Experimental study on bubble dynamics subject to buoyancy

• Published in: Journal of fluid mechanics Vol. 776; pp. 137 - 160
• Main Authors: Zhang, A. M., Cui, P., Cui, J., Wang, Q. X.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 10.08.2015
• Subjects: Boundary layer; Bubbles; Buoyancy; Fluid mechanics; Free surfaces; Vapor pressure; drops and bubbles; bubble dynamics; cavitation
• DOI: 10.1017/jfm.2015.323
• ISSN: 0022-1120

This paper is concerned with the dynamics of large bubbles subject to various strengths of buoyancy effects, which are associated with applications for underwater explosion. The bubble is produced by electric discharge in a low-pressure tank to enhance the buoyancy effects. Experiments are carried out for a bubble in an infinite field, below a free surface and above a rigid boundary. The effects of buoyancy are reflected by the dimensionless parameter ${\it\delta}=\sqrt{{\it\rho}gR_{m}/(p_{amb}-p_{v})}$ , where $R_{m}$ , $p_{amb}$ , $p_{v}$ , ${\it\rho}$ and $g$ are the maximum bubble radius, ambient pressure, saturated vapour pressure, density of water and the acceleration of gravity respectively. A systematic study of buoyancy effects is carried out for a wide range of ${\it\delta}$ from 0.034 to 0.95. A series of new phenomena and new features is observed. The bubbles recorded are transparent, and thus we are able to display and study the jet formation, development and impact on the opposite bubble surface as well as the subsequent collapsing and rebounding of the ring bubble. Qualitative analyses are carried out for the bubble migration, jet velocity and jet initiation time, etc. for different values of ${\it\delta}$ . When a bubble oscillates below a free surface or above a rigid boundary, the Bjerknes force due to the free surface (or rigid boundary) and the buoyancy are in opposite directions. Three situations are studied for each of the two configurations: (i) the Bjerknes force being dominant, (ii) the buoyancy force being dominant and (iii) the two forces being approximately balanced. For case (iii), we further consider two subcases, where both the balanced Bjerknes and buoyancy forces are weak or strong. When the Bjerknes and buoyancy forces are approximately balanced over the pulsation, some representative bubble behaviours are observed: the bubble near free surface is found to split into two parts jetting away from each other for small ${\it\delta}$ , or involutes from both top and bottom for large ${\it\delta}$ . A bubble above a rigid wall is found to be subject to contraction from the lateral part leading to bubble splitting. New criteria are established based on experimental results for neutral collapses where there is no dominant jetting along one direction, which correlate well with the criteria of Blake et al. (J. Fluid Mech., vol. 170, 1986, pp. 479–497; J. Fluid Mech., vol. 181, 1987, pp. 197–212) but agree better with the experimental and computational results.