Numerical Study of the Droplet Bag Breakup Behaviour*

We present novel numerical simulations investigating the secondary atomisation of water droplets with a focus on the bag breakup regime, which has important implications for many physical processes, especially for understanding the generation of ocean sprays under high-wind conditions. We solve the two-phase incompressible Navier-Stokes equation using the adaptive mesh refinement (AMR) technique, with interface reconstruction by a volume-of-fluid (VOF) method. We first show good agreement with theoretical predictions at early time, and clarify the viscous effect on drop deformation. Next, in numerical simulations of the bag breakup regime, a bag film is developed at late time and is susceptible to spurious mesh-induced breakup, which has prevented previous numerical studies from reaching grid convergence of fragment statistics. We therefore adapt a recently developed manifold death (MD) algorithm which artificially perforates thin films once they reach a prescribed critical thickness independent of the grid size, and we vary the parameters of the hole perforation mechanism to examine the relative effects of Ohnesorge and Weber numbers on the subsequent fragmentation behaviour of the bag. We show grid convergence of the fragment size distribution when utilising the MD algorithm, and identify the physical mechanisms involved in the fragmentation process. These results highlight the utility of this numerical method for multiphase atomisation problems, and pave the way for physics-based numerical investigations into spume generation at the air-sea interface.