Reduction of airfoil turbulence-impingement noise by means of leading-edge serrations and/or porous materials

The paper is about the sound produced as turbulence impinges on the leading edge of an airfoil and its reduction by means of either leading-edge serrations (tubercles) or the use of porous materials. The first part describes a series of experiments performed on a NACA-12 airfoil in a low-speed open-jet anechoic wind tunnel. The airfoil is held between end-plates and the sound is measured in the far field in the mid-span plane The chord-based Reynolds number ranges from 1.3 105 to 2 105. Various versions of the airfoil are tested and compared to the baseline. Sound reduction is achieved by both serrations and porosity in a wide frequency range. The second part is devoted to dedicated prediction techniques. A new analytical model of the response of a serrated leading-edge is proposed, extending Amiet's theory, in the limit of arbitrary large chord. Preliminary numerical modeling is also discussed for the response of a porous aifoil to incident disturbances, based on a panel method combined with a locally-reacting impedance model.