Dynamic stall is an airspeed and maneuver limiting event which occurs on helicopter retreating blades at high advance ratios and is associated with aerodynamic flutter and large negative pitch moments. The potentially violent dynamic stall sequence amplifies pitch link stress and can lead to loss of aircraft control. Steady Vortex Generator Jet (VGJ) blowing has proven to delay the onset of dynamic stall. This work presents results of an experimental investigation into active flow control of a Sikorsky SSC-A09 airfoil undergoing periodic pitching motion in a variety of flow conditions including, steady incompressible, steady compressible, and time-varying compressible freestream, representative of a helicopter rotor system in flight. The airfoil was evaluated at reduced pitching frequencies of 0.025 and 0.050 with a nominal angle of attack schedule, a=9.5°-10.5°cos(wt). Flow conditions were at steady Mach numbers of M=0.2 and M=0.4 and time-varying phase-locked freestream oscillations at Mach number M=0.4+0.07cos(wt), at Reynolds numbers Re=1.5 M and Re=3.0 M. Flow control was achieved through a spanwise row of jets located at 10% chord, oriented normal to the surface, with an effective activated control width of 75% airfoil span. Blowing flow control was evaluated at a jet mass flux ratios from Cq=0.002 to Cq=0.005.
Flow control enhancements evaluated include stall penetration, lift and moment improvements, reduction in negative damping, and flow reattachment angle. Quantitative measurements of lift and moment coefficients were calculated through the integration of airfoil surface pressure taps. Qualitative, time-resolved Background Oriented Schlieren (BOS) supplemented surface pressure measurements to assess spanwise averaged dynamic stall vortex progression as well as shock interaction.
No optimal mass flux ratio completely controls dynamic stall, but VGJs delayed boundary layer separation, consistently improved cycle average moment, and increased cycle average lift. VGJs triggered an earlier flow reattachment which reduced hysteresis and circuits of clockwise rotation on the CM curve related to negative damping. BOS imagery confirmed the presence of leading edge shock formation and showed VGJ capability to delay shock-induced flow separation. The effectiveness of VGJ flow control is primarily a function of maximum angle of attack, pitching frequency, and freestream compressibility.
A comparison of VGJ flow control evaluated on a pitching airfoil in a steady compressible freestream at M=0.4 versus a pitching airfoil in a time-varying compressible freestream at M=0.04+0.07cos(wt) at matched mean reduced frequency and Reynolds number, experienced similar quantitative improvements. Comparison of BOS imagery reveal the same physical VGJ to shear layer interaction between the steady and time-varying freestream cases. Thus, performance measurements based on active VGJs in a steady compressible freestream provide a good prediction of the expected performance measurements when blowing is applied to an airfoil in a low amplitude, time-varying compressible freestream. At low freestream oscillations, airfoil pitching frequency is the dominant factor influencing VGJ effectiveness.