Experimental investigation of the precessing vortex core in annular swirling jet flows in the transitional regime

© 2019 Elsevier Inc. In this paper, we study the flow structures and associated dynamics of an annular swirling jet in the transitional flow regime at Re = 520. In literature, studies on fully laminar or turbulent jets are plentiful, but detailed quantitative data for transitional jets are very scarce. Time-resolved stereoscopic particle image velocimetry measurements are employed to study the flow dynamics in combination with a spectral proper orthogonal decomposition analysis to extract the large-scale motions. Three different swirl numbers Sw are measured: Sw = 0.38, 0.47 and 0.57, with the lowest one very close to the critical swirl number for vortex breakdown to occur. Swirl has a significant impact on the flow field. For the lowest swirl number (Sw = 0.38), a vortex breakdown bubble appears/disappears periodically corresponding to a Strouhal number St = 0.023. If the swirl is increased to Sw = 0.47, the breakdown bubble remains permanently present in the flow field, accompanied with a large-scale vortical structure called the Precessing Vortex Core (PVC). The PVC precesses around the central axis of the jet at a Strouhal number of 0.23. It was observed that the precessing frequency of this spiral vortex core decreases to St = 0.146 as the swirl is increased to Sw = 0.57. This decrease of precession frequency finds its origin in the increased radial spreading of the jet, reducing the rotation rate of the flow near the centre. The 3D structure of the PVC is reconstructed from the planar stereo-PIV measurements by rotating the measurement plane around the central axis of the jet geometry in synchronisation with the precession. It is shown that the PVC is wrapped around the central vortex breakdown bubble and the windings are directed opposite to the swirl direction. This geometrical structure is also found in fully laminar or turbulent swirling jets and this paper shows it is the same for transitional jets.

Publication year:2019

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Authors from:Higher Education

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