Reduction of flow induced airfoil tonal noise using leading edge sinusoidal modifications

Significant tonal noise reduction has been achieved using sinusoidal protuberances, also known as tubercles, on the leading edge of a NACA 0021 airfoil for a Reynolds number, Re ~ 120,000. It has also been observed that the overall broadband noise is reduced for a considerable range of frequencies surrounding the peak in tonal noise. It is postulated that tonal noise elimination is facilitated by the presence of streamwise vortices generated by the tubercles and that the spanwise variation in separation location is also an important factor. Both characteristics modify the boundary layer stability, altering the frequency of velocity fluctuations in the shear layer near the trailing edge. This affects the coherence of the vortex generation downstream of the trailing edge, hence leading to a decrease in trailing edge noise generation. An additional effect is the confinement of the suction surface separation bubble to the troughs between tubercles, which may reduce the boundary layer receptivity to external acoustic excitation. Investigations have also revealed that the smallest wavelength and largest amplitude tubercle configuration have the lowest associated tonal and broadband noise.