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Community and Ramp Aircraft NoisE

Final Report Summary - CRANE (Community and Ramp Aircraft NoisE)

It is well known that high or sustained noise levels can result in partial or total hearing loss, tinnitus, sleeping disturbance. Recent epidemiological studies have documented the strong link between exposure to road and air traffic noise and hypertension and ischaemic heart disease [1]. It is estimated that today in Europe [2]:
• 113 million people are exposed to noise levels sufficient to induce serious health problems;
• 10 million people are exposed to ambient noise levels that can lead to hearing loss;
• 30 million people are exposed to occupational noise (i.e. at the work place) that can damage their hearing.
It is also estimated that in western Europe a large number of healthy years of life (due to premature death or disability) are lost every year due to the negative impact of environmental noise [1], including 61 000 years for ischemic heart diseases, 45 000 years for cognitive impairments of children, 903 000 years for sleep disturbances, 22 000 years for tinnitus, and 654 000 years for annoyance. Obviously, this loss has a substantial economic impact on the EU population and the European health care systems [5].
Aircrafts are one of the few sources of noise that contribute significantly to both community noise and occupational noise for the ground crew. Commercial aviation is a crucial component of the European economy with half a million jobs and a yearly turnover of 100 billion euros [3]. As there is a strong interplay between noise emission, fuel efficiency and air pollution, it is clear that low-noise technologies will be crucial in enabling more environmentally-friendly aircraft. For instance, contra-rotating open rotors are a different type of engine design with the potential of reducing fuel consumption by 20 to 30%. However, this technology will only be deployed if the associated noise emissions can be kept within acceptable limits.
It is therefore recognized that a major challenge faced by commercial aviation is the reduction of aircraft noise [4]. This is also supported by the European Commission, as one of the objectives set out in its 2050 vision for the aviation sector is to reduce the noise emission of flying aircraft by 65% [3]. With an expected growth rate of 5% in air traffic per year and the increasing constraints and regulations not only for noise emission but also for fuel efficiency and air pollution, significant efforts in research and development will be needed for the European aeronautic industry to remain competitive and sustainable.
Computer simulations play a crucial role in the acoustic design of aircraft airframes and engines, by selecting novel concepts, optimizing final designs, and avoiding costly flight and static tests. But current computational tools are not optimal to perform large acoustic simulations over the complete frequency range. Furthermore, they are not taking into account the full geometrical and physical complexity. This limitation is a major roadblock for the industry to reach the objectives of the European aviation sector.

Further improvements in computational aero-acoustics require novel numerical schemes, better integration with other engineering design tools, and also stronger alignment with the needs of private sector end-users. To achieve this, Siemens Industry Software and ISVR have been leading this training and research program, together with Rolls-Royce and KULeuven University as associated partners. The research activities included the improvement of high-order finite element methods, discontinuous Galerkin methods, as well as novel techniques for accurate geometry description, and extensions of the Boundary Element methods. These improved methodologies will be used for the acoustic design of nacelles and Auxiliary Power Units (APU) which feature complex geometries and flow configurations, and for predicting installation effects (acoustic interaction between the engine and the wing) which currently requires vast amount of computational resources.

This research programme has been supported by a range of courses that combine the complementary expertise of the project partners (aero-acoustics, computational methods, CAD methods, aircraft noise). Such a multi-disciplinary training is sought to have brought to the young researchers the appropriate blend of expertise to support the future evolutions of aircraft noise predictions.

A total of 4 young researchers have been hired and trained through this research programme: Alice Lieu (France), Michael Williamschen (United States), Verena Schmid (Germany) and Simone Mancini (Italy). They equally shared their time between University of Southampton in UK and Siemens Industry Software in Belgium.

ESR1 (Verena Schmid) investigated the impact of the geometrical error on the overall computation error. Her findings confirmed that if a poor geometrical representation is used, the global error is quickly dominated by the geometrical error. An error estimate was proposed that relates the geometric inaccuracies (measured using the Hausdorff distance) with the global minimal error that a given model can achieve. An innovative mesh-curving approach has also been proposed to increase the geometrical accuracy of high-order coarse meshes. The high order curved mesh is obtained through a projection on a refined low-order mesh, such that it can be operated even when the exact geometry description cannot be accessed [6].

ESR2 (Michael Williamschen) investigated the accuracy and robustness of time discontinuous Galerkin method (DGM) applied to the Linearized Euler Equations (LEEs) for solving turbofan exhaust problems. The LEEs are required for turbofan exhaust due to the presence of a strong shear layer in the jet. Some numerical issues associated with the DGM have been further investigated, notably the aliasing-driven instabilities [7]. Novel mapping technologies have also been developed to improve the mean flow representation [8]. The proposed method has finally been applied to assess the impact of chevrons on tonal noise propagation in a turbofan exhaust [9].

ESR3 (Alice Lieu) investigated frequency-based numerical methods applied to the Linearized Potential Equations, which have the benefit of being stable and can allow a downright modeling of frequency-dependent liners. This methodology is typically used for the inlet because of the absence of shear layer. But the presence of flow leads to strong meshing requirements which can significantly impair the performance of finite element method (FEM). An initial study was conducted to benchmark the performance of two class of methods for acoustics propagation and identify the most efficient method [10]. Then, a sub-structuring algorithm was proposed to efficiently parallelize this method and was applied to resolve large scale problems, including in the presence of flow.

ESR4 (Simone Mancini) investigated the installation effect and noise propagation around the aircraft and in the far field using the Boundary Element Method (BEM). A novel approach which better includes the effect of background potential flows on the acoustic propagation has been developed and investigated. It is based on a combination of both Taylor transform and Lorentz transform [11]. Hybrid methodologies which can combine 1) detailed local FEM or DGM methods in the near-field of the engine and 2) BEM in the far-field have been examined. Rules of good practice were investigated to allow an efficient FEM/BEM or DGM/BEM coupling and the relevant method was applied to the propagation around the aircraft for occupancy noise and in the far-field for community noise analysis [12].

The project has resulted in 4 Ph.D. theses and in more than 20 publications/communications. It contributed to significantly pushing the state-of-the-art in the field of computational methods for aircraft engine noise prediction. The methods developed in this project will allow to improve the simulation workflow and will allow engineers to design quieter aeroengines, leading to a quieter environment. The 4 highly skilled researchers will now focus on the research for a quieter world.

More info can be found on

[1] Burden of disease from environmental noise; Quantification of Healthy life years lost in Europe. World Health Organization, 2011. ISBN 978 92 890 0229 5
[2] Noise in figures. European Agency for Safety and Health at Work, 2005. ISBN 92-9191-150-X
[3] Flightpath 2050 – Europe's vision for aviation. Report of the high level group on aviation research. European Commission, 2011.
[4] Aeronautics and Air transport: beyond vision 2020 (towards 2050). Advisory Council for Aeronautics research in Europe (ACARE), June 2010.
[5] Assessing the environmental impacts of aircraft noise and emissions. A. Mahashabde et al. in Progress in Aerospace Sciences, vol. 47, 2011.
[6] Ziel, V. S., Bériot, H., Atak, O., & Gabard, G. (2017). Comparison of 2D boundary curving methods with modal shape functions and a piecewise linear target mesh. Procedia Engineering, 203, 91-101.
[7] Williamschen, M., Gabard, G., & Bériot, H. (2015). A study of aliasing error in DGM solutions to turbofan exhaust noise problems.
[8] Williamschen, M., Gabard, G., & Bériot, H. (2016). Impact of the Mean Flow Representation on DGM Simulations of Flow Acoustics. In 22nd AIAA/CEAS Aeroacoustics Conference, Lyon, France (Vol. 5, pp. 3-2).
[9] Williamschen, M., Gabard, G., & Bériot, H. (2017). Diffraction of Tonal Noise by Chevrons in a Turbofan Exhaust. In 23rd AIAA/CEAS Aeroacoustics Conference (p. 3032).
[10] Lieu, A., Gabard, G., & Bériot, H. (2016). A comparison of high-order polynomial and wave-based methods for Helmholtz problems. Journal of Computational Physics, 321, 105-125.
[11] Mancini, S., Astley, R. J., Sinayoko, S., Gabard, G., & Tournour, M. (2016). An integral formulation for wave propagation on weakly non-uniform potential flows. Journal of Sound and Vibration, 385, 184-201.
[12] Mancini, S., Sinayoko, S., Astley, R. J., Gabard, G., & Tournour, M. (2016). Boundary element formulation for wave propagation in weakly non-uniform potential flows. In Proc. 22nd AIAA/CEAS Aeroacoustics Conference (Vol. 30).