Fourier Analysis of Beamforming Data at the Tip of an Axial Fan Rotor

Abstract

Axial fans operate in large numbers, often in the close vicinity of humans, therefore reducing their noise is a primary concern. In this paper, beamforming is applied to get information about the location and strength of noise sources on the rotor. This information is invaluable, however, difficult to interpret because of some limitations of the beamforming method. The broadband noise radiated in the upstream direction by an axial fan rotor has been examined by means of a phased array microphone system, and the recorded data have been processed with the use of the Rotating Source Identifier beamforming algorithm. In order to reduce the complexity of the source maps, spatial Fourier analysis has been applied to the beamforming-based circumferential source strength distribution taken at the tip radius of the rotor, where the complexity of the local aerodynamic phenomena makes the evaluation of the aeroacoustic noise source data especially challenging. As part of the processing method, a criterion has been introduced for identifying the significant components of the noise sources. The Fourier analysis enabled the effective enhancement of distinct noise sources out of the beamforming database, despite the presence of signal perturbations and limitations. The noise sources have been located and quantified with use of the Fourier transformation and with help of a criterion based on the physical properties of the noise generation mechanisms.