Numerical Simulation of the Agglomeration of Micron-Sized Cloud Droplets under the Action of Thunder Using Computational Fluid Dynamics and Discrete Element (CFD-DEM) Method

The occurrence of thunderstorms in nature is often accompanied by rainfall. This study focuses on the effect and the mechanism of thunder on rainfall, and numerically simulates cloud droplet agglomeration under the action of sound waves. The action of sound waves creates an air disturbance, and the oscillation of the local air causes the micron-sized cloud droplets in the cloud layer to move. The interaction between cloud droplets and air under the action of sound waves is a typical two-phase flow problem, which is numerically solved by coupling computational fluid dynamics (CFD) and discrete element methods (DEMs) in this study. The movements of micron-sized cloud droplets are simulated, as well as their agglomeration. The evolution process of the average particle size and the number of multimers are obtained, and the influence of different thunder frequencies, sound pressure level (SPL), and droplet spacing on droplet agglomeration are studied. It is found that the promotion effect of low-frequency sound waves on cloud droplet agglomeration is significantly higher than that of high-frequency sound waves, and the sound wave promotion effect of high SPLs is better than that of low SPL. In addition, the concept of the average agglomeration time required to quantify the acoustic agglomeration speed is proposed, and it is found to be positively correlated with sound frequency and particle spacing, while being negatively correlated with SPL.