Control of High-Reynolds-Number Turbulent Boundary Layer Separation Using Counter-Flow Fluid Injection

Abstract

A combine d computational and experimental study has been carried out to assess a novel concept for airfoil separation control. This concept involves the use of a thin, counter flowing wall jet, injected on the pressure surface of an airfoil in the counter -streamwi se direction. The wall jet provides several boundary layer energization benefits beyond what can be accomplished using conventional streamwise blowing on the airfoil suction surface. The computational portion of the study was carried out using the Comput ational Fluid Dynamics (CFD) CFL3D Navier -Stokes code for a two -dimensional airfoil operating over a Mach number range of 0.2 to 0.4. The calculations were used to study the effects of the counter -flow injection slot geometry and placement, blowing rate, and details of the flowfield with and without separation control. The CFD calculations indicated such promising results that, at the conclusion of this portion of the work, a validation experiment was carried out in the United Technologies Research Center Large Subsonic Wind Tunnel using a two dimensional section of a full -scale Sikorsky SSCA09 airfoil. The CFD results were utilized to guid e the set -up of the experiment, including slot size, location, and plenum pressure . Results of this study indicate t hat the counter -flowing wall jet concept provides dramatic separation control for an airfoil operating at normally stalled, high -angle -of -attack conditions. This flow control was achieved using relatively low blowing momentum coefficients, on the order of 0.01 and lower. Maximum lift coefficient was increased by approximately 25%, and stall angle was increased on the order of 4 – 5 degrees relative to the baseline airfoil operating without flow control. Excellent agreement was obtained between predicted and measured airfoil lift performance in the presence of the counter flowing wall jet injection, and CFD results indicate that this approach requires substantially reduced amounts of blowing air when compared to conventional, streamwise upper -surface slot injection. CFD calculations were also carried out to perform an initial assessment of the ability to extend the counter -flow fluid injection concept to three -dimensional flows relevant to gas turbine engine inlet geometries and operating conditions .