2 results
Experiments on vertical plane buoyant jets in shallow water
- Jannis Andreopoulos, Ananda Praturi, Wolfgang Rodi
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- Journal:
- Journal of Fluid Mechanics / Volume 168 / July 1986
- Published online by Cambridge University Press:
- 21 April 2006, pp. 305-336
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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.
A stereoscopic visual study of coherent structures in turbulent shear flow
- Ananda K. Praturi, Robert S. Brodkey
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- Journal:
- Journal of Fluid Mechanics / Volume 89 / Issue 2 / 28 November 1978
- Published online by Cambridge University Press:
- 19 April 2006, pp. 251-272
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A visual study of a turbulent boundary-layer flow was conducted by photographing the motions of small tracer particles using a stereoscopic medium-speed camera system moving with the flow. In some experiments, dye injection at the leading edge of the flat plate helped to delineate the outer edge of the boundary layer. The technique allowed the three-dimensional aspects of the flow to be studied in some detail, and in particular allowed axial vortex motions in the wall region to be identified.
The flow was found to exhibit three characteristic regions which can be roughly divided into the wall and outer regions of the boundary layer and an irrotational region, unmarked by dye, outside the instantaneous edge of the boundary layer. Briefly, the outer region of the boundary layer was dominated by transverse vortex motions that formed as a result of an interaction between low-speed and high-speed (sweep) fluid elements in that region. The present results clearly show that bulges in the edge of the boundary layer are associated with transverse vortex motions. In addition, the transverse vortex motions appear to induce massive inflows of fluid from the irrotational region deep into the outer region of the boundary layer. The outer edge of the boundary layer thus becomes further contorted, contributing to the intermittency of the region. Furthermore, the outer-region motions give rise to the conditions necessary for the dominant wall-region activity of ejections and axial vortex motions. It is not the energetic wall-region ejections that move to the outer region and give rise to the contorted edge of the boundary layer as has been suggested by others.
The wall-region axial vortex motions were intense and lasted for a time short compared with the lifetime of outer-region transverse vortex motions. The present results strongly suggest that wall-region vortex motions are a result of interaction between the incoming higher-speed fluid from the outer region of the boundary layer and the outflowing low-speed wall-region fluid. This is in direct contrast to all models that suggest that axial vortex pairs in the wall region are the factor that gives rise to the outflow of low-speed fluid trapped between.
Although all the elements necessary to make up a horseshoe vortex structure riding along the wall were present, such a composite was not observed. However, this could be visualized as a possible model to represent the ensemble average of the flow.
Finally, the massive inflows from the irrotational region were observed to precede the appearance of low- and high-speed fluid elements in the boundary layer, thus completing the deterministic cycle of individual coherent events.