| In actual wind farms,due to the existence of wake effects,the overall power generation efficiency of the wind farm is weakened.In order to arrange more wind turbines and increase power generation efficiency within a limited area of land,this paper focus on the impact of wind turbine tailings recovery(mainly velocity and turbulence)through the installation of blade tip winglets.The TR-PIV system was utilized to conduct experiments in the closed section of the low-speed wind tunnel with the experimental objects of two-bladed wingletted horizontal axis wind turbines,including the airfoils of NACA4415 and SD and the winglet of types S and type V,and the range of wake information in the 4.5 times of the diameter of rotor was collected.The effects of tip vane on variation of wake velocities,vorticity,turbulence and coherent structure were compared by analysis of qualitative and quantitative.Through the analysis and comparison of the velocity field of the wind turbine wake,it is found that the recovery of axial velocity about the wingletted wind turbines with two kinds of airfoils is better than that of without winglet(similar to the incoming wind velocity),indicating that it is advantageous to recover the wake velocity.Firstly,the radial velocity of the wake of the wind turbine reflects the transport and blend of wake and external mainstream.From the rule of the radial velocity of the wake,it can be known that the acting as a ?carrier? for transport,the region of the ?effect of tip vortex inducing?(0.9R,?R?is the radius of rotor)is able to continuously transport high-speed fluid from the area of external mainstream to the area of central wake and then mix with it,which result in the gradually recovery and expanding of axial velocity of the fluid in the wake area.Compared with the axial velocity without winglets,the value of radial velocity in the wake with winglet is negative at the relative radius of 0.1R,indicating that more external fluid is entrained in the wake area,which makes the blending of axial velocity in the central wake area more uniform,and gradually increasing with the development of the wake and the increase of the radial distance.In the far wake,it is closer to the velocity of inflow.It is suggested that the existence of winglet is likely to vary the flow structure of the region of the ?effect of tip vortex inducing,thereby enhancing the ability of fluid transport and blending.For different airfoils,at the relative radius of 0.5R,the values of axial velocity of NACA4415 continues to be higher than the SD with any of two winglets;while at 0.9R,the values of axial velocity of NACA4415 exceeds SD until at 3.5D(with S-type)and 2.5D(with V-type),respectively.However,it is universally acknowleged that the distance between wind turbines in wind farms is generally greater than 5D.Therefore,it is believed that the airfoil of NACA4415 with winglet is more beneficial to the recovery of wake velocities,thereby increasing the value of wind velocity of the downstream wind turbines and enhancing their workability.At the same time,it also shows that the V-type make the recovery of wake velocity more quickly.By analyzing region of the ?effect of tip vortex inducing,the winglet does change its flow structure in two ways: the number of vortex cores and the geometry;the magnitude of the vorticity value.First of all,for the two types of airfoil,the geometry of the tip vortex is indirectly ?broken? by the winglet downstream of the wake,and the area of the vortex concentration is enlarged,leading to the increasing on range of fluid transport and more external fluid to enter,thus promoting the recovery of wake velocity(the degree on tip vortex ?broken? of the airfoil of SD is more than the airfoil of NACA4415);On the other hand,for the airfoil of SD,the presence of the winglet genious has the effect of weakening the tip vorticity,which is beneficial to reduce the aerodynamic noise of the wind turbine.The effect of winglets on the recovery of turbulence in the wake of wind turbine for both airfoils is related to the selected airfoil:The RMS and turbulent kinetic energy in the wake of the NACA4415 airfoil are both increased with winglet;while the airfoil of SD is both reduced significantly,which indicates that the degree of fluidic disturbance near the tip vortex of the NACA4415 airfoil is intensifying,promoting the exchange of energy and momentum between the fluid inside and outside the wake,thus the capacity of the transporting fluid is enhanced;The disturbance of the blade tip of the airfoil of SD is reduced,which is beneficial to improving the aerodynamic performance of downstream wind turbine.In combination with the influence of two kinds of airfoil on the recovery of velocity,vorticity and turbulent structure,two factors affecting the recovery of the wake are obtained: the number of transport ?vehicles? and the degree of disturbance of the fluid in the region of the ?effect of tip vortex inducing?(more important),which is related to the airfoil of the selected blade,so the degree of wake recovery is also constrained by different airfoil structures.For the coherent structure(spatial correlation coefficient),the presence of winglet degrades the correlation,and the coherent structure dissipates in advance and no longer exist in the far wake.Correspondingly,the complete tip vortex vortex core is ?split up? into small eddies(increasing of transport carriers),proving that the dissipation of the coherent structure is the dissipation of large-scale vortex structures.In short,the wind turbine of airfoils of NACA4415 and SD with two kinds of winglets has various impacts on wake recovery,but due to the limitations of the airfoil itself,the wake cannot be completely recovered in both velocity and turbulence energy.The future research can be studied and optimized for the geometric parameters of the airfoil and the winglet itself,so as to obtain the airfoil and the winglet that are most favorable for wake recovery. |
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