Unsteady Flows in the Vortex Region Behind a Circular Cylinder Started Impulsively : 1st Report, Feeding Mechanism of Vorticity and Onset of Turbulence

Abstract

A detailed visualization experiment was made on unsteady flows in the vortex-region behind a circular cylinder which was started impulsively from rest at the Reynolds number of 1, 200 referred to the diameter of the cylinder. An improved hydrogen bubble technique was employed in order to visualize streaklines from the unsteady boundary layer on the surface of the cylinder. Also a new method to estimate accurately the vorticity distribution in unsteady vortex region was developed. By use of these methods, the mechanism of vorticity produced on the surface of the cylinder and fed to the vortex region and the mechanism of onset of turbulence in the vortex region were made clear in relation to the vorticity distribution and the flow patterns.