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 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 < Fr < 7, but the constants of proportionality are smaller. llustrate the relevance of the work for the design of mixing and aeration systems in freshwater reservoirs.
Iran E. Lima Neto, Silvana S. S. Cardoso and Andrew W. Woods, Journal of Fluid Mechanics, Volume 802, R3 (2016)