## 2 Experimental Methods
### Triangular Prisms and Validation  
The experimental prisms are all isosceles triangular prisms made of 10 mm Plexiglas plate. The characteristic dimensions \(D\) of the prisms are all 10 cm; the lengths \(L\) are approximately 0.9 m.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \(\boldsymbol{K/N\cdot m^{-1}}\) & \(\boldsymbol{\zeta_{\rm m}}\) & \(\boldsymbol{c_{\rm m}/N\cdot s\cdot m^{-1}}\) & \(\boldsymbol{f_{\rm n}/Hz}\) & \(\boldsymbol{m/\text{kg}}\) \\ \hline
800 & 0.161 & 51.940 & 0.791 & 32.43 \\
1000 & 0.139 & 50.300 & 0.881 & 32.63 \\
1200 & 0.132 & 52.223 & 0.966 & 32.56 \\
1400 & 0.120 & 51.360 & 1.044 & 32.55 \\
1600 & 0.111 & 50.469 & 1.124 & 32.10 \\ Average & - & 51.258 & - & 32.45 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The complete results in free-decay test by varying the system stiffness.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \(\boldsymbol{m_{\rm add}/\text{kg}}\) & \(\boldsymbol{\zeta}\) & \(\boldsymbol{c/N\cdot s\cdot m^{-1}}\) & \(\boldsymbol{f_{\rm n}/Hz}\) & \(\boldsymbol{m/\text{kg}}\) & \(\boldsymbol{m_{\rm o\text{-}cal}/\text{kg}}\) & \(\boldsymbol{m^{*}}\) \\ \hline
0.00 & 0.132 & 52.223 & 0.966 & 32.56 & 15.94 & 8.36 \\
3.91 & 0.122 & 51.528 & 0.907 & 36.92 & 16.39 & 9.47 \\
7.80 & 0.119 & 52.980 & 0.860 & 41.08 & 16.66 & 10.54 \\
11.73 & 0.114 & 53.209 & 0.819 & 45.32 & 16.96 & 11.63 \\ Average & - & 52.485 & - & - & 16.49 & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: The complete results of the free-decay test by varying the mass of oscillation system.  
In order to reduce the boundary effects on the prisms, two end plates are attached on the two end sides of the prisms, and the gaps between the end plates and the walls of the channel are less than around 3 cm [32; 33].  
In this paper, the isosceles triangular prisms with different section aspect ratios \(\alpha\) are investigated. The section aspect ratio \(\alpha\) is the ratio of the height of the triangular cross-section to the length of the base.  
\[\alpha=H/D \tag{15}\]  
Four triangular prisms with different section aspect ratios--0.5, 0.866, 1 and 1.5--are produced. When the aspect ratio is \(\alpha=0.866\), the section of the prism is the regular triangular. The prisms and the variation of aspect ratios are shown in Figure 8.  
For triangular prisms, the incident angles strongly affect the responses because of the non-axis symmetry of the cross-section. This paper does not focus on the influence of incident angles of the oscillation, so only one incident angle, in which the base of the cross-section is vertical to the coming flow, is taken into account. Fully developed transverse galloping can be achieved at this incident angle when the flow velocity is high enough.  
It should be noted that the differences in section aspect ratios may cause the differences in oscillation mass. In order to keep the same natural frequency \(f_{\mathrm{n}}\) for different prisms, additional weights are used to balance the differences in the prisms' mass.  
In the present experiment, we used the established oscillation system, which had been introduced by [PERSON] et al. [28]. The details of validation had also been described in [PERSON] et al.'s research [28].