A Large-Eddy Simulation Study on Effects of Wave Shape and Phase-Lag in Oscillatory Sheet Flow

The effect of wave shape on sediment transport in the sheet flow regime is investigated using a large-eddy simulation (LES) Eulerian two-phase flow model. The numerical model is first validated with measured data from oscillating water tunnel for medium and fine sand sheet flows driven by velocity-skewed and acceleration-skewed flows. Simulation results are used to explain observed distinct behaviors of a net positive (or negative) transport rate for medium (or fine) sand driven by velocity-skewed flows and an increase of positive transport rate in fine sand driven by acceleration-skewed flows relative to that of medium sand. Model results show that the phase-lag effect typically used to explain these behaviors not only depends on the magnitude of settling velocities but also correlated with sheet flow instabilities occurred at different instants, depending on wave shapes, which enhance sheet flow layer thickness and the importance of suspended load. The phase-lag effect also causes an imbalance of pickup and deposition flux between the crest and trough intervals and the resulting net transport is related to the phase when the deposition flux becomes dominant.