Turbulence Intensity Effects on Laminar Separation Bubbles Formed over an Airfoil

The effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigated experimentally for angles of attack between 0 and 20 deg, chord Reynolds numbers between 100,000 and 200,000, and freestream turbulence intensities between 0.09 and 2.03%. The results show that increasing freestream turbulence intensity results in earlier transition and reattachment, contributing to an overall decrease in separation bubble length. At lower angles of attack, this is accompanied by a minor decrease in lift, whereas at prestall angles of attack and higher turbulence intensity levels, lift increases and stall is delayed. Spatial amplification rates of disturbances in the separated shear layer are shown to decrease at elevated levels of turbulence intensity, indicating that the earlier transition is attributed solely to the larger initial amplitude of perturbations. At elevated turbulence intensity levels, a broader range of unstable frequencies is detected in the separated shear layer, with the central frequency of the unstable band showing moderate variations with the level of freestream perturbations. The results indicate a change in the transition process at higher freestream turbulence intensity levels, indicative of bypass transition in the boundary layer. The degree of influence of freestream turbulence intensity on the separation bubble is shown to decrease as the chord Reynolds number is increased.