## 1 Introduction  
In the past few decades, the increasing exploitation of nonrenewable fossil fuels, such as coals, oils and natural gases, have caused a serious energy crisis and environmental pollution [1; 2; 3], while developing and utilizing clean renewable energy [4] can relieve the pressure of fossil energy demand and environmental pollution. Wave energy is a kind of marine renewable energy with wide distribution and huge reserves. However, due to the limited technology, it is difficult to make use of wave energy fully and efficiently [5; 6; 7; 8], especially the low-frequency part of wave energy [9; 10]. Therefore, it is desired to develop a new technology to harvest low-frequency wave energy efficiently and conveniently. Recently, [PERSON] [11] proposed the concept of triboelectric nanogenerator (TENG) based on the conjugation of triboelectric effect and electrostatic induction. The TENG technology is considered as a promising way for harvesting low-frequency energy [12; 13; 14; 15; 16], including energy generated by human movement [17; 18; 19; 20], wind [21; 22; 23], rainfall [24; 25; 26], etc. Up to now, the TENG has been successfully used to scavenge various types of energy, such as water wave [27; 28] at a very low frequency (0.1-1 Hz). By connecting multiple TENG units in parallel, the TENG array has shown great potential in solving the problem of harvesting large-scale wave energy efficiently. However, while TENGs do excellent performance in harvesting wave energy, most of the work focuses on the design of the structure, but lacks the basic researches on TENGs, such as the hydrodynamic and energy capture properties of TENGs. The hydrodynamic properties of TENGs reflect on how TENGs move with waves. The energy capture properties reflect correlation between energy capture and motion. These two properties are related to the fundamental issues, which is conductive to improve the performance of TENGs. Focusing on this fundamental problem, we design a simple structure, which is mainly composed of a hollow cylinder and a mass block. The simplest structure is convenient for calculation and verification. The other advantage of this structure is that it can make full use of the vertical motion in the wave, which is the concentrated part of wave waves, so that the wave energy can be converted into electrical energy more effectively. Considering that the TENG is installed inside the ocean buoy, the hydrodynamic properties of TENG are consistent with the ocean buoy. In the simulation calculation, the mass block is connected with the crane through the fixing device. Based on this consistency, only hydrodynamic properties of the ocean buoy need to be calculated. There are many kinds of CFD software used to calculate the hydrodynamic properties of structures, such as Fluent, HydroSTAR and Star-CCM+. The CFD software we used in this paper is Star-CCM+. The hydrodynamic simulation of buoys by Star-CCM+ have been proven to be basically consistent with the experiments. [PERSON] [29] conducted numerical simulations by means of LES to reveal 3D flow hydrodynamics (i.e., mass/momentum fluxes and turbulent structure) in lateral embayments of open-channel flows. [PERSON] [30] carried out numerical simulation and experimental research on the horizontal and vertical motion of the composite wave-inspired ocean buoy in the wave tank, derived the motion equation of the ocean buoy, and verified that the response of the horizontal and vertical motion of the buoy basically conforms to the simulation results. [PERSON] [31] also carried out a two-dimensional nonlinear analysis of the untether ocean buoy under wave load by means of equation derivation and experiment, and obtained that the buoy had a good single harmonic sinusoidal response in both directions of X and Z.