# Experimental Study on Geysers Induced by the Release of Trapped Air in Storage Tunnel Systems
## 3 Results and Discussion  
### Flow Regime  
Figures 2 and 3 show the typical flow regime of the tunnel and the geyser flow regime during the release of the trapped air pocket, respectively. After the control valve upstream was opened, the trapped air moved to the side of the shaft. The air pocket head was a smooth, curved water-air interface, as shown in Figure 2a, and the shape was consistent with the interface of the air intrusion water body described by [PERSON] [21]. The propagation velocity of the air pocket head was approximately \(0.52\sqrt{gD_{0}}\). The tail of the air pocket was a free surface bore formed by reflected wave due to the water flow striking the end wall as shown in Figure 2b. The interface of the bore was highly aerated flow, and the tail has a long and narrow air chamber. The head shape and tail shape of the air pockets in mode A and mode B were consistent. According to [PERSON]'s [21] theoretical analysis, the velocity of air pocket propagation in stagnant water depends on the shape of the air pocket, and is independent of external pressure; therefore, the velocity of air pocket propagation under two modes has no significant discrepancy. When the air pocket head moves to the shaft location, under the effect of buoyancy, some of the air enters the shaft and forms an air column. As the amount of air entering the shaft gradually increases, the length of the water column in the shaft decreases accordingly, and the rise speed of the water-air interface gradually increases. The upper remnant water column is rapidly lifted and ejected out of the shaft in the role of high-speed gas flow. Under the effect of inertia, the ejection continues to rise and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Operation** & **Diameter of Shaft,** & **Length of Air** & **Water Head of Tank/Water** \\
**Mode** & **D (m)** & **Pocket, \(L_{0}\) (m)** & **Depth of Shaft, \(H_{0}\) (m)** \\ \hline A & 0.05 & 0.60; 1.20; 1.80; 2.40 & 0.525; 0.675 \\ B & 0.05; 0.075; 0.10; 0.15 & 0.60; 1.20; 1.80; 2.40 & 0.225; 0.375; 0.525; 0.675 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The operation conditions of the geyser experiment. Figure 2: Typical flow regime in tunnel: (**a**) head of the air pocket; (**b**) tail of the air pocket; (**c**) unstable free surface waves. spread to the surrounding air. The geyser evolution process is shown in Figure 3a-d. The geyser is mainly caused by gas-flow shock, so it is called a \"gas-flow geyser\". After the gas-flow geyser occurs, the air column is broken, and the pressure at the shaft position drops sharply. When the air pocket volume is large enough, the air pocket in the tunnel will be connected to the atmosphere, and the water flow in the tunnel will translate into open channel flow. When the air pocket volume is small, the air column in the shaft and the air pocket in the tunnel are not connected, but the local low-pressure area is still formed at the shaft position. Under the action of external pressure, water from a high-pressure water tank will rapidly fill the tunnel, quickly driving residual air out of the tunnel and significantly increasing the air discharge. When the water level in the tunnel is close to the pipe crown, the system pressure fluctuation intensifies the instability of the free surface and produces unstable wave flow and slug flow in the tunnel (as shown in Figure 2c). Under the action of inertia, the water flow continues to fill into the tunnel and strike the water-air mixture, which produces a water hammer effect and causes the system pressure to surge. Subjected to instantaneous high pressure, the water-air mixture pours into the shaft and rises rapidly. When the extreme pressure is large enough, the water-air mixture will jet out of the shaft and form a geyser. The geyser evolution process is shown in Figure 3e-h. The geyser is caused by the instantaneous pressure surge of the system, so it is called the \"surge-type geyser\".