Impulsively Started Flow Over a Bluff Body of Square Cross-Section

The phenomenon of an impulsively started flow about a square cylinder for different orientations was studied experimentally. It was found that separation occurred almost instantaneously from two or three of the body corners. The shedding of the first two or three vortices had a dramatic effect on the drag coefficient. The first two or three vortices displayed strengths far in excess of the vortices that were shed during steady state. Initially, the body experienced a drag force which was several times the steady state value. After the shedding of the first vortex the drag force dropped phenomenally and in some cases assumed a negative value for a short period of time, presumably due to the effect of the large vortex moving across the afterbody. This sequence of events was observed to re-occur with the shedding of the next two vortices but to a lesser degree.