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We present results from experiment and direct-numerical-simulation(DNS), on the statistics of relative velocity(δv) between small-heavy particles in the dissipative scales of turbulent flow. The experimental flow was nearly homogenous and isotropic at Taylor- scaled Reynolds number around 200. The particles were small liquid droplets (d < 0.1 η) and have Stokes numbers (St) in the range of 0.04 to 0.51.The simulation was tuned to match the Reynolds, Stokes and Froude number of the experiments. Comparison showed that DNS reproduced all qualitative trends of the experiments. These included the stretched-exponential form of the tails of the distribution of δv, its skewness, its growth with Stokes number and particle separation. Good quantitative agreement were found for the negative δv (approaching particles) for sufficiently large St. We discuss the remaining quantitative discrepancies in terms of mismatch of intermittency between the experiment and DNS. We show that the tails of the distribution of δv are accounted for by the sling effect – a mechanism in which turbulent fluctuations causes the droplets to decouple from the background fluid and move toward each other with Stokes-dragged ballistic motions. We attempt to reproduce the forms of these tails and relate them to fluid flow statistics via the sling-start-scales – particle separations at the initiation of slings.
Author(s):
Ewe-Wei Saw
Max Planck Institute for Dynamics and Self Organization
Germany
Gregory P. Bewley
Max Planck Institute for Dynamics and Self Organization
Germany
Samriddhi S. Ray
Max PlaLaboratoire Lagrange UMR\,7293, Universit\'e de Nice-Sophia Antipolis, CNRS, Observatoire de la C\^ote d'Azurnck Institute for Dynamics and Self Organization
France
Holger Homann
Laboratoire Lagrange UMR\,7293, Universit\'e de Nice-Sophia Antipolis, CNRS, Observatoire de la C\^ote d'Azur
France
Jeremie Bec
Laboratoire Lagrange UMR\,7293, Universit\'e de Nice-Sophia Antipolis, CNRS, Observatoire de la C\^ote d'Azur
France
Eberhard Bodenschatz
Max Planck Institute for Dynamics and Self Organization
Germany