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Making droplets glow in turbulence

Humberto Bocanegra Evans1,2, Nico Dam3, Guus Bertens1,4, and Willem van de Water1,5,*

  • 1Physics Department, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
  • 2School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA
  • 3Mechanical Engineering Department, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
  • 4Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany
  • 5Laboratory for Aero and Hydrodynamics, Delft University of Technology and J.M. Burgers Centre for Fluid Dynamics, 2628 CD Delft, The Netherlands

  • *Corresponding author: w.vandewater@tudelft.nl; present address: Laboratory for Aero and Hydrodynamics, Delft University of Technology and J.M. Burgers Centre for Fluid Dynamics, 2628 CD Delft, The Netherlands.
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Phys. Rev. Fluids 5, 044303 – Published 7 April, 2020

DOI: https://doi.org/10.1103/PhysRevFluids.5.044303

Abstract

We present a new technique to study preferential concentration of droplets in a turbulent air flow. Preferential concentration is the tendency of droplets to cluster in regions of strain, while avoiding regions of rotation. We study the properties of the droplet concentration field in zero mean flow turbulence that was created using an array of synthetic jets. The droplets are made of a phosphorescent solution of Europium chelate. They are excited by a laser sheet from a pulsed UV laser, after which the glowing droplets are followed using a high-speed intensified camera. We quantify preferential concentration through measurement of moments of the coarse-grained local droplet density. At the Stokes numbers studied (St≈2) the fractal dimension, a scaling property of this coarse-grained density field, points to clustering. Clustering is a consequence of the compressibility of the droplet velocity field. We also quantify the dynamical behavior of clustering by moving with this velocity field. We find a preference for clustering in the Lagrangian frame during the time interval set by the decay of the phosphorescence.

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Phys. Rev. Fluids 5, 044303– Published 7 April, 2020
  • Received 2 June 2017
  • Accepted 3 March 2020
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