A combined method consisting of mixed finite element method (MFEM) for the pressure equation and expanded mixed finite element method with characteristics(CEMFEM) for the concentration equation is presented to solve the coupled system of incompressible miscible displacement problem. To solve the resulting nonlinear system of equations efficiently, the two-grid algorithm relegates all of the Newton-like iterations to grids much coarser than the original one, with no loss in order of accuracy. It is shown that coarse space can be extremely coarse and our algorithm achieve asymptotically optimal approximation when the mesh sizes satisfy H = O(h 1 4 ). Numerical experiment is provided to confirm our theoretical results.
K E Y W O R D Scharacteristic expanded mixed finite element method, miscible displacement, mixed finite element method, two-grid method
INTRODUCTIONNumerical model for incompressible two-phase flow in porous media was investigated extensively in the past three decades, 1-13 due to its wide applications in hydrology and petroleum reservoir engineering. Standard Galerkin methods tend to generate unacceptable nonphysical and oscillations in the concentration approximations, since the concentration equation is convection dominated. The method of characteristics is more effective for solving such a coupled system. 2,7,8,10,11,14,15 Characteristic method was introduced and analyzed by Douglas et al. for a single convection-dominated diffusion equation. 14 Later, the method was extended to the nonlinear miscible displacement problem. 7,10,11,14 There are various efficient numerical algorithms for nonlinear problems. 12,[16][17][18][19][20][21][22][23][24][25][26][27][28][29] Many of them are in spirit of domain decomposition in general. For instance, Bornemann and Deuflhard 29 developed the usual cascadic multigrid method without the coarse grid corrections for elliptic problems. Later, Shi et al. 30 proposed cascadic multigrid method for parabolic problems and developed 31 economical cascadic multigrid method. Recently, we have extended two-grid method to miscible displacements problems, 10,11,32,33 due to two-grid method that relegates all of the Newton-like iterations to grids much coarser than the original one, with no loss in order of accuracy. In Reference 11, we proposed two-grid algorithm-based mixed finite element method (MFEM) and mixed finite element method of characteristics (CEMFEM) for miscible displacements problems, furthermore, this method was extended to miscible displacement problem with dispersion term. 32 However, the standard MFEM is not suitable for problems with small tensor coefficients since we need to invert the tensor D. 19,20,34 As a continued work of our work, 11,32 the purpose of this paper is to propose two-grid Numer Linear Algebra Appl. 2020;27:e2292. wileyonlinelibrary.com/journal/nla