Prediction Methods for Propagation in Bypass Ducts and Comparison with Measured Data

A hybrid numerical procedure is presented for predicting the propagation and radiation of fan noise from aero-engine bypass ducts. In this approach, an accurate in-duct flnite element model which captures well the physics of multimode propagation within the duct, is coupled to a more approximate radiation model in the external domain. The objective is to develop a relatively simple scheme which can predict the efiects of bypass duct geometry and acoustic treatments on the radiated sound at modest computational cost. Predictions of the attenuation and the far fleld directivity for no-∞ow prototype model bypass ducts are validated against test data. A hybrid Numerical approach is presented for predicting the propagation and attenuation of fan noise in an annular bypass duct and its radiation to the far fleld. The method combines an in-duct FE computation with an approximate radiation model for the far fleld directivity. The efiectiveness of this approach is assessed in the current article by comparing predicted and measured values of insertion loss and sound pressure level directivity to measured data from a 1/6th scale rig for idealised bypass duct conflgurations in the absence of ∞ow. The noise path of sound radiating from a turbofan bypass duct is shown in Figure 1. Tone and broadband noise are generated by the unsteady aerodynamics associated with the ∞ow over the fan and the stator. The resulting sound fleld propagates through the bypass duct where it is scattered and attenuated by mean ∞ow gradients and by lined internal surfaces. The sound radiating from the duct exit is then modifled and refracted by the external geometry of the nacelle and by the bypass shear layer. The prediction of aft fan noise has generally received less attention than the complementary problem of forward radiation from the intake. Aft radiation is somewhat more di‐cult to deal with due to the presence of rotational mean ∞ow in the shear layer between the bypass stream and the external ∞ow. In-duct analysis is complicated by 3-D features such as the presence of the pylon and the lower bifurcation within the bypass duct itself. These render the assumption of an axisymmetric geometry and consequent uncoupling of azimuthal components of the acoustic fleld more problematic for bypass ducts than for intakes. In high and ultra-high bypass ratio engines, however, aft fan noise is the dominant noise source at cutback and approach, and its accurate prediction has become a pressing necessity in attempts to mitigate the nuisance of aircraft noise at takeofi. Prediction methods which have been applied to the calculation of aft fan noise include axisymmetric Finite/Inflnite Element (FE/IE) models in which the bypass mixing layer is modelled by a vortex sheet; 4 time-domain and frequency-domain, high order Finite Difierence (FD) codes based on the Linearised Euler Equations (LEE), 11,9 and application of the parabolic wave equation. 3,8