Wake-Induced Boundary Layer Transition on an Airfoil at Moderate Free-Stream Turbulence

Wake-induced boundary-layer transition on a NACA0012 airfoil with zero angle of attack is experimentally investigated in periodically passing wakes under the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensities (Tu(infinity)) at the leading edge of the airfoil are 0.5 and 3.5 %, respectively. The Reynolds number (Re-C) based on chord length (C) of the airfoil is 2.0x10(5), and Strouhal number (St(C)) of the passing wake is about 1.4. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The patch under the high free-stream turbulence (Tu(infinity)= 3.5%) grows more greatly in laminar-like regions compared with that under the low turbulence (Tu(infinity)= 0.5%) in laminar regions. The former, however, does not greatly change the turbulence level in very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually loses its identification, whereas the latter keeps growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and with the receding wakes.