Insight into Hysteretic Drying and Wetting in Unsaturated Granular Soil from Fully Resolved Computational Fluid Dynamics Analysis

The hysteresis of soil water retention curves (WRCs) originates from multiple factors including the pore geometry, contact angles at the solid–liquid interface, and pore-water morphology. This study exploits high-resolution computational fluid dynamics (CFD) to explore the mechanisms underlying this hysteresis in wetting and drying and the development of scanning curves. We consider the role of pore geometry on the processes, i.e., the ink-bottle effect, and we focus on capillary rise. Drying and wetting in single pores were simulated using fully resolved CFD, and the results are summarized in the form of WRCs. A detailed analysis of the force balance in the system reveals the role of the angle between the fluid–fluid interface and the vertical direction as the water surface moves into the bulb during drying. In wetting, residual water remaining in the pore after a drying cycle plays a key role. This difference in mechanisms contributes to the observed hysteresis. Scanning curves emerged from simulations of a single pore and a composite pore comprising two types of double-bulb systems. The analyses were expanded to simulate wetting and drying in a face-centered cubic packing of uniform spheres. The study shows that CFD simulations can elucidate the mechanisms at key transition points and capture the influence of factors including residual water on the emergent WRCs.