Multiscale simulations for multi-continuum Richards equations

Abstract

In this paper, we study a multiscale method for simulating a dual-continuum unsaturated flow problem within complex heterogeneous fractured porous media. Mathematically, each of the dual continua is modeled by a multiscale Richards equation (for pressure head), and these equations are coupled to one another by transfer terms. On its own, Richards equation is already a nonlinear partial differential equation, and it is exceedingly difficult to solve numerically due to the extra nonlinear dependencies involving the soil water. To deal with multiple scales, our strategy is that starting from a microscopic scale, we upscale the coupled system of dual-continuum Richards equations via homogenization by the two-scale asymptotic expansion, to obtain a homogenized system, at an intermediate scale (level). Based on a hierarchical approach, the homogenization's effective coefficients are computed through solving the arising cell problems. To tackle the nonlinearity, after time discretization, we use Picard iteration procedure for linearization of the homogenized Richards equations. At each Picard iteration, some degree of multiscale still remains from the intermediate level, so we utilize the generalized multiscale finite element method (GMsFEM) combining with a multi-continuum approach, to upscale the homogenized system to a macroscopic (coarse-grid) level. This scheme involves building uncoupled and coupled multiscale basis functions, which are used not only to construct coarse-grid solution approximation with high accuracy but also (with the coupled multiscale basis) to capture the interactions among continua. These prospects and convergence are demonstrated by several numerical results for the proposed method.

arXiv comment45 pages, 4 figures, 2 tables, major revision. This is the accepted manuscript by Journal of Computational and Applied Mathematics (2021). The published journal article is available at https://doi.org/10.1016/j.cam.2021.113648 (2021)

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Cited by in corpus (4)

• Theory of functional connections applied to quadratic and nonlinear programming under equality constraints
• Prediction of numerical homogenization using deep learning for the Richards equation
• Constraint Energy Minimizing Generalized Multiscale Finite Element Method for multi-continuum Richards equations
• Analysis of Non-local Multicontinuum Upscaling for Dual Continuum Model