On the attempts to reduce parasitic viscous forces in the oil-air capillary bridge simulations

Setting up a stable multiphase Volume of Fluid (VoF) numerical simulation to quantify capillary bridge forces acting on soil particles in the pendular regime presents significant computational challenges. Non-physical currents in these surface tension dominated simulations generate parasitic viscous forces that compromise the total force predictive capability of the numerical model. This study examines a two-sphere system with an intervening capillary bridge as its primary test case. OpenFOAM’s Geometric (PLIC, isoAdvector) and Algebraic (MULES) VoF approaches using the CSF Surface Tension (ST) model proved insufficient, necessitating modifications to the ST model. Two specific advanced ST models for the algebraic VoF were investigated: Laplacian Smoothing (SCSF) and Sharpening with Smoothing (SSF) of the VoF field. The parameters of the ST model require tuning based on the specific fluid combination considered. For oil-air fluid combinations characterized by a high density ratio, high viscosity, and weak surface tension forces compared to water-air systems, careful calibration of solver parameters, mesh division, and time step are essential to minimize spurious currents while preserving interface curvature accuracy.