A recently completed European research project has enabled engineers to understand the way in which so-called "buzz-saw" noise is generated and propagates along the nacelle of a jet engine, according to Rolls-Royce, one of the participants in the "Fanpac" research programme.
Buzz-saw noise is caused by shock waves formed at the front of the fan. They propagate along the engine's inlet duct, generating a distinctive tone during take-off and cruise. Engine and airframe manufacturers are keen to tackle the problem of buzz-saw noise, as it accounts for a large proportion of the overall noise levels produced by modern "big fan" engines which have very high bypass ratios.
"Acoustic liners and active noise- suppression systems provide part of the solution, but Fanpac enabled us to start looking at the source of buzz-saw noise and how to design for low-noise in the future," says Dave Henshaw, project engineer for installations engineering at Rolls-Royce.
The team built a scale model of a typical wide-chord fan, fitted with tiny probes and transducers, to measure the "characteristic shape" of the noise it produced.
"We have also been able to study turbulence from each fan blade to better understand the noise generated when this wake hits the inlet guide vanes immediately behind," says Henshaw. "This 'wake interaction' is key in any approach to reducing fan noise," he says.
Also taking part in the three-and-a-half year-long project were Aerospatiale, Alenia, Daimler-Benz Aerospace, Short Brothers, and Snecma, together with several European research organisations.
R-R says that a proposal for a follow-on programme will be submitted to the European Commission early this year.
Source: Flight International
