Investigation of Noise Emission on the ECL5 Fan Stage by Means of Sector LES Simulations

In the present study, the broadband noise of an ultra-high bypass ratio fan/OGV stage, developed at École Centrale de Lyon, is analyzed. Wall-modeled large eddy simulations (LES) over a periodic angular sector have been performed at nominal speed, with a flow in the transonic regime. The noise emissions are directly obtained by the fully compressible LES using a well refined unstructured mesh. The good level of comparison with experimental data motivates a deeper analysis devoted to the acoustic waves within the stage. Upstream of the rotor blades a low frequency hump between 750 Hz and 1200 Hz is detected. A dynamic mode decomposition of pressure fluctuations is performed for this frequency range. The decomposition suggests that the modal structure of this low frequency noise emission corresponds to the first radial mode, and is homogeneous in the azimuthal direction. A mode-matching strategy in duct engine is performed to investigate further those acoustic duct modes in the entire stage. This hump is also detected in the inter-stage. The frequency range of the of this modal structure is cut-off downstream of the stator vanes.