## 1 Introduction  
The scholars' research on the application of piezoelectrics has never ceased since the piezoelectric effect was discovered in the 19 th century. Based on the piezoelectric effect, vibrational energy can be converted into electric power using piezoelectric materials. In the early 1980s, [PERSON] et al. [1] proposed a prototype energy power-generating device based on the flutter phenomenon. [PERSON] et al. [2] proposed an energy-harvesting generator from vibration using a piezoelectric membrane. Under an acceleration of 2 \(g\) and a load resistance of 56 k\(\Omega\), the energy-harvesting generator generated a maximum power of 1.8 mW. The piezoelectric energy-harvesting system as a sustainable power source can be a substitute for small batteries which have a finite life span [3]. Fluid-induced vibration as a style of vibration can be found in smokestacks, suspension bridges, oil pipe lines, power transmission lines, television antennas, heat exchanger tubes, nuclear fuel assemblies, submarine periscopes and hulls, etc. Containing vast amounts of vibration energy, fluid-induced vibration is often used as an energy source for piezoelectric energy harvesters. Wake-induced vibration (WIV) [4; 5], vortex-induced vibration (VIV) [6; 7; 8], and flutter-induced vibration (FIV) [9; 10; 11; 12] all belong to fluid-induced vibration. Piezoelectric energy harvesters based on vortex-induced vibration [13; 14; 15; 16; 17; 18] and galloping [19; 20; 21] have been extensively studied in the past several years. [PERSON] et al. [22]investigated a piezoelectric energy harvester from vortex-induced vibration in air flow. A maximum of 0.1 mW non-rectified electrical power was attained at a flow speed of about 1.192 m/s in the wind tunnel test. [PERSON] et al. [23] evaluated different parameters of a piezoelectric energy harvester from vortex-induced vibration in water flow. They found that the amplitude of the signal changed according to the variability of the vortexes' generation. Furthermore, the performance of galloping-based piezoelectric energy harvesters with different cross-section geometries has also been investigated in a few studies [24; 25; 26; 27; 28; 29; 30]. [PERSON] et al. [31] reviewed the previous work on aeroelastic energy harvesting, and qualitatively and quantitatively compared used designs. Then they introduced an enhanced stability characterization for an aeroelastic energy harvester by using both the normal form of the Hopf bifurcation and the shooting method [32]. It can be seen that most piezoelectric energy harvesters from vortex-induced vibration are of a single piezoelectric energy harvester. In practice, however, a group of energy harvesters will probably be needed at the same time to make the best use of space and generate more energy. [PERSON] et al. [33; 34; 35] investigated three main configurations of two circular cylinders, namely the tandem, side-by-side, and staggered configurations. They found that the mean aerodynamic forces on both cylinders depend largely on the spacing ratio. [PERSON] et al. [36; 37] also indicated that the characteristics of flow and fluid force changed with the staggered configuration. [PERSON] et al. [38] experimentally investigated a novel piezoelectric energy harvester using the macro-fiber composite cantilever with a bicylinder in water. Two cylinders were connected with a piezoelectric beam in this pattern. Wake-induced vibration happened on the downstream cylinder. [PERSON] et al. [39] experimentally investigated two piezoelectric energy harvesters in a tandem array in the airflow. It was found that the downstream harvester had no output of energy when the diameter of the upstream cylinder was larger. Placing a smaller circular cylinder upstream of the harvester, wake galloping of the upstream circular cylinder took place when the distance between the two harvesters exceeded a critical value. Furthermore, they concluded that wake galloping could be used to increase the average harvested power of a piezoelectric energy harvester.