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UBC Theses and Dissertations

Radial spreading of vertical buoyant jets in shallow water MacLatchy, Michael R


Many wastewaters are discharged from single vertical outlets located at the bottom of shallow water bodies. Often these effluents are buoyant with respect to the receiving environment. In many jurisdictions, regulations require that a certain dilution is achieved within a specified mixing zone. In shallow water it is likely that a portion of this dilution will have to be achieved in the radially spreading surface region of the jet. The degree of dilution obtained in the radial surface buoyant jet region will depend upon the buoyancy and velocity of the effluent, and the depth of water available. Prior to the installation of an outfall, the dilution that will be achieved and behaviour of the flow must be modeled in developing the design to meet regulatory and other requirements. Unfortunately, radially spreading flows are not clearly understood, and their behaviour is the subject of some controversy. This study was intended to identify the details of the structure of the radially spreading upper layer associated with the discharge of a vertical buoyant jet in shallow water. Collection and analysis of detailed numerical data was not the focus or intent of this study, flow visualization was relied upon to investigate the radially spreading surface flow. Experiments to study the mechanisms of the radially spreading surface flow were conducted with a series of vertical buoyant jets discharged into a shallow tank. This experimental tank was specially designed to simulate an infinite ambient, and avoid downstream control effects. A range of flow rates and port diameters were utilized to determine the nature of the flow structure in the surface region. Flow visualization was used to examine the flow and obtain an understanding of the mechanisms present to aid in modeling. Four regions of flow are present when a buoyant jet is discharged vertically in shallow water. The first of these is the vertical buoyant jet region, in which the vertical jet is entraining fluid as the flow moves upward to the surface. Next is a surface impingement region, where the vertical flow arrives at the surface, and is redirected to an outward, radially spreading surface flow. The first of two surface regions is the radial buoyant jet region, in which the momentum of the fluid dominates. This study determined that in this region the surface flow exists as a series of billows or vortices into which ambient fluid is entrained as they grow and move radially outward. Eventually this flow will have spread out, and entrained enough fluid, that the interfacial shear will decrease to the point where buoyancy will dominate. The large scale billows will collapse, to be replaced by smaller less vigorous interfacial instabilities, as the second surface region, the radial buoyant plume region, is entered. Contrary to the assumptions made by some researchers, no radial internal hydraulic jumps were detected in the radially spreading surface flow regions.

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