بررسي تاثير بلوك هاي مياني حوضچه آرامش در حالت پرش هيدروليكي مستغرق بر الگوي جريان

Numerical Study of Performance of Baffle Blocks in Submerged Hydraulic Jumps

در اين مقاله به بررسي عددي گردابه‌هاي به وجود آمده و مقدار چرخش در پرش هيدروليكي مستغرق شكل گرفته در حوضچه آرامش بدون و با بلوك‌هاي مياني پرداخته شده است. پرش هيدروليكي مستغرق تحت تاثير بلوك‌هاي مياني حوضچه آرامش، تنها به صورت دو نوع رژيم اتفاق مي افتد؛ جريان منحرف شده به سطح يا به‌گونه مختصر رژيم (DSJ)، به صورت جت ديواره‌اي دوباره متصل شوند و يا به‌گونه مختصر رژيم (RWJ). در مطالعه حاضراز نرم افزار فلوينت براي شبيه سازي جريان استفاده شده است. ميدان محاسباتي با استفاده از شبكه منشوري گسسته شده است. براي دقت بيشتر از مدل آشفتگي هفت معادله‌اي RSM استفاده شد كه در صحت سنجي نتايج بهتري توليد كرد. مقايسه براي سه مدل به ترتيب در شرايط پرش هيدروليكي بدون بلوك به شكل جت ديواره اي مستغرق، پرش هيدروليكي مستغرق در شرايط رژيم DSJ و پرش هيدروليكي مستغرق در شرايط رژيم RWJ انجام گرفته و گردابه‌هاي شكل گرفته حول سه محور x، y و z و مقدار چرخش بررسي شده است. نتايج بيانگر اين است كه گردابه‌هايي كه حول محور Zها در گردش هستند، با توجه به قدرت و ابعاد بزرگتري كه دارند، رژيم اصلي جريان را تعيين مي‌كنند. اين گردابه ها در رژيم DSJ قوي‌تر بوده و باعث استهلاك بيشتر انرژي مي‌شوند.

Hydraulic jumps occur in natural systems like streams and rivers as well as manufactured systems. Samples of the latter occurance are jumps in water distribution and irrigation networks formed downstream of hydraulic structures such as spillways, sluice gates, and drops. These structures are usually designed for a specific tailwater depth. Stilling basins with baffle blocks are frequently used as energy dissipators downstream of hydraulic structures. Baffle blocks are often used to stabilize the jump, decrease its length and increase the energy dissipation. If the flow rates become more than the design discharge, the tail water depth will be greater than the one required for a free jump. These situations are common in low head hydraulic structures including low diversion dam spillways and gates. Under such conditions the hydraulic jump will be submerged. The performance of the blocks in submerged jump (SJ) condition differs from the free jump (FJ) case. According to some factors such as Froude number, block shape and location and submergence factor, flow regimes on baffle blocks in condition of submerged hydraulic jumps which occurs in stilling basins, are classified into two regimes, the deflected surface jet (DSJ) and reattaching wall jet (RWJ). In this article a numerical study was conducted to investigate flow pattern, vortexes and the magnitude of vorticity in submerged hydraulic jumps with baffle blocks downstream of a sluice gate. The results were compared to ones in same conditions without blocks. 3D RANS simulations have been applied by Fluent software. RSM turbulence model were used which illustrated much precise results in verification. Three numerical models have been created; Submerged wall jet without blocks, submerged hydraulic jumps with baffle blocks in the condition of deflected surface jet flow regime and submerged hydraulic jumps with baffle blocks in the condition of reattaching wall jet flow regime. Flow pattern has been exhibited for each model and results were compared with each other. Vortexes formed in such situations classified into three groups according to axis which they whirl around. It was observed that deflected surface jet regime has more vortexes in comparison to the two other conditions. In addition, by measuring the average magnitude of vorticity in cross-sections it has been concluded that z-vortexes –vortexes which rotate around z axis– much more powerful than x- and y-vortexes as they determine the kind of flow regime. Furthermore, this magnitude is about two times larger in deflected surface regime than two other situations. This fact leads to more turbulence in the flow that makes deflected surface jet regime the desirable condition in which baffle blocks perform more efficiently as energy dissipators in comparison to two other investigated models. In order that, from energy vantage point, conditions should be provided in a way to form submerged hydraulic jump as deflected surface jet regime