Crown formation from a cavitating bubble close to a free surface

TL;DR

This study uses axisymmetric simulations to uncover the mechanism behind crown formation in cavitating bubbles near free surfaces, revealing the roles of pressure distortion and flow reversal, and explores how various parameters influence this phenomenon.

Contribution

It provides a detailed simulation-based explanation of crown formation mechanisms, which were previously not well understood, and analyzes the effects of key parameters on the process.

Findings

Crown formation results from pressure distortion and flow reversal.

Higher Weber numbers lead to weaker secondary crowns.

The study identifies the influence of multiple parameters on jet and crown dynamics.

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, that uses a geometric Volume-of-Fluid method (VoF) 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.