Flow past a dual step cylinder has been investigated using experimental flow visualization methods. The dual step cylinder model is comprised of a small diameter cylinder (d) and a large diameter cylinder (D) mounted at the mid-span of the small cylinder. The experiments have been performed for ReD = 1050, D/d = 2, and a range of large cylinder aspect ratios (L/D). The focus of the study is on vortex shedding and vortex interactions occurring in the large and small cylinder wakes. A flow visualization study completed using hydrogen bubble technique and planar laser induced fluorescence has shown that the flow development is highly dependent on the aspect ratio of the large cylinder, L/D. The results identify four distinct flow regimes: (i) for L/D ≥ 17, three vortex shedding cells form in the wake of the large cylinder, one central cell and two cells of lower frequency extending over about 4.5D from the large cylinder ends, (ii) for 7 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, whose shedding frequency decreases with decreasing L/D, (iii) for 2 ≤ L/D ≤ 7, vortex shedding in the wake of the large cylinder is highly three-dimensional, such that each vortex deforms while it is shed into the wake, (iv) for 0.2 ≤ L/D ≤ 1, only small cylinder vortices are shed in the wake and often form vortex connections across the wake of the large cylinder.

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