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کارکرد سرريزهاي لبه‌کوتاه کرامپ در پيچ يک آبراهه
عنوان (انگلیسی): Performance of crump weirs in a channel bend
نشریه: پژوهش آب ايران
شماره: پژوهش آب ايران (دوره: ۱۰، شماره: ۱)
نویسنده: حسيني مبرا، سيد عرفان ، ياسي، مهدي
کلیدواژه‌ها : پيچ آبراهه ، سرريز با تاج شيب‌دار ، سرريز كرامپ ، مقطع كنترل
کلیدواژه‌ها (انگلیسی): River bend , Sloping crest weir , Crump weir , Control section.
چکیده:

احداث سرريزها در محدوده پيچ رودخانه،‏ مشكلاتي را از نظر اختلاف توزيع جريان در عرض و غيريکنواختي راندمان آبگيرهاي در دو ساحل چپ و راست رودخانه پديد مي‌آورد. يکي از فرضيه‌ها براي برقراري توزيع يکنواخت جريان،‏ تساوي بده جريان در واحد عرض تاج سرريز،‏ از طريق تغيير پروفيل تاج سرريز از حالت افقي به حالت شيب‌دار است. در اين پژوهش،‏ کارکرد سرريزهاي کرامپ با تاج افقي و شيب‌دار،‏ با هدف همگرايي توزيع بده واحد جريان در عرض پيچ يک آبراهه،‏ ارزيابي شده است. آزمون تجربی جريان در پيچ يک آبراهه در يک فلوم آزمایشگاهی با پيچ 90 درجه انجام شد و خصوصیات جريان در استقرار سرريزهای با تاج افقی و شيب‌دار،‏ با ارتفاع متغیر و در مقاطع مختلف پيچ کانال،‏ مورد بررسی قرار گرفت. نتايج نشان مي‌دهد که استقرار سرريزهاي با تاج افقي در مقاطع ورودي و خروجي پيچ،‏ مناسب‌تر است. در محدوده مياني پيچ،‏ تاج شيب‌دار با شيب 85º<θ<88º (θ زاويه تاج سرريز با امتداد قائم،‏ به سمت ديواره خارجي پيچ) باعث همگرايي بيشتر توزيع بده جريان در واحد عرض مي‌شود. بهترين موقعيت استقرار سرريز،‏ به ‌ترتيب در مقاطع زاويه‌اي 30،‏ 60 و 45 درجه از ابتداي پيچ است. در اين بررسي،‏ معادله جريان آزاد روي سرريز كرامپ در محدوده پيچ،‏ با تلفيق روش تحليل رياضي و ابعادي ارائه شده است. ضريب بده جريان آزاد،‏ با آزمون تجربي ارزيابي شده است.

چکیده (انگلیسی):

Construction of weirs and diversion dams in river bends has been inevitable in some cases around the world, and many examples are available to demonstrate the use of conventional types of weirs in the channel bends. The lack of design criteria for the construction of weirs on channel bends is evident. However, the operational experiences of these weirs in river bends indicate the non-uniformity of water surface levels, sediment distribution, and flow rates across the channel bend, hence, the poor performance of intake structures on both sides of the river. The main hypothesis for providing uniform distribution of flow across the bend is the change of the weir-crest profile from horizontal to sloping crest. Review of previous studies indicates a few researches on the performance of sloping-crested weirs in channel bends. The aim of the present study was to test the performance of horizontal and sloping Crump weirs in a channel bend, in order to achieve the most uniform distribution of the unit flow rates across the river bend. In this study, the theory of the design of a weir crest in a channel bend is presented. Flow equation was derived from mathematical and dimensional analysis. The discharge coefficient, Cd, is related to the relative curvature of the bend, the position of the weir along the bend, the geometry of the weir and the upstream head water. Flow characteristics were evaluated in a laboratory flume consisting of a 90 degree bend with a relative radius of curvature 3, with and without the standard Crump weirs. The experiments were carried out using 3 different heights of weir, 4 different weir crest slopes (3 sloping crests, and 1 horizontal crest), and at 7 different positions of the channel (2 cross sections in straight reach and 5 cross sections in curvature reach of the channel). All experiments were repeated under 3 different flow conditions. A total of 252 tests (63 with horizontal weirs and 189 experiments with sloping crested weirs) were carried out. Flow discharge, 2D velocity and water surface profiles at the control sections of weirs were measured.
The results indicated that horizontal crested weirs are appropriate for straight reaches upstream and downstream of the channel bend, and are still sufficient at the entrance and exit of the bend. The control section is parallel to the weir crest. It is located at a distance of (2-4.7)h upstream the weir at upstream and bend’s entrance sections (h is the upstream head). For the horizontal weirs at the downstream of the bend, the control section is located at (1-3.45)h, and on 2B-Downstream of the bend is at (2.1-3.5)h. In the channel bend, sloping crest weirs provide better convergence of the unit flow rates across the river. The slope of the weir crest is recommended to be in the range of 85º<θ<88º (θ is the angle between the crest and the vertical axis, towards the outer bank of the bend). Higher flow rates and/or head waters, and lower weir heights result in more uniformity in the flow distribution across the bend, upstream of the weir. The best location for the weir inside the bend is to be at the angular section of 30, 60 and 45, respectively. The direction of the control section is diagonal with an angle of ω (the angle between the control section line and weir direction). The angle of ω varies for different weirs with the slope of the crest at different cross sections. However, for horizontal-crested weirs inside the bend, it is located at (1.0-4.8)h upstream the weir on the outer bank side, with an angle in the range of 0º<ω<8º. The corresponding values for sloping-crested weirs inside the bend are (1.0-4.8)h and 0º<ω<16º.
In this study, the equation of flow over both the horizontal- and sloping- crested Crump weirs are also presented. For the best weir-crest configuration, the discharge coefficient, Cd, is determined for a wide range of free to submerged flows as a function of the position of the weir along the bend, the geometry of the weir, the upstream head water, and the downstream tail water. The average value of the Cd for free flows over horizontal and sloping Crump weirs is 0.70 and 0.54, respectively. The modular limit for free to submerged flows over the weir is determined by the ratio of tail water to the weir height in the outer bank (Hd/P1).
Keywords: Crump weir, Sloping crest weir, River bend, Control section.

فایل مقاله : [دریافت (317.2 kB)] ‏217 دریافت تاكنون
صاحب امتیاز:
دانشگاه شهرکرد
مدیر مسئول:
دکتر حسين صمدی
سردبیر:
دکتر منوچهر حيدرپور
مدیر داخلی:
دکتر محمدعلی نصراصفهانی