Inflow turbulence distortion for airfoil leading-edge noise prediction for large turbulence length scales for zero-mean loading

Turbulence distortion due to airfoil finite thickness is an important but not fully understood phenomenon that affects the airfoil radiated noise, resulting in inaccurate noise predictions. This study discusses the turbulence distortion in the leading edge (LE) region of an airfoil aiming to obtain more accurate LE noise predictions. Wind tunnel experiments were performed for National Advisory Committee for Aeronautics (NACA) 0008 and NACA 0012 airfoils at zero angle of attack subjected to large turbulence length scales (between 10 and 43 times the airfoil LE radius) generated by a grid and a rod. Hot-wire and surface pressure measurements were performed in the LE region. Results show that the root mean square of the velocity fluctuations u rms and the turbulence integral length scale Λ f at the stagnation line decrease considerably as the LE is approached. Rod-airfoil radiated noise was measured and compared with Amiet's model. The predicted noise overestimates the LE noise for high frequencies. However, the prediction agrees well with measurements when the turbulence spectrum based on the rapid distortion theory is used in Amiet's model, with as inputs the u rms and Λ f values measured close to the LE. This work's main contribution is to demonstrate that more accurate noise predictions are obtained when the inputs to the model consider the turbulence distortion effects.