Flow around a scoured bridge pier: a stereoscopic PIV analysis

We performed stereoscopic particle image velocimetry of the turbulent flow inside a scour hole around a cylinder in a sandy bed. At two planes, symmetry plane and $$45^\circ$$ with respect to the approach flow, the flow and its turbulence structure were investigated. We used two Reynolds numbers (20, 000 and 39, 000) based on the cylinder diameter and the depth-averaged velocity in the symmetry plane. The flow is characterized by a strong down-flow in front of the cylinder, a large horseshoe vortex inside the scour, and an upstream directed wall jet underneath. The values of vorticity in the horseshoe vortex and of the velocity in the wall jet are larger than in a comparable configuration on a flat bed. Enhanced levels of turbulent kinetic energy are found around the horseshoe vortex and in the shear layer detaching from the rim. The orientation of the main axis of the velocity fluctuations changes when the flow enters the scour hole: from about wall-parallel in the detaching shear layer to vertical at the horseshoe vortex. The production of turbulent kinetic energy shows a maximum upstream of the horseshoe vortex centre with considerable production in the shear layer and in the wall jet underneath the horseshoe vortex. Furthermore, strong wall-parallel velocity fluctuations are visible in this region, and bimodal velocity distributions are found, but not anywhere else. The time-averaged wall-shear stresses are largest under the horseshoe vortex and most likely larger than in a corresponding flat-bed configuration.