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

Abstract

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.

Control of vortex shedding on two- and three-dimensional aerofoils

“…The total duration of the transient response was long when compared with the actuation, lasting several hundred actuation cycles, or about 10 convective time units (t + = tU /c = 10). The response to step changes in actuation has also been studied [29,38,39] and shows similarly long transient times when toggling between fully separated and controlled flow states. Similar to the burst of actuation, the forced reattachment first results in a negative vortex followed by the shedding of large, positive vortices.…”

“…At lower Reynolds number, the response is more oscillatory, and seems to consist of consecutive shedding of largescale vortices from the flap. Toggling off control results in transients of similar duration, but the early response includes the formation of a large (positive) vortex which Darabi & Wygnanski [39] liken to the dynamic stall vortex. Similar experiments for Re = 10 4 -10 5 have been conducted in the semicircular aerofoil in the IIT wind tunnel with leading-edge pulsed-jet or synthetic jet actuators, and they reveal strikingly similar transients to the two-dimensional aerofoils and flaps previously studied.…”

Alleviating Unsteady Aerodynamic Loads with Closed-Loop Flow Control

“…The separated flow behavior in response to actuator input can be nonlinear when there is formation of a dynamic stall vortex or actuator pulses occur in rapid succession. Furthermore, the characteristic times of the separated flow attachment [40] and separation [41] processes are much longer than the characteristic time of the actuator.…”

Switching of a Bistable Diverter Valve with Synthetic Jet Actuators

“…Simultaneous actuation of the two jets, with amplitude modulation to create opposing reduced frequency strategies, in order to potentially promote forced reattachment and forced separation 22 , on opposing valve sides, is an interesting proposal for increased valve response. The geometric properties of the valve that govern the timescales involved in the transition of flow from one channel to the other are of particular importance, as this may also have a role in defining the maximum achievable frequency.…”