Efficient Highly Parallel Multigrid Methods for the Convection Equation.

Boris Diskin

Institute for Computer Applications in Science and Engineering (ICASE), Mail Stop 132C, NASA Langley Research Center, Hampton, VA 23681-2199.


Abstract

The convergence properties of multigrid algorithms are defined by two factors: (1) the smoothing rate which describes the reduction of high-frequency error components and (2) the quality of the coarse-grid correction which is responsible for dumping of smooth error components. In elliptic problems, all the fine-grid smooth components are well approximated on the coarse grid built by standard (full) coarsening.  In nonelliptic problems, however, some fine-grid  components that are much smoother in the characteristic direction than in other directions, cannot be approximated with standard multigrid methods. We present a novel multigrid approach to the solution of nonelliptic problems. This approach is based on semicoarsening and well-balanced explicit correction terms, added to coarse-grid operators to maintain on coarse grids the same cross-characteristic interaction as on the target (fine) grid. Multicolor relaxation schemes are used on all the levels, allowing a very efficient parallel implementation. Applications to the 2-D constant-coefficient convection operator discretized on vertex and cell centered grids are demonstrated. The resulting multigrid algorithms demonstrate the "textbook multigrid efficiency", meaning the solution to the governing problem is attained in a computational work which is a small multiple of the operation count in the discretized problem itself. Extensions to the three dimensions, to variable coefficients, and to convection-diffusion problems are discussed.