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The flow at a channel bifurcation is turbulent, highly three-dimensional (3D) and has many complex features. There is transverse motion accompanying the main flow and an extensive separation zone that develops in the branch channel. There are two complex flow regions along the intake channel: one is the separation zone and the other is the region in which helical motion of water particles forms. This separation occurs because the flow entering the branch channel has considerable momentum in the direction of the main channel flow. This zone causes hydraulic and sedimentation problems that must be known before designing the system. This necessitates a deeper insight into the flow patterns and shear stress distributions near the solid boundaries. In this research, 3D flow patterns at lateral diversion were investigated experimentally and numerically. The experimental investigation was carried out at a T-junction, formed by two channels with rectangular cross-sections. The width of lateral intake to the main channel was 0.4. 3D velocity measurements were obtained using Acoustic Doppler Velocimeter at junction region for 11%, 16% and 21% discharge ratios. Fluent mathematical model was then used to investigate the dividing open-channel flow characteristics. Turbulence was modeled by Two Equation (k-ε, k-ω) and Reynolds Stress (RSM) turbulence models. The predicted flow characteristics were validated using experimental data and the proper model was selected for hydrodynamic and parametric studies. Within the main channel, good agreement was obtained between all models prediction and the experimental measurements, but within the lateral channel, the RSM predictions were in better agreement with the measured data, and k-ω predictions was better than those of k-ε. The comparison of experimental and numerical streamlines at different elevations showed that the selected model is capable to simulate the most important features of flow at diversions. The study of the velocity contours at different elevations showed that the velocity magnitude decreases at main channel, just downstream corner of lateral intake at the near bed levels and this causes the sedimentation in movable beds. The results showed that the width of separation zone at lateral intake will decrease and the distance of dividing stream surface from left bank of the main channel will increase by increasing of the discharge ratio. Investigation of the flow pattern at the entrance of the lateral intake showed that the secondary flow will form at this section. The dimension of the secondary flow at near bed elevation will increase by increasing of the discharge ratio and this causes entering of more bed load into the lateral channel.

Keywords: secondary flow, Lateral intake, Experimental investigation, Fluent CFD model, Turbulence model

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