Strategies for parallel and numerical scalability of CFD codes

In this article we discuss a strategy for speeding up the solution of the N avier-Stokes equations on highly complex solution domains such as complete aircraft, spacecraft, or turbomachinery equipment. We have used a finite-vo lume code for the (non-turbulent) Navier-Stokes equations as a testbed for implementation of linked numerical and parallel processing techniques. Spee dup is achieved by the Tangled Web of advanced grid topology generation, ad aptive coupling, and sophisticated parallel computing techniques. An optimized grid topology is used to generate an optimized grid: on the bl ock level such a grid is unstructured whereas within a block a structured m esh is constructed, thus retaining the geometrical flexibility of the finit e element method while maintaining the numerical efficiency of the finite d ifference technique. To achieve a steady state solution, we use grid-sequen cing: proceeding from coarse to finer grids, where the scheme is explicit i n time. Adaptive coupling is derived from the observation that numerical sc hemes have differing efficiency during the solution process. Coupling stren gth between grid points is increased by using an implicit scheme at the sub -block level, then at the block level, ultimately fully implicit across the whole computational domain. Other techniques include switching numerical s chemes and the physics model during the solution, and dynamic deactivation of blocks. Because the computational work per block is very variable with a daptive coupling, especially for very complex flows, we have implemented pa rallel dynamic load-balancing to dynamically transfer blocks between proces sors. Several 2D and 3D examples illustrate the functioning of the Tangled Web approach on different parallel architectures. (C) 1999 Elsevier Science S.A. All rights reserved.