Modelling of Flow Effects on Propagation and Radiation from Bypass Ducts

Abstract

A hybrid numerical procedure is presented for predicting the propagation and radiation of fan noise from an aero-engine bypass duct in the presence of mean flow and a bypass shear layer. The objective is to develop a relatively simple scheme which can predict the effects of bypass duct geometry and acoustic treatment on the radiated sound at modest computational cost. To achieve this, a relatively detailed in-duct numerical model, which captures the effects of the mean flow, liners and non-uniform geometry is coupled to an approximate radiation model which includes the effect of refraction through the bypass shear layer. Two different approaches are proposed for the radiation model. Results are presented which compare the predictions of each scheme for a generic, bypass duct with an impedance distribution characteristic of a modern high bypass ratio turbofan engine.