As the aviation industry continues to expand, there is growing concern over the noise and nuisance caused by jet engines, particularly as aircraft pass over urban and residential areas. In an effort to reduce such impacts, organisations around the world are focusing on reducing jet engine noise in both civilian and military applications. So, what are some of the most recent developments? And what are likely to be the key innovations and trends in technology to reduce such noise?
The noise emitted by jet engines comes from a variety of different sources, including the fan, the turbine and the core of the engine itself, as well as the thrust-producing jet exhaust. Sensitive human ears can also detect the tiny pressure fluctuations generated as air surges over the aircraft itself as well as through its engines.
In the UK, one of the premier research centres focusing on the issue is the Airbus Noise Technology Centre (ANTC) based at the University of Southampton. The centre specialises in computational modelling using the cutting-edge Iridis4 computing cluster, as well as in experimental measurements using the various wind tunnels available at the university. Dr David Angland, head of the ANTC, says the centre collaborates with its industrial sponsor Airbus to try to improve the understanding and prediction of various aspects of aircraft noise.
“The main themes of research at the ANTC are the prediction of broadband rotor or fan noise, and the prediction of landing gear noise. A greater understanding of both of these noise sources is required for future aircraft concepts, such as those using contra-rotating open-rotor engines (CRORs),” he says.
“Historically, aircraft engine noise was dominated by jet noise. However, in today’s high-bypass ratio turbofan engines, the leading-edge noise from the fan has become dominant and should be better understood and reduced as much as possible,” he adds.
Blade geometry
Dr James Gill, research fellow at the ANTC, says this noise mechanism is also of interest because it forms a significant element in CROR designs – prompting the centre to use its expertise to improve understanding of engine fan noise and to answer questions that are difficult to study in an industrial context. One ANTC project investigated the effects of rotor blade geometry on the leading-edge fan noise. Industrial models tend to predict this noise source by assuming that the rotor blades are infinitely thin, but the ANTC has shown that this assumption will cause noise over-predictions at high frequency, and that the error incurred by making this simplification has been quantified, says Gill.
“This work, and other examples, are producing fast noise-prediction models that will allow the design of quieter aircraft in the future,” he adds.
Another such initiative in the UK is Sustainable Aviation, a collaboration between airlines, airports, manufacturers and air navigation service providers that takes a collective approach to ensuring a sustainable future for global aviation. Joe Walsh of Rolls-Royce, who is also an associate fellow in noise engineering at Sustainable Aviation, says the organisation’s research and development involves sophisticated measurement techniques, often involving hundreds of microphones, in an attempt to isolate and understand each of the engine components that can produce noise, and to identify the most effective way of developing quieter engine designs.
“We have sound-absorbing material in the engine casing, and use advanced computational design tools to get the best acoustic performance from our engines,” he says. “We have also introduced innovative manufacturing techniques to achieve a perfectly ‘acoustically smooth’ fan case, which has improved noise absorption without changing the size of the engine casing.”
Meanwhile, in the US, a team of researchers at Virginia Tech is using novel measurements of jet engine exhausts in an attempt to better understand the physical mechanisms that lead to noise reduction. The team’s speciality is a measurement technique that can resolve the rapidly evolving turbulent flow eddies that contribute most intensely to supersonic jet noise of the type generally found in military-style engines, and predicted for use in future commercial supersonic transport aircraft.
Critical factors
The group has carried out measurements in lab-scale jets at a mid-scale facility at the Nasa Glenn Research Center, as well as in a research turbofan at the university. Project co-leader Todd Lowe, who is associate professor in the department of aerospace and ocean engineering at Virginia Tech, says the ultimate vision is to obtain the first measurements of turbulence in full-scale, high-performance engines, providing insights needed for transitioning from the lab to full-scale applications.
“Our results have revealed information about the cause-and-effect relationships between engine conditions, such as temperature and geometry, and the speed of turbulent disturbances in the exhaust plume,” he says. “The speed of these disturbances relative to the ambient air around the plume is a critical parameter related to the intensity of noise produced by engines.
“Generally, concepts under development by others have sought to alter the velocity and stream-wise vorticity profiles within the nozzle. These alterations are intended to slow the speeds of the largest turbulent eddies or shift dominant turbulent energy from large turbulent eddies to smaller eddies. According to prevailing theories, both of these approaches reduce the peak noise in the spectrum,” he says.
The X-plane arrives
Also in the US, a team at Nasa is working on breaking down some of the barriers to successful commercial supersonic flight, including environmental factors such as sonic booms caused by the presence of shock waves in supersonic flow. Peter Coen, manager of Nasa’s Commercial Supersonic Technology project, says that for the past three years the agency has been investigating conceptual designs for a low-boom flight research aircraft – an X-plane – that could prove boom reduction technology in flight in a safe and cost-effective manner. In the most recent development in this design process, Lockheed Martin has been given a contract for the preliminary design of the X-plane – a step that will essentially set the shape of the aircraft and many other important details, says Coen.
“We’ll do analysis and wind-tunnel testing to verify the performance, and should be through with that by the middle of 2017. If the project gets final approval and funding is OK-ed, we’ll take the next step, which would be for the final design and the fabrication phase,” he says. If things go according to plan, the team is looking forward to a first flight at the end of 2019 or early 2020 – with initial flights being carried out to ensure it is safe and prove that the low-boom design was correct.
For Coen, the keys to the noise and sonic boom reductions made in the initial designs are better computational fluid dynamics tools, as well as better and faster computers and design processes, coupled with innovative thinking about how to formulate design objectives, which allow designers to carefully shape the 3D geometry of the aircraft so that the strength and position of each shock wave are controlled.
“The result is that, as the shocks travel towards the ground, they do not coalesce,” he says. “Instead, they are greatly reduced in strength and sharpness. All that is left when the wave reaches someone on the ground is a soft ‘thump-thump’ sound – a supersonic heartbeat instead of a sonic boom.”
Challenges ahead
Looking ahead, Lowe at Virginia Tech believes there are several practical challenges involved in reducing supersonic jet noise. One is that with military and supersonic transport applications performance is key, so methods for noise reduction must not affect the peak performance of the engines, in order to preserve the technological advantages of the performance-focused engine subsystems, he says.
“The developments today are moving toward simplified approaches with few or no moving parts. These provide both operational and financial benefits, and will almost certainly be the characteristics of future approaches,” he says.
In Lowe’s view, detailed knowledge of the fundamental drivers for supersonic jet noise emissions is also critical to any elegant approach. He also stresses the importance of obtaining “actionable information” about the behaviour of turbulent flows in realistic supersonic jets, to ease the transition from lab concepts to full-scale implementations.
“I expect to see noise reduction on military aircraft happen incrementally in the next few years,” he says. “Each concept at the early research stage must progress through the technology readiness levels until it is ready for military engine deployment.”
Some of the current research concepts, such as chevrons, can be retrofitted onto existing engines and airframes – making them attractive for near-term testing – but he argues that more significant modifications to the nozzle geometry or architecture of future engines may be necessary for more dramatic reductions, and will not be tractable for implementation on current platforms.
That said, Lowe still believes we are living through a critical time in jet noise research, with a few promising noise reduction strategies, complemented by new capabilities to measure precisely how these approaches alter the noise-producing turbulence.
“The most direct overlap for military and civilian aviation will be in the commercial supersonic transport concepts being developed by Nasa,” he says. “These concepts share many of the same elements as military engines. In addition, some concepts being developed for noise shielding and redirection will find dual-use applications.”
Jet engines are complex machines operating at high pressures and temperatures which must also achieve several requirements simultaneously, including low noise emissions. So Walsh at Sustainable Aviation says a key challenge is ensuring that the industry continues to deliver even lower noise emissions without it detrimentally affecting other requirements such as fuel burn or carbon dioxide emissions. “With the increasing use of cutting-edge computational models, we can gain further insight into the physics of noise generation to help develop even quieter future designs,” he says.
Ambitious targets
Although the aviation industry has made progress over many decades, with modern aircraft being much quieter than their predecessors, Walsh admits there is still more to do.
“The aerospace industry has set ambitious targets to further improve product environmental characteristics including noise, such as those in the Advisory Council for Aviation Research in Europe which look as far forward as 2050. A vital part of future innovations will come from our ability to continue working closely with aircraft manufacturers, so we can understand and support opportunities from new aircraft concepts. For example, aircraft design might be able to incorporate noise reduction by using parts of the aircraft to shield some of the engine noise from making its way to the ground,” he says.