconditioned by an ideal block diagonal preconditioner, leading to a condition number κ bounded by 4 for any time-step size, any approximation order and any positive self-adjoint spatial operators. Numerical experiments demonstrate the low condition numbers and fast convergence of the algorithm for both ideal and approximate  preconditioners, and show the feasibility of the high-order solution of large problems.

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Internal seminar of the SERENA team, Thursday 29 Octobre, 3pm-4pm in building 13: Sarah Ali Hassan: A posteriori error estimates for domain decomposition methods Abstract: This work develops tight a posteriori error estimates for the Domain Decomposition (DD) method with Robin transmission conditions in mixed finite element discretizations. An interface problem is solved iteratively where at each iteration local subdomain problems are solved, and information is then transferred to the neighboring subdomains. By estimating the error of the DD method as well as the discretization error, an effective criterion to stop the DD iterations is developed. The a posteriori estimates are based on the reconstruction techniques for pressures and fluxes.

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