Near wake flows behind two- and three-dimensional bluff bodies

The flow in the near wake region behind a bluff body, incorporating the vortex formation region, plays an important role in determining the steady and unsteady forces acting on the body. Even behind two-dimensional geometries this flow exhibits certain strong three-dimensional properties. A brief review is presented of the characteristic three-dimensional features found in the wakes of bluff bodies. In the case of three-dimensional bluff bodies a distinction is made between bodies where the geometry is mildly three-dimensional and strongly three-dimensional. The behaviour of the vortex formation length behind a mildly three-dimensional body is discussed. Results from a two-dimensional numerical simulation are used to show how the vortex formation length varies through the lock-in range for a circular cylinder oscillating transverse to a flow. Finally, velocity fields are presented for the wake of complex bluff form, a car model, obtained by using particle image velocimetry which show that the instantaneous flow is quite different to the time-average flow.