## 3 Results and Discussion
### Energy-Harvesting Performance of The Downstream Harvester  
The downstream harvester is placed in the wake flow field of the upstream harvester. The characteristics of the flow field around the downstream harvester are different from that of the single harvester. As a result, their dynamic responses and performances are different. Figure 8a shows the time history of the output voltage of the downstream harvester when the spacing ratio is \(L/D=3.3\), and water speed is \(U=0.306\) m/s. From Figure 8a, we find that the beat vibration happens in the downstream harvester. It can be concluded that the output voltage signal of the downstream harvester comprises several simple harmonic components with similar frequencies. Figure 8b shows the FFT analysis result of the output voltage. Figure 8a shows that the output voltage comprises mainly two simple harmonic components with the frequencies of 3.787 Hz and 3.891 Hz. Furthermore, we compare the frequency of the output voltages of the downstream harvester to that of the upstream harvester at different water speeds in Figure 8c. We find that the frequencies of the output voltages of the downstream harvester contain or are equal to that of the upstream harvester. This is because the vortex shedding from the upstream harvester exerts a force, whose frequency equals that of the output voltage of the upstream harvester, on the downstream harvester. Figure 8d shows the variations of the output power of the downstream harvester with different water speeds. Inspecting this figure, we find that increasing the water speed always results in an increase in the output power of the downstream harvester. It is different from the upstream harvester. In the low water speed range (\(U\leq 0.272\) m/s), the increasing of the output power with the water speed is relatively small. However, this variation increases significantly in the higher water speed range. Figure 7: Variations of the output power and the frequency of vortex shedding from the upstream harvester, respectively, with the water speed and the spacing ratio: (**a**) Output power as a function of the water speed and the spacing ratio; (**b**) Frequency of vortex shedding from the upstream harvester as a function of the water speed and the spacing ratio. Figure 9 shows the output powers of the downstream harvester and the single harvester as a function of the water speed for eight different spacing ratios. We find that the performance of the downstream harvester is totally different from that of the single harvester for a specific spacing ratio. When the water speed is \(U\)! 0.306 m/s, the increase of the output power with the water speed is relatively slow compared to the single harvester and, therefore, the performance of the downstream is weaker than that of the single harvester. With the water speed continuing to increase, this increase becomes significantly faster, but the performance of the single harvester then declines. As a result, the performance of the downstream harvester becomes better than that of the single harvester. We conclude that the upstream harvester has dual effects on the performance of the downstream harvester: in the low water speed range, the performance of the downstream harvester is weakened compared to the single harvester, while in the higher water speed range, the performance is enhanced, conversely. Furthermore, with the spacing ratio increasing, the performance of the downstream harvester will decrease in the higher water speed range. Finally, the performance of the downstream harvester will be equal to that of the single harvester if the spacing ratio is big enough.