Turbulent exchange mechanisms in bubble plumes

Turbulent transport and exchange mechanisms in an unstable oscillating bubble plume are investigated using a novel Large-Eddy PIV measurement technique providing simultaneous gas and liquid velocity fields. The flow was generated in a 2 m-diameter vessel with a water depth of 1.5 m for void fractions up to 4% and bubble size of the order of 2.5 mm. Sub-scale turbulent properties of the flow were estimated, revealing an enhanced anisotropic transport of turbulence and energy redistribution mechanism and with strong shear-induced characteristics. It is shown that the plume dynamics contain distinct flow zones, each associated with specific flow events and featuring specific energy exchange mechanisms: the plume-core zone where the flow accelerates, the peak zone where the plume contracts, and a residual external zone where the stresses are relaxed. The turbulent kinetic energy is found to be dominated by the vertical stresses, along the flow direction, notably in the peak zone, a phenomenon associated with the meandering of the central bubble plume. The quantitative estimation of the production terms new explanations of the energy exchange mechanisms between the mean flow and the sub-scale motions within the three flow zones identified. During plume contraction the longitudinal normal- and cross-production terms act as pure generation terms extracting energy from the mean flow; when the contraction is terminated, the shear-induced contribution competes with the normal stress-induced term, which tends to restitute energy back to the mean flow. In the plume-core zone instead, both terms are negligible, except at lower elevations where, because of the strong vertical acceleration of the flow, the contraction occurs and the normal stress-induced restitutes energy to the mean flow.