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Abstract

A rapidly growing bubble close to a free surface induces jetting: a central jet protruding
outwards and a crown surrounding it at later stages. While the formation mechanism
of the central jet is known and documented, that of the crown remains unsettled. We
perform axisymmetric simulations of the problem using the free software program
BASILISK, where a finite-volume compressible solver has been implemented, which uses a
geometric volume-of-fluid (VoF) method for the tracking of the interface. We show that the
mechanism of crown formation is a combination of a pressure distortion over the curved
interface, inducing flow focusing, and of a flow reversal, caused by the second expansion of
the toroidal bubble that drives the crown. The work culminates in a parametric study with
the Weber number, the Reynolds number, the pressure ratio and the dimensionless bubble
distance to the free surface as control parameters. Their effects on both the central jet and
the crown are explored. For high Weber numbers, we observe the formation of weaker
‘secondary crowns’, highly correlated with the third oscillation cycle of the bubble.