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Aerodynamic Control of Static and Pitching Airfoils using Distributed Active Bleed

A novel, scalable approach to aerodynamic control of lifting surfaces using distributed active bleed is investigated experimentally and theoretically in collaboration with Professor Anthony Leonard at Caltech.  Control is achieved by large-area air bleed that is driven by the pressure differences between airfoil surfaces and is regulated by addressable, low-power, surface-integrated louver actuators.  The joint numerical-experimental investigation focuses on the flow mechanisms of the interaction between the bleed and the cross flow and the aerodynamic effects of unsteady bleed on a 2-D Clark-Y wing model.  Particular emphasis is placed on the generation and regulation of vorticity concentrations that alter the wing’s apparent aerodynamic shape and thereby its aerodynamic erformance over a range of (static) angles of attack.  The resulting time-dependent forces and moment are measured over a wide range of angles of attack from pre- to post-stall using load cells, and the induced changes in surface vorticity concentrations are measured using PIV.  Trailing edge bleed at low angles of attack effects nearly-linear variation of the pitching moment (±12%) with minimal drag penalty.  Leading edge bleed at high angles of attack leads to large variations in lift and pitching moment by vectoring the separated shear layer towards or away from the suction surface (see figure).  Here bleed can also be used to extend the stall margin using time-periodic louver actuation by re-attaching separated flow.  Theoretical and computation investigations, including a Navier-Stokes solver using an immersed boundary method and a penalty method, accompany the laboratory experiments.  These show that the global effect of bleed near the trailing edge is due to a change in airfoil circulation required to maintain the Kutta condition.  This change in circulation also corresponds to reduction of lift due to bleed and accounts for five sevenths of the total change in lift coefficient with a center-of-pressure at quarter chord, in accordance with classical theory. 

In addition, the aerodynamic loads on a VR-7 airfoil model that is pitching beyond its static stall margin are controlled in wind tunnel experiments by regulation of surface vorticity flux using distributed bleed actuation.  The airfoil model is based on a two-dimensional VR-7 configuration that is pitching time harmonically over a broad range of reduced frequencies and angles of attack (up to 22°).  Bleed is driven by pressure differences between surface ports upstream of the pressure side trailing edge and downstream of the suction side leading edge and is regulated by integrated low-power piezoelectric louver actuators.  The time-dependent evolution of the outer flow over the airfoil during the pitch cycle is investigated in the absence and presence of bleed using high-speed PIV to resolve transitory formation and shedding of vorticity concentrations during the onset and termination of dynamic stall.  The timing of the dynamic stall vorticity flux into the near wake and its effect on the flow field are analyzed in the presence and absence of bleed using proper orthogonal decomposition (POD).  It is shown that bleed actuation alters the production, accumulation, and advection of vorticity concentrations near the surface with significant effects on the evolution, and, in particular, the timing of the dynamic stall vortex.  These changes are manifested by alteration of the lift hysteresis and pitch stability during the cycle.  The time-periodic changes in lift during the up- and downstroke segments of the pitch cycle are accompanied by mitigation of sharp excursions of the pitching moment in the base flow, and in complete reversal of the “negative damping” while keeping the cycle-averaged lift to within 5% of the base flow level.  Such control of the pitch stability can lead to significant improvement of the stability of flexible wings and rotor blades.

Supported by AFOSR and GT-VLRCOE