اثر هندسه ديوار جدا كننده كانال فرعي بر الگوي جريان يك كانال باز بزرگ مقياس با تقاطع °90

Effect of Separating Wall Geometry on Flow Pattern of a LargeScaled 90° OpenChannel Junction

تقاطع كانال‌هاي باز استفاده وسيعي در كارهاي آبي دارد. شكل‌گيري ناحيه كم فشار با جريان بازچرخشي (ناحيه با پتانسيل رسوب‌گذاري بالا) و نيز ناحيه پر سرعت (ناحيه با پتانسيل آبشستگي بالا) از مهم‌ترين مشخصه‌هاي جريان در اين كانال‌ها به حساب مي‌آيند. تاكنون تحقيقات وسيعي در زمينه شناسايي الگوي جريان در اين كانال‌ها و رفع مشكلات هيدروليكي مذكور صورت گرفته است. در اين تحقيق نيز استفاده از ديوار جداكننده به عنوان راه‌كاري جهت كاهش پتانسيل آب‌شستگي و رسوب‌گذاري مورد بررسي كامل قرار گرفته است. ابتدا مدل عددي يك نمونه آزمايشگاهي موجود توليد شده و نتايج آن با در نظرگرفتن و بدون در نظر گرفتن اثرات هوا صحت‌سنجي شده‌اند. در ادامه، اثر راهكار مذكور بر الگوي جريان مورد بررسي قرار گرفته و طرح‌هايي جهت بهبود هندسه آن ارائه و ارزيابي شده‌است. در ميان طرح‌هاي بررسي شده ديوارجداكننده با لبه دايره‌اي بهترين عملكرد را به لحاظ كاهش حداكثر سرعت پس از تقاطع نشان داده است. نتايج اين تحقيق كاهش قابل ملاحظه‌اي را در پتانسيل آب‌شستگي و رسوب‌گذاري جريان نشان مي‌دهند.

Abstract: Openchannel 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 branchside bank is one of the most distinctive characteristics of a flow in openchannel junctions. The recirculating flow in this lowpressure 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. Openchannel junctions have a broad application in civil and environmental engineering. Formation of a lowpressure zone with recirculating flow (high sedimentation) accompanied by a highvelocity 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 openchannel 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 openchannel junction. Initially, the numerical twophase model of a previous experimental study, is prepared and its results are validated. Following that, a onephase 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 onephase and twophase 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 (onephase) and experimental studies and considerable increase in computation time of twophase analysis, onephase analysis is implemented instead of twophase one in the rest of this research. Finally, effects of separating wall are evaluated in a largescaled industrial 90° openchannel 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 openchannel junctions. Keywords: Openchannel junction, Separating wall, Numerical model, Erosion, Sedimentation.