Effects of turbofan engine intake droop and length on fan tone noise

摘要

Numerical study is carried out to investigate the impacts of the three-dimensional geometry and length of the intake on the far-field noise propagation, radiation, and the acoustic liner efficiency, primarily for buzz-saw noise. The study has been conducted by using a commercial FE/IE tool ACTRAN TM and is based on a scale rig test with a drooped intake. The axisymmetric intake models and shorter intake models which are artificially defined from the rig intake geometry are also used for comparison. Firstly detailed observations are made for the circumferential mean flow distortion due to the intake droop, acoustic propagation of a rotor-locked single mode noise within the intake with distorted flow, and far-field noise radiation at Blade Passing Frequency (BPF) for various intake configurations. Simulations are then extended to cover wide range of engine orders. The difference in the far-field noise attenuation caused by the intake droop and the short intake length is demonstrated for both single mode rotor-locked tone noise and multimode noise component. The influence of the intake length in the radiated noise is more significant for lined intakes than hard-walled cases, with a change in polar directivity pattern where multiple lobe angles corresponding to different radial orders. The radiated noise pattern is quite complex for realistic drooped intakes and the impact of shorter intake length is not only reduced noise attenuation but also changes in directivities. Although the geometries used in this study were simple modifications of an existing rig intake which models currently-operating turbofan engines, the results implies the complex and sensitive nature of noise propagation through distorted flow and radiation from three-dimensional geometry. The results indicate that liner insertion loss is decreased due to the intake droop over low engine orders which are important for the consideration of buzz-saw noise.