Study On Hydrodynamic Characteristics Of Pendulum Wave Energy Device Based On Floating Breakwater

Nodding duck wave energy converter(Wave energy converter,WEC)is a wave energy converter with high conversion efficiency,but its stability in the ocean is poor,which severely restricts its commercial development.Since the nod-duck WEC needs to rotate around a fixed axis when working,it needs a structure connected to the fixed axis.Because of its special shape,it will be subjected to a large wave load in the actual sea waves,which is easy to overturn,and the breakwater can be withstanding greater wave loads.The combination of the WEC and the breakwater can improve the stability of the WEC.At the same time,the breakwater reflects back part of the waves transmitted from the nodding duck WEC,which can increase the wave energy conversion efficiency.Therefore,this article combines the nodding-duck WEC with the breakwater and carries out a series of research work,which has important scientific significance and engineering value.This paper first uses the high-order boundary element calculation software WAFDUT2D based on the frequency domain linear theory developed by Dalian University of Technology to study the hydrodynamic performance and energy conversion performance of the nod-duck WEC.The grid convergence verification is carried out and the present results are compared with the published experimental results.The influence of the angle of attack and the center of rotation on the hydrodynamic performance and energy conversion performance of the nod-duck WEC is studied under the optimal PTO damping.Secondly,using the Star-CCM+software based on CFD theory,a nod-duck WEC model with the same parameters with the potential flow theory is established,and the convergence verification of grid step length,time step length and computational domain length are carried out.The comparison with the published experiments results verify the accuracy of the present numerical simulation.The effects of the angle of attack,the center of rotation and the wave height on the hydrodynamic performance and energy conversion performance are studied.Finally,the nodding-duck WEC and the breakwater are combined into a hybrid system.The power generation performance and wave attenuation performance of the single nod-duck WEC,the single breakwater and the hybrid system are compared.The influence of the WEC angle of attack,the distance between the breakwater and the nod-duck WEC,and the wave height are further analyzed.The research results show that the linear frequency results are different from the experimental results and CFD results,and the CFD results are closer to the experimental results.This is because the linear frequency domain method ignores the nonlinearity and viscosity.The selection of the initial attack angle and the center of the rotation axis of the nod-duck WEC is importrant for the energy conversion performance of the WEC.Through CFD calculation,the optimal initial angle of attack and the position of the axis of rotation can be obtained.After the breakwater is added on the backside of the nod-duck WEC,the wave energy conversion efficiency of the WEC increases at small periods,but decreases at some periods,due to the reflection of the breakwater and the narrow gap resonance between the two bodies.The transmission coefficient,which is a measure of wave attenuation performance,is closely related to the draft of the structure.Because the draft of the breakwater is much larger than that of the nod-duck WEC,the wave attenuation performance of the hybrid system is close to the single breakwater.The gap distance has a little effect on the wave energy conversion efficiency of the WEC.As the wave height increases,the viscous dissipation increases,and the wave energy conversion efficiency of the single nod-duck WEC and the hybrid system both decreases.