Unsteady flow near a circular cylinder oscillated sinusoidally in uniform flow. I - Vortex-shedding mechanism in synchronization phenomena

The flow past a circular cylinder undergoing transverse oscillation in a uniform flow is investigated by means of flow visualization technique. The experimental conditions are R(e) = 1100, A/D = 0.2, and S(c) = f(c)D/U = 0.2, where R(e), is the Reynolds number based on the diameter D of the cylinder, A and f(c) the maximum amplitude and frequency of the oscillation of the cylinder, respectively, and U the speed of the uniform flow. Oscillations of the separation points and the stagnation point, and the rate of vorticity fluxes through the section near the separation points were examined during a cycle. The phenomenon that the vortex sheds periodically in the same frequency as the cylinder was found to be caused by a significant change in the vorticity distribution in the vortex region, which was induced by vorticity feeding from the third separation point found behind two ordinary separation points.