## 3 Results and Discussion
### Energy-Harvesting Performance of The Upstream Harvester  
Figure 5a shows the effect of the water speed on the output power when the resistance is a constant at \(R=100\) k\(\Omega\), and the spacing ratio \(L/D=3.3\). It can be found that the output power strongly depends on the water speed. In the low water speed range (\(U\leq 0.193\) m/s), the output power of the upstream harvester is relatively small. This value increases significantly in the middle range of the water speed values and yields a peak for the output power when the water speed is 0.306 m/s. In the higher water speed range, the output power drops again to values that are close to those obtained in the low water speed range. When the value of the water speed is 0.306 m/s, the time history of the output voltage of the upstream harvester and its Fast Fourier Transform (FFT) analysis are shown in Figure 5b,c. It notes that the output voltage can be regarded as a simple harmonic wave with a frequency of 3.891 Hz. On the other hand, the output voltage is generated because of the oscillation of the cylindrical vibrator, which is stimulated by the vortex shedding from the cylinder. So the frequency of the vortex shedding from the cylinder equals that of the output voltage, 3.891 Hz. This frequency is very close to the natural frequency of the harvester, 3.92 Hz. As a result, the harvester resonates at the water speed of 0.306 m/s. That is the reason why the output power reaches a peak. Figure 6 shows the output powers of the upstream harvester and the single harvester as a function of the water speed for eight different spacing ratios. It notes that, with the water speed increasing, the changing trend of the output power of the upstream harvester is simi  
Figure 5: Performance of the upstream harvester at the spacing ratio \(L/D=3.3\): (**a**) Output power of the upstream harvester as a function of the water speed; (**b**) Time history of the output voltage at the water speed of 0.306 m/s; (**c**) FFT analysis of the output voltage at the water speed of 0.306 m/s. for specific spacing ratios. In other words, the downstream harvester has no effect on the vibration form (vortex-induced vibration) of the upstream harvester. However, the performance of the upstream harvester is different from that of the single harvester. This is because the wake flow field of the upstream harvester is changed by the vibration of the downstream harvester. The changing of the performance varies with the value of the water speed. In the lower and higher water speed range (\(U\leq 0.230\) m/s, \(U\geq 0.412\) m/s), the output power is nearly equal to that of the single harvester. In the middle range of the water speed, however, the output power is greater than that of the single harvester. In addition, the enhancement of the performance is related to the spacing ratio for a specific water speed. In a low spacing ratio range, the performance of the upstream harvester is significantly enhanced compared to that of the single harvester. Particularly, the output power is the largest when the spacing ratio is near \(L/D=2.5\). With the spacing ratio increasing, the enhancement of the performance is weakened. Finally, the output power of the upstream harvester is close to that of the single harvester and stabilizes. This is because the vortex shedding from the upstream harvester will directly attach to the downstream harvester when the space between the two harvesters is smaller than the size of the vortex. The movement of the downstream harvester will block the vortex and increase the stream turbulence. The speed difference between the two sides of the upstream harvester and the force becomes greater than that of the single harvester. As a result, the upstream harvester has a larger power output. With the increase of the spacing ratio, the effect of the downstream harvester on the vortex shedding from the upstream harvester becomes smaller, and the output power of the upstream harvester is close to that of the single harvester.