| Under the action of bend circulation and water sediment exchange,the natural bend channel forms a compound bend channel.The flow is affected by many factors and presents a complex three-dimensional character.In the dry season,the flow mainly flows in the main channel of the bend,and its flow characteristics only have the characteristics of the flow in the bend;in the flood season,the overflow increases to form the flood plain flow,and its flow characteristics not only have the characteristics of the flow in the bend,but also have significant changes in the flow structure due to the influence of the flow exchange in the channel.Because of its special boundary conditions,the flow structure of curved compound channel becomes more complex,and its research is of great significance to the safety of flood control engineering and the evolution and management of natural river channel.On the basis of previous studies,this paper takes the existing bending compound channel test as the verification basis,RNG k-? turbulence model is used to calculate the flow characteristics of curved compound channel under different cross-section forms,and the characteristics of velocity distribution and secondary flow of curved compound channel under different flood plain flow conditions are analyzed,and analyze the influence of the slope coefficient,water depth and beach width of the main channel on the flow characteristics,and compare the flow characteristics with the straight compound channel.The main conclusions are as follows:(1)Under the condition of floodplain,the distribution of transverse water surface profile presents a concave curve,and the concave degree decreases with the increase of water depth,and increases with the increase of slope coefficient of main channel or the decrease of beach width.(2)Under the condition of floodplain,the secondary flow structure produced by bend channel is changed by the flow action of shoal channel.The size and direction of secondary flow vortex generated by adjacent upstream and downstream curved top sections are the same and the direction is opposite,and the longitudinal velocity contour of main channel is spiral and opposite;the secondary flow intensity at the junction of beach and trough increases with the increase of slope coefficient of main channel,and increases slowly with the decrease of beach width(3)From the vertical average velocity,turbulence intensity and turbulent kinetic energy distribution,it can be seen that the flow exchange between the main channel area and the beach area has little influence on the flow in the main channel area and the beach area,but has a greater impact on the flow at the junction of the beach and the channel(4)The formation mechanism of secondary flow in straight compound channel is different from that in curved compound channel.In the former,shear layer is formed at the junction of channel and beach due to turbulence and mixing between water bodies;in the latter,vertical vortex is generated in both main channel and beach due to the comprehensive effect of centrifugal inertia force and flow exchange between channel and channel(5)The shear force in straight compound channel is caused by the velocity difference between the main channel and the beach and the momentum exchange at the junction of the channel and the channel;the shear force in the curved compound channel is caused by the interaction of the two layers of water due to the interaction of the centrifugal inertia force and the momentum exchange between the beach channel and the main channel.In addition,the shear stress difference of the main channel decreases slightly with the increase of the slope coefficient of the main channel or the decrease of the beach width(6)Compared with the straight compound channel,the flow in curved compound channel is affected by centrifugal inertia force,and the turbulence intensity along water depth distribution is greater than that in straight compound channel.With the increase of slope coefficient of main channel,the mixing effect of beach channel flow is enhanced,and the turbulence intensity at the junction of beach and channel increases. |
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