Two-dimensional and line jets in a weak cross-flow

Abstract

The behavior of two-dimensional (2D) and line jets in a weak cross-flow are studied in the laboratory. A laser-induced fluorescence system is employed to visualize the flows and provide quantitative information about their behavior. This information includes data for the mean properties, such as jet dilution, spread and trajectory, and turbulent properties, such as root-mean-square and intermittency of tracer concentration fluctuations. A predictive model is developed for 2D weakly advectedjets, where it is assumed that the flow behavior is similar to that of a jet in a still ambient fluid. Comparisons between the experimental data and model predictions show the location of the transition from weakly advected to strongly adverted behavior. In the weakly advected region, the spread of the 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 0.012. Beyond the transition to strongly advected behavior, the dilution rate increases rapidly and model predictions are no longer valid. These comparisons also confirm 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 weak cross-flow, the drag coefficient C D has a value of 0.8. Additional experiments on line jets show that an equivalent unconfined slot discharge responds more readily to the presence of an ambient current.