## 4 Results and Discussions
### Pressure Fluctuation Analysis of Bladeless Region
It can be seen that the variation period of 5 f\({}_{\rm n}\) was about 90\({}^{\circ}\), which means that if the circumference area is divided into four quadrants, the distribution of this component in each quadrant is roughly the same. The variation period of 10 f\({}_{\rm n}\) and 20 f\({}_{\rm n}\) varied with a period of 22.5\({}^{\circ}\), which was the same as the angle between each blade. It shows that the blade frequency and frequency doubling amplitude values were mainly affected by the different positions of the monitoring points relative to the adjacent guide vanes. In addition, the amplitude variation of the main frequency 10 f\({}_{\rm n}\) at the monitoring points GR037-GR039 was different from that at other points. Figure 15 shows the velocity distribution around GR001-GR003 and GR037-GR039 at 1.0 Q\({}_{\rm d}\). It can be seen in Figure 16 that the high-speed flow field at the outlet of the blade impinged the guide vane, resulting in delamination on the trailing edge of  
Figure 14: Pressure distribution of GR001 at different times at 1.0 Q\({}_{\rm d}\). the guide vane. The velocity direction was toward monitoring point GR002, so this monitoring point was greatly affected by the blade frequency. At monitoring points GR001 and GR003, at which the occurrence and attenuation of stay guide vane rotor-stator interaction occurred, respectively, the blade frequency amplitude was small. The variation trend of blade frequency amplitude at monitoring points GR037-GR039 was the same as that at monitoring points GR001-GR003. However, the upstream channel was near the tongue, so the insufficient overcurrent capacity led to a small velocity in the upstream, the runner's outflow was squeezed, and the velocity of the channel at the monitoring point increased. At monitoring point GR038, the velocity along the guide vane passage direction was relatively high while the upstream velocity was relatively low. The rotor-stator interaction effect generated after hitting the trailing edge of the guide vane was relatively small, resulting in almost no increase in the blade frequency amplitude at GR037 and GR038. Therefore, the blade frequency amplitude of GR038 and GR039 was obviously lower than that of the other channels at the same position. As shown in Figures 17-19, the pressure fluctuation distributions at different vane heights under different flow rates were significantly different. The section near the roof was span0.1, span0.5 was the middle section, and span0.9 was the section near the bottom ring. At a low flow rate at 0.75 Qd, the pressure fluctuation intensity distribution at different guide vane heights had the most significant  
Figure 16: Velocity distribution around the monitoring points at 1.0 Qd. Figure 15: Circumferential distribution of rotor–stator interaction amplitudes at 1.0 Qd. difference. Pressure fluctuation in span0.1 was small, and the strong fluctuation region was mainly concentrated below the trailing edge of the guide vane. The pressure fluctuation under the trailing edge of the guide vane at the middle height was significantly intensified, which affected the pressure fluctuation distribution in the bladeless region. The pressure fluctuation under the trailing edge of the active guide vane in span 0.9 weakened, but the pressure fluctuation intensity in the bladeless region near the outlet of the runner was greater. In addition, due to the strong flow separation near the trailing edge of the guide vane at a low flow rate at 0.75 Qd, the pressure fluctuation in the channel above the trailing edge of the guide vane was also obvious, especially near the bottom ring. With the increase of flow, the overcurrent capacity in the cascade channel increases, and the pressure fluctuation greatly reduces. The strong fluctuation region was mainly concentrated in the bladeless region near the runner's outlet, and the pressure fluctuation intensity near the bottom ring was relatively large.