# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 2 Siphon Drainage Test
### Siphon Flow Period  
During siphon drainage, many flow patterns exist, including bubble flow, bubbling flow, and slug flow. In D-3-5 pipes, slug flow is the dominant flow pattern, while in D-6.5-8 pipes, slug flow and churn flow are both spotted.  
The shape and size of the bubbles in the siphon drain's vertical rise section determine the size of the water cross section and the main flow pattern in the horizontal section. The air pressure inside the pipe gradually decreases when the liquid is lifted in the rising section. [PERSON]'s law shows that the reduction of air pressure will cause the precipitation of the original dissolved gas in the water if temperature and other environmental factors remain unchanged. Therefore, with the continuous forward motion of the fluid in the siphon tube, the flow is transformed from a single fluid to a gas-liquid two-phase mixed fluid motion. In Schemes 41-50, as the height rises, round bubbles gradually appear and are accompanied by trembling motion, while the tube wall has attached many tiny stationary bubbles. However, in Schemes 51-52, the stationary bubbles gradually evolved into moving spherical and long ellipsoidal bubbles with diameters ranging from 3 to 7 mm as the observed height increased.  
In the D-3-4 pipes, the horizontal section of the siphon is mainly in the form of segmental slug flow: the front of the segmental slug bubble is curved with some curvature, and the tail is closer to a flat surface. The type of slug flow in the tube can be further classified as dry slug flow. When the siphon flow in the D-5 pipe enters the horizontal section, the main flow pattern of gas-liquid two-phase flow is mainly wet slug flow, with occasional appearance of dry slug flow. The difference between dry and wet slug flow manifestations is determined by the bubbles, which are composed of only the gas-liquid interface in wet slug flow, while the bubbles in dry slug flow are composed of both gas-liquid interfaces as well as the pipe wall. In horizontal sections of D-6.5-8 pipes, due to the effect of gravity and liquid surface tension, the churn and the slug flow are dominant flow pattern, with bubbles deviating from the midline and adhering to the upper wall of the tubes. In addition, many tiny stationary bubbles can be observed in the D-8 pipe attached to the wall but keep merging with the later moving long bubbles. It should be noted that the bubbles in the D-6.5-8 pipes are irregular and their longitudinal profiles are asymmetric up and down, as shown in Figure 5c,d; while the bubbles in the D-3-5 pipes are more regular in shape and longitudinal profiles are symmetric up and down, as shown in Figure 5a,b.  
A sudden acceleration of the fluid can be observed at the junction of the horizontal and vertical descending section: the bubble deformation elongates, and the shape becomes irregular. However, this process lasts only for a short period, and the flow pattern of the vertical section soon returns to the same as that of the horizontal section. The bubbles may merge during the motion but not redissolve in the water. Eventually, the siphon flow will still be discharged from the siphon tube as a gas-liquid two-phase flow.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **D/[mm]** & **\(L_{hor}\)/[m]** & **\(H_{lift}\)/[m]** \\ \hline
1–10 & 3, 4, 5, 6.5, 8 & 0 & 3, 5 \\
11–20 & 3, 4, 5, 6.5, 8 & 0 & 7, 8 \\
21–28 & 3, 4 & 80, 160 & 4, 6 \\
29–32 & 5 & 80, 160 & 3, 6 \\
33–40 & 6.5, 8 & 80, 160 & 3, 5 \\
41–44 & 4 & 80, 160 & 7, 8 \\
45–48 & 5 & 80, 160 & 7, 8 \\
48–50 & 6.5 & 160 & 7, 8 \\
51–52 & 8 & 160 & 7, 8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental schemes of siphon drainage (T = 25 \({}^{\circ}\)C, \(P_{la}\) = 100.53 kPa).