Analysis of lowest-order characteristics-mixed FEMs for incompressible miscible flow in porous media

TL;DR

This paper establishes optimal second-order error estimates for the lowest-order characteristics-mixed finite element method applied to incompressible miscible flow in porous media, improving upon previous first-order accuracy results.

Contribution

The paper introduces an elliptic quasi-projection technique to achieve optimal error estimates, including second-order accuracy for concentration and first-order for pressure/velocity.

Findings

Proves second-order convergence in L^2 norm for concentration.

Achieves first-order accuracy for pressure/velocity.

Numerical results confirm theoretical error estimates.

Abstract

The time discrete scheme of characteristics type is especially effective for convection-dominated diffusion problems. The scheme has been used in various engineering areas with different approximations in spatial direction. The lowest-order mixed method is the most popular one for miscible flow in porous media. The method is based on a linear Lagrange approximation to the concentration and the zero-order Raviart-Thomas approximation to the pressure/velocity. However, the optimal error estimate for the lowest-order characteristics-mixed FEM has not been presented although numerous effort has been made in last several decades. In all previous works, only first-order accuracy in spatial direction was proved under certain time-step and mesh size restrictions. The main purpose of this paper is to establish optimal error estimates, $i.e.$, the second-order in $L^2$-norm for the concentration and the first-order for the pressure/velocity, while the concentration is more important physical component for the underlying model. For this purpose, an elliptic quasi-projection is introduced in our analysis to clean up the pollution of the numerical velocity through the nonlinear dispersion-diffusion tensor and the concentration-dependent viscosity. Moreover, the numerical pressure/velocity of the second-order accuracy can be obtained by re-solving the (elliptic) pressure equation at a given time level with a higher-order approximation. Numerical results are presented to confirm our theoretical analysis.