Rolling and lifting thresholds of sediment entrainment: Effects of bed particle arrangement, particle exposure and slip-spinning motion

We explore the rolling and lifting thresholds of sediment entrainment in turbulent flow, focusing on loosely packed bed sediment particles in tetrahedral and square pyramid arrangements. In steady uniform flow, we thoroughly analyse the forces acting on a target particle to determine the threshold Shields parameters in both rolling and lifting modes for various relative exposures. The effects of slip-spinning motion of the target particle on rolling and lifting thresholds are examined through numerical experiments, comparing cases without and with slip-spinning motion. In both cases, for a given relative exposure, the lifting threshold for a given particle parameter is higher than the rolling threshold. In rough flow, both thresholds increase with a decrease in relative exposure. For smaller relative exposures, the rolling and lifting thresholds with slip-spinning motion are lower than those without slip-spinning motion. However, for larger relative exposures, the effects of slip-spinning motion on rolling and lifting thresholds are minimal. The influence of bed particle arrangement is limited to the rolling threshold. For non-uniform sediment, in rough flow, both thresholds corresponding to the average relative exposure increase as the relative particle size increases. The present study satisfactorily captures the experimental data of both uniform and non-uniform sediment.