ERCOFTAC Bulletin - Centre Acoustique

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Figure 6: Modal content injected in the CAA computations. From left to right: PDA (MTU predictions); RFN (reconstructed amplitudes on Kulite positions), SAF (RPMI technique) Figure 7: Acoustical results for global evaluation of shielding effects. PDA: transversal and lateral RMS pressure distribution. RFN: noise scattered by the rear empennage (up), RMS pressure comparisons between measurements and numeric (down). SAF: instantaneous view of pressure fluctuations (up), far field directivity (down). the positive azimuthal angles due to the relative engine aircraft position. However, a detailed comparison of the levels, either observed or computed, in the flyover direction (right side plot) shows that the computation underestimates the experimental level by 7-8 dB. Two points are still very encouraging. It may be observed that the same modulation is preserved between the simulation and the experiments, and also the same slope of directivity. Within these considerations, the engine can be now moved in its axial direction to find the optimum position. In the future, the differences between the prediction and the measurement for this axial position, may be reduced by increasing the amount of cut-on modes, limited in this simulation, and propagating them over a more realistic internal mean flow (the bifurcation thickness in not taken into account in the CFD). The recent implementation of the mean flow in the BEM solver now allows taking into account some flow gradients. Finally, a supplementary effect could be added by also taking into account the inlet fan propagation as shown in [15]. 3. Scarfed aft-fan concept The instantaneous view of nearfield pressure fluctuations (Figure 7, right, up) shows the acoustic pattern but does not provide a global evaluation in the far field domain. To overcome this problem, a semi-spherical observation surface was placed at a distance of about 400 R from the centre of the fan exhaust plane. The farfield results were averaged in un-correlated way between the 10 simulated RPMI events. In Figure 7 (right, down), the angular extension of the observation surface around the engine is represented. On this visualization, it is clear that the scarfed nozzle globally radiates lower levels. In order to quantify this overall noise reduction, an azimuthal integration of the RMS pressure levels on the semi-spherical surface was computed, showing that for almost all axial angular positions the attenuation is of between 1 and 3 dB. 5 Conclusions Based on three concepts of future innovative aircraft, the present work gives insights on today’s possibilities to numerically investigate the potential of installation effects for aircraft noise reduction. Most of the presented acoustical tools have now reached some maturity, and they may be successfully used for industrial cases. For all three studied configurations, the shielding effects obtained by different rigid surfaces may be considered as effective (of course with possible consequences on other aircraft performances). For example, in the 26 ERCOFTAC Bulletin 90
case of the scarfed nozzle an almost invisible extension of the lower nacelle may induce significant noise reduction. In all numerical simulations, one critical point is the description of the noise sources, an issue which is particularly addressed in this study, mainly based on assumptions driven by experimental data. Comparisons with analytical solutions or simplified configurations also allowed isolating and understanding phenomena (see references [1, 2, 3]). In the case of the RFN configuration, the proposed hybrid methodology is particularly adapted to parametric studies of the installation effects, especially the relative position of the engine and the aircraft. For that purpose, the NACRE fan noise experimental database remains a valuable tool for further validations of numerical codes and methods. This will be also the case in OPENAIR, where specific measurements will be devoted to the investigation of installation effects. Acknowledgments The authors deeply acknowledge the helpful contribution from their colleagues at Onera, Cyril Polacsek, Thomas le Garrec and Gabriel Reboul (Aeroacoustics Team) and Xavier Juvigny (High Performance Computing Team) for fruitful discussions about the modal description of sound fields in nacelles and the computational process. References [1] Mincu D.C., E. Manoha, C. Parzani, J. Chappuis, S. Redonnet, R. Davy and M. Ecsouflaire, Numerical and experimental characterization of aft - fan noise for isolated and installed configurations, AIAA-2010-3918, 16th AIAA/CEAS Aeroacoustics Conference, Stockholm, Sweden, June 2010. [2] Manoha E., Mincu D.C., Numerical simulation of the fan noise radiated through a semi-buried air inlet, AIAA Paper AIAA-2009-3293, AIAA/CEAS Aeroacoustics Conference, Miami, June, 2009. [3] Mincu D.C., E. Manoha, G. Reboul, S. Redonnet, S. Pascal, Numerical Simulation of Broadband Aft Fan Noise Radiation for Turbofan with Scarfed Nozzle, AIAA-2011-2941, 17th AIAA/CEAS Aeroacoustics Conference, Portland, USA, June 2011. [4] Redonnet S., Desquesnes G., Manoha E., Numerical Study of Acoustic Installation Effects through a Chimera CAA Method, AIAA-Paper 2007-3501, 13th AIAA/CEAS Aeroacoustics Conference, 21 - 23 May 2007 Roma (Italy) [5] C. Polacsek, S. Burguburu, S. Redonnet and M. Terracol, Numerical Simulations of Fan Interaction Noise using a Hybrid Approach, AIAA Journal, Vol. 44, n ◦ 6, June 2006. [6] Redonnet S., Manoha E. and Sagaut P. “Numerical Simulation of Propagation of Small Perturbations interacting with Flows and Solid Bodies“, AIAA Paper, 2001-2223, 7th CEAS/AIAA Aeroacoustics Conference, Maastricht, The Netherlands, 28-30 May, 2001. [7] Juvigny, X., “A Fast Algebraic Boundary Integral Solver“, Eccomas Paper, Venise, 1998. [8] Rokhlin, V.,“Rapid solution of integral equations of classical potential theory“. Journal of Computational Physics, 60, 187–207, (1985) [9] Bebendorf, M.,“Approximation of boundary element matrices“, Numer. Math. 86, 565-589 (2000) [10] Grasedyck, L.,“Adaptive Recompression of Hmatrix for BEM“, Technical Report 17, Max- Planck- Institut fur Mathematik in den Naturwissenschaften, Leipzig (2004) [11] S. Redonnet, D.-C. Mincu and E. Manoha, Computational AeroAcoustics of Realistic Co-Axial Engines, AIAA Paper 2008-2826, 14th AIAA/CEAS Aeroacoustics Conference, Vancouver, Canada, May 2008. [12] Tyler J.M. and Sofrin T.G., “Axial Flow Compressor Noise Studies“, Trans. SAE, n ◦ 70, pp. 309-332, 1962. [13] Polacsek C., Desquesnes G., Reboul G., "An Equivalent-Source Model for Simulating Noise Generation in Turbofan Engines", J. Sound Vib., Vol. 323, Issues 3-5, pp 697-717 (2009). [14] G. Reboul and C. Polacsek, “Towards Numerical Simulation of Fan Broadband Noise Aft Radiation from Aero-engines“, AIAA Journal, 2010, Vol. 48, no. 9, pp. 2038–2048. [15] Polacsek, C., Barrier, R., “Numerical simulation of counter-rotating fan aeroacoustics“, 13th AIAA/CEAS Aeroacoustics Conference, Rome (Italie), 21-23 mai 2007. ERCOFTAC Bulletin 90 27
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