## 4 Results
### Pressure Pulsation Analysis
The effect of rotation on the pressure pulsation was extremely pronounced. As opposed to the operation of the unit under the pump condition, the pressure pulsation change at the interface between the front guide vane and the runner was also quite severe when it was operated under the turbine condition, and its periodic change was obviously related to the number of movable guide vanes. When the pump was running in the forward direction, it was running under the pump condition. Inside the runner chamber, the flow was influenced by the axial water thrust. At this time, the mechanical energy was converted into the kinetic energy of the water body. When it flowed into the guide vane area after passing through the movable guide vane, kinetic energy was converted into pressure energy. When the pump was running in reverse, the unit was equivalent to a water turbine. The fluid flowed through the guide vanes and entered the runner chamber, converting kinetic energy into mechanical energy so the impeller rotated. In the guide vane area, it was regulated by the movable guide vanes. The pressure energy  
Figure 9: Time–domain diagram of the pressure pulsation under pump conditions. **(i)** 0.6Q. **(ii)** 0.8Q. **(iii)** 1.0Q. **(iv)** 1.2Q. was converted into kinetic energy, so the pressure pulsation changes were also very obvious and related to the number of guide vanes. A Fourier transform was performed on the pressure pulsation coefficient data obtained by numerical simulation, and the distribution of the pressure pulsation coefficients on the frequency spectrum was obtained. Figures 11 and 12 are the frequency domain diagrams of the pressure pulsation under the pump condition and the pressure pulsation under the turbine condition, respectively. It can be seen from the figure that when the unit was running under the pump condition, under different flow conditions, the frequency at the inlet of the runner was the BPF and its multiplier, in which the main frequency was the BPF, and the secondary frequency was the rotation frequency. Under the design flow condition and the large flow condition, the primary and secondary frequencies of the pressure pulsation frequency distribution of each monitoring surface were obvious, and there was no obvious clutter. Under the small flow condition, however, especially deviation from the design flow under the low flow condition in the low frequency band, there were a many secondary frequencies and clutter, the spectrum was extremely complex, and there was no obvious main frequency, indicating that the internal flow state of the unit was poor at this time, with serious decurrent, backflow, etc. Comparing the frequency domain diagrams under different flow conditions, it can be found that on the monitoring surface of the runner chamber, the main frequency was the BPF, and when it was far away from the runner chamber, the main frequency was the rotation frequency. This indicated that the dynamic and static  
Figure 10: Time–domain diagram of the pressure pulsation under turbine conditions. **(i)** 0.6Q. **(ii)** 1.0Q. interference area was obviously affected by the rotation of the blades, while in the non-blade area, it was mostly influenced by the rotation. Figure 11: _Cont._  
Figure 11: Frequency domain diagram of the pressure pulsation under pump condition. (**i**) 0.6Q; (**ii**) 0.8Q; (**iii**) 1.0Q; (**iv**) 1.2Q. Figure 12: Frequency domain diagram of the pressure pulsation under turbine condition. (**i**) 0.6Q. (**ii**) 1.0Q. When the pump was running under turbine condition, the amplitude of the pressure pulsation coefficient under the low flow rate condition was larger than that under the design flow rate condition, but the frequency domain distribution of the pressure pulsation was basically the same, reflecting the influence of the flow on the pressure pulsation. In Figure 12, when in the reverse operation, at the outlet of the guide vane, the main frequency was the guide vane passing frequency, the secondary frequency was the rotation frequency and the multifold vane passing frequency.