Numerical investigation of particle saltation in the bed-load regime

This paper numerically investigates particle saltation in a turbulent channel flow having a rough bed consisting of 2–3 layers of densely packed spheres. In this study, we combined three the state-of-the-art technologies, i.e., the direct numerical simulation of turbulent flow, the combined finite-discrete element modelling of the deformation, movement and collision of the particles, and the immersed boundary method for the fluid-solid interaction. Here we verify our code by comparing the flow and particle statistical features with the published data and then present the hydrodynamic forces acting on a particle together with the particle coordinates and velocities, during a typical saltation. We found strong correlation between the abruptly decreasing particle stream-wise velocity and the increasing vertical velocity at collision, which indicates that the continuous saltation of large grain-size particles is controlled by collision parameters such as particle incident angle, local rough bed packing arrangement, and particle density, etc. This physical process is different from that of particle entrainment in which turbulence coherence structures play an important role. Probability distribution functions of several important saltation parameters and the relationships between them are presented. The results show that the saltating particles hitting the windward side of the bed particles are more likely to bounce off the rough bed than those hitting the leeside. Based on the above findings, saltation mechanisms of large grain-size particles in turbulent channel flow are presented.