An improved semi-resolved CFD-DEM method for particle systems with wide mesh/particle size ratios

Solid-liquid two-phase flows with particles of a wide size range are widespread in geotechnical engineering. The CFD-DEM method is valid for solid–fluid coupling analysis, but traditional methods are limited in size applicability. This study proposes an improved semi-resolved CFD-DEM method capable of simulating systems with wide mesh/particle size ratios ( L / d ). In the method, a dynamic coupling strategy is adopted for different L / d : when the mesh size is much larger than the particle diameter, a gradient-based interpolation method is used to reconstruct the fluid velocity around the particles; when the mesh size is comparable to or smaller than the particle diameter, inter-phase forces are corrected through an extended domain. The proposed method is validated through the simulation of two typical cases, including single particle settling and collapse of granular piles, and is applied to upward seepage in sandy soils. Simulation results show that the method not only accurately reflects macroscopic phenomena, but also effectively captures the characteristics of the local flow field around particles in wide L / d systems, thereby revealing the mesoscopic mechanisms of particle–fluid interactions. Furthermore, the simulation of upward seepage indicated that the non-uniformity of the flow field drives the preferential migration of fine particles, which subsequently induces piping in gap-graded soils.