## 4 Experimental Results and Discussion
### Experimental Study of the Output Performance of the Energy Harvester
#### 4.2.2 Output Performance of FEH  
The FEH is composed of a cantilever beam (the same as VFPEH-30) and two airfoils fixed at both ends of the long axis and symmetrically distributed. The cantilever beam and the long axis are fixed by connectors obtained by 3D printing. Since the airfoil is small in size and formed by 3D printing, the material is light in mass and placed perpendicular to the air flow direction, so the influence of its gravity is ignored. Figure 7 illustrates the prototype of the FEH and Table 5 shows the material parameters and dimensional parameters of the FEH. Figure 6: VEH-30 at different wind velocities: (**a**) Output voltage graph; (**b**) Voltage versus time graph. The experiments were carried out in a wind tunnel with one end open, and the maximum wind velocity was 14.5 m/s. Figure 8 is a point line diagram of the output voltage of FEH under different wind velocities and a waveform diagram of the output voltage over time. As can be seen from Figure 8a, with the increase in wind velocity, the output voltage increases nonlinearly and finally reaches a stable state. When the wind velocity is less than 8.5 m/s, the vibration of FEH structure is small and unstable, and it is in a quasi-static  
Figure 8: FEH output voltage: (**a**) Output voltage curve with wind velocity; (**b**) Voltage waveform with time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Properties** & **Substrates** & **Piezoelectric Sheets** & **Connector** & **Shaft** & **Bearings** \\ \hline Materials & Copper alloy & PZT-5H & PLA & Copper & Bearing steel \\ Density (kg/m\({}^{3}\)) & 8900 & 7500 & 1250 & 8700 & 7810 \\ Young’s modulus (GPa) & 105 & 66 & 3.5 & 119 & - \\ Poisson’s ratio & 0.35 & 0.3 & - & 0.32 & - \\ Length (mm) & 140 & 60 & - & 150 & - \\ Width/Diameter (mm) & 26 & 20 & - & 0.3 & 6 \\ Thickness (mm) & 0.4 & 0.2 & - & - & 2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Materials and parameters of each part of the FEH. Figure 7: Enlarged view of FEH. state [45]. This is because the ground vibrates during the operation of the fan, which affects FEH, resulting in small voltage output and instability. Figure 8b shows the voltage waveform when the wind velocity is 7.9 m/s (less than the starting wind velocity), and the output voltage is 0.2 V. When the wind velocity is 8.5 m/s, the cantilever beam of the FEH vibrates with a large amplitude, and the output voltage of the FEH is 3.3 V. Combine this with Figure 8a, this wind velocity is considered as its starting wind velocity. When the wind velocity is greater than the starting wind velocity of 8.5 m/s, the output voltage gradually increases with the increase in wind velocity. After the wind velocity is 14.0 m/s, the voltage reaches stable output. When the wind velocity is 14.5 m/s, the output voltage is 9.53 V. Shown below is Figure 9, it is a phase diagram of the output voltage of FEH under different wind velocities. It can be seen from Figure 9 that FEH produced limit cycle oscillation during the experiment. When the wind velocity is 7.90 m/s, the starting wind velocity is not reached, and the aerodynamic force on the symmetrically distributed airfoil components is uneven, resulting in more chaotic phase diagrams at the current velocity. The phase portraits of output voltage appear approximately as a single cycle, while the phase portrait does not appear to be very smooth, especially at 7.9 m/s. The reason is that there exist the unexpected disturbances in the experiment. When the wind velocity is 8.5 m/s, the  
Figure 9: Phase diagram of FEH at different wind velocities: (**a**) Wind velocity of 7.9 m/s; (**b**) Wind velocity of 10.2 m/s; (**c**) Wind velocity of 11.5 m/s; (**d**) Wind velocity of 14.0 m/s. aerodynamic damping of the energy trap exceeds the wind damping of the structure, resulting in aeroelastic motion in two degrees of freedom and corresponding limit cycle oscillation (LCO) [46].