upwind Petrov-Galerkin, Galerkin/Least-Squares (GLS) and sub-grid scale (SGS). Least-squares stabilization, on the contrary, leads to a symmetric matrix. Hence, we should solve thousands of linear algebraic systemswhere the matrix A is large, sparse, symmetric, positive definite and constant at each time-step n. A finite element discretization described in [5] is applied. Owing to the formation of an abrupt front, the global solution varies only in a small part of the complete domain at each time-step. Our proposal is to exploit this fact by combining a domain decomposition method with the activation and inactivation of subdomains depending on how the hydrocarbon concentration front moves.We decided to use the multiplicative Schwarz method (MSM) because it converges faster than the additive Schwarz method [6], and its convergence rate depends on both the overlap size and the number of subdomains [7,8]. We want to emphasize that the MSM is used directly as a block Gauss-Seidel solver [9] rather than as a preconditioner.Since it is important to order the subdomains (and nodes) in the flow direction [6, 10], the nodes of the mesh are reordered following the typical block-interface structure of many activated-carbon filters, see [5]. Therefore, a block-tridiagonal global matrix is obtained. In addition, for each block, the reverse Cuthill-McKee algorithm [11] is used to reduce the bandwidth of the matrix.This article is organized as follows. In Section 2, we define the subdomains that are used in the MSM. In Section 3, we introduce the appropriate criteria to activate or inactivate subdomains. In Section 4, we present our proposal: the MSM with active subdomains. In Section 5, we show some numerical experiments illustrating the performance of the new algorithm, as well as some comparisons with the classical direct and iterative solvers. We close the paper with some concluding remarks.
DEFINITION OF SUBDOMAINS FOR THE MSM