Flow and Turbulence Structure Around an In-Stream Rectangular Cylinder with Scour Hole

Abstract

An eddy resolving technique is used to reveal the unsteady dynamics of the coherent structures present in the flow field around an in-stream vertical cylinder with a scour hole at a channel Reynolds number of 240000. Such an investigation is important as most of the erosion around obstacles present in alluvial streams takes place after a scour hole of sufficiently large dimensions to stabilize the large-scale oscillations of the horseshoe vortex (HV) system has formed. The cylinder has a rectangular section and is placed perpendicular to the incoming flow. The geometry of the scour hole is obtained from an experiment. The mechanisms driving the bed erosion during the advanced stages of the scour process around the vertical plate are discussed. Simulation results demonstrate the critical role played by these large-scale turbulent eddies and their interactions in driving the local scour. Results show that important changes in the structure of the wake (e. g. the wake loses its undular shape due to suppression of the anti-symmetrical shedding of the roller vortices) and the nature of the interactions between the necklaces vortices of the HV system and the eddies present inside the detached shear layers (DSLs) occurs as the scour process proceeds. This means that information on the vortical structure of the flow at the initiation of the scour process or during its initial stages are insufficient to understand the local scour mechanisms.