A Unified Multiscale Model for Pore‐ScaleFlow Simulations in Soils

Pore‐scale simulations have received increasing interest in subsurface sciences to provide mechanistic insights into the macroscopic phenomena of fluid flow and reactive transport processes. The application of pore‐scale simulations to soils and sediments is challenging, however, because of the characterization limitation that often allows only partial resolution of pore structure and geometry. A significant proportion of the pore spaces in soils and sediments is below the spatial resolution, forming a mixed medium with pore and porous regions. The objective of this research was to develop a unified multiscale model (UMSM) that can be used to simulate fluid flow and transport in mixed media containing pore and porous regions. The UMSM modifies the classic Navier–Stokes (N‐S) equations by adding a Darcy term to describe fluid momentum and uses a generalized mass balance equation for saturated and unsaturated conditions. A series of simulations of water flow in pore, porous, and mixed pore and porous regions were performed to evaluate the UMSM by comparing with other numerical approaches. A water imbibition experiment was conducted in a soil column to compare theoretical predictions with experimental measurements. The results indicated that the UMSM is numerically equivalent to the N‐S equations in pore regions, becomes Darcy's law in porous regions, and is equivalent to a model coupling the N‐S and Darcy's law in a mixed medium containing pore and porous regions. The UMSM‐simulated water imbibition also matched well with experimental measurements in the soil column, with its pore structures characterized from X‐ray tomography. The UMSM approach allows direct simulation of fluid flow at the voxel resolution of characterization in realistic soils and sediments under both saturated and unsaturated conditions.