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

• Published in: Journal of fluids and structures Vol. 15; no. 3-4; pp. 399 - 413
• Main Authors: KIYA, M., MOCHIZUKI, O., ISHIKAWA, H.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: London Elsevier Ltd 01.04.2001; Elsevier
• Subjects: Exact sciences and technology; Fluid dynamics; Fundamental areas of phenomenology (including applications); Physics; Rotational flow and vorticity; Separated flows; Flat rolling; Measuring methods; Ring vortex; Momentum; Eigenfrequency; End vortex; Leading edge; Zone plates; Side chain; Near wake; Drag; Flow velocity; Vortex flow; Interactions; Separated flow; Active control; Shear layer; Surface flow; Fundamental frequency; Flat plate
• ISSN: 0889-9746; 1095-8622
• DOI: 10.1006/jfls.2000.0353

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 andΓ . The momentum defect appears to attain a minimum at a particular frequencyFc /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.