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Aeroacoustics

Published online by Cambridge University Press:  04 July 2016

J. E. Ffowcs Williams
J. E. Ffowcs Williams*
Affiliation:
Department of EngineeringUniversity of CambridgeCambridge, UK
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Summary

The high noise levels of early turbojet aircraft forcedattention to be given to the mechanics of the noisegeneration process and ways were urgently sought forsilencing their high speed propulsive flows. Thesubject of aeroacoustics was born with this problem,the subject being the variety of ways in which soundand flow interact, with the early emphasis being onsound generated aerodynamically. Sound was thoughtto be a by-product of the flow arid believed to havea negligible back-reaction on it.

The search for noise suppression has been extremelysuccessful, so much so, that engine noise is nolonger a problem likely to constrain future aircraftoperations. Silencing technology has made enormousstrides by avoiding as much as possible the veryhigh speed jet flows that were a feature of earlyengines. But that avenue is not available tosupersonic aircraft, which still wait for ways ofmaking their operation acceptably quiet to beidentified.

Aeroacoustics has brought into being a much deeperinsight into the interactions of sound and flow, andsome of the early presumptions about the mechanismsof noise creation have had to be rethought. Some ofthe advances have come from entirely new directions,for example, from anti-sound, a silencing idea basedon the destructive cancellation of interferingwaves, which has advanced to a state that provides auseful new technology. Some new understandingfollows the discovery that flow-acousticinteractions are not always the one-way processpreviously assumed. Sound can affect flow,especially so when the flow is unstable. It is ofcourse the very unstable flows that make most noiseso, taken together, these developments indicate thatcontrol might one day be exercised by activelymanaging the unsteady flows, making them more usefuland desirable than before. Indeed examples are nowknown where control has made possible the avoidanceof a powerful performance-limiting instability thatpreviously made that flow regime a prohibitedoperational zone, a development of aeroacousticsthat is probably far more significant than its noisesuppression role. That line of thought is expandedin this paper, which concludes with the speculationthat active control might now be offering an avenuefor approaching die high speed jet noise problemfrom a completely new direction. Of course it is fartoo early to know whether that approach can providea way for removing the take-off noise constraintcurrently threatening the viability of supersonictransports, but it might; that makes it veryimportant indeed, for it is hard to see anyalternative.

Information

Type
Research Article
Information
The Aeronautical Journal , Volume 100 , Issue 1000 , December 1996 , pp. 530 - 537
Copyright
Copyright © Royal Aeronautical Society 1996 

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