This paper discusses experiments and a theoretical model for the convective circulation driven by a surface buoyancy flux in a horizontal layer of fluid. The layer is closed at one end and, at the other end, the buoyancy has a fixed value over a given depth. Such circulation occurs in side arms of cooling lakes used for waste-heat disposal from power generation. Some geophysical circulations, such as in the Red Sea, are also of the above type.The experiments were done in a 35 ft long flume using heat transfer between the heated water and the atmosphere to generate the surface buoyancy flux. The observed circulation was characterized by two distinct layers flowing in opposite directions and separated by a density interface. For upper-layer depths less than about half the total depth at the open end, the downflow was observed to be concentrated near the closed end. Circulation flowrates and vertical temperature profiles were measured.The theoretical model uses a ‘two-layer’ approach. The mass, momentum, and buoyancy conservation equations are integrated vertically on each side of the interface. Mass and buoyancy transfer across the interface are neglected. The interfacial shear stress is proportional to the square of the difference of the average layer velocities. For small layer densimetric Froude numbers, the free surface is shown to be approximately horizontal and the problem reduces to one ordinary differential equation for the thickness of the upper layer. General solutions of this interface equation are presented for horizontal and sloping bottoms. Different configurations are possible depending on the nature of the singular points which occur in the phase plan.For the convective circulation, the interface is shown to go through a singular point. This condition leads to a simple analytical solution for the circulation flowrate in the case of constant surface buoyancy flux and horizontal bottom. This solution compares well with the experimental data and with measurements on the Red Sea circulation.
The Deep Tunnel Sewerage System (DTSS) is aimed at providing a robust and efficient means of catering to Singapore's used-water needs. DTSS2 is the second phase of this project, comprising an approximately 30-km long South Tunnel, a 10-km long Industrial Tunnel, 60-km of Link Sewers and a new Water Reclamation Plant integrated with a NEWater facility. In contrast with tunnels designed to store overflows in combined sewer systems, the DTSS tunnels convey used-water all the time from Singapore's separated system. This paper describes hydraulic analyses that were conducted during the feasibility study and preliminary design. The topics covered include hydraulic modelling of the entire system with the main goal of ensuring system resilience, air management to avoid odours at ground level, and isolation of tunnel section using gates for potential maintenance or repair. The resilience analyses concentrated on the system functionality in case of a failure, to ensure that used-water can be safely conveyed to a treatment plant. The air management system included several odour control facilities and air jumpers to avoid escape of odorous air from the system and the isolation gates requires detailed hydraulic analyses to cater to the high heads involved.
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