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Regarding significant reduction in costs and operating problems, three-sided spillway in comparison with other spillways, attracts crucial attention of designers of these structures. Three-side spillways are a type of outlet works at dams that despite their hydraulic limitations and construction problems, under specific topographical conditions selected as one of the best options in storage dams. This spillway is applicable in areas concerned with limitation of available space for overall width of spillway and where excess volume for flood overload. Also when modification and capacity increase in existing spillways are necessary, this structure is recommended. On the other hand, inappropriate conditions in water channel, such as flow turbulence and impact of water on bed and lateral walls of channel result in poor performance of these structures. In the present study, firstly 3D flow pattern of a U-shaped spillway, the channel and the end sill have been evaluated using computational fluid dynamics software (FLOW-3D). RNG k-ɛ model was implemented for simulation of turbulence. Comparison of numerical results with experimental data showed that this model has a good ability to predict three dimensional flow patterns over this kind of spillways. Hydraulic performance with targeting to reduce pressure fluctuations in side channel is an important issue in this type of spillways design. Regarding important effect of air entrance in hydraulic structures, two-phase analysis has been performed in this study. Numerical results show that two-phase analyses have a better performance compared to one-phase simulations. Studies show that by changing the inlet flow rate, the maximum error in the estimation of water level and pressure profiles at bottom of the channel occurred at low discharges. Also the maximum numerical error in computing observed in the area where bulge is. Then, taking into account the actual dimensions of the model, scale effects have been studied on physical model scales. The findings have some major implications of civil, environmental and sanitary engineering, because most hydraulic structures, storm water systems and water treatment facilities operate with Reynolds numbers within ranging from 106 to over 108. In a physical model, the flow conditions are said to be similar to those in the prototype flow conditions if the model displays similarity of form, similarity of motion and similarity of forces. The present results demonstrated quantitatively that the dynamic similarity of two-phase flows cannot be achieved with a Froude similarity unless working at full-scale. So that physical models are not good at predicting air entrainment and the amount of air entering is dependent on Reynolds number and does not follow Froude similarity. The largest amount of free surface profile variation due to aforementioned reason has been observed in air entrance and bulge formation zones. This variation decreases as flow moves toward downstream or as discharge value increases.

Keywords: Flow 3D, numerical simulation, Three-sided spillway, two-phase flow, Scale effect, Flow 3D

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