A time-periodic blowing/suction is provided to control turbulent separation in a boundary layer using direct numerical simulation. The blowing/suction is given just before the separation point, and its nondimensional forcing frequency ranges from F*= fLb/U = 0.28–8.75, where f is the forcing frequency, Lb is the streamwise length of uncontrolled separation bubble, and U is the freestream velocity. The size of separation bubble is minimum at F*= 0.5. At low forcing frequencies of F*≤ 0.5, vortices generated by the forcing travel downstream at convection velocity of 0.32–0.35 U, bring high momentum toward the wall, and reduce the size of separation bubble. However, at high forcing frequencies of F*≥ 1.56, flow separation disappears and appears in time during the forcing period. This phenomenon occurs due to high wall-pressure gradients alternating favorably and adversely in time. A potential flow theory indicates that this rapid change of the wall pressure in time occurs through an inviscid mechanism. Finally, it is shown that this high-frequency forcing requires a large control input power due to high pressure work.

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