Experimental investigation of airfoil turbulence-impingement noise reduction using porous treatment

The present work aims at evaluating the effectiveness of the use of porous materials for reducing airfoil turbulence-impingement noise. To pursue this objective, a porous NACA-0024 profile filled with melamine foam has been designed for comparison with a solid model. The porous media constituting the airfoil has been fully characterized in order to determine the parameters that describe the material. The two profiles have been tested in a rod airfoil configuration in the anechoic chamber of von Karman Institute for Fluid Dynamics. A free-stream velocity of 30 m/s (corresponding to a chord-based Reynolds number of 3.2·10⁵) and an angle of attack of 0^◦ have been considered for the tests. Hot-wire anemometry measurements have been performed with the aim of characterizing the boundary layers around the two airfoils and investigating the eventual nose mitigation mechanisms that occur due to the porous treatment. Moreover, the static pressure distributions along the two surfaces have been studied to find correlations with the velocity profiles. The results show that the use of a porous treatment leads to a decrease of turbulence-impingement noise in the low-frequency range and to an increase in the high-frequency one, mostly due to surface roughness noise. The application of an inverse beamforming technique has lead to a qualitative comparison of noise distribution maps at different one-third octave band frequencies for the two cases. Furthermore, the pressure values of each map point within a rectangular region surrounding the leading edge have been summed in order to retrieve the integrated one-third octave band spectra.