Effect of Airfoil thickness on low-frequency gust-airfoil interaction noise

Interaction between turbulence and an airfoil is a significant aerodynamic noise source for many engineering applications when turbulence in the wake of upstream blades interacts with the leading edge of downstream blades. Modeling the oncoming turbulence as harmonic gusts is a common approach to studying the noise generated by turbulence-airfoil interaction. The airfoil can be considered a compact noise source, resulting in a dipole-like sound directivity pattern when the acoustic wavelength of gust-airfoil interaction is much larger than the airfoil chord length. However, the interaction becomes more complex when the acoustic wavelength is comparable to or smaller than the airfoil chord, as the airfoil cannot be treated as a compact source anymore and multiple lobes appear in the radiated sound directivity. This paper studies airfoil thickness effects using a computational aeroacoustics approach based on the spectral/hp element method for low-reduced frequency gusts. It is found that the sound pressure decreases with the increase of thickness for low reduced frequency gusts. To reveal its mechanism, a semi-analytical method and the convective FW-H equation are used for low reduced frequency to analyze the radiated noise. The interaction between sound sources on the airfoil surface is crucial for the thickness effect.