The effect of sinusoidal airfoil geometry on the reduction of flow induced tonal noise

Sinusoidal protuberances on the leading edge of an airfoil, also known as tubercles, have led to the elimination of tonal noise for a NACA 0021 airfoil at a Reynolds number, Re ∼ 120,000. It has also been found that the overall broadband noise is reduced for a considerable range of frequencies surrounding the peak in tonal noise. It is believed 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 stability characteristics of the boundary layer, 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 coustic excitation. Investigations have also revealed that the smallest wavelength and largest amplitude tubercle configuration demonstrate the lowest noise generation. The feedback loop mechanism of tonal noise generation is also investigated through consideration of the separation characteristics of the unmodified NACA 0021 airfoil. The feedback loop is defined between the trailing edge and positions where the boundary layer receptivity to external acoustic excitation is expected to be greatest. The separation point on the suction surface at the onset of a separation bubble is found to be the most receptive location since the predicted tonal frequency for a feedback loop of this length corresponds most closely to the measured value.