Near-field acoustic assessment of open rotor aero-engines

Abstract

The focus of this research, part sponsored by Rolls-Royce plc., is to both model and gain an understanding of the acoustics in the near-field of the open rotor aero-engine concept. The eventual aim of such work is to develop accurate and fast prediction models which can be used to optimise engine design and operation, so that the noise produced by the open rotor can be minimised to sit well under regulation noise levels. This work will also be used to help understand the results of important experimental testing of the open rotor. Finally, such work may be useful for understanding what cabin noise levels may be, which is a strong factor in passenger comfort and is therefore another consideration for the client.

Outlined in this thesis are a number of tools and methods aimed at helping to predict the open rotor’s acoustic near-field in a fast and reliable fashion. As a primer, analytical models for the tonal noise of an open rotor in the free-field are first derived. The models are based on frequency domain methods, such as those proposed by Hanson in the 1980s. All models in this thesis may be used during engine development to make acoustic predictions at various operating conditions and gain insight into how certain rotor geometric features and operating conditions affect the generated noise level and propagation. Methods for projecting noise levels from the near-field to the far-field are also presented, which is useful when validating noise measurements from experiments. The effect of scattering from the open rotor centrebody and the effect of reverberation when contained in a closed test-section wind tunnel are also modelled. Centrebody scattering and tunnel reverberation have been shown from data analysis to have an impact on open rotor acoustic measurements. The analysis presented in this thesis has led to some novel mitigation techniques and suggestions for improved open rotor experiment design. Some limited comparisons to scaled open rotor experiments are provided, in order to validate the models and highlight further issues that would occur during real engine operation.

The business case for and the environmental benefits of the open rotor are clear. Efficient operation is a key driver in the development of aircraft propulsion technologies, since it can significantly drive down operational costs and the open rotor aero-engine promises a step change improvement in fuel efficiency. Another important objective during engine development is mitigating the generation of unwanted noise, in order to meet aircraft noise regulations

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