## 2 Experimental Conditions  
Experiments are conducted in a low-speed water channel platform. Figure 1a,b illustrate the schematic diagram and physical picture of the water channel platform, respectively. The experimental system is composed of five parts, including an open channel, a pump (Shimge Co., Ltd(r); Hangzhou, China), a frequency converter (Shenzhen Junhui Electronics Co., Ltd(r); Shenzhen, China), a cellular device (Qingdao Tonglide plastic buzzer hives Co., Ltd(r); Qingdao, China) and several damping meshes (Hangzhou Kuangshi Co., Ltd(r); Hangzhou, China). Water cycle in the experimental system with closed loop. Water is poured into the open channel from the inlet by the pump at a certain speed, which can be controlled, from 0 to 1 m/s, by changing the working frequency of the pump with the frequency converter. The water speed can be measured by the flow measuring instrument. The roles of cellular device and damping meshes are weakening the stream turbulence so as to provide a steady current for experiments. According to the experiences in the low speed wind tunnel design, the cellular device with small holes can effectively break vortex and smooth airflow if the angle between the free-stream flow and the hole axis is less than 10\({}^{\circ}\) and the length is 5-10 times more than the diameter of the hole. Furthermore, several damping meshes are always placed in the downstream of the cellular device to further break smaller vortex. In this paper, the holes of the cellular device are hexagon, and their equivalent diameter and length are \(\Phi\)8 mm and 40 mm, respectively, and three damping meshes are used to weaken stream turbulence and provide a steady current. Furthermore, the contraction section is designed to increase the water speed and improve the quality of the flow filed. Finally, water steadily passes through the test section, runs into the outlet and pours into the open channel. There is a limit plate to ensure the depth of water. Figure 2a shows the schematic of the two tandem identical piezoelectric energy harvesters system, and Figure 2b shows the composition of the energy harvester, what is made up of a piezoelectric beam (Baoding Hengsheng Acoustics Electron Apparatus Co., Ltd(r), Baoding, Hebei, China) and a circular cylinder (Guangzhou Huaying Plexiglass Co., Ltd(r), Guangzhou, China). The piezoelectric beam has the advantages of simple construction, easy to produce, low cost, high energy density and low natural frequency. According to the theory of fluid dynamics, the fluid will exert a force on the object with non-streamlined boundary when the fluid flows through it. Hence the piezoelectric beam will oscillate caused by the circular cylinder whose most part is immersed in flowing water. Furthermore, adding the cylinders is mean to decrease the natural frequency. The harvesters in the paper are used at low water speed. The frequency of the vortex shedding from the cylinder is low. Hence the harvesters with low natural frequency are easy to resonate, and have larger power output. The circular cylinder of 12 mm (diameter, \(D\)) \(\times\) 80 mm (length) is vertically equipped at the end of the piezoelectric beam, which contains a substrate layer and a piezoelectric layer. The two layers of the  
Figure 1: Schematic diagram of the experimental set-up and a picture of the experimental platform: (a) Schematic diagram of the experimental set-up; (b) Experimental platform. beam are the same in size, 80 mm (active length) \(\times\) 20 mm (wide) \(\times\) 0.2 mm (thickness). The material of substrate layer is aluminum and that of piezoelectric layer is PZT-5H. Fixing the other end of the piezoelectric beam, putting the circular cylinder in flowing water, output voltage can be obtained across the resistor connected with the beam. The experiment data is entered into a computer using a data sampling card of NI corporation (National Instruments, Austin, TX, USA). Table 1 lists the detailed material parameters of the energy harvester prototype.