Abstract
In this study, three-dimensional flow structures at a Reynolds number of 16 000 are measured by stereoscopic particle image velocimetry to reveal the dynamic structures of a dual-stage co-rotating annular swirling jet with a blunt separating wall. The swirler number of the outer swirling jet is fixed at around 0.5 and that of the inner swirling jet is varied from 0 to 0.7. Spectral proper orthogonal decomposition is used for extracting organized structures and the topological evolution from a spatiotemporal flow field. The evolution of mean-flow topography is initially depicted as the flow transition from the center body wake to the presence of a central recirculation zone, with an increase in the inner swirl number varying from 0 to 0.7. Two precessing vortex cores (PVCs) with different frequencies are observed in the presence of a central recirculation zone, and the temporal independence of the two PVCs is identified. A transition region exists between the two PVCs because of the different dominant axial regions of the two PVCs, which is located between the central recirculation induced by inner swirling and that by the merged swirling flow. The conservation of circulation for both PVCs was confirmed in the transition region, and the two PVCs exhibited independent single-helical modes. Furthermore, the main frequencies of the two PVCs are proportional to the inner swirl number; however, they are higher than those of the corresponding single swirling jet. As predicted by the Landau equation, both PVCs had the same critical swirl number, suggesting that the two structures occurred simultaneously.
Published Version
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