Study Of The Hydrodynamic Performance Of Ocean Current Energy Harvesters Based On Flapping Wing

The ocean is abundant in renewable energy resources,including current energy,wave energy,offshore wind energy,osmotic energy,ocean thermal energy,etc.To harvest the current energy by oscillating wings is a new concept inspired by the locomotive ability of aquatic animals.Com-pared with the facilities based on rotary turbines,this new strategy has several advantages,such as easy deployment in shallow water,better space utilization,friendly to aquatic animals,less ob-struction on navigation et al.According to the present research progress on the flapping foil based energy harvesting system,this thesis performs systematical numerical studies by considering the complicated fluid-structure-interaction between the foil and its ambient flow.First,we identify the optimal regime within the(f*,?0)parametric space for high energy harvesting efficiency,where f*is the flapping frequency and ?0 is the pitching amplitude.The feasibility of enhancing the perfor-mance by adopting non-sinusoidal pitching profiles has also been evaluated.The effects of inertia and damping on the performance of the system are investigated.Further,the three-dimensionality aroused by either the tips and the inherent 3D instability have also been studied.Finally,the effects of free surface on the flapping foil energy harvesters have been numerically investigated,in which a fully-active model has been considered.First,for the effects of non-sinusoidal pitching motion,we identify the optimal parameters of?0=75° and f*= 0.16 for the sinusoidal pitching motion as the basis for further investigation,with the highest energy harvesting efficiency of 32%being recorded.Then we study the non-sinusoidal pitching motion,with a gradual change from a sinusoid to a square wave as the control parameter for pitching profile ? is increased from one.We find that the efficiency enhancement is limited for the parameters approaching their optimal values,and the upper boundary of the efficiency appears not to be increased.In detail,we report that when the pitching amplitude is small,non-sinusoidal pitching motions can indeed improve the performance of the system,which is consistent with the results reported by previous researchers.However,when both the pitching amplitude and the flapping frequency are close to their optimal values,non-sinusoidal pitching motions contribute negatively to the harvesting efficiency.We suggest that a non-sinusoidal profile,at least a simple trapezoidal-like one is ineffective in the semi-active system.Second,for the effects of inertia and damping,at a fixed mass ratio of r = 1,we identify an optimal set of parameters,at which a maximum efficiency of ?= 34%is achieved.Further studies with r ranging from 0.125 to 100 are performed around the optimal parameters.The results show that for the semi-passive flapping-foil energy harvester,the efficiency decreases monotonically with increasing mass ratio.We also notice that the total power extraction stays at a high level with little variation for r<10;therefore,if we concern more about the amount of power extraction rather than its efficiency,the inertial effects can be neglectable for r<10.Moreover,we conjecture that it is possible for the semi-active system to automatically determine its optimal phase difference.We note that the optimal phase difference ?=82° has been well determined,which leads to a good timing of vortex-foil interactions.With the increase of damping,the energy harvesting efficiency first increases and then drops,with the optimal damping within c*? 0.5-0.7 identified.This conclusion is a little different from that of linear theory because the later does not take the leading edge vortex into consideration can thereby only apply to the situation of extremely large Reynolds numbers.Third,for the effects of three-dimensionality,we consider the foils with finite aspect ratio,as well as that with periodic conditions imposed on the spanwise boundaries of the computational domain.According to our simulation,the energy harvesting efficiency declines with the decrease of aspect ratio,and the sensitivity to aspect ratio varies between various flapping frequencies.The cases with higher flapping frequencies are more likely to be affected by the aspect ratio.We con-clude that the optimal flapping frequency increases with the aspect ratio.Moreover,we report that there are two contributions to the three-dimensionality of the flow:the tip effects and the in-herent three-dimensional instabilities.We perform the Floquet stability analysis to investigate the effect of Reynolds number on the inherent three-dimensional instabilities in the wake,and a crit-ical Reynolds number is identified.It is further confirmed by three-dimensional direct numerical simulations.Third,for the effects of free surface,we choose three Froude numbers according to the real.situation in some potential deploying locations in the ocean.We find that the free surface affects the energy harvesting efficiency of the foil severely when the submergence of the system is small,i.e.d/c<8.The energy harvesting efficiency increases as the foil approaches the free surface.Moreover,we investigate the effect of Froude number.When Fr<1.2,the energy harvesting efficiency varies little,the effect of Froude number can therefore be neglected.However,if the Froude number increases further it will lead to the decline of energy harvesting efficiency.