| Construction diversion and closure is a very complex system engineering.Success or failure of construction diversion and closure directly affects the construction safety,period and cost of a hydro-junction project.Therefore,the study on construction diversion and closure has always been a very important subject in the construction of a hydro-junction project.In the stage diversion and closure project,the discharge structures on the dam built in the early stage diversion are needed to discharge flows for later stage diversion project.The flow pattern in the discharge structures may be open channel flow,pressure flow,or transition flow.Therefore,the stage diversion and closure project can not be simulated by a 2D model.Though the 3D model can reproduce the flow field of the diversion channel and discharge structure in the stage diversion project,the scale of the discharge structures is usually much smaller than that of the river channel.Accordingly,the grid size and time step of the 3D model limited by the discharge structures will quite small,which increases the time consumption leading to unacceptable computing time.Therefore,the 3D model is not suitable for the application of practical hydraulic engineering.Besides,the boundary condition changes dramatically with the continuous advance of the berm in the river closure,which makes the hydraulic condition at the closure gap quite complex.Therefore,it is difficult to simulate the river closure,especially the unsteady river closure.The flow field of stage diversion and the dynamic river closure of unsteady flow can not be simulated by the existing numerical models.To solve this problem,a 1D-3D coupled model of construction diversion and closure,which can simulate the dynamic advancing of the berm,is established based on a coupling algorithm and dynamic boundary condition.In the model,the 3D RANS and standard k-ε equations are solved to simulate the river flow,and the 1D transition flow equations are solved to simulate the flow in the discharge structure to reduce the grid number and time-consumption.Then the 1D and 3D model are coupled by an algorithm of mass and momentum conservation.In addition,gradually advance of the berm and the dynamic change of the flow field can be realized by modifying the boundary condition dynamically.Finally,the whole flow field of stage construction diversion and closure is obtained.The model is validated by comparing the numerical results of the water level,velocity and flow rate with the experimental data,analytical solutions,MIKE solution,and Fluent solution.Some or all of the flow have been discharged through the discharge structures built in early stage diversion projects is another important problem in the stage diversion and closure project.The flow velocity in the discharge structures is quite fast,and the kinetic energy is quite large.In addition,deformation of the free surface is large,and there is broken of the free surface in the discharge structures.Therefore,it is necessary to take an appropriate method to study the downstream energy dissipation of the discharge structures to prevent cavitation,vibration and erosion damage of buildings caused by the discharge flow.However,the mesh based Euler model needs complex free surface capture scheme to accurately calculate the free surface due to the complex flow field in the discharge structure.Moreover,the mesh needs to be refined at the channel bottom or the energy dissipation area to improve the accuracy of the calculated results.All of these makes it difficult to solve the model.Besides,the efficiency of the Euler model will be lower too.Therefore,a GPU based 3D SPH model of discharge and energy dissipation is established by introducing an open boundary condition to DualSPHysics,which is an open-source code,to achieve the steady and unsteady energy dissipation investigation of the second staged diversion and closure of a real Hydro-junction Project.Due to the Lagrangian characteristics of the SPH method,the model can automatically capture the free surface without special treatment.Therefore,it is convenient to simulate the surface rolling,free surface breaking and air entrainment in the hydraulic jumps.The calculated water level and velocity of the open channel flow,dam break with partial breach,and 3D hydraulic jumps test cases are in good agreement with the experimental data,demonstrating the capability of the model in resolving inflow,outflow boundary conditions,and complex 3D flows.The maximum acceleration of the model by GPU device can reach 12.which is about equal to the theoretical maximum acceleration,relative to a parallel CPU.Then the discharge capacity of the stage diversion and the scheme of the second stage river closure of the Dateng Gorge Hydro-junction Project are investigated and compared by using the 1D-3D coupled model.The upstream flow bypasses the cofferdam and leads to the downstream from the narrow riverbed on the right bank after the construction of the first stage cofferdam through the analysis of the first stage diversion flow field,and the flow velocity and water level at the measuring point.The construction of the first stage cofferdam causes the rise of the upstream water level and the decrease of the downstream water level,and the increases of the velocity in the narrowed riverbed on the right bank,which makes the flow field more complex and increases the impact damage of the cofferdam.For the second stage river closure of the Dateng Gorge Hydro-junction Project,the calculated results show that the flow capacity of the closure gap is smaller,the diversion ratio of the discharge low hole is larger,and the maximum velocity of the closure gap is lower when the angle between the axis of the berm and the flow direction is an obtuse angle.Therefore,the axis of the berm should be designed to form a certain angle with the flow direction.The closure flow rate should be less than 2380m3/s(after November).The final drop is 2.5m,and the maximum velocity of the closure gap is 6.6m/s that is achieved when the closure gap is about 30-40m.The second stage construction diversion of Dateng Gorge Hydro-junction Project has a good capacity of flow discharge.The project can discharge 48h flood,and meet the requirement of the construction diversion.The comparative study on diversion flow field and closure scheme based on the 1D-2D coupled construction diversion and closure model can provide theoretical basis and technical guidance for the reasonable design of construction diversion and closure and ensure the construction safety for similar practical hydro-junction project.The application of GPU based SPH model in discharge and energy dissipation of the real large-scale Hydro-junction project is achieved by using the SPH model to investigate the effect of the stilling sill on the energy dissipation of the stilling basin downstream the built discharge and energy dissipation structures of the second stage diversion and closure of the Dateng Gorge Hydro-junction Project.This application provides a new solution for the study of discharge and energy dissipation of similar hydro-junction project.Comparing the velocity field,jump toe position,and energy dissipation rate of the stilling basin with and without stilling sill,the stilling sill can obviously shorten the length of high-speed flow area in stilling basin and improve the efficiency of energy dissipation.In addition,a coupled model of finite volume staged diversion and closure model with SPH discharge and energy dissipation model is established by an external coupling method.In the model,the average water level and velocity at the upstream and downstream coupling boundaries,that are used as the inflow and outflow boundary conditions of the SPH discharge and energy dissipation model,are obtained using the large-scale finite volume model.The steady flow in stilling basin downstream the discharge structure built at the first stage diversion project of the Dateng Gorge Hydro-junction project is studied using the coupled model.In addition,3D meshless simulation can be carried out on the position with complex large deformation flow,which is conducive to the reasonable simulation and analysis of 3D details of complex flow field. |
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