Abstract
The vortex system around the step surface of a step cylinder with a diameter ratio D/d=2 at Reynolds number (ReD) 3900 was investigated by directly solving the three-dimensional Navier–Stokes equations. Formation mechanisms and vortex dynamics of the complex vortex system were studied by performing a detailed investigation of both the time-averaged and instantaneous flow fields. For the time-averaged flow, including the known junction and edge vortices, in total, four horseshoe vortices were observed to form above the step surface in front of the upper small cylinder. The crossflow width of the four horseshoe vortices varies differently as they convect downstream. Moreover, we captured a pair of base vortices and a backside horizontal vortex in the rear part of the step surface behind the small cylinder. For the instantaneous flow, hairpin vortices were found to form between the legs of two counter-rotating horseshoe vortices located on the same side of the step cylinder. Furthermore, in the small step cylinder wake, Kelvin–Helmholtz vortices were observed to shed at an unexpectedly high shedding frequency