Numerical investigation of the true sources of jet noise
People living close to airports have become very sensitive to noise pollution, which affects their health and quality of life. Unfortunately, air traffic is predicted to increase significantly in the next decade. Noise pollution is likely to follow the same trend unless aircrafts become quieter. As a result, regulations have been reinforced and aircraft manufacturers are now required to decrease aircraft noise below stringent levels.
One of the main source of aircraft noise is jet noise. Jet noise is generated by the high speed flow at the exit of turbofan engines. In 1950, sir James Lighthill wrote a paper in which he showed that jet noise increases dramatically with jet velocity. This crucial result led to the design of wider engines, which generate slower flows, thereby greatly reducing jet noise. Unfortunately, it has now become impossible to further increase the width of the engines due to the limited space available below the wings of the aircraft. Aircraft manufacturers have to find innovative ways to control jet noise.
Jet noise sources
An important problem in reducing jet noise is the lack of understanding of the source of noise. This problem originates from the smallness of noise fluctuations compared to other fluctuations present in the flow. Sound is therefore hidden within the flow and the source of sound is inaccessible. Not being able to identify the source has prevented a good understanding of the mechanisms of sound generation in jets.
In 2005, Goldstein proposed an innovative method to identify the sources of jet noise. The method relies on identifying the silent part of the jet. Assuming that this identification is possible, Goldstein was able to derive a mathematical expression for the sources of jet noise. Our objective is to show that it is indeed possible to identify the silent part of a jet. This will then allow us, using Goldstein's expression, to extract the sources of sound in a jet.
To achieve our objective, we will simulate a jet numerically. We will then identify the silent part of the jet by using signal processing techniques.
This project will allow aircraft manufacturers to access the sources of jet noise. It will help them to better understand the physical mechanisms of sound generation. Such understanding is crucial for them to further reduce jet noise.
This project is funded by the Engineering and Physical Sciences Research Council (EPSRC), with additional support from Rolls-Royce plc. Rolls-Royce is currently funding a University Technology Centre (UTC) on aeroacoustic noise at the Institute of Sound and Vibration Research (ISVR).