## 3 Results and Discussion  
Firstly, we place only one harvester in the test section of the water channel to investigate its performance. The corresponding result is used to choose the optimal resistance and compare it to the performances of the two tandem harvesters in the following section. The plotted curves in Figure 3 show variations of the output power with different load resistances at three water speeds, namely \(U\) = 0.230 m/s, \(U\) = 0.272 m/s, \(U\) = 0.306 m/s. Clearly, the output power first increases and then decreases with the load resistance increasing. In other words, there is an optimal resistance at which the output power is at its maximum. In this case, the optimal resistances are around 100 k\(\Omega\) at all of the water speeds.  
Then, two harvesters with identical geometry are placed in the test section of the water channel to investigate their energy-harvesting performances. The two harvesters are arranged in a tandem array, as shown in Figure 2a. By setting the electrical load resistance at a constant value of 100 k\(\Omega\), and varying the spacing ratio (\(L/D\)) between the two tandem harvesters from 1.7 to 12.5 and the water speed from 0.1 m/s to 0.5 m/s, different dynamic responses are found in the two harvesters. Figure 4 shows the relationship between the output powers of the two tandem harvesters and the water speed in the case of eight spacing ratios namely, \(L/D\) = 1.7, 2.5, 3.3, 3.75, 4.6, 6.25, 8.3 and 12.5.  
Figure 4: _Cont._  
Figure 3: Variations of the output power with resistors at three water speeds.  
From Figure 4, on the one hand, it is easy to find that, with the water speed increasing, the changing trends of the output power of the upstream harvester are similar for all the spacing ratios. The trends of the output power are increased at first and then decreased. The dynamic response of the upstream harvester is vortex-induced vibration. On the other hand, the performance of the downstream harvester is totally different from that of the upstream harvester. The output power of the downstream harvester only increases with the water speed increasing for the spacing ratio \(L/D<6.25\). The dynamic response of the downstream harvester is wake-induced vibration which is a divergent oscillation. It is because the downstream harvester is in the wake flow field and is influenced by the vortex shedding from the upstream harvester. However, the effect of the upstream harvester on the downstream one is being impaired with the spacing ratio expanding and it eventually disappears. Hence, we conclude that the performance of the upstream harvester has been less affected by the downstream one, but it is the very opposite of the downstream one in that the performance of the downstream harvester strongly depends on the upstream one. We also find that the sum of the output power of the two tandem harvesters is more than double that of a single harvester, even much more in some cases. It shows that the interactions between the two tandem harvesters have some effect on their performances.  
Therefore, in order to further investigate the energy-harvesting performance of each harvester, we respectively analyze them and compare their own performances to the single harvester (without an upstream or downstream harvester) in two ways: one is by fixing the spacing ratio and varying the water speed, the other one is by fixing the water speed and varying the spacing ratio.  
Figure 4: Output powers of two harvesters as a function of the water speed for eight spacing ratios: (**a**) \(L/D=1.7\); (**b**) \(L/D=2.5\); (**c**) \(L/D=3.3\); (**d**) \(L/D=3.75\); (**e**) \(L/D=4.6\); (**f**) \(L/D=6.25\); (**g**) \(L/D=8.3\); (**h**) \(L/D=12.5\).