| Construction diversion can be divided into dry season diversion and flood season diversion according to the diversion time.Because diversion process involves the diversity of diversion buildings and the complexity of construction features,it is difficult to accurately describe the complex hydraulic characteristics of diversion flow field and its influence on buildings and rivers by specific theoretical methods or experimental methods.In this paper,based on the diversion project of Nyerere hydropower station,the hydraulic characteristics of the diversion tunnel in dry season and the combined diversion tunnel and dam reserve gap in flood season are studied by means of physical model test and numerical simulation.The main research results are as follows:(1)It is found through the dry period diversion test that the flow capacity of the diversion tunnel can meet the design requirements,the flow state of the tunnel body is good,and the torsional surface on the left side of the outlet connects with the body shape,which makes the outlet scouring range larger and the energy dissipation problem more serious.The flow capacity of the diversion tunnel is modified to meet the design requirements.When there is pressure flow,the flow coefficient is greater than the original body shape.After the outlet adopts double torsional surface to connect the body shape,the energy dissipation effect can meet the engineering requirements.Based on flow-3D software,RNG k-εturbulence model and scour model were used to simulate the 1:80 model of the energy dissipation zone at the outlet of the modified diversion tunnel.It was proved that the numerical simulation can well reduce the Flow field and scour of the experiment,and the scour trend can be predicted quickly and well by simulating the Flow velocity at the bottom of fixed bed.(2)In order to understand the flood diversion in the different periods,first of all,the first and second flood control model test of two gap sin elevation,indicate two flood diversion tunnel and reserved gap joint flood control dam discharge capacity meet the design requirements,and analyzes the characteristics of flow field in hydraulic,scour,two gaps can satisfy the requirements of flood control,flood season for project points out the key protection area and protective measures.Secondly,in order to analyze the reliability of numerical simulation of combined flood control of diversion buildings based on a large range of accurate terrain and the influence of diversion tunnel roughness on hydraulic parameters,the model(converted to prototype roughness n_p=0.019)and prototype(roughness n=0.015)simulation of diversion tunnels with different roughness participating in flood control were carried out.Compared with the experimental results,the consistency of the flow field between the numerical simulation and the model test is fully verified.However,the calculation results of the flow flow of the prototype diversion tunnel in the first and second floods are about 10%larger than the experimental values,indicating that the 1:80 scale model test diversion tunnel is made of plexiglass and the test results should be modified.At the same time,it is found that the distribution law of roof pressure in outlet section of diversion tunnel is consistent in both flood season and flood season,which decreases first and then increases.The minimum position of roof pressure in outlet section is unchanged,while the negative pressure occurs in outlet section of diversion tunnel in flood season,but its cavitation number is greater than the critical cavitation number.The upper cofferdam of the first flood and the top of the downstream gap of the second flood show large negative pressure,and the cavitation number of the second flood gap is less than the critical cavitation number,which may lead to cavitation erosion risk.Finally,the range of isoline larger than the anti-scour velocity is highly consistent with the experimental scour range,and it is feasible to evaluate the scour range and scour intensity of the riverbed by means of flood discharge greater than the anti-scour velocity range and shear stress.(3)According to the above method,the partially modified flood control scheme can be predicted and simulated by numerical simulation.The results show that the position of the gap is biased to one side,the width decreases,and the elevation increases,which causes the upstream water level to rise.The diverging ratio of diversion tunnel increases,which increases the cavitation erosion risk and the possibility of vertical vortex.Collapsible flow is easy to occur in the stilling pool,which is unfavorable to the energy dissipation and anti-scour of the bed at the end of the pool,especially in the absence of cofferdam and berm downstream.Through the calculation,the possible unfavorable parts in the project are found out,and the protective measures are put forward to reduce the loss of the project and ensure the safety of the flood. |
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