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In modern helicopter engines, the core noise generated by the combustor and turbine stages dominates the core sound radiated from the engine exhaust. Identi cation of the speci c contributions of each rotor-stator stage to the core noise is impeded by the lack of available space and the complex duct geometry within the engine. Modal decomposition is an advanced technique which can be used to analyse detailed information on the modal content of the core noise radiating from the engine. This paper advances previous work whereby a technique of identifying noise source location in ducts using the coherence function on a modal basis was developed. A proof of concept was developed using analytical data, with a full radial modal decomposition performed at two axial locations upstream and downstream in the duct. For the case of a real helicopter engine however, there is not enough space to perform a full modal decomposition upstream of the turbine. In this paper, the aforementioned technique is applied to experimental data in order to verify its usefulness for application inside a real turboshaft engine. The technique is also advanced to calculate the coherence between the modes found from a full modal decomposition close to the exit plane of the test duct in the ow (analogous to the engine exhaust) and a single pressure measurement upstream (analogous to a point inside the engine close to a noise source of interest). This better represents a realisable technique within the small con nes of an engine, as only one pressure transducer is required in the region of each source location of interest. Experimental data measured within an FP7 European research project by one of the consortium partners (German Aerospace Centre (DLR)) was be used to validate this technique.

Spatial Noise Source Identification of Tonal Noise in Turbomachinery Using the Coherence Function on a Modal Basis