Spectral Broadening in Aeroacoustics
Lead Research Organisation:
University of Southampton
Department Name: Faculty of Engineering & the Environment
Abstract
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.
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.
Planned 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.
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.
Publications
Clair V
(2018)
Spectral broadening of acoustic waves by convected vortices
in Journal of Fluid Mechanics
Clair V.
(2015)
Computational study of the Spectral broadening of an acoustic tone by turbulence
in 22nd International Congress on Sound and Vibration, ICSV 2015
Clair V.
(2016)
Numerical investigation on the spectral broadening of acoustic waves by a turbulent layer
in 22nd AIAA/CEAS Aeroacoustics Conference, 2016
Clair V.
(2016)
Numerical assessment of the scattering of acoustic waves by turbulent structures
in 21st AIAA/CEAS Aeroacoustics Conference
McAlpine A
(2020)
Spectral Broadening of Tonal Sound Propagating Through an Axisymmetric Turbulent Shear Layer
in AIAA Journal
McAlpine A.
(2016)
A weak-scattering model for tone haystacking caused by sound propagation through an axisymmetric turbulent shear layer
in 22nd AIAA/CEAS Aeroacoustics Conference, 2016
| Description | Spectral broadening refers to the scattering of tonal sound fields by turbulence. This occurs in jet engines, when turbine and fan tones radiate sound propagating through the exhaust jet shear layers. New laboratory experimental measurements on this project have provided measurements of spectral broadening owing to tonal sound radiated from a jet pipe. The source of the sound is upstream of the jet nozzle. As the sound propagates and is radiated from the jet pipe it passes through the turbulent jet and sound energy is scattered into adjacent frequency bands, thus providing examples of spectral broadening. Owing to the small scale of the experiment, since the radius of the jet is small, it is necessary to utilise an ultrasonic (high frequency) source so that the length-scales associated with the jet and the sound are the appropriate scaling relative to each other, New theoretical analysis on this project has led to a new analytical prediction scheme which can predict the spectral shape of the scattered sound field. Also a new theoretical framework has been developed which can be applied to "strong" spectral broadening which is when the tonal sound is scattered substantially such that the resulting spectral shape exhibits a broadband hump in effect masking the original frequency of the tone. We aim to publish this work in scientific journals (in 2017 or 2018). New computational work on this project has investigated scattering by a single vortex to better understand the physics of the scattering process. For a moving vortex, the spectral broadening effect can be linked to the Doppler effect (the vortex is acting as a moving source of the scattered sound field relative to an observer). Scattering by a single vortex has been replaced by simulations of the scattering by a turbulence layer, where the turbulence is generated by a stochastic method. At very high frequencies, simulations of "strong" scattering have been achieved and published for the first time. These results have already been presented at international conferences, and we aim to publish this work in scientific journals (in 2017 or 2018). |
| Exploitation Route | A key area for future study will be to develop a procedure to simulate the inverse problem, i.e. how to calculate the original tonal sound field prior to the scattering by turbulence and the resulting spectral broadening. |
| Sectors | Aerospace, Defence and Marine,Transport |
| URL | http://www.southampton.ac.uk/engineering/research/projects/scattering_of_turbines_tones.page |