External forcing on a wing-tip vortex can affect its instability, and therefore an optimal perturbation can improve the aerodynamic performance of the wing. The present study examined the unsteadiness of the wing-tip vortex under periodic wing-tip vibration, and revealed its effect on the aerodynamic performance of the wing. A 3D-printed vibrating wing-tip model was prepared, which was driven by a sheet-type piezo actuator. Phase-averaged stereo particle image velocimetry (PIV) measurements clarified that the averaged position of the vortex depends on the phase of the wing-tip vibration, and the vortex shifted further from the wing as the actuation frequency increased. The phase-averaged velocity distributions indicate that the velocity deficit inside the vortex is significantly enhanced near the end of the downstroke of the wing-tip motion. The wing-tip vortex is weakened in the mid-upstroke, and its impact depends on the actuation frequency. This is because the motion of the wing is in the same direction as the flow rolling up from the pressure side, which prevents the formation of the vortex. In the mid-upstroke phase, the turbulence quantities, e.g., the turbulent kinetic energy and the Reynolds shear stress, are significantly suppressed; these effects depend monotonically on the actuation frequency. These arguments are supported by time-resolved recordings of the flow and the wing motion. The force measurements reveal that the vibration of the wing-tip brings a positive effect on the lift-to-drag ratio.
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