Flow Over the Spillway of AlAdhiam Dam Under Gated Condition

T he Al Adhaim Dam is located 133 kilometers northeast of Baghdad. It is a multipurpose dam and joints the Iraqi dam system in 2000. It has a storage capacity of 1.5 billion m 3 . The dam has an ogee spillway with a length of 562 m, a crest level of 131.5 m.a.m.s.l. and a maximum discharge capacity of 1150 m 3 /s at its maximum storage height of 143 m.a.m.s.l. This research aimed to investigate the hydrodynamics performance of the spillway and the stilling basin of AlAdhiam Dam by using numerical simulation models under gated situations. It was suggested to modify the dam capacity by increasing the dam's storage capacity by installing gates on the crest of the dam spillway. The FLUENT program was used to simulate the flow over the spillway. The free surface was calculated using the volume of fluid (VOF) method. To deal with turbulence, the SST k-ω turbulence model was used. The study showed that the spillway is capable of carrying the designed flood discharge and the modified conditions with negative pressure behind the gate and at some points along the spillway. Hydraulic jumps occur at various distances throughout the plunge pool depending on the incoming velocity in the flip bucket.


INTRODUCTION
The Al Adhaim Dam was built on the Al Adhaim River t 133 km north of Baghdad City. It is a multifunctional dam that provides power, flood control, and water storage for irrigation and other uses. The dam body is made up of a 3800m long earth fill dam with a clay core with a height of 76.5 m and a top width of 12 m. The upstream side of the dam body is protected by concrete block riprap. The reservoir storage volume is 1.5 billion m 3 covering an area of 120 km 2 . Its spillway was designed to release flood waves at a rate of 1150 m 3 /s. The Ministry of Water Resources intended to increase the dam's storage capacity by installing gates on the dam spillway to increase the water level of the dam reservoir. The performance of the spillway and the stilling basin of the dam must be investigated under this condition. A numerical simulation model can be developed by using CFD software. This software provides detailed hydrodynamics calculations of the flow over the spillway and within the Al Adhaim Dam stilling basin under the gated case.
CFD was used to simulate flow over ogee spillways by many studies and showed to be accurate as the results were compared to data acquired from physical models or field measurements. (Daneshkhah and Vosoughifar, 2012), investigated the effects of various turbulence models on the appropriate flow parameters over an ogee spillway. The results show that the RNG k-ε turbulence model, which was used, had significantly higher accuracy than the other models. (Zhenweia et al., 2012) investigated by using numerical 3D simulation of the spillway flow field using VOF method. The standard k-ε turbulence model was applied, which employed turbulent flow with high velocity in the chute. The results show that the single hole scheme is better than the two holes scheme because of the effect of the pier and the bigger deflected angle in the two holes scheme. (Kumcu, 2016) studied the flow over the Kavsak Dam spillway modeling as well as a comparison of experimental data with CFD analysis. FLOW-3D CFD software was used for the numerical simulation. For the turbulence model, k-ε was employed. The results of the physical model and the numerical mode were compared using discharge rating curves, velocity patterns, and pressures. It shows that the difference between the CFD and the physical models is 3.2%. (Mohammed et al., 2017), used the commercial CFD software to test an ogee-crested spillway at three different heights. The results revealed that the data from physical and numerical models for water surface profiles agreed well, despite some pressure differences. (Zawawi et al., 2018) investigated several open gates of a dam spillway. ANSYS program to create the 3D flow. Three dam spillway models were created for the simulation, with each model having a different gate opening height. The result showed that the area of the gate gap would affect the speed of the water released from the dam and the impact of the water discharge to the bottom downstream. conducted a hydrological study on modifying the Adhiam Dam Spillway by installing gates in the spillway to save more water. Two gate installation options were modeled. Installing a pair of gates near the lake outlet was one option. Installing a pair of gates above the spillway crest is the second option. The study found that the obtained maximum discharges agreed with that obtained design discharges, and installing gates is a good suggestion to increase the capacity of the Adhiam Reservoir.

DESCRIPTION OF THE STUDY CASE
Two 6 m slice vertical gates with lip face having a 45-degree angle will be installed on 15 m of the spillway wide, the width of each gate 0.2 m, and the height of the gate is 13 m with a 2 m wide pier between them. Five gates opening to be examined between 2m and 10m opening.

THE GOVERNING EQUATIONS
FLUENT software was used to study the hydraulic performance of the free surface flow over the ogee spillway and the stilling basin of Al Adhaim Dam. The simulation model's geometry was plotted by using ANSYS Parametric Design Language, APDL. The output file of APDL is inserted through the read case of FLUENT. FLUENT is the most common and suitable software of CFD that offers a huge advanced physical model for fluid flow. The finite volume method can solve the 3D differential equations and algebraic equations. In this program, the volume of fluid (VOF) model was used to determine the free surface of the flow. (Nichols and Hirt, 1981), presented a model for determining the common surface of a two-phase fluid, which is useful in a variety of hydraulic issues. The equations governing the flow over the spillway are Reynolds averaged Navier Stokes. Among the governing equations are the continuity and momentum equations, as follows (Daneshfaraz et al., 2016).
= 0 (1) The Reynolds stresses, which operate on the fluid and apply the effects of turbulence vortices, are the parameter: Where: t =is time, s ui = the velocities in the xi directions, m/s ρ =is the volume-fraction-averaged density, kg/m 3 g = acceleration due to the gravity, m/s 2 v = kinematic viscosity, N.s/m 2 p= pressure, pa ij= Kronecker delta and this will be 1 if i = j and 0 otherwise.
Where ₖ represents the generation of turbulence kinetic energy due to mean velocity gradients. ω represents the generation of ω. ω and ₖ represent the effective dissipation of and due to turbulence. ₖ and ω are user-defined source terms. ₖ and ω are the turbulent Prandtl numbers for and , respectively. Dω represents the cross-diffusion term.

BOUNDARY CONDITIONS
The problem is one of the most important stages of numerical flow field analysis. The CFD software provides a variety of boundary conditions that can be used to determine the domain solution. The upstream boundary is the pressure inlet. The downstream boundary is the pressure outlet which is the atmospheric pressure for both upstream and downstream sides, channel walls are the bottom boundary, and side walls and blocks are defined as nonslip walls in this study's numerical modeling of the ogee spillway. The no-slip boundary indicates that the velocity at the walls is zero, that the roughness height ks for smooth concrete is 0.5mm (Engineering ToolBox, 2003) and that the default roughness constant cs is 0.5.

ANALYSIS METHOD
Meshing is the first step in mathematical analysis after the geometry has been prepared. As shown in Fig.2, a tetrahedral mesh suitable for use with a 3D model was used. For all the gated conditions: the largest grid size was 8 m. A smaller mesh with a minimum size of 0.01m was used to obtain accurate results in areas where flow conditions are expected to be highly variable. The bed of the spillway has a maximum face size of 0.1 m. The total number of cells obtained is estimated to be around 3.5 million. Skewness was measured is 0.77, which is considered acceptable (ANSYS Fluent Theory Guide). The aspect ratio calculates how much the cell has been stretched, which is 1.16 for the minimum value. The governing equations are discretized using the PRESTO pressure discretization scheme and the finite volume method. The PISO technique, which is only used for transient simulations, could be used to couple the pressure and velocity (ANSYS Fluent Theory Guide). Under-relaxation factors are used to stabilize the convergence behavior of the outer nonlinear iterations in the pressure-based solver by starting arbitrary amounts of  into the system of discretized equations. The under-relaxation factors, such as the pressure coefficient, k, momentum coefficient, and ω are all set to their default values of 0.3, 0.7, 0.7and 0.8, respectively. In the vast majority of cases, the default values are close to ideal. The size of the time step depended on the gated condition.

RESULTS AND ANALYSIS
The velocity, surface of the water level, and pressure variation over the spillway and along the stilling basin of AlAdhiam Dam were obtained under different gate openings at the maximum capacity of the spillway at 1150 m 3 /s. The rating curve shown in Fig. 3 was obtained based on the result of runs under different conditions of the gate opening. The discharges vary from 257.32 to 1036.3 m 3 /s, corresponding to the gate opening 2 and 10m, respectively.               The pressure at the inlet side of the spillway varied between (7.2 -183) kPa depending on the gate opening. The pressure behind the gate is reduced to reach low negative pressure; a minimum value of the negative pressure is -54.05 to -58.59 kPa, which corresponds to gate openings of 2 and 10 m, respectively. After that, the pressure will be positive along the stilling basin except for some points on the drops in elevations and at the bucket. The pressure at the drops elevation at the 2m gate opening increased and varied from -25.45 to -28.28 kPa. At the 4 m gate opening, the drops elevation varied from -35.5 to -47.49 kPa, and at the 6m gate opening, the drops elevation also varied -50.8 to -57.32 kPa, at the 8 m gate opening the pressure at the drops elevation varies from -48.22 to -57.45 kPa and the pressure at the drops elevation at 10 m gate opening vary between -52.60 to -56.30 kPa. When the gate opens 10 m at the end of the flip bucket, the maximum value of the minimum negative pressure occurs, with a value of -459 kPa. At 2m gate opening, the lowest value of minimum negative pressure at the end of the flip bucket is -4.9 kPa.

CONCLUSIONS
In this study, the flow over the Al Adhaim Dam ogee spillway was simulated in 3D using CFD techniques after being gated, and turbulence was modeled in the flow using an SST k-ω with the VOF approach. A tetrahedral mesh of 8 m was employed with the grid size. The result of this investigation of a spillway and the stilling basin of Al Adhaim Dam after gated conditions indicated the following conclusions: