## 3 Experimental Analysis at Different Oil Pressure Levels
### Dual-Rotor Experimental Analysis
#### 3.2.1 Oil Pressure Level of 0.2 MPa  
Figure 10 shows the frequency waterfall chart extracted by the rotating disk along the vertical direction with a higher oil pressure level of about 0.2 MPa. Figure 11 shows the rotating shaft along the same direction with a higher oil pressure level of about 0.2 MPa. In terms of the results presented in Figures 10 and 11, the vibration amplitude gradually increases with the increment of the rotating speed in the range of 0 to 283 rad/s. The amplitude is abruptly dropped when the rotating speed exceeds 283 rad/s. Accelerating the speed up to 480 rad/s, the amplitude is significantly characterized by a half doubling frequency, which means the rotor has converted to the status of oil whip, and the component of the amplitude corresponding to the half doubling frequency is gradually elevated.  
Figure 8: System response at different speeds when the oil pressure is 1.0 MPa.  
Figure 9: Three-dimensional spectrum waterfall diagram of a dual-rotor system.  
Figures 12 and 13 show that with a higher oil pressure level of about 1.2 MPa, the trajectory of the shaft center with the Poincare map and the power spectrum for the rotating disk and the shaft at different rotating speeds. At lower rotating speeds, the trajectory of the disk is visualized in oval, while the track of the shaft displays a quasi-circle. The distribution of the Poincare map indicates that the rotor system features a multicycle under operation, which is also confirmed by the figure of the power spectrum where the amplitude reaches the maximum only at the frequency equaling to the rotating frequency (1.0 fr).  
Accelerating the speed to the status of the oil whip, the whirl orbits of the disk and the shaft are both disorderly, and the scattered points are distributed discretely and irregularly in the Poincare map. The rotor motion cycle changes under different oil pressures, as shown in Table 1. Except for the domain frequency equaling to the rotating frequency, the amplitude at the frequency of double rotating frequency is also remarkable, and the amplitude for the disk is higher than that of the shaft. From what has been discussed above, the rotor system is in a chaotic state at this time.  
Figure 11: Frequency waterfall chart of the rotating shaft along a vertical direction at oil pressure of 0.2 MPa.  
Figure 10: Frequency waterfall chart of the rotating disk along a vertical direction at oil pressure of 0.2 MPa.  
At the test condition in the vicinity of the maximum rotating speed, the whirl orbits of the disk and the shaft feature oval in shape. In addition, the radius of the trajectory is the maximum among the three speeds in Figures 12 and 13. Compared with the frequency characteristic at lower speeds, the domain frequency at 649 rad/s is calculated as half of the rotating frequency. At the same time, all amplitudes at the three test conditions are comparable. The analysis results can give some insight into that the system of the rotor test operates in quasi-periodic mode.  
Figure 12: Trajectory of shaft center with the Poincaré map (**left side**) and the power spectrum (**right side**) for the rotating disk at different rotating speeds.