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Deformation and orientation statistics of neutrally buoyant sub-Kolmogorov ellipsoidal droplets in turbulent Taylor–Couette flow

Published online by Cambridge University Press:  14 November 2016

Vamsi Spandan ,
Detlef Lohse  and
Roberto Verzicco
Vamsi Spandan
Affiliation:
Physics of Fluids, University of Twente, Enschede, PO Box 217, 7500 AE, Netherlands
Detlef Lohse
Affiliation:
Physics of Fluids, University of Twente, Enschede, PO Box 217, 7500 AE, Netherlands Max Planck Institute for Dynamics and Self-Organization, 37077 Göttingen, Germany
Roberto Verzicco*
Affiliation:
Physics of Fluids, University of Twente, Enschede, PO Box 217, 7500 AE, Netherlands Dipartimento di Ingegneria Meccanica, University of Rome ‘Tor Vergata’, Via del Politecnico 1, Rome 00133, Italy
*
†Email address for correspondence: [email protected]
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Abstract

The influence of the underlying flow topology on the shape and size of sub-Kolmogorov droplets dispersed in a turbulent flow is of considerable interest in many industrial and scientific applications. In this work we study the deformation and orientation statistics of sub-Kolmogorov droplets dispersed into a turbulent Taylor–Couette flow. Along with direct numerical simulations (DNS) of the carrier phase and Lagrangian tracking of the dispersed droplets, we solve a phenomenological equation proposed by Maffettone and Minale (J. Non-Newtonian Fluid Mech., vol. 78, 1998, pp. 227–241) to track the shape evolution and orientation of approximately $10^{5}$ ellipsoidal droplets. By varying the capillary number $Ca$ and viscosity ratio $\hat{\unicode[STIX]{x1D707}}$ of the droplets we find that they deform more with increasing capillary number $Ca$ and this effect is more pronounced in the boundary layer regions. This indicates that along with an expected capillary number effect there is also a strong correlation between spatial position and degree of deformation of the droplet. Regardless of the capillary number $Ca$, the major axis of the ellipsoids tends to align with the streamwise direction and the extensional strain rate eigendirection in the boundary layer region while the distribution is highly isotropic in the bulk due to the strong mixing provided by the large-scale vortical structures. When the viscosity ratio between the droplet and the carrier fluid is increased we find that there is no preferential stretched axis which is due to the increased influence of rotation over stretching and relaxation. Droplets in high viscosity ratio systems are thus less deformed and oblate (disk-like) as compared to highly deformed prolate (cigar-like) droplets in low viscosity ratio systems.

JFM classification

Drops and Bubbles: Drops and Bubbles Multiphase and Particle-laden Flows: Multiphase flow
Type
Papers
Information
Journal of Fluid Mechanics , Volume 809 , 25 December 2016 , pp. 480 - 501
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Copyright
© 2016 Cambridge University Press 

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