Aircraft noise is a major environmental and societal problem, and aircraft engine noise is currently the most prominent noise source. Noise generation by aircraft engines and propagation from the engine to a receiver (control microphone or citizen) is a complex process.
To assess possible solutions, the industry requires accurate and flexible simulation tools able to predict the noise generated by a given engine and to compare, both critically and objectively, the efficiency of different quietening solutions.
Aircraft engine noise consists of various contributions: fan, compressor, turbine and jet noise. Fan noise is then separated further into forward (inlet) and rearward (exhaust) noise components. Simulation techniques now exist for fan forward noise propagation and radiation, for instance those developed in the earlier AROMA 5th Framework Programme project. These techniques are, however, limited in performance (especially for 3D geometries at mid to high frequencies). Moreover, exhaust fan noise raises specific questions, notably on the propagation of noise disturbances through shear layers generated by the coaxial jets behind the engine. MESSIAEN will target the development of new methods designed to meet these specific challenges.
The chosen method is based on the solution of linearised Euler equations (LEE) using discontinuous Galerkin methods (DGM). The objectives of MESSIAEN, expressed in terms of accuracy, performance and robustness, are based on specifications drawn by the industrial members of the consortium.
The work programme consisted of six related Work Packages:
- Work Package 1 defines the expectations of the industrial end-users in terms of applicability and performance. Detailed specifications were drawn for each targeted application: aircraft engine (from the standpoint of the air-framer, engine and nacelle manufacturers), turbo-shaft engine nozzle, air system and axial compressor.
- Work Package 2 works on source modelling and on the recovery of computational fluid dynamics (CFD) results for source quantification. Noise is generated by the flow through the fan, which is calculated by dedicated CFD tools. Work Package 2 considers both the physical and mathematical aspects related to noise generation but also the data interfacing aspects.
- Work Package 3 investigates sound propagation in the near field of the source including the effect of liners. The method chosen in MESSIAEN is based on the solution of linearised Euler equations (LEE) using discontinuous Galerkin methods (DGM). This time-domain approach raises delicate issues related to the integration of frequency dependent impedance boundary conditions and of modal excitations, which are tackled in specific tasks. Alternative approaches (e.g. pseudo time-stepping) are also investigated.
- Work Package 4 looks into the problem of sound radiation in the far field of the source. The DGM method only models the near field, and specific techniques like the Ffowcs-Williams and Hawkings method need to be implemented. An analytical approach (TEARS) and a simplified approach specifically tailored to air systems are also developed within Work Package 4.
- Work Package 5 applies the developments to end-user applications: engine nacelle/bypass/exhaust applications, turbo-shaft engine nozzle applications, air systems applications and axial compressor applications.
- Work Package 6 is concerned with project management, dissemination and exploitation.
MESSIAEN tackled tonal (as opposed to broadband) sources of noise because the relevant theoretical models have reached a sufficient level of maturity to support industrial processes at the end of the MESSIAEN project. MESSIAEN focused on the solution of linearised Euler equations by a discontinuous Galerkin method (DGM) on unstructured grids. This method, modelling the near field of the source, is coupled with the Ffowcs-Williams and Hawkings integral method to predict the far-field acoustic pressure. Besides the development of the solver itself, the following topics have been addressed:
- Use of parallel processing techniques for tackling real industrial challenges, i.e., three-dimensional configurations over a representative frequency range in a CPU time compatible with industrial constraints.
- Non-reflecting boundary conditions were used to minimize the acoustic waves reflected when disturbances leave the computational domain.
- Stability issues of the Linearised Euler equations in the presence of strong shear have been assessed.
- Effective modelling techniques for including impedance boundary conditions (liners) in time-domain solution schemes were developed.
Techniques for extracting aero-acoustic source terms from CFD results were enhanced.
The MESSIAEN project has led to:
- the improvement of the ACTRAN/TM acoustic computer aided engineering (CAE) software program, and
- to the development from scratch of a complementary CAE software tool called ACTRAN/DGM.
- is modelling the sound generation and propagation on the fore-side of the engine where the flow is smooth (potential flow) and nearly isothermal;
- solves a convected wave equation using the finite and infinite element method.
- is used to model sound generation and propagation on the aft-side of the aircraft where the flow is complex, rotational and definitely non-isothermal;
- solves the linearised Euler equations using the discontinuous Galerkin method.
Both tools are now, 6 years after the end of the research project, used by every single aircraft engine manufacturer in the world to design and optimise aircraft engine nacelle liners. One can therefore say that MESSIAEN has had a real impact on design methodologies for silent aircraft and has helped reduce the acoustic pollution around airports throughout the world. The total revenue generated from MESSIAEN has been over € 6 million so far giving the project a huge return on investment.