Flow and turbulence structure around an in-stream rectangular cylinder with scour hole

Most of the erosion around obstacles present in alluvial streams takes place after the formation of a scour hole of sufficiently large dimensions to stabilize the large-scale oscillations of the horseshoe vortex (HV) system. The present paper uses eddy resolving techniques to reveal the unsteady dynamics of the coherent structures present in the flow field around an in-stream vertical cylinder (e. g., bridge pier) with a large scour hole at a channel Reynolds number defined with the channel depth and the bulk channel velocity of 2.4 x 10(5). 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 conducted as part of the present work. 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. The paper analyzes the changes in the flow and turbulence structure with respect to the initial stages of the scour process (flat bed conditions) for a cylinder of identical shape and orientation. Results show the wake loses its undular shape due to suppression of the antisymmetrical shedding of the roller vortices. Also, the nature of the interactions between the necklace vortices of the HV system and the eddies present inside the detached shear layers (DSLs) changes as the scour process evolves. 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. The paper also examines the effect of the shape of the obstruction on the dynamics of the vortical eddies and how it affects the bed erosion processes during the advanced stages of the local scour. In particular, the paper provides an explanation for the observed increase in the maximum scour depth for bed-mounted cylinders of rectangular section compared to cylinders of same width but of circular section. This increase is explained by the larger coherence of the HV system, the increased regularity of the interactions between the legs of the necklace vortices and the eddies shed in the DSLs, and the stronger coherence of the wake eddies, during both the initial and the later stages of the local scour process, for cases in which the obstruction has sharp edges that fix the separation point on the in-stream obstacle at all flow depths (e.g., rectangular cylinder).