PIV characterization of the horseshoe vortex in front of a wall-mounted cylinder in a developing turbulent boundary layer

A cylindrical obstacle intercepting the full depth of an open-channel flow changes the pressure field in its vicinity. In the upstream side of the cylinder, the adverse pressure gradient originates separation of the down-flow in front of this and near its base. The separated flow is organised in a system of vortexes dominated by a horseshoe vortex. Herein, the turbulent separated flow dominated by the horseshoe vortex in front of the cylinder is characterized in several locations along a developing turbulent boundary layer using instantaneous velocity fields obtained with Particle Image Velocimetry (PIV). The flow in a scour hole, namely the downflow, the horseshoe vortex system and the separation vortex are characterized. Mean velocities, Reynolds stresses, vorticity and streamlines of the flow field are discussed. It is shown that the intensity of the equivalent horseshoe vortex, defined as the vortex calculated for the mean velocity field, does not change considerably with Reynolds number based on the boundary layer thickness. Previously unreported results concerning the maximum size of the horseshoe vortex are presented and discussed. The intermittent nature of the main vortex is discussed and related with the occurrence of organised turbulent bursting structures. The relevance of the system of vortexes for the initiation of local scour is also assessed.