Active Flow Control for High Lift Airfoils: Separation Versus Circulation Control

Abstract

*† ‡ § Wind tunnel investigations were conducted with the goal of gaining a better understanding of the mechanisms governing Active Flow Control (AFC) for flows dominated by large separated regions and its effects on separation control versus circulation control. Inviscid solutions were calculated to gain insight into the pressure distribution around the leading edge and flap shoulder as well as set goals for the AFC application. This series of tests utilized the ADVINT 5%-scale airfoil model. Steady blowing AFC was used extensively in conjunction with a surface tangent, downstream facing slot located immediately upstream of the flap on the trailing edge of the main element. AFC amplitude sweeps were performed for several flap deflections and free stream velocities over a wide range of angles of attack. The data were compared to the inviscid solutions to show that significantly enhanced lift can be obtained without controlling flow separation, and lift can be enhanced beyond inviscid levels through control of separation in addition to circulation enhancement for an extremely high energy input. The tests also reveal two other important mechanisms for control of aerodynamic forces on lifting bodies: the direct momentum vectoring of the AFC and “super inviscid” performance along surfaces with the aid of high-speed attached jets. Optimizing the performance with AFC requires understanding these two mechanisms in addition to separation and circulation control, and the dependence of all four on the various control parameters. Understanding previous results, including those referred to as “super circulation” or Coanda type upper-surface blowing, requires the type of insight revealed by the present experiments. The results are also very helpful in deciding on the choice of AFC methodology between steady and unsteady suction or blowing and zero mass flux (ZMF).