## 4 Experiments and Analysis
### Voltage Frequency Response Analysis  
In order to comprehensively evaluate the performance of the S-type generator beam in two environments (The up-and-down vibration excitation and the wind-induced vibration excitation), a specific mode (3 st mode, 7.45 Hz) and a typical low-frequency environmental excitation (6.0 Hz and 9.0 Hz) are applied to the two energy harvesters containing the S-type generator beam. The experimental results are shown in Figures 6 and 7.  
In the up-and-down vibration excitation environment, when the excitation frequency is 6 Hz, the output peak voltage of the generator beam is 8790 mV (Figure 6a). At this time, the generator beam fails to break through the potential energy barrier and only moves within the well. The above conclusion can be drawn from the displacement data of the two motion observation points set on the generator beam, as shown in Figure 6d. When the environmental excitation frequency rises to the third natural frequency (3 rd mode, 7.45 Hz) of the S-type generator beam, the motion state of the beam changes obviously, mainly manifested in the motion process of the beam breaking through the potential energy barrier and generating inter-well movement (snap-through). The amplitude increases substantially, and some local high-frequency vibration occurs at the same time (Figure 6d). Thus, in terms of voltage output, the peak voltage is increased to 14,350 mV, resulting in increased burrs in the voltage curve due to local high-frequency vibration. When the ambient excitation frequency continues to increase to 9.0 Hz, the motion state of the beam changes from inter-well motion to in-well motion again, as shown in Figure 6d. At this time, the amplitude is slightly larger than that of the excitation frequency of 6 Hz but far smaller than that of the resonant frequency 7.45 Hz, and the peak voltage output at this time is 8680 mV (Figure 6c).  
In the condition of the wind-induced vibration excitation, when the environmental excitation frequency reaches 6.0 Hz, 7.45 Hz and 9.0 Hz respectively, the motion variation law of S-type generator beam is similar to the above-mentioned law of the up-and-down vibration excitation, as shown in Figure 7. At this condition, the peak output voltage is 9610 mV, 17,630 mV and 10,020 mV respectively. Compared with the up-and-down vibration excitation, both the peak value and average value of the output voltage of the generator beam at the condition of wind-induced vibration are slightly larger (as shown in Figure 8). This is because the vorticity induced by the alternating vortex behind the blocked fluid can more effectively act on the middle of the generator beam in the form of torque in the wind environment.  
Figure 5: Experimental prototype and set-up.  
Figure 6: Test results of vibration excitation: (**a**) F = 6 Hz (The excitation frequency); (**b**) F = 7.45 Hz; (**c**) F = 9.00 Hz; (**d**) Displacement of two vaults of generator beam at different excitation frequencies (Different colors enhance visual comparison of measurements at different positions/frequencies).  
Figure 7: Test results of wind field excitation: (**a**) F = 6 Hz (The excitation frequency); (**b**) F = 7.45 Hz; (**c**) F = 9.00 Hz; (**d**) Displacement of two vaults of generator beam at different excitation frequencies (Different colors enhance visual comparison of measurements at different positions/frequencies).