نوع مقاله : مقالات پژوهشی
نویسندگان
1 شهید چمران اهواز
2 دانشگاه آزاد اسلامی واحد علوم و تحقیقات اهواز
چکیده
جریان در قوس تحت تأثیر دو نیروی گرادیان عرضی فشار و گریز از مرکز قرار می‌گیرد که باعث به وجود آمدن جریان‌های حلزونی می‌شود. چنین الگوئی عامل عمده‌ی فرسایش در قوس خارجی و رسوب گذاری در قوس داخلی است و اصلاح این الگو با روش‌های مختلفی امکان پذیر است که از جمله می‌توان به نصب سرریز W اشاره کرد. در تحقیق حاضر میزان فرسایش دیواره داخلی، خارجی و میانه فلوم در شرایط با حضور سرریز W و نصب آن در موقعیت های 30، 60 و 90 درجه در قوس 90 درجه تند بررسی شده است. حضور سرریز W سبب توزیع یکنواخت تر جریان در بالادست محل نصب سرریز شده و اثر دو نیروی گرادیان عرضی فشار و گریز از مرکز بگونه‌ای اصلاح شده که هیچگونه فرسایشی مشاهده نمی شود. نتایج این تحقیق نشان داد که در موقعیت 30، 60 و 90 درجه بیشترین مقدار عمق فرسایش در عدد فرود 206/0 نسبت به عدد فرود 137/0 به ترتیب به میزان 84، 90 و 118 درصد در محدوده دیواره خارجی افزایش داشته است. همچنین در هر دو عدد فرود نصب سرریز در زاویه 30 درجه بیشترین درصد کاهش فرسایش بستر را نسبت به زوایای 60 و 90 درجه به همراه داشته است.
کلیدواژه‌ها
عنوان مقاله [English]
Experimental Study of the Effect of W-weir on Reduction of Scour Depth at 90 Degree Sharp Bend
نویسندگان [English]
- Vida Atashi 1
- Mahmood Shafai-Bajestan 1
- Ideh Golrokh 2
1 Shahid Chamran University, Ahwaz
2 Islamic Azad University, Ahwaz
چکیده [English]
Introduction: Flow patterns within the river bend is three dimensional. Occurrence of secondary flow due to centrifugal force and formation of helicoidally vortex in river bend usually causes the outer bank of river erodes whilst the sediment are deposited in inner bend which appears in the form of point bars. To reduce the river bank scour, many techniques have been developed which may be classified as covering technique and modified flow patterns methods. The W-weir is among such structures. In the present paper, by measuring three components of flow velocity with and without presence of W-weir, variation of flow patterns and shear stress distribution in a 90-degree sharp bend have been investigated. The main purpose of this study is to see the installation of different locations of W-weir in the bend on reduction of outer bank scour. In the present paper, by measuring three components of flow velocity with and without presence of W-weir, variation of flow patterns and shear stress distribution in a 90-degree sharp bend have been investigated. The analyses of data showed more uniform flow upstream of the weir and also revealed that the effect of transverse and centrifugal forces are modified in such a way that the secondary flow is diminished. The results showed that for 30, 60 and 90-degree bends maximum erosion depth in the vicinity of the outer bank with Froude number of 0.206 in comparison with 0.137 has increased up to 84, 90 and 118 % respectively. In both Froude numbers, installation of W-Weir in 30 degree has the most reduction in bed in comparison with 60 and 90 degree.
Materials and Methods: To reach the goal of this study a physical model of 90 degree sharp bend was constructed in the hydraulic lab of Shahid Chamran university of Ahvaz. The ratio of R(radius)/b(flume width) was less than 2 which shows a sharp bend. The W-weir was built with 1mm galvanized steel. Flume bed was covered with sediment of D50=1.5mm. The W-weir was installed at three different locations of 30, 60 and 90 degrees from the bend entrance. Two sets of tests were carried out with and without weir. For each location two different flow discharges (Fr= 0.137 and Fr=0.206) were studied. The flow depth for all tests were kept constant equal to 15cm. At the end of each test the flume was drained and bed topography was recorded using laser meter. Measured bed topographic data were used in SURFER and TECPLOT software to compare the results of the W-weir location
Results and Discussion:
a)W-weir in 60 degree location
b)W-weir in 30 degree location
a)W-weir in 90 degree location
Fig.1 Bed topography after W-weir installed(Q=10l/s)
The results showed that W-weir concentrated the flow toward the flume center thus the bed only will scour at the downstream of W-weir whilst the bed at upstream is neither of weir nor eroded. This is because the flow patterns within the bend has been modified in such a way that diminishes the strength of helicoidally vortex upstream of the weir thus the scour or deposition will not occur. The results of tests with Q=15 l/s also was similar with the exception that in these tests the Froude number is higher and the scour depth downstream of weir is much larger. The results also showed that the scour depth is much higher when the weir is installed at 60 degrees. The scour depth for weir at 90 degree location showed reduction of about 33% and 39% compare to the weir in 30 and 60 degree location respectively.
Conclusion: In this research, by assessing the cross velocities and the scour depth downstream of weir in 90 degree sharp bends and studying the impacts of w-shape weir on those parameters, following results were obtained: The W-weir can modify the flow patterns within the flume bend in which no scour and deposition is observed upstream of the weir. The scour downstream of weir with higher depth closed to the outer bank is observed in all tests. The scour depth is much higher when the weir is installed at 30 degree location whilst is minimum when the weir is installed at 90 degree location.
کلیدواژه‌ها [English]
- 90 Bend
- Erosion
- Flow Patterns
- Scour Depth
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