Skip to main content

Good vibrations: Quieter skies ahead with Clean Sky

Quiet skies in recent months have accentuated public awareness of noise. It's as noticeable when it's not there as when it is present. Hence, as the air travel industry gets ready to reboot, it's now more important than ever that Europe's high-level targets of reducing noise (and vibration, collectively referred to as N&V in the industry) are brought into focus.

Chorochronic LES of an Open Rotor - adimensional density times axial velocity @ CERFACS
Chorochronic LES of an Open Rotor - adimensional density times axial velocity @ CERFACS

‘Noise is a matter of comfort for passengers and crew,’ says Clean Sky project officer Francesco Paolo Mancini. ‘For people living in the neighbourhood of an airport, noise is a big concern. So with all this effort that Clean Sky is investing into better understanding, prediction and reduction of noise, this is something that will be beneficial for the general public.’

‘In an aircraft, vibrations are generated by engines that are rotating at high speeds, and by the aerodynamic loads on the fuselage – and needless to say, the engine is also a major source of noise,’ says Clean Sky project officer Alexandra Gurau.

‘The aviation industry is developing larger, more powerful and efficient engines, such as the UHBR (Ultra High Bypass Ratio), or rear-mounted engines for business jets, which means that the intensity of engine and airframe vibration is increased, while at the same time we're developing lightweight airframes, which means that the noise and vibration damping capacity of the fuselage is reduced. The challenge is to develop new N&V reducing technologies but with only minimal weight addition,’ adds Gurau.

To tackle N&V Clean Sky has a range of projects that are bringing improved understanding of aircraft noise to enable Europe's aeronautics industry to build more environmentally compliant, and more competitive airplanes. A trinity of such projects, TAVAC, SCONE and ADAPT, are compelling examples of Clean Sky's recent and ongoing efforts to address EU noise-reduction objectives. 

The challenge is to develop new noise and vibration reducing technologies but with only minimal weight addition

Passenger cabin noise cancellation with TAVAC (Technologies for Active Vibration and Acoustic Comfort) 

Part of the TAVAC project aimed to address noise discomfort in the passenger cabin. The objective was to generate a 'quiet zone' around the passenger's head in the area of the passenger seat headrests.

Miltech simulated in its cabin test environment a setup where microphones were strategically placed to collect the cabin ambient noise as well as the noise from engines. The collective input of these noises were used to create an opposite phase noise which was relayed to loudspeakers in the passenger seat headrests to cancel out the noise – a technical approach not unlike the setup used in noise cancelling headphones.

‘We managed to produce a 15 decibel reduction in the frequencies of 110 hertz and 400 hertz, and these are the typical frequencies of the noise produced by the engines in an aircraft,’ explains Michael Loupis, Technical Director at Miltech Hellas S.A. and Project Coordinator of TAVAC. ‘So, this works and we actually produced a laboratory test environment which simulated the aircraft cabin.’

Overall three individual lightweight systems were designed for the TAVAC project, achieving TRL5, and with very good results in terms of system weight. 

‘Nowadays customers expect noise level in an aircraft cabin to be as quiet as in their living rooms,’ says Yann Revalor, R&D Engineer at Dassault. ‘Indeed, for business jets as for airliners, users ask for long and not tiring travel to take full advantage of a business or a holiday trip. As part of noise comfort, one should also consider reducing vibrations which are not only a source of fatigue by themselves but also an important source of noise.’

Solutions were modelled on a full aircraft virtual demonstration of a Falcon business jet provided by the Topic Manager, Dassault Aviation. The project was also supported by the University of Athens and the University of Patras.

Now that the relevance and efficiency of the solutions are well established, Revalor concludes that ‘TAVAC could be about to propose active noise and vibration control systems that are sufficiently light and adaptive enough to change the paradigm.’

TAVAC could be about to propose active noise and vibration control systems that are sufficiently light and adaptive enough to change the paradigm

Predicting airframe noise with SCONE (Simulations of CrOr and fan broadband NoisE with reduced order modelling)

Noise has always been an issue with aero-engines, but the larger diameter and higher operation of current and near-future UHBR and Open Rotor engines makes the issue more pressing, especially as increasingly stringent noise legislation comes into play. 

For both of these categories of engines there are typically two main types of noise emitted. There is ‘tonal noise,’ characterised by having sharp peaks at several frequencies corresponding to the frequency of the engine's blades, and ‘broadband noise,’ which is emitted from the trailing edge of the rotor during its rotation and also from the wake behind the rotor, which in turn impacts either the stator for UHBR, or the second propeller on a contra-rotating open rotor. Broadband noise is the noise that affects people living around airports, and Clean Sky’s SCONE dealt with both types. 

The project focused on enhancing analytical techniques for predicting airframe noise, providing a better understanding of the physical mechanisms of sound generation and proliferation caused by boundary-layer turbulence scattering at the fringe of an aerodynamic profile. 

‘What we did in the project is the acquisition of a high fidelity flow database on the rotor’s profile using high fidelity aerodynamic computation with a huge number of points read, focusing on the trailing edge of the rotor,’ explains Bastien Caruelle, Aeroacoustics Engineer at Airbus, the Topic Manager of the SCONE project.

A second part of the project was based on improvements of a trailing edge model for noise using this database, and also using some recent and innovative theoretical research on how to use these inputs in order to predict noise better. 

‘What's new is that we have been able to extend the database to much higher Mach numbers, so we had much higher velocity of the airflow, and this is really not easy to produce experimentally — that’s the innovative part of it,’ says Caruelle. 

The results were applied to a full 3D open-rotor configuration and full 3D fan configuration to show that it is possible to make a full demonstration using this method. The last part of the project was dedicated to full 3D open-rotor and fan tonal and broadband noise computations using reduced order Large Eddy Simulations, allowing for a drastic reduction in computational time which made this kind of simulation possible.

What's new is that we have been able to extend the database to much higher Mach numbers, so we had much higher velocity of the airflow, and this is really not easy to produce experimentally — that’s the innovative part of it

Carrying out acoustic measurements with ADAPT (ADvanced Aeroacoustic Processing Techniques)

One of the main issues when carrying out acoustic measurements to capture noise — either by attaching microphones to the aircraft fuselage and measuring during flight, or by installing microphones in wind-tunnels — is the presence of background (hydrodynamic) noise. 

Although inflight analysis of noise using real aircraft with microphones attached to the fuselage can provide many useful pieces of information, they cannot provide the full picture, due to the signatures created by the microphones themselves. To calculate the noises from a full-size aircraft in flight, mainly represented by the sound interferences between the engine and the wing, or the engine and the fuselage, requires simulation. 

That's where Clean Sky's ADAPT project comes in. ADAPT, which ended in August 2020, focused on devising post-processing tools to allow engineers to efficiently remove turbulent boundary layer noise from aeroacoustic measurements and to determine absolute levels of low-noise aircraft components in both wind tunnels and in flight. 

From the Topic Manager perspective, Acoustics Engineer Emmanuel Julliard at Airbus explains that ‘the aim for us was to give status on the acoustic sources and to better understand them, and afterwards to give these inputs to our colleagues to improve the noise in the passenger cabin and around the airport.’ 

The team carried out tests on the Contra Rotative Open Rotor (CROR), using a variety of de-noising approaches. 

‘The radiation of sound from the CROR is a bit like having the noise of a helicopter, but doubled,’ says Christophe Picard, Acoustics R&D Innovator at MicrodB, the Lyon-based SME that coordinated the ADAPT project, with the assistance of the Laboratoire Vibrations Acoustique from the School of Engineering INSA Lyon. ‘And the issue for engineers is to understand the interaction between the two blades. Until now there were no tools to do that, but with the results we obtained from this project, we can now extract the different noise contributions of each blade from the acoustic signals.’

With the results we obtained from this project, we can now extract the different noise contributions of each blade from the acoustic signals

Picard says that the most innovative aspect of ADAPT has been combining niche specialisations and analysis techniques from the ‘separate worlds of aeroacoustics and signal processing (applied mathematics)’ to create a hybrid approach. 

ADAPT’s wind-tunnel testing was largely successful, and the next step is to go to flight tests. Here, it will be much noisier, and the aim will be to test the methodology on more realistic data. The results of the project will also have other applications outside of aircraft:

‘The ADAPT project’s results are transverse,’ explains Clean Sky's Mancini, ‘they don’t just apply to aviation but also to other industries. Microdb for example, mainly serves the automotive sector where the results of the ADAPT would be likely to have relevance.’