Numerical Investigation of Flow in 180-degree Bend with Different Radius and Lateral Intakes by SRH-2D Model

Introduction
In nature, different types of river bends such as simple, complex, sharp, mild, etc. can be seen. The hydraulic behavior of the flow in the place of water intake from the river bends is more complicated than the straight path. Due to the presence of secondary flow in the bends, the outer wall of the bend is a suitable place for taking water from the river.

Methodology
Using SRH-2D numerical model, the behavior of flow in a 180-degree bend with side intake and three ratios of bend radius to channel width (R/B) were investigated. In this way, three ratios of bend radius to the channel width, equivalent to 2 (sharp bend), 3 and 4 (mild bend) were used. The hydraulic parameters such as flow velocity and streamline and spanwise water level were taken for sections of 100, 110, 115 and 125 degrees along the main channel and one section at the beginning of the side channel.

Results and discussion
Validation of the numerical model was done using the laboratory data of previous research. The results showed that the model correctly simulates the different parts of the flow in a bend with the presence of a side intake. The calculated and measured velocity values have an R2 coefficient equal to 0.94. According to the results obtained from this research, in the 115-degree section which corresponds to the axis of the side intake, with the increase of R/B ratio from 2 to 4, the velocity of the flow entering the intake increases by 6%. In the area of the outer wall of the bend, before the water intake, flow velocity in the sharp bend is higher than in the mild bend, and in the section immediately after the water intake, the flow velocity in the mild bend is higher than in the sharp bend. Regardless of the relative radius of the bend, before the water intake opening, the position of the maximum flow velocity is in the vicinity of the outer wall of the bend, while after the intake opening, it is drawn towards the inner wall of the bend. In the 125-degree section, with the relative radius of the bend increasing from 2 to 4, the flow velocity in the outer wall has increased by 5%, but in the inner wall, the average velocity has decreased by 4%. At the 115-degree section, the maximum amount of water level can be seen around the center line of the bend, and the sharper the size of the bend, the higher the amount of water level rises around the center line of the bend.

Conclusions
According to the obtained results, the SRH-2D model was able to simulate well the flow in a 180-degree bend with a lateral intake. Also, the different flow areas including the point of stagnation, the minimum velocity after the intake which tends to the inner wall bend, the minimum velocity, the separation zone and the maximum flow area inside the intake channel were correctly modeled. In taking and deviation of water from a mild bend, the entering velocity into the intake is higher than that from a sharp bend, which can be effective in reducing sedimentation in the intake opening. In the cross-sections before and after the lateral intake, the highest value of flow depth occurs near the outer wall of the bend and the lowest value occurs near the inner wall of the bend. The sharper the bend, the greater spanwise slope of the water surface.