Intermittency in the relative separations of tracers and of heavy particles in turbulent flows
Abstract
Results from Direct Numerical Simulations of particle relative dispersion in three dimensional homogeneous and isotropic turbulence at Reynolds number $Re_\lambda \sim 300$ are presented. We study pointlike passive tracers and heavy particles, at Stokes number St = 0, 0.6, 1 and 5. Particles are emitted from localised sources, in bunches of thousands, periodically in time, allowing to reach an unprecedented statistical accuracy, with a total number of events for twopoint observables of the order of $10^{11}$. The right tail of the probability density function for tracers develops a clear deviation from Richardson's selfsimilar prediction, pointing to the intermittent nature of the dispersion process. In our numerical experiment, such deviations are manifest once the probability to measure an event becomes of the order of or rarer than one part over one million, hence the crucial importance of a large dataset. The role of finiteReynolds effects and the related fluctuations when pair separations cross the boundary between viscous and inertial range scales are discussed. An asymptotic prediction based on the multifractal theory for inertial range intermittency and valid for large Reynolds numbers is found to agree with the data better than the Richardson theory. The agreement is improved when considering heavy particles, whose inertia filters out viscous scale fluctuations. By using the exittime statistics we also show that events associated to pairs experiencing unusually slow inertial range separations have a non selfsimilar probability distribution function.
 Publication:

Journal of Fluid Mechanics
 Pub Date:
 October 2014
 DOI:
 10.1017/jfm.2014.515
 arXiv:
 arXiv:1411.3985
 Bibcode:
 2014JFM...757..550B
 Keywords:

 Physics  Fluid Dynamics
 EPrint:
 22 pages, 14 figures