Volume 18, Issue 3 (2018)                   MCEJ 2018, 18(3): 1-11 | Back to browse issues page

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Oftad S, ershadi C. Effects of side walls and Froude number on flow field over ogee spillway in axial arc condition. MCEJ 2018; 18 (3) :1-11
URL: http://mcej.modares.ac.ir/article-16-20174-en.html
1- Department of civil Engineering, Azad University, Bandarabad
Abstract:   (7925 Views)
Weirs are one of the common structures for regulating and measuring of flow. Also, this structures with different forms such as normal weir, side weir and ogee spillway are widely used for various purposes. In some cases, due to practical restrictions the ogee spillways with curvature in plan are designed. An ogee spillway is located at the top of the reservoir of dam. Dams may also have bottom outlets with valves or gates which may be operated to release flood, and a few dams lack overflow spillways and rely entirely on bottom outlets. In such situations study of flow distribution over the ogee spillway and other related parameters, might be important. In this study, the flow field over thw ogee spillways in axial arc condition was simulated by FLOW-3D software. In the FLOW-3D model, the Navier-Stokes and continuity equations were discretized using the finite difference method. The computational domain was divided into a mesh of rectangular cells. All variables (except for velocity values) were placed at the center of the computational cells (staggered grid arrangement). To solve the governing equations, control volumes were defined around each dependent variable. The surface fluxes, body forces and surface stresses were computed in terms of surrounding variables. Most terms in the governing equations were explicitly evaluated. To solve the flow field of a non-compressible fluid, the continuity and the Navier-Stocks equations were solved. On the other hand, to validate the numerical results, the experimental measurements that were performed in Soil Conservation and Watershed Management Research Institute at reservoir with dimensions 1.4 m length, 0.30 m width and 0.18 m height. The experimental model was made of plexiglas plates which was a model of prototype at the scale of 1:75. Moreover, to measure the flow discharge, a sharp triangular weir with apex angle of 90˚ in the output of channel was used. The flow field turbulence was modeled using the standard and the RNG turbulence model. According to numerical model results, the RNG turbulence model had more accuracy than the standard turbulence model. Also, variations of flow free surface were reconstructed by volume of fluid (VOF) scheme. In this numerical model, the effects of the side symmetrical walls of ogee spillway were examined for cases 60o, 90o and 120o in discharges 34, 34 and 22.6 lit/s, respectively. The applied boundary conditions were chosen according to the physical model. Therefore, the depth and discharge specific values were for inlet boundary condition. At the outlet boundary of the model, the outlet boundary condition was used. All the solid walls of the model were defined as the “Wall” boundary conditions. Also, a symmetry plane was determined at the top layer of the computation field. According to numerical results, the acceptable agreement was obtained between numerical results and experimental measurements. For example, the relative error percent of longitudinal profiles of flow free surface were calculated 12.83, 13.60 and 3.48 percentage for cases 120o, 90o and 60o, respectively. Also with increasing angle of axial arc, the height of rooster tail increased significantly. In addition, by increasing Froude number, the height of rooster tail reduced.
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Article Type: Original Manuscript | Subject: Earthquake
Received: 2017/05/11 | Accepted: 2017/12/30 | Published: 2018/09/15

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