Noise source identification for ducted fan systems

Abstract

Coherence based source analysis techniques can be used to identify the contribution of combustion noise in the exhaust of a jet engine and hence enable the design of noise reduction devices. However, when the combustion noise propagates in a non-linear fashion the identified contribution using ordinary coherence methods will be inaccurate. In this paper, an analysis technique to enable the contribution of linear and non-linear mechanisms to the propagated sound to be identified is reported. An experimental rig to study the propagation of noise through a rotor/stator set-up using a vane-axial fan mounted in a duct so that non-linear interactions between a sound source and the fan could be investigated is described. The technique which is used to identify a non-linear tone generated by the interaction of the rotor and a propagated tone is reported. The identification procedures are then applied to data from full scale turbo-fan engine tests instrumented with pressure transducers at the combustor can and in the hot-jet pipe with microphones in the near-field. At a particular power setting, the interaction between the combustion noise and the high pressure turbine was measured in the hot jet pipe. The analysis techniques enabled non-linear interactions to be identified and linear and non-linear coherent output powers to be determined.