Reevaluating Noise Sources Appearing on the Axis for Beamforming Maps of Rotating Sources

B1 - Signal processing for noise source location/characterization

Beamforming investigations focusing on the dominant noise sources of turbomachinery have become very common. As legislations and regulations have become more stringent along with the expectations of the customers, the amount of research in the field of turbomachinery aeroacoustics has progressively increased. Beamforming processes developed specifically for rotating sources, such as the Rotating Source Identifier (ROSI) method [1] and the Rotating Beamforming method [2], have provided a nonintrusive means by which the noise sources can be localized. What these methods have in common is that they apply a so called dedopplerization step in order to place the rotating noise sources into a rotating reference frame. Utilizing phased array microphones and these advanced beamforming algorithms we are therefore able to collect data for identifying aeroacoustic noise sources, which is becoming a common practice [1-4]. The results, on the other hand, are not so easily understood. Most beamforming algorithms assume that the noise is generated by compact stationary incoherent noise sources, in most cases resulting in beamforming maps which localize the noise sources to their true source locations. On the other hand, if the noise sources are coherent, then the results can be misleading. The publications of Horváth et al. have recently shown that this is also true for rotating coherent noise sources, which play a key role in complicating the beamforming results of rotating noise sources, pinpointing noise sources to their respective Mach radii rather than their true locations [5].
The present investigation looks at ROSI beamforming maps for an axial flow fan test case investigated from the axial direction. The focus of the investigation is the noise source appearing on the axis of the fan. In many similar investigations noise sources located on the axis have been associated with motor noise [1, 4]. Taking into account what is now known about rotating coherent noise sources appearing at their respective Mach radii, this investigation shows that the results have in some cases been misinterpreted, providing an explanation as to why these noise sources appear on the axis as well as providing information as to their true noise source locations.

[1] Sijtsma, P., Oerlemans, S., and Holthusen, H., “Location of rotating sources by phased array measurements,” National Aerospace Lab., Paper NLR-TP-2001-135, 2001.
[2] Pannert, W., and Maier, C., “Rotating beamforming – motion-compensation in the frequency domain and application of high-resolution beamforming algorithms,” Journal of Sound and Vibration, Vol. 333, Issue 7, 2014, pp. 1899-1912.
[3] Kennedy, J., Eret, P., Bennett, G., Sopranzetti, F., Chiariotti, P., Castellinni, P., Finez, A., and Picard, C., “The application of advanced beamforming techniques for the noise characterization of installed counter rotating open rotors,” 19th AIAA/CEAS Aeroacoustics Conference, Paper AIAA 2013-2093, 2013.
[4] Benedek, T., and Tóth, P., “Beamforming measurements of an axial fan in an industrial environment,” Periodica Polytechnica: Mechanical Engineering, Vol. 57, No. 2, 2013, pp. 37-46.
[5] Horváth, Cs., Envia, E., Podboy, G. G., “Limitations of phased array beamforming in open rotor noise source imaging,” AIAA Journal, Vol. 52, No. 8, 2014, pp. 1810-1817.