Investigations On Technologies Of Ocean Current Energy Harvesting Based On Flow-Induced Motions

Fluid-induced motion(FIM)is a common fluid-structure interaction(FSI)phenomenon.When the fluid flows through the cylinder,the alternating fluid forces will act on the surface of the cylinder.If the cylinder is elastically mounted,the FIM phenomenon will appear.In this paper,the ocean current energy harvesting technology is based on the FIM phenomenon.Two typical FIM phenomena,Vortex-induced vibration(VIV)and galloping,are employed in this sudy.In the process of FIM,the cylinder converts the kinetic energy of fluid into its mechanical energy,and further drives the generator to generate electrical energy.In this paper,the design of the ocean current energy harvesting device,the numerical simulation of the two-way fluid-structure interaction,the experimental test platform,the vortex shedding mode in FIM,the performance analysis of the energy harvesting device under complex conditions and the multi-unit layout optimization are studied systematically.The research results and innovations of the research can be found as follows:(1)The numerical simulation of the two-way fluid-structure interaction is established,and the experimental test platform for the FIM phenomena is built.The two-way FSI means the two-way transfer of CFD result data and the cylinder motion data: the CFD results in simulations will be output to the motion equation of cylinder;the solution of the motion equation will also be input to the CFD simulations,in which the position of cylinder will be updated in real time in the CFD grid.Therefore,the dynamic flow can be accurately simulated.In addition,in order to verify the effectiveness of the ocean current energy harvesting device,the experimental test platform is built on the basis of the preliminary CFD simulations.A series of towing tank experiment are carried out to test the energy conversion performance of the device under the condition of large damping in the "Unmanned Underwater Vehicle Laboratory" of NWPU.The feasibility of the FIM energy harvesting device is verified.(2)The unified formula of the vortex shedding mode in FIM is put forward.In the early studies,the vortex shedding modes are generally named according to the number and relationship of vortices shedding in one cycle.This means that it is necessary to count the number of vortices and distinguish the relationship of vortices for each different case.In this paper,based on the CFD flow visualization technology,the mathematical relationship between the FIM frequency and the vortex shedding frequency is derived from a macro point of view.The unified formula of the vortex shedding mode of n P+2S is proposed.In this equation,n will be obtained by calculating the quotient of the secondary frequency and the main frequency in FIM.This formula has been confirmed in the follow-up study,and it can accurately describe the vortex shedding law in FIM phenomenon.(3)The influence of the cross section shape of the cylinder on FIM is studied.The influence of the number of sides of polygon section on FIM and current energy collection is analyzed in detail.The FIM responses of the cylinders with triangular,square,hexagonal,octagonal,twenty-four and circular sections are discussed.It is found that in the region of VIV,the FIM response of the circular cylinder is the strongest.In the fully developed dance area,reducing the number of edges in the cross section will enhance the FIM amplitude and improve energy harvesting performance.Under the measured velocity conditions,the energy harvesting performance of the square cylinder is relatively low,but the vibration stability is significantly better than that of other cylinder.Therefore,the responses of rectangular cylinders are analyzed.The effect of aspect ratio of the cylinder on the FIM and energy harvesting is further focused and discussed.A small aspect ratio of cross-section can promote the FIM responses,expand the velocity coverage range of VIV,and delay the onset velocity of galloping.It is also observed that the excessive aspect ratio will lead to the disappearance of the FIM responses.(4)The ocean current energy harvesting performance of cylinder in FIM near the free are studied.Through the study of the VIV responses of the circular cylinder and the FIM responses of the square cylinder at different submergence depths,the interaction between the vibrating cylinder and the free surface is discussed.For the VIV responses of the circular cylinder,when the submergence depth is small,the amplitude will be obviously suppressed,and the velocity coverage range will be correspondingly reduced.With the increase of submergence depth,the deformation degree of free surface decreases,and the amplitude and the energy harvesting efficiency increase.At the deep submergence depth,the maximum energy conversion efficiency of the cylinder reaches up to 35%.For the FIM responses of the square cylinder,the vibration law is similar to that of a circular cylinder.It should be noted that when the square cylinder is in the galloping state,the height of the free surface above the square cylinder will increase significantly,and even waves will be generated.However,if the square cylinder is still completely surrounded by the flow field,due to the good stability and robustness of the galloping response,the effect of the free surface can be ignored,which is significantly different from the VIV response.(5)The influence of multi-cylinder interaction and the layout optimization method of "Goose style" are explored.Inspired by the migration of geese by arranging in the “I” or“V”-shaped formation,a "Goose style" layout scheme based on the idea of recovering wake energy is proposed.In this method,the downstream cylinder is on both sides of the wake.The vortex energy o can be used by the downstream cylinder to increase the pressure difference in the movement direction thereby promoting the vibration of the downstream cylinder.According to the CFD simulation results,the Kriging surrogate model is constructed with the spacing parameter as the independent variable and the energy harvesting power as the target variable.Finally,the PSO algorithm can search the optimal layout solution.The results show that compared with the downstream cylinder with the same spacing X in tandem arrangement,the downstream cylinder with the optimal layout is far away from the low-speed and unstable disturbance of the wake.As a result,the FIM amplitude approaches or even exceeds that of the upstream cylinder.Compared with side-by-side cylinders with the same spacing Y,the cylinders with the optimal layout are not disturbed and constrained by other nearby cylinders.Therefore,the FIM response is much stronger,and the energy harvesting performance is enhanced by 19.6%.