## 4 Results and Discussion  
### Flow Field Analysis of Straight Outlet Conduit  
To explore the influence of a division pier on the hydraulic performance of the straight outlet conduit, according to the idea in [18], three kinds of division pier design schemes were designed as shown in Figure 11, which were all placed at the inlet of the conduit and continuously extended to the inside of the conduit, to analyze the flow characteristics of a straight outlet conduit with different lengths of division pier. The length of the division pier in design scheme 1 was the length of the round pipe section of the straight outlet conduit, the length of the division pier in design scheme 2 was extended to half of the round square diffusion section of the straight outlet conduit based on design scheme 1, and the length of the division pier in design scheme 3 was the diffusion section of the straight outlet conduit.  
The three-dimensional numerical simulation of the straight outlet conduit under the three schemes was carried out, and the three-dimensional streamline diagram of the straight outlet conduit under the three kinds of division pier length under the optimal working condition was drawn. It can be seen from Figure 12 that the streamline at the starting position of the division pier under the three schemes was relatively smooth, but there was a large range of unstable flow on both sides of the middle and rear position of the division pier; the unstable flow gradually improved with the increase of the division pier length. This is because the main flow in the straight outlet conduit changed direction after passing through the \(90^{\circ}\) elbow; under the action of circulation and flow inertia, the main flow deviated to the upper left side of the division pier, and there was a low-speed zone below the path.  
Figure 11: Design scheme of straight outlet conduit.  
Figure 10: Decomposition tree of wavelet packet in four layers.  
Table 2 shows the flow distribution on both sides of the division pier under the three schemes; when there was no division pier, the hydraulic loss of straight outlet conduit was 0.027 m under the optimal condition. It can be seen from Table 2 that the hydraulic loss of the straight outlet conduit was increased by adding the division pier; with the increase of the length of the division pier, the hydraulic loss gradually decreased, and in the process from scheme 1 to scheme 3, the hydraulic loss decreased by 7.4% compared with that in scheme 1. The main flow deviations of the three division piers with the same starting position but different lengths to the straight outlet conduit were concentrated on the left side, and the flow distribution ratios were 1.46, 1.45 and 1.48, respectively. It can be seen that the main flow deviation in the straight outlet conduit was mainly concentrated on the left side of the division pier, and there was only a small difference between the left and right sides of the division pier with the same starting position but different lengths. Combined with the previous research results, the division pier mainly caused the flow to deviate to the left side of the division pier, and the length of the division pier extending backward had an impact on the hydraulic loss of the straight outlet conduit, but the different lengths of the division pier had little impact on the flow distribution on the left and right sides of the straight outlet conduit.