  Topological singularities occur in a broad range of physical systems,
including collapsing stars and pinching fluid interfaces. They are important
for being able to concentrate energy into a small region. Underwater air
bubbles in particular appear in many practical applications, including new
technologies to reduce skin drag on cargo ships. Previous theories show that
just before an air bubble pinches off, the neck looks like a cylinder at its
very smallest point. Unusually, however, the neck approaches this shape so
gradually that the theoretical cylinder solution is not reached in practice;
the singularity spends its entire lifetime in a transient phase. Therefore, in
order to understand the evolution, we study the transient effects in detail.
This paper details the simulation results of bubbles with initial conditions
far from the cylindrical solution: squat, up--down asymmetric neck shapes, with
imposed vertical flow. We find that the asymmetry is transient: the neck
quickly shifts vertically to become up--down symmetric. Importantly, we find
that the resulting symmetric singularity is a blend of the initial top and
bottom sides, with a weighting factor that is tunable by adjusting the airflow
through the neck. This effect should have implications for the later stages of
evolution, including the generation of satellite bubbles and the formation of
the Worthington jet.

Fluid Dynamics