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This question already has an answer here:

How does a normal aircraft exhaust nozzle (base model) generates noise and, whats the difference between the operation of the normal base model of aircraft exhaust nozzle and the new design of aircraft nozzle with an add of chevrons ? Is the chevrons effects the aircraft or engine performance in any way ?

Base model aircraft exhaust nozzle

aircraft chevrons nozzle

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marked as duplicate by Simon, ymb1, mins, SMS von der Tann, Federico Mar 19 '17 at 13:07

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Those "chevrons" are vanes that modify the air and exhaust flow (those are turbo-fan- engines in those two images) and create air vortexes that quiet the noise but also create some drag. They are engineered to maximize noise reduction with a minimum of drag.

enter image description here
(popularmechanics.com) Quiet engine air brake.

There is a company called ATA Engineering that is now developing a product called a "quiet engine air brake" that create more drag but is stowable during cruise flight. The R&D work was commissioned by NASA. They use the same vortex flow concept. Because most of the noise from airliners during approach comes from things that stick out in the airflow, like flaps and spoilers, landing gear doors, etc., this allows the aircraft designer to generate drag to slow down the aircraft while at the same time reducing noise.

Its a pretty nifty concept, if you ask me. Over time I think we will see the design of exhaust systems on jet engines become stranger and stranger! 8)

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