Near field evolution of wingtip vortices under synthetic-jet based control

The present research focuses on the experimental investigation of the effectiveness of synthetic jet actuation on a pair of counter-rotating vortices generated by an unswept, low aspect ratio, squared-tipped wing in order to preserve the mutual induction during their evolution. The synthetic jet is actuated at Crow and Widnall's instability frequencies ( ) and at a fixed momentum coefficient with the goal of reducing the vortex strength and the induced circumferential velocity. The effect of the jet exit section area, and thus the characteristic jet velocity, has been investigated by testing three wing models equipped with a synthetic jet issuing through a rectangular slot of constant length and three different values of the height, equal to 0.01, 0.02, and 0.04 chord lengths, respectively. A phase-locked stereoscopic particle image velocimetry setup has been designed and implemented to carry out a parametric study in the near wake of the wing models at four downstream distances from the wing trailing edge, namely equal to 0.1, 0.5, 1 and 2 chord lengths. The slot with a height to chord ratio of 4%, yields the minimum vorticity level, the maximum vortex diffusion with a diameter up to 3 times greater than the baseline reference value, and lower values of the vortex circulation. This effect, together with the periodic motion along a direction experienced by the vortex after the synthetic jet blowing, is beneficial in terms of an anticipated instability of the tip vortices as well as of the mitigation of the blade-vortex-interaction in propellers.