Towards quantitative prediction of droplet collision outcomes: A dual-VOF approach with rarefied gas effect and augmented van der Waals force

A dual-VOF (volume of fluid) approach incorporating with the adaptive mesh refinement (AMR) algorithm, the rarefied gas effect (RGE), and the augmented van der Waals (vdW) force was proposed and validated in the present study for predicting droplet collision outcomes, particularly the bouncing and coalescence. RGE is introduced by modifying the gas phase viscosity within the gas film, while vdW force is incorporated using a disjoining pressure model. The results show that the present method can successfully predict the non-monotonic trend for head-on collision that collision outcome transfers from soft coalescence to bouncing then to hard coalescence with increasing the Weber number. This trend has been well observed in many previous experimental works. Although the adopted Hamaker constant was substantially augmented compared with the physically realistic one, it is by three orders of magnitude smaller than those adopted by the previous simulation works. The present method is demonstrated to be a small but firm step toward quantitatively predicting droplet collision in gaseous medium, a well-recognized multi-scale, multi-physics problem.