A note on vortex shedding from axisymmetric bluff bodies

Abstract

The linear parallel and incompressible stability of a family of axisymmetric wake profiles is studied in the range of Reynolds numbers where helical vortex shedding from bluff bodies of revolution is observed. The family of mean flow profiles allows for the variation of the wake depth as well as for a variable ratio of wake width to mixing-layer thickness. It is found that, even without reverse flow, the first helical mode is absolutely unstable in the near wake for Reynolds numbers, based on wake diameter and free-stream velocity, in excess of 3.3 × 103. A survey of the region of local absolute instability as a function of profile parameters and Reynolds number suggests that the large-scale helical vortex shedding, which is observed between Reynolds numbers of 6000 and 3 × 105for spheres, may be ‘driven’ by a self-excited oscillation in the near wake.