## 1 Introduction
Recently, [PERSON] [28] established a combined system of oscillation and power take-off (PTO) to investigate the energy extraction of an equilateral triangular prism in water flow. More than expected, the maximum amplitude of the prism reached 2.71\(D\) in galloping zone and the maximum energy conversion efficiency reached 40.44%. These results indicated that the triangular prism might harness more energy from FIM. In both [PERSON]'s research [26] and [PERSON]'s research [28], an interesting galloping phenomenon was observed. When the cylinder was applied with a threshold initial displacement, which was approximately one diameter, the hard galloping (HG) response was observed at a high velocity (in VIV lower branch). However, for an initial displacement less than one diameter, HG could not be observed. That means that HG was not a self-excited galloping. In contrast, when the edge angle of PTC is at an appropriate value or the damping is low enough, the circular cylinder with PTC or the triangular prism could be self-initiated to galloping with no initial displacements or external forces. This phenomenon was named soft galloping (SG) as opposed to the HG. The information currently available about HG and SG is still scarce. As far as we know, only [PERSON] [26] and [PERSON] [28] occasionally found HG under certain conditions. Fewer studies about the relationship between SG and HG were performed. Moreover, while the configuration of the oscillation system was changed, prisms presented two different HG modes: hard galloping 1 (HG1) and hard galloping 2 (HG2). This phenomenon indicated that the physical parameters (such as damping, mass, stiffness, and cross-section aspect ratio) had great impacts on the oscillation mode of triangular prisms. As reported by previous research [12; 13; 14; 25; 26; 27; 28], triangular prisms have great advantages in energy extraction, especially at the galloping branch. However, HG cannot be self-initiated without external excitation, which means the energy in HG can hardly be harnessed. Consequently, the lack of investigation on the occurrence conditions of SG and HG seriously restricts the development in the energy extraction in FIM of triangular prisms. The objective of the present study is to investigate the characteristics of SG and HG responses of triangular prisms in water by experimental methods. In Section 2, the PTO system is introduced to adjust the system damping coefficient \(c\); a spring system is established to adjust the system stiffness \(K\); additional weights are used to adjust the system mass \(m\); and different triangular prisms are used to achieve the variation in section aspect ratio \(\alpha\). Next, the corresponding parameters of the experimental setup are deduced and verified. The complete SG and HG responses under conditions of increasing or decreasing flow velocity and applying or not applying a threshold initial displacement are exhibited in Section 3. In Section 4, the evolutions of oscillation mode of triangular prisms with the variations of \(c\), \(K\), \(m\) and \(\alpha\) are analyzed. In Section 5, the occurrence conditions of SG and HG are revealed. Finally, the conclusions are summarized.