Near-wake instabilities and vortex structures of three-dimensional bluff bodies: a review

Instabilities and vortical structure in the near wake of a sphere and a circular disk are reviewed, together with those of an elliptic disk and rectangular plate which have two length scales. The linear global instability analysis (J. Fluid Mech. 254 (1993) 323) yields critical Reynolds numbers of regular and Hopf bifurcations of the sphere wake, which are in excellent agreement with direct numerical simulations and experiments. The Strouhal number of shedding of hairpin-like vortices in the sphere wake is significantly dependent on Reynolds number, reflecting changes in the wake such as transition from laminar to turbulent wakes. The vortex structure at high Reynolds numbers still remains to be clarified. Two periodic components of velocity fluctuations are found in the wake of elliptic disks and rectangular plates, being likely to be caused by global instability in the near wake.