Active management of naturally separated flow over a solid surface. Part 2. The separation process

The controlled separation of flow from an inclined straight flap at high inclination angles was investigated experimentally. The separation process was initiated by an abrupt change in the excitation emanating from a slot at the flap shoulder. A complete cessation of the actuation resulted in formation of a large vortex above the flap akin to the familiar 'dynamic stall vortex' (DSV) seen over oscillating airfoils in pitch. The DSV temporarily increased the aerodynamic load over the flap before it dropped to its low separated value. The duration of this overload decreased as the flap inclination increased. The use of periodic excitation during separation slowed down the rate of separation and changed its character depending on the amplitude and the frequency used. Forcing separation by switching the excitation to a high frequency (3 < F⁺ < 8) reduced or even eliminated the increase in flap loading that is associated with the DSV. A switch to low frequencies (F⁺ < 1) extended the duration of separation and increased the transient overload during the initial stage of the process.