Flow past elliptical cylinders at low Reynolds numbers

Abstract

The vortex structures behind 2D elliptical cylinders at low Reynolds numbers were investigated for a Reynolds numbers range of 30 to 200. By varying the aspect ratio of an elliptical cylinder, the geometry varies between the extremes of a circular cylinder and a flat plate normal to the flow. It is ascertained that as the aspect ratio is decreased, the shedding behind the elliptic cylinder changed from steady Karman vortex shedding to a surprising different flow with two distinct regions. The first region situated directly behind the cylinder contained two rows of vortices rolling up from the cylinder with a region of relatively dead flow in between. A convective instability in the flow further downstream causes the two rows of vortices to interact strongly and results in the formation of downstream secondary vortices. It is found that at the transition Reynolds number between the normal Karman vortex wake and the occurrence of secondary vortex shedding, there is a peak in the Strouhal number for a given aspect ratio as well as a local minimum in the drag coefficient. Furthermore, the value of the Reynolds number at the onset of periodic vortex shedding decreased as the aspect ratio decreased.