| For the low-head pivot project,it has the characteristics of large kinetic energy,large flow,low Froude number,complicated flow pattern,and low energy dissipation efficiency,and it is easy to cause erosion and damage to downstream riverbeds and buildings.Therefore,the accurate prediction of the flow field information of the downstream riverbed,the range of the scouring pit and the depth of the scouring pit,has a guiding role in the safety design of the downstream structure of the flood discharge sluice.Our research group Cheng Ye is based on the dynamic grid technology in the Fluent software,combined with the secondary development of the user-defined function(UDF),and adopts the bed load sediment transport rate formula based on the randomness of water flow and bed sediment distribution.According to the principle of sediment transport balance,The riverbed deformation equation is programmed into UDF using C language programming,and the starting shear stress is used as the criterion for sediment scour and starting transportation to reflect the elevation change of the bed surface grid center point,which is controlled by the UDF macro of the moving grid The movement of the corner points of the boundary grid of the movable bed.Because the dynamic mesh UDF macro controls the elevation change of the bed surface grid corner points,and the river bed deformation equation calculates the river bed grid center point elevation change,this is also the case when Cheng Ye's grid deformation method predicts the bed surface elevation.The cause of the error.Therefore,a new dynamic mesh deformation method is constructed,which converts the elevation change of the bed surface grid center point obtained by the river bed deformation equation calculation into the corresponding grid corner elevation change,and controls the bed surface grid through the dynamic grid DEFINE?GRID?MOTION()macro The movement of the corner points can more accurately predict the deformation of the grid.Based on the ANSYS FLUENT software platform,combined with the new mesh deformation method,the downstream three-dimensional flow field and the local river bed scouring process of the Dangka Hydropower Station are simulated,and the following conclusions are obtained.(1)Using the RNG k-?model,combined with the SIMPLE algorithm,simulations have obtained the discharge flow of Dangka Hydropower Station,the distribution law along the water surface line,and the vertical velocity distribution of the section,which are in good agreement with the test results.(2)A new grid construction method is used to simulate the scouring process of the downstream riverbed.The simulation results of this paper under the condition of a flow rate of 345.36m~3/s and a particle size of 100mm are improved compared with the simulation results of Cheng Ye of this research group.Among them,the depth of the scouring pit The accuracy of the scouring pit has been increased from 6%to 2%,and the accuracy of the length of the scouring pit has been increased from 10.14%to 2.74%;The simulation results of the scouring pit shape are closer to the measured results of the physical model,and the position of the maximum scouring pit depth has also been improved.Basically the same;the simulation results under the three working conditions of flow rate 345.36m~3/s,400.48m~3/s,sediment particle size 100mm,and 50mm are compared with the hydraulic model test data.The relative error range of scouring pit depth is between 1.79%?2.91%,the relative error range of the scouring pit length is between 2.38%and 5.06%,which is in good agreement with the measured results.(3)Analyzed the changes of the scouring pit shape in different periods under the conditions of a flow rate of 345.36m~3/s and a particle size of 100mm:the shape of the scouring pit in the downstream riverbed changes at any time.In the initial stage of the scouring,the range and depth of the scouring pit develop the fastest;In the middle of the erosion,the rate of change of the depth of the erosion pit decreases,and the range of the erosion pit continues to expand downstream;when the limit erosion equilibrium state is reached,the range and depth of the erosion pit no longer change.(4)Under the conditions of flow rate 345.36m~3/s and particle size of 100mm,the change law of bed surface shear stress in different periods is analyzed:bed surface shear stress increases first and then decreases along the process in different periods;bed surface shear stress affects the shape of scour pits The main factor of the erosion is that the bed surface shear stress is relatively large at the beginning of the erosion,and the range and depth of the erosion pits increase rapidly at this stage;in the middle of erosion,the bed surface shear stress gradually decreases,the rate of change of the erosion pit depth decreases,and the erosion pit range continues to expand downstream,But the expansion speed is reduced;the peak bed shear stress in the ultimate scouring equilibrium stage is 1.1993 Pa,which is less than the flat slope sediment initial shear stress 1.3124 Pa.The sediment in the entire downstream riverbed no longer starts to move,and the scouring pits are not in shape.Change again.(5)Analyzed the changes of turbulent kinetic energy and turbulent energy dissipation rate in different periods under the conditions of a flow rate of 345.36m~3/s and a particle size of 100mm:with the development of the range and depth of the downstream riverbed scour pit,the turbulent kinetic energy and turbulent energy dissipation The rate gradually decreases.In the initial stage of scouring,the turbulent kinetic energy and turbulent energy dissipation rate in the scouring area of the downstream riverbed is relatively large;with the development of the scouring pit shape,the turbulent kinetic energy and turbulent energy dissipation rate gradually decrease;after reaching the limit scouring equilibrium stage,the scouring pit Both the turbulent kinetic energy and the turbulent energy dissipation rate within the range no longer change.(6)Analyzed the influence of different discharge flow rates and different bed sand particle size conditions on the range and depth of the scour pits:under the same discharge flow and different particle sizes,the range and depth of the scour pits in the downstream riverbed vary with the bed surface sediment size.The increase shows a decreasing trend;under the same particle size and different discharge flow conditions,the range and depth of the scouring pits in the downstream riverbed increase with the increase of the discharge flow. |
PDF Full Text Request