Experimental study on the multimodal dynamics of the turbulent horseshoe vortex system around a circular cylinder

Abstract

A turbulent horseshoe vortex (HV) system is generated around a wall-normal cylinder when the approaching boundary layer separates from the wall. This study investigates the dynamics of the turbulent HV system around a circular cylinder in open channel flows with cylinder Reynolds numbers ranging from 8600 to 13900. The velocity fields in the upstream symmetry plane of the cylinder are measured using time-resolved particle image velocimetry. The joint probability density function of the streamwise and vertical velocities in the HV system region is found to exhibit three peaks, indicating that three major types of flow events are induced by the turbulent HV system. The conditional averaged velocity fields based on the characteristic velocity vectors of these events are obtained by using the method of linear stochastic estimation. The estimated flow fields reveal that the turbulent HV system interplays mainly among the back-flow, intermediate, and zero-flow modes. These modes are present for the smallest, moderate, and largest percentage of time, respectively, within the present Reynolds-number range. The major mechanism for triggering the zero-flow mode is the occurrence of an inrush of high-momentum fluid from the inner region of the approaching flow. The intermediate mode appears when the inrush of fluid is weaker than the reverse flow below the primary HV or a tertiary vortex approaches the primary HV.