The behaviours of a two-dimensional and line jets in a weak cross-flow are studied in the laboratory and through the development of analytical and numerical models....[ Read more ]

The behaviours of a two-dimensional and line jets in a weak cross-flow are studied in the laboratory and through the development of analytical and numerical models.

Laser Induced Fluorescence (LIF) and Image Processing (IP) techniques are employed to visualize the flow and provide quantitative information about the jet behaviour. This information includes data for the mean properties, such as jet dilution, spread and trajectory, and turbulent properties, such as Root-Mean-Square (RMS) of tracer concentration fluctuation and intermittency (ITY) of concentration distribution.

Predictive models are developed for 2D weakly-advected jets, where it is assumed that the flow behaviour is similar to that of a jet in a still ambient fluid. Both numerical and analytical solutions are obtained. Analytical solutions require the control volume surfaces through which the jet flows to be perpendicular to the direction of the initial discharge. The numerical solution allows a control volume that is perpendicular to the excess velocity profile and it therefore rotates as the discharge is deflected. Comparisons of these two models show that in the weakly-advected region the solutions are very similar and the relatively simple analytical solution is adequate.

Comparisons between the experimental data and model predictions show that the transition from weakly-advected region to strongly-advected region happens when z/Zws=0.4. In weakly-advected region, the spread of the weakly-advected jet is significantly larger than that of a jet in a still ambient fluid. Although there appears to be a weak dependence on the strength of the ambient current, a significant increase in spread rate is evident at initial velocity ratios (ambient to discharge) as low as 0.0117. Beyond z/Zws=0.4, dilution rate increases rapidly and model predictions are no longer valid. These comparisons also show that in order to predict the location of the jet it is necessary to incorporate the impact of pressure variations across the jet. Hence, A drag term is incorporated into the model for this purpose. For a two-dimensional jet in a week cross-flow, the drag coefficient C_D has a value of 0.8.

The experiments also enable comparisons to be made between the behaviour of line jets and two-dimensional jets. These comparisons show the ambient current dominates the line jet behaviour much closer to the source, when compared to an equivalent two-dimensional jet in a cross-flow.