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One reason why jets are so loud is the energy they need to ignite the air-gas mixture. This process creates turbulence and noise. The exhaust that comes from the jet’s engine also creates noise. While subsonic jets are quieter, they are not without their own share of noise.
Subsonic jets are quieter
There is a huge debate on whether supersonic planes should be banned for making noise. In the past, the US government banned supersonic flights over land and has been working to develop supersonic aircraft that are quieter. Now, NASA is starting work on a quieter supersonic passenger jet.
While the noise levels caused by supersonic flight were previously considered prohibitive, new technological advances have made it possible to build a new generation of quiet supersonic jets. American companies Aerion Supersonic and Spike Aerospace are developing new models and say they will meet subsonic noise standards. Another supersonic airliner, Boom Supersonic, is currently working on a 55-seat jet design. The company is confident that the technology has advanced enough to make supersonic airplanes commercially viable. However, the FAA has yet to remove the over-land speed limit, which makes it difficult to build and maintain supersonic planes.
However, in 2007, Gulfstream claimed its Quiet Spike trials had been successful. It claimed these demonstrations had taken the company “one step closer” to developing a commercial supersonic jet. However, the design of the Quiet Spike was not tested on a real aircraft, so there is no way to know if the Quiet Spike would have made any difference in the aircraft’s sonic boom.
Turbulent eddies create more noise
There are several causes for ocean eddies. They include winds blowing over the water’s surface and upwelling along coasts. Both contribute to the noise produced by jets. LES-based jet noise predictions often underpredict high frequencies. The CLST method aims to model the missing high frequencies. This technique combines large-eddy simulations with a synthetic eddy method. The theory behind this method relies on linearized Euler equations (LEE) formulation.
Turbulent eddies are thought to contribute to jet noise by interacting with the shear layer of the jet and generating crackle emissions. These eddies are often small in size, but their high-frequency components are present close to the jet. These high-frequency components are similar to the Mach-wave emission created by eddies.
In the CLST method, synthetic eddies are simulated with the use of an LEE solver and high-order numerical algorithms. The resulting near-field noise spectra are compared with measurements from previous studies. However, the LES solver predicts noise that is not found in published results.
More energy needed to ignite the air-gas mixture produces more noise
The process of igniting the air-gas mixture in jet engines creates noise. It involves compressing and igniting the gas mixture within the engine, and then expelling it. This process is also known as the “black art” of jet engines.
More energy is needed to expel the exhaust from the engine
In order to generate jet thrust, jet engines must produce enough energy to burn the fuel and expel the exhaust. A typical jet engine will consume about 1,500 kilograms of air per second. However, the air density at sea level is only 1.2 kilograms per cubic meter. A typical UPS truck is 23 cubic meters. Therefore, a jet engine must pull in the equivalent of 53 UPS trucks. Pratt & Whitney engineers have been working on propulsive efficiency for 20 years.
The exhaust from the jet engine is produced by a turbine that utilizes the high-energy airflow in the combustor. The turbine is connected to a shaft that drives the compressors. The fan is located at the front of the turbine. This turbine catches the exhaust gases and creates a high-speed exhaust flow. It is designed to dissipate more exhaust than it is consumed.