بررسي عملكرد مدل‌هاي مختلف آشفتگي در جريان عبوري از جام پرتابي مثلثي

Effects of Different Turbulence Models on Flow over a Triangular FlipBucket

جريان عبوري از جام پرتابي، جرياني به شدت آشفته و دو فازي مي‌باشد. . آشفتگي يكي از محدودكننده‌ترين عوامل در شبيه‌سازي عددي جريان سيالات مي‌باشد، به طوري كه با ثابت فرض كردن ابعاد شبكه و روش گسسته‌سازي، فاكتور موثر در زمان محاسبات، مدل آشفتگي خواهد بود. هدف از انجام اين پژوهش، استفاده از روش عددي در تحليل جريان دو فازي و آشفته عبوري از جام‌ پرتابي مثلثي به منظور مقايسه آن با نتايج آزمايشگاهي و بررسي عملكرد مدل‌هاي مختلف آشفتگي براي اين نوع جريانات مي‌باشد. به منظور دست‌يابي به اهداف مذكور با استفاده از نرم افزار فلوئنت، معادلات رينولدز درحالت دوبعدي و گذرا براي جريان عبوري از جام پرتابي حل شده‌اند. براي مدل كردن جريان آشفته از مدل‌هاي مختلف آشفتگي kε، kω و RSM و به منظور شبيه‌سازي جريان دوفازي آب و هوا از روش حجم سيال استفاده شده است. در بررسي پارامتر حداكثر فشار ديناميكي بر روي جام مدل kε استاندارد حالت عدد رينولدز پايين (LRN) و مدل RSM Stress Omega جواب‌هاي مشابه و دقيقي را در بين مدل‌هاي بررسي شده بدست مي‌آورند. مدل SST kω جواب بهتري نسبت به مدل kω استاندارد و جواب نسبتاً مشابه با مدل kε در محاسبه حداكثر فشار ديناميكي بر روي جام مي‌دهد. همچنين اين مدل در محاسبه پروفيل جت خروجي از جام عملكرد بهتري نسبت به ساير مدل‌هاي مبتني بر لزجت گردابه‌اي ارائه مي‌دهد. در نهايت مدل kω SST با در نظر‌گيري زمان و هزينه محاسبات به عنوان مدل مناسب براي جريان عبوري از جام پرتابي مثلثي معرفي شده است.

Spillways have long been of practical importance to safety of dams, therefore these structures have to be built strong, reliable and highly efficient. Ski jump dissipator is one the flow energy dissipators which is applicable downstream of spillway chutes with velocity over 20 m/s. Flow over a flipbucket is a twophase and strongly turbulent flow. Turbulence modeling is one of the most limiting factors in accurate computer simulation of flows. By fixing the grid resolution and the discretization scheme, the difference of computation time is mainly attributed to the turbulence model. The choice of turbulence model depends on factors such as the physics encompassed in the flow, the level of accuracy required, the available computational resources, and the amount of time available for the simulation. It is a fact that no single turbulence model is universally accepted as being superior for all classes of problems. The main purpose of the present study is numerical investigation of twophase turbulent flow over a triangular flipbucket to evaluate effects of different turbulence models in this type of flow. Hence, using FLUENT® software, two dimensional Reynolds averaged NavierStockes equations have been solved in unsteady state. Different turbulence models consist of kε, kω and RSM; have been used. To simulate twophase flow, volume of fluid (VOF) method has been applied. Standard kε and stressomega RSM models with lowReynolds number modifications have the best performance among the other turbulence models. In standard kε model when lowReynolds number modification was activated, the effects of molecular viscosity were taken into account in nearwall regions. Therefore, in lowReynolds number kε model, maximum dynamic pressure over the bucket was predicted more accurately in comparison with standard kε model. Regarding modification in strainpressure terms in turbulence equations, effects of anisotropic Reynolds stress tensor were taken into account in stressomega RSM model with lowReynolds number modifications. Thus, compared to other turbulence models, numerical results of this model are in a better agreement with experimental results. Different kε models could not predict the jet trajectory after the bucket very well. Due to blending function in SST kω model, this turbulence model effectively blended the robust and accurate formulation of the kω model in nearwall regions with the freestream independence of the kε model in the far field. In estimation of maximum dynamic pressure over the bucket, this model had a better performance than standard kω model and relatively similar results to kε model. In addition, SST kω model has shown the best prediction of the jet trajectory among other turbulence models. Eventually, with respect to computation cost and accuracy of results, SST kω turbulence model has been introduced as the most suitable turbulence model to predict the flow pattern of a triangular flipbucket.