On the effects of vortex trapping on the velocity statistics of tracers and heavy particle in turbulent flows

The Lagrangian statistics of heavy particles and of fluid tracers transported by a fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. The Lagrangian velocity structure functions are measured in a time range spanning about three decades, from a tenth of the Kolmogorov timescale up to a few large-scale eddy turnover time. Strong evidence is obtained that fluid tracers statistics are contaminated in the time span 1-10 times the dissipative time scale by a bottleneck effect due to vortex trapping. This effect is found to be significantly reduced for heavy particles which are expelled from vortices by inertia. These findings help in clarifying the results of a recent study by H. Xu et al. Phys. Rev. Lett. 96, 024503 (2006), where differences between experimental and numerical results on scaling properties of fluid tracers were reported.