  An array of twelve circular-orifice synthetic jet actuators (SJAs) was used
to provide the unsteady forcing required for flow separation control over a
National Advisory Committee for Aeronautics (NACA) 0025 airfoil at a
chord-based Reynolds number of 100000 and an angle of attack of 10{\deg}. Two
distinct high- and low-forcing frequencies corresponding to the shear layer and
wake instabilities were used at an identical blowing strength for flow control.
Particle image velocimetry (PIV) was used to measure the velocity fields at the
centerline of the airfoil. The results showed the presence of a turbulent shear
layer stretching from the edge of the reattached boundary layer to the
irrotational flow with an invariant mean spanwise vorticity in the wall-normal
direction. It was revealed that the coherent structures for the high-frequency
controlled case are advected along the boundary of the rigid-body rotation
shear layer and the irrotational flow, whereas for the low-frequency actuation,
some structures directly pass through the rigid-body rotation region,
disrupting the wall-normal balance of vorticity. Analytical expressions were
derived for the variation of the mean spanwise vorticity in the rigid-body
rotation region and the curvature-multiplied mean angular momentum in the
irrotational flow region based on order-of-magnitude analysis and
semi-empirical grounds. The resulting patterns showed an excellent agreement
with the measured experimental data.
