مطالعه عددي تأثير موقعيت و زاويه آبگيري بر الگوي جريان در كانال قوسي با استفاده از نرم افزار SSIIM2

Numerical investigations on effect of intake location and diversion angle on flow pattern in a channel bend by SSIIM2 Software

آبگير جانبي همراه با كانال انحرافي يكي از روش هاي آبگيري از رودخانه است. با توجه به اينكه بيشتر رودخانه هاي در طبيعت به حالت مئاندري است بنابراين تعيين موقعيت و زاويه آبگيري از اهميت زيادي برخوردار مي باشد. در اين مقاله با شبيه سازي عددي ميدان جريان در اطراف آبگير در يك قوس 180 درجه با استفاده از مدل عددي SSIIM2 در چهار موقعيت 45، 90، 115و 135درجه با چهار زاويه آبگيري 30، 45، 60 و 90 درجه به بررسي ابعاد صفحه تقسيم جريان و ابعاد ناحيه جداشدگي در كانال فرعي و اصلي پرداخته شده است. بدين منظور ابتدا مدل SSIIM2 نسبت به مدل هاي آشفتگي kε استاندارد و RNG حساسيت سنجي شده و نتايج با نتايج آزمايشگاهي منتصري و همكاران (1387) صحت سنجي شده است. نتايج حاصل نشان مي دهد در همه موقعيت ها، زاويه آبگيري 90 درجه داراي بيشترين ابعاد نواحي جداشدگي،‌ بيشترين عرض صفحه تقسيم جريان در تراز بستر و كمترين عرض صفحه تقسيم جريان در تراز نزديك سطح را دارد و در عوض زاويه آبگيري 30 درجه داراي كمترين ابعاد نواحي جداشدگي جريان و كوچكترين عرض صفحه تقسيم جريان در تراز بستر و بيشترين عرض صفحه تقسيم جريان در تراز سطح را دارد. همچنين در موقعيت 135درجه، در تمام زواياي آبگيري، عرض صفحه تقسيم جريان در سطح بيشترين مقدار و در كف كمترين مقدار را دارا مي باشد.

The use of lateral intake is one method of providing water from river. The most important issue in branch channel gets the maximum of water and lowest sediment. The rivers rarely run on straight paths in nature, and most rivers have meandering forms. In a river bend the presence of centrifugal force leads to the formation of secondary flow. As a result water particles near the surface are driven outward. The secondary flow advects the main flow, leading to high velocity at the outer bank of the bend. On the other hand the flow at the bed of a channel is directed toward the inner bank. The interaction of the main flow with the secondary flow forms the socalled helical flow in the bend. This flow system has important consequences in the longitudinal, transverse, vertical velocity distributions, transport of momentum and streamlines at different levels of water. Therefore layout the intake outer bank of bend is one of the ways to reduce sediment input to the lateral intake. Combining the helical flow and complex flow pattern in front of the lateral intake is added complexity of this three dimensional flow pattern. The flow approaches the intake; it is accelerated laterally by the suction pressure at the end of the branch channel. This causes the flow to divide so that a portion enters the branch channel with the remainder continuing downstream in the main channel. The portion withdrawn by the branch is delineated by a curved shearlayer surface, denoted as the dividing stream surface Because of the streamwise curvature of the dividing stream surface, the diverted flow experiences an imbalance between the transverse pressure gradient and shear and centrifugal forces that initiates a clockwise secondary motion cell. This secondary motion interacts with the separation zone along the inner wall of the branch channel. In design of lateral intakes, determination of appropriate intake location and diversion angle is very important. In this paper, we simulated lateral intake at different location and different angel by using the SSIIM numerical model to investigate dividing stream surface and separation zone at main and branch channel. For this purpose the flow is simulated using standard kε model and RNG model. For calibration of model we used the result of the Montaseri et.al (2008) investigation. The results show that in the curved channel the dividing stream surface has a completely different structure than the lateral diversion in a straight one. In other words, wide of dividing stream surface near bed is smaller than near surface. Furthermore, in any locations dividing stream surface width near the bed and separation zone has largest dimension at 90 degree diversion angle and has smallest dimension at 30 degree diversion angle. Also, in 135 degree location, dividing stream surface width near the bed has smallest dimension and dividing stream surface width near the surface has largest dimension at any diversion angle.