Quote:
Originally Posted by pascal28
Serrated nozzles on aircraft engines are for noise reduction. They have a slight efficiency penalty. They were put on the engines (Trent 1000 & GEnX) powering the Boeing 787 due to concerns that its composite fuselage would transmit more noise than a conventional aluminum one. They reduce noise by encouraging mixing between the hot & bypass flow and between the bypass and free-stream air. This reduces the shear force and consequential noise generation.
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I believe that this technology is an outgrowth of NASA's acoustic and body-morphing research of raptors,specifically the peregrine falcon and one type of owl.(please see PBS' NOVA's 'Raptor Force')
Critter-cams attached to both birds revealed damping abilities owed to aft-body feather and down architecture.
Earliest winglet technology was also modeled after buzzard wingtip feathers which not only cut drag,but also attenuated noise propagation from the reduced shearing forces.