Measurement of the noise generated by wall-mounted airfoils of different thickness

The flow around a wall-mounted finite airfoil with natural transition can lead to complex tonal characteristics in the corresponding aeroacoustic noise spectra. While many of the flow features and noise generating mechanisms are well understood, there are still open questions, for example regarding the influence of the airfoil shape. In the present paper, the influence of the thickness of a wall-mounted finite airfoil on the noise generation is examined experimentally. To this end, detailed measurements were performed on a NACA0012 airfoil and a NACA0018 airfoil in an open jet aeroacoustic wind tunnel at various flow speeds and angles of attack. This includes acoustic measurements with a planar microphone array as well as measurements of the surface pressure fluctuations using flush-mounted pressure capsules. In addition, surface flow visualization experiments were conducted on the NACA0012 airfoil. The results show that the thickness of the airfoil has a notable influence on the tonal noise generation, which is visible both in the sound pressure level spectra as well as in the wall pressure spectra. At small geometric angles of attack and high flow speeds, the thinner NACA0012 generates a strong tone with weaker side tones, whereas the NACA0018 generates a set of equispaced tones. At higher angles the NACA0012 does not radiate tonal noise, while the NACA0018 now generates this strong tone with weaker side tones.