Flow about a rotating circular cylinder is one of the classical problems of fluid mechanics. The Karman vortex street in the wake of a still circular cylinder is one of the most well-known phenomena in fluid mechanics. It is of interest how the vortex street is affected by the rotation of the cylinder. It is so far known by experiments that the Strouhal number of vortex shedding becomes larger as the rotating speed becomes higher and that the meandering of the wake due to the Karman vortex street disappears when the rotating speed of the cylinder is high enough, that is, when the spin parameter, which is defined as the ratio of the peripheral speed of the cylinder surface of the uniform velocity, is about 2. The Reynolds number in which these experiments have been done is of order 104-105 so that it is rather high. Kimura and Tsutahara simulated these phenomena by the discrete vortex method. Their simulation corresponds to very high Reynolds number flows because the diffusion of the vorticity is neglected. For a rather wide range of the Reynolds number, the initial stage of the flows about rotating circular cylinders has been studied numerically and experimentally by Badr and Dennis and Badr and Coutanceau. Who state that at a Reynolds number of 103 a periodic variation appears in the time variation of the lift for the flow at the spin parameter of unity, but no periodic variation appears for the flow at a spin parameter of 3. However, the effect of the Reynolds number on these phenomena has never been explicitly described. In this study, the effect of the rotation of the cylinder and that of the Reynolds number are investigated by experiments and numerical simulations.
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