Prediction of BPF tones emitted by the inlet of an aero-engine model using in-duct angular mode spectrum measurements

Abstract : The goal of this study was to realize predictions of tonal noise generated by a SNECMA turbofan mock up with a heterogeneous stator and radiated from the inlet using as input a modal decomposition made in the inlet duct wall. A numerical approach based on the CAA (Computational AeroAcoustics) code sAbrinA.V0 with an equivalent source term recently implemented is used as well as an analytical methodology using a Wiener-Hopf technique and an uniform flow assumption. Even if directivities from raw experimental data exhibit significant variations of noise level in the azimuthal direction proving that correlation effect between modes seems to be not negligible for tonal noise, the hypothesis of incoherent modes inducing axisymmetric directivities allow to compare reasonably well with measured data averaged azimuthally. Comparisons between theoretical and numerical predictions confirm the practical interest of using simplified analytical methods even if clear improvement are provided by more complex and time consuming methods like CAA. Especially, for polar angles between 20° and 70°, a very good agreement is obtained with a maximum discrepancy of 0.7 dB on overall sound pressure level for the three first tones.