مقایسه الگوی جریان زیربحرانی‌ و فوق بحرانی در کانال‌‍‌‍‍ٍ‌های مثلثی دارای سرریزجانبی

Triangular channel is widely used in irrigation and drainage systems. Also, the triangular channels with alongside weirs are used to control and measure the flow in irrigation and drainage networks and floodwater spreading projects. In this study, the pattern of the spatially-varied flow with decreasing discharge was investigated and compared in subcritical and supercritical flow regimes for triangular channels alongside weirs. Flow pattern in triangular channels alongside weirs is numerically simulated using the FLOW-3D software for both flow regimes. Also, variations of the free surface flow over alongside weirs are modeled by the volume of fluid (VOF) scheme. Furthermore, the turbulence of the flow field in triangular channels alongside weirs is numerically simulated using the RNG k-? turbulence model. In the numerical study by these models, the specified amounts of discharge and flow depth are used for the "inlet" boundary condition of the triangular channel. Also, the specified amounts of the flow depth and the pressure are used in the main channel outlet boundary condition and all of the solid walls are defined as the "wall" boundary conditions. Then, the top layer of the air phase is considered as the "symmetry" boundary condition. The whole computational domain is gridded by a non-uniform mesh block consisted of rectangular elements. The distance from the first cell-wall was chosen so that the calculations below the viscous layer are eliminated. For this purpose, the first cell is located where the dimensionless parameter y+ is greater than 30. The results obtained from the numerical simulation in this study were verified by comparing them to the experimental measurements provided by Uyumaz (1992). A good agreement was obtained between the results of the numerical simulation and the experimental measurements for both flow regimes. The RMSE of the simulated water surface profile for the subcritical and supercritical flow regime was calculated 11.46% and 7.90%, respectively. Also, for free surface flow, the average percent relative error for both flow conditions were computed 10.8% and 6.87%, respectively. The specific energy was measured in the beginning of the side weir which was equal to 0.1661m and 0.2116m in subcritical and supercritical flow condition, respectively. The numerical model predicted the specific energy equal to 0.1565m and 0.2283m for both flow regimes. The specific energy relative error percent was 5.78% and 7.89% which showed suitable accuracy of the numerical model in predicting the specific energy. Generally, the flow depth increases from the upstream end towards the downstream end of the weir in subcritical conditions and decreases from the upstream end towards the downstream end of the side weir in supercritical conditions. A drop in the free surface in the first third of the side weir span and a surface jump in the final third of its length occurs. After this drop, the flow depth increases towards the downstream end of the side weir in subcritical flow regime, while in the supercritical flow condition the water depth gradually decreases towards the downstream end of the weir. The surface jump with a stagnation point occurs at the downstream end of the side weir for both flow regimes. Along the mentioned surface jump the amount of the kinetic energy increases and the potential energy decreases. Also, the water elevation is the highest at the stagnation point location. According to the simulation results, the maximum longitudinal velocity for subcritical regime occurred at the first third of the side weir length and for supercritical flow, almost in the middle of the side weir span. In both subcritical and supercritical flow regimes, the maximum transverse velocity occurred at the last third of the side weir length. According to the numerical simulation, the transverse velocity before the side weir was predicted negligible. In the vicinity of the side weir crest, the transverse velocity increased by flowing toward the span of the side weir. As the flow passed from the plane of the side weir, the amount of the transverse velocity decreased. The angle of the spilling jet was close to 90° at the upstream and downstream of the side weir in both flow regimes. The minimum angle of the spilling jet for subcritical and supercritical flow regimes occurred at the last third and the middle of the side weir length, respectively.