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Noise mitigation strategies
Published in Emily S. Nelson, Dhanireddy R. Reddy, Green Aviation: Reduction of Environmental Impact Through Aircraft Technology and Alternative Fuels, 2018
Jet noise has been studied since the 1950s when turbojets were introduced for commercial aircraft. It is dominant for lower BPR turbofans during takeoff and is characterized as a broadband source with peak directivities aft of the engine. It is worth noting that the low-frequency rumble associated with jet noise can persist over long distances. The primary cause of jet noise is the turbulent mixing of the core and bypass flow streams exiting the engine. It is a distributed noise source external to the engine that persists for 15 to 20 nozzle diameters until the turbulence sufficiently decays. This makes jet noise reduction a challenge since modifications to the nozzles need to favorably alter turbulence well downstream of the engine. Higher frequency noise can sometimes be generated inside the nozzles by mixing devices and can impact the PNL.
Turbofan Engines
Published in Ahmed F. El-Sayed, Aircraft Propulsion and Gas Turbine Engines, 2017
Moreover, both of the jet velocities of the cold air from the fan nozzle and the hot gases from the turbine nozzle are less than the jet velocity of turbojet engines. This represents an additional advantage of a turbofan engine over simple turbojet engines, as the jet noise is correlated to the jet velocity. The intensity of jet noise has been shown to be proportional to the eighth power of the velocity of the jet relative to the ambient air, thus small reductions in velocity means significant reductions in noise. It is well-known that the noise of the jet engine, especially those noise components with frequencies disagreeable to humans, appear to come from the compressor.
Sound Power and Sound Pressure Level Estimation Procedures
Published in David A. Bies, Colin H. Hansen, Carl Q. Howard, Engineering Noise Control, 2018
David A. Bies, Colin H. Hansen, Carl Q. Howard
Furnace noise is due to a combination of three noise producing mechanisms: jet noise produced by the entering fuel gas;jet noise produced by the entering air; andnoise produced by the combustion process.
Experimental investigation on effect of exit shape of nozzle on the flow Mach number and noise
Published in International Journal of Ambient Energy, 2021
R. Sarweswaran, P. Manivannan, P. Booma Devi
Myung and Rokaf (2013) explained about Technologies of jet Noise Abatement for the Republic of Korea Air Force and they introduced the various nozzle designs from the Advanced Subsonic Transport (AST) program such as Doublets, scar fed, tabbed, chevrons, tongue, lobed nozzles. They completely showed the Schematic diagram of a localized arcing plasma actuator system. Shriwas, Prashant Singh, and Kaurase (2015) completely explained about the decline of jet noise in exhaust nozzle owing to the high turbulence structure created in the jet shear. They showed different types of nozzles images with specifications such as cone, bell, spike, E-D, R-F, H-F nozzles. They also mentioned various probable techniques to decrease jet noise such as chevron nozzles, nano-technology, aircraft engine configuration, acoustic liner, configuration of nozzle and increasing the bypass ratio in turbo fans.