Evolution of Hydrodynamic Characteristics with Scour Hole Developing around a Pile Group

This study concerns the evolution of flow field and hydrodynamic characteristics within the developing scour hole around a four-pile group with 2 x 2 arrangement. The instantaneous velocities in scour holes at four typical stages during the scouring process were measured by an acoustic Doppler velocimeter (ADV). The evolution and spatial distribution of the time-averaged flow field, turbulence, and the corresponding hydrodynamic characteristics within scour holes were compared. The time-averaged flow field shows that the reverse flow, downward flow, and horseshoe vortex are formed in the upstream of the pile group. During the scouring process, the mean components of flow characteristics (i.e., mean velocity, vorticity, and bed shear stress) around the pile group decrease while the fluctuating components (i.e., turbulence intensity) intensify simultaneously. Similarity of turbulence intensity profiles was found within different scour holes. The horseshoe vortex at upstream of each pile merges and the shear layer in the gap region extends when the dimension of the scour hole increases to that of equilibrium scour status, indicating that the four piles behave more like a single bluff body. With the development of scour holes, the large-scale horseshoe vortex system becomes more stable and the dissipation of small-scale eddies becomes more significant.