A Two-Point Active Flow Control Strategy for Improved Airfoil Stall/Post-stall Aerodynamics

Abstract

A numerical study is conducted to assess the benefits of a new “hybrid” active flow control (AFC) strategy based on the simultaneous use of a pulsed suction jet and a pulsed blowing jet - for improving the stall/poststall aerodynamics of the 12% thick VR-7 helicopter blade airfoil. Predictions are made using a modified version of the NASA Langley “CFL3D” unsteady thin layer Navier-Stokes flow solver. Here, unsteady transpiration boundary conditions are used to emulate the temporal variations of the pulsed jet velocities. Predicted flowfields that provide insight into the mechanics of the interaction between the pulsed jets and the developing boundary layer over the airfoil are presented. For a free stream Mach number of 0.30 (representative condition where maximum lift is sought), it is shown that the use of the present hybrid AFC strategy results in aerodynamic benefits that exceed those obtained from the use of a pulsed suction jet or an oscillatory zero-net-mass jet. Specifically, the overall benefits are shown to be a direct result of: a) using the pulsed suction jet to partially reattach the otherwise separated boundary layer flow over the airfoil and, b) using the pulsed blowing jet to fragment the vortex structure in the remainder of the separated flow region to locally create a favorable pressure gradient that promotes flow attachment.