Noise and emissions (carbon dioxide and nitrogen oxides) from jet engines are a major issue, with public expectations of quieter and cleaner skies, despite the rapid growth in commercial air transportation. Research on aircraft noise is of major importance to many stakeholders in the UK. London Heathrow enforces some of the most stringent noise regulations of any of the world's major city airports. Also Rolls-Royce, one of the UK's premier engineering companies, currently has a 30% share of the civil-engine market, making it the world's second largest supplier of civil aircraft engines.

In addition to the economic benefits, reducing aircraft noise and emissions also benefits society, improving the quality of life, and in some instances the health, of people living and working near airports. One of the principal aims in the ACARE (Advisory Council for Aeronautics Research in Europe) 2020 vision is a 50% reduction in perceived average noise levels. Notwithstanding the significant investment in aircraft noise research in Europe and the U.S. during the last two decades, this vision will still require considerable technological advances to make airplanes substantially quieter.

The key application of the majority of research in aeroacoustics is aircraft noise. 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.

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. Open-rotor noise testing conducted in free-jet wind-tunnels can be affected by the presence of the wind-tunnel 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. 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 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.

In summary, the research project will be the first comprehensive study on spectral broadening in aeroacoustics, with key applications directly linked to noise emissions from both turbofan and open-rotor aircraft engines.

Potential Impact:
Outside the academic community, the impact of the research will principally benefit the aerospace industry. The key application of the majority of research in aeroacoustics is aircraft noise. Noise and emissions (carbon dioxide and oxides of nitrogen) from jet engines are a major issue, and research on aircraft noise is of key importance to many stakeholders in the UK. London Heathrow - the world's busiest airport for international passengers - enforces some of the most stringent noise regulations of any of the world's major city airports. Rolls-Royce, one of the UK's premier engineering companies, currently has a 30% share of the civil-engine market, making it the world's second largest supplier of civil aircraft engines. Other key aerospace industries located in the UK include Airbus UK, Bombardier Shorts and GKN aerospace. Also at Farnborough, QinetiQ houses one of the world's premier jet noise test facilities.

Owing to the continued growth in commercial air transportation, lowering noise emissions from aircraft engines will alleviate the detrimental impact on community noise from the expanding numbers of aircraft in service. Aircraft engines with reduced emissions will be more competitive in the global aviation market. However, in addition to the economic benefits, reducing aircraft noise and emissions also benefits society, improving the quality of life, and in some instances the health, of people living and working near airports. One of the principal aims in the ACARE (Advisory Council for Aeronautics Research in Europe) 2020 vision is a 50 % reduction in perceived average noise levels. Notwithstanding the significant investment in aircraft noise research in Europe and the U.S. during the last two decades, this vision will still require considerable technological advances to make airplanes substantially quieter.

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. 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 and fan tones.

Recently 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. Noise testing to measure the tones generated by advanced open-rotor designs conducted in free-jet wind-tunnels will be affected by the presence of the wind-tunnel jet shear layers. The jet shear layers will cause spectral broadening of the tones (which would not occur on an aircraft).

The key outputs from the research project will be robust, validated simulation methods (theoretical and computational) to predict spectral broadening in aeroacoustics. The other key output will be the acquisition of a model-scale experimental database of measurements of spectral broadening obtained in the laboratory. The capability to predict strong scattering is the key aim of the research project; currently there are no prediction methods available to predict strong scattering of tones from turbofan aircraft engines. Also these type of prediction methods are required to appraise noise measurements from free-jet wind-tunnels, in particular for testing open rotors which generate highly protrusive tonal sound fields that can be significantly affected by scattering caused by the wind-tunnel jet shear layers. In addition to the development of new prediction methods and a new experimental database, the research will provide new knowledge and understanding of this scattering phenomenon, as this project will be the first comprehensive study on spectral broadening in aeroacoustics.