Airfoil Turbulence-Impingement Noise Reduction by Porosity or Wavy Leading-Edge Cut: Experimental Investigations

Airfoil turbulence-impingement noise generation is a mechanism by which the kinetic energy of vortical patterns in a flow is partly converted into sound under the effect of compressibility. It contributes to the broadband noise radiated by fans, turbomachines and high-lift devices. In many instances the turbulent field is inherent to the installation of a fan and cannot be directly controlled. An alternative is to modify either the geometry or the material of the blades in such a way that the sound radiation is reduced whereas the performances remain unaffected. Two possible techniques are addressed in the paper: replacing the straight leading edge of a blade by a wavy leading edge, on the one hand, and manufacturing part of the blade with a porous material, on the other hand. Both are tested on modified versions of the same baseline rigid-airfoil mockup (NACA-0012), by placing the mockup in grid-generated turbulence in the flow delivered by an open-jet anechoic wind tunnel. A step-by-step procedure is described to subtract background noise and trailing-edge noise sources in order to unambiguously quantify the benefit of the reduction techniques. Both are found to reduce the noise by amounts between 4 to 8 dB in substantial frequency ranges.