Acoustic noise from the trailing edge of 2D airfoils is known to be discrete at certain moderate Reynolds numbers. The most widely accepted explanation for this phenomenon is the acoustic linkage between acoustic radiation and Tollmien-Schlichting (T-S) disturbances growing in the airfoil boundary layer. This paper has two objectives: (1) is to devise a technical method for suppressing or abating trailing-edge noise from a practical viewpoint; (2) is to obtain evidence whether or not T-S instabilities are associated with the frequency selection mechanism mentioned in the literature. From previous experiments (Atobe, T., et al., Trans. Jpn. Soc. Aeronaut. Space Sci., Vol. 52, pp. 74–80, 2009), generation of trailing-edge noise is dominated primarily by suddenly amplified unsteady disturbances in the presence of reverse flow on the pressure side of the airfoil rather than the suction side. Therefore, the boundary layer near the trailing edge on the pressure side is disturbed artificially with distributed roughness elements. As a result, the separation is swept away, and the trailing-edge noise is drastically suppressed. We reconfirmed that under the natural configuration with sound emission, most unsteady disturbances are not amplified by T-S instabilities, but rather by other inflectional-type instabilities. The frequency selection mechanism in the tonal noise generation process remains unsolved.
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