Quantification of flow structures generated by an oscillating fence actuator in a flat plate laminar boundary layer

Particle image velocimetry was used to quantify the flow structures generated by an oscillating fence actuator in a flat-plate laminar boundary layer, as well as to confirm the classification of the fence-generated structures by the ratio of the fence oscillation frequency to the fundamental shedding frequency of the static fence. This actuator has demonstrated the capability of high authority relative to other flow control devices. The results indicate that the subcritical flow regime (frequency ratio < 0.1) sheds vortices due to vorticity saturation behind the fence. The vortices of the critical flow regime (frequency ratio ~ O(1)) gain vorticity during the fence upstroke and are forced to shed once the fence begins to descend. The supercritical flow regime (frequency ratio > O(1)) sheds one unsteady vortex per fence oscillation cycle. These unsteady vortices coalesce to form large vortices at the fundamental shedding frequency of the static fence. The transitional flow regime (frequency ratio ~ 0.1 - O(1)) structures are two-dimensional during the fence upstroke and evolve to three-dimensional once the fence begins to descend. The ability of this actuator to produce a required structure, by adjustment of the frequency ratio, suggests its potential as an effective flow control device.