Unsteady Flow Past a Circular Cylinder Started Impulsively

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The growth of the vortex-pair developed behind a circular cylinder was investigated experimentally, when the cylinder was moved impulsively in a water tank. The experimental conditions were Re = Ud/ν = 10^2~2×10^3, d/h =0.04~0.67 where U is the cylinder speed, d the cylinder diameter and h the distance between side walls of the tank. The detailed patterns of the vortex sheets and path lines around the cylinder were visualized by means of electrolysis and Al-dust methods. When the ratio d/h is decreased to 0.1, the effects of the tank walls on the flow are substantially negligible. The vortex sheets roll up near the separation points when Re> 520 and a secondary small vortex-pair is formed on the upstream side of the main vortices. The pattern of the vortex sheets shedding from the secondary vortices changes as Re is increased. This phenomenon is closely related to the manner of growth and collapse of the main vortices. The wall effects were examined within the range of d/h = 0.1~0.67. When d/h>0.5, the separation of flow occurs also at the side walls and "wall vortices" are formed. These vortices deform the pattern of main vortices behind the cylinder.
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