Category: News

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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