Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence

Abstract

We present a numerical study of two-particle dispersion from point-sources in 3D incompressible Homogeneous and Isotropic turbulence, at Reynolds number Re \simeq 300. Tracer particles are emitted in bunches from localized sources smaller than the Kolmogorov scale. We report the first quantitative evidence, supported by an unprecedented statistics, of the deviations of relative dispersion from Richardson's picture. Deviations are due to extreme events of pairs separating much faster than average, and of pairs remaining close for long times. The two classes of events are the fingerprint of complete different physics, the former being dominated by inertial subrange and large-scale fluctuations, while the latter by the dissipation subrange. A comparison of relative separation in surrogate white-in-time velocity field, with correct viscous-, inertial- and integral-scale properties allows us to assess the importance of temporal correlations along tracer trajectories.