Study Of The Performance Of Energy Harvesting And Propulsion Of Flapping Foils

Flapping foil device originates from bionics.Its motion trajectory is two freedoms of degree movement with combined heaving and pitching motion.Based on the relative velocity of the coming flow and the foil and the effective attack angle,flapping foil can extract energy from the coming flow;it is named the energy harvesting mode.The flapping foil can develop thrust like the wings or fish tails;it is named propulsion mode.Flapping foil has a good application prospect in the field of wind and tidal energy utilization and under water vehicle propulsion.Considering the progress at present,this paper studies the energy harvesting of single foil,the energy harvesting and propulsion of tandem foils and the mechanism of the vortex interaction.The unsteady fluid dynamic model of the boundary element method(BEM)and viscous CFD model are established firstly.The validity of these models is verified.Considering the advantages and disadvantages of the design of several existing flapping foil prototype,a new sound fully passive flapping foil power generator has been proposed.The device adopts the crankshaft structure and pairs of gears to amplify the pitch angle.The prototype is built and tested in a tunnel.It has relative higher energy efficiency and low mechanical losses.The flapping foil and the flow field have been simulated through CFD;good agreement of the experimental data and the CFD simulation results has been found.Simulations with different pitch amplitudes and various Strouhal number have been further carried out.The vortex flow field and the instantaneous forces have been analysed;the mechanism of the energy harvesting efficiency of single foil have been investigated.The highest efficiency of the device is up to 32%when pitch amplitude is 70 degree.The difference of the BEM and CFD exists mainly because of the viscosity at higher St and the flow separation at lower St.The energy harvesting and propulsion of tandem flapping foil mainly consider the vortex interaction of the upstream foil and the downstream foil.The parameter global phase shift is introduced to systematically describe the relative position of tandem foils.For given motion frequency and heave amplitude,the global phase shift is dominated by the phase difference of heave motion and the longitudinal spacing.The variations of the performance of energy harvesting and propulsion against global phase shift are analysed based on numerical simulation.There vortex interaction modes of low,medium and high efficiency has been found in the energy harvesting of tandem foils.For the propulsion of tandem foils,there vortex interaction modes for lowest thrust and efficiency,highest thrust and highest efficiency have been found.It is observed that the vortices of upstream foil changes the local flow velocity and the effective attack angle of the downstream foil.The results from the CFD and BEM can observe the influence of the upstream vortices on the downstream foil;similar vortex interaction mechanism can be found.