A numerical study of the particle-bed collision process during aeolian transport on moist sand surfaces

Introduction
The collision between saltating particles and the sand bed is widely recognized as the primary mechanism driving particle entrainment in aeolian transport. Sand moisture, a critical factor in natural environments, significantly alters particle dynamics during these collisions, thereby influencing transport patterns. The commonly used laws to describe this process, known as splash functions, are extensively employed in the numerical modeling of aeolian sediment transport as a bridge between the particle scale and subsequent larger scales (Anderson & Haff, 1988; Kok & Renno, 2009). However, these splash functions are primarily derived from experiments involving single-particle impacts on static, dry or moist sediment beds in the absence of wind (Beladjine et al., 2007; Ralaiarisoa et al., 2022). Recent studies, however, highlight substantial deviations in the ejection processes observed during saltation events with the presence of wind compared to controlled single-particle impact experiments without wind (Jia & Wang, 2022; Tholen et al., 2023; Jiang et al., 2024). Despite these findings, the influence of moisture on ejection dynamics during full saltation events remains poorly understood, creating a critical gap in our understanding of moisture-modified aeolian processes.

Objective and Methods
This study aims to investigate the influence of sand moisture on rebound and ejection dynamics during particle-bed collisions in aeolian transport using discrete particle simulations. Moist sand particles are represented by an advanced particle model incorporating evolving liquid bridges. A wind flow model is two-way coupled to the particle model, accounting for fluid-particle interactions. By simulating the transport evolution from initially a few mobile particles and driven by wind towards steady-state transport, extensive particle statistics are obtained. A detailed analysis identifies the moments of particle collisions with the bed and subsequent ejections. The coefficient of restitution for saltating particles and the properties of ejected particles are then evaluated as functions of impact velocity and moisture content, providing the basis for a new moisture-dependent splash function during saltation.

Results
The statistical analysis reveals that in the steady state of aeolian transport, rebound and ejection properties exhibit minimal sensitivity to moisture content. However, during the transient phase, the coefficient of restitution for saltating particles increases with moisture content. Additionally, moisture reduces the average number of ejected particles per impact while simultaneously increasing the mean ejection velocity. Based on these findings, a new splash function was developed from the simulation data, offering a promising improvement for predicting aeolian transport dynamics on moist surfaces.