Noise Generation by Porous Airfoils

The noise generated by the flow around airfoils continues to be one of the important aeroacoustic noise sources. Innovative techniques for airfoil noise reduction are, therefore, of great importance. The present experimental study deals with the use of porous material for the construction of airfoils and the subsequent noise reduction. The use of such material controls the flow around the airfoil and has an influence on the sound generation. The analysis focuses on the characterization of the influence of the porous material parameters, especially the flow resistivity and the porosity, on the noise generation. Results of aeroacoustic wind tunnel tests on several porous and non-porous SD 7003 model scale airfoils are compared. Using microphone array measurements, it was found that not only the overall sound pressure level, but also the spectral characteristics depend on the parameters of the porous material. While the overall sound pressure level decreases in the order of a few decibel, at lower frequencies the reduction is considerable larger and extends 10 dB in some cases. Additionally, the influence of the material parameters on drag and lift is discussed. The aerodynamic performance of the porous airfoils is in general inferior to that of non-porous airfoils. However, there appear to be sets of material parameters that provide a considerable decrease in sound generation and only a minor degradation of aerodynamic eciency.