## 5 Discussion  
### Flow Field Distribution  
When the service gate height is small, the flow field behind the emergency gate is mostly full flow in the closing process. While the service gate height is large, the flow field behind the gate usually goes through the stages of the vortex, backflow, open flow and jet flow. For the upstream water depth of 65 m and service gate height of 4.0 m, Figure 9 exhibits the flow field behind the emergency gate. In the initial stage, the vortex shedding appears and mainly concentrates in a small area near the gate bottom, as displayed in Figure 9a. When the fluid flows through the gate bottom, the streamlines are separated. Afterwards, the vortex generates, detaches and moves into the downstream. The gate is subjected to the vortex action and forms the vertical vibration, which is similar to the vortex-induced vibration of marine pipelines [22]. As the gate continuously falls, the vortex shedding gradually transforms into the backflow. As depicted in Figure 9b, the backflow with a large range presents at the top of the spillway tunnel. The cause of the backflow  
Figure 8: Flow discharge and velocity: (**a**) service gate height 1.0 m; and (**b**) service gate height 4.0 m.  
is the shear action of the outflow and the flow draining from the gate shaft. Due to the separation of the flow draining downstream, there is a flow passage between the backflow and the gate downstream plate. As the gate drops further, the fluid in gate shaft drains completely, and air subsequently flows into the pressurized tunnel. At this stage, the flow pattern behind the gate transforms from full-flow to open-flow with strong turbulence, as displayed in Figure 9c. With the emergency gate being further closed, the flow jets out from the gate bottom and forms a submerged water jump with strong swirling and rolling in the pressurized channel, as shown in Figure 9d. At last, the emergency gate is completely closed and the flow cuts off.  
[PERSON] et al. [23] established the fluid-structure interaction numerical simulation for the emergency gate through Fluent and studied the gate vibration in a fixed gate opening. They believed that turbulent pressure and multi-vortex mixing were the main reasons for the gate flow-induced vibration. Moreover, some researchers pointed out that vortex shedding behind the gate bottom is the cause of the gate vibration [6; 7]. Those results, nevertheless, were mostly obtained for low-speed flow conditions or in fixed gate openings. We found that there was no large-scale and continuous vortex shedding phenomenon at the gate bottom for high-speed flow, and vortex shedding only existed in a small range. The characteristics of gate vertical vibration in the closing process is different from that in a fixed gate opening. Thus, we have reason to believe that the gate vertical vibration is a motion  
Figure 9: Flow regime of behind the gate at different stages: (**a**) vortex; (**b**) backflow; (**c**) open flow; and (**d**) jet flow.  
induced vibration due to the variation in fluid boundary and gate body loads caused by the gate active closing process. Meanwhile, a severe flow pattern would aggravate the gate vibration. Thus, some studies have tried to artificially create a good flow pattern to control the gate vibration. [PERSON] and [PERSON] (2018) pointed out that the vibration amplitude dropped to nearly zero when a rigid plate was placed horizontally behind the gate. [PERSON] et al. (2018) came up with a measure to effectively suppress hydrodynamic loads through placing a penetrating orifice at the gate bottom. [PERSON] et al. (2018) modified the shape of the curved gate bottom and added a guide plate to stabilize the flow. When the gate opening height is 0.08 m, the vibration reduction can reach 75%. [PERSON] et al. (2019) mounted the horizontal porous baffle below the free surface to reduce the vortex in the gate downstream, which can effectively restrain the gate vibration.