| Water flow characteristics and influencing factors of bifurcated channels play an important role in the design of open channel water and sand transport and irrigation channel system.However,there are some problems in bifurcated channels,such as water flow structure disorder,local erosion,sediment deposition and large head loss.This research takes the channel in the Hetao irrigation area of Inner Mongolia as the research object.A technical way combining laboratory channel experiment and numerical simulation is used to study the effects of different water diversion angles,lateral channel bed height,channel width ratio and splitting ratio on the hydrodynamic characteristics of open channel branch area,such as water level,three-dimensional velocity,circulation intensity,turbulent kinetic energy,turbulent burst,Reynolds shear stress,flow dynamic axis and water diversion width.The characteristics of bed surface scour and siltation under different bed sand types(silty loam,sandy loam,sandy soil)were defined,the flow structure,flow turbulence characteristics and influencing factors in the open channel branching area were determined,and the rules of bed surface scour and siltation in the open channel branching area were revealed.The main results were as follows:(1)The variation law of open channel branch water level is expounded,and the influence of diversion ratio,diversion angle,side channel bottom height,channel width ratio and other factors on the variation of open channel branch water level under the condition of inflow from different main channels is clarified.When the main channel has a constant flow rate,the effect law of vertical and horizontal water level change is similar for the branch area.The amplitude of the longitudinal water level fluctuation of the main channel is inversely proportional to the discharge ratio,and the larger the discharge ratio,the smaller the amplitude of the water level fluctuation and the less obvious the discharge and siltation phenomenon.When the water diversion angle is greater than 60°,the water level of the side channel fluctuates greatly and gradually increases with the increase of the water diversion angle.The fluctuation range of the water level in the main channel is inversely proportional to the bottom height of the side channel and reaches the maximum when the bottom height of the side channel is zero.When the channel width ratio is greater than 0.5,the water level in the side channel fluctuates greatly,and the water flow structure at the entrance of the side channel is relatively disordered,which is not conducive to the migration of sediment particles in the side channel.The maximum relative water distribution width was located near the bottom of the channel,and the relative water distribution width of each flow layer gradually decreased with the increase of water distribution angle,channel bottom height and channel width ratio,and the diversion volume of the side channel also decreased.(2)The distribution law of flow velocity at the bifurcation outlet of the main channel was studied,and the influences of branching ratio,branching angle,side channel bottom height,channel width ratio and other factors on the flow structure and turbulence characteristics of the bifurcation outlet were clarified.The vertical distribution of velocity is mostly logarithmic.The area of greater circulation intensity appeared near the lower position of the portal,and the velocity distribution was disordered,resulting in the transport of more sediment particles on the bed surface.The turbulent energy in the lower portion of the portal region is the greatest,and the area of higher value occurs mostly within 0.2 times the width of the channel.When the shunt ratio is less than 0.3,the turbulent activity of the jet and down sweep occurs more frequently.When the shunt ratio is greater than 0.3,the turbulent activity in the near-bottom region gradually evolves from jet and downs weep events to inside-out interactions.The Reynolds shear stress in the facade near the lower position of the branch area is greater than in other directions,the vertical momentum transfer is greater,and local scour is easy to occur in this area.When the water separation angle is less than 30°,it is easy for siltation to occur near the lower position of the branch area;when the water separation angle is between 30° and 45°,it is easy for scour to occur near the upper position of the branch area;when the water separation angle is greater than 60°,the phenomenon of backwater and siltation is obvious.When the water separation angle is 45°,the deflection of the dynamic axis of the bifurcation is greatest and the effect of the main channel is better.The region of turbulent energy in the lower water body increases as the water separation angle increases,resulting in a decrease in the water transport capacity of the side channel and obvious local scouring and silting.When the bottom height of the side channel is zero,the maximum amount of sediment enters the side channel.When the bottom height of the side channel is not zero,the deflection of the dynamic axis of the flow is large and has little change,and the amount of silt entering the side channel is correspondingly small.When the bottom of the side channel is higher than 2 cm,the kinetic energy of the turbulence at the bifurcation point decreases with the increase of the bottom of the side channel,and local siltation is easily formed.The flow difference between the outlet and both sides decreases as the channel width ratio increases,flow turbulence is weakened,sediment particles are prone to deposition,and concave bank walls are heavily stressed,causing serious structural damage.(3)The distribution law of velocity at the entrance area of the side channel is clarified,and the influence of different water distribution angles,side channel bottom height,channel width ratio and other factors on the flow characteristics at the entrance area of the side channel is clarified.When the water separation angle is greater than 60°,the lateral and vertical velocity of the side channel changes greatly along the course,and the velocity direction changes,resulting in obvious reflux at the inlet area.The width of the return zone of each flow layer increases as the water separation angle increases.The width of the return zone near the bottom is small,and the maximum return zone width appears in the middle or near the surface of the flow.The lateral flow velocity difference increases with the increase of the side channel bottom height,and the maximum return zone width is proportional to the side channel bottom height,and most of them are near the flow surface.The velocity difference between the two sides of the side channel increases with the increase of the channel width ratio,and the probability of the return area also increases.When the channel width ratio is greater than 0.3 for high sediment flow,the side channel sediment deposition is severe.The impact force of the water flow near the branching point on the side wall of the channel increases with the increase of the water separation angle,and the phenomenon of erosion and breakage is easy to occur.(4)The influence of sediment particles on the turbulence characteristics of the branching outlet of the main channel is clarified,and the rules of moving bed washing and siltation in the branching area of the open channel are revealed.The vertical velocity of the main channel is affected by the deposition of sediment at the bottom of the main channel,while the lateral velocity and vertical velocity are concentrated in the area near the branch area.When sand is deposited at the bottom of the main channel,the turbulence strength of the water flow is weakened at the outlet and the turbulence dynamics become uniform.Bed surface sediment particles affect the near-bottom flow turbulent burst activity greater than the bleeder,and the inner and outer interaction frequency is significantly restricted from the side of the bleeder channel door,and water flows up and down is higher frequency.Sand ridges in the form of tongues,chains and crescents appear on the bed surface of the acceleration area near the inlet bank of the main channel and the undercurrent acceleration area.The diffusion deceleration area near the central axis has obvious straight sand ridges.When sand is placed at the bottom of the channel,the effect of water separation and sediment discharge in the main channel is less affected by sediment particles.When silty loam was used,the water level was raised significantly and the water transported by side channels was low.When sandy loam is used,the drop in the bed surface is large and a scour pit is easily formed near the branch area,reducing the effect of side channel diversion. |
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