Turbulent vortex shedding from a dual-step cylinder: Influence of diameter ratio and aspect ratio

In the present study, the influence of aspect ratio (L/D) and diameter ratio (D/d) on the flow development past a dual-step cylinder is investigated experimentally for ReD = 2100, 1.33 ≤ D/d ≤ 2.67, and 0.2 ≤ L/D ≤ 3. Experiments are carried out in a water flume facility using Laser Doppler Velocimetry (LDV), Particle Image Velocimetry (PIV), and hydrogen bubble flow visualization. The results show that four distinct vortex shedding regimes can be identified based on changes in large cylinder wake development. Depending on the geometrical parameters of the dual-step cylinder, the wake of the large cylinder may involve (i) vortex shedding at a frequency lower than that expected for a uniform cylinder at the same Reynolds number, (ii) irregular shedding, (iii) vortex shedding at a frequency higher than that for a uniform cylinder, and (iv) suppression of large cylinder vortex shedding. Based on the present results and those from previous studies on relevant geometries, a map of wake regimes for a dual-step cylinder has been constructed.