The characteristics of the flow around a 50mm circular cylinder surrounded by a permeable outer cylinder were investigated by Particle Image Velocimetry (PIV) and flow visualization techniques in order to control the unsteady flow structure downstream of the cylinder in shallow water. The effect of outer permeable cylinder with a porosity of P=0.4 on the flow control was studied using five different diameters; D=60, 70, 80, 90, 100mm. Depth-averaged free stream velocity was kept constant as U=170mm/s corresponding to a Reynolds number of Re=8500 and the water height was adjusted to h_w=25mm throughout the study. The results clearly showed that the outer permeable cylinder significantly affects the flow structure of the inner cylinder. It was found that by the existence of outer cylinder, the frequency of unsteady vortex shedding is reduced, vortex formation region is elongated and fluctuations are attenuated which are good indications of effective flow control. Owing to the results, optimum parameters were defined and suggested for the suppression of vortex-induced vibrations on bluff bodies.
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