Domain decomposition for a finite volume method on non-matching grids

We are interested in a robust and accurate domain decomposition method with Robin interface conditions on non-matching grids using a finite volume discretization. We introduce transmission operators on the non-matching grids and define new interface conditions of Robin type. Under a compatibility assumption, we show the equivalence between Robin interface conditions and Dirichlet–Neumann interface conditions and the well-posedness of the global and local problems. Two error estimates are given in terms of the discrete H¹-norm: one in O(h^1/2) with operators based on piecewise constant functions and the other in O(h) (as in the conforming case) with operators using a linear rebuilding. Numerical results are given. To cite this article: L. Saas et al., C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

A non-overlapping domain decomposition method for the Stokes equations via a penalty term on the interfaceDécomposition de domaines sans superposition pour les équations de Stokes via une pénalisation dans l'interface

The purpose of this Note is to perform a theoretical analysis of the domain decomposition method introduced in [2]. We motivate and introduce an improvement of this method and carry out the analysis when it is applied to solving the Stokes equations. Our method is based on a penalty term on the interface between subdomains that enforces the appropriate transmission conditions and may be seen as variation of the Robin method. We obtain strong convergence results for velocity and pressure in the standard H¹ and L² norms and precise rates of convergence, together with error estimates. These error estimates are of optimal order with respect to the precision of the interpolation. We conclude with some numerical tests. To cite this article: T. Chacón Rebollo, E. Chacón Vera, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 221–226.     

Convergence analysis of finite element methods for H(curl; Ω)-elliptic interface problems

In this article we investigate the analysis of a finite element method for solving H(curl; ??)-elliptic interface problems in general three-dimensional polyhedral domains with smooth interfaces. The continuous problems are discretized by means of the first family of lowest order Nédélec H(curl; ??)-conforming finite elements on a family of tetrahedral meshes which resolve the smooth interface in the sense of sufficient approximation in terms of a parameter ?? that quantifies the mismatch between the smooth interface and the triangulation. Optimal error estimates in the H(curl; ??)-norm are obtained for the first time. The analysis is based on a ??-strip argument, a new extension theorem for H ¹(curl; ??)-functions across smooth interfaces, a novel non-standard interface-aware interpolation operator, and a perturbation argument for degrees of freedom for H(curl; ??)-conforming finite elements. Numerical tests are presented to verify the theoretical predictions and confirm the optimal order convergence of the numerical solution.     

Improved interface conditions for a non-overlapping domain decomposition of a non-convex polygonal domain

We propose a local improvement of domain decomposition methods which fits with the singularities occurring in the solutions of elliptic equations in polygonal domains. This short presentation focuses on a model elliptic problem with the decomposition of a non-convex polygonal domain into convex polygonal subdomains. After explaining the strategy and the theoretical design of adapted interface conditions at the corner, we present numerical experiments which show that these new interface conditions satisfy some optimality properties. To cite this article: C. Chniti et al., C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

A mortar element method for hyperbolic problems

A non-conforming finite element method based on non-overlapping domain decomposition is extended to linear hyperbolic problems. The method is based on streamline-diffusion/discontinuous Galerkin methods and the mortar element method. A weak flux continuity condition at the inflow interface is enforced by means of Lagrange multipliers. This weak flux continuity condition replaces the usual mortar condition for elliptic problems, and allows non-matching grids at the subdomain interfaces. To cite this article: Y. Bourgault, A. El Boukili, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

A particle model for fluid–structure interactionUne méthode particulaire pour le couplage fluide–structure

We propose a particle method to handle fluid–structure interactions on a 1D model problem. Interactions between fluid and solid particles implicitly enforce the continuity of stresses on the interface. Comparisons with results obtained by ALE methods allow one to evaluate the robustness and accuracy of the method. To cite this article: G.-H. Cottet, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 833–838.     

Singular perturbation of an infinite interval linear state regulator problem in optimal control

The corresponding problem on a finite interval has been studied by O'Malley and Kung using two different methods, namely: (1) the two-point boundary value method (O'Malley and Kung, SIAM J. Control13 (1975), 327–337) and (2) The Riccati gain method (O'Malley and Kung, J. Differential Eqs.16 (1974), 413–427). For the infinite interval, the two-point boundary value method is no longer relevant. However, the Riccati gain method can be applied. The conditions are changed slightly from those for the finite interval case. Some conditions are eliminated and some new conditions are added.     

Control,observation and polynomial decay for a coupled heat-wave system

This Note is devoted to study the control, observation and polynomial decay of a linearized 1-d model for fluid–structure interaction, where a wave and a heat equation evolve in two bounded intervals, with natural transmission conditions at the point of interface. These conditions couple, in particular, the heat unknown with the velocity of the wave solution. The controllability and observability of the system through the wave component are derived from sidewise energy estimate and Carleman inequalities. As for the control and observation through the heat component, we need to develop first a careful spectral high frequency analysis for the underlying semigroup, which yields a new Ingahm-type inequality. It is shown that the controllable/observable subspace for both cases are quite different. Also, we obtain a sharp polynomial decay rate for the energy of smooth solutions. To cite this article: X. Zhang, E. Zuazua, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Efficiency of approximate boundary conditions for corner domains coated with thin layers

In this Note, we consider an interface problem posed in a bounded domain with thin layer. In the case of a smooth domain, approximate boundary conditions (also called impedance conditions) are known to approximate in a precise way the effect of the layer, as its thickness goes to zero. We investigate here the efficiency of such conditions when the domain has a corner; we show that it deteriorates when the opening of the corner angle grows, giving optimal estimates thanks to multiscale asymptotic expansions. Numerical results are given, which illustrate these estimates. To cite this article: G. Vial, C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Méthode d'éléments finis avec hybridisation frontière pour les problèmes de contact avec frottementFinite element method with Lagrange multipliers for contact problems with friction

In this Note, we propose a finite element method with Lagrange multipliers in order to approximate contact problems with friction. The discretized normal and tangential constraints at the candidate contact interface are expressed by using continuous piecewise linear Lagrange multipliers in the saddle-point formulation. An optimal error estimate is established and several numerical studies corresponding to this choice of the discretized normal and tangential constraints are achieved. To cite this article: L. Baillet, T. Sassi, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 917–922.     

A unified fictitious domain model for general embedded boundary conditions

This Note addresses the analysis of a new fictitious domain method for elliptic problems in order to handle general embedded boundary conditions (E.B.C.): Fourier, Neumann and Dirichlet conditions on an immersed interface. Our method is based on a recent model of fracture combining flux and solution jumps on the interface Σ separating the original domain $\stackrel{?}{\Omega }$ from the auxiliary exterior domain ${\Omega }_e$. A class of methods is derived within the same unified formulation with either no penalty or exterior control in ${\Omega }_e$, or surface penalty on Σ, volume $H^1$ or $L^2$ penalty in ${\Omega }_e$, or both. The consistency (no penalty) or optimal error estimates with respect to the penalty parameter are proved for such methods. To cite this article: Ph. Angot, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Simple components of group algebras Q[G] central over real quadratic fields

Let k be a real quadratic field, and $\text{U}$ a central division quaternion algebra over k. In this paper sufficient conditions are given to insure that $\text{U}$ appears in a simple component of the group algebra Q[G] of some finite group G over the rational field Q. In particular, when k is assumed to be Q(√2) or Q(√5), the necessary and sufficient conditions for $\text{U}$ to appear in some Q[G] are given.     

Algorithme de Neumann–Dirichlet pour des problèmes de contact unilatéral : Résultat de convergenceNeumann–Dirichlet algorithm for unilateral contact problems: Convergence results

In this Note, we propose and we prove the convergence of a Neumann–Dirichlet algorithm in order to approximate a Signorini problem between two elastic bodies. The idea is to retain the natural interface between the two bodies as numerical interface for the domain decomposition and to replace the Dirichlet problem in [4] by a variational inequality. To cite this article: G. Bayada et al., C. R. Acad. Sci. Paris, Ser. I 335 (2002) 381–386.     

Polynomial decay and control of a 1−d model for fluid–structure interaction

We consider a linearized and simplified 1?d model for fluid–structure interaction. The domain where the system evolves consists in two bounded intervals in which the wave and heat equations evolve respectively, with transmission conditions at the point of interface. First, we develop a careful spectral asymptotic analysis on high frequencies. Next, according to this spectral analysis we obtain sharp polynomial decay rates for the whole energy of smooth solutions. Finally, we prove the null-controllability of the system when the control acts on the boundary of the interval where the heat equation holds. The proof is based on a new Ingham-type inequality, which follows from the spectral analysis we develop and the null controllability result in Zuazua (in: J.L. Menaldi et al. (Eds.), Optimal Control and Partial Differential Equations, IOS Press, 2001, pp. 198–210) where the control acts on the wave component. To cite this article: X. Zhang, E. Zuazua, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Approximation par la méthode des éléments finis de la formulation en domaine régulier de problèmes de fissures

The discretization by various mixed finite element methods of a new variational formulation of crack problems is considered. The new formulation, called the smooth domain method, is derived for crack problems in the case of a simplified model of an elastic membrane. Inequality type boundary conditions are prescribed at the crack faces. The resulting model takes the form of an unilateral contact problem on the crack. The mathematical analysis for the method leads to optimal convergence rates, as given in this Note. To cite this article: Z. Belhachmi et al., C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

A domain embedding method for mixed boundary value problems

We propose a domain embedding (fictitious domain) method for elliptic equations subject to mixed boundary conditions, and prove the sharp convergence rate. The theory provides a unified treatment for Dirichlet, Neumann, and Robin boundary conditions. To cite this article: S. Zhang, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

Algorithme RLS en deux étapes pour l'estimation d'un modèle ARCH

The aim of this Note is, on the one hand, the development of a recursive method, in two stages, for estimating ARCH models, and, on the other hand, the analysis of the statistical properties of the estimators provided by this method. We show that these estimators are asymptotically Gaussian and that the estimator of the second stage is asymptotically efficient. To cite this article: A. Aknouche, H. Guerbyenne, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

On linear integral operators with nonnegative kernels

Let K be an eventually compact linear integral operator on $\text{L}{}^{\mathrm{p}}\text{(Ω, μ), 1 ? p < ∞}$, with nonnegative kernel k(x, y), where the underlying measure μ is totally σ-finite on the domain set Ω when p = 1. In considering the equation λf = Kf + g for given nonnegative $\text{g ? L}{}^{\mathrm{p}}\text{(Ω, μ), λ > 0}$, P. Nelson, Jr. provided necessary and sufficient conditions, in terms of the support of g, such that a nonnegative solution $\text{f ? L}{}^{\mathrm{p}}\text{(Ω, μ)}$ was attained. Such conditions led to generalizing some of the graph-theoretic ideas associated with the normal form of a nonnegative reducible matrix. The purpose of this paper is to show that the analysis by Nelson can be enlarged to provide a more complete generalization of the normal form of a nonnegative matrix which can be used to characterize the distinguished eigenvalues of K and K¹, and to describe sets of support for the eigenfunctions and generalized eigenfunctions of both K and K¹ belonging to the spectral radius of K.     

A model of fracture for elliptic problems with flux and solution jumps

A new model of fracture for elliptic problems combining flux and solution jumps as immersed boundary conditions is proposed and proved to be well-posed. An application of this model to the flow in fractured porous media is also proposed including the cases of “impermeable fracture” and “fully permeable fracture” satisfying the so-called “cubic law”, as well as intermediate cases. A finite volume scheme on general polygonal meshes is built to solve such problems. Since no unknown is required at the fracture interface, the scheme is as cheap as standard schemes for the same problems without fault. The convergence of the scheme can be proved to the weak solution of the problem. With weak regularity assumptions, we also establish for the discrete H¹₀ and L² norms some error estimates in $\text{O}\text{(h)}$, where h is the maximum diameter of the control volumes of the mesh. To cite this article: Ph. Angot, C. R. Acad. Sci. Paris, Ser. I 337 (2003).