Coupled effects of bed structure and neighboring particles on forces, torques, and incipient motion of exposed particles above a particulate bed

Understanding the hydrodynamic forces on exposed particles above a particulate bed is crucial for predicting particle resuspension and sediment transport. This study uses the lattice Boltzmann method to quantify the forces and torque acting on particles located above structured porous beds exposed to a linear shear flow, as a function of bed structure and number density of exposed particles. Increasing gap distance between bed particles (increasing bed porosity) causes the exposed particles to sink downward in the bed, which decreases associated forces and torque due to enhanced shielding from upstream bed particles. Multiple exposed particles reduce the near-bed flow, so when multiple particles sink lower in a bed with large gap distances, the flow interference is reduced. This has the effect of increasing the local fluid velocity incident on the particles and increasing the particle forces and torque. These two opposing mechanisms largely offset one another, resulting in drag and lift forces on exposed particles that are nearly insensitive to bed porosity at sufficiently high exposed-particle number densities. A torque balance analysis of the particle rolling pivot axis demonstrates that the drag force is primarily responsible for incipient particle rolling, although the torque induced by the drag force decreases with increasing particle gap distance. A computation of the critical Shields parameter yields results within the range of experimentally observed values. The findings highlight the local gap distance between bed particles as a key parameter influencing the incipient motion of exposed particles, through its effects on both torque balance and the critical Shields parameter.