Numerical Aero-Acoustics of Fans Radiation Downstream of a Dual-Flow Turbojet (S. Redonnet, E. Manoha)
The purpose of Airbus’s Low Noise Aircraft 2 (LNA2) project is to characterize, experimentally and numerically, the downstream acoustic radiation noise of the fan, to reduce it by installation effect (specifically, an acoustic masking effect by the tail unit and rear fuselage).
Begun in January 2005, this project directly involves Onera/DSNA (BREC unit), which was assigned a series of acoustic propagation simulations, simulations concerning the various components (or modes) characterizing the fan downstream noise generated by a conventional dual-flow turbojet.
The difficulty here resides in the complexity of the geometries and – still more – of the aerodynamic flows (see following figure) making up the background of the aero-acoustic simulations to be conducted. This is why a study of this kind is conducted using the sAbrinA computation platform. Developed at the DSNA on the basis of several previous CFD and CAA solvers, this high-performance tool is used to deal with very complex aerodynamic and/or aeroacoustic computations.
Aerodynamic field of mean axial velocity (RANS calculation, elsA software, for Airbus France) bathing the nozzle in takeoff (left) and landing (right) conditions.
The simulations conducted in LNA2 relate to a great many acoustic modes that were retained for their pertinence to the industrial problem at hand. Each is notably characterized by a spatial periodicity in the azimuth (rank m) and radial (rank n) directions, and by a harmonic frequency of rotation (up to twice the blade passage frequency). These modes are generated by the fan (rotor-stator interactions) and are propagated numerically inside and outside the nozzle, both in the presence of the takeoff aerodynamic flow and in the environment at rest.
LNA2 project: Instantaneous acoustic pressure fields obtained (sAbrinA program) for various fan downstream noise modes, emitted at the blade passage frequency. Here, mode (0, 3) propagated in the environment at rest and mode (0, 3) propagated in the flow (takeoff).
