It has been proposed that axisymmetric buoyant jets discharged vertically into a horizontal turbulent boundary layer flow undergo a transition from self-induced mixing to an ultimate state where mixing is dominated by the shear-flow turbulence. Both plume mixing and ambient shear-flow mixing have been separately well characterized by many previous studies and can be thought of as asymptotic mixing regimes. This investigation focuses on the transition between the two asymptotic regimes that is not well understood and that is often of particular engineering interest.
In this work, we present the results of a detailed experimental analysis of buoyant jet mixing in a turbulent shear flow. Our purpose is to obtain a detailed picture of the turbulent velocity field and the concentration distributions throughout the various mixing regimes in order to discern the effects of changes in various flow parameters on the predominant mixing mechanisms. The experimental technique employs buoyant jets whose fluid is optically homogeneous with that of the ambient shear flow. This enables the combined use of laser-Doppler velocimetry and laser-induced fluorescence to measure the velocity and concentration profiles, respectively.
Dimensional analysis indicates that the cross-flow shear velocity and the plume specific buoyancy flux are the parameters controlling the transition from plume mixing to diffusion mixing. Quantitative analysis of the experimental results indicates that the mixing is dominated entirely by diffusion, or shear-flow mixing, even close to the point of discharge. Further, we observe that within the diffusive mixing regime, a transition occurs from a region where the turbulent mixing coefficient is proportional to the local elevation to a region where the turbulent mixing coefficient is proportional to the boundary layer thickness. Detailed instantaneous spatial concentration distributions indicate that regions of dilution far below mean values persist well into the mixing regime dominated by shear-flow turbulence. This indicates that both plume mixing and diffusion-type mixing models may provide a false sense of security with regard to the absolute minimum dilutions observed in actual flow situations since both methods focus on the minimum average dilution.
