## 3 Results and Discussion
### The Effects of Load Resistances and Spacing Ratios on Energy Conversion
#### 3.3.1 Effect of Damping Ratio on Energy Conversion  
(a) For UTP (_1 st_), the active power curves crossed each other, and the rules were similar. As the load resistances decreased, the active power and the efficiency increased considerably for 11 \(\Omega\leq R_{L}\leq\) 21 \(\Omega\), as shown in Figure 16. For 16 \(\Omega\leq R_{L}\leq\) 41 \(\Omega\), SG occurred; the UTP experienced the VIV initial branch, followed by the VIV-galloping transition branch and ended with the galloping branch. For \(U_{r}\)\(\geq\) 8.5, the active power gradually increased from 1.93 W\(\sim\)3.11 W to 6.14 W\(\sim\)13.44 W. For 11 \(\Omega\leq R_{L}\)  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{\(\mathbf{U_{r}}\)} & \multirow{2}{*}{\(\mathbf{U}\) (m/s)} & \multicolumn{3}{c}{**Active Power (W)**} & \multirow{2}{*}{**Ratio**} & \multicolumn{2}{c}{**Efficiency (\%)**} & \multirow{2}{*}{**Ratio**} \\  & & **1 st Pri.** & & **2 nd Pir.** & & **Tandem** & & **Single** & **Tandem** & **Single** \\ \hline
10.375 & 1.12 & 5.91 & 3.87 & 9.78 & 6.98 & 1.40 & 6.17 & 4.33 & 1.42 \\
11 & 1.19 & 7.41 & 5.80 & 13.21 & 8.30 & 1.59 & 6.41 & 3.97 & 1.61 \\
11.625 & 1.26 & 8.79 & 7.98 & 16.77 & 10.98 & 1.53 & 6.32 & 4.08 & 1.55 \\
12.25 & 1.33 & 10.41 & 11.20 & 21.61 & 12.52 & 1.72 & 6.61 & 3.78 & 1.75 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Comparison of energy conversion for UTP+DTP of \(L/D=6\) to STP in SG.  
Figure 15: The instantaneous voltage and active power of different reduced velocities for SG: (a) Upstream prism; (b) Downstream prism.  
\(\leq 13\)\(\Omega\), as the velocity increased, the VIV occurred; for \(U_{r}\geq 10.375\), HG occurred. The active power of UTP changed from 8W (\(R_{L}=13\)\(\Omega\)) to 16.63 W (\(R_{L}=11\)\(\Omega\)) at \(U_{r}=12.25\). However, the efficiency did not exhibit an upward trend but maintained a stable value. As the load resistances decreased, the active power and efficiency increased. In the tests, the active power and efficiency of UTP at the load resistance of \(R_{L}=8\)\(\Omega\) were lower than the corresponding values at \(R_{L}=11\)\(\Omega\).  
(b) For DTP (\(2 nd\)), in the VIV responses, the active power showed a slight upward trend as the velocity increased. The active power changed from 1.67 W (\(R_{L}=8\)\(\Omega\)) to 5.0 W (\(R_{L}=13\)\(\Omega\)) and at \(U_{r}=12.25\) was higher than that of UTP. In the VIV lower branch, for 16 \(\Omega\leq R_{L}\leq 41\)\(\Omega\), the active power was also higher than that of UTP. In the galloping branch, the active power increased from 5.21 W (\(R_{L}=41\)\(\Omega\)) to 12.47 W (\(R_{L}=11\)\(\Omega\)) at \(U_{r}=12.25\) as the load resistances decreased to a certain value \(R_{L}\) (\(11\)\(\Omega\)).  
At the load resistances of 11 \(\Omega\leq R_{L}\leq 41\)\(\Omega\), decreasing the load resistance resulted in increases in efficiency; the efficiency of DTP changed from 1.52% (\(R_{L}=41\)\(\Omega\)) to 3.67% (\(R_{L}=11\)\(\Omega\)) respectively, as shown in Figure 17.  
Figure 16: Energy conversion of UTP under various load resistances: (**a**) Active power; (**b**) Energy conversion efficiency.  
Figure 17: Energy conversion of DTP at various load resistances: (**a**) Active power; (**b**) Energy conversion efficiency.