| Phased diversion is a commonly used diversion method in the construction of large-scale water conservancy and hydropower projects.In order to ensure that hydraulic buildings can be constructed on dry land,it is necessary to build cofferdams on the river channel in stages during the diversion period to enclose the construction of the foundation pit and direct the river water to the narrow river channel outside the foundation pit.After the construction of the cofferdam,the water passage space of the river channel is narrow,and the flow pattern near the cofferdam is more complicated,which may cause erosion at the foot of the cofferdam slope and adversely affect flooding and navigation.In order to reveal the complex flow patterns in the narrow section of the cofferdam,this paper conducts a three-dimensional numerical simulation study on the water flow motion in the narrow section of the cofferdam.Firstly,the basic theories and methods of Delft3D and Fluent software,which are commonly used in 3D water flow simulation,are introduced,and then the Standard k-ε model in Delft3D and the Standard k-ε model,RNG k-ε model and Realizable k-ε model in Fluent are used to simulate the water flow in the narrow channel of the cofferdam.The applicability of the above models in the numerical simulation of 3D water flow in the narrow channel of the staged diversion weir was evaluated based on the measured data from the flume test.The main findings are as follows:(1)For the simulation of the 3D water flow in the narrow channel of the weir bundle,the RNG k-ε model in Fluent has higher accuracy than the other three models.(2)It is found that the flow within the contracted channel near the inlet exhibits strong three dimensionality,with a significant water stage difference observed along longitudinal and horizontal directions.Part of the flow is deflected to the right bank and rapidly swept towards the channel bottom owing to the cofferdam,which leads to the occurrence of lateral flow circulation at the inlet of the contracted channel.Flow separation occurs at the outlet of the contracted channel and recirculating flow is generated within the channel side close to the cofferdam in the channel downstream.The size of the recirculating flow region near the bottom of the flume is smaller than that near the water surface.The distribution of turbulence kinetic energy is mainly concentrated in four regions,including the in-channel side adjacent to the cofferdam,the bottom of the contracted channel,the downstream recirculating flow region,and the area near the interface between the mainstream and recirculating flow region in the channel downstream.(3)A higher channel contraction ratio appears to have several effects on flow dynamics,such as a larger water-level difference along longitudinal and horizontal directions,a greater flow deflection angle and an increased flow circulation at the contracted channel inlet,an extended recirculating flow region downstream,as well as a higher intensity and larger size of turbulence kinetic energy along the channel.Therefore,in order to mitigate the impact of the narrowed riverbed on the flow motion,it is recommended that the contraction ratio of the cofferdam should be controlled within 60%.Additionally,protective measures should be implemented at the positions of the cofferdam corner and the contracted channel,where flow patterns change rapidly.The findings of this study offer valuable insights into cofferdam configuration with a stage-dependent diversion scheme. |
