Influence of the Free-stream Turbulence on an Airfoil’s Wall-pressure Fluctuations

The inflow turbulence increases the wall-pressure fluctuations close to the airfoil leading edge producing broadband far-field noise. An inflow with sufficiently high turbulence intensity presents turbulent structures that can penetrate the boundary layer increasing the wall-pressure fluctuations up to the trailing edge of the airfoil. Measurements of the wall-pressure fluctuations along the chord and span of a NACA 0008 were conducted at the Aeroacoustics Wind Tunnel of the University of Twente. Two passive grids generated turbulence intensity of 10% and 20% at the airfoil’s leading edge plane with integral length scales of 48 mm and 68 mm, respectively. The results showed that the inflow turbulence increases the wall-pressure fluctuations up to the trailing edge, i.e., x/c = 97%. For the case of 10% inflow turbulence, the near-wall generated turbulence dominates the wall-pressure fluctuations, whereas, for the case of 20% inflow turbulence, the wall-pressure fluctuations are dominated by the penetration of the free-stream turbulence. The influence of the free-stream turbulence in the wall-pressure spectrum is stronger in the low-and high-frequency ranges and minimum in the mid-frequency range. The penetration of the free-stream turbulence inside the boundary layer highly depends on the size of the turbulent structures inside the boundary layer. The Reynolds number and angle of attack were varied to discuss this effect. As the Reynolds number is increased, the effect of the free-stream turbulence is increased in the low-frequency range. Oppositely, as the angle of attack increases, the effect of the free-stream turbulence is reduced in the low-frequency range and increased in the high-frequency range. The spanwise coherence was reduced downstream of the airfoil chord in the low-frequency range for both turbulence intensities, contrary to the case of uniform inflow. For the case of 20% inflow turbulence, the coherence was increased in the high-frequency range downstream of the airfoil chord, demonstrating a growth of the small turbulent structures due to the strong interaction between the near-wall generated turbulence and the penetration of the free-stream turbulence.