## 3 Valve Opening Characteristics
### Results and Discussion  
Figure 8 shows the pressure and velocity distribution contours at three instances during the valve opening process of the CFFCS: (a) the beginning of coupling (t = 0.5 s, the valve is not opened); (b) the moment the valve has just started opening (t = 1 s); (c) the valve is fully opened (t = 2 s). It can be seen from Figure 8a that the two chambers are not connected and the pressure is evenly distributed in the high-pressure chamber on the left side of the valve and in the low-pressure area on the right. At this time, the flow velocity in the two chambers is almost zero. Since at least one layer of mesh is required in the dynamic mesh setting, it is inevitable that a large flow velocity will be generated in the gap in the simulation results. In the actual opening process, due to the presence of the seal, such a large flow velocity does not exist in the gap. This study only observes the characteristics of the flow in the fluid domain during the coupling process, thus the influence of such unavoidable limitations of the simulation methodology on the research results is ignored. The left and right chambers start to connect during the leftward movement of the valve (as shown in Figure 8b). At this time, the gap between the valve and glove reaches its smallest value. In this state, the fluid is pushed by the high pressure in the left chamber to pass through the gap at a great velocity and form an obvious velocity gradient in the low-pressure area. At this time, the pressure distribution of the two chambers still does not change greatly. Figure 7: Meshing of the two models, (**a**) CFFCS and (**b**) NFFCS. At t = 2.0 s, CFFCS completes the coupling process, and the entire hydraulic circuit is fully connected (as shown in Figure 8c). In this state, the pressure distribution in the entire watershed tends to be uniform, the inlet maintains a high pressure, and there is a high and even pressure gradient. The overall flow velocity distribution corresponds to the pressure distribution. Figure 9 shows the pressure and velocity distributions of the NFFCS at different moments during the coupling process. The five most representative time points were selected for observation and analysis. As shown in the left side image in Figure 9a, at the initial time (t = 0 s) the valve in the coupler is closed. In this state, it can be observed from the pressure distribution that the pressure accumulation phenomenon occurs at the inlet and forms an obvious high-pressure zone. The pressure in the outlet chamber is low and there is no obvious pressure gradient. Similar to CFFCS, it can be observed that the entire watershed is divided into two chambers by the pin and the valve, as shown in the right side image in Figure 9a, and the velocity in both the chambers is zero. As the gap of the pin valve becomes larger (t = 0.2 s), the fluid in the high-pressure side begins to flow into the outlet chamber through the pin valve gap and the pressure difference between the inlet and the outlet is reduced. Since the gap is small, the flow velocity in the gap is large (as shown in the right side image of Figure 9b), which results in very low pressure in the gap (as shown in the left side image of Figure 9b). Due to  
Figure 8: Instantaneous pressure and velocity distributions at different times during the coupling process of the CFFCS, (a) t = 0.5 s, (b) t = 1.0 s and (c) t = 2.0 s.  
high-speed flow at the end of the gap, the fluid accumulates at the corners and is squeezed by the following high-speed fluid, which forms a local high pressure region. Figure 9: Instantaneous pressure and velocity distributions at different times during the coupling process of the NFFCS, (**a**) t = 0 s, (**b**) t = 0.2 s, (**c**) t = 0.4 s, (**d**) t = 1 s and (**e**) t = 2 s.  
After the pin valve is fully opened (t = 0.4 s), the pressure at the inlet and outlet becomes almost equal (as shown in the left side image of Figure 9c).