**1. Introduction**

In recent years, the environmental problems caused by the burning of fossil fuels such as petroleum and coal have become more and more serious, and fossil energy is increasingly depleted. It is an important research focus to obtain clean and renewable energy from the environment [1]. Previous studies have shown that vortices may be generated alternately from the two side surfaces of a bluff body that is immersed in the flow, which results in the phenomenon of flow-induced motion (FIM) [2]. Although FIM may endanger the safety of structures, it can be potentially exploited for collecting energy from the environment [3]. To this end, various energy harvesters and technologies have been developed. It is also expected that such environmental energy harvesters can be utilized in practices to power micro-electromechanical systems (MEMS) and wireless sensor systems so that a more convenient realization of structural health monitoring, industry sense and detection, military track, and environmental monitoring [4] can be achieved.

In reference to wind energy harvesting techniques, the majority of related harvesters were developed using the principles of vortex-induced vibration (VIV), galloping, flutter, and buffeting, the main forms of wind-induced induced VIV and galloping. For the VIV energy harvester, the flow rate range of high-efficiency power generation requires that the vortex shedding frequency of the harvester is consistent with the natural frequency. Therefore, it is commonly used for fluid energy harvesting. Williamson's team [5,6] conducted a

**Citation:** Liao, P.; Fu, J.; Ma, W.; Cai, Y.; He, Y. Study on the Efficiency and Dynamic Characteristics of an Energy Harvester Based on Flexible Structure Galloping. *Energies* **2021**, *14*, 6548. https://doi.org/10.3390/ en14206548

Academic Editor: Sandro Nizetic

Received: 22 September 2021 Accepted: 8 October 2021 Published: 12 October 2021

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lot of research on cylindrical VIV, identified various branches of the VIV amplitude, and summarized the vortex shedding mode into S and P modes. Ding et al. [7] conducted numerical simulation of VIV for different bluff bodies and studied their energy harvesting efficiency. An et al. [8] analyzed and studied the effect of CFD technology on the plate length of the VIV energy harvester on the flow velocity, dynamics, and performance of the wake structure. Zhang et al. [9] used the two-dimensional Reynolds number method to study the VIV of four staggered cylinders, and conducted a series of numerical simulations for energy harvesting. Akaydin et al. [10] developed a cylindrical bluff body VIV piezoelectric energy harvester. When the flow speed is 1.19 m/s, the harvester with a resonance frequency of 3.14 Hz can output a maximum power of 0.1 mW to an optimized load of 2.46 MΩ. Galloping is a typical self-excited vibration phenomenon caused by aeroelastic instability. It mostly occurs in rectangular, angular, and flexible structures, and is usually characterized by low-frequency and high-amplitude oscillations [11]. Due to the greater vibration and higher output power, this aerodynamic instability may be more suitable for energy harvesting than VIV [12]. Barrero-Gil et al. [13] were first to theoretically analyze the potential of using a single-degree-of-freedom (SDOF) system to harvest energy using lateral gallop. Javed et al. [14] used a distributed parameter pattern to study the influence of various aerodynamic force expressions on galloping. Zhao et al. [15] studied the influence of bluff wind exposure area, load resistance, mass of bluff, and piezoelectric sheet length on the output power of a galloping energy harvester. On the other hand, Hu et al. [16] examined the influence of aerodynamic configuration on wind harvesting performance, and found that the VIV of a cylinder could be transformed into galloping if the cylinder was treated via corner modification techniques. Additionally, Sirohi et al. [17] proposed a harvester based on triangular section rods attached to the cantilever beam.

The results from previous studies have shown that transmission lines can be covered with ice on cold days, and their cross section may change to a non-circular shape [18]. Under certain wind speed and wind attack angles, the incoming flow on both sides of the bluff body can produce vortices and shed backwards, and generate an alternate aerodynamic load, which results in the galloping of the transmission line [19]. Previous studies also showed that the tension of the wire can influence galloping significantly, and greater tensions tends to favor the occurrence of galloping. Meanwhile, many galloping energy harvesters were developed by using columns with a square section, as prisms with a square section have more complex cross-section geometric characteristics compared to cylinders. Since Den Hartog first studied and explained the galloping phenomenon, numerous studies have shown that galloping can be widely observed on bluff bodies with a square section [20,21]. Therefore, the square section is usually preferred for the study of galloping energy harvesters.

In this study, a square section energy harvester based on the galloping principle of an iced transmission line was developed. The performance of the harvester was examined via both experimental tests and CFD simulations. The CFD technique was utilized, since it provides a powerful tool to explore the characteristics of flow motions, and to further understand the working mechanism of the harvester. The remainder of the article is organized as follows: Section 2 introduces the design and modeling of the harvester, Sections 3 and 4 detail the CFD method, and the experimental method, respectively. The specific results are presented and discussed in Section 5, and the main findings and conclusions are summarized in Section 6.
