The unsteady characteristics of velocity field around the tongue of the volute in a centrifugal fan with forward-curved blades were investigated by time-resolved particle image velocimetry and mode decomposition analysis, i.e., proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD). Both POD and SPOD analysis reveal the existence of two types of unsteadiness in the flow field in the volute, one is the large-scale fluctuations at rotation frequency and its high-order harmonics in the main outlet flow, the other is the jet-wake structures at blade passage frequency in the cutoff clearance region. Additionally, SPOD analysis reveal a third type of disturbances, which are characterized as strip-like velocity structures at intermediate frequency. The geometric and dynamic features of these coherent flow structures are inspected by the eigenfunction and their reduced-order reconstruction. Compare with the POD analysis, SPOD analysis enables the examination of the spatial pattern of each frequency component due to its dual-orthogonality nature. These structures direct impact onto the tongue, thus might be responsible for the generation of strong wall-pressure fluctuation on the nose of tongue. Due to the frequency mixing limitation of POD analysis, they cannot be easily identified from the leading-order POD modes. Furthermore, the frequency range of these three groups of disturbances, as well as their spatial extension, are determined via SPOD analysis and reduced-order reconstruction.

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