Development of Micro-Flap for Active Control of Separated Flows

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The purpose of this project is to develop micro-flaps for active control of separated flows. The target of the active control is to prevent separation of flow from the surface of solid bodies such as airfoils or airfoils cascades or to reduce as much as possible the region of separated flows if the flow separation is unavoidable. Main results are as follows.(1) An array of micro-flaps arranged along the separation edge can efficiently reduce the spatial extent of a leading-edge separation region at a particular forcing frequency if the forcing amplitude is fixed. The reduction of the separation zone is greater for higher forcing amplitude if the velocity fluctuation induced by the micro-flap is less than 20% of the oncoming-flow velocity. A flow model is constructed to interpret this most effective frequency and the associated reattachment length.(2) An interactive control of the leading-edge separation region is made, in which velocity fluctuation at a representative position in the separation region is fedbeck to the input of the micro-flaps by changing the phase lag between the velocity fluctuation and the input of the micro-flaps. The reattachment length is found, for the first time, to reduce by more than 50% by the change of the phase lag.(3) A chain of vortex rings is introduced from outside into the separated shear layer of a stalled airfoil. This active control can significantly reduce the height of the separated region on the suction side of the airfoil. The reduction is greater for higher frequency of micro-flaps to generate the vortex rings, saturating at sufficiently high frequencies of the vortex-ring generation.