## 4 Experimental Study
### Experimental Results of PIPRs with Three Different End Faces  
The results of the PIV experiment for PIPRs with three different end faces are shown in Figures 24-26. It can be seen from Figure 24 that the surrounding flow field is relatively stable, benefiting from the end face optimized by IWOA. Before the plugging process, there are no vortices of varying sizes in the flow field. At the end of the plugging operation, there is no vortex or backflow phenomenon in the experiment. From the velocity vector distribution of numerical simulation, a symmetrical vortex appears at the tail of PIPR. And fluid particles still smoothly transition along the surface of the elliptical end face. Figure 25 shows the unstable flow field and vortex structure for the improved PIPR model before optimization. When fluid particles pass through a narrow pipeline wall, there is no smooth transition, which is caused by fluid backflow. For the original PIPR model with a footstep-shaped end face in Figure 26, the vortex structure is more obvious, and fluid particles are more densely distributed. When the plugging operation is nearly completed, most of the in-pipe fluid particles are basically in a static state, and the original vortex structure and flow field characteristics at the tail of PIPR gradually disappear, which is consistent with the numerical simulation results. The tail flow field of the PIPR with a curved end face is stable, and the vortex evolution has finished before the plugging operation is completed. Because the curved structure can make fluid particles flow more smoothly compared to a footstep-shaped structure, there  
Figure 24: The results of the PIV experiment for improved PIPR’s end face optimized by IWOA. (**a**) Before the plugging process; (**b**) After the plugging process. Figure 25: The results of the PIV experiment for improved PIPR’s end face before optimization. (**a**) Before the plugging process, (**b**) After the plugging process. Figure 26: The results of the PIV experiment for the original footstep-shaped PIPR’s end face. (**a**) Before the plugging process, (**b**) After the plugging process. are no obvious flow field characteristics like the tail of PIPR with a footstep-shaped end face. For the flow field characteristics at the tail of PIPR, the tail flow field of improved PIPR's end face optimized by IWOA is more stable than the other two models, which can ensure the stable development of in-pipe flow field and the thorough evolution of vortex structure. In summary, the optimization of PIPR's end face can reduce the impact caused by turbulent flow field excitation. To verify the similarity between experimental and simulation results, the pressure values of monitoring point A during the plugging process are obtained and compared for the original footstep-shaped PIPR's end face, as shown in Figure 27. For the leakage that exists in the experimental setup, the experimental results are slightly smaller than the simulation results, but the maximum relative errors are only 10.33%. Therefore, the plugging experiment and numerical simulation are consistent. In order to observe the pressure fluctuation of the flow field for the above three PIPR's end faces, namely the improved PIPR's end face optimized by IWOA (Model 1), improved PIPR's end face before optimization (Model 2), and original footstep-shaped PIPR's end face (Model 3), the pressure values of seven observation points (A-G) are compared, as shown in Figure 28. Due to the similarity experiment, the pressure of the flow field is smaller than that of the experiment. Before the plugging operation, the pressure values of Model 1 were stable and smaller than that of Model 2. The upstream pressure and downstream pressure of Model 3 are larger, and the pressure variation is more drastic. After the plugging operation, the pressure values of the plugging area and downstream of Model 1 are smaller in general. The pressure oscillation of Model 2 and Model 3 is larger than that of Model 1.