Numerical investigation of binary droplet collisions in all relevant collision regimes

A numerical investigation of binary droplet collisions in a gaseous phase has been conducted. A volume of fluid (VOF) based interface capturing method, which is characterized by introducing an extra artificial compression term into the volume-fraction transport equation, is employed to capture the liquid-gas interface. The full Navier-Stokes equations coupled with the volume-fraction transport equation are discretized on a fixed, uniform grid using the finite volume method. The solution of the volume-fraction transport equation is bounded by an explicit universal multidimensional limiter. The simulations cover five major regimes of binary droplet collisions: (I) coalescence with minor deformation, (II) bouncing, (III) coalescence with major deformation, (IV) reflexive separation and (V) stretching separation. A ghost cell method is introduced in order to simulate bouncing and retarded coalescence in head-on collisions. Especially, for retarded coalescence collisions, the volumefraction boundary condition on the symmetry plane is switched to numerically control coalescence at a critical time which is determined by tracking the interface front. The simulated results are compared with the attainable experimental data and provide detailed pictures exhibiting the droplet collision processes. The inter-droplet pressure, defined by the static pressure on the symmetry plane in head-on collisions, is also evaluated and analyzed.