Effect of porosity on the wall-pressure fluctuations on a wing profile immersed in a turbulent flow

The turbulence impinging on an airfoil generates aeroacoustic noise, also referred to as leading-edge noise, which plays an important role in industrial sectors such as wind energy, aviation and cooling/ventilation. For its mitigation, one of the proposed solutions involves the integration of porous materials into the structure of the wing profile. Since the physical mechanisms responsible for the noise reduction have not been defined yet, the aim of this work is to investigate how porosity affects the surface-turbulence interaction and the evolution of the wall-pressure fluctuations radiating into the far field to provide new information for a better understanding of this phenomenon. The post-processing activity has been carried out on the large-eddy simulations data, provided by a group of researchers from the RWTH Aachen University: these data are based on an experimental setup already implemented at VKI, which involves the comparison between a porous NACA-0024 profile equipped with melamine foam and an identical solid configuration, both subjected to a turbulent flow generated by an upstream circular rod. Wall-pressure fluctuations are reduced by 6 dB along the porous surface, particularly just downstream of the stagnation point, and flow structures show less spanwise coherence than the solid design. Results of the power spectral densities indicate an attenuation of the pressure and velocity fluctuations in the porous model, which mainly affects the low-frequency range. The most important result comes from the calculation of surface-normal velocity, whose PSD shows for the porous case a similar reduction to the wall-pressure fluctuations in the same frequency range: this correspondence demonstrates that the attenuation of turbulence distortion due to porosity is one of the mechanisms associated with leading-edge noise reduction, at least for thick airfoils. For the experimental validation, a fast and in-situ calibration procedure for microphones embedded on the surface of both solid and porous airfoils has been designed and implemented to investigate how the wall pressure fluctuations are affected by porous media. The results are promising in terms of repeatability, but require further investigations in order to be used for a quantitative analysis.