# Experimental Study on Geysers Induced by the Release of Trapped Air in Storage Tunnel Systems
## 3 Results and Discussion
### Transient Pressure  
Figure 4 shows the variation of system transient pressure during the release of trapped air pockets under operation modes A and B, in which, \(P/\rho_{w}g\) represents the pressure head.  
According to the transient pressure variation characteristic, the air pocket release process can be divided into three stages: the stage of air pocket propagation in the tunnel, the stage of air pocket release, and the stage of inertia oscillation, among which the geysers occur in the stage of air pocket release.  
At the stage of air pocket propagation in the tunnel, affected by air pocket compressibility, the system pressure presents a law of periodic oscillation. Under mode A, the pressure of PT1 is significantly reduced due to the reduction of the water level in the shaft. As the water level of the shaft gradually stabilizes, the period of pressure oscillation is short, and the amplitude decays quickly. The pressure of PT2 increases rapidly from zero and maintains wide oscillation due to the compression and expansion effect. As the air pocket propagates downstream, the pressure of two measuring points gradually tends to be consistent. On the contrary, in mode B, under the excitation of the external pressure head, the compression and expansion effect of the air pocket is significantly enhanced, the pressure oscillation period of each measuring point increases, and the amplitude attenuation rate becomes rather slow. The pressure of the two measuring points was consistent.  
At the stage of air pocket release, with the length of the air column increasing gradually, the system pressure drops rapidly. The greater the gas flow velocity, the faster the pressure drop. When the air column is broken, the system pressure drops to a minimum. At this moment, the water flow in the tunnel transforms into open channel flow, and the system pressure maintains a high-frequency oscillation at a low pressure.  
Figure 4: Variation of transient pressure during the release of a trapped air pocket. (**a**) Operation condition: mode A, \(D\) = 0.05 m, \(H_{0}\) = 0.675 m, \(L_{0}\) = 2.4 m; (**b**) operation condition: mode B, \(D\) = 0.05 m, \(H_{0}\) = 0.675 m, \(L_{0}\) = 2.4 m.  
During the process of the air column rising and breaking, the tendency of pressure variation in mode A is the same as that in mode B, but no geyser is formed, which indicates that the pressure drop is mainly caused by the rise of the air column and the decrease of the water column length, and the external pressure plays an important role in the formation of the gas-flow geyser.  
When the water-air mixture is near the tunnel crown, it will be squeezed, and the system pressure increases rapidly. The maximum pressure is up to 3-5 times the external pressure. In mode A, no pressure surge can be generated due to the absence of external pressure.  
After a geyser occurs, the instantaneous high pressure in the system is released, and the system pressure continues to oscillate periodically under the action of external pressure and inertia, but the oscillation amplitude is significantly lower than the maximum surge pressure. In this stage, the air pocket is rarely released. When the residual air content in the system is still relatively high, under the action of pressure oscillation and water-air interaction, multiple geysers may be produced.