Multigrid as a Preconditioner for Conjugate Gradients in
             the Numerical Simulation of Groundwater Flow on the
                     Cray T3D Massively Parallel Computer

                        S. F. Ashby and R. D. Falgout

                                   Abstract

The numerical simulation of groundwater flow plays an important role in the
modeling of various environmental remediation procedures.  In this paper, we
will discuss the use of multigrid as a preconditioner for conjugate gradients
in the computation of the fluid pressure for single phase, saturated flow.
The problems of interest are three-dimensional, and the subsurface medium is
heterogeneous in composition and spatial distribution.  To adequately resolv e
the fine-scale heterogeneities (which can have a profound impact on
contaminant migration), it is necessary to use computational grids with
millions of spatial zones.  We will investigate the effect of different
smoothers, boundary condition types, and coarse grid operators on the
convergence of the MGCG solver in the context of this application area.  In
addition, we will demonstrate the code's scalability on the Cray T3D and show
a video that summarizes the role of this type of simulation in environmental
modeling.