Flow Condition at Sudden Slope Change of Chutes

One of the important parts of many large dams is flood release chute spillway. Aerators are installed on chute spillways to prevent cavitation phenomenon under very high speed water flow. An aerator consists of a ramp providing a sudden slope change that separates water from the bed and permits airto mix with water. For the study of flow condition in aeration zone and for having an insight knowledge about the aeration, the study of flow condition on the ramp is a prerequisite. The change in the slope of the bed is common in spillways due to the change in the topographical slope and at the connection of the spillway and the stilling basin. In the present study, the flow condition at the place of sudden slope change was numerically simulated by FLUENT software. FLUENT is powerful software in simulation of water flow, in which the effects of turbulent are well considered. In this software, the Navier-Stokes equations are numerically solved. Volume of fluid method is applied to simulate the free surface water accurately. The results of the model in terms of pressure distribution at bed, pressure distribution in depth, velocity profile and water surface profile were compared with the results of an analytical model of streamline method as well as ith the available experimental data. The analytical model is a streamline method based on conformal mapping with the basic assumption of ideal flow. The results of the model were compared in various slopes and water velocities. The study showed that numerical model can predict water surface profile and dynamic pressure properly and the results of numerical model are more accurate as compared to analytical model for the two important parameters of pressure distribution at the bed and the velocity profile. The pressure distribution was reasonably predicted by the numerical model at the bed of channel before the ramp for all the studied degrees and at the bed on the ramp for smaller degrees. The maximum error was nearly 5-10 percent. The velocity profile calculated by the numerical model was very close to the experimental data and the maximum error was nearly 5 percent. The analytical method was found quite capable in prediction of dynamic pressure and water surface profile, and less accurate in predicting the velocity profile. The study also showed that before and after the slope change, the influence length, where the dynamic pressures are significant, is dependent on the degreeof sudden slope change and also on the amount of normal depth. Furthermore, the water surfaceprofiles calculated by the numerical model and the analytical model were quite coincident, showing that the effects of gravity, viscosity and turbulence parameters on the water surface profile are insignificant.