Numerical and Experimental Characterization of Fan Noise Installation Effects
D.C. Mincu, E. Manoha (Onera)
Within the context of the two major European Research Projects, NACRE and OPENAIR, the potential of acoustic installation effects on the aft fan noise radiated by innovative installations of coaxial turbofans are evaluated. Three different installation concepts are considered: a semi-buried engine, a rear-fuselage nacelle and, finally, a scarfed nozzle. The main objective of these concepts is to reduce the acoustic radiation of fan noise through the engine nozzle towards the ground, without significant losses in the aerodynamic performance. This evaluation relies on numerical simulations achieved with Onera’s solvers, namely sAbrinA-V0 (CAA) and BEMUSE (BEM). The nozzle configurations are typical of coaxial turbofans with a large bypass ratio, including 3D effects from the internal bifurcation and, possibly, the external pylon or fuselage. To obtain a representative fan noise effect, several levels of complexity are used to numerically model the fan noise sources. The most advanced acoustic computations rely on Random Phase Multi-modal Injection (RPMI), an innovative technique based on the optimization of the modal phases, in order to obtain, with a minimum number of CAA computations, the contribution of all cut-on modes with evenly distributed acoustic power, summed in an un-correlated way. Noise propagation also accounts for the refraction effects, due to the large velocity gradients in the coaxial flow. For this purpose, non-homogeneous RANS mean flows were computed by Onera, AIRBUS and SNECMA respectively, for the reference (isolated) and the installed configurations, allowing their respective aerodynamic performances to be checked. For all three configurations, the installation effect is evaluated as a combination of the result of the CAA computation in the near-field and an extrapolation in the far-field, using the BEM or Kirchhoff integral methods to take into account the acoustic scattering on different fuselage parts. Undeniable benefits in noise reduction by the use of such installations are demonstrated. However, additional studies are still required to confirm these benefits, especially by improving the modeling of the fan noise sources and optimizing the acoustic shielding process.