Effects of bed roughness on a horseshoe vortex in overland water flowing past a cylinder

Abstract

A horseshoe vortex (HV) system is formed around a bed-mounted circular cylinder in overland water flowing past the cylinder; however, the effect of bed roughness on HV still lacks clear recognition. The present study experimentally investigated the characteristics of HV upstream of a bed-mounted circular cylinder in a flume, where sand of four grain sizes was glued to the flume bed to produce the bed roughness. A particle image velocimetry system was mounted on the flume to facilitate a high-resolution (63 pixel/mm) observation of a flow field. A total of 28 cases with flow depths varying from 0.39 to 1.09 cm and cylinder Reynolds numbers (ReD) varying from 2700 to 5800 were selected to evaluate the effects of the bed roughness on HV in overland flow conditions. Measurements were then conducted on the upstream flow of a cylinder in the symmetry plane. Bed roughness significantly affected HV, in turn destroying the three vortices. Subsequently, time-averaged primary HV features on the three beds with lower roughness levels were analyzed in terms of the separation point, location, radius, and swirling strength. With an increase in the bed roughness, the separation point moved toward the cylinder, and the primary HV moved toward a free surface; however, the effect on the streamwise location was nonsignificant. A positive relationship was observed between the radius of the primary HV and the bed roughness, whereas a negative relationship was observed between the swirling strength and bed roughness. For beds with the highest roughness levels, all vortices were destroyed, and there was no clear HV topology in the time average sense. Finally, the motion process of the HV was summarized as showing an increase in the bed roughness. The present study investigated HV features with different bed roughness levels and has the potential to enhance our theoretical understanding of the relationship between HV kinematic characteristics and the bed roughness.