Boundary Layer Transition in High-Intensity Free-stream Turbulence at Aerodynamically Low Chord Reynolds Numbers*

Aircraft and turbomachines that operate in the atmospheric boundary layer often experience free-stream turbulence intensities of 10% or greater. Our goal in this experimental study is to understand the effect of high-intensity turbulence on the aerodynamic loading of lifting surfaces at aerodynamically low chord Reynolds numbers where the lifting surface performance is particularly sensitive to the level of free-stream turbulence. Free-stream turbulence intensities of up to 15% and varying integral length scales were generated in a wind tunnel using an active turbulence grid. We investigate the effect of increased turbulence intensity on flow separation and laminar-to-turbulent transition on an airfoil model using particle image velocimetry and correlate it with the changes in aerodynamic loads measured using a force balance. Under conditions of elevated freestream turbulence, we find intermittent changes in the transition process that lead to notable fluctuations in the lift and drag forces, whose characteristics depend on the freestream turbulence parameters.