## 4 Experimental Results and Discussion
### Experimental Study of the Output Performance of the Energy Harvester
#### 4.2.3 Output Performance of VFPEH-30  
The output performance test of VFPEH-30, a hybrid energy trap for vortex-induced flutter of symmetric airfoils, was carried out under the same experimental conditions as EVH-30 and FEH. We extracted the experimental data, drew the VFPEH-30 output voltage curve (as shown in Figure 10) with the change in wind velocity, as well as the VFPEH-30 output voltage waveform (as shown in Figure 11) with different wind velocities, and the VFPEH-30 output power spectral density diagram (as shown in Figure 12).  
Figure 11: Time story of output voltage for VFPEH-30 at different wind velocities: (**a**) 5.1m/s and 6.2m/s; (**b**) 9.0m/s and 11.7 m/s.  
Figure 10: Output characteristics of the VFPEH-30: (**a**) Output voltage curve at different wind velocities; (**b**) Output voltage waveform.  
Figure 10a shows that with the increase in wind velocity, the output voltage of the VFPEH-30 changes nonlinearly, first increasing and then gradually decreasing. The output voltage of VFPEH-30 is significantly higher than that of VEH-30 and FEH. When the wind velocity is less than 4.8 m/s, the output voltage is small. At this time, the captured energy is mainly generated by the vortex-induced vibration of the cylindrical blunt body, and the airfoil does not swing at low velocity [45]. When the wind velocity is 4.8 m/s, the VFPEH-30 has a large amplitude vibration. The airfoil starts to vibrate under the action of the wind. The output voltage of the VFPEH-30 is 5.3 V, and this wind velocity is its starting wind velocity. When the wind velocity is greater than 4.8 m/s, with the increase in wind velocity, the vibration amplitude of VFPEH-30 becomes larger and larger, and the output voltage increases rapidly; when the wind velocity is 8.0 m/s-9.0 m/s, the output voltage reaches the peak range. When the wind velocity is 9.0 m/s, the output voltage is the largest, 60.2 V. After that, with the increase in wind velocity, the output voltage of the VFPEH-30 decreases rapidly until the wind velocity is greater than 11.7 m/s, and the output voltage tends to be stable, both of which are greater than 30 V. Figure 11 shows a time story of the output voltage for the VFPEH-30 at wind speeds of 5.1, 6.2, 9.0 and 11.7 m/s. Compared to 6.2 m/s and 9.0 m/s, the irregular vibration response at 5.1 m/s is due to the relatively  
Figure 12: Power spectral density (PSD) plot of the VFPEH-30 at different wind velocities: (**a**) 5.1 m/s; (**b**) 6.2 m/s; (**c**) 9.0 m/s; (**d**) 11.7 m/s.  
low vibration of the airfoil at lower wind speeds, 5.1 m/s, which causes less disturbance to the VFPEH-30. Whereas at higher wind velocities, the vibration of the airfoil is relatively higher, disturbing the VFPEH-30 more and subjecting it to higher aerodynamic forces, i.e., the vibration response is irregular.  
From Figure 12, it was found that the output performance of VFPEH-30 is relatively poor when the wind velocity is 5.1 m/s, and the output voltage is 18.2 V. When the wind velocity is 9.0 m/s and 11.7 m/s, a higher quasi periodic output voltage can be generated, and the peak voltage is 60.2 V at 9.0 m/s. After fast Fourier transformation, the power density map can be obtained, as shown in Figure 12. As can be seen from Figure 12, with the increase in the air flow velocity, the fundamental frequency oscillation frequency does not change much and the frequency domain amplitude increases rapidly. The main harmonic is 4 Hz or 4.5 Hz under the four wind velocities, and the second and third harmonic oscillations occur, which indicates that VFPEH-30 is affected by aerodynamic nonlinearity in the process of energy capture, thus showing weak nonlinearity.