Vortex dislocation characteristics of a dual-step cylinder in low-Re flow

Abstract

Flow around a dual step cylinder for D/d = 2 and L/D = 5 at a low Reynolds number of ReD = 200 is numerically investigated utilising the immersed boundary method. The vortex dislocation caused by two steps is investigated. It was found that a pair of streamwise vortex - the centre vortex exists at the downstream of the large cylinder. Meanwhile, due to the barrier effect of the centre vortices, a very weak L-cell exists behind the centre of the large cylinder. In a complete dislocation cycle, three positive and negative half-loop connections appear alternately. This cycle is related not only to the phase realignment of the different vortex cells, but also to the connection realignment. Moreover, the lateral velocity is always dominated by the vortex shedding frequency. While in the streamwise velocity spectra, the vortex shedding frequency gradually dissipates in the downstream, leaving lower frequencies of dislocation.