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 bench-test 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 and validated using this rig. The techniques are subsequently applied to data generated from within an FP7 Collaborative European Aeroacoustics Project: TEENI. The analysis techniques enabled non-linear interactions to be identified and linear and non-linear contributions to be determined.