John Kiedaisch

Active Flow Control Applied to High-Lift Airfoils Utilizing Simple Flaps

An extensive series of small-scale wind tunnel tests was conducted at the Illinois Institute of Technology as part of the Boeing/DARPA/AFRL/NASA-sponsored ADVINT program. In these tests, Zero Mass Flux (ZMF) Active Flow Control (AFC) was applied to a high-lift airfoil with a highly-deflected simple flap. Three different leading edge geometries were evaluated, and the simple flap performance was compared to the performance of various slotted flap configurations. The wind tunnel model was tested in both 2-D and 3-D configurations. It was found that on this airfoil, flow separation near the leading edge occurred at low angles of attack with droop and cruise leading edges. This resulted in flow conditions upstream of the AFC location at the flap shoulder that were not amenable to effective control. The addition of a slat leading edge improved the upstream flow conditions and resulted in significant performance enhancement with AFC. The AFC applied at the flap shoulder effectively increased the circulation around the airfoil without fully reattaching the flap. The improvement in lift with AFC was found to be proportional to the ratio of peak ZMF jet velocity to the free stream velocity. It was determined that values of this ratio ≥ 2 were sufficient to meet the performance goals of the ADVINT program. It was also found that replacing the simple flap with a slotted flap provided even greater performance. Finally, it was found that 3-D effects did not diminish the effectiveness of the AFC, and near the tip, the AFC effects were even enhanced.

Control Techniques for Flows with Large Separated Regions: A New Look at Scaling Parameters

*† ‡ § Two wind tunnel investigations were conducted with the goals of gaining a better understanding of the mechanisms governing Active Flow Control (AFC) for flows dominated by large separated regions, identifying the proper parameters to use for characterizing the performance of the AFC system under these conditions, and determining the scaling relationships needed to develop AFC systems for large-scale tests and, eventually, for fullscale applications. The first series of tests utilized the ADVINT 5%-scale airfoil model, and the second series of tests utilized the ADVINT pseudoflap model. In both tests, key operational parameters were varied systematically, independently of one another with minimal variations in the model geometry. The parameters that were varied include the free-stream velocity, the AFC amplitude (characterized by the peak jet velocity Uj), the AFC forcing frequency, and most importantly the slot width. On the 5% airfoil model, where AFC primarily affected the flow by controlling circulation, it was found that the key parameter for scaling the AFC effects was the ratio Uj/U∞, and a new parameter, H, incorporating this ratio was proposed. The results of the pseudoflap test indicated that suction was more effective for controlling separation than blowing, and that the optimum locations for suction and blowing were not necessarily the same, indicating that ZMF AFC is not the best choice for controlling flows of this type. Finally, both tests showed that for flows of this type, the AFC effects do not scale with the often used momentum coefficient, Cµ, for ZMF, suction, or blowing AFC methods.

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

*† ‡ § 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).

Active Flow Control for High Lift Airfoils: Dynamic Flap Actuation

An extensive series of small-scale wind tunnel tests was conducted at the Illinois Institute of Technology as part of the Boeing/DARPA/AFRL/NASA-sponsored ADVINT program. Various methods of Active Flow Control (AFC) were applied to a high-lift airfoil with a highly-deflected simple flap. The major goal of the program is to significantly increase aerodynamic performance during takeoff and landing by utilizing an affordable, easily maintainable simplified flap system that relies on AFC to control separation at high flap deflections, and enables the development of next generation extreme short takeoff and landing (XSTOL) transport aircraft. A small but important subset of these tests, presented here, investigated unsteady flap effects on the performance of the AFC system. The model was modified to allow dynamic deflection of the simple flap at controlled rates. The primary objective of this test series was to investigate the effectiveness of AFC applied to the simple flap as it deflects from a high-drag to a high-lift deflection angle as part of a conceptual takeoff maneuver for future XSTOL aircraft. By studying the behavior of the pressure coefficient at key locations on the airfoil under these unsteady flow conditions, it was found that the lift enhancement due to the effective control of flow separation and globalcirculation enhancement for the static-flap case, is also realized when the same control is applied to a dynamically deflecting flap, and that the AFC performance is generally independent of flap deflection rate. However at high deflection angles, when the flap transitions to a flow regime where the AFC system can no longer maintain separation control, any local flow effects of the AFC that develop on the static flap may be lost during the dynamic flap deployment.