Predicting fan noise propagation in aeroengine bypass ducts

Abstract

This thesis explores the prediction of rearwards fan noise propagation within the bypass duct and its radiation into the far feld. Two recently developed models: B-induct and GXMunt, are exploited in application to real engine bypass ducts and their performance is evaluated. These methods are an improvement on current industry standards, allowing realistic duct geometry and flow conditions to be modelled with reasonable computation and time demands. The main focus is on the model b-induct.
B-induct predictions for bypass attenuation are integrated into an industry standard whole engine model, and predictions of far-feld noise are obtained for a modern high bypass-ratio engine. These predictions compare more favourably with measured data from full-scale static engine tests than similar predictions made using a standard uni-form rectangular duct model for the bypass attenuations, indicating that b-induct is an improvement over the current model.
Initial studies on the effect of duct geometry on noise propagation suggest a noise benefit for a duct with higher curvature when compared to a typical Baseline design. This suggestion is confirmed using measured data from zero-flow rig tests. Predictions for three-dimensional duct geometries are also performed to show the effect of scattering due to bifurcations within the duct.
B-induct allows for the specific bypass geometry and liner positions to be taken into account when performing impedance optimisations. A new optimisation procedure is proposed in which b-induct predictions are used within an existing whole aircraft noise prediction model. This procedure is used to select liner impedances for a modern engine bypass duct. B-induct is demonstrated to be a promising new tool within the engine design process, for both analysis of the impact of rear fan noise on whole engine noise, and assessment of potential low noise bypass configurations.

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