What you should know about…exhaust nozzle shapes
Airports are busier than ever – thousands of airplanes taking off and landing every day. Passengers and people living around airports are reminded that the airplane is not the quietest mode of transportation. One of the most recent noise-reducing technologies shepherded through the research process by NASA and now making a difference on commercial jet engines is “chevrons”.
“Chevrons” are the sawtooth pattern seen on the trailing edges of some jet engine nozzles. The nozzle is the end module of the engine which accelerates the flow of exhaust gases out of the engine. As hot air from the engine core mixes with cooler air blowing through the engine fan, the shaped edges serve to smooth the mixing, which reduces turbulence that creates noise.
Who makes the noise?
Most of the civil airliners use high bypass turbofan engines, which produce a significant amount of noise especially under high thrust conditions. Aircraft noise is especially critical during the takeoff and approach phases as it affects the people in the area around the airport and also the ground crew. The engine noise is perhaps the most important contributor for the total aircraft noise. Thus, reduction of engine noise becomes paramount. Most of the high bypass turbofan engines have two flow regions - the central core and the surrounding fan. These regions normally don't mix with each other in the short nozzle. Usually, high bypass ratios reduce noise. The main source of the jet noise is the turbulent mixing of the shear layers in the engine exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound.
NASA research generated sawtooth shape
One of the ideas to reduce noise has been to mix the core and bypass gases effectively. NASA carried out research in this area and evaluated a number of different configurations. They developed and tested different shapes: Doublets, scarfed, tabbed, tongue, lobed, as well as sawtooth.
Over 30% noise reduction compared to previous versions
So the main purpose of the chevrons in the engines is not to appear as an elegant solution, but to reduce the engine noise. The new Boeing 787 with its GEnx or Trent 1000 engine is among the most modern jets relying on chevrons to reduce engine noise levels. It sports chevrons on the nacelles, or fan housings. The Boeing 747-8 has chevrons on both the nacelles and inner core engine nozzles.
The chevron geometry reduces engine noise significantly: For the 747-8, this means around 30% less noise compared to the previous engine model.
Look out for them during your next flight!
Interested in our tools for engines? Contact your local Area Sales Manager