Bag film breakup of droplets in uniform airflows

We present novel numerical simulations investigating the bag breakup of liquid droplets. We first examine the viscous effect on the early-time drop deformation, comparing with theory and experiment. Next, a bag film forms at late time and is susceptible to spurious mesh-induced breakup in numerical simulations, which has prevented previous studies from reaching grid convergence of fragment statistics. We therefore adopt the manifold death (MD) algorithm which artificially perforates thin films once they reach a prescribed critical thickness independent of the grid size, controlled by a numerical parameter L-sig. We show grid convergence of fragment statistics when utilising the MD algorithm, and analyse the fragment behaviour and bag film disintegration mechanisms including ligament breakup, node detachment and rim destabilisation. Our choice of the critical thickness parameter L-sig is limited by numerical constraints and thus has not been matched to experiment or theory; consequently, the current simulations yield critical bag film perforation thicknesses larger than experimentally observed. The influence of the MD algorithm configuration on the bag breakup phenomena and statistics will be investigated in future work. We also study the effects of moderate liquid Ohnesorge number (0.005 = Oh = 0.05) on the bag breakup process and fragment statistics, where a non-monotonic dependency of the average diameter of bag film fragments on Oh is found. These results highlight the utility of the MD algorithm in multiphase simulations involving topological changes, and pave the way for physics-based numerical investigations into spume generation at the air-sea interface.

Automated summary

In this study, we conducted novel numerical simulations to investigate the process of bag breakup in liquid droplets. We examined the early-time drop deformation and the late-time formation of a bag film in comparison with theory and experiment. By adopting the manifold death (MD) algorithm, we achieved grid convergence of fragment statistics and analyzed the mechanisms of bag film disintegration. Furthermore, we investigated the effects of liquid Ohnesorge number on the bag breakup process and found a non-monotonic relationship between the average diameter of bag film fragments and Oh.