Coarse Mesh Rebalance Acceleration Applied to an Iterative Domain Decomposition Method on Unstructured Mesh

An iterative domain decomposition method (DDM) is implemented inside the APOLLO3 S_n transport core solver MINARET. Based on a block-Jacobi algorithm, the method inherently suffers a convergence penalty in terms of both computing time and number of iterations. An acceleration method has to be developed in order to overcome this difficulty. This paper investigates a nonlinear coarse mesh rebalance (CMR) method that favors the way information propagates through the core when domain decomposition is used. The fundamental idea involves updating each sub-domain boundary condition thanks to a core-sized low-order calculation on a coarse spatial mesh. The numerical convergence is sped up. Performances are meeting the expectations since the CMR acceleration systematically succeeds in overbalancing the domain decomposition additional cost. The aim of such a DDM + CMR algorithm is eventually to introduce more parallelism when solving the spatial transport equation. Nevertheless, parallel computing is not addressed in this paper. (authors)