Aerodynamic Control of a Pitching Airfoil by Active Bleed

Abstract

The aerodynamic loads on an airfoil 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 timeperiodic 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. I. OverviewThe interactions between the outer flow over the aerodynamic surfaces and controlled distributed aerodynamic surface bleed that is engendered by pressure differences in flight can result in significant alteration of the global flow and therefore of the global aerodynamic loads. 1-3 Exploiting this approach for aerodynamic control is attractive because the primary actuation power is derived from the motion of the vehicle, and control is applied by regulation of the bleed flow through surface ports and therefore requires relatively little power. Aerodynamic flow control using bleed has been demonstrated in a number of earlier investigations using continuous, time-invariant bleed through passive porous surfaces by leveraging high-and low-pressure domains over lifting surfaces. The bleed flow results in modifications of the local pressure distributions and thereby alters the global aerodynamic forces. Early investigations of passive (nearly time-invariant) bleed for flow control realized lift enhancement and drag reduction using multiple slots in wings, 4,5 while more recent efforts have demonstrated control of boundary-layer instabilities, 6 rotor blade tip vortices, 7 and leading edge...

Rotating Wing Dynamic Stall: State of the Art and Future Directions

“…120), flow-through holes (bleed or centrifugal pumping) (Ref 27),. or embedding of devices (synthetic jets, COMPACT jets) (Refs 121,122)…”