Integral models proposed to simulate positively buoyant jets are used to model jets with negative or reversing buoyancy issuing into a calm, homogeneous or density-stratified environment. On the basis of the self-similarity assumption, ‘top hat’ and Gaussian cross-sectional distributions are employed for concentration and velocity. The entrainment coefficient is considered to vary with the local Richardson number, between the asymptotic values for simple jets and plumes, estimated from earlier experiments in positively buoyant jets. Top-hat and Gaussian distribution models are employed in a wide range of experimental data on negatively buoyant jets, issuing vertically or at an angle into a calm homogeneous ambient, and on jets with reversing buoyancy, discharging into a calm, density-stratified fluid. It is found that geometrical characteristics such as the terminal (steady state) height of rise, the spreading elevation in stratified ambient and the distance to the point of impingement are considerably underestimated, resulting in lower dilution rates at the point of impingement, especially when the Gaussian formulation is applied. Reduction of the entrainment coefficient in the jet-like flow regime improves model predictions, indicating that the negative buoyancy reduces the entrainment in momentum-driven, negatively buoyant jets.

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Experiments on concentration measurements in round turbulent negatively buoyant jets discharging into a calm homogeneous fluid at three different angles are presented. Detailed measurements using a micro-scale conductivity probe were carried out at the horizontal location of the terminal rise height, mostly along the vertical of the axial jet plane and in the transverse direction. The dimensionless vertical distribution of the mean concentration is found to be asymmetric, whereas the transverse distribution is approximately Gaussian. At the same horizontal location, the vertical distribution of the concentration turbulent intensity has a maximum at a point systematically higher than the jet axis. The width of the mean concentration distribution, the heights to the location of the maximum mean concentration and to the maximum turbulent intensity as well as the minimum dilution are determined. From measurements in the region where the jet returns to the source elevation, the minimum dilution and the horizontal distance to the return point are estimated.

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