Non-linear Identification Applied to Broadband Turbomachinery Noise

Coherence based source analysis techniques can be used to identify the contribution of broadband noise sources in the exhaust of a gas turbine and hence enable the design of noise reduction devices. However, when the broadband noise source 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 non-linear noise contributions generated by the interaction of the rotor and propagated narrowband noise is reported. The analysis techniques enabled non-linear interactions to be identified and linear and contributions to be determined.