Modelling of Boundary Layer Effects on the Attenuation of Buzz-Saw Noise in Lined Ducts

摘要

Buzz-saw noise is a major contributor to the noise from a modern high-bypass ratio turbofan engine during take-off and climb. In this paper, a hybrid method combines a combination of prediction tools to predict the in-duct sound field, including the effects of non-linear propagation and attenuation by an acoustic liner. The first of these tools is a Computational AeroAcoustics (CAA) code, using the finite element method. This is used to predict linear propagation and attenuation in an acoustically lined intake duct. Non-linear effects, which are significant for high amplitude noise, are accounted for by applying a correction to the linear predictions by using a model of one-dimensional shock wave propagation. The CAA code used in this work is based on the Mohring formulation. The acoustic liner is modelled by the Myers boundary condition, in which the boundary layer of the flow is represented by a vortex sheet. This can result in an over-prediction of the linear attenuation for upstream noise propagation. In reality, a boundary layer of finite thickness refracts the upstream propagating sound away from the liner, thus reducing the attenuation. Solutions of the Pridmore-Brown equation can provide more accurate predictions by accounting for a finite thickness boundary layer. However, higher computational expense limits its use to predict the attenuation of modes in an axial parallel flow in a uniform duct. In this article, an 'effective impedance' is introduced, which provides identical phase and attenuation predictions to a Pridmore-Brown solution for an acoustic mode when used in the Myers boundary condition. This method is validated against measurements from an axisym-metric rig intake test. The use of an effective impedance significantly improves predictions without the additional computational cost associated with resolving the boundary layer in the flow profile.