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This dissertation, Global Behavior of a Round Buoyant Jet in a Counterflow by Wing-yan, Lee, 李永仁, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Global Behavior of a Round Buoyant Jet in a Counterflow Submitted by Lee Wing Yan for the degree of Master of Philosophy at the University of Hong Kong in January 2006 In this study, time-averaged global spreading patterns of a round buoyant jet discharging horizontally into a counterflow were investigated with laser-induced fluorescence (LIF) measurements in a laboratory flume. Time-averaged mean concentration fields of jet effluent were obtained from LIF on the central vertical plane of the buoyant jet for different combinations of Froude number and jet-to-current velocity ratio. The densimetric Froude number (Fr) characterizes the ratio between the initial buoyancy force on the jet effluent and the initial jet momentum. The jet-to-current velocity ratio (R) compares the initial jet exit velocity to the speed of the counterflow. Experiments were carried out at Froude numbers Fr = 3 to 10 and velocity ratios R between 2.5 and 15. The concentration fields were analyzed with a number of image processing algorithms to study the spreading and dilution pattern. Quantitative information on the flow pattern was obtained, and this including the trajectory of the jet centerline, the development of jet widths, the drop of centerline concentration and the radial concentration profiles. It was found that the jet flow can be divided in two flow regions. In the first region near the jet exit, the forward jet momentum dominated and the jet penetrated into the counterflow primarily along the horizontal direction. The jet width increased due to interaction with the counterflow. When the jet forward velocities decreased farther downstream, the effect of buoyancy became obvious and gradually increasing vertical rise of the jet was observed. At penetration point, the forward jet velocities dropped to zero and the counterflow velocity took over and advected the still buoyant jet effluent backwards. In this second region of backward flow, the jet behaved like a buoyant line thermal, being advected backwards by the counterflow. The width of the jet increased at a faster rate than in the first region, but the concentration levels were much lower. Radial profiles of mean effluent concentration at successive stations along the jet centerline were found to follow reasonably well the Gaussian distribution. The predictions of the Lagrangian model JETLAG were compared with the experimental data for this complex situation of a buoyant jet in counterflow. The Lagrangian integral model JETLAG was found to give satisfactory overall predictions of jet centerline trajectories and concentrations for most situations. However, the model tended to over-predict the horizontal and vertical locations of the penetration point. In particular, predictions in the second flow region did not agree well with data in the weak counterflow situations. This discrepancy can be traced to the break down of the boundary layer approximations (inherent in an integral jet model) near and beyond the penetration point, when the flow changes from parabolic to more elliptic in nature. JETLAG also over-predicted the centerline concentration before penetration point in strong counterflows. This may be due to simplified treatment of the jet potential core in the calculation. Length scale analysis was conducted on the jet centerline trajectories and dilution from experiments a

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