Abstract: It is the objective of this paper to summarize the authors' recent work on flapping foils, including the application of flapping foils for boundary layer and flow separation control. Water tunnel experiments on sinusoidally plunging foils are described which elucidate the change in vortical wake pattern shed from the foil's trailing edge. These experiments were carried out using dye flow visualization and laser-Doppler velocimetry. It is found that the wake pattern is a strong function of the non-dimensional plunge velocity, with the wake topology changing from a typical Kármán vortex street to an inverse Kármán vortex street and to an asymmetric wake structure as the non-dimensional plunge velocity increases. The transition points between the various structures is dependent on scaling effects and wing and flow quality. These results are partly reproducible with inviscid panel code and Navier-Stokes code predictions. Additional interesting features are obtained if two degrees of freedom are permitted (pitch-plunge motions) or two airfoils in close proximity to each other are studied. Finally, experimental and computational results are presented which demonstrate the use of flapping foils for boundary layer and flow separation control.

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