Effect of Separating Wall Geometry on Flow Pattern of a Large-Scaled 90° Open-Channel Junction

Abstract: Open-channel junctions are common structures in hydraulic and environmental engineering. The interaction between the main and branch flows, causes the main flow to be diverted toward the opposite bank and create a separation streamline at the downstream corner of the junction. Formation of a 3D separation zone with lower pressure immediately near the branch-side bank is one of the most distinctive characteristics of a flow in open-channel junctions. The recirculating flow in this low-pressure zone not only prepares a suitable space for sedimentation, but also increases the velocity near the opposite bank of the main channel and its bed and thereby leads to local erosion. Reducing the sedimentation in recirculation zone and eliminating the erosion near the opposite wall of the channel are two major concerns of designers of these structures. Open-channel junctions have a broad application in civil and environmental engineering. Formation of a low-pressure zone with recirculating flow (high sedimentation) accompanied by a high-velocity zone (high erosion) are the most characteristic features of flow in junctions. Numerous experimental, numerical and analytical researches have been conducted to explore the flow structure in the junctions and investigate on methods for reducing the effects of sedimentation and erosion on channels. A large number of researches have been performed to grasp the complicated concept of hydraulic phenomena and correct the related deficiencies in junction of open-channel flows. Implementing a separating wall in the middle of the branch channel is thoroughly investigated in this research to understand its effect on the flow pattern and its contribution in reducing the sedimentation and erosion potential in an open-channel junction. Initially, the numerical two-phase model of a previous experimental study, is prepared and its results are validated. Following that, a one-phase model is prepared based on the fact that the difference between maximum and mean elevation of water free surface is less than 10%. According to the negligible (less than 3%) discrepancy between numerical one-phase and two-phase results in predicting the maximum U* after the junction in channel, acceptable similarity between U* contours and U* variation along the flow depth in numerical (one-phase) and experimental studies and considerable increase in computation time of two-phase analysis, one-phase analysis is implemented instead of two-phase one in the rest of this research. Finally, effects of separating wall are evaluated in a large-scaled industrial 90° open-channel junction and some new geometrical efficiency measurements are devised and assessed to improve the wall performance. Among proposed geometrical modifications on the wall head, circular profile had the best performance in reducing the maximum velocity after the junction. The results show a considerable improvement in flow pattern which causes a significant reduction in sedimentation and erosion potential in open-channel junctions. Keywords: Open-channel junction, Separating wall, Numerical model, Erosion, Sedimentation.