## 1 Introduction  
The phenomenon of flow-induced motion (FIM) can be frequently observed in many engineering structures such as long-spanned bridges [1], marine risers [2], power-transmission cables [3], etc. Vortex-induced vibration (VIV) is one of the most common FIM phenomena, which usually occurs due to the alternating shedding of vortices from either side of the bluff body [4]. VIV is a self-excited and self-limited motion with a limited amplitude and limited resonant flow velocity range (lock-in range). It is usually observed in the motion of circular cylinders immersed in steady flow. Numerous publications have already investigated the characteristics of VIV for an elastically mounted cylinder in steady flow, illustrating that the VIV response was strongly dependent on the mass, the damping, and Reynolds number [5; 6; 7; 8; 9]. In addition to the circular cylinder, other prisms such as square prism and triangular prism also presented VIV responses. [PERSON] et al. [10; 11] reported that the incident angle had a significant influence on the performance of square prisms. When the incident angle was 45 degrees (diamond cross-section), the square prism presented a self-limited motion (which was VIV). In [PERSON]'s numerical results [12], the triangular prism with a low mass ratio could undergo the VIV initial branch at a low flow velocity. This phenomenon has been confirmed by [PERSON] [13] and [PERSON] [14]. Contrary to VIV, galloping, as another common FIM phenomenon, is a self-excited but not self-limited motion, which is characterized with a higher amplitude, lower frequency, and broader resonant flow velocity range. It usually occurs in the motion of prisms with sharp sections at a high flow velocity, and is commonly induced by the instability of lift force caused from the periodic variation of attack angle [15]. Thus far, the studies of galloping were not as complete as the studies of VIV, owing to the diversity of the cylinder cross-sections and the complexity of the galloping itself. Most researchers such as [PERSON] et al. [10, 11] and [PERSON] et al. [16] concentrated on the studies of square or rectangular prisms, while only limited studies focused on the triangular prism due to the complexity of this prism. Actually, research on the dynamics of triangular prism is very valuable in many engineering fields such as aerodynamics, fluid-structure interactions (FIS), offshore structures and renewable energy fields. [PERSON] et al. [17, 18, 19] focused on the responses of triangular prisms based on the Glauert-Den Hartog criterion [20] and dynamical tests in wind tunnels. They concluded that the stability of galloping for triangular prisms strongly depended on the incident flow orientation and the aspect ratio as well as the mass and damping. In view of the huge destruction caused by FIM on engineering structures, many researchers [1; 2; 3] have devoted a great deal of effort to predicting and suppressing FIM over the past few decades. Nevertheless, researchers found that the high-quality renewable energy in FIM could be harnessed by using some new devices such as the piezoelectric transducer [21, 22] or VIVACE (Vortex-Induced Vibration for Aquatic Clean Energy) in recent years [23, 24]. Following that, an increasing number of FIM academics began studying approaches to enhancing the FIM (especially the amplitude) of cylinders. From 2011 to 2013, [PERSON] [25], [PERSON] [26], and [PERSON] et al. [27] developed the PTC (passive turbulence control) system to alter the surface roughness of circular cylinders in order to convert more hydrokinetic energy to electrical energy in a broader velocity range. By adjusting height and edge angle of the roughness trip, the circular cylinder presented the galloping response at a high velocity and the power efficiency was raised from 22% [24] to 28% [27]. Compared with the circular cylinder with PTC, the triangular prism exhibited a great advantage in energy extraction. [PERSON]'s [12] numerical results have shown that the harness efficiency could reach 20%.