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Collision rate of ice crystals with water droplets in turbulent flows

Published online by Cambridge University Press:  27 April 2018

Aurore Naso Open the ORCID record for Aurore Naso [Opens in a new window] ,
Jennifer Jucha ,
Emmanuel Lévêque  and
Alain Pumir
Aurore Naso*
Affiliation:
Université de Lyon, Ecole Centrale de Lyon, Université Claude Bernard, CNRS, INSA de Lyon, Laboratoire de Mécanique des Fluides et d’Acoustique, 69134 Ecully, France
Jennifer Jucha
Affiliation:
Forschungszentrum Jülich, Projektträger Jülich, 52425 Jülich, Germany
Emmanuel Lévêque
Affiliation:
Université de Lyon, Ecole Centrale de Lyon, Université Claude Bernard, CNRS, INSA de Lyon, Laboratoire de Mécanique des Fluides et d’Acoustique, 69134 Ecully, France
Alain Pumir
Affiliation:
Université de Lyon, ENS de Lyon, Université Claude Bernard, CNRS, Laboratoire de Physique, 69342 Lyon, France
*
†Email address for correspondence: Aurore.Naso@ec-lyon.fr
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Abstract

Riming, the process whereby ice crystals get coated by impacting supercooled liquid droplets, is one of the dominant processes leading to precipitation in mixed-phase clouds. How a settling crystal collides with very small water droplets has been mostly studied in laminar conditions. The present numerical study aims at providing further insight on how turbulent flow motion affects the riming of ice crystals. We model the crystals as narrow oblate ellipsoids, smaller than the Kolmogorov elementary scale. By neglecting the effect of fluid inertia on the motion of the crystals and droplets, and using direct numerical simulations of the Navier–Stokes equations in a moderately turbulent regime, over a range of kinetic energy dissipation $1~\text{cm}^{2}~\text{s}^{-3}\lesssim \unicode[STIX]{x1D700}\lesssim 256~\text{cm}^{2}~\text{s}^{-3}$ , we determine the collision rate between disk-shaped ice crystals and very small liquid water droplets. Whereas differential settling plays the dominant role in determining the collision rate at small turbulence intensity, the role of turbulence becomes more important at the large values of $\unicode[STIX]{x1D700}$ simulated, an effect that can be partly attributed to the increased role of inertia. We always find that collisions occur with a large probability on the rim of the ellipsoids, a phenomenon that can be explained to a large extent by kinematic considerations. The difference in the settling velocity of crystals and droplets induces a strong asymmetry in the probability of collision between the faces of the ellipsoids. Our results shed light on the physical mechanisms involved in the riming of ice crystals in clouds.

JFM classification

Geophysical and Geological Flows: Atmospheric flows Multiphase and Particle-laden Flows: Multiphase and Particle-laden Flows Turbulent Flows: Turbulent Flows

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Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 845 , 25 June 2018 , pp. 615 - 641
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© 2018 Cambridge University Press 

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