Merging buoyant jets in stationary and flowing ambient fluids
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The behaviour of an array of merging buoyant jets has been investigated. Time averaged properties and temporal properties, including the concentration fluctuations and the intermittency, were measured using planar laser-induced fluorescence. A flapping motion in the trajectory of the merged flow region was shown to be caused by the relative position of large scale vortices on either of the flow boundaries. The effect of the intermittent flapping events on the flow properties was examined, and the flapping was shown to cause the greater spread rate of plane buoyant jets as compared to round buoyant jets. The experimental results were used to modify and verify a numerical model that predicted the behaviour of a single row of either vertical or horizontal merging buoyant jets. The efficiencies of different wastewater diffuser configurations were examined, and a single row of horizontal ports was recommended as the most efficient design. The effect of an ambient fluid velocity, flowing perpendicular to the array of merging buoyant jets, has also been examined. In this situation, the merged buoyant jets were either weakly deflected and the plume-like behaviour was preserved, or strongly deflected such that they had a horizontal trajectory. The transition between the two regimes was determined by the cross-flow number. For a limited range of the cross-flow number, the behaviour of the merging buoyant jets was shown to be largely independent of the array configuration and mostly determined by the cross-flow number. Where the horizontal buoyant jets were discharged into a co-flow, the dilution growth with depth of the strongly deflected merging buoyant jets was greater than that of a single buoyant jet. Experimental findings were used to develop theoretical models that were successful in describing the behaviour of a single horizontal buoyant jet in a co-flowing current and merging horizontal buoyant jets in a weak co-flowing current. The ambient velocity was shown to be additive to the entrainment velocity field, and this lead to the derivation of a new term in the momentum equations. A pressure force term, with a coefficient determined from experiments, was introduced to account for the effect of the pressure difference either side of the merged buoyant jets caused by flow separation in the ambient fluid.