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Droplet dynamics and fine-scale structure in a shearless turbulent mixing layer with phase changes

Published online by Cambridge University Press:  08 February 2017

Paul Götzfried ,
Bipin Kumar ,
Raymond A. Shaw  and
Jörg Schumacher
Paul Götzfried*
Affiliation:
Institut für Thermo- und Fluiddynamik, Postfach 100565, Technische Universität Ilmenau, D-98684 Ilmenau, Germany
Bipin Kumar
Affiliation:
Indian Institute of Tropical Meteorology, Dr Homi Bhaba Road, Pashan, Pune 411008, Maharashtra, India
Raymond A. Shaw
Affiliation:
Department of Physics, Michigan Technological University, Houghton, MI 49931, USA
Jörg Schumacher
Affiliation:
Institut für Thermo- und Fluiddynamik, Postfach 100565, Technische Universität Ilmenau, D-98684 Ilmenau, Germany
*
†Email address for correspondence: paul.goetzfried@tu-ilmenau.de
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Abstract

Three-dimensional direct numerical simulations of a shearless mixing layer in a small fraction of the cloud–clear air interface are performed to study the response of an ensemble of cloud water droplets to the turbulent entrainment of clear air into a cloud filament. The main goal of this work is to understand how mixing of cloudy and clear air evolves as turbulence and thermodynamics interact through phase changes, and how the cloud droplets respond. In the main simulation case, mixing proceeds between a higher level of turbulence in the cloudy filament and a lower level of turbulence in the clear air environment – the typical shearless mixing layer set-up. Fluid turbulence is driven solely by buoyancy, which incorporates feedbacks from the temperature, the vapour content and the liquid water content fields. Two different variations on the core set of shearless mixing layer simulations are discussed, a simulation in a larger domain and a simulation with the same turbulence level inside the filament and its environment. Overall, it is found that, as evaporation occurs for the droplets that enter subsaturated clear air regions, buoyancy comes to dominate the subsequent evolution of the mixing layer. The buoyancy feedback leads initially to downdraughts at the cloudy–clear air interface and to updraughts in the bulk regions. The strength of the turbulence after initial transients depends on the domain size, showing that the range of scales is an important parameter in the shearless mixing layer set-up. In contrast, the level of turbulence in the clear air is found to have little effect on the evolution of the mixing process. The distributions of cloud water droplet size, supersaturation at the droplet positions and vertical velocity are more sensitive to domain size than to the details of the turbulence profile, suggesting that the evolution of cloud microphysics is more sensitive to large-scale as opposed to small-scale properties of the flow.

JFM classification

Phase change: Condensation/evaporation Multiphase and Particle-laden Flows: Particle/fluid flow Mixing: Turbulent mixing
Type
Papers
Information
Journal of Fluid Mechanics , Volume 814 , 10 March 2017 , pp. 452 - 483
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© 2017 Cambridge University Press 

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