03.1 - Aerodynamics – CFD Methods and Validation
METHODS OF DIRECTIONAL COARSENING FOR UNSTRUCTURED GRIDS AND APPLICATIONS
Z.-Y. Xiao¹, L. Zhang¹, X.-D. Cui¹, B. Mou¹; ¹China aerodynamics research and development center , China
Coarse grids are generated for geometric multigrid by agglomeration of cells. In the simulation of high Reynolds number flows, highly stretched grids in boundary layer tend to cause low quality coarse grid cells in the aspects of skewness and convexity, which decrease the acceleration effect of multigrid. For this issue, directional coarsening methods are proposed to construct coarse grid levels in boundary layer. Firstly original grids are divided into isotropic and anisotropic parts by line searching of marching cells along the direction normal to wall surface. Then for the anisotropic grids, directional coarsening is applied to cells normal to the wall, and surface agglomeration is applied to grids on the wall, both results are integrated to generate volume cells on coarse levels. Various parameters are studied to improve the coarse grid quality. By agglomeration, coarse grid cells are polyhedrons consisting of many faces, to reduce the complexity of flux computation, multiple faces between two neighbor cells are merged, and new grid connections are built. Highly stretched unstructured grids are generated on a cube, M6 wing and CHNT-1 model in this paper, which are successively coarsened for geometric multigrid computation, results show that directional coarsening improves the qualities of coarse grids in orthogonalities and aspect ratios, and by RANS based simulation, the speeding up effects and accuracies of multigrid are demonstrated.