Computation of fan noise radiation through an engine exhaust geometry with flow

Description/Abstract

This paper outlines a computational model of noise radiation from a realistic engine exhaust
geometry with flow. The computational model described allows acoustic waves, propagating
inside the bypass duct of a generic aircraft engine, to be admitted into a computational domain
that includes the aft duct section, the exit plane of the duct, and the jet flow immediately
downstream. The method has three parts: a matching process to admit acoustic waves into the induct
propagation region; near field propagation inside the duct and diffraction at the lip of the
exhaust duct; and an integral surface for far field directivity. In this model the near field
propagation is determined by a numerical solution of a 2.5D form of the linearised Euler
equations. The mean flow about which the equations are linearised is assumed to be axisymmetric.
The proposed method is illustrated through a case study on the radiation of a typical fan
assembly generated acoustic wave from a generic engine bypass duct. Inside the duct, an acoustic
wave of circumferential order m = –13 and comprising five radial modes (n = 1 – 5) is admitted
into the model as inputs on the boundary of the computation domain. The radiation of the acoustic
wave through the exhaust geometry and mean flow is determined, with the effect of acoustic
treatment through the inclusion of lined duct sections also examined.