Numerical Simulation of Binary Droplet Collision Using CFD Software Tools

We have computationally investigated the head-on collision between two identical droplets by using commercial software tools based on the volume-of-fluid (VOF) method. The results were compared to that obtained by the front-tracking method and experiments. The simulated collision images were found to agree with the experimental observations only at a low or moderate Weber number (We). If We became so large that the surfaces were substantially deformed, specifically when the regimes showing separation and breakup were attained, prominent disagreements were generated. In particular, the simulation for phenomena of separation was characterized by exaggeratedly created bubbles near the coalesced interfaces, whereas that for break-up scenario could be affected much by the evaluation of surface curvature and forces as well as the grid construction. Furthermore, the predictions of the transition boundaries between the coalescence and bouncing regimes at lower We fidelity. This is because the merging process involves multi-scale physics that cannot be simply resolved by the artificial treatment of the interfaces in the present methodology based only on a macroscopic description of hydrodynamics. This work thus provides examples not suitable for these tools and provokes cautious consideration for those who apply such commercial packages to relevant problems.