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 air-core 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 free-water surface enters a closed area such as a shaft pipeline or morning-glory 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 anti-vortex 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 anti-vortex plates on reduction of swirling flow over the intake.


Methodology
The aim of this study is to make the model of anti-vortex plates at vertical shaft in Flow-3D software. Flow-3D software is Computational Fluid Dynamic (CFD) software with the ability of assimilation of fluid flow on free surface. The effects of different anti-vortex plate geometries, including numbers, dimensions, positioning angles and also the shape of anti-vortex plates on the weir flow discharge coefficient are taken into account. Three different configurations, (D×D, 2D×2D, 3D×3D), of square and circular anti-vortex plates in four angles, 0‎‏◦‏‎ ‎‎(single), 90‎‏◦‏‎ (pair), 120‎‏◦‏‎ (triad) and 180‎‏◦‏‎ (four anti-vortex plates), were investigated. Previous experimental study suggested that optimal size for square anti-vortex 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 (Kabiri-Samani and Borghei, 2013). In present study, the same position R=D/2 and Z=0 for anti-vortex plate is considered.


Results and Discussion
In order to model-validation of the no-plate model, one D×D anti-vortex plate (p1) and two 2D×2D anti-vortex plates (P2) were considered in 10 cm, and 7.5 cm diameters of the shaft, respectively. In no-plate 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 flow-3D software has good performance in numerical simulation with error less than 8%.
In the present study, square anti-vortex 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 anti-vortex plates) for square and 180‎‏◦‏‎ for circular. ‎The results showed that in four square anti-vortex 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 anti-vortex 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 anti-vortex plates (n=4) for D×D, 2D×2D and 3D×3D. The results shows that the square anti-vortex plate with D×D dimension has a lower water level which indicate passing more water through the shaft. Circular anti-vortex 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 anti-vortex 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 anti-vortex 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 anti-vortex plates based on model simulation.
Based on results obtained from numerical modeling, application of four D×D square anti-vortex plates is more effective compared to one, two, or three square plates and also four circular anti-vortex plates. Head-discharge diagrams indicate that four D×D square anti-vortex plates in similar discharges, has the minimal water height above the spillway. So the four D×D square anti-vortex plates are introduced as the optimal case and circular anti-vortex plate can be applied as the second choice.

Keywords: Shaft Spillway, Anti-Vortex Plates, Vortex flow, Velocity component.