A 3-D finite element conjugate gradient model of subsurface flow with automatic mesh generation.

The 3-D flow modelling of groundwater systems of realistic size generally requires a big effort for the preparation of the input data as well as large computational costs. A numerical finite element model (MAITHREE) is developed for the efficient analysis of the steady and unsteady behaviour of natural confined 3-D basins. Starting from an initial triangular grid the code automatically generates a set of tetrahedral elements in each of the geologic units or subunits specified by the user in the vertical profile. The original element incidences list is then rearranged in order to provide conforming 3-D elements throughout the domain. The model is designed with a view to saving much of the labour involved in setting a 3-D grid and to providing flexibility as well as economical convenience through a high computational efficiency. The latter task is achieved by the aid of a solver based on the modified conjugate gradient (MCG) method which has proved to be an excellent technique for the solution of large linear finite element sets of sparse 3-D subsurface equations. Some examples derived from both hypothetical and real-world situations are discussed to illustrate the innovative features of MAITHREE and its computational performance.