A SEMI-COARSENING STRATEGY FOR UNSTRUCTURED MULTIGRID BASED ON AGGLOMERATION

Extending multigrid concepts to the calculation of complex compressibl e flow is usually not straightforward. This is especially true when no n-embedded grid hierarchies or volume agglomeration strategies are use d to construct a gradation of unstructured grids. In this work, a mult igrid method for solving second-order PDE's on stretched unstructured triangulations is studied. The finite volume agglomeration multigrid t echnique originally developed for solving the Euler equations is used (M.-H. Lallemand and A. Dervieux, in Multigrid Methods, Theory, Applic ations and Supercomputing, Marcel Dekker, 337-363 (1988)). First, a di rectional semi-coarsening strategy based on Poisson's equation is prop osed. The second-order derivatives are approximated on each level by i ntroducing a correction factor adapted to the semi-coarsening strategy . Then, this method is applied to solve the Poisson equation. It is ex tended to the 2D Reynolds-averaged Navier-Stokes equations with approp riate boundary treatment for low-Reynolds number turbulent flows. (C) 1998 John Wiley & Sons, Ltd.