تاثیر فراسنجه های هندسی صفحات ضدگرداب بر هیدرولیک جریان در سرریزهای مدور قائم

یکی از انواع مهم و کاربردی سرریزها، سرریز مدور قائم است. یکی از مشکلات اصلی سرریزهای مدور قائم ایجاد گرداب‌های قوی در دهانه آن‌ها است. این امر باعث ورود هوا و ایجاد جریان‌های چرخشی در داخل مجرای انتقال می‌شود، که به دنبال آن مشکلات دیگری مانند کاهش ظرفیت آبگیری، افزایش احتمال وقوع کاویتاسیون، لرزش و سر و صدا را در بر خواهد داشت. یکی از روش‌های کاهش اثرات گردابه های مخرب، استفاده از صفحات ضدگرداب است که باعث به‌ تاخیر انداختن جریان گردابی و هدایت خطوط جریان به سمت مرکز شفت می‌شود. مطالعات محدودی روی صفحات ضدگرداب چهارگوشه انجام شده است، این در حالی است که ارزیابی دقیق پارامترهای هندسی صفحات ضد گرداب و تعیین شکل بهینه جهت طراحی نیاز به مطالعات بیشتر دارد. در تحقیق حاضر هیدرولیک جریان روی 25 مدل سرریز مدور قائم به همراه گرداب‌شکن‌های چهارگوشه و دایره‌ای در نرم‌افزار flow-3d شبیه‌سازی شد. جهت کالیبراسیون مدل از نتایج آزمایشگاهی موجود برای دو مدل مربع و مستطیلی با دو قطر شفت 10 و 7.5 سانتی‌متر استفاده گردید. نتایج نشان داد نرم افزار flow-3d با خطای کمتر از 8 درصد، قادر به مدل‌سازی جریان است. شبیه‌سازی‌ها برای صفحات ضدگرداب مربعی و دایره‌ای در دبی‌های مختلف انجام گردید و تاثیر ابعاد، تعداد و زاویه قرارگیری آن‌ها مورد بررسی قرار گرفت. نتایج نشان داد که استفاده از صفحات ضدگرداب مربعی در ابعاد d×d و تعداد چهار صفحه (4= n) بهترین نتیجه را از نظر کمترین ارتفاع آب روی شفت، عدم وجود هسته هوا و فشار منفی به خود اختصاص داده است.

The effects of geometrical parameters of anti-vortex plates on flow field in vertical intakes

Introduction Shaft spillways are one of the major hydraulic structures in dam engineering, influencing the flow field inside the dam reservoir, by inducing swirling flow and aircore vortices. Formation of vortex at vertical shaft is one of the critical problems encountered in these kinds of shafts. Intake vortices are the result of angular momentum conservation at the flow constriction, where angular velocity increases with a decrease in the cross sectional area. This phenomenon usually happens when a freewater surface enters a closed area such as a shaft pipeline or morningglory spillway. Swirling flows at vertical shaft spillways have the potential to engender flow fluctuations, reduce efficiency, and cause structural damages. In order to make the vortex weakness or totally omitted, the strength of the vortex must be limited by some obstacles in its formation point and caused uniform flow toward the spillway. To this end, using the antivortex plates will increase both discharge coefficient and spillway efficiency, while decrease the water flow oscillation. Due to numerous design variables, the optimal hydraulic design of such structures is not deeply understood. In the present study, a comprehensive investigation by simulated model is conducted to examine the effects geometrical parameters of antivortex plates on reduction of swirling flow over the intake. Methodology The aim of this study is to make the model of antivortex plates at vertical shaft in Flow3D software. Flow3D software is Computational Fluid Dynamic (CFD) software with the ability of assimilation of fluid flow on free surface. The effects of different antivortex plate geometries, including numbers, dimensions, positioning angles and also the shape of antivortex plates on the weir flow discharge coefficient are taken into account. Three different configurations, (D×D, 2D×2D, 3D×3D), of square and circular antivortex plates in four angles, 0‎‏◦‏‎ ‎‎(single), 90‎‏◦‏‎ (pair), 120‎‏◦‏‎ (triad) and 180‎‏◦‏‎ (four antivortex plates), were investigated. Previous experimental study suggested that optimal size for square antivortex plate is 2D×D if R=D/2 (R is the horizontal distance between the shaft center and plate edge) and Z=0, i. e. the vertical distance between shaft edge and palate center (KabiriSamani and Borghei, 2013). In present study, the same position R=D/2 and Z=0 for antivortex plate is considered. Results and Discussion In order to modelvalidation of the noplate model, one D×D antivortex plate (p1) and two 2D×2D antivortex plates (P2) were considered in 10 cm, and 7.5 cm diameters of the shaft, respectively. In noplate numerical simulation, six different discharges (0.00604 – 0.0019 m3/s) was considered for each diameter individually, and in P1 and P2 simulation the variation of discharge is 0.0125 to 0.0019 (m3/s). The results of verification showed that flow3D software has good performance in numerical simulation with error less than 8%. In the present study, square antivortex plates with D×D, 2D×2D, 3D×3D dimensions and circular ‎with diameter D, were modeled at the angles 0‎‏◦‏‎ ‎‎(single), 90‎‏◦‏‎ (pair), 120‎‏◦‏‎ (triad) and 180‎‏◦‏‎ (four antivortex plates) for square and 180‎‏◦‏‎ for circular. ‎The results showed that in four square antivortex plates (n=4) and size D×D negative pressure on vertical intake and high pressure around the tank shells was not observed. Thus, the four square antivortex plates (n=4) with D×D dimensions is the best configuration in terms of negative pressure and air core. Water free surface elevations have been investigated for the best number of square antivortex plates (n=4) for D×D, 2D×2D and 3D×3D. The results shows that the square antivortex plate with D×D dimension has a lower water level which indicate passing more water through the shaft. Circular antivortex plates in comparison with square type, in the same condition, shows the higher water level. That is proof weak performance of the circular model versus the square. Investigation of velocity components over the intake indicate that the fluctuation of vertical component is more than the radial and tangential velocity components. The maximum velocity value is obtained for radial velocity component of square antivortex plates. Conclusion Vertical shaft is one of the most important and the most practical one among the spillways. In the present study, different configuration of antivortex plates has been considered to reduce or minimize swirling flow strength and air entrainment at the vertical shaft spillways. A parametric study was carried out on the main geometric variables of the antivortex plates based on model simulation. Based on results obtained from numerical modeling, application of four D×D square antivortex plates is more effective compared to one, two, or three square plates and also four circular antivortex plates. Headdischarge diagrams indicate that four D×D square antivortex plates in similar discharges, has the minimal water height above the spillway. So the four D×D square antivortex plates are introduced as the optimal case and circular antivortex plate can be applied as the second choice. Keywords: Shaft Spillway, AntiVortex Plates, Vortex flow, Velocity component.