Frequency Selection Mechanism of Airfoil Trailing-Edge Noise

An experimental investigation was conducted with the aim to reveal the frequency-selection mechanism of the acoustic noise emanated from the trailing edge of two-dimensional airfoils, and a single splitter plate was proposed for suppressing trailing-edge noise. It was observed that broadband disturbances caused by Tollmien-Schlichting instabilities in the transitional boundary layer increase if trailing-edge noise is suppressed with the proposed device. In addition, by the introduction of artificial acoustic feedback, for which the initial disturbances are obtained from a sensor for unsteady pressure near the trailing edge, a particular discrete Tollmien-Schlichting wave is selected from among the broadband disturbances caused by Tollmien-Schlichting instabilities. This result demonstrates that the process of frequency selection is clearly ascribable to a positive feedback loop between instabilities in the boundary layer and trailing-edge noise. Also, the application of artificial acoustic feedback shows that the airfoil boundary layer favors a particular frequency, which results in steplike structures as a function of the distance between the loudspeaker and the trailing edge that are similarly observed in conventional environments with natural trailing-edge noise emission.