On the generation of airfoil tonal noise at zero angle of attack and low to moderate Reynolds number

Abstract

The generation of tones from a NACA 0012 airfoil at zero angle of attack and Reynolds numbers of 50,000 to 150,000 was investigated using acoustic beamforming and surface flow visualization techniques in an anechoic wind tunnel. Boundary layer separation lengths were obtained using surface flow visualization techniques and XFOIL software and were input into a feedback loop equation to determine if the measured tones were generated by a feedback mechanism. Using the maximum point of velocity on the airfoil surface as a feedback length provided the best agreement with the measured tones. Boundary layer tripping on one airfoil surface resulted in a decrease in the primary tone amplitude and some of the secondary tone amplitudes. Applying a trip to both surfaces resulted in a significant noise decrease and no tones were observed.