Modal identification of a small-scale ducted fan
Résumé
The subject of this paper is the experimental investigation of the noise radiated by a ducted rotating machine. A modal identification approach is used to decompose the radiated sound field into duct modes from acoustic pressure measured by wall-flush mounted microphones. Both azimuthal and radial decompositions are computed by means of an array with optimized microphone arrangement. The optimized array ensures a low condition number of the matrix relating modal coefficients to acoustic pressure over a wide frequency band, up to the second harmonic of the blade passing frequency. Above this frequency the number of cut-on modes is comparable to the number of microphones and the modal matrix suffers from ill-conditioning. A regularization procedure is then introduced to increase the high-frequency limit of the method. Results are presented for both tonal and broadband components of the radiated sound field. The difficulty in the broadband regime is that pressure fluctuations measured by in-duct microphones are strongly affected by hydrodynamic noise associated to the turbulent boundary layer (TBL). A technique to suppress the TBL related noise is thus applied prior to the modal identification, its interest is shown on experimental data from an academic test bench.