ارزيابي مدل هاي دوفازي اويلري و لاگرانژي در شبيه سازي ساز و كار انتقال رسوب در كانال قوسي 180 درجه با آبگير جانبي

Evaluation of Two Phase Models for Numerical Simulation of Sediment Transport in a 180 degree Bend with Lateral Intake

در اين تحقيق با استفاده از نرم افزار فلوئنت به ارزيابي مدل هاي دوفازي اويلري و فاز گسسته لاگرانژي در مدل سازي پديده انتقال رسوب و مكانيسم هاي ورود رسوبات به آبگير جانبي واقع در موقعيت 115 درجه از يك كانال قوسي 180 درجه با روش تزريق رسوب پرداخته شده است. براي بررسي عملكرد مدل هاي دو فازي، روند حركت رسوبات در زمان هاي مختلف از شروع تزريق در مدل هاي عددي با مدل آزمايشگاهي مقايسه گرديده اند. نتايج نشان دهنده عملكرد مناسب هر دو مدل در تعيين مسير حركت ذرات بستر در زمان هاي مختلف در كف كانال و مكانيسم ورود رسوبات به داخل آبگير مي باشد اما مدل اويلري از عملكرد بهتري برخوردار است. در شبيه سازي توپوگرافي بستر نيز عملكرد روش اويلري بسيار بهتر از مدل فاز گسسته بوده است. در عوض روش فاز گسسته توانايي تعيين درصد ذرات منحرف شده به داخل آبگير را دارا مي باشد. در اين تحقيق اثر زاويه انحراف آبگير و درصد دبي آبگيري بر مكانيسم شكل گيري توپوگرافي بستر، مكانيسم ورود رسوبات به آبگير و درصد رسوب انحرافي مورد مطالعه قرار گرفته است. مكانيسم ورود رسوبات به ابگير در دبي هاي آبگيري مختلف متفاوت است اما همواره دو پشته رسوبي در مجاورت قوس داخلي در نيمه اول و دوم قوس ايجاد مي شود. افزايش دبي آبگيري باعث كاهش نسبي ابعاد پشته هاي رسوبي مي گردد. مكانيسم ورود رسوبات به آبگير تحت تاثير زاويه انحراف آبگير نيز ميباشد. در زاويه آبگيري 50 درجه كمترين مقدار رسوب در كليه دبي هاي آبگيري وارد آبگير مي گردد.

A large number of flow types - that are encountered in nature and technology – consist of phases. Advances in computational fluid mechanics have provided the basis for further insight into the dynamics of multiphase flows. Currently, there are two approaches for the numerical calculation of multiphase flows: the Euler-Euler approach and the Euler-Lagrange approach. In the Euler-Euler approach, different phases are treated mathematically as interpenetrating continua. In FLUENT, three different Euler-Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. For sedimentation, Eulerian model must be used. The Eulerian multiphase model in FLUENT allows modeling multiple separate, even interacting phases. The phases can be liquid, gas, or solid in nearly any combination. The Lagrangian discrete phase model (DPM) in FLUENT follows the Euler-Lagrange approach. The fluid phase is treated as continuum by solving the time-averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a large number of particles through the calculated flow field. Sediment transport by fluid flow is one of the most important two-phase flows in the nature. Due to existence of secondary current in channel bends, the mechanism of flow and sediment transport in these channels is much complex and locating lateral intake at outer bank of the bends decreases this complexity. In this paper, mechanisms of sediments transport into the intake have been evaluated in a 180 degree bend channel with lateral intake. These mechanisms are simulated by the Eulerian and Discrete phases models in fluent software. The intake is located at the outer bank of an 180o bend at position 115° with 45° diversion angle. The effect of diversion discharge rate and diversion angle on mechanism of sediment entry to the intake are also considered. Turbulence model is k-ε model. Models were run in different times and the results are compared with laboratory results. The result in Qr=40% shows that the mechanism of sediment entry consists of continuous entrance from downstream edge of intake and periodic entrance from upstream of the intake. However, in Qr=25%, the mechanism of sediment entry only consists of continuous entrance from downstream edge of intake. The two models (Eulerian and Discrete phases) have shown good results. The rout mean square errors for outer boundary of the path of the particle at the channel’s bed are measured for two discharges (25% and 40%). Number of particles in discrete phases is limited; therefore, this model cannot display the depth of sediment. The Eulerian model displays the bed topography very well. Measuring mean square errors shows that the model operation for topography simulation is very well. This model accurately shows the location of intermittent dune and location of sediment accumulation. The discrete phase model can show the particle trapped place more proper than the Eulerian model. Due to increase in intake discharge, dimensions of sediment accumulation is decreased. Mechanism of sediment entry to lateral intake is affected by diversion angle of intake. Minimum sediment is entered to lateral intake at diversion angle equal to 50 degree.