New Findings of Flow Velocity and bed Shear Stress at an Open Channel Junction

Flow velocity and bed shear stress distributions are important in open channel junctions. In this study, subcritical flow at a 90º four-branch open channel junction was simulated using Flow-3D. In order to validate the modeling, the numerical results were compared with the experimental results which has been conducted by corresponding author. Comparison of the experimental and numerical models for flow depth showed a good agreement so that the RMSE for the main and tributary channels was less than 0.016 m and 0.029 m, respectively. The results showed, with increase of inlet flow ratio, the flow velocity and bed shear stress increased at the junction area. At the middle of the junction, along with tributary channel, for \({\text{Q}}_{\text{yi}}/{\text{Q}}_{\text{xi}}=1\) ${\text{Q}}_{\text{yi}}/{\text{Q}}_{\text{xi}}=1$ , the maximum u (velocity in x direction), w (velocity in z direction) and v (velocity in y direction) were about 0.2 m/s, -0.0175 m/s and 0.194 m/s, respectively, which happened for c/b = 0.377. Along the tributary channel, for c/b = 0.377, \({Q}_{yi}/{Q}_{xi}=1\) $Q_{\mathit{yi}}/Q_{\mathit{xi}}=1$ , the maximum and minimum values of the bed shear stress were around 0.47 N/m² and 0.04 N/m², respectively. Secondary currents had more effects on the flow velocity and bed shear stress at the middle and after the junction. Also, increase of the height ratio of the outlet weirs, led to decrease of the flow velocity and bed shear stress. So, along the main outlet channel, after the junction, changing \(c/b\) $c/b$ , from 0.377 to 0.632, the maximum bed shear stress decreased about 55%. The intensity of the secondary and eddy currents was proportional to collision of the inflows. Rotation of the flow into the outlet channels caused recirculation flow and formation of the separation zone, which its dimension affected by inlet flow ratio and height ratio of the outlet weirs.