## 3 Results and Discussions  
### Hydrodynamic Properties of Ocean Buoy  
Since the water sail has little effect on the vertical motion of floating buoy and the influence of water sail on TENG can be ignored, only the motion of the sphere needs to be considered. As shown in Figure 3a, in order to simplify the calculation, the mass block is connected with the crane through the fixing device. Therefore, the motion of the sphere is the relative motion between the C-TENG and the buoy. The C-TENG is installed inside the ocean buoy and its hydrodynamic properties are consistent with that of the ocean buoy, so we only need to study the hydrodynamic properties of the ocean buoy. The calculation model adopted this time is mainly an ocean buoy with a diameter of 600 mm and a cuboid with a computational domain of 10 m \(\times\) 2 m \(\times\) 5 m, as shown in Figure 3b. The method we adopted for grid setting was automatic grid generation with base size of 0.5 m. In order to more accurately study the motion pattern of the ocean buoy, the water surface and waves are encrypted with three grids. In addition, we also carried out mesh encryption for the ocean buoy for three times. The boundary conditions are mainly set for the computational domain and sphere. The calculation domain mainly sets six boundary conditions. The Bottom, we need to make the wall impermeable; For inlet is a speed inlet; Outlet is pressure outlet; Symmetry is the plane of symmetry; Top stands for pressure outlet. The boundary condition Settings for the Sphere are mainly Sphere Surface (wall) and Block Surface. In addition to setting basic physical quantities such as air and water, the initial conditions mainly require setting the parameters of waves. BUOY-41 is used in the Yellow Sea and Bohai Sea. According to the data, the average wave height of the Yellow Sea and Bohai Sea is about 0.4 m, and the period is between 2 s and 4 s. Therefore, the 0.4 m wave height and 2 s period are selected as the wave condition. Waves are mainly fifth-order stokes waves with a wave period of 2 s, a wave height of 0.4 m and a water depth of 5 m. In addition, the Volume Fraction of all parameters is set as composite. Air field function is Volume Fraction of Light Fluid of fifth-order VOF wave, water field function is Volume Fraction of Heavy Fluid of fifth-order VOF wave, and Pressure field function is Hydrostatic Pressure of fifth-order VOF wave.  
For the hydrodynamic properties, we mainly calculated and analyzed the vertical displacement variation and buoyancy variation. The calculated step length was 0.02 and the maximum limiting time was 85 s. Figure 4a,b, respectively, represent the vertical displacement change and 6-DOF buoyancy change of the ocean buoy. From Figure 4a, it is observed that the motion of the ocean buoy tends to be stable from about 70 s, and the maximum displacement can reach 0.19 m. By comparing the buoyancy diagram of the ocean buoy, we found that the force variation of the ocean buoy tends to be stable from 37.74 s, and the maximum buoyancy reaches 13.62 N. Through the above simulation  
Figure 2: Model of C-TENG. (**a**) model of the assembly; (**b**) cutaway view of C-TENG.  
results, we found that under the condition of stable force, the ocean buoy could produce a good displacement, so we could import the displacement function into COMSOL for further simulation.