The patterns of the periodic vortex-shedding in the wake flows of bluff cylinders are sensitively affected by the shape of afterbody of the cylinder. Sometimes the Strouhal numbers afe jumped due to the variation of the depth/breadth ratio of the bluff cylinder. Here the separated flow around a bluff cylinder with various sectional shapes is solved by the finite difference technique of the incompressible Navier-Stokes equations. The mechanism of the unstable and drastic changes in vortex-shedding is investigated, through the comoarison between the experimental and computational data. We also bring into focus the behaviors of the separated shear layers from the cylinder and their following vortex formation process in the near wake.
