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. INTRODUCTION The present investigation is focused on the flow development over dual-step cylinders. A dual-step cylinder, shown in Fig. 1, consists of a large diameter cylinder (D) of low aspect ratio (L/D) attached coaxially to the mid-span of a small diameter cylinder (d). It is instructive to consider the dual-step cylinder geometry in limiting cases of D/d and L/D. As D/d approaches unity, the dual-step cylinder approaches the geometry of a uniform circular cylinder. On the other hand, for large D/d, the dual-step cylinder resembles a cylinder with two free ends. Hence, it is expected that, depending on the geometrical parameters of the model, the flow development for a dual-step cylinder may be similar to that found for a low aspect ratio uniform cylinder (e.g., Norberg, 1994), a cylinder with free ends (e.g., Inoue & Sakuragi, 2008), or a coin-like cylinder (e.g., Zdravkovich et al., 1998). For uniform circular cylinders mounted between endplates, the frequency of vortex shedding tends to decrease with decreasing aspect ratio for about L/D < 30 (Norberg, 1994). For L/D < 7, the coherence and strength of the vortex shedding decreases, with regular vortex shedding being replaced by the formation of less coherent wake structures (Norberg, 1994). Zdravkovich et al. (1989, 1998) and Inoue & Sakuragi (2008) investigated the flow development over uniform cylinders with two free ends. In the laminar vortex shedding regime, Inoue & Sakuragi (2008) found that three wake patterns can occur depending on ReD and L/D: (i) spanwise vortex shedding, (ii) a steady wake consisting of two counter-rotating streamwise vortices, and (iii) alternate shedding of streamwise vortex pairs from the flat cylinder ends. Zdravkovich et al. (1989) performed experiments for 6000 ≤ ReD ≤ 26000 and 2 ≤ L/D ≤ 8. For all models investigated, spanwise vortex shedding occurred in the wake, however, it was intermittent, and the shedding frequency varied with time. Zdravkovich et al. (1998) investigated the flow development over coin-like cylinders for 200000 ≤ ReD ≤ 600000 and U0 L D d