Parallel Modeling of Three-dimensional Variably Saturated Ground Water Flows with Unstructured Mesh using Open Source Finite Volume Platform Openfoam

Abstract We report the development of a three-dimensional open source model suGWFoam, based on the multiphysics platform OpenFOAM for variably saturated flow in porous media. The nonlinear Richards equation is spatially discretized on unstructured meshes using finite volume method in conjunction with the modified Picard iterative method, which inherently guarantees conservation of mass. A switching algorithm between different forms of the Richards equation is implemented and three choices of convergence criteria suitable for different simulation scenarios are provided. The linearized equation results in a symmetric matrix system which can be solved with one of the following Krylov subspace linear solvers: preconditioned conjugate gradient (PCG), incomplete Cholesky preconditioned conjugate gradient (ICCG) and geometric agglomerated algebraic multigrid (GAMG). Various preconditioners are runtime selectable. Parallel computation through domain decomposition shows good speedup for large scale simulations. Comparing to other available codes for ground water, the merits of this model reside in the architecture of OpenFOAM. Built upon the modularized and hierarchical design of the platform, this model has less than several hundreds of lines of code on its top level and yet has the full functionalities for three-dimensional unstructured mesh, parallel computing, and numerous choices of spatial and temporal discretization schemes. This liberates the researchers in the water resource community from the tedious and long process of coding. It is also extremely easy to couple groundwater flow with surface water hydrodynamics using existing flow solvers (such as channel flows and waves) in OpenFOAM. This model is fully functional and validations show excellent agreement with data in literature for a wide range of applications. It can be used for both fundamental researches and real world applications.

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