The absorber is known to be vertical axisymmetric for a single-point wave energy converter (WEC). The shape of the wetted surface usually has a great influence on the absorber’s hydrodynamic characteristics which are closely linked with the wave power conversion ability. For complex wetted surface, the hydrodynamic coefficients have been predicted traditionally by hydrodynamic software based upon the BEM. However, for a systematic study of various parameters and geometries, they are too multifarious to generate so many models and data grids. This paper examines a semi-analytical method of decomposing the complex axisymmetric boundary into several ring shaped stepped surfaces based on the boundary discretization method (BDM) which overcomes the former difficulties. In such case, using the linear wave theory based upon eigenfunction expansion matching method, the expressions of velocity potential in each domain, added mass, radiation damping and wave excitation forces of the oscillating absorbers are obtained. The good astringency of the hydrodynamic coefficients and wave forces are obtained for various geometries when the discrete number reaches a certain value. The captured wave power for a given same draught and displacement for various geometries are calculated and compared. Numerical results show that the geometrical shape has great effect on the wave conversion performance of the absorber. For absorbers with the same outer radius and draught or displacement, the cylindrical type shows fantastic wave energy conversion ability at some given frequencies, while in the random sea wave, the parabolic and conical ones have better stabilization and applicability in wave power conversion.

In intermediate water depths (100~300m), an array of floating point-absorbing Wave Energy Converters (WECs) may be employed for extracting efficiently ocean wave energy. In such case, it may be more feasible and convenient to connect the absorbers array with a semi-submersible bottom-moored platform, whose function is to act as the seabed. An array of identical floating symmetrically distributed cylinders in a coaxial moored platform is proposed in this study. The Power Take-Off (PTO) system is assumed to be composed of a linear/nonlinear damper activated by the buoys heaving motion. Linear hydrodynamic analysis of the examined floating system is implemented in frequency domain. Hydrodynamic interferences between the oscillating bodies are accounted for in the corresponding coupled equations. The array layouts under the constraint of the platform, incidence wave directions, separating distance between the absorbers and the PTO damping are considered to optimize this kind of WECs. Numerical results with regular waves are presented and discussed for the axisymmetric system utilizing heave mode with these impact factors, in terms of a specific numbers of cylinders and expected power production.

将浮式垂直轴叶轮在波浪中运动的水动力学问题简化为其在均匀流中受到强迫摇荡运动。通过二次开发ANSYS-CFX软件建立了这种强迫摇荡的水动力学模型，计算了垂直轴叶轮绕自身主轴旋转的同时发生单自由度受迫横摇运动时的水动力性能，分析了不同受迫横摇运动频率对叶轮推力和侧向力的影响，通过对计算的推力和侧向力进行傅里叶级数展开并进行最小二乘法拟合，得到了与水轮机受迫横摇运动速度和加速度相关的阻尼和附加质量傅里叶级数系数。计算结果表明：受迫横摇运动下的叶轮效率、推力和侧向力曲线的上下包络线明显按照横摇频率发生周期性波动；随着横摇频率的增加，推力和侧向力系数包络线波动幅值会随之增大，但平均值几乎不变；不同横摇频率下，在推力中阻尼项有较大的占比，而在侧向力中阻尼力和附加质量力几乎相当。浮式立轴叶轮在波流作用下的摇荡运动对潮流能装置的结构强度和疲劳特性有很大影响。本文对波流作用下的浮式潮流能装置水动力特性的研究有巨大的工程和学术意义。

Point absorber wave energy device with multiple degrees of freedom (DOF) is assumed to have a better absorption ability of mechanical energy from ocean waves. In this paper, a coaxial symmetric articulated point absorber wave energy converter with two degrees of freedom is presented. The mechanical equations of the oscillation buoy with power take-off mechanism (PTO) in regular waves are established. The three-dimensional numerical wave tank is built in consideration of the buoy motion based upon the CFD method. The appropriate simulation elements are selected for buoy and wave parameters. The feasibility of the CFD method is verified through the contrast between the numerical simulation results of typical wave conditions and test results. In such case, the buoy with single DOF of heave, pitch and their coupling motion considering free (no PTO damping) and damped oscillations in regular waves are simulated by using the verified CFD method respectively. The hydrodynamic and wave energy conversion characteristics with typical wave conditions are analyzed. The numerical results show that the heave and pitch can affect each other in the buoy coupling motion, hydrodynamic loads, wave energy absorption and flow field. The total capture width ratio with two coupled DOF motion is higher than that with a single DOF motion. The wave energy conversion of a certain DOF motion may be higher than that of the single certain DOF motion even though the wave is at the resonance period. When the wave periods are high enough, the interaction between the coupled DOF motion can be neglected.

振荡运动下立轴叶轮有限的展弦比会使叶片梢端出现绕流，从而影响叶片的性能。为了研究振荡运动下立轴水轮机的三维效应影响，采用CFX软件三维模拟立轴水轮机在不同展弦下的振荡运动（横荡、纵荡），和二维模拟的立轴水轮机作对比，并通过最小二乘法拟合计算得到的水轮机推力和侧向力系数时域结果，分析得到不同展弦比下水轮机阻尼系数和附加质量系数。结果表明：随着展弦比增大，叶轮潮流能的转换效率逐渐增大；三维振荡叶轮的水动力载荷与叶轮展长呈正相关，当展弦比达到10时其水动力载荷及能量转换效率和二维的计算结果相当；三维水轮机水动力系数常数项和阻尼项随展弦比增大而增大。研究成果可以为浮式立轴水轮机设计提供参考帮助。

为了研究浪流联合环境中，浮式立轴水轮机的水动力特性及能量转换效率，本文将浮式垂直轴叶轮在波浪中运动的水动力学问题简化为在均匀流中强迫摇荡运动，并利用ANSYS-CFX软件二次开发建立了这种强迫摇荡的水动力学模型，计算了垂直轴叶轮单自由度横摇运动时的不同横摇幅值下的叶轮推力、侧向力的水动力性能变化，并将叶轮的推力系数和侧向力系数的时历曲线用最小二乘法进行拟合，分析得到了水轮机阻尼系数和附加质量系数。结果表明：横摇运动下，叶轮推力和侧向力曲线的上下包络线明显随着横摇运动发生周期性波动；随着横摇幅值的增加，推力、侧向力系数的平均值变化不大，但是瞬时波动值明显增大；在横摇推力中，阻尼力比附加质量力所占的比例要大，而横摇侧向力受到阻尼力和附加质量力的影响。本文的研究对浪流联合作用下浮式潮流能装置水动力特性研究有巨大的工程和学术意义。

为解决波能转换装置向深水环境推进过程中存在的系统稳定性和能量转换效率问题，借鉴海洋工程中常用的稳性辅助构件形式，在现有的点吸式波能装置基础上引入阻尼板。基于线性微幅波假设，通过特征函数展开和边界匹配的势流半解析方法，并结合多自由度振动理论，探索阻尼板的存在及其构型参数变化对获能系统水动力、运动响应及能量转换效率的影响。计算结果表明，阻尼板会降低浮子受到的波浪激励力，阻尼板与浮子间的相互作用水动力大于浮子自身运动受到的水动力，且主要体现在惯性载荷部分；阻尼板会使系统出现两个耦合共振频率，且新出现的共振频率对阻尼板半径更加敏感，在较小共振频率处的最优波能转换效率均随着阻尼板半径和浸没深度增加先增大后减小。研究结果可为深水波浪能利用的工程应用提供理论基础，为后续振荡浮子波浪能发电装置优化提供依据。

动力输出系统(Power Take-Off, PTO)作为波浪能转换装置(Wave Energy Converter, WEC)的主要构件之一，对系统运动及能量转换至关重要。本文首先基于势流理论，运用特征函数展开法得到圆柱形浮体所在流域的速度势函数级数表达式，进而通过边界匹配法得到作垂荡运动浮子的附加质量、阻尼系数及波浪激励力的解析表达式。针对阻尼器特性，分别研究线性和非线性PTO阻尼作用下，浮子的运动及波能转换特性，重点研究了线性PTO作用下的过阻尼问题。计算结果表明，低速度指数的 PTO 系统对装置运动的影响主要体现在PTO 阻尼系数上，随着阻尼系数增大，波能装置的共振频率逐渐减小，但减小幅度很小；PTO 系统的非线性特性并不能改变浮子的最优转换效率，但是较大的速度指数能有效改善 PTO 系统的阻尼容量；在较低频和较高频时，通过解析算法得到的最优 PTO 阻尼会使得装置处于过阻尼工作状态，且在低频部分需要进行最优 PTO 修正的最高频率和在高频部分需要进行修正的最低频率均随着半径和吃水的增大而逐渐减小。

To simulate the hydrodynamic performance of a floating current turbine in combined wave and flow environment is important. In this paper, the ANSYS-CFX softwareis used to analyse the hydrodynamic performance of a vertical-axis turbine with various influence factors such as tip speed ratio, pitching frequency and amplitude. The time-varying curves of thrust and lateral forces are fitted with the least square method; the added mass and damping coefficients are refined to analyse the influences of the former factors. The simulation results demonstrate that, compared with the non-pitching and rotating turbines under constant inflow, the time-varying load of rotating turbines with pitching exhibit an additional fluctuation. The pitching motion of the turbine has a positive effect on the power output. The fluctuation amplitudes of thrust and lateral force’s envelope curves have a positive correlation with the frequency and amplitude of the pitching motion and tip speed ratio, which is harmful to the turbine structural strength. The mean values of the forces are slightly affected by pitching frequencies and amplitudes,but positively proportional to the tip speed ratio of turbine. Based upon the least square method, the thrust and lateral force coefficients can be divided into three components, uniform load coefficient, added mass and damping coefficients, and the middle one is significantly smaller than other two.Damping force plays a more important role in the fluctuation of loads induced by pitching motion. These results can facilitate the study of the motion response of floating vertical-axis tidal current turbine system in waves.

To further solve the problems of system stability and energy conversion efficiency in the development of wave energy utilization to deep water, the perforated damping plate is considered based on the existed point-absorber wave energy converter in this study. A new form of dual-buoy with supported column and perforated plate is proposed. Based upon the hypothesis of linear micro-amplitude wave theory, the influence of the perforated damping plates and the variation of configuration parameters on the hydrodynamic, motion and power conversion efficiency of the system are explored by means of the semi-analytical method of eigenfunction expansion and boundary matching and the multi-degree-of-freedom vibration theory. The results show that the opening radius of the damping plate will reduce the wave excitation force on the float. The float, the damping plate and the coupling radiation force will decrease with the increase of the opening radius of the damping plate. Its increase can also promote the relative motion of float and damping plate. The damping plate will make the system appear two coupling resonance frequencies. The optimal wave energy conversion efficiency at the smaller resonance frequency increases first and then decreases with the increase of the opening radius of the damping plate. The research results can provide a theoretical foundation for the engineering application of wave energy utilization in deep water and provide a basis for the optimization of oscillating-buoy wave energy converters.

Due to the effect of fluid viscosity exerted on the motions for a cylindrical structure, a series of complex changes will occur in the motion variations, especially for the motion of single degree of freedom. However, the computation accuracy of traditional computing methods has been limited, with no thought for the fluid viscosity. In this paper, a three-dimensional numerical wave tank is established based on the theory of viscous fluid mechanics. Firstly, a series of convergence analysis of different grid resolutions are carried out, and the parameters of numerical models are optimized. After that, taking the fluid viscosity into consideration, the heave motions of different cylinders with various sizes are investigated via the optimized models. Meanwhile, the equation of wave-cylinder coupling movement is established based on the potential flow theory and linear gravity water wave theory. And the analytical solution is obtained. Finally, through comparing the analytical solution and the numerical results, the basic moving laws of float has been validated and the variations of heave motion of cylindrical structures is analyzed systematically.

The power take-off (PTO) damping mechanism is very important to the motion and power conversion for the wave energy converters (WECs). Based upon the potential flow theory, the series expression with unknown Fourier coefficients of velocity potential function of the basin where the cylindrical floating buoy is located is obtained by using the eigenfunction expansion method. According to the characteristics of the PTO damper, the motion and wave energy conversion characteristics of the float under the linear and nonlinear PTO damping are studied respectively, and the over-damping problem under the linear PTO damping is emphatically explored. The results show that the influence of PTO system with low velocity index on the motion of the device is mainly reflected in the PTO damping coefficient. With the increase of damping coefficient, the resonance frequency of the wave energy device decreases gradually, but the decrease amplitude is very small. The nonlinear characteristics of PTO system cannot change the optimal capture width ratio of the float, but the large velocity index can effectively improve the damping capacity of PTO system. At lower and higher frequencies, the optimal PTO damping obtained by the analytic algorithm will make the device in an over-damped state. The highest frequency in the low frequency part and the lowest frequency in the high frequency part which need to be modified will gradually decrease with the increase of radius and draught.

With the intensification of the world's energy demand, clean and efficient marine energy has received more attention, and the energy conversion device is particularly important. As an important part of the oscillating wave energy conversion device, the hydrodynamic characteristics of cylindrical absorbers have a great impact on the wave energy conversion efficiency. When the cylindrical buoys with different bottom configurations heave under the action of waves, the oscillating motion and flow field become more complex. At present, the potential flow theory is usually used to predict the motion response of cylindrical buoys in waves. Although the calculation speed is fast, there will be large errors due to the lack of consideration of the effect of fluid viscosity. To explore this error range, a three-dimensional numerical wave pool is established based on the viscous fluid theory and STARCCM+ software to study the response of the buoy and wave coupling action considering the viscous effect. At the same time, based upon the potential flow theory, the analytical solution of the cylindrical buoy oscillating in waves is established. Combined with the CFD numerical simulation results, the oscillation laws of the buoys with different bottom configurations in waves with different periods are compared, and the effects of viscosity on the buoys’ motion are also explored. According to the calculation results of floating buoy movement under viscous and non-viscous fluid, the statistical correction algorithm is adopted to obtain the viscous hydrodynamic correction algorithm of floating buoy movement based on the potential flow theory, and the feasibility is verified by viscous numerical simulation under other wave periods.

To obtain the mechanical energy of waves from arbitrary directions, the vibration absorbers of wave energy converters (WEC) are usually vertically axisymmetric. In such case, the wave-body interaction hydrodynamics is an essential research topic to obtain high-efficiency wave energy. In this paper, a semi-analytical method of decomposing the complex axisymmetric boundary into several ring-shaped stepped surfaces based upon the boundary approximation method (BAM) is introduced and examined. The hydrodynamic loads and parameters, such as the wave excitation forces, added mass and radiation damping of the vertical axisymmetric oscillating buoys, can then be achieved by using the new boundary discretisation method. The calculations of the wave forces and hydrodynamic coefficients show good convergence with the number of discretisation increases. Comparison between the constringent results and the results of the conventional method also verifies the feasibility of the method. Then, simulations and comparisons of the hydrodynamic forces, motions and wave power conversions of the buoys with series draught and displacement ratios in regular and irregular waves are conducted. The calculation results show that the geometrical shape has a great effect on the hydrodynamic and wave power conversion performance of the absorber. In regular waves, though the concave buoy has the lowest wave conversion efficiency, it has the largest frequency bandwidth for a given draught ratio, while in irregular waves, for a given draught ratio, the truncated cylindrical buoy has the best wave power conversion, and for a given displacement of the buoy, the concave buoy shows the best wave power conversion ability.