Experiments on vertical plane buoyant jets in shallow water

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Abstract
The paper reports on measurements of the flow generated by a plane buoyant jet discharging vertically into shallow water. The study comprises visualization experiments, mean-velocity and turbulence measurements with a two-channel laser-Doppler anemometer and temperature measurements with thermistor probes. According to the previous investigation of Jirka & Harleman (1979) (JH) the flow may be either stable with the heated discharge water leaving the near field in a warm water layer adjacent to the surface, or unstable with flow recirculation and re-entrainment of heated water into the jet. The stable situation usually involves an internal hydraulic jump associated with a roller. Both stable and unstable situations were investigated, the limiting case of a non-buoyant jet representing the unstable one. In order that a roller representing an internal hydraulic jump developed in the relatively short test channel in the buoyant situations, a strong downstream control had to be imposed by inserting a slightly submerged weir. Most experiments were carried out at a depth-to-discharge-width ratio of 100, and in this case the strong upstream control caused the hydraulic jump to be flooded for both of the densimetric Froude numbers studied (F = 9.9 and 21). In each case, a thick upper layer of nearly uniform temperature developed, with a larger layer thickness for F = 21. Below this layer was a relatively thin interface with temperature gradients and below this a counterflow of cold ambient water. For both Froude numbers, the flow was stable in the sense of JH, but only marginally so in the higher-Froude-number case. The observed trends of the flow behaviour follow the stability analysis of JH, but the dilution of the heated water, which was determined from the temperature measurements, is different from that predicted by the JH mixing analysis. The dilution is much lower in the present case with the flooded jump than in the JH analysis and experiments without specific downstream control and with a much longer test channel and thus no flooded jump.

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