## 4 Model Validation  
Figure 3 shows the comparison between the calculated and measured values of the water surface line of the curved section, and the water surface lines of D0, D45, D90 and D135 are obtained. It can be seen from the figures that the calculation results from grid 1 and grid 2 are in good agreement with the measured values, while the calculation results from grid 3 deviate from the measured values. For example, the D45 section, D90 section and D135 section of the water surface line deviation is larger, especially near the convex bend. It shows that the LES Model used in this article could accurately simulate the distribution characteristics of the profile of water surface of the bend flow. In order to illustrate the accuracy of the numerical calculation more clearly, the simulated values of the longitudinal time-averaged velocity distribution along the vertical line on the concave bank and the center line and the convex bank of four typical sections are analyzed. Figure 4 show the comparison between the calculated results and the measured values of the longitudinal velocity the latter of which is performed utilizing acoustic Doppler velocimetry (ADV). It can be observed that the calculated values from grid 1 and grid 2 are in good agreement with the measured values. The deviation between the calculated value and the measured value from grid 3 is large, especially in the cross section and the surface layer of the bend, such as the concave bank and the convex bank of the D90 section and the convex bank of the D135 section. The reason for the large deviation is that the simulation of the water flow structure near the wall, especially near the boundary, is not accurate enough under the condition of a coarse grid affected by the wall. Therefore,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \(\mathbf{Q}\) (L/s) & \(\mathbf{U}\) (m/s) & \(\mathbf{B}\) (m) & \(\mathbf{H}\) (m) & \(\mathbf{R}\) (m) & \(\mathbf{Re}\) & \(\mathbf{R}\)/\(\mathbf{B}\) & \(\mathbf{B}\)/\(\mathbf{H}\) & \(\mathbf{R}\)/\(\mathbf{H}\) \\ \hline
37.5 & 0.47 & 0.4 &the simulation of the flow velocity near the concave and convex sides has a large deviation from the measured value. Since the accuracy of numerical simulation depends on the discrete method, solution method and truncation error, there is a certain error between the calculated value and the experimental value. It can be seen from the comparison between the calculated and measured values of water level and longitudinal velocity in grid 1 and grid 2 that the calculated values of the LES model are basically consistent with the measured values, which verifies the credibility of the model. In order to observe the cross-section mainstream distribution from the three grids more clearly, the cross-section flow velocity along the way is extracted, as shown in Figure 5. After entering the bend, the mainstream of the water flow under the three grid conditions shifts to the convex bank and shifts to the middle position of the curved top section. After the curved top section, the mainstream shifts to the concave bank, and the mainstream remains in the concave bank to flow out of the bend. From the diagram, it can be observed that in the straight section of the entrance, the flow velocity of the cross section in Figure 5a-c is basically the same, and the difference begins to appear in the D45 section. In Figure 5c, the mainstream concentration is not high, but the cross-section velocity is relatively uniform. This is due to the large grid size and to the insufficient information of the bend flow structure. To sum up, under three grid conditions, the results of the curved flume test on the water surface line and the longitudinal velocity were verified. The results show that the influence of the near wall on the water flow structure reduce the ability of grid 3 with the lowest grid resolution to capture the details of the flow field, which affects the accuracy of the calculation, resulting in a large deviation between the calculated results and the measured values.