Section: New Results

Multirate methods

Participants : Alain Dervieux, Bruno Koobus, Emmanuelle Itam, Stephen Wornom.

This study is performed in collaboration with IMAG-Montpellier II. It addresses an important complexity issue in unsteady mesh adaptation and takes place in the work done in the ANR Maidesc. Unsteady high-Reynolds computations are strongly penalized by the very small time step imposed by accuracy requirements on regions involving small space-time scales. Unfortunately, this is also true for sophisticated unsteady mesh adaptive calculations. This small time step is an important computational penalty for mesh adaptive methods of AMR type. This is also the case for the Unsteady Fixed-Point mesh adaptive methods developed by Ecuador in cooperation with the Gamma3 team of Inria-Saclay. In the latter method, the loss of efficiency is even more crucial when the anisotropic mesh is locally strongly streched. This loss is evaluated as limiting the numerical convergence order for discontinuities to 8/5 instead of second-order convergence. An obvious remedy is to design time-consistent methods using different time steps on different parts of the mesh, as far as they are efficient and not too complex. The family of time-advancing methods in which unsteady phenomena are computed with different time steps in different regions is referred to as the multirate methods. In our collaboration with university of Montpellier, a novel multirate method using cell agglomeration has been designed and developed in our AIRONUM CFD platform. A series of large-scale test cases show that the new method is much more efficient than an explicit method, while retaining a similar time accuracy over the whole computational domain. The comparison with an implicit scheme shows that the implicit scheme is in most cases one order less accurate due to higher time steps and higher dissipation. For the applications to massively parallel computing, an accurate study has been undertaken in order to analyse the impact of the mesh partitioning on the parallel efficiency. Three options have been considered. The usual partition, minimizing communication under the unique constraint of uniform load over the whole domain is optimal for a part of the algorithm but performs very poorly for the other part. We have also applied the multi-constraint partitioning of Metis which relies on both whole domain balancing and fine-mesh subdomain balancing. This strategy significantly improves the efficiency, but we observed that the balancing of the whole domain phase was not perfect. A third set of experiments relied on a geometrical-based optimal multi-contraint partition which we could apply to most of our geometries and which gave a notable further improvement. An article is submitted to a journal on the basis of the second part of the thesis of Emmanuelle Itam.