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Spatial crest overflow
Published in Willi H. Hager, Anton J. Schleiss, Robert M. Boes, Michael Pfister, Hydraulic Engineering of Dams, 2020
Willi H. Hager, Anton J. Schleiss, Robert M. Boes, Michael Pfister
A siphon is a ducted overflow structure with either free-surface or pressurized flow. Siphons are used as spillway either in parallel or in addition to other flood releases, or as intake of small power plants, as described by Xian-Huan (1989). In the following, the latter are excluded. Siphons either correspond to a saddle siphon or a shaft siphon (Figure 3.64). Under increasing discharge, both behave hydraulically similar to a weir. At a certain discharge, priming occurs so that the flow becomes pressurized under higher discharges. The transition between free-surface and pressurized flows depends mainly on the aeration and de-aeration features of the siphon crest.
Energy Equation
Published in Ahlam I. Shalaby, Fluid Mechanics for Civil and Environmental Engineers, 2018
Applications of the Bernoulli equation for ideal flow from a tank (or a water source) include siphon flows, which are a result of gravity flow from an open tank with a siphon. A siphon is a small-diameter hose that is commonly used to transfer liquid from one tank to a second tank at a lower elevation without the use of a pump, as illustrated in Figure 4.16. In order for the siphon to work properly, there are several criteria that must be met. First, one end of the siphon is inserted into the tank full of fluid, and the free end of the siphon (point 3) must be at an elevation that is lower than the level of the fluid in the full tank (point 1) and may be inserted into a second tank at a lower elevation. Second, in order to siphon the fluid from the full tank into the second tank, a suction in the siphon must be established/initiated. One may initiate a suction in the siphon by temporarily suctioning (for instance, by sucking on the free end of the tube) the free end of the siphon tube. As a result, the pressure difference created between the atmospheric pressure at point 1 and the negative pressure (vacuum) at point 2 causes the fluid to flow from the tank into the siphon tube, and then finally out of the siphon tube into the second tank. And, third, in order to maintain the flow in the siphon, the elevation of tubing at point 2 must be high enough above point 1 to maintain the established negative pressure (vacuum) at point 2. However, if the elevation of point 2 is too high above point 1, the pressure at point 2 (suction end of the siphon) is decreased to the liquid's vapor pressure, and cavitation will occur. Cavitation is unintended boiling of a liquid in motion at room temperature. Furthermore, when evaporation of the liquid occurs during cavitation, the vapor may cause a pocket to form in the suction line of the siphon and thus stop the flow of the liquid. Therefore, in the design of a siphon, there is a maximum allowable height for point 2 in the tubing in order to avoid a pressure drop below the vapor pressure of the liquid and thus avoid cavitation and restriction of flow. As such, determination of the maximum allowable height for point 2 assumes that the pressure at point 2 is set equal to the vapor pressure of the liquid in the tank (see Example Problem 4.17).
Laboratory modeling of siphon drainage combined with surcharge loading consolidation for soft ground treatment
Published in Marine Georesources & Geotechnology, 2018
Hongyue Sun, Gang Wu, Xu Liang, Xin Yan, Dongfei Wang, Zhenlei Wei
The aforementioned methods have disadvantages including energy cost, complex operation, and being environmentally unfriendly. The siphon phenomenon is caused by liquid molecular gravity and potential energy difference, so it can be used to achieve water-free power and efficient transportation. Previous researchers have applied siphon drainage technology in slope engineering and achieved efficient drainage effects (Cambiaghi and Schuster 1989; Govi 1989; Bertolini, Guida, and Pizziolo 2005; Bomont 2008; Gillarduzzi 2008; Mrvik and Bomont 2010; Cai et al. 2014). The siphon drainage method is economical, highly efficient, power free and has a convenient operation. Siphon drainage combined with surcharge loading is potentially a good alternative to drain water from soft clay economically and environmentally with a vertically installed PVD and siphon tube. The proposed method has both a relationship and a difference with well point dewatering method and sand-drained consolidation method. The groundwater table can be controlled at a certain depth below the surface by reasonable arrangement of siphon tube spacing, while the groundwater table controlled by sand-drained method is at the soil surface. It has been shown in the previous literature (Cai et al. 2014) that the siphon lift is about 10 m. The groundwater table can be controlled at 8–10 m depth below the soil surface. And about 10 m thickness of unsaturated zone can be formed by siphon drainage. Unsaturated zone can produce rapid compression deformation and reduce the hydrostatic pressure of the soil below unsaturated zone. The buoyant unit weight over the flow profile is changed to natural unit weight, which procedures a surcharge loading effect on the soil below the flow profile. Compared with siphon drainage, well point dewatering consolidation has high energy costs and needs manual real-time maintenance. Due to the low permeability of soft ground (about 10−7 cm/s), water seepage into the well is a very slow process in soft soil area, and the water in the well may be pumped out for a few minutes by well point dewatering method. However, siphon drainage can work continually without manual real-time maintenance when the preliminary work is completed well. And the arrangement of well point is not as intensive as that of siphon tube. In addition, siphon is energy-free, so less postoperation and maintenance are needed for siphon drainage-combined surcharge loading.