## 3 Results and Discussion
### Oscillation of Two T-Section Prisms in Tandem
#### 3.1.1 Typical Oscillation Responses of Hard Galloping (HG)  
In the tests, under increasing velocity and no manual external excitation, UTP and DTP vibrated freely. Then, after applying a threshold initial displacement (exceeding 1 \(\times\) D) at \(U_{r}=12.25\), the prisms entered the hard galloping branch accompanied with a high amplitude and low frequency. As velocity decreased (\(U_{r}=10.375\)), the prism suddenly dropped in the VIV lower branch. The oscillation responses are described below. (1) Amplitude Response:  
For \(R_{L}=11\ \Omega\ (\zeta_{total,LTP}=0.246\), \(\zeta_{total,DTP}=0.251\)), the UTP and DTP experienced the VIV initial branch, followed by the VIV upper branch, ending with the VIV lower branch, indicating that the prisms only present the characteristics of VIV. That was similar to the STP, as shown in Figure 8. For \(4.75\leq U_{r}\leq 7.25\), \(A^{*}\) of UTP increased rapidly from 0 to 0.5 and approached that of STP regardless of the spacing ratio in Figure 8a. However, \(A^{*}\) of DTP increased to 0.15\(\sim\)0.45 at \(U_{r}=7.25\), and the amplitude response of DTP was similar to that of STP but lower than that of STP in Figure 8b. The suppression can be observed at all the spacing ratios for DTP. In the range of \(7.25\leq U_{r}\leq 8.5\), \(A^{*}\) of UTP kept increasing with an uptrend to about 0.7, and the amplitude of UTP was close to that of STP. For \(8.5\leq U_{r}\leq 10.375\), the oscillation gradually weakened, and the amplitude of UTP was lower than that of STP, which indicated that the oscillation response of UTP was similar to that of STP and UTP was negatively influenced by DTP as \(U_{r}\geq 8.5\). For \(U_{r}\geq 10.375\), the UTP experienced the VIV lower branch, and the amplitude decreased to about 0 as the velocity increased. In the VIV upper branch, the oscillation response of DTP was similar to that of STP for \(L\)/\(D\) = 3\(\sim\)6, and \(A^{*}\) increased to about 0.48\(\sim\)0.59 at \(U_{r}\) = 9.125 and then started to decrease. In the range of \(L\)/\(D\) = 9\(\sim\)12, the wake vortex of UTP had a positive influence on DTP, so that the amplitude of DTP increased with a slight upward trend to 0.15\(D\)\(\sim\)0.6\(D\). In addition, \(A^{*}\) of DTP kept increasing with a slight uptrend; the amplitude varied from 0.45\(D\) to 0.75\(D\) for \(L\)/\(D\) = 15. As \(U_{r}\)\(\geq\) 10.375, the amplitude response of DTP was higher than that of STP for \(L\)/\(D\) = 6\(\sim\)15, which showed that the DTP was positively promoted by UTP. The VIV lower branch did not appear in the wake-induced motion (WIM) region. In the WIM region, the maximum amplitude of DTP varied from 0.4\(D\) (\(L\)/\(D\) = 6) to 1.47\(D\) (\(L\)/\(D\) = 15) at \(U_{r}\) = 12.25. For 10.375 \(\leq U_{r}\)\(\leq\) 12.25, there was an initial displacement applied to the UTP and DTP. A stable growth in the amplitude of UTP can be observed as the velocity increases. Amplitude varied from 1.35\(D\) (\(L\)/\(D\) = 9) at \(U_{r}\) = 10.375 to 1.94\(D\) (\(L\)/\(D\) = 12) at \(U_{r}\) = 12.25. If \(U_{r}\) continued to increase, the oscillation would remain in the galloping branch. Furthermore, the amplitude of DTP was lower than that of UTP and STP in general due to the wake of UTP, while the \(A^{*}\) of DTP varied from 1.05 (\(L\)/\(D\) = 3, \(U_{r}\) = 10.375) to 1.94 (\(L\)/\(D\) = 6, \(U_{r}\) = 12.25). (2) Frequency Responses:  
In the VIV initial branch (4.75 \(\leq U_{r}\)\(\leq\) 7.25), the frequency responses of both UTP and DTP were close to that of STP regardless of the spacing ratios in Figure 9a; \(f^{*}\) for all the conditions increased gradually from 0.6 to 0.75. In the range of 7.25 \(\leq U_{r}\)\(\leq\) 10.375, the \(f^{*}\) of UTP was a little higher than that of STP, and the \(f^{*}\) of UTP continued to increase from 0.75 to about 1\(\sim\)1.35 with a strong upward trend. In the VIV upper branch, the \(f^{*}\) of DTP dropped and increased with different spacing ratios, as shown in Figure 9b. The \(f^{*}\) of DTP was close to that of STP until \(U_{r}\) = 9.125.