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Caustics-induced coalescence of small droplets near a vortex

P. Deepu1,*, S. Ravichandran2,†, and Rama Govindarajan3,‡

  • 1Department of Mechanical Engineering, Indian Institute of Technology, Palakkad 678557, India
  • 2TIFR Centre for Interdisciplinary Sciences, Narsingi, Hyderabad 500075, India
  • 3International Centre for Theoretical Sciences, TIFR, Bangalore 560089, India

  • *deepu@iitpkd.ac.in
  • †ravis@tifrh.res.in
  • ‡rama@icts.res.in
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Phys. Rev. Fluids 2, 024305 – Published 28 February, 2017

DOI: https://doi.org/10.1103/PhysRevFluids.2.024305

Abstract

How droplets grow rapidly from 10 to 50 μm is an outstanding question in cloud physics. We show theoretically and numerically that caustics, locations of multivalued droplet velocity, of small droplets near a single planar steady vortex offer one route through this bottleneck. Such a vortex serves as a simple model for the more complicated turbulence field existing in clouds. Within a special radial distance rc from the vortex center, droplets closer to the vortex can centrifugally overtake those farther out and coalesce. Small polydispersity increases rc dramatically, enabling repeated collisions at short time intervals and formation of large droplets. Our results show that caustics brought about in a polydisperse suspension could offer a mechanistic explanation of accelerated rain initiation.

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Phys. Rev. Fluids 2, 024305– Published 28 February, 2017
  • Received 30 June 2016

©2017 American Physical Society

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