GLOBAL DYNAMICS OF SYMMETRICAL AND ASYMMETRIC WAKES

The two-dimensional wake-shear layer forming behind a rectangular-base d forebody with independent ambient streams on either side of the fore body is examined by direct numerical simulation. Theoretical aspects o f global modes and frequency selection criteria based on local and glo bal stability arguments are tested by computing local stability proper ties using local, time-averaged velocity profiles obtained from the nu merical simulations and making the parallel-flow approximation. The th eoretical results based on the assumption of a slightly non-parallel, spatially developing flow are shown to provide a firm basis for the fr equency selection of vortex shedding and for defining the conditions f or its onset. Distributed suction or blowing applied at the base of th e forebody is used as a means of wake flow modification. The critical suction velocity to suppress vortex shedding is calculated. It is show n that local directional control (i.e. vectoring) of the near-wake flo w is possible, but only when all global modes are suppressed.