Investigating the characteristics of horseshoe vortex around a vertical circular cylinder in waves using particle image velocimetry: An experimental study

This study experimentally investigates the characteristics of the horseshoe vortex (HSV) surrounding a vertical circular cylinder under wave conditions, with a particular emphasis on the impact of Keulegan-Carpenter (KC) numbers and the variation of the HSV during a wave period. Utilizing time-resolved particle image velocimetry, the evolution of the HSV is analyzed using the swirling strength λci, with circulation strength serving as a measure of vortex strength. The findings highlight a marked presence of an HSV upstream of the cylinder, which evolves into a triple-vortex formation at KC numbers exceeding 6. The downstream HSV exhibits a lower intensity compared to its upstream counterpart. Interestingly, the primary HSV's strength is closely correlated with wave phase changes, exhibiting an initial decreasing and then increasing trend within a wave period.