FILTERING NON-SOLENOIDAL MODES IN NUMERICAL-SOLUTIONS OF INCOMPRESSIBLE FLOWS

Solving the incompressible Navier-Stokes equations requires special ca re if the velocity field is not discretely divergence-free. Approximat e projection methods and many pressure Poisson equation methods fall i nto this category. The approximate projection operator does not dampen high frequency modes that represent a local decoupling of the velocit y held. For robust behavior, filtering is necessary. This is especiall y true in two instances that were studied: long-term integrations and large density jumps. Projection-based filters and velocity-based filte rs are derived and discussed. A cell-centered velocity filter, in conj unction with a vertex-projection filter, was found to be the most effe ctive in the widest range of cases. (C) 1998 John Wiley & Sons, Ltd.