LIF Study of Plane Jet Bounded in Shallow Water Layer

Rivers, estuaries, and coastal waters represent shallow water layers because the water depths are normally much smaller than the horizontal scales. Pollutant and waste heat disposal often takes the form of a plane jet discharging into such shallow water layers, which is denoted herein as a shallow jet. To predict their mixing characteristics is important for environmental protection. In this experimental study, the field distributions of instantaneous concentration have been measured with a laser-induced fluorescence (LIF) system developed as an attractive and convenient technique for the measurement of concentration fields of both gases and liquids. The instantaneous concentration field illustrates the overall meandering of the plane jet and the role of large-scale turbulent structures in the mixing process. Field distributions of mean concentration and RMS of concentration fluctuation have been obtained by averaging hundreds of digital image frames. The following major results can be concluded from the analysis of these images: (1) All concentration properties of the shallow jet are in scale with the water depth h rather than the slot width B, as is typical for the unbounded planar jet (this result is in agreement with the earlier results of Giger et al. on the velocity field); (2) the concentration half-width grows linearly but faster than for the unbounded case; and (3) the turbulent properties show a lack of self-preservation with increasing distance, a high degree of fluctuation greatly exceeding that of the unbounded case, and a spectral behavior indicative of two-dimensional turbulence.