Editing Turbofan (section)

=== Noise ===
[[File:Boeing 747-8 VQ-BSK at LSZH (17754867239).jpg|thumb|Chevrons on a 747-8's [[General Electric GEnx|GEnx-2B]]]]
Most of the air flow through a high-bypass turbofan is lower-velocity bypass flow: even when combined with the much-higher-velocity engine exhaust, the average exhaust velocity is considerably lower than in a pure turbojet. Turbojet engine noise is predominately [[jet noise]] from the high exhaust velocity. Therefore, turbofan engines are significantly quieter than a pure-jet of the same thrust, and jet noise is no longer the predominant source.<ref name="Kempton2011">Kempton, A., [http://www.win.tue.nl/ceas-asc/Workshop15/CEAS-ASC_XNoise-EV_K1_Kempton.pdf "Acoustic liners for modern aero-engines"], 15th CEAS-ASC Workshop and 1st Scientific Workshop of X-Noise EV, 2011. Win.tue.nl.</ref> Turbofan engine noise propagates both upstream via the inlet and downstream via the primary nozzle and the by-pass duct. Other noise sources are the fan, compressor and turbine.<ref>{{Cite journal| title = Softly, softly towards the quiet jet | first = Michael J. T. | last = Smith | journal=[[New Scientist]] | date = 19 February 1970 | at = fig. 5}}</ref>

Modern commercial aircraft employ high-bypass-ratio (HBPR) engines with separate flow, non-mixing, short-duct exhaust systems. Their noise at takeoff is primarily from the fan and jet.<ref>{{cite book |last1=Huff |first1=Dennis |last2=Envia |first2=Edmane |date=October 2007 |editor-last=Crocker |editor-first=Malcolm |title=Handbook of Noise and Vibration Control |publisher=Wiley |pages=1096–1108 |chapter=Chapter 89: Jet Engine Noise Generation, Prediction, and Control |url= https://www.wiley.com/en-us/Handbook+of+Noise+and+Vibration+Control-p-9780471395997 |isbn= 978-0-471-39599-7}}</ref> The primary source of jet noise is the turbulent mixing of shear layers in the engine's exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound. To reduce the noise associated with jet flow, the aerospace industry has sought to disrupt shear layer turbulence and reduce the overall noise produced.<ref name="chevron technology">{{cite journal |url= https://www.researchgate.net/publication/273550214 | journal= Proceedings of the 13th Asian Congress of Fluid Mechanics 17–21 December 2010, Dhaka, Bangladesh | title=Evolution from 'Tabs' to 'Chevron Technology'–a Review | format= PDF | last1 =Zaman | first1 = K. B. M. Q. | last2 =Bridges | first2 = J. E. | last3 =Huff | first3 = D. L. | date=17–21 December 2010 | publisher=b[[NASA Glenn Research Center]] | place = Cleveland, [[Ohio|OH]] | access-date= January 8, 2025}}</ref>

Fan noise may come from the interaction of the fan-blade wakes with the pressure field of the downstream fan-exit stator vanes. It may be minimized by adequate axial spacing between blade trailing edge and stator entrance.<ref>{{cite journal |title=Designing the JT-9D Engine to meet Low Noise Requirements for Future Transports |last1=Kester |first1=JD |last2=Slaiby |first2=TG |journal=SAE Transactions |date=1968 |volume=76 |issue=2 |id=paper 670331 |page=1332 |jstor=44565020 |doi=10.4271/670331}}</ref>
At high engine speeds, as at takeoff, shock waves from the supersonic fan tips, because of their unequal nature, produce noise of a discordant nature known as "buzz saw" noise.<ref>{{cite magazine |title=Quiet Propulsion |first=M.J.T. |last=Smith |magazine=Flight International |date=17 August 1972 |page=241}}</ref><ref name= buzz_saw>{{Citation | first = A. | last = McAlpine | url = http://www.southampton.ac.uk/engineering/research/projects/buzz_saw_noise_and_non_linear_acoustics.page | title = Research project: Buzz-saw noise and nonlinear acoustics | publisher = University of Southampton}}</ref>

All modern turbofan engines have [[acoustic liner]]s in the [[nacelle]] to damp their noise. They extend as much as possible to cover the largest surface area. The acoustic performance of the engine can be experimentally evaluated by means of ground tests<ref name="Schuster2010">{{Citation | last1 = Schuster | first1 = B. | last2 = Lieber | first2 = L. | last3 = Vavalle | first3 = A. | title = Optimization of a seamless inlet liner using an empirically validated prediction method | work = 16th AIAA/CEAS Aeroacoustics Conference | year = 2010 | place = Stockholm, [[Sweden|SE]] |doi=10.2514/6.2010-3824| isbn = 978-1-60086-955-6 | s2cid = 113015300 }}</ref> or in dedicated experimental test rigs.<ref name="Ferrante2011">{{Citation | last1 = Ferrante | first1 = P. G. | last2 = Copiello | first2 = D. | last3 = Beutke | first3 = M. | title = Design and experimental verification of 'true zero-splice' acoustic liners in the universal fan facility adaptation (UFFA) modular rig | work = 17h AIAA/CEAS Aeroacoustics Conference | year = 2011 | id = AIAA-2011-2728 | place = Portland, OR |doi=10.2514/6.2011-2728| isbn = 978-1-60086-943-3 }}</ref>

In the [[aerospace]] industry, <!--link target-->{{anchor|Chevrons}}'''chevrons''' are the "saw-tooth" patterns on the trailing edges of some [[jet engine]] nozzles<ref name=NASA>{{cite web |url= http://www.nasa.gov/topics/aeronautics/features/bridges_chevron_events.html |title= NASA Helps Create a More Silent Night |last1=Banke |first1=Jim |date= 2012-12-13 |publisher=[[NASA]] |access-date= January 12, 2013}}</ref> that are used for [[noise control|noise reduction]]. The shaped edges smooth the mixing of hot air from the engine core and cooler air flowing through the engine fan, which reduces noise-creating turbulence.<ref name=NASA/> Chevrons were developed by GE under a [[NASA]] contract.<ref name="chevron technology" /> Some notable examples of such designs are [[Boeing 787]], [[Boeing 737 Max]] and [[Boeing 747-8]]{{snd}} on the [[Rolls-Royce Trent 1000]] (787), [[General Electric GEnx]] (787, 748-8) and [[CFM LEAP]] (1B variant only; 737 Max) engines.<ref>{{Citation | archive-date = 2014-03-25 | place = [[China|CN]] | work = 13th ACFM | title = Invited | url = http://www.afmc.org.cn/13thacfm/invited/201.pdf | publisher = AFMC | archive-url = https://web.archive.org/web/20140325205124/http://www.afmc.org.cn/13thacfm/invited/201.pdf}}</ref>