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 Lsig. 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 Lsig 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.
