Numerical hydraulic and hydrodynamic investigation of flow passing through multiple elliptic lopac gates with Flow 3D software

Introduction
One of the water level control and regulation structures is lopac gates, which is known as a dam or flow depth control structure. lopac gates are in the form of two rectangular sheets that are connected to the channel wall. This structure has the ability to adjust the water level upstream of the valve and regulate the distribution of passing water downstream of the valve in different flow rates and openings. lopac gates are divided into rectangular and elliptic categories.
 
Methodology
 Experimental model
The experiments conducted by (Pilbala et al, 2021) were conducted on an open rectangular channel with a length of 10 meters, width and height of the laboratory channel equal to 0.8 meters, with a horizontal rectangular channel bed slope and at Shahid Chamran University of Ahvaz. In total, the laboratory model in 45 tests considering three flow rates (25, 35, and 45 liters per second), five opening angles (35, 40, 45, 50, and 52.5 degrees), and three intake ratios (70, 80 and 90 percent) by (Peel Bala et al., 2021).
 
 Verification of the physical model
In this research, in order to validate and increase accuracy in numerical simulation results of a rectangular Lopac gate (Pilbala et al, 2021) with a flow rate of 25 liters per second, an opening of 35 degrees and an extraction rate of 70% in the number of mesh cells and different turbulence models were simulated in Flow3D and compared with laboratory data. The results show that among the simulations regarding the selection of the number of meshing cells and different disturbance models, the best performance, compared to the measured laboratory data, is by RNG models, and the number of meshing cells is 1000000. The RNG disturbance model and the number of meshing cells used for all simulations were used in the results of this research. Also, according to the convergence diagram, 40 seconds of time was used for the purpose of simulation.
 
Results and discussion
The effect of changes in Froude number (Fr) on maximum velocity (V _max)
When using multiple elliptic lopac gates in submerged flow mode, the range of velocity distribution and numerical values of velocity are reduced compared to multiple elliptic lopac gates in free flow conditions. It can be seen that with the increase of the Froude number in fixed openings, the maximum velocity value has increased, and also with the increase of the opening angle of the valve (reduction of narrowness), this parameter has been continuously increased. It can be seen that the maximum velocity value has an inverse relationship with the absorption percentage. In such a way that with the increase in the absorption percentage of the current, the maximum velocity decreases continuously. Investigations show that the maximum flow velocity created on multiple elliptic lopac gates in submerged flow conditions is far lower than when multiple elliptic lopac gates are used in free flow conditions.
 
Effect of dimensionless opening (bg/B) on discharge coefficient (Cd)
It can be seen that the value of the flow coefficient during the operation of multiple elliptic lopac gates in submerged flow conditions was lower than that of multiple elliptic lopac gates in free flow conditions. Evaluations show that when multiple elliptic lopac gates are used in submerged flow conditions, compared to multiple elliptic lopac gates in free flow conditions, the flow coefficient at the opening of 30 degrees, on average, in flow rates of (20, 40, and 60) liters per second for the absorption rate of (70, 80 and 90) percent, respectively 8, 26 and 45 percent, in the opening of 45 degrees, in the flow rates of (20, 40 and 60) liters per second for the absorption rate of (70, 80 and 90) percentages are 16, 34 and 49%, respectively, and at 60-degree opening, at flow rates of (20, 40 and 60) liters per second, for the amount of absorption (70, 80 and 90)%, respectively, 17, 36 and 54%  will be reduced.
 
Flow pattern in multiple elliptic lopac gates
 It was found that when multiple elliptic lopac gates are used in free flow conditions, high-strength vortices are formed compared to multiple elliptic lopac gates in submerged flow conditions. This phenomenon can cause problems such as increased energy loss, reduced water absorption rate, reduced efficiency of hydraulic machines, and their improper performance. However by examining the flow pattern of multiple elliptic lopac gates in submerged flow conditions, the size of the vortices has decreased and as a result, its strength has decreased.
 
Conclusions
In this research, using laboratory data and Flow3D software, the effect of using multiple elliptic lopac gates in submerged flow conditions compared to multiple elliptic lopac gates in free flow conditions, maximum flow velocity, flow coefficient, and flow vortices were evaluated. The results showed that the maximum velocity have a direct relationship with the Froude number and the opening angle. Investigations showed that the flow coefficient (Cd) has a direct relationship with the flow rate and gate opening value. The results showed that the flow is diverted to the center or around the channel when passing through multiple elliptic lopac gates, which creates large vortices with high longitudinal elongation near the flume wall.