Open-Fan Noise Characterization Using Source-Mode-Formulation-Based Microphone Array Processing

This study presents the development of a tone noise model dedicated to an open-fan engine. The model is intended for implementation in microphone array techniques to better characterize the dominant sources in wind tunnel experiments and to segregate rotor and stator contributions. It is based on Goldstein’s formulation written in the frequency domain. A decomposition into angular modes through equivalent fixed phase-shifted sources is proposed. The predicted sound field takes into account different contributions investigated in this study: thickness noise, steady and unsteady loading noise (from rotor and stator) in advancing flight configurations, and near-field effects. The unsteady loading harmonics were obtained from unsteady aerodynamic computations. Initial predictions were compared with reference solutions from a time-domain solver at multiple operating points. A drastic reduction in the number of sources through compactness was investigated, with compactness forms adjusted to ensure acceptable discrepancies with previous reliable solutions. The selected compact source-mode propagation model was then used as a steering function in standard array processing tools. Finally, a preliminary validation of the array processing using a deconvolution algorithm and a numerically based cross-spectral matrix with correlated sources is presented.