High-Resolution Simulation of Turbulent Collision of Cloud Droplets

A novel parallel implementation of hybrid DNS (Direct Numerical Simulation) code for simulating collision-coalescence of aerodynamically interacting particles in a turbulent flow has been developed. An important application of this code is to quantify turbulent collision-coalescence rate of cloud droplets, relevant to warm rain formation, under physically realistic conditions. The code enables performing high-resolution DNS of turbulent collisions so the simulation results can be used to begin addressing the question of Reynolds number dependence of pair and collision statistics. The new implementation is based on MPI (Message Passing Interface) library, and thus the code can run on computers with distributed memory. This development enables to conduct hybrid DNS with flow field solved at grid resolutions up to 5123 while simultaneously track up to several million aerodynamically-interacting droplets. In this paper we discuss key elements of the MPI implementation and present preliminary results from the high resolution simulations. The key conclusion is that, for small cloud droplets, the results on pair statistics and collision kernel appear to reach their saturation values as the flow Reynolds number is increased.