Modal identification of a small-scale ducted fan A. Pereira * , E. Salze † and P. Souchotte ‡ ´ Ecole Centrale de Lyon Laboratoire de M´ ecanique des Fluides et d’Acoustique, UMR CNRS 5509 36, avenue Guy de Collongue, 69143 ´ Ecully, France A. Finez § MicrodB/Vibratec 28 Chemin du petit bois, ´ Ecully, 69134, France Q. Lecl` ere ¶ INSA de Lyon, Laboratoire Vibrations Acoustique 25 bis avenue Jean Capelle, F-69621, Villeurbanne Cedex, France 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 radi- ated 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 con- dition 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 fre- quency 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. I. Introduction T he investigation of noise generation mechanisms from turbomachinery is a current problem in aeroacous- tics. Several approaches, either analytical, 1–3 numerical or experimental 4–7 have been proposed in this context. The interest in this paper is the experimental characterization of a ducted fan system based on microphone array measurements. In this context two different approaches with respect to the microphone array configuration may be distinguished: (i) a first one in which the microphones are installed outside the fan inlet, in the near field; (ii) and a second one in which microphones are placed flush-mounted on the duct inner surface. The advantage of an external microphone array is that measurements are less perturbed by the hydrodynamic noise (associated to the turbulent boundary layer (TBL)) and thus modal decomposition may be readily applicable to both tonal and broadband components. One difficulty, however, is that the propagation model relating measurements to modal amplitudes may not be available analytically and one has to resort to numerical models 5 of the radiated field. This approach has been addressed by Castres et * Postdoctoral Researcher, antonio.pereira@ec-lyon.fr † Research Engineer, edouard.salze@ec-lyon.fr ‡ Research Engineer, pascal.souchotte@ec-lyon.fr § Project & Service Engineer, MicrodB, arthur.finez@microdb.fr, ¶ Assistant Professor, quentin.leclere@insa-lyon.fr, 1 of 12 American Institute of Aeronautics and Astronautics Downloaded by Antonio Pereira on June 6, 2016 | http://arc.aiaa.org | DOI: 10.2514/6.2016-3063 22nd AIAA/CEAS Aeroacoustics Conference 30 May - 1 June, 2016, Lyon, France AIAA 2016-3063 Copyright © 2016 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Aeroacoustics Conferences