Validation and Application of a Hybrid Prediction Scheme for Bypass Duct Noise

Abstract

A prediction scheme for noise propagation and radiation from aeroengine bypass ducts is validated against measured data and applied to a generic bypass duct to study the efiects of bypass duct geometry on modal scattering. The objective has been to develop a relatively simple scheme which can predict the efiects of bypass duct geometry and acoustic treatment at acceptable computational cost. To achieve this, the in-duct and radiation problems are uncoupled and separate methods applied to each. Two difierent radiation models are implemented and compared. The approach is validated by a comparison of predicted and measured data for the NASA ANCF (Active Noise Control Fan) ∞ow test rig. A well deflned interference tone is used as a source. The rig is of simple geometry but incorporates the efiects of mean ∞ow and refraction through the bypass shear layer. Numerical predictions for aft radiated noise are shown to be in a good agreement with measured data, and both radiation models are self-consistent except at small angles from the aft axis. This persists at higher frequencies for which measured data are not available. The model is also used to study in-duct geometry efiects on modal scattering and attenuation in a generic lined bypass ducts. I. Introduction The prediction of aft radiated fan noise from turbofan aero-engines is challenging for conventional numerical schemes, more so than for the corresponding intake problem. The major complicating factor is the presence of a shear layer between the bypass stream and the external ∞ow. This precludes the straightforward use of frequency-domain flnite and inflnite element (FE/IE) analysis, 1 an approach which has proven efiective for intake problems. 2 This approach, and others based on the solution of a convected wave equation for the acoustic potential, cannot readily be applied to exhaust problems due to the presence of irrotational mean ∞ow. 3,4 High order schemes based on the numerical solution of the Linearised Euler Equations (LEE) in the time domain have however been applied both to intake and exhaust problems . 5,6 The intrinsic instability of the shear layer gives rise however to unstable transient solutions in the linearised Euler solution for exhaust ∞ows which must be removed or flltered from the numerical solution. 7{9 Agarwal et al. 10 have demonstrated that this instability can be suppressed if the governing equations are solved by a direct solver in the frequency domain. Parabolic approximations, 11 have also been applied to both intake and exhaust problems, 12,13 but do not deal well with propagation at high mode angles. In all of the above numerical schemes, the discrete solution is terminated by a non-re∞ecting condition which must be imposed at a flnite boundary. This must be placed close to the near fleld if the overall problem size is to be contained within acceptable limits. This constraint is particularly demanding in cases where a full three-dimensional numerical model is required. In the current paper, a hybrid modal/numerical/analytic prediction scheme for the noise propagation and radiation from aeroengine bypass ducts is presented, validated against ∞ow data and applied to a generic lined bypass duct. The objective of the current approach has been to develop a scheme which can predict the efiects of bypass duct geometry and acoustic treatment at acceptable computational cost. To achieve this, the in-duct and radiation problems are uncoupled and treated separately. The in-duct problem is treated by a Finite Element (FE) scheme. The radiation problem is treated using two approximate methods. An outline of the method was presented at the 11th AIAA/CEAS Aeroacoustics conference 14 and numerical predictions