Numerical Simulation Study On Flow Characteristics Of Wide And Narrow River Channels In Mountain Area

There are a large number of typical wide and narrow alternate river channels in mountain areas of China,especially in southwest China,where there is much rain.The sudden relaxation and contraction of river channels cause backwater and backflow of the water flow,which makes this type of river channel present complex flow characteristics,thereby affecting its water and sediment movement and riverbed deformation.So far,there are few three-dimensional numerical simulations of the wide and narrow channel flow field.Therefore,the three-dimensional numerical simulation is helpful to understand the flow characteristics of wide and narrow channels and the distribution of shear forces on the bed surface and wall surface,which can provide a theoretical basis for flood control and river regulation in mountainous areas.In this paper,the 3D model of the channel is generalized by measuring the data of the wide and narrow interphase channels in the Xiaoqingshui River in Ya’an City.The Ansys Fluent 16.0 software is used to solve the RNG-k-?turbulence model,the VOF two-phase flow model and the Open Channel Flow model.This paper simulates the three-dimensional flow characteristics in wide and narrow channels.The main work and conclusions are as follows:(1)When the initial discharge Q=114 m~3/s,the average velocity of the three wide and narrow sections in the continuous wide narrow section decreases,which indicates that the upstream wide and narrow section will produce a water beam effect on the downstream wide and narrow section,reduce the discharge and average velocity of the downstream section,and reduce the shear force of the bed surface and the bank wall of the downstream section.(2)The simulation results show that the distribution of bed shear stress and bank wall shear stress in the backwater area and non-backwater area in the wide section of the river channel are quite different.Affected by backwater,the wide backwater area is easy to form a high shear stress distribution area on the bed surface and the bank wall.(3)The influence of channel width on flow characteristics is studied when the discharge and slope are constant.The results show that:when the inlet flow Q=114m~3/s and the slope is 10‰,the width to width ratio of the reach decreases from 2:1 to 3:2,and the proportion of backwater area in the whole wide section is reduced from 33%to 13%.It shows that the smaller the width ratio is,the smaller the backwater area is,and the corresponding backwater starting position will also move downstream.(4)In this paper,the influence of slope change on the flow characteristics of river channel is studied under the condition of medium and small discharge(Q=114 m~3/s).The results show that when the ratio of width to narrow is 2:1 and the slope is 5‰,the backwater area in the river channel accounts for 83.3%of the whole wide section area.With the increase of slope to 20‰,there is no obvious backwater phenomenon in the wide section of the river,and the backwater area in the river only accounts for 10%of the whole wide section area.This shows that the larger the slope is,the smaller the backwater area is in the wide section of the river,and the corresponding backwater starting position will also move downstream.The maximum shear force on the bed surface and the maximum shear force on the bank wall of the wide section with a slope of 20‰are 52%and 44%of the 5‰slope,respectively.The results show that in the same wide section,with the increase of river slope,the shear force of river bed surface and the shear force of river bank wall will decrease.Meanwhile,the high peak area of bed shear force concentrates to the center of backwater area of wide section of river,and the erosion of river bed and both banks is greatly weakened.(5)The influence of only the variation of river discharge on the characteristics of river flow is studied under the condition of constant slope.The results show that:when the width ratio is 2:1 and the slope is 5‰,with the increase of initial discharge from114 m~3/s to 337 m~3/s,almost the whole river is in backwater area,and the backwater depth in the river is higher than the initial water depth;when the slope is 10‰,the proportion of backwater in the wide section of the river increases from 20%to 40%.The results show that both sides of wide and narrow river channel in mountainous area are easy to be submerged by flood in low slope.With the increase of flow rate,the backwater area will also increase,and the corresponding backwater initial position will move to the upstream of the river.The maximum shear force on the bed surface and the maximum shear force on the bank wall of the wide section with discharge Q of 114.3m~3/s are 47%and 34%of the discharge of 337.5m~3/s,respectively.This shows that the increase of discharge will increase the shear stress on the bed surface and the bank wall in the same slope and narrow section,and intensify the erosion on the riverbed and both sides of the river.