شبيه سازي جريان و رسوب در پيچ و آبگيري از پيچ

Simulation of flow and sediment in bend and lateral intake of bend

مطالعه طرح هاي ساماندهي يا مديريتي در رودخانه ها نيازمند تحليل جريان و رسوب در آن‌هاست. ماهيت سه بعدي جريان در پيچ رودخانه ها، تحليل جريان و رسوب را در آنها مشكل مي نمايد. توسعه مدل‌هاي عددي سه بعدي و پيشرفت قدرت محاسبات رايانه ها، منجر به استفاده روزافزون از مدلسازي عددي در بررسي هاي هيدروديناميكي جريان و رسوب شده است؛ اما اكنون هنوز صحت و دقت نتايج مدل‌هاي عددي در شرايط پيچيده جريان به ويژه در جريان همراه با رسوب مشخص نشده است. در اين پژوهش به منظور شبيه سازي هيدروديناميك جريان و رسوب در پيچ و آبگيري از پيچ مدل عددي سه بعدي SSIIM2 مورد استفاده قرار گرفته و با مقايسه نتايج مدل عددي با داده هاي آزمايشگاهي يا صحرايي، دقت مدل عددي در هر مورد بررسي شده است. نتايج نشان مي دهد مدل ياد شده در شبيه سازي هيدروديناميك جريان و پيش بيني جريان ثانويه در قوس، هيدروديناميك جريان در قوس به همراه آبگير و آستانه و همچنين پيش بيني سطح آزاد در آبگيري از قوس از دقت قابل قبولي برخوردار است. خطاي مدل در پيش بيني توپوگرافي بستر در آبگيري از قوس نسبتا زياد است، اما زماني كه هندسه‌هاي پيچاني و جريان ثانويه قوي در مورد مطالعاتي وجود نداشته باشد، مي توان به نتايج تغييرات توپوگرافي پيش بيني شده مدل عددي SSIIM2 اطمينان نمود استفاده از مدل آشفتگي دو معادله اي ?-k و خطاي موجود در معادلات انتقال رسوب از عوامل ايجاد خطاي مدل عددي است.

Flow and sediment transport analysis is required for planning or managing research projects in river engineering. Due to fully three-dimensional nature of the flow in the river bend, it is difficult to analyze flow pattern and sediment transport in such a condition. Development of three-dimensional numerical models and advanced computing power of computers have led to the increasing use of numerical modeling studies of hydrodynamics and sediment transport. However, accuracy and validity of numerical results of flow pattern and especially in conjunction with the sediment transport at complex geometry should be determined first. In the present study, in order to simulate the flow pattern and sediment transport in the bend and lateral intake of bend, three-dimensional numerical model SSIIM2 has been used. SSIIM2 solve Reynolds equation by finite volume method, using Power-law scheme and SIMPLE algorithm for pressure coupling. To close governing equations, k- ? two equations turbulence model is used. Simulation and verification consisted of two stages, first only bend simulation and then bend with lateral intake along with entrance sill. Each stage includes both rigid and erodible bed simulation. Simulations show that the accuracy of model results of flow pattern, and especially the secondary flow in the bend, flow field in the lateral intake of the bend, and water surface elevations in the lateral intake of the bend compared to experimental results are acceptable. Maximum error in simulation of flow pattern occurs in the region of intake entrance where the helicoidally flow in bend and secondary flow are present. In predicting bed topography, in the case of bend itself, the maximum error is about 33%. However, when intake is considered along with the bend, the error in simulating the bed topography increases. Model error in predicting the topography of the bed in the bend with lateral intake is relatively high. The error of the numerical model may be due to using two-equation turbulence model k- ?, mismatch of numerical cells at the junction of lateral intake and main bend channel while using multi-block mesh arrangement, and the empirical sediment transport equations. As another test case the flow pattern and bed topography are simulated for the junction of two rivers for which field data were available. Comparison of bed topography simulation results show agreement with field data, unless in the vicinity of junction. Therefore it is concluded that SSIIM2 model can be used for flow pattern and bed topography simulation in cases where the strong secondary flow is not present.