Bursting phenomenon created by bridge piers group in open channel flow

Abstract

Bridge pier is a common feature in hydraulic structure. Its impact to the river usually occurs in group form rather than a single pier, so this challenging pier-group influence towards river hydraulics and turbulence needs to be explored. In this paper, the measurements were conducted using an Acoustic Doppler Velocimeter (ADV) to study velocities in three dimensions (longitudinal, transversal, and vertical). Based on the experimental data, we have observed reversed depth-averaged velocity vector after each pier in the group of three-pier. The analysis has been conducted on the contribution of each bursting event to Reynolds shear stress (RSS) generation, in order to identify the critical bursting events and turbulence structures around the piers. In the upstream near-wake flow in the bed-wall layer, strong sweep and ejection events have been observed; while at downstream, sweeps were more dominant. The pattern of bursting changed in the outer layer of flow, where ejections were more dominant. Furthermore, the contribution fractional ratio to RSS variation at hole size H = 0 indicates that sweeps and ejections were significantly generated at the near wake-flow in upstream.