Visual Observations of the Flow past a Circular Cylinder Performing a Rotatory Oscillation

The flow around a circular cylinder performing a rotatory oscillation about its axis in a uniform flow has been investigated experimentally by means of flow visualization methods at Reynolds numbers between 30 and 300 and Strouhal numbers between 0 and 55. It has been found that the dead water region is reduced as the frequency of the oscillation is increased, and that the critical Strouhal number at which the dead water region vanishes is approximately proportional to the reciprocal of the angular amplitude of oscillation, but is independent of the Reynolds number. It has also been found that when the Strouhal number is much higher than the critical Strouhal number the flow past the cylinder is strongly accelerated.