Interaction between vortex rings and a separated shear layer: towards active control of separation zones

摘要

The separation zone of an inclined flat plate was reduced by bombarding rolling-up vortices in the separated shear layer with a chain of vortex rings introduced from the side of the main flow. The reduction was realized because a compact and strong vortex is successively formed near the leading edge, transporting high-momentum fluid of the main flow towards the surface. Momentum defect in the near wake, which serves as a measure of effectiveness of reduction in the separation zone and can be approximately interpreted as the drag of the plate, generally decreases with increasing frequency of introduction of the rings F and their circulation F, saturating at sufficiently large values of F and F. The momentum defect appears to attain a minimum at a particular frequency Fc/U{sub}∞≈4, where C is the length of the plate and U{sub}∞ is the main-flow velocity. This frequency can be interpreted as the fundamental frequency of the shedding-type instability of the separated flow. Efficiency, which is defined as decrease in loss of power in the wake divided by the power required to generate the vortex rings, attains a maximum approximately at the same frequency Fc/ U{sub}∞≈4, and at a particular value of the circulation F/ U{sub}∞≈0.32, which is approximately 16 times the circulation of the shear-layer vortices in the region of interaction. Thus, the steady jet which corresponds to F=∞ is not the best choice in terms of the efficiency.