Fingering of a cavitation bubble in a thin gap: Ejection of the reversed boundary layer into the bulk flow

Abstract

The flow around a bubble, precipitously expanding in a thin gap between flat walls, was found to have a peculiar feature: distinct fingering occurs at the bubble wall, which was observed through the ultra-high speed optical visualization. The effect is attributed to the reversal of the flow within boundary layers, which provokes the growth of instabilities at the inflection point and, when the surface tension is low enough (the local Weber number is high enough), leads to the fingering. In this paper, we show the high speed recordings of the fingering and model the evolution of the radial velocity to quantitatively confirm feasibility of the proposed instability mechanism.