## 3 Results and Discussion
### Oscillation of Two T-Section Prisms in Tandem
#### 3.1.2. Typical Oscillation Responses of Soft Galloping (SG)  
In the tests, under increasing velocity and no manual external excitation, UTP and DTP vibrated freely. Then, after applying an external force at \(U_{r}=12.25\), UTP entered VIV lower and DTP entered the WIM branch. As velocity decreased to a certain value (\(U_{r}=10.375\)), UTP and DTP suddenly jumped in the VIV-galloping transition branch. The oscillation responses are described below.  
(1) Amplitude Responses:  
For \(R_{L}=21\)\(\Omega\) (\(\zeta_{total,UTP}=0.153\), \(\zeta_{total,DTP}=0.152\)), the STP oscillated freely and experienced the VIV initial branch, followed by the VIV-galloping branch and ended with the soft galloping branch, as shown in Figure 10.  
In Figure 10a, the amplitude response of UTP was similar to that of STP, and the amplitude of UTP was a little higher than that of STP as \(U_{r}\leq 9.75\). In the galloping branch, the amplitude of UTP was lower than that of STP, but the amplitude of different spacing ratios was similar. In Figure 10b, the suppression can be observed at all the spacing ratios for DTP; as the spacing ratio increased, the negative influence gradually decreased for \(6\leq L/D\leq 15\) in the VIV initial branch and transition branch.  
Figure 10. Amplitude responses for UTP and DTP in SG: (a) Amplitude ratio responses of UTP; (b) Amplitude ratio responses of DTP.  
Figure 9. Frequency responses of UTP and DTP in HG: (a) Frequency ratio responses of UTP; (b) Frequency ratio responses of DTP.  
For \(7.25\leq U_{r}\leq 10.375\), the amplitude of \(L\)/\(D\) = 3 increased sharply from \(0.2D\) to \(1.4D\), but the amplitude was still lower than that of STP because the disturbance from the wake of UTP on DTP was stronger. In the galloping branch (\(10.375\leq U_{r}\leq 12.25\)), the amplitude curves of DTP intersected with each other and for all spacing ratios were also lower than those of STP. In addition, in the WIM region, the amplitude of DTP was much higher than that of STP for \(6\leq L\)/\(D\leq 15\). The \(A\)* of DTP varied from \(0.59\) (\(L\)/\(D\) = 6) to \(1.69\) (\(L\)/\(D\) = 15) at \(U_{r}\) = 12.25. For \(L\)/\(D\) = 3, the amplitude of DTP was lower than that of STP.  
(2) Frequency Responses:  
The frequency responses of UTP were similar to those of STP, and the \(f\)* of UTP almost remained at \(0.65\)-\(0.75\) as the velocity increased. In the VIV upper branch, the \(f\)* of STP was a little higher than that of UTP. The frequency response of DTP was also similar to that of STP, but there were some fluctuations as the velocity increased. In the WIM region, the \(f\)* of DTP was lower than that of STP and similar to the \(f\)* of the galloping branch for \(6\leq L\)/\(D\leq 15\), as shown in Figure 11.  
In summary, the amplitude and frequency of UTP were similar to those of STP in both HG and SG with the variation of flow velocity, indicating that the oscillation responses of UTP and STP were basically the same. The amplitude of DTP in VIV (initial branch, upper branch) and galloping was less than that of UTP and STP, indicating that the oscillation intensity of DTP was weakened by the wake of UTP. In the WIM region, there was an evident upward trend in the amplitude of DTP with the increase of the flow velocity, and the uptrend became more significant as the spacing ratio increased, indicating that DTP experienced WIM due to the influence of the wake, and as the spacing ratio increased, the WIM became stronger.