Global random walk solvers for fully coupled flow and transport in saturated/unsaturated porous media (extended version)

TL;DR

This paper introduces global random walk algorithms for solving coupled flow and transport in porous media, effectively handling nonlinearity and degeneracy, with high accuracy and minimal numerical diffusion, validated through benchmark comparisons.

Contribution

The paper develops novel GRW L-schemes for coupled flow and transport in porous media, addressing nonlinearity and degeneracy with convergence guarantees and high accuracy.

Findings

GRW L-schemes converge with iterations

Solutions are first-order in time, second-order in space

Methods are practically free of numerical diffusion

Abstract

In this article, we present new random walk methods to solve flow and transport problems in unsaturated/saturated porous media, including coupled flow and transport processes in soils, heterogeneous systems modeled through random hydraulic conductivity and recharge fields, processes at the field and regional scales. The numerical schemes are based on global random walk algorithms (GRW) which approximate the solution by moving large numbers of computational particles on regular lattices according to specific random walk rules. To cope with the nonlinearity and the degeneracy of the Richards equation and of the coupled system, we implemented the GRW algorithms by employing linearization techniques similar to the $L$-scheme developed in finite element/volume approaches. The resulting GRW $L$-schemes converge with the number of iterations and provide numerical solutions that are first-order accurate in time and second-order in space. A remarkable property of the flow and transport GRW solutions is that they are practically free of numerical diffusion. The GRW solutions are validated by comparisons with mixed finite element and finite volume solutions in one- and two-dimensional benchmark problems. They include Richards' equation fully coupled with the advection-diffusion-reaction equation and capture the transition from unsaturated to saturated flow regimes. For completeness, we also consider decoupled flow and transport model problems for saturated aquifers.