Separation of Tonal and Broadband Noise Components by Cyclostationary Analysis of the Modal Sound Field in a Low-speed Fan Test Rig
2.2 Experimental Methods for Localizing / Characterizing Sources
The development of axial turbomachinery such as ventilators and aero engines imposes increasing requirements regarding a reduction of the tonal and broadband sound field components. The analysis of tonal sound field components can be performed by use of the well-established Radial Mode Analysis, which allows the determination of the sound power of all cut-on modes on the basis of sound pressure measurements using in-duct microphone arrays. Recently, the application of the Radial Mode Analysis to determine broadband mode amplitudes for experimental data from a laboratory low-speed fan test rig was presented. The method assumes that the broadband sound field can be modelled as a quasi-stationary process. In this study, another approach will be investigated which is based on the assumption, that the statistical properties of the broadband sound field are periodic in time with respect to the rotor revolution, i.e. the assumption of a cyclostationary process.
In contrast to the quasi-stationary analysis, the cyclostationary analysis allows the separation between tonal and broadband sound field components without affecting the amplitude and phase relations of the respective signal components. The effect of this approach on resulting mode spectra, sound powers and mode coherences will be discussed using experimental data measured at a laboratory low-speed fan test rig. Furthermore, the potential benefits of the cyclostationary analysis for the development of a source localization technique for rotating sources will be examined. Based on the cyclostationary analysis the Wigner-Ville spectrum can be computed, which describes the development of the statistical signal properties such as auto- and cross-power spectra over the rotor revolution. As a result, the positions and strengths of rotating sources can be tracked. The identification of the modulation of sources and the separation of rotating and static sources in the case of a rotor-stator stage are two main advantages that result from the cyclostationary analysis.