## 3 Results and Discussion
### The Effects of Load Resistances and Spacing Ratios on Energy Conversion
#### 3.3.3 Optimal Energy Conversion of UTP + DTP  
During the texts, under increasing velocity and no manual external excitation, UTP and DTP vibrated freely. For different load resistances, external excitation or suppression was applied to the prism. In general, the active power of UTP+DTP increased with the flow velocity and exhibited drops and jumps, as shown in Figure 22.  
Figure 21: Energy conversion of DTP at various spacing ratios for \(R_{L}=13\)\(\Omega\): (a) Active power; (b) Energy conversion efficiency. Figure 20: Energy conversion of downstream prisms at various spacing ratios for \(R_{L}=21\)\(\Omega\): (a) Active power; (b) Energy conversion efficiency. (1) For \(R_{L}\) = 8 \(\Omega\), the active power of UTP + DTP increased as the velocity increased until \(U_{r}\) = 8.5; the local maximum active power was 4.72 W (\(L\)/\(D\) = 15). As the velocity continuously increased, the active power showed a downward trend at \(U_{r}\) = 8.5 which was consistent with the STP case. For most spacing ratios, the active power decreased to about 0. With the variation of the spacing ratio, the'active power showed a similar response. (2) For \(R_{L}\) = 11 \(\Omega\), as the velocity increased, the VIV phenomenon occurred for all spacing ratios. The active power increased as the velocity increased and then decreased at \(U_{r}\) = 8.5; the local maximum active power was 5.68 W, which appeared at \(L\)/\(D\) = 12. The same also occurred at \(L\)/\(D\) = 12 and \(L\)/\(D\) = 15 for \(R_{L}\) = 13 \(\Omega\). In addition, if there was an initial displacement applied to UTP and DTP, the prisms sharply entered the galloping branch with large amplitude and had more output power. The optimal spacing ratio of active power existed for the two T-section prisms. In particular, at a lower spacing ratio, there was a negative effect on the active power. The DTP was disturbed by the alternate shedding vortex from UTP, affecting the oscillation responses. In the tests, the optimal active power of \(P_{harn}\) = 30.12 W (\(L\)/\(D\) = 6) was that of a single prism (20.12 W), i.e., 1.5 times for \(R_{L}\) = 11 \(\Omega\). (3) For 16 \(\Omega\) \(\leq R_{L}\leq\) 41 \(\Omega\), the active power increased as the flow velocity increased for 4.75 \(\leq U_{r}\leq\) 12.25. Lower load resistances resulted in higher active power. In general, the active power of \(L\)/\(D\) = 6 performed best, and as spacing ratio increased, the variation of active power was slight. For \(R_{L}\) = 16 \(\Omega\), the local maximum active power of UTP + DTP at different spacing ratios at \(U_{r}\) = 12.25 was 21.7 W (\(L\)/\(D\) = 3), 25.67 W (\(L\)/\(D\) = 6), 23.2 W (\(L\)/\(D\) = 9), 23.61 (\(L\)/\(D\) = 12), and 20 W (\(L\)/\(D\) = 15); these values are less than twice the value of STP. In summary, the comparison of active power for UTP+DTP to STP in the galloping branch for each spacing ratio with the same configuration is presented in Table 7. A few observations can be drawn. For 4.75 \(\leq U_{r}\leq\) 8.5-9.125, the active power variation of UTP + DTP was similar to that of STP; the system started to output power at a flow velocity of 0.516 m/s in the tests. A steady growth was \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{\(U_{r}\)} & \multirow{2}{*}{\(U\) (m/s)} & \multicolumn{6}{c}{**UTP + DTP (W)**} & \multirow{2}{*}{**STP (W)**} \\  & & & \(L\)/\(D\) = 3 & \(L\)/\(D\) = 6 & \(L\)/\(D\) = 9 & \(L\)/\(D\) = 12 & \(L\)/\(D\) = 15 \\ \hline
10.375 & 1.13 & 14.39 & 13.80 & – & – & 13.46 & – \\
11 & 1.20 & 20.38 & 21.02 & 17.83 & 17.74 & 17.52 & – \\
11.625 & 1.26 & 23.38 & 25.90 & 24.67 & 23.79 & 23.26 & 16.72 \\
12.25 & 1.33 & 25.36 & 30.12 & 27.10 & 29.10 & 28.04 & 20.17 \\ \hline \end{tabular}
\end{table} 
Table 7: Comparison of active power for UTP + DTP to STP at \(R_{L}\) = 11 \(\Omega\). Figure 22: Active power and efficiency at \(K\) = 1400 N/m: (**a**) Active power; (**b**) Energy conversion efficiency. observed as the velocity increased until \(U_{r}\) = 8.5-9.125, and then a drop occurred.