Research On Numerical Calculation Of Flood Discharge Atomization Of High Dam Based On Water-air Two-phase Flow

Flood discharge atomization is a kind of water mist dispersion phenomenon that occurs during the flood discharge of water conservancy projects.The heavy rainfall and strong wind generated by the flood discharge atomization will cause great harm to the normal operation of the hydropower station,slope stability,traffic safety,etc.The high dam project in China has the characteristics of high head,large flow,narrow river valley and high slope,etc.Many indicators are in the forefront of the world,and the problem of atomization associated with flood discharge is especially prominent,which makes the safety protection of flood discharge atomization more difficult.The research on of flood discharge atomization,the construction of accurate prediction model under complex flood discharge environment,the quantitative analysis of the influence of various factors on flood discharge atomization,and the exploration of the water and air migration law in the process of flood discharge are of great engineering practical needs and scientific theoretical value for promoting the research of flood discharge atomization from experience to science and ensuring the construction and long-term safety operation of major water conservancy and hydropower projects.Compared with the method of prototype observation,physical model test,theoretical analysis and calculation,the numerical simulation method has the advantages of economic efficiency,freedom from model similarity law,and low dependence on prototype observation data.In this paper,based on the theory of water-air two-phase flow,the numerical simulation method is used to study the flood discharge atomization of high dams.The related research work mainly focuses on the mathematical description,parameter definition,numerical solution and water-air movement laws of the flood discharge atomization.Based on the theory of water-air two-phase flow,a mathematical model describing the water-air movement in the process of flood discharge atomization is developed.The numerical solution method and technology of the mathematical model of flood discharge atomization are studied,and the corresponding finite element calculation program is compiled.According to the needs of numerical simulation,the dynamic viscosity model of water air two-phase mixture considering the influence of mixing degree is proposed The mathematical model is verified by the atomization prototype observation data,and the influence of Valley width,initial discharge velocity and downstream water depth on flood discharge atomization is analyzed quantitatively.The main research work and content of this paper are as follows:1)Based on the theory of water-air two-phase flow,a mathematical model describing the process of flood discharge atomization movement is developed.The model is composed of the mass and momentum conservation equation of water air mixture fluid,the momentum conservation equation of gas phase and the mass conservation equation of water.The fluid pressure solution equation can be derived through the mass conservation equation of the mixed fluid,which realizes the direct solution of the pressure and improves the stability and convergence during the numerical solution process.The momentum conservation equation describing the motion of the gas phase contains the influence of the force between the water and air,which can describe the difference of water air motion.The concentration transfer equation is derived from the mass conservation equation of water,and the rainfall intensity in the atomization process can be solved by combining the fog rain conversion formula.The construction of the mathematical model of flood discharge atomization makes it possible to numerically simulate the large-scale flood discharge atomization process.2)The finite element method is used to study the solution method and technology of the flood discharge atomization mathematical model.The numerical solution method of large-scale highly nonlinear partial differential equations is developed.The problem of numerical stability and convergence in the process of numerical simulation of the flood discharge atomization is solved,and the numerical simulation of flood discharge atomization process is realized.In the numerical solution of flood discharge atomization,the finite element method is used to discretize the partial differential equations in the mathematical model of flood discharge atomization,and the large eddy simulation(LES)method is used to deal with the turbulence in the flood discharge atomization.In order to ensure the stability and convergence of the numerical solution,a proper pressure-velocity coupling solution strategy is sought,which avoids the simplification of traditional commercial software for solving difficult problems.Furthermore,a three-dimensional finite element calculation program is written,and the correctness and effectiveness of the program are examined.3)In this paper,the research on the dynamic viscosity of water-air mixture fluid is carried out,and a test method for testing the dynamic viscosity of water-air mixture fluid is developed.The dynamic viscosity of water-air mixture fluid under different air volume and mixing degree is tested,and then the dynamic viscosity model of water-air mixture fluid including air volume and mixing degree is deduced.By means of combining physical test and numerical simulation,the viscous force of the object moving in the mixed fluid is tested through the test,and the viscous force of the object under different viscous conditions under the same moving state is simulated through the numerical simulation.The correlation between the test and numerical simulation is established,and then the test of the dynamic viscosity of the water-air mixture fluid is realized.Based on the analysis of the force of the object in the water-air mixture fluid,the dynamic viscosity model of the water-air mixture fluid considering the air content and the mixing degree is deduced,and then the influence of the air content and the mixing degree on the dynamic viscosity of the water air mixture is analyzed.4)The mathematical model of flood discharge atomization based on water-air two-phase flow is adopted to realize the three-dimensional numerical simulation of flood discharge atomization of Shuibuya Hydropower Station.Relying on the monitoring data of flood discharge atomization of Shuibuya Hydropower Station,the calculation results of the model were examined and verified to be correct.The influence of the discharge and gate combination mode on flood discharge atomization of the hydropower station is studied.According to the comparative analysis of the flood discharge atomization monitoring data and the model calculation results,the numerical simulation method has a good predictive ability for the wind speed and rainfall intensity during the flood discharge atomization process,among which the prediction deviation of wind speed is within ± 15%,and the prediction deviation of rainfall intensity is within ± 20%.The distribution of the wind field and rain field during the process of the flood discharge atomization of Shuibuya Hydropower Station with different discharge and gate combination is predicted,and the laws of water and air movement during the process of flood discharge atomization are analyzed.The wind speed and rainfall intensity during the flooding and atomization process of Shuibuya Hydropower Station both increase continuously with the increase of the discharge flow.When the discharge is small,the relationship is basically linear.When the discharge flow is increased to a certain value,the growth rate of the maximum atomization wind speed is slowing down,and the maximum rain intensity is increasing exponentially.At the same time,the influence range of strong wind and rainfall is increasing rapidly.5)Using the mathematical model of flooding and atomization based on water-air two-phase flow,the water and air migration law of flood discharge atomization of Shuibuya Hydropower Station was studied.The results show that the movement of water air is obviously restricted by the topography.From the perspective of near-surface water air movement,it is easy to form "backflow" phenomenon in locally blocked areas.From the perspective of water and air movement at higher elevations,the water and air at the different elevation has both downward and upward movement areas.When the elevation reaches a certain height,the water and air movement direction is downward,indicates that the water mist will not rise to this elevation.According to the distribution of velocity and pressure in the process of water and air movement,the flood discharge atomization is divided into three areas,namely: water air mixing area,water mist generation area and water mist diffusion area.In the water air mixing area,the pressure in the water body is less than the external atmospheric pressure,and the air in the atmosphere is continuously mixed into the water body to form the water air mixing fluid;when the air mixing water falls into the downstream river,the water body pressure increases rapidly,a large number of bubbles in the water body escape,and the bubbles break to produce many tiny droplets,forming the water mist;the escaping air in the water body continuously moves towards the high altitude and the downstream river direction,and "wraps" the droplet movement,thus forming a common atomization phenomenon.6)Aiming at the way of energy dissipation in the spillway,the effects of valley width,initial discharge velocity and downstream water depth on the temporal and spatial distribution of atomization wind speed and rainfall intensity are studied by threedimensional finite element numerical simulation.The results show that the width of the river valley has a significant effect on the atomization wind speed.The narrower the river valley,the wider the vertical propagation of the atomization,and the greater atomization wind speed near the dam.In this study,when the width of the river valley increases five times,the atomization wind speed drop can reach more than half when the flood discharge is stable.The influence of initial discharge velocity on atomization wind and atomization rainfall is different.When the initial discharge velocity is greater than 5 m/s,the further increase of velocity will not cause the significant increase of atomization wind velocity in the core area of the flood discharge,but the range affected by atomization wind will increase significantly;the intensity of atomization rainfall will increase with the increase of initial discharge velocity,which is basically exponential function.The increase of the downstream water depth is beneficial to reduce the atomization wind speed of flood discharge.In this study,the atomization wind speed decreases about 0.2 m/s when the downstream water depth increases by 1 m.The downstream water depth has a greater influence on the pressure of the channel floor at the point where the water tongue falls.The deeper the water depth,the smaller the pressure increase of the channel floor caused by flood discharge.When the downstream water depth is suitable,the increment of bottom plate pressure caused by flood discharge is zero.