Spectral, modal and coherence analysis of sum and difference scattering of narrowband noise in turbomachinery

Abstract

The phenomenon of sum and difference frequency scattering of narrowband noise as it interacts with ducted rotating turbomachinery is studied experimentally in this article. This phenomenon will affect the spectral and modal distribution of the noise downstream of rotating turbomachinery, which must be considered in order to optimise any acoustic mitigation methods installed inside the engine. In the experimental set-up, narrowband noise is generated by a loudspeaker located at the upstream end of the test duct which then propagates through a rotor stage. A microphone array located downstream of the rotor is used to examine the spectral and modal distribution of the acoustic pressure field at a single axial location. The results from this analysis are compared with pre-existing theory which had not previously been experimentally validated in the literature. This theory may be used to predict the modal content of the scattered noise. A coherence analysis is also presented using microphones upstream and downstream of the rotor stage in order to assess whether sum and difference scattering is best predicted by a linear or non-linear model. The results from this study serve to enhance our current understanding of sum and difference scattering of narrowband noise through turbomachinery stages, and highlight interesting results from this interaction such as potential overlapping of incident and scattered noise which may otherwise be overlooked.