Flow transitions in collisions between vortex-rings and density interfaces
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R E G UL A R P A P E R
K. W. B. Yeo • J. Y. Koh • J. Long • T. H. New
Flow transitions in collisions between vortex-rings and density interfaces
Received: 25 November 2019 / Revised: 19 February 2020 / Accepted: 2 March 2020 Ó The Visualization Society of Japan 2020
Abstract Flow transitions and vortical developments during vortex-ring collisions with a sharp water–oil density interface are studied using planar laser-induced fluorescence and time-resolved particle-image velocimetry techniques. Circular vortex-rings at Reynolds numbers of Re ¼ 1000; 2000 and 4000 colliding with a density interface characterized by an Atwood number of approximately A ¼ 0:045 were investigated. Results show that at Re ¼ 1000, collision with the density interface produces vortical structures and flow transitions that are relatively similar to those for a solid-boundary collision. However, the dynamics underlying the present vortical formations and behaviour are different from those associated with solidboundary collisions, in that the former are driven by baroclinic vorticity generation. Flow behaviour at Re ¼ 2000 shows more significant deformation of the density interface by the vortex-ring but overall behaviour remains comparable. Last but not least, at Re ¼ 4000, the largest Reynolds number investigated here, the vortex-ring penetrates the density interface almost completely. However, buoyancy effects eventually limit its penetration and reverse its translational direction, such that it crosses back into the oil layer again with its vortex core rotational senses reversed as well. At the same time, vortex-ring fluid is shed and a significant trailing-jet is left in the former’s wake. Keywords Vortex-ring Collision Vortex dynamics Laser-induced fluorescence Particle-image velocimetry
1 Introduction Collisions of vortex-rings with solid boundaries and density interfaces often reveal important information on vortex–boundary layer interactions, which in turn aid the research community in better understanding more complex real-world flow scenarios. For example, such scenarios will be particularly useful for applications associated with jet impingement for heat transfer purposes or fluid mixing enhancements. Generally speaking, such collisions involve generating a discrete and coherent vortex-ring and allowing it to propagate towards and collide with a surface at a pre-determined translational velocity. Despite the seemingly simple initial flow conditions and configurations, experimental and numerical studies in the past have shown that the collision results in complex three-dimensional flow behaviour with a multitude of vortical structures. While the flow behaviour of vortex-rings colliding with solid boundaries is well-studied and understood both experimentally as well as numerically (Lim 1989; Lim et al. 1991; Shariff and Leonard 1992; Chu et al. 1995; Lim and Nickels 1995; Fabris et al. 1996; Orlandi and Verzicco 2006; Cheng et al. 2010; Couch and Krueger 2011; Xu and Wang 2016; Nguyen et al. 2019; Oishi et al. 2019), flo
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