Coupling Analysis Of Energy Conversion And Hydrodynamic Characteristic Based On Parallel-oscillating Hydrofoils Hydrokinetic Turbine

Tidal energy,as an important component of marine energy,has a good development prospects because of its advantages.Such as resource-rich,high energy density,load stability,predictability,cleanliness and so on.The most concentrated areas of tidal energy resources are the Strait,the narrow waterways and the Gulf entrance.Oscillating hydrofoil turbine is an important form of tidal power generation technology.At the same depth of sea condition,the swept area of the oscillating hydrofoils is three times of traditional impeller turbine.So,it has obvious advantages in shallow water.At present,the main research methods of the oscillating hydrofoil technology are theoretical analysis,experimental study and CFD numerical simulation.With the rapid development of the computer technology,many complex flow problems are solved by numerical simulation method.The dual-oscillating hydrofoil model is based on the coupling of the two hydrofoil pitching and plunging movements.The two hydrofoils are in parallel arrangement.An hydraulic transducer system is established based on the hydrofoils and its pitching-plunging movement.In this paper,CFD numerical simulation is used to solve the two-dimensional unsteady and incompressible Navier-Stokes equations around the dual-oscillating foils.The mesh model is based on the dynamic grid technology and sliding mesh technology.The simulation is in the excessive and high Reynolds number which is more suitable for the actual sea conditions.The criteria,which is to evaluate the performance,hydrodynamic characteristics and stability of the dual-oscillating hydrofoils system,is established.The coupling effect of geometrical parameters(airfoils,axis position),motion parameters(plunging amplitude pitching amplitude,and oscillating frequency)and flow conditions on the oscillating system is studied.The energy extraction performance,vortex characteristics and pressure distribution characteristics under different parameters are synthetically analyzed.The results show that the parameters will affect the vortex and boundary layer distribution.The NACA0018 foil has the best power extraction performance under the motion parameters among the selected foils.The effect of pitching axis position on the oscillating wing system is varies with the motion parameters.Putting the axis position closer to the trailing edge of the foil will improve energy efficiency,while worse the hydrodynamic characteristics.The plunging amplitude affects the energy extraction efficiency mainly via the plunging velocity.Non-uniform flow conditions will reduce the hydrodynamic characteristics of the oscillating hydrofoils.Under lower reduced frequency,it will improve the power extraction performance.In summary,the optimal parameters are selected as follows,NACA0018 hydrofoil,the pitching axis located at c/3,the plunging amplitude is c,the pitching amplitude is 75 degree,the reduced frequency is between 0.14 and 0.18.The choice of the dual-oscillating hydrofoil model and the optimal combined parameters will allow the oscillating system have better conversion and dynamic characteristics.