Steady blowing to control the lift and drag on a free shear layer airfoil*

We report on the use of flow control to modulate the aerodynamic performance of a family of “free streamline” airfoils. The airfoil consists of two flat plates, the shorter plate (flap) is joined at an angle to the longer plate at the trailing edge. The flow separates at the leading edge, but the use of a steady wall jet, located on the flap close to the leading edge facilitates flow reattachment toward the back of the flap. The lift and drag generated by the free shear layer wing have been measured at low Reynolds numbers (~26,000) for different relative lengths and angles of the two plates. Without actuation, high lift coefficients (CL ~ 1.2-1.5) are obtained at high angles of attack but this comes with a high drag penalty (CD ~ 0.3-0.6).  Steady blowing can increase the maximum lift of the airfoil to values of CL = 2.8, while delaying stall. Blowing also reduces the drag coefficient (CD ~ 0.1-0.2) at small angles of attack (α up to 15°), although it is not effective at high angles of attack. The drag reduction at small angles of attack is primarily due to a pressure reduction in the cavity between the plates and also to the thrust generated by the jet.