Direct noise predictions of a 360° full fan stage using LES

Abstract

In the present study, the broadband noise of an ultra high bypass ratio fan/OGV stage, developed at École Centrale de Lyon, is studied numerically. Wall-modeled large eddy simulations (LES) over a 360° full fan stage are performed for the first time, at approach conditions. The noise emissions in the fan stage are directly obtained by the fully compressible LES, using a well refined unstructured grid. Additionally, a LES over a periodic sector of the fan stage is performed using the same configuration and numerical setup as for the 360° LES. The results of the 360° and the periodic sector configurations are compared in terms of mean and turbulent flow parameters and acoustic pressure spectra. The impact of the periodic boundary conditions, usually used for high-fidelity simulations of turbofan engines, is thus assessed. A slightly lower pressure difference between the pressure and suction sides of the blade is obtained in the 360° LES in the blade tip region. The acoustic azimuthal decomposition, which can only be estimated using the 360° LES configuration, is presented. The evolution of the coherence function in the azimuthal direction is also studied, particularly between monitor points located in separate blade channels. The coherence functions in the radial and azimuthal directions between monitor points located in the same blade channel are compared between the 360° and the periodic sector configurations. Lower coherence levels are obtained in the 360° LES mainly at low and mid frequencies.