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 Science PicturesFan Noise radiated downstream a Coaxial Jet Engine
The reduction of aircraft noise: a socio-economic stakeThe reduction of aircraft noise in the airport areas has become a major socio-economic stake. Indeed, because of continuous increase in the number of flights but also in the aircraft capacity - and thus, size -, the air traffic is in constant growth. Due to the strong noise levels emitted by the aircraft at takeoff and approach, this growth compromises the harmonious integration of the commercial aeronautical activity within the environment. This is why airframe (Airbus, Boeing, etc.) and engine (Snecma, Rolls-Royce, etc.) manufacturers become more and more concerned about the reduction of the sound emissions of their future products. The reduction of aircraft noise : a scientific and technical challengeIf the reduction of aircraft noise is a clear concept for everybody, it proves to be a problem of most difficulty. Indeed, the various sound sources on an aircraft at takeoff or approach involve complex physical mechanisms which are still not well known to date. So this problem is a real scientific and technical challenge, since prediction and reduction of the sound emissions assume a fine comprehension of the involved phenomena. Comprehension of aircraft noise for its reductionIt is possible to identify and quantify the main phenomena responsible for the strong levels of noise induced by the aircraft at take-off and landing. One can coarsely separate them in two components, which are (1) the airframe noise and (2) the engine noise. Airframe noise (1), is due to the strong interactions of the flow with the solid appendices of the aircraft (landing gears, leading edge slats, trailing edge flaps, etc...), interactions resulting in the generation of strongly turbulent, and thus very noisy, phenomena. In the case of a traditional turbojet, engine noise (2) integrates mainly the jet noise (due to the fast hot gas ejection by the nozzle), and the fan noise (due to the fluid/structure interactions generated by the rotating blades feeding the secondary duct). At landing, the plane being in high lift configuration (landing gears down, slats and flaps extended) and the engines near idle condition, airframe noise is of the same magnitude order as propulsion noise, which is dominated by fan noise. At takeoff, the engine thrust is with its maximum and then fan noise and jet noise equally contribute to the total noise. This explains why fan noise has become a major concern for engine and airframe manufacturers. For a long time, the latter have been only working on the problem of prediction/reduction of the fraction of fan noise emitted upstream, via the air intake. Recently, they have become also interested in the problem - more complex – of predicting the "downstream" component which is emitted through the nozzle and the strongly heterogeneous jet flow. Thus the Acoustics & Environnement Department at Airbus France recently launched several projects aiming to characterize, both experimentally and numerically, the fan noise radiated downstream a coaxial engine.
The numerical simulation as a tool for prediction / reduction of fan noise radiated downstream a coaxial engineBecause of the particularly severe aero- and thermodynamic conditions at the nozzle exit, such a prediction problem remains out-of-reach to theoretical modeling. Moreover, if the experimental approach remains necessary, it proves very expensive. This is why, in addition to this last way, one also resorts to the numerical approach - a way that the recent explosion of computing capacities made very promising. Indeed, with the numerical simulation, it is the whole acoustic chain (including the noise generation, its propagation in ambient conditions, then its radiation in far field) which is virtually reproduced. Within ONERA, this activity constitutes one of the missions of the Computational Fluid Mechanics and Aeroacoustics Department (DSNA), in which the research and development in Computational AeroAcoustics (CAA) has had a very strong growth these last years. Numerical simulations of the fan noise of a coaxial turbojetThe Airbus project "Low Noise Aircraft 2 - LNA2" aims at the experimental and numerical characterization of the fan acoustic radiation downstream of the nozzle, aiming at its reduction through installation effects (in fact, the effect of acoustic masking by the empennage and the rear fuselage). Begun in January 2005, this project directly implies ONERA/DSNA to which a series of simulations of acoustic propagation was entrusted, simulations relating to the various components (or "modes **") characterizing the downstream fan noise for a traditional coaxial turbojet. The difficulty of this task lies in the complex geometries, and the even more complex streamline flows (see following figure) constituting the basic background for the propagation of acoustic waves. These computations are performed using the sAbrinA software, developed within the DSNA on the basis of several CFD and former CAA solvers. This powerful tool makes it possible to treat very complex calculations of aerodynamics and/or aeroacoustic. The simulations carried out within the LNA2 framework handled a great number of acoustic modes which were retained for their relevance to the industrial problem. In particular, each mode is characterized by a space periodicity in the azimuthal (order m The following figures show several simulations carried out for the modes (m, N) = (10, 1) and (0, 3) at frequency BPF (meaning an acoustic wavelength - normalized by the engine radius R - equal to Kr = 2
On all views in these figures, one clearly distinguishes the acoustic wave fronts radiating from the duct exit into the ambient space. The comparison of the results relating to the mode ** (0,3) propagated in absence, then in the presence, of the heterogeneous flow, makes it possible to well highlight the significant effects of acoustic refraction induced by the jet on the acoustic propagation. Authors: Stéphane Redonnet, Eric Manoha
* Method RANS (Reynolds Averaged Navier-stokes) ** Acoustic modes
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Last Update: 17 March 2006 - © ONERA 2009 - Terms of use |
20) and radial (order N 
R/
~ 25). Once emitted upstream of the secondary duct, these modes propagate inside and outside the duct, this propagation being carried out either in the presence of the streamline flow at takeoff, or in a medium at rest.
