Numerical simulation of broadband aft fan noise radiation for turbofan with scarfed nozzle

In the context of the on-going European research project OPENAIR, the acoustic potential of the concept of scarfed nozzles for coaxial turbofan is evaluated. The objective is to reduce the fan noise radiation through the engine nozzle towards the ground, without significant losses in the aerodynamic performances. This evaluation relies on CAA computations achieved with Onera's CAA solver sAbrinA-V0. The nozzle configuration is typical of a coaxial turbofan with large bypass ratio, including 3D effects from the internal bifurcation and external pylon. From a reference configuration, a scarfed configuration has been designed by SNECMA with the constraint of minimizing the loss in aerodynamic performance. The acoustic computations rely on the Random Phase Multimodal 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, in an un-correlated way, of all cut-on modes with evenly distributed acoustic power. The noise propagation accounts for (i) the 3D geometric details of the secondary duct, including the bifurcation and (ii) the refraction effects due to the large velocity gradients in the coaxial nozzle. For this purpose, non-homogeneous RANS mean flows were computed by SNECMA for both the reference and the scarfed configurations, allowing to check their respective aerodynamic performances. The CAA computations provide acoustic fields on a surface enclosing the engine and pylon, then the farfield directivity of the isolated engine is evaluated using a Kirchhoff integral method. Both configurations are compared at large distance in flyover and sideline directions corresponding to certification points. The acoustical benefit of the scarfed nozzle is demonstrated, especially in the flyover direction.