Spinning mode analysis of the acoustic field generated by a turboshaft engine

The spinning mode analysis of a ducted acoustic field is a powerful means for advancing the understanding of noise sources in turbofan or turboshaft engines, and it is also the main information needed to predict the far-field directivity radiated by the inlet or the nozzle. Tests were made with a circular array of fixed microphones in a nozzle cross section of a Turbomeca TM 333 turboshaft engine to determine the modal characteristics of its acoustic field. The main difficulty in data processing comes from the high background noise due to the very turbulent internal flow. A conventional method based on the angular Fourier transform of the cross-spectral matrix may therefore be inadequate. Several improvements are discussed and validated by numerical simulations, such as the use of a three-signal coherence technique to reduce the output noise. A further gain is achieved by the reduction of the estimate bias due to the finite number of statistical averages. The experimental TM 333 wave-number spectra are presented in the last section at several frequencies, corresponding to the broadband combustion noise and to the tones emitted by the high-pressure turbine and the free turbine. The observed spinning modes are explained by taking into account the nozzle cut-off properties and the radiation mechanisms of transonic rotors.