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Aerodynamic Control of a Pitching Airfoil using Discrete Pulsed Actuation

Flow control technologies can improve the aerodynamic performance and efficiency of rotorcraft systems by enabling alleviation of retreating blade stall (RBS) and by using induced aerodynamic forces for structural stabilization and to better match hover and cruise design conditions.  In particular, RBS continues to limit rotorcraft speed, lift capacity, and maneuverability.  This phenomenon is transitory in nature, and involves complex interactions between unsteady 3D flow and blade structural dynamics in a rotating environment, and therefore active flow control technologies present unique opportunities for its mitigation.  Transitory control and regulation of trapped vorticity concentrations to control the aerodynamic loads are investigated in wind tunnel experiments on a dynamically-pitching VR-12 model. 

 

 

 

 

 

 

 

 

 

 

In these experiments, COMPACT (COMbustion Powered ACTuation) actuators were used to perform the transitory flow control.  COMPACT produces a momentary, high-velocity jet for flow control actuation via the ignition of a mixture of gaseous fuel and oxidizer in a small (cubic centimeter scale) combustion chamber with no moving parts in the chamber.  COMPACT’s millisecond-scale bursts create pulsed actuation on time scales that are an order of magnitude shorter than the characteristic convective time scale of the base flow.  The effects of the transitory actuation on the aerodynamic characteristics of the dynamically-pitching airfoil are assessed using time-resolved measurements of the global lift force and pitching moment, and PIV is acquired phase-locked to the actuation waveform.  The baseline flow field for the VR-12 with pitching frequency of k = 0.06 for 10° to 20° indicates a typical time-periodic stall beginning around 17° on the upstroke and reattaching around 12° on the downstroke as shown in sample PIV results in the figure.

The significant disparity between the time scales of flow attachment and subsequent separation [O(Tconv) and O(10Tconv), respectively] is exploited for controlling the global aerodynamic loads and pitch stability on a pitching airfoil using individual actual pulses and strings of successive bursts.  Flow reattachment with each actuation burst follows a similar process of severing the separated shear layer with a momentary reattachment as a result and is shown to produce both stall delay on the upstroke and earlier reattachment on the downstroke with subsequent improvements in lift.  Lift and pitching moment coefficients for sample actuation programs consisting of separate bursts on the upstroke and downstroke of the pitching cycle are shown in the figure. 

A variety of actuation patterns have been tested aimed at investigating optimization of both lift and damping parameters.  The figure below shows one breakdown of these tests plotting change in damping factor versus change in lift.  Proper timing of the actuation pulses at key angles over the pitch cycle has shown increases of up to 5.6% in the cycle averaged lift (using actuation covering only 20% of the model span) while strongly enhancing the pitch stability (lower “negative damping”) that is typically associated with the presence of dynamic stall.
 

Supported by NASA and UTRC