Weakly Advected Jets in Cross-Flow

We present the results of an experimental investigation into the behavior of weakly advected jets in a cross-flow. Concentration profiles are measured using a laser-induced-fluorescence system. The data show that weakly advected jets can be modeled effectively assuming the profiles are Gaussian and self-similar. A simple advection model is developed based on the assumption that weakly advected jets behave in essentially the same way as jets in a still environment but that they are in addition advected by the ambient current. Concentration profiles of still and weakly-advected jets are shown to be similar. However, in the weakly advected jet experiments there are profile distortions near the edges of the flow and some detachment of tracer from the main body of the jet. Comparisons of spread, dilution, and trajectory data with predictions from the simple advection model show that the behavior of the discharge resembles a still jet and that it is the average velocity that locates the weakly advected jets (not the previously suggested centerline velocity). This indicates the significant role that large-scale turbulent structures play in determining the mean flow behavior of weakly advected jets. A comparison of flucturation statistics shows that the strength and intermittency of the concentration fluctuations in still and weakly advected jets are similar in the central regions of these flows.