Experimental and numerical investigation of turbulence-airfoil noise reduction using wavy edges

Clair, V., Polacsek, C. and Le Garrec, T. et al. (2013) Experimental and numerical investigation of turbulence-airfoil noise reduction using wavy edges. AIAA Journal, 51 (11), 2695-2713. (doi:10.2514/1.J052394).

Abstract

A passive leading-edge treatment based on sinusoidal serrations aimed at reducing turbofan interaction noise has been recently studied in the framework of a European project (FLOCON). The turbulence-airfoil interaction mechanism is achieved using a turbulence grid located upstream of an isolated NACA airfoil tested in the Institute of Sound and Vibration Research anechoic open jet wind tunnel. The experimental setup with several airfoils designed and manufactured by ONERA is first presented with main acoustic results, highlighting the sound power level reductions obtained for all studied flow speeds (about 3–4 dB reduction) without altering the aerodynamic performances (as shown by available measurements and Reynolds-averaged Navier–Stokes calculations). The experimental investigations are supplemented by numerical predictions in order to assess the acoustic performances of the serrations. The method described in the second part of the paper is based on a computational aeroacoustics code solving the nonlinear Euler equations applied to the disturbances and is coupled to a Ffowcs-Williams and Hawkings formulation. Convection effects are achieved from a Reynolds-averaged Navier–Stokes solution or approximated by a fully uniform flow. The upstream turbulence is synthesized by means of a stochastic model and injected into the computational domain through an adapted boundary condition. It is first validated in two and three dimensions against academic flat-plate configurations by comparison with Amiet solutions (exact in such cases). Finally, three-dimensional computations are applied to FLOCON configurations. The present methodology is shown to reproduce the measured spectra and far-field directivities with a reasonably good precision. Radiated sound level attenuation due to the serrations versus frequency is fairly well assessed too.

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