An Investigation into the Use of Synthetic Jets for the Control of Surface Unsteadiness due to Vortex Breakdown

This paper details the findings of experiments that have attempted to identify the fluid mechanical mechanisms that allowed an array of synthetic jet actuators to alter the unsteadiness levels in the post-burst region of the flow over a 60° angle of sweep, delta wing configuration. The experiments made use of an array of synthetic jet actuators embedded in the wing leading edge to try to alter the shear layer that separates from the leading edge before rolling up to form the vortex which subsequently bursts at a location downstream of the wing apex. The frequencies used to actuate the flow were related to, and were an order of magnitude larger than the dominant frequencies found in the burst region that was to be altered. Following on from earlier experiments that had shown that this technique was able to reduce the surface rms pressures in the post-burst region by up to 30%, this paper details the findings of a series of PIV and surface flow visualisation experiments that examined the burst vortex itself and the associated leading edge separation line. The findings showed that the effect of actuation was in fact to produce time-averaged separation delays in the leading edge separation line, local to each actuator. These delays produced kinks (or ripples) in the separated shear layer that, in turn, led to a series of variations in the vortex diameter. From these results it has been postulated that this variable diameter feature of the vortex leads to the coherent spiral structure known to exist within the burst flow being de-stabilized due to the production of self induced Biot-Savart velocities. Additionally it is shown that actuation directly into the post-burst region is less effective at producing the rms reductions previously seen, than actuation of the pre-burst area of the vortex.