Tonal Noise Prediction of a Turbofan with Heterogeneous Stator and Bifurcations

Abstract

The interaction of wakes generated by a fan with the outlet guide vanes occurring at the blade-passing frequency and its harmonics is mainly responsible for aeroengine tonal noise emission in approach conditions. Conventional rotor–stator interaction models assume axisymmetric rows and quasi-annular ducts. However, the stator of new engines is characterized by nonidentical vanes (so-called heterogeneous outlet guide vane) and integrates two internal bifurcations up to the nacelle outlet. These new technologies invalidate the existing tools adopted by engine manufacturers at the design stage. For this reason, hybrid methodologies based on a three-dimensional unsteady Reynolds-averaged Navier–Stokes simulation considering the complete geometry of a modern Snecma engine model are investigated in this paper. Unsteady Reynolds-averaged Navier–Stokes simulation output data are postprocessed using different integral methods to assess the impact of the heterogeneity on angular mode distribution (compared to an idealized homogeneous stator) and the effect of bifurcations on downstream propagation. In particular, the Goldstein formulation is extended to loading sources distributed over nonidentical vanes. The harmonic sound power predictions derived from loading noise equations are also compared to a direct calculation of the acoustic intensity obtained by extracting the perturbation fields over prescribed cross sections behind the outlet guide vanes. This method requires a suited spatial filtering process in order to reduce the contribution of vortical waves known for polluting the acoustic field. A modal decomposition of the extracted perturbations on a Fourier–Bessel basis is also proposed to avoid this difficulty. Finally, computational fluid dynamics solutions and acoustic predictions compare reasonably with the available experimental data issued from a half-scale turbofan test.