Air of optimism

Residents living near airports have complained and campaigned about jet noise for years, and it is a problem that the industry is beginning to take seriously.

A research project, which is part-funded by Rolls-Royce and will take place at Southampton University, will use a supercomputer to produce more accurate models of how jet engines create noise in a bid to help solve the problem. The research will take place at the university’s Institute of Sound and Vibration Research (ISVR).

While engine manufacturers have made great progress in recent years to reduce engine noise, one of the project’s co-investigators, Dr Anurag Agarwal, believes that further improvements are only possible through a better understanding of how the sound is produced.

‘Jet noise is a big problem and even though research has been going on in this area for around 50 years we still haven’t made much progress because we don’t understand the fundamental mechanism of noise generation,’ he said. ‘As long as we don’t know that, it will be really hard to reduce the noise substantially.’

He added: ‘Despite the millions spent on this, the only thing we know for sure is that the noise can be reduced if the plane’s velocity is also reduced. That is rather basic.’

Agarwal has experience in working on noise reduction for aircraft. Before joining ISVR he was on Cambridge University’s Silent Aircraft Initiative. This ambitious project aimed to design a completely new concept aircraft that would be so quiet as to be almost imperceptible beyond the airport’s perimeter. Now, Agarwal aims to look at the problem in a slightly more immediate context.

To accurately model how noise is created in jet engines the team will use the Edinburgh-based HPCx supercomputer. This machine, 43 in the world’s top 500, is one of the UK’s most powerful academic supercomputers. This will be the first time that one has been used in this way.

The computer will be used to model the physics involved in the way air is emitted from a jet engine. There is a theory that it is engine turbulence which causes the most noise. According to Agarwal, when the air first leaves the engine nozzle it is in a ‘clean’ straight column. But as it interacts with outside air, small perturbations cause it to swirl and interact with the jetstream, leading to it becoming turbulent and noisy.

‘It’s like cigarette smoke,’ explained Agawal. ‘At first it rises smoothly and then becomes turbulent due to small interactions of air. In jet engines this is what causes the noise. If we can reduce the turbulence — or at least delay its onset — we can reduce the noise.’

Airflow simulations will be run which model smooth air when it first leaves the engine, through to when it is disturbed and when it becomes turbulent. How these different instabilities interact will then be accurately modelled.

After this computational approach, Agarwal said it may be possible to carry out some physical testing on different ideas that can alter or manipulate the airflow in some way, based on the simulation data. Recently aircraft manufacturers have tested innovative designs that can alter the airflow to reduce noise.

Last year, for instance, Boeing worked with NASA and General Electric to develop serrated edges for jet engines known as chevrons which were used to control the jet blast and are said to reduce noise. Boeing claimed that the success of the chevrons meant that it was able to shed several hundred pounds of sound insulation. However, Agarwal pointed out that although these chevrons might work, no-one can really explain why — a problem this project could help to resolve.

While this project, which is also receiving EPSRC funding, is based on understanding the physics behind jet noise, Agarwal plans on taking its findings and using them in practical applications. He is currently applying for funding for a new European project — in collaboration with Airbus — which will look into ways of controlling jet noise and could result in radical new engine designs, he said.