Experimental study of flow field and sediment suspension in the gap area of tandem piles under oscillatory flows

This study experimentally investigates the local scour mechanism around tandem piles in oscillatory flows with emphasis on the flow characteristics and sediment suspension during the scouring process. The instantaneous flow field is measured by the particle image velocimetry (PIV) technology meanwhile the sediment transport is measured by an image processing method. A preprocessing technique for the same PIV particle image is employed to identify the water particle and sediment particle, thereby realizing the simultaneous measurement of flow field and sediment movement. It shows that with the temporal development of scour, the scour profiles tend to be symmetrical, and the equilibrium scour depth is expected increased with the increase of KC number. Suspended sediment is observed significant in the gap area of the piles with large magnitude of suspended sediment concentration, which contributes most to the bed scour. From the instantaneous flow field, a noticeable upward flow is occurred in the gap area, which is identified as the primary driving force lifting the sediment. From the time-averaged flow field, a vortex is formed, inducing a strong rising flow with high velocity magnitude below it, which is responsible for the suspension of sediment. It is found that there exists a linear correlation between flow velocity and sediment motion velocity.