The effects of forced Oscillations on the performance of airfoils

A. Seifert, A. Daraby, B. Nishri, I. Wygnanski

Research output: Contribution to conferencePaperpeer-review

19 Scopus citations


The effects of oscillatory blowing emanating from a leading edge of a flapped NACA 0015 airfoil are discussed and compared to similar experiments carried out on the same airfoil when the blowing emanated from the flap-shoulder located at 75% of the chord. The purpose of these experiments was to improve our understanding of the observations made earlier and to establish a proper scaling of the variables involved. By moving the blowing location from the flap-shoulder to the leading edge, the primary distance used in scaling the wave-length of the oscillations was increased four fold but the reduced frequency based on this distance was unchanged. The actual frequencies imposed at identical Reynolds numbers were thus 4 times lower. Neither coarse roughness strips, placed near the leading edge of the airfoil, nor an eight fold increase of Reynolds number had any significant effect on the benefits derived from the forced oscillations or on the most effective reduced frequency. Oscillatory blowing from the leading edge of a flapped airfoil, however, is not as effective as the same blowing applied to the flap-shoulder from which the flow separates in the absence of active control. The possible interactions among the characteristics of the upstream boundary layer, the pressure gradient and the imposed oscillations were also investigated on a large deflected surface simulating the flow over a “generic flap”. The excitation was done by a mechanically actuated ribbon (flaperon) calibrated in terms of an oscillatory, input momentum coefficient. The absence of circulation and flow over the lower surface (as is the case around the airfoil), made the analysis of the flow simpler and enabled one to isolate the leading parameters governing the flow. It was observed that the size of the separation bubble, originating above the flap shoulder and spanning most of the flap-chord at high deflection angles, was greatly reduced and even eliminated by the imposed periodic motion. The magnitude of the normal force coefficient and the moment coefficient about the hinge serve as a measure of the flow attachment.

Original languageEnglish
StatePublished - 1993
EventAIAA 2nd Shear Flow Conference, 1993 - Orlando, United States
Duration: 6 Jul 19939 Jul 1993


ConferenceAIAA 2nd Shear Flow Conference, 1993
Country/TerritoryUnited States


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