INTERACTION BETWEEN VORTEX RINGS AND A SEPARATED SHEAR LAYER: TOWARDS ACTIVE CONTROL OF SEPARATION ZONES

Abstract 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 Γ , saturating at sufficiently large values of F and Γ . The momentum defect appears to attain a minimum at a particular frequency Fc / U ∞ ≈4, where c is the length of the plate and U ∞ 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 ∞ ≈4, and at a particular value of the circulation Γ / U ∞ c ≈0·32, which is approximately 1·6 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.