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A quiet revolution

Reducing noise in air travel

Published: 1 June 2021

Southampton Spotlight shines a light on the impact our University is having across the world, through the achievements of the individuals that make up our community.

Our skies are becoming busier. Despite the huge impact of the COVID-19 pandemic on air travel, the predictions are that the numbers of flights will continue to grow in years to come.

The International Air Transport Association predicts air travel will increase from the end of this year, returning to pre-pandemic 2019 levels by 2024, and then continue growing. That means more noise for all of us, especially those living under flight paths or near airports.

Or does it? For two decades, the University of Southampton has been Rolls-Royce’s main research partner for aircraft engine and airframe noise. Teams based in the University’s Institute for Sound and Vibration Research (ISVR) have developed noise modelling, measurement and mitigation technology that has been applied to every Rolls-Royce Civil Aerospace engine project since the early 2000s.

We can always fly higher (and quieter)

A group of researchers is continuing the revolutionary journey to ensure that an increase in air travel won’t bring with it an increase in noise on the ground.

A new grant from Innovate UK, worth £2m to the University over the next four years, will enable the Rolls-Royce University Technology Centre (UTC) in Propulsion Systems Noise to take its pioneering research collaboration with Rolls-Royce to the next level. The project is called FANTASIA (Future Noise Technologies and Systems Integration Analytics).

Aircraft noise, if we do nothing, will increase substantially simply because of the preicted increase in air traffic. FANTASIA is looking at aircraft noise, specifically engine noise, and is aimed at the next generation of aeroplane engines

Alec Wilson - Professor in Computational Aeroacoustics and Director of the UTC

“It’s an exciting project that is only able to happen thanks to our strong and well-established relationship with Rolls-Royce.”

FANTASIA follows on from the ACAPELLA project set in train research that will be used in the development of UltraFan, Rolls-Royce’s next generation of engines. UltraFan engines, for medium to large aircraft, have a large diameter fan that improves aircraft propulsion efficiency.

Rolls-Royce is the lead partner on FANTASIA. Juan Vera, former PhD student at the UTC and now Aeroacoustics Specialist at Rolls-Royce, is managing the FANTASIA project. He said: “FANTASIA seeks to develop, model and validate noise technologies to ensure integrated propulsion systems that will achieve the required noise levels for the novel UltraFan engine architecture, as well as for future hybrid-electric offerings.

“Multidisciplinary optimisation techniques will be developed to design for the optimal noise and emissions levels. Computational fluid dynamics and source separation techniques will be enhanced to replace expensive testing and give early indications of design suitability.”

The team from the UTC is working towards achieving noise and carbon emission targets set by ACARE (the Advisory Council for Aviation Research and Innovation in Europe).

Ultra Fan
Our researchers are reducing engine noise

“We are working to reduce noise,” said Alec. “The ACARE targets are coming down and have to keep coming down in order to reduce, or even maintain, the noise annoyance because, COVID aside, the number of flights in the world is going up all the time. At the same time as reducing noise, though, we have to respect Rolls-Royce’s other design objectives, which include high aerodynamic efficiency and low carbon emissions. It’s only by working closely with industry that we can ensure that our research provides maximum benefit when integrated into the wider multi-objective design process.”

The noise basics

It’s not just the mechanical noise from the engine that creates the noise when a plane takes off, flies and lands. The entire propulsion system, plus the components of the plane moving through the air, contribute to the volume of noise.

Alec explained: “When a plane takes off, most of the noise comes from the propulsion system which is working at full power. But when a plane lands about half of the noise comes from the aircraft itself, such as the wing flaps and the landing gear.”

Unlike cars, making planes electric is not the answer to reducing the noise they create. “Making things electric, which makes things quieter for cars, is not a panacea for aircraft because the main sources of noise from aircraft are aerodynamic as opposed to mechanical,” said Alec.

Another element of the FANTASIA project will be to produce the preliminary design for a new model scale aero-engine fan rig. “It will allow us to simulate the engine conditions on a plane coming into land,” explained Alec. “When it’s built, this rig will be a great facility and will really push us forward in what we can do.”

There are about 20 major noise sources to be considered when calculating what a plane will sound like to someone on the ground, depending on where they are stood in relation to the aircraft.

Outlining an aspect of component noise, Alec said: “For example, at the back of the engine you have the jet that interacts with the pylon that holds the engine to the wing, and its interactions with the wing. One of the key things about the new generation of engines is they tend to be bigger diameter, so that means the engine gets closer to the wing in order to maintain ground clearance – so interactions between the engine and the wing become more significant.”

Assessing the psychoacoustics

New areas of focus for the UTC under the FANTASIA project are whole aircraft noise and psychoacoustics.

Whole aircraft noise is where the noise from individual components, together with interaction between components, is integrated into the noise spectrum that is heard on the ground during a flyover event. The observed noise varies strongly depending on where the observer is standing, and the work in FANTASIA is contributing to a suite of programs that convert estimates of noise source strength into ‘noise footprints’, which show graphically where the highest noise levels will be observed on the ground.

There is a well-established method for assessing human annoyance from whole aircraft noise spectra, but that relationship, between the physical noise spectrum and the annoyance it generates, has to be kept under review as the industry develops in terms of aeroplane and engine design. The psychoacoustics element of FANTASIA will look in closer detail at how that relationship might change in the future as we move to next generation turbofan and potentially to hybrid-electric or electrically powered aircraft.

Rod Self, Professor of Aeroacoustics, is leading the psychoacoustics element of FANTASIA. He said: “One would think that reducing the amount of noise an aircraft makes would necessarily make it less annoying. However, this is only partly true. People find different types of noises annoying to differing degrees, so the type of noise made by an aircraft, or its noise signature, is just as important as the quantity of noise it makes.

“Understanding these relationships is the objective of psychoacoustics and will play an important part in FANTASIA. As aircraft design changes, so too does the noise signature and we need to understand how people are likely to respond so that we can best advise designers to minimise the annoyance.”

Read more research stories in Re:action, the University’s research and enterprise magazine.

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