Study On Hydrodynamic Characteristics And Energy Harvesting Mechanism Of Oscillating Foils

In order to reduce the dependence on traditional fossil energy,renewable energy should be developed.Tidal energy is a kind of clean renewable energy with abundant reserves.The hydrofoil is capable of harvesting energy from incoming flow when it undergoes a combination of pitching and heaving motions.Turbines based on an oscillating foil posse many advantages,such as no limitation in span direction and low cost,which have a bright application prospects in tidal energy utilization.An in-depth study of the energy harvesting mechanism of the oscillating foil could provide scientific theoretical guidance for the development of an oscillating foil turbine with high efficiency,stable output power and reliable operation.In this study,the fully-passive,semi-passive and fully-induced motion models were studied mainly using Fluent.Based on the fully-passive oscillation motion model,the prototype test was performed in wind tunnel.The experimental results were compared with the numerical results,and the numerical results were in good agreement with the experimental results if the errors caused by various factors were taken into account.The results of different aspect ratio hydrofoil indicated that the influence of the wing tip vortex was mainly within one chord length from the end of the hydrofoil.The oscillating foil energy harvester usually adopted a large aspect ratio hydrofoil.The wing tip vortex had a little influence on the hydrodynamic performance.Installing the endplate on the end of the hydrofoil could further reduce the influence of the wing tip vortex.In order to analyze the semi-passive and fully-induced oscillation models,the added mass and moment of the foil was theoretically calculated.For the semi-passive motion model based on a spring-damper system,an iteration procedure using Runge-Kutta method was used to solve the heaving motion in Fluent.The power coefficient and efficiency of the oscillating foil were mapped in the parametric space(0,f*)at high Reynold number.When two hydrofoils had a tandem configuration,there was flow interference between the hydrofoils.The positive wake vortex interference usually occurred at high oscillating frequencies,which had no practical significance.The negative wake vortex interference increased the swept height and reduced the energy harvesting efficiency.Compared to the spring-damping system,the oscillating foil model based on the hydraulic system was more realistic.The hydrodynamic of the hydrofoil and the performance of the hydraulic system were analyzed by Fluent-AMESim co-simulation.Different from the heaving response of the spring-damper system,the hydrofoil would stop briefly at the endpoint of heaving motion.For the stability of the system,a spring element added in the hydraulic system.When the stiffness coefficient of the spring was small,the irregular heaving response causes the decreased of the energy harvesting efficiency.The parameters of hydraulic cylinder and loads had a significant impact on the heaving response,but had a little impact on the hydrodynamic performance.The oscillation motion had dead points for the single hydrofoil.Hydraulic pipes were used to connect the pitching motion of the hydrofoil with the heaving motion of the other hydrofoil to achieve the linkage of two hydrofoils.The mathematical equation was established in terms of the movement and hydrodynamic of the hydrofoils.An iteration procedure using Runge-Kutta method was used to solve the motion responses in Fluent.The initial pitch angle affected the system response slightly,because the response of the system was its inherent property.The hydrofoils experienced attenuation,unstable and stable responses according to the matching of hydraulic system parameters.The hydrofoils experienced an attenuation response at small a and P because the pitching motion consumed more energy.With increasing ? and ?,the response of the hydrofoils changed from an attenuation response to unstable response and to stable response.When two hydrofoils had a tandem configuration,the power coefficient of upstream hydrofoil was always higher than that of the downstream hydrofoil because there was no strong wake vortices interference between two tandem hydrofoils.However,it accounted for no more than 60%of the total power coefficient of the system.The results of Fluent-AMESim co-simulation further demonstrated that the hydraulic linkage system could achieve the fully flow induced oscillation motion of the two hydrofoils.Compared with the semi-passive and fully flow-induced motion models,the fully passive motion had a stronger adaptability to working conditions.According to the mechanism of oscillating foil harvesting energy,the flexible leading-edge foil,non-harmonic pitching motion and shrouded foil models were respectively proposed to improve the energy harvesting performance in terms of foil,motion and flow velocity.The flexible leading-edge foil could adjust the effective angle of attack and increase the lift.However,it only works at large pitching amplitude and high oscillation frequency when phase difference of the leading-edge foil and the foil was ?.The non-harmonic pitching motion could maintain a large effective angle of attack for a long time,which could increase the maximum power coefficient of the oscillating foil harvested.The shroud enhanced the flow velocity over the foil.The nozzle shroud increased the power coefficient but decreased the energy harvesting efficiency.The diffuser enhanced the power coefficient noticeable,and increased the maximum energy harvesting efficiency by 20%.