The study of turbulent collision of cloud droplets requires simultaneous considerations of the transport of background air turbulence (i.e., geometric collision rate) and influence of droplet disturbance flows (i.e., collision efficiency). In recent years, this multiscale problem has been addressed through a hybrid direct numerical simulation (HDNS) approach (Ayala et al., J. Comp. Phys. 225 (2007) 51-73). This approach, while currently is the only viable tool to quantify the effects of air turbulence on collision statistics, is computationally expensive. In order to extend the HDNS approach to higher flow Reynolds numbers, here we developed a highly scalable implementation of the approach using 2D domain decomposition. The scalability of the parallel implementation was studied using several parallel computers, at 512 3 and 1024 3 grid resolutions with O(10 6 )-O(10 7 ) droplets. It was found that the execution time scaled with number of processors almost linearly until it saturates and deteriorates due to communication latency issues. To better understand the scalability, we developed a complexity analysis by partitioning the execution tasks into computation, communication, and
['Orlando Ayala', 'Hossein Parishani', 'Liu Chen', 'Bogdan Rosa', 'Lian-Ping Wang']
DNS of hydrodynamically interacting droplets in turbulent clouds: Parallel implementation and scalability analysis using 2D domain decomposition