Crown formation from a cavitating bubble close to a free surface

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.

Automated summary

The study reveals that the formation mechanism of the crown is a combination of pressure distortion and induced flow focusing on the curved interface, as well as flow reversal caused by the second expansion of the toroidal bubble. A parametric study with control parameters such as Weber number, Reynolds number, pressure ratio, and dimensionless bubble distance to the free surface shows their effects on both the central jet and the crown formation. High Weber numbers lead to the formation of weaker 'secondary crowns', highly correlated with the third oscillation cycle of the bubble.