Hydrodynamics of fluid around a collapsing bubble in the spark bubble droplet generation process

Abstract

Summary This article aims to study in detail the bubble oscillation near confined free surfaces and the ensuing droplet formation using a combined boundary element-finite difference method. Three configurations are considered: (i) the bubble oscillation near a circular aperture made in a flat plate, (ii) the bubble oscillation inside and near the top opening of a vertical cylinder and (iii) the bubble oscillation between a perforated flat plate and a horizontal solid boundary. The effects of standoff distance on the bubble dynamics and on the surrounding fluid dynamics are examined. Completely different bubble shapes, free surface motions, jetting patterns and pressure distributions under different standoff distances could be observed in the present work. In addition, it was found that for the configurations (i) and (iii), the bubble reentrant jet is always directed away from the free surface. For the cylinder case, however, a critical standoff distance was found for which the pressure distribution in the fluid above and below the bubble are the same and the bubble takes the shape of an hourglass at the time of jet impact. For subcritical standoff distances the bubble reentrant jet is always away from the free surface. However, for supercritical standoff distances the direction of the reentrant jet is interestingly directed towards the free surface. Finally, the effect of plate and cylinder configurations on the resulting droplet dynamics was investigated. It was found that the driving force for the droplet formation is the initial velocity induced by the bubble reentrant jet rather than the pressure difference inside the liquid. Besides, in the cylinder case the droplet size is smaller and its pinch-off happens earlier than the plate case.