Predicted low frequency structures in the wake of elliptical cylinders

The vortex structures in the wake of 2D elliptical cylinders at low Reynolds numbers are investigated for a Reynolds numbers range of 75 to 175 using direct numerical simulation. By varying the aspect ratio of an elliptical cylinder, the geometry is varied between the extremes of a circular cylinder and a flat plate normal to the flow. The power spectrum analysis of the vertical velocity along the horizontal centreline shows the presence of secondary and tertiary frequencies in the far region of the wake. As the Reynolds number is increased and/or the aspect ratio is decreased, the lower frequencies in the far wake become more dominant and their inception point occurs closer to the elliptical cylinder. This research suggests that the low frequency unsteadiness behind normal flat plates does not need to be due to the vortex interaction, but, rather, can result from the presence of a two-dimensional instability of the mean wake. (C) 2003 Elsevier SAS. All rights reserved.