Abstract
In this study, the coherent structure dynamics and entrainment capability of elliptical jets emitted from an elliptical nozzle with aspect ratio (AR) values of 1 (i.e., a circular jet), 2, and 4 at a fixed Reynolds number of 3000 were experimentally characterized by flow reconstruction using tomographic particle image velocimetry and modal decomposition using the time-domain spectral proper orthogonal decomposition method. Statistical analysis indicated that the elliptical synthetic jet had a greater entrainment rate and momentum flux than the circular jet. The temporal dynamics of the coherent structure showed that all the jets emitted from the elliptical nozzle at AR = 1 (i.e., a circular jet), 2, and 4 had a dominant frequency at Strouhal number (St) = 0.39, representing the leading Kelvin–Helmholtz (K–H) vortex ring in each jet. The frequency of the trailing vortex was at St = 0.28 at AR = 1 and 2, whereas that at AR = 4 was at St = 0.70. It was found that vortex ring pairing and merging, as well as axis switching, are common in elliptical jets, while the merging process was not strictly repetitive, sometimes a single vortex stretched and disintegrated, without coalescing. When AR was 4, there was also a large-scale single vortex ring between the two merged vortex rings, this large-scale single vortex alone underwent axis transformation and breakage. The merger always occurred in the major plane in elliptical jets because in the major plane, the leading and trailing vortices approached each other as a result of self-induction and mutual induction. The entrainment rate was strongly correlated with the K–H vortex ring dynamics in circular jets and weakly correlated with the K–H vortex ring passing in elliptical jets. The entrainment appeared in the upstream part of the K–H vortex ring structures in circular jets, whereas it was enhanced in the axis-switching region in elliptical jets. An analysis of the contribution of each mode to mass entrainment showed that the entrainment rate of the elliptical nozzle was better than that of the circular nozzle, mainly because many streamwise vortices were generated.
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The authors thank the financial support from the National Natural Science Foundation of China (12272231, 12227803).
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XZ and YZ did this experiment and analyzed experimental data, XZ wrote the main manuscript text and plotted all figures. CH guided the methods of experimental measurement and data analysis, and revised the paper. YL provided the overall idea of the paper and experiment. All authors reviewed the manuscript.
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Zeng, X., Zhang, Y., He, C. et al. Dynamics and entrainment mechanism of the jet flows from an elliptical nozzle: time-resolved tomographic PIV measurements. Exp Fluids 64, 142 (2023). https://doi.org/10.1007/s00348-023-03683-y
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DOI: https://doi.org/10.1007/s00348-023-03683-y