بررسي تاثير تغيير ابعاد مجرا بر خصوصيات هيدروليكي جريان در تخليه‌كننده تحتاني سد

The effect of bottom outlet geometry changes on hydraulic characteristics of flow

در اين مطالعه تخليه‌كننده تحتاني سد جگين با مقياس 1 به 10 به صورت عددي مدل شده و نتايج آن با مقادير اندازه‌گيري شده از مدل آزمايشگاهي در موسسه تحقيقات آب مقايسه شد. نتايج نشان‌دهنده عملكرد مناسب مدل عددي در پيش‌بيني الگوي جريان است. هدف اين پژوهش بررسي تاثير نسبت عمق به عرض مجرا بر هوادهي و نرخ جريان ورودي به مجرا است. براي اين كار مدل‌هاي عددي متفاوتي در نرم‌افزار ساخته شد كه وجه تمايز آن با مدل اوليه تفاوت در مقدار پارامتر عمق به عرض مجرا بود. اين تغييرات در دو حالت عمق ثابت- عرض متغير و عرض ثابت- عمق متغير صورت پذيرفت. نتايج نشان مي‌دهد با افزايش عمق و عرض مجرا، نرخ جريان ورودي و ميزان هوادهي جريان افزايش مي‌يابد، به گونه‌اي كه از نقطه نظر هيدروليكي محدوده 5/1 الي 2 براي h/b، محدوده مناسبي است و ساير نسبت‌هاي h/b روي كاهش هوادهي و ضريب ? موثر است.

Outlet conduits are one of the important parts of dams. Due to the high flow rate and pressure drop, problems such as cavitation can affect these structures. Considering these problems, detailed design is necessary. Laboratory researches which are expensive are usually carried out. However, numerical models for determining complex flow characteristics have attracted the attention of the designers. In this study, the numerical simulation of Jegin dam outlet conduit in south of Iran with the scale of 1:10 is provided and the results are verified by experimental information taken from physical model built and tested at Water Research Institute. Gate opening in this research is always 70% and the water head is constant with value of 38.6m. The research is focused on the intake gate and not the service gate, so the service gate is always fully opened. FLUENT computer code is considered for the numerical model studies. In the numerical simulation the Finite volume mixture two phase flow scheme and k–? turbulence model are used. The flow discharge and air supply from the air vent downstream of the gate is then computed by 3D numerical model for different channel geometries. Reasonable agreement between the numerical model and experimental results shows reliable performance of the numerical model. This study showed the ability of the numerical model to simulate the complex air water flow in high speed gated tunnels. This study also includes the effect of the height to width ratio of the conduit on flow discharge and aeration downstream of the gate. To do this, different numerical models are simulated among which the height to width ratio of the conduit is changed. Height and width of the conduit are measured at the gate section and changes are applied in two cases of constant height (depth) and variable width, and constant width and variable height. Results show more aeration and more flow discharge while heightening and widening the outlet. Flow discharge has also been determined as function of the height to width ratio of the conduit at gate section of the channels. One of the important results is that in comparison with the width changes, height changes of the channel affect hydraulic characteristics of flow more and the diagram rates vary more sharply. Results show that 1.5 to 2 ranges for height to width ratio is the best range based on hydraulic performance and other ranges have effect on reducing aeration and air demand ratio B (B in this research agrees more with the relation presented by Kalinske and Robertson), so the pressure in a conduit may fall considerably below atmospheric pressure which may result in cavitation and vibration. To avoid these problems, it is suggested not to heighten the conduit more than a specific value.