Sensitivity of Suspension Pattern of Numerically Simulated Sediments to Oscillating Periods of Channel Flows over a Rippled Bed

Abstract

In this study, we investigated the sediment suspension process over a ripple, using large eddy simulation and a Lagrangian particle tracking model to numerically calculate the individual motion of sediment grains in turbulent boundary layer flows. We examined the relationship of the sediment convection process with the formation of lee eddies at the back of the ripple. We tested three flow conditions by changing the oscillating period while keeping other conditions. In all three cases lee eddies developed, but they formed at different flow phases and differed in their duration. These lee eddies significantly influenced the turbulent kinetic energy distribution and sediment suspension pattern as they showed phase differences between the cases, according to their lee eddy formation. We compared two types of sediment suspension rates by counting the number of suspended sediment grains. Both these rates showed that maximum sediment pick-up occurred earlier in the oscillating flow phase as the period increased, but the magnitude of the pick-up rate decreased with increasing period. The results of our study indicate that the suspension pattern may vary significantly with the period. Therefore, formulas for sediment pick-up rates, which usually give maximum rates at the time of flow reversal over ripples, may require an additional controlling factor-the oscillation period. The sediment suspension pattern, however, shows some discrepancies from that observed by van der Werf et al. (2007) as the sediments were not suspended at the time of flow reversal. This is likely due to the low steepness of the ripple and the employment of an unrealistically small sediment size in our study experiment.