INERTIAL PARTICLES IN TURBULENT FLOWS

I will review some recent advancements about the small-scale statistical prop- erties of dilute suspensions of inertial particles, dispersed in turbulent flows. These are very small, but finite size impurities, with a density contrast with respect to the carrier fluid. Examples of heavy inertial particles - with den- sity much larger than the fluid-, are liquid water droplets in air clouds, or dust and chemicals dispersed in the atmosphere. Inertial particles do not simply follow fluid streamlines, and in the simplest approximation they respond to flow fluctuations via a Stokes viscous drag. I will discuss the relative dispersion of pairs of particles, and contrast the Richardson diffusion behaviour observed for Lagrangian tracers, to the in- ertia related regimes observed for heavy particles. These regimes appear because of the uncorrelation between relative velocities for larger and larger inertia. Locally uncorrelated velocities have an impact also on the estimation of the collision rate of same-size particles, that I will compare to the classical Saffman-Turner formula. The results presented are obtained from Direct Numerical Simulations of in- compressible, homogeneous and isotropic 3D turbulent flow, characterised by a Taylor-scale based Reynolds number between Re? ? 200 and Re? ? 400, seeded with inertial particles.