Large Eddy Simulation And Experiment Study Of Hydraulic Characteristics In A Confluent Meander Bend Channel

River confluences are complex hydrodynamic environments in fluvial rivers.Flow structures in a confluent meander bend channel have great effect on sediment transport and bank erosion,and such studies are essential for river engineering and environment projects.In this paper,physical model test combined with large eddy simulation are carried out for the study of hydraulic characteristics at confluent meander bend channel.Analysis is focused on the influence of the longitudinal and transverse slope of the water surface along the main bend channel,velocity distribution,helical structures,velocity circulation,turbulence kinetic energy,characteristics of separation zone and shear stress near the riverbed.The results are showed below:(1)Junction angle and discharge ratios have great effect on longitudinal slope of water surface at the area of cross-section ?=60° to ?=120°.High junction angle and discharge ratios correspond to a low elevation of longitudinal water surface at confluent meander bend.There is a downturn in the average longitudinal gradientat upstream of the junction,but upward at downstream(cross-section ? 100° to ?= 120°)of the junction.(2)The water surface increases along the radial direction and the amplitude increases with junction angle and discharge ratios near the inner bank.A high junction angle correspond to a high water surface along the radial direction near the outer bank at upstream(?=60°)of the junction.The water surface decrease along the radial direction and the amplitude of water surface increases with junction angle and discharge ratios at junction cross-section near the outer bank.At downstream(?=150°)of the junction,the water surface increase along the radial direction and the amplitude of water surface decrease with junction angle,but incerase with discharge ratios.The average surface transverse gradient is positive resulting in the water surface increase along the radial direction.A high discharge ratio corresponds to a high water surface from cross-section ?=30° to ??90°.But a high discharge ratio corresponds to a low water surface sloop from cross-section ?= 100° to ?=150°.(3)The existence of the separation zone of the downstream is determined by the size of the junction angle.Junction angle and discharge ratios nearly have little effect on velocity distribution at the upstream of the junction.Large junction angle and discharge ratios correspond to a high streamwise velocity near the inner bank,but large junction angle and discharge ratios correspond to a low streamwise velocity near the inner bank.Large junction angle and discharge ratios correspond to a high transverse velocity at downstream of the junction and induce more helical structures near the outer bank.The direction of the transvers velocity is changed with high discharge ratio.The largest mean streamwise velocity move form outer bank to inner bank with junction angle increased.The position of flow dynamic axis is changed and induce the second flow dynamic axis at downstream of the junction with increase of junction angle and discharge ratio increased.(4)The basics of turbulence research suggest that the shear layer is a three-dimensional structure:a shear layer consisting of an inner layer(rough surface)and an outer layer(smooth surface).The turbulent structure in the shear layer is related to turbulence intensities.Thus,the inner layer is rough,which is influenced by the maximum value of turbulence velocity.The high TKE moves from outer bank to inner bank with increasing discharge ratio and junction angle at downstream of the junction.A high junction angle and discharge ratios correspond to a large area of high TKE near the outer bank.A high discharge ratio result in low TKE,but a high junction angle corresponds to a high TKE near the inner bank.(5)Discharge ratio and junction angle has little effect on the size of helical flow at upstream cross-sections of the junction.At downstream cross-sections and the junction cross-section,a high discharge ratio and junction angle corresponds to large size of helical flow near the outer bank at the downstream of the junction.A large junction angle and discharge ratios correspond to small size of helical flow near the inner bank.Transverse slope at downstream cross-sections of the junction is determined by the size and position of secondary flow.A high discharge ratio and junction angle corresponds to high strong velocity circulation along the river and reach its maxmum at junction(p=90° cross-section.(6)The length and width of a separation zone increase with discharge ratio and junction angle.However,the width of a separation zone gradually trends to constant when the junction angle increases from 60° to 90°.For three discharge ratios and three junction angles,the dimensionless results show that the field of the separation zone has similar shape.Using the polynomial fitting method,the equations of relative width and relative length for separation zone are obtained firstly.(7)The high range of bed shear stress move from outer bank to inner bank with increase of discharge ratio at downstream of the junction.When the discharge ratio and junction angle reach them maximum,a high bed shear stress appears near the both outer and inner bank at the cross-section of the outlet of bend.