Initial Flow Past an Impulsively Started Circular Cylinder

Abstract

A visualization study was made on flow separation s around a circular cylinder which was started impulsively from rest in a water tank. The experimental conditions were Re=Ud/v= 250 ∼ 1200, d/h=0.1 ∼ 0.63, where U is the cylinder speed, d the cylinder diameter, v the kinematic viscosity, and h the distance between side walls of the tank. The hydrogen bubble techniques and electrolysis methods were used to observe the behaviour of boundary layers and near wakes. Thickness of the boundary layers and reversed flow and the scale of the vortex formed were measured in course of motion. The measured values of location and time of the reversed flow appearing initially are in good agreement with those predicted by existing theories for flow separation. In the first stage, however, the reversed flow does not lead to an abrupt thickening of the boundary layer or a large scale break away of fluid. To examine these phenomena, the convective transfer of vorticity generated at the cylinder was traced by use of visualization techniques. The detail of vortex formation due to the flow separation was explained in terms of vorticity transfer and velocity field.