  We present novel numerical simulations investigating the bag breakup of
liquid droplets. We first examine the viscous effect on drop deformation before
the onset of bag breakup, 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 $L_{\rm sig}$. 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 $L_{\rm sig}$
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 ($0.005 \leq Oh \leq 0.05$) 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.
