Numerical Simulation And Experiment Study On Flood Discharge And Energy Dissipation Of High Dam With Large Discharge

Flood discharge and energy dissipation are important parts of hydropower projects. When dam and reservoir are built, the river must discharge normal flood as it did. Flood discharge is the aim, energy dissipation is the means by which to obtain the former. The main means of flood discharge and energy dissipation of hydropower projects are outlets through dam body, near bank spillways and tunnels. Among traditional energy dissipation types, jet energy dissipatior has many advantages such as less investment and simple structure, so it is the most widely used one in the hydropower projects. The main characteristics of many big or great hydropower projects are high water head, large flood discharge with narrow river valleys. Flood discharge and energy dissipation are critical technological problems in the projects design. In order to meet the need of flood discharge and energy dissipation of these projects, many of them use multiple-jet flows type, water plunge pool is built downstream behind the dam for energy dissipation. Multiple-jet flows make the water disperse along crosswise and lengthways to lessen jet water volume into the water plunge pool per unit surface, which will lessen the impact pressure onthe bed of water plunge pool to protect the bed.Numerical simulation of jet flow and tunnel with free surface are difficult in computational fluid dynamics( CFD) due to their water-air two phases flow problem. Although there are some research achievements about impinging jet flows of the water plunge pool and aeration for lessening cavitation destroy of tunnel with high speed flow at present, many of them were obtained by experiment way, especially for the latter. The reason caused this condition is that the water-air two phase flow problem is very complicated itself, meanwhile, boundaries of jet flow in air are unknown in advance, it is difficult to make certain its boundaries. In this paper, the author simulates the jet flow and tunnel with high velocity and free water surface, combining with physical model experiment studies base on the review and summarization of existing research achievements concerned at present. The main contents and conclusions of this paper are as follows:1. The fractional volume of fluid (VOF) model of the water-air two phase flow is introduced to the 3-D K— ￡ double equations turbulent model, combined with geometry reconstruction scheme to simulate free water surface. The unstructured grid is adopted to treat the irregular boundaries. By these methods, turbulence flow field of multiple-jet flows from upstream to downstream and No.3 tunnel with free water surface of Xiluodu hydropower station which is only second to the Three Gorges hydropower station in China are simulated successfully. No simulation achievements like this have been reported in the world at present.2. The hydraulic characteristics of the water plunge pool under the conditions of 2-D single jet and 3-D multiple jets are simulated. The distributions of their velocity, pressure, turbulence kinetic energy and turbulence dissipation rate are obtained. The simulation results show, under some conditions, the impact pressure on the bed of the water plunge pool can be controlled to the same level as that caused by the small jet flow though the multiple jet water volume is several times bigger than the small one due to "moving water cushion" effect. Pressure on the bed of the water plunge pool under the water-air two phases flow and single phase (water) conditions is simulated respectively, which shows the former is 15 percent less than the latter. Thenumerical simulation results are test by the physical model experiments. They are identical well.3. Methods of grid adaptation itself and simulated result transplant and interpolation are used to make more grid in the key simulated regions concerned. Which will improve simulation accuracy and efficiency. This method has not been report in China at present.4. 3-D numerical simulation and experiment studies are done to the hydraulic characteristics of the anti-arch water plunge pool. By physical model experiments, The distributions of turbulence kinetic energy, mean pressure and impulse pressure on upside and downside surface of arch circle, pressure-time curve and its probability density are obtained. The results show that in the view of hydraulic, because of the is relative smooth combination of the bed and side walls of the anti-arch water plunge pool, under same flood discharge volume, anti-arch water plunge pool can support bigger impact pressure, which means it is not easy to be destroyed and is more safe relatively. The study indicates that the distributions of the pressure probability density on the upside and downside surface are almost normal distribution. The study results supply the hydraulic theoretical proofs for projects to use anti-arch water plunge pool this new type energy dissipation5. No.3 tunnel with aeration facilities and free water surface of Xiluodu hydropower station is simulated by VOF method from reservoir upstream to its outlet downstream. Air space height, length of aeration region and its height, wind volume of aeration well, water height distribution of the outlet of the runnel have been obtained, the results of simulation are identical with that of the experiments. The simulation indicates, complicated flow pattern of jet flow and 3-D tunnel with aeration facilities and free water surface can be simulated successfully by the combination of the K — e double equations turbulent model, VOF method, structured and unstructured grid. The simulation supplies a new measure and method for the study on the flood discharge and energy dissipation of high dam projects with large discharge.The studies of this paper indicate, water-air two phases flow numericalsimulation can be a effective method to study the water-air two phases flow problems of hydropower projects.