Wakes behind blunt bodies

years. These results are based on further research into the existen ce of absolutely unstable regions in the near-wake region and the reformulation of the classical von Karman stability theory on near and far fields in the von Karman vortex street. The existence of absolutely unstable regions at supercritical Reynolds numbers makes effcctivc wake control accessible. Figure I explains the differing instabilities in shear layers. The top diagram shows a classical example of a shear layer that was formed by the velocities uland U2 at the upper and lower sides of a splitter plate. The unstable waves of the shear layer traveling downstream have different characteristic phase velocities C1• In general, the shear layer is convectively unstable. Perturbations that are introduced at specific locations in the shear layer then move downstream. The waves do not, with increasing time, influence the source of the disturbances. A hydroacoustical resonance results when a second body is introduced into the shear layer, producing compression waves that travel upstream (middle diagram), so that self­ sustained oscillations can be achieved. For particular distances between the two objects a resonance can be triggercd, which induccs an acoustical wake tone. The shear flow is still locally convectively unstable, but the