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Research project: Scattering of turbine tones

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Analysis of frequency scattering of turbine tones radiated from turbofan aircraft engines

Project Overview

Frequency scattering of fan and turbine tones
Scattering in turbofan engine

Spectral broadening refers to the scattering of tonal sound fields by turbulence, whereby the interaction of the sound with a random, time-varying, turbulent flow results in power lost from the tone and distributed into a broadband field around the tone frequency. When the proportion of scattered power is small relative to the power that remains in the tone, this is termed "weak scattering". However, spectral broadening can lead to the disappearance of the tone itself, replaced by a broadband hump: this is termed "strong scattering".

The advent of the high-bypass-ratio turbofan engine led to a significant step-change reduction in noise from jet engines, principally due to lower levels of jet noise. A consequence of this reduction in jet noise was that, relative to other sources, fan, core and turbine noise became more important noise sources. In turbofan engines, spectral broadening occurs due to the aft radiated sound propagating through the exhaust jet shear layers. This affects the radiation of turbine tones, and to a lesser extent fan tones.

Frequency scattering in free-jet windtunnel
Scattering in wind tunnel

It is likely that in order to generate another step-change reduction in aircraft engine noise, radical changes to the engine's design will be required. Currently advanced open-rotor contra-rotating propeller concepts are being reappraised due to the significant fuel efficiency savings they can provide. However open-rotors generate a multitude of tones, and historically they have been perceived as being noisier compared to turbofan engines. Testing conducted in free-jet windtunnels can be affected by the presence of the windtunnel jet shear layers through which the sound propagates because open-rotors generate highly protrusive tonal sound fields. The shear layers cause spectral broadening of the tones.

The development of robust, validated prediction methods (theoretical and computational) will be a key output from this research project. The capability to predict strong scattering is the key aim; currently there are no prediction methods available to predict strong scattering of tones from turbofan and open-rotor aircraft engines.

The ISVR Doak-jet laboratory
The ISVR Doak-jet laboratory

The acquisition of a model-scale experimental database of measurements of spectral broadening obtained in the laboratory will be the other key output from this research. There is currently no such database available; the data will be used for validation purposes, as well as to improve our understanding of the scattering phenomenon.

Preliminary results from a pilot experiment in the ISVR's Doak-jet laboratory have revealed a clear example of spectral broadening of a tonal sound field by a turbulent jet.

Experiment to measure frequency scattering in the laboratory
Experiment
Tonal sound field with no turbulent jet
Laboratory experiment
Tonal sound field scattered by the turbulent jet
Laboratory experiment

Importantly this research project will be a comprehensive study of spectral broadening in aeroacoustics, with key applications directly linked to noise emissions from both turbofan and open-rotor aircraft engines.

Previous work at the ISVR on this topic was funded by the European project TURNEX. New work at the ISVR commencing in 2013 will be funded by the EPSRC and Rolls-Royce plc. 

Related publications

A weak-scattering model for turbine-tone haystacking outside the cone of silence
By C.J. POWLES, B.J. TESTER, and A. McALPINE.
International Journal of Aeroacoustics, 10(1):17–50, 2011.

A weak-scattering model for turbine-tone haystacking
By A. McAlpine, C.J. Powles and B.J. Tester.
Journal of Sound and Vibration, 332(16):3806-3831, 2013.

Related research groups

Acoustics Group

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