Numerical Investigation of the Flow Pattern in the Bottom Outlet of Seymareh Dam

Bottom outlets as the related structures play very important role in receiving water from reservoir and delivering it to downstream of dam. They involve with some undesirable phenomenon during operation period such as local vortices, intensive pressure fluctuation, cavitation, and control gate vibrations which consequently lead to reduce the coefficient of flow discharge and to harm outlet’s wall and control gates. It is necessary to appropriately understand the flow pattern in the bottom outlets, in order to identify and to decrease harmful effects. Seymareh dam is a concrete double-arch dams where located at 40 Km of Darreh Shahr, Ilam, Iran. The dam has two bottom outlets in body (No.1 and No. 2) in which the entrance levels are respectively 620 and 640 m above the sea level and 20 and 40 m above the river bed. The entrance of bottom outlets’ dimension is 17.85 * 9.56 m (height *width) and the emergence and service gate are respectively slider and radial. In the present research, pattern of flow has been investigated numerically by using FLOW 3D software, in No. 1 bottom outlet. This outlet has designed for maximum flow rate (654 m3/s) per 111.5 upstream water head and has 45.4 m length, so that on 100% service gate opening, the length of pipe flow is 36.5 m and free flow is 8.9 m. To simulate of flow’s hydraulic in FLOW 3D software, first the geometry of model has been prepared at Auto cad 3D in real size, then was called with stl format in FLOW3D software. Intended fluid in simulation is incompressible and single – phase and has been used K-ε (RNG) turbulence model because it has additional terms in k and ε transport equations. Also, time of the analysis was considered 30s. Out flow, wall and specified pressure condition respectively were used as boundary condition of outflow and walls of conduit and entrance condition which is supply reservoir head. In order to select the upstream reservoir dimensions (width, length) in the numerical model, the model has been performed with various dimensions on maximum head (111.5) and by checking of velocity profiles at different sections in conduit, 30*29.53 m were selected as reservoir dimension. The laboratorial results obtained from relevant physical model applied for verification of the numerical model performance. To verify of numerical model, the results of average pressure and flow discharge were used. For the parameter of average pressure per 30, 70 percent service gate opening and for the discharge parameter per: 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 percent gate opening in the normal head (100 meters), were validated. Good agreement with the results of numerical model and laboratory values indicate high ability of model to simulate flow field. The research results indicate that the flow velocity is gradually increasing on the flow direction while the flow pressure is reversely reducing. The flow velocity reaches to maximum 45.6 m/s and the flow pressure decreases to minimum -4.1 kPa when the bottom operates at normal head and complete service gate opening condition. The mean velocity and wall pressure between immediately upstream and downstream of the gate is sharply changed for small opening of gate. The maximum mean velocity of flow equal to 39.97 m/s (at 30% gate opening), and minimum mean wall pressure equal to 21.3 Pa (at 10% gate opening) are seen in the vicinity of service gate. For H /D < 1.2, the pipe flow in the bottom outlet changes to free surface flow condition. As result in the vertical sections, the uniform velocity and hydrostatic pressure distribution are observed approximately. The maximum velocity of flow equals to 8.4 m/s and the minimum wall pressure equals to 17.2 kPa. Also, the water surface profile varies rapidly due to expansion occurred on the flow section downstream of gate. By checking of longitudinal and lateral flow surface are observed longitudinal surface flow per smaller opening service gate, at the end of conduit is increasing because of located energy dissipation block. By increasing of gate opening, the effect of energy dissipation block decreased and flow surface profiles after passing the gate, take downward trend and per smaller gate opening lateral surface profiles have downward arc state. With increasing of gate opening, the process has changed and become to upward arc.