Development of an octree based grid coarsening and multigrid flow solution

Mahmutyazıcıoğlu, Emel
The multigrid technique is one of the most effective techniques to achieve the reduction of the CPU cost for flow solvers. The multigrid strategy uses the multilevel grids which are the coarsening subsets of fine grid. An explicit solver rapidly reduces the high frequency errors on the computational grids. Since high frequency errors on coarse grids correspond to low frequency errors on fine grids, cycling through the coarse grid levels rapidly reduces the errors ranging from high-to-low frequency. The aim of this study is, therefore, to accelerate SENSE3D solver developed by TUBITAK-SAGE by implementating multigrid concept. In this work, a novel grid coarsening method suitable for cell-centered hybrid/unstructured grids is developed to provide the cells with high aspect ratio. This new grid coarsening technique relies on the agglomeration of cells based on their distribution on octree data structure. Then, the multigrid strategy is implemented to the baseline flow solver. During this implementation, the flux calculation along the face loops is modified without changing cell-centered scheme. The performance of the coarsening algorithm is investigated for all grid types in two and three dimension. The grid coarsening algorithm produces well defined, nested, body fitted coarser grids with aspect ratios of one and the coarse grids have similar characteristics of Cartesian grids. Then, the multigrid flow solutions are obtained at inviscid, laminar and turbulent flows. It is shown that, the convergence accelerations are up to 14 times for inviscid flows and in a range of 4 to 110 fold for turbulent flow solutions.
Citation Formats
E. Mahmutyazıcıoğlu, “Development of an octree based grid coarsening and multigrid flow solution,” Ph.D. - Doctoral Program, Middle East Technical University, 2010.