The Sediment's Incipience Mechanism Under Complex Flow

As one of the most basic problems in the sediment transport theory, the mechanism of the sediment incipience is also needed to be understood for the study of local scouring process.Critical incipient conditions for sediment motion have been studied extensively in the past years.While those works are mainly focused on the channel flows. The sediment incipience downstream of the backward-facing step flow is chosen to study the sediment incipient mechanism under complex flow in this paper. A series of experiments were conducted in the low-turbulence tunnel and the flow structures under three different conditions were studied. The evolution of the flow field downstream of the backward-facing step is obtained by the PIV measurement. The location of reattachment point, mean velocity profile and the root-mean-square fluctuant velocity were presented by the statistical analysis of the experimental data.The instantaneous flow fields downstream the backward-facing step were studied numerically by the large-eddy simulation method. The simulated results agree well with the experiment, which shows the lower mean velocity and the higher root-mean-square fluctuant velocity near the reattachment point. In the recirculation zone, with increasing distance from the step, the pressure gradient increases. The maximum value of the pressure gradient occurs at the reattachment point. The pressure gradient downstream of reattachment point almost becomes a constant, which is much lower than that at the reattachment point.An image process method was presented to measure the probability of sediment incipience quantitatively. The sediment incipient probability under the channel flows and the backward-facing step flows are measured. The measurement of sediment incipience under the channel flow shows that friction velocity is the most important mechanism contributed to the sediment incipience and the probability of sediment incipience increases with increasing friction velocity. For the backward-facing step flows, the largest sediment incipient probability occurs at the reattachment point, where the mean friction velocity is zero. In this paper, a series of experiment about the evolution of sediment bed in the backward-facing step flows were conducted in order to confirm the effects of complex flows on sediment incipience. Both the most serious erosion and the largest probability of sediment incipience occur at the location near the reattachment point.The effects of the seepage on the sediment incipience under complex flows are also studied. It is found that the probability of the sediment incipience with considering the seepage is 50% higher than that without considering the seepage.