A study of the effect of airfoil thickness on the tonal noise generation of finite, wall-mounted airfoils

The noise generated by finite, wall-mounted airfoils strongly affects the total noise from aerodynamic applications such as small unmanned aerial vehicles, micro wind turbines and cooling fans. In an attempt to understand the effect of airfoil thickness on the trailing edge tonal noise generated by such airfoils, detailed wind tunnel experiments have been performed on two symmetric NACA-type airfoils with aspect ratio of 1.2 and nominal thicknesses of 12% and 18% of the chord length. The measurements included microphone array measurements, hot-wire measurements in the near wake of the airfoils and surface flow visualizations. It was found that the thickness has a strong impact on the tonal noise generation. At non-lifting conditions, the thinner airfoil generates tonal noise, while the thicker airfoil radiates broadband noise. At a slightly higher angle of attack (6 degrees), both airfoils generate tonal noise, but with different frequency and side-tone characteristics. Further increasing the angle (to 12 degrees) resulted in the generation of tonal noise from the thicker airfoil, while the far field spectrum obtained for the thinner airfoil has broadband character. This is attributed to the tip flow having stronger influence over the airfoil span in the case of the thinner airfoil leading to suppression of the tonal noise mechanism. Maps of the sound source locations, flow visualizations as well as detailed hot-wire measurements also provide further insight into the sound source mechanisms. (C) 2021 Elsevier Masson SAS. All rights reserved.