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Christopher Morton
Department of Mechanical & Mechatronics Engineering University of Waterloo 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada; Department of Mechanical & Manufacturing Eng. University of Calgary 2500 University Dr. NW, Calgary, Alberta, Canada

Serhiy Yarusevych
Mechanical and Mechatronics Eng. University of Waterloo 200 University Avenue West Waterloo, Ontario, N2L 3G1 Canada


Vortex shedding in the wake of dual step cylinders was investigated experimentally using flow visualization and Laser Doppler Velocimetry (LDV). The dual step cylinder model consists of a small diameter cylinder (d) and a large diameter cylinder (D) mounted at the mid-span of the small cylinder. Experiments were performed for ReD = 1050, D/d = 2, and large cylinder aspect ratios in the range of 0.2≤L/D≤17. Hydrogen bubble flow visualization results show that the wake development is highly dependent on the aspect ratio of the large cylinder, L/D. Spectral analysis of streamwise velocity signals obtained in the dual step cylinder wake was used to estimate the shedding frequency variation along the model span. 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, (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, large cylinder vortex shedding is highly three-dimensional, and the frequency of the shedding continues to decrease with L/D, (iv) for 0.2 ≤ L/D ≤ 1, the large cylinder induces vortex dislocations between small cylinder vortices whose frequency of occurrence decreases with L/D.