A stalled airfoil can exhibit up to three natural frequencies associated with the separated flow: that of the shear layer, the separation bubble, and the wake. This work investigates these flow phenomena using a simplified canonical setup and targets their frequencies with zero-net mass-flux (ZNMF) actuation in order to effectively and efficiently reduce the extent of the separation. First, boundary layer separation is created on a flat plate model, devoid of curvature that would otherwise include effects particular to the type of aerodynamic body, by imposing an adverse pressure gradient via a ZNMF suction/blowing boundary condition on the tunnel ceiling. At a chord Reynolds number of 10~5, the nominal two-dimensional characteristics of the flow are verified. The uncontrolled flow is characterized, including identification of the key flow frequency content, prior to strategically targeting this content with unsteady actuation. The identified natural frequencies of the shear layer and wake are then targeted via open-loop ZNMF sinusoidal and burst-modulated (BM) forcing. The results clearly indicate the ability to reattach the separated flow using significantly reduced C_μ values via BM forcing compared to sinusoidal forcing at the actuator resonance frequency.
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