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Vortex rings produced by non-parallel planar starting jets

Published online by Cambridge University Press:  21 September 2020

Ben Steinfurth Open the ORCID record for Ben Steinfurth [Opens in a new window]  and
Julien Weiss Open the ORCID record for Julien Weiss [Opens in a new window]
Ben Steinfurth*
Affiliation:
Institute of Aeronautics and Astronautics, Technische Universität Berlin, Marchstr. 12-14, 10587Berlin, Germany
Julien Weiss
Affiliation:
Institute of Aeronautics and Astronautics, Technische Universität Berlin, Marchstr. 12-14, 10587Berlin, Germany
*
†Email address for correspondence: ben.steinfurth@tu-berlin.de
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Abstract

Experimental methods are employed to investigate vortex rings generated by impulsively emitting pressurised air through a rectangular outlet of high aspect ratio into quiescent surroundings. The flow is characterised by a rapid transverse expansion as thick-cored, almost spherical vortex rings with a diameter more than 40 times the outlet width and entrainment rates of $\eta >0.9$ are generated. They continue to absorb vorticity far beyond the universal formation time of $t^*\approx 4$ applicable to axisymmetric parallel starting jets introduced by Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121–140). Here, the maximum circulation and the corresponding formation time depend on the magnitude of over-pressure in the outlet plane. After momentarily reaching a non-dimensional energy close to or even below a value of $\alpha =0.16$ associated with Hill's spherical vortex, vorticity is continuously shed into a trailing jet, and the vortex rings evolve into unsteady thinner-core states. No separation between the vortex ring and the trailing jet (pinch-off) is observed. The present study provides new insights into the flow physics of non-parallel planar starting jets that significantly differ from parallel starting flows investigated previously. The potential for active flow control applications is discussed.

JFM classification

Flow Control: Mixing enhancement Vortex Flows: Vortex dynamics Wakes/Jets: Jets

Information

Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 903 , 25 November 2020 , A16
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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References

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Steinfurth and Weiss supplementary movie 1

Flow visualisation of vortex rings generated with circular outlet

Download Steinfurth and Weiss supplementary movie 1(Video)
Video 8.4 MB

Steinfurth and Weiss supplementary movie 2

Flow visualisation of vortex rings generated with square outlet

Download Steinfurth and Weiss supplementary movie 2(Video)
Video 10.1 MB

Steinfurth and Weiss supplementary movie 3

Flow visualisation of vortex tubes generated with slit-shaped outlet

Download Steinfurth and Weiss supplementary movie 3(Video)
Video 8.8 MB

Steinfurth and Weiss supplementary movie 4

Flow visualisation of vortex tubes generated with slit-shaped outlet; cross section at $x = 20\,\mathrm{mm}$ is shown

Download Steinfurth and Weiss supplementary movie 4(Video)
Video 363.6 KB