Turbulence has an important influence on the transport of small particulate matter like dust in the atmosphere, fuel droplets in combustion chambers and small biological organisms in marine environments.
The relative separation of small tracer particles is strongly influenced by the multi-‐scale space and time correlations of the turbulent velocity field. While in applications it is often necessary to model the smallest turbulent scales, this may have an important impact on particles’ dynamics and in particular on their relative dispersion.
We present recent results on the dispersion of particle pairs studied by means of fully resolved high-‐ resolution and high-‐statistics direct numerical simulations [1,2]. We further discuss a recently proposed subgrid Lagrangian model capable of reproducing the effect of unresolved eddies on single particle dynamics (absolute dispersion) as well as on the relative separation of tracers. The model is simple, computationally efficient and is capable to reproduce Richardson separation dynamics between pairs in a cloud composed by an arbitrary number of tracers .
 Scatamacchia, R., Biferale, L. & Toschi, F. (2012). Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence. Physical Review Letters, 109(14):144501.  Biferale, L., Lanotte, A.S., Scatamacchia, R. & Toschi, F. (2014). Intermittency in the relative separations of tracers and of heavy particles in turbulent flows. Journal of Fluid Mechanics, 757, 550-‐ 572.  Mazzitelli, I.M., Toschi, F. & Lanotte, A.S. (2014). An accurate and efficient Lagrangian sub-‐grid model. Physics of Fluids, 26(9), 095101-‐1/17.