Laser-induced fluorescence measurements of a turbulent plume

The laser-induced fluorescence (LIF) measurement technique is discussed in the context of environmental fluid mechanics. The measurement equipment and procedures employed in our laboratory are described in detail. The technique is applied to an isokinetic chemical plume (neutrally buoyant) in a turbulent open channel flow. A nonuniform laser sweep rate is employed to take full advantage of the dynamic range of the digital camera over the entire spatially varying concentration field. Statistical measures of the concentration field, such as the average and variance, are presented and discussed. Comparisons are made with theoretical models and previous experimental observations. The plume width grows at a greater rate in the downstream direction than that predicted by the analytical model of a point source release into a uniform flow. Probability density function distributions do not resemble Gaussian distributions, which reflects the highly intermittent nature of the concentration time record. The current measurements suggest that LIF is a valuable technique for nonintrusively recording the scalar field evolution in turbulent flows.