Effect of single and multiple bubbles on a thin vortex ring

Biswas, Subhajit and Govardhan, Raghuraman N. (2020) Effect of single and multiple bubbles on a thin vortex ring. Journal of Flow Visualization and Image Processing, 27 (1). (doi:10.1615/jflowvisimageproc.2020031014).

Abstract

The interactions of bubbles with turbulence involves multiple bubbles interacting with multiple eddies. In the present work, we experimentally study an idealization of this, namely, the interaction of single and multiple air bubbles with a single vortex ring formed in the water medium at different vortex ring strengths. The effect of single and two bubbles on the vortex ring is studied keeping the ratio of the total captured bubble volume to vortex ring core volume (VR) to be the same (VR ≈ 0.1). In both cases, the ring's convection speed is seen to be significantly reduced at lower ring strengths, with the reduction being larger for the two-bubble case. This reduction is linked to the fragmentation of the vortex core and an about 50% enstrophy reduction from PIV measurements. From the bubble dynamics side different bubble elongation and breakup dynamics are seen depending on the ring strength. The interaction of the vortex ring with a bubble swarm is studied for three bulk volume fractions (VBulk = 0.003, 0.006, and 0.014), with the actual volume of bubble captured by the ring changing with time, and the corresponding VR being significantly larger than VBulk. In this case, we again see reduction in the ring convection speed for the lower ring strength, with this being linked again to vortex core fragmentation and corresponding large enstrophy reduction. The present results show that the interactions of a ring with a single bubble, two bubbles, and a large number of bubbles are qualitatively similar, with significant reduction in convection speed for the lower ring strength cases, which is linked to fragmentation of the vortex core and corresponding reduction in its enstrophy.

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Contributors

Author: Subhajit Biswas

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