NUMERICAL ASSESSMENT OF A MODE DETECTION ARRAY FOR AZIMUTHAL DECOMPOSITION
Résumé
The efficiency of a mode detection array for azimuthal mode decomposition in turbofan nacelle experiments is numerically investigated by analyzing acoustic solutions computed using the commercial finite-element code AC-TRANTM. First, irrotational inviscid mean flow fields are computed for sideline conditions, with and without an external flow at a Mach number of 0.2. Then, the upstream propagation of duct modes representing fan noise components is performed for three frequencies by solving the Möhring equation in the frequency domain using ACTRANTM. The acoustic solutions are interpolated to the microphone locations of the mode detection array. In all cases, the azimuthal structure of the interpolated data is obtained using Fourier transforms. It is compared to a reference one, determined by a duct mode decomposition of the full acoustic solution. The results provide guidelines for the analysis of ongoing experiments. They indicate that the detection antenna will be able to efficiently capture the azimuthal mode distributions, but not the radial structure. The procedure can be applied to design improved microphone arrays by defining judicious axial and radial positions for azimuthal and radial mode decompositions. Moreover, the approach can determine the optimum discretization for azimuthal mode decomposition.
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