Experimental Study of Droplet Relative Velocity Statistics in Turbulent Cloud—a Test of the Simple Stokes Drag Model

I will present results obtained in collaborations with former colleagues in which we compare experiments and direct numerical simulations to evaluate the accuracy of the Stokes-drag model, which is used widely in studies of cloud droplets in turbulence. In the Stokes drag model, force on the particle/droplet is modeled simply as a linear function of the slip velocity between particle and fluid. 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 some dynamical effects. Nevertheless, the scaling behavior of the extreme events in both the experiments and the simulations can be captured by the same multi-fractal model. I will also present results pertaining bi-disperse case where the particle pairs has different sizes.