On mixing a density interface by a bubble plume

Neto, Iran E. Lima and Cardoso, Silvana S. S. and Woods, Andrew W. (2016) On mixing a density interface by a bubble plume. Journal of Fluid Mechanics, 802. ISSN 0022-1120, ESSN: 1469-7645 DOI https://doi.org/10.1017/jfm.2016.454

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We describe new experiments in which a bubble plume, produced from a point source of bubbles, rises through an ambient fluid composed of two layers of fluid of different density. In the lower layer, the speed of the plume exceeds the bubble rise speed and the motion is well described using classical theory of turbulent buoyant plumes. As the mixture enters the upper layer, it is either buoyant and rises to the top of the layer, or is negatively buoyant and forms a fountain. In our experiments, in which a fountain forms in the upper layer, the bubble rise speed exceeds the characteristic speed of this fountain, and a separated flow develops. The bubbles rise to the top of the system, while the lower layer fluid in the fountain rises a finite distance into the upper layer, entrains some of the upper layer fluid, and then collapses. This mixture of fluids then feeds a growing layer of density which is intermediate between the upper and lower layer. The height of rise of the fountain scales with the square of the Froude number of the fountain and the rate of entrainment of upper layer fluid into the fountain is directly proportional to the height of the fountain. This is analogous to the scaling for single-phase fountains with Froude numbers in the same range, $1 , but the constants of proportionality are smaller. We illustrate the relevance of the work for the design of mixing and aeration systems in freshwater reservoirs.

Item Type: Article
Uncontrolled Keywords: 2016AREP; IA70
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
99 - Other
Journal or Publication Title: Journal of Fluid Mechanics
Volume: 802
Identification Number: https://doi.org/10.1017/jfm.2016.454
Depositing User: Sarah Humbert
Date Deposited: 06 Jun 2017 16:15
Last Modified: 06 Jun 2017 16:15
URI: http://eprints.esc.cam.ac.uk/id/eprint/3977

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