Acoustics and Thrust of Separate Flow Exhaust Nozzles for High Bypass Ratio Engines Acoustics and Thrust of Separate Flow Exhaust Nozzles With Mixing Devices Investigated for High Bypass Ratio Engines Typical installed separate-flow exhaust nozzle system. The jet noise from modern turbofan engines is a major contributor to the overall noise from commercial aircraft. Many of these engines use separate nozzles for exhausting core and fan streams. The illustration shows a typical exhaust nozzle system for a bypass ratio of about 5. Tabs are severe protrusions into the flow at the nozzle exit plane.
Chevrons are also protrusions, but of much less severity than tabs.
Other nozzle designs attempted to shield the core flow by using a scarf fan nozzle and an offset fan nozzle. Several exhaust nozzle systems reduced jet noise by more than 2.5 EPNdB, calculated for a 1500-ft level flyover, without significant thrust loss either at takeoff or at cruise. The following photos show two of the best exhaust nozzle systems. Chevrons on fan nozzle and tabs on core nozzle. Because of increasingly stringent restrictions near airports, the noise of an aircraft, much like its range and payload capability, has become a competitive factor.