Effect of Leading-Edge Thickness on High-Speed Airfoil-Turbulence Interaction Noise

Abstract

Airfoil-turbulence interaction noise is studied to understand how thickness changes in the neighborhood of an airfoil leading edge can modify noise generation. The broadband interaction noise is generated by placing an airfoil in the turbulent boundary layer of a wind tunnel’s rectangular duct test section. Four airfoil leading edge configurations are tested at four different airfoil positions, relative to the duct exit plane. Local changes in mean flow properties are produced by both modifying leading edge thickness, and translating the airfoil axially to positions internal and external to the duct. In each configuration, the maximum airfoil thickness, camber distribution, and aerodynamic suction surface are unchanged. Leading edge thickness changes are confined to nearly the first ten percent of the airfoil chord, along the airfoil pressure surface only. Numerical results for the mean flow show that when the airfoil is internal to the duct, changes in thickness result in small changes in incidence between the flow and airfoil leading edge. However, with changes in the airfoil axial position, more significant changes in incidence occur, with large flow angles exhibited at the furthest external position. Measurements show that noise is reduced by several decibels as leading edge thickness increases, over a frequency range of 2-4 kHz when the airfoil lies within the duct, and 1-4 kHz when the airfoil lies external to the duct. The maximum noise reduction occurs at frequencies where the reduced frequency, based on leading edge thickness, is order one. The difference in the spectral noise reduction when the airfoil is translated internal versus external to the duct, suggests that changes in leading edge flow incidence may contribute to the noise reduction observed, and that the mechanism for noise reduction is not purely controlled by the relative turbulence scale to leading edge thickness relationship. To examine this hypothesis a simple model for turbulence-airfoil interaction noise that includes thickness effects is compared with data. Results show that agreement between the theory and the measurements are quite good when the airfoil lies within the duct and the incidence is small. When the airfoil lies external to the duct, significant variation between the theoretical model, which does not account for incidence, and measurements exists.