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Aerodynamic Control at Low Angles of Attack by Trapped Vorticity Concentrations

The design and performance of transport aircraft are influenced substantially by the effectiveness of the aerodynamic control system, affecting characteristics such as maximum takeoff weight, required runway length and fuel consumption.  Many conventional control systems are complex and consist of multiple elements that are designed to maximize performance and efficiency.  Active flow control (AFC) provides a means for reducing weight, part count and fabrication cost while improving cruise efficiency, creating the potential for significantly enhanced performance compared to what is possible using conventional technologies.

AFC is used to fluidically manipulate the aerodynamic characteristics of an airfoil based on a commercial aircraft configuration at low angles of attack (cruise conditions, with a fully attached global flow) without moving control surfaces, resulting in effects including drag reduction and pitching moment control.  Actuation is provided by synthetic jet actuators that are integrated into the airfoil which function by engendering and manipulating concentrations of vorticity near the surface, affecting the accumulation and shedding of vorticity around the airfoil.  The effectiveness of the actuation and the resulting changes in concentrations of trapped vorticity are varied by altering the momentum coefficient and other actuator operating conditions, leading to changes in the flow around the entire airfoil and the associated aerodynamic forces and moments. 

In one configuration tested, it is demonstrated how actuation can be used to reduce the pressure drag with virtually no penalty in lift.  An actuator located on the pressure surface of the airfoil near the leading edge (x/c = 0.21) is used to manipulate the trapped vorticity concentrations that are induced farther downstream near the surface of the airfoil.  The actuator leverages the presence of a small passive obstruction having a characteristic scale of O(0.01c) to increase the effectiveness of an integrated, low-power synthetic jet actuator [Cm = O(10-3)] by using the jet to manipulate the strength and streamwise extent of the trapped vortex that forms downstream of the obstruction.  Doing so leads to a substantial reduction in pressure drag (~50%) with minimal penalty in lift and therefore to an increase of ~30% in the overall lift to (total) drag ratio.  By controlling the streamwise extent of the induced concentrations of trapped vorticity, actuation can be used to reduce the pressure drag of the airfoil and concomitantly increase its lift to drag ratio.

Supported by Boeing