Comparison of different 2 order formulations for the solution of the 2D groundwater flow problem over irregular triangular meshes

Mixed and Mixed Hybrid Finite Elements (MHFE) methods have been widely used in the last decade for simulation of groundwater flow problem, petroleum reservoir problems, potential flow problems, etc. The main advantage of these methods is that, unlike the classical Galerkin approach, they guarantee local and global mass balance, as well the flux continuity between inter-element sides. The simple shape of the control volume, where the mass conservation is satisfied, makes also easier to couple this technique with a Finite Volume technique in the time splitting approach for the solution of advection-dispersion problems. In the present paper, a new MHFE formulation is proposed for the solution of the 2D linear groundwater flow problem over domain discretized by means of triangular irregular meshes. The numerical results of the modified MHFE procedure are compared with the results of a modified 2 spatial approximation order Finite Volume (FV2) formulation [2], as well as with the results given by the standard MHFE method. The FV2 approach is equivalent to the standard MHFE approach in the case of isotropic medium and regular or mildly irregular mesh, but has a smaller number of unknowns and better matrix properties. In the case of irregular mesh, an approximation is proposed to maintain the superior matrix properties of the FV2 approach, with the consequent introduction of a small error in the computed solution. The modified MHFE formulation is equivalent to the standard MHFE approach in both isotropic and heterogeneous medium cases, using regular or irregular computational meshes, but has a smaller number of unknowns for given mesh geometry. Key-Words: groundwater, finite elements method, mixed hybrid finite elements method, finite volumes method, positive-definite matrix, M-property, Raviart-Thomas basis function