Three-dimensional numerical simulation of tandem droplets accelerated by continuous uniform airflow

In a dense droplet environment, droplets influence each other's motion, deformation, and breakup behavior. The tandem droplet is a particularly relevant case for the study of its unsteady dynamic behavior. A three-dimensional numerical simulation study was conducted to investigate the deformation process of tandem droplets under different conditions. The input parameters included Reynolds numbers (150-1500), liquid/gas density ratios (100-1000), Weber numbers (30-90), and initial relative distances (4-12). The volume of fluid method was employed on an adaptive octree grid combined with the dynamic Smagorinsky turbulence model. This method has passed validity verification and grid independence verification, and it requires significantly fewer computational resources than the direct numerical simulation method when droplet breakup is not considered. The results of the research show that under conditions of high density ratio and a significant Reynolds number, the edge morphological characteristics of droplets are predominantly influenced by the Rayleigh-Taylor instability. In the case of low density ratios, the pressure drag force on the leeward side exerts a dominant influence on the accelerated motion of the leading droplet. The shape of the droplet is significantly influenced by the vortex ring present in the recirculation region. The perturbation of the liquid edge induces the vortex ring to split into secondary vortex rings, which act back on the droplet, thereby affecting its morphological characteristics. The trailing droplet is subject to a reduction in cross-flow radius, drag coefficient, minimum length, and expansion speed of the liquid bag due to the influence of the wake of the leading droplet. The decrease in Reynolds number and relative distance leads to a stronger suppression effect, while the decrease in density ratio shortens the length of the recirculation region, thereby weakening the suppression of trailing droplets. Finally, a predictive model was proposed to describe the time evolution of the radius of tandem droplets.