Implication of horizontal force moments for the threshold of bed entrainment in an open-channel flow

This paper reports on a numerical study of the drag moments exerted on bed gravels and the effect of near-bed coherent structures on the moments in turbulent open-channel flow. We employ the result of a numerical direct simulation (DNS) for the calculation. The particles are represented as a hexagonal arrangement of spheres and the depth of the flow is four times their diameter. The drag moments experienced by the spheres are calculated in two directions: streamwise (MD1) and spanwise (MD2). MD2 was found to be an important component for the horizontal moment, which is traditionally considered to be neglected. The majority of MD2 originates from pressure whilst the viscous force is as important as pressure for MD1. Further, the contributions of the forces from different vertical regions of the sphere to effective rotational moment are investigated. The quasi-streamwise vortices and the high-speed fluid between the vortices are strongly associated with MD2. The horizontal, rather than vertical, fluctuating velocities of both ejections and sweeps are shown to obviously affect MD1.