Influence of near-bed coherent structures on the entrainment of bed gravels

Abstract

This paper presents the results of a numerical investigation into the forces acting on bed particles in fully developed turbulent open-channel flow. A direct numerical simulation (DNS) has been carried out using a parallel complex-geometry code with the bed particles represented as a hexagonal arrangement of spheres and the depth of flow four times their diameter. The horizontal forces experienced by the spheres are calculated in the streamwise (FD1) and spanwise (FD2) directions respectively. It was found that the moment produced by FD2 is of the same order of magnitude as the moment of FD1, which is traditionally considered as the main drag force. The contributions of the forces at different heights of sphere to the effective rotational moment are investigated. Moreover, the results indicate that FD2 is associated with near-bed quasi-streamwise vortices and the high speed fluid between these vortices. They also reveal that ejection events are more favorable than sweeps for entrainment whilst the latter accounts for the large FD1 and makes the bed stable.