Extreme fluctuations of the relative velocities between droplets in   turbulent airflow
  We compare experiments and direct numerical simulations to evaluate the accuracy of the Stokes-drag model, which is used widely in studies of inertial particles in turbulence. We focus on statistics at the dissipation scale and on extreme values of relative particle velocities for moderately inertial particles (St < 1). The probability distributions of relative velocities in the simulations were qualitatively similar to those in the experiments. The agreement improved with increasing Stokes number and decreasing relative velocity. Simulations underestimated the probability of extreme events, which suggests that the Stokes drag model misses important dynamics. Nevertheless, the scaling behavior of the extreme events in both the experiments and the simulations can be captured by the same multi-fractal model. 
Ewe-Wei Saw, Gregory P. Bewley, Eberhard Bodenschatz, Samriddhi Sankar   Ray and J\'er\'emie Bec
physics.flu-dyn