Broadband Noise Source Localisation in Turbomachinery using a Conditioned Spectral Analysis Technique Coupled with Modal Decomposition

Methods are presented for the identification, viz., relative magnitude and spatial origin, of broadband and tonal noise sources within aeroengines. The methods require dynamic pressure measurements from sensors located within the engine. These techniques can be used to assess the contribution of internal noise sources to sound radiated from an aeroengine exhaust, for example, to the far field. Three coherence based techniques are evaluated: the coherent output power, the signal enhancement technique and a five sensor partial coherence method. The techniques are applied to data generated by a small scale experimental rig within an EU FP7 programme – TEENI. The experimental test rig design simulates the most relevant acoustic features of the downstream part of a turbo-shaft engine, i.e. the combustion noise source, the turbine stages and the relevant propagation processes to the exhaust exit. The application of these techniques to turbo-shaft engines is of particular interest where corenoise is a far more significant contributor to far field sound than jet noise. Results show how broadband noise from a rotor/stator stage, broadband noise from a single loudspeaker (simulating the combustor) and a tone from the rotor/stator stage can all be decomposed from measurements and how these contributions can be localised. In addition, the five sensor method is significantly enhanced to include acoustic modal decomposition. This allows the contribution of sources to individual radial modes to be measured and the source location identified. Specific radial modes were generated and measured with upstream loudspeaker and downstream microphone arrays respectively.