Study On Aerodynamic And Aeroelastic Characteristic Of Swept-back Airfoil Model Based On Wind Tunnel Experiments

In recent years,the large-scale development trend of wind turbines has caused the blade loads to increase sharply,thereby affecting the economy and safety of large units to a certain extent.Therefore,it is urgent to develop innovative load control methods.As an adaptive passive load alleviation technology,the swept-back blade has received extensive attention.Its control idea is to use the geometric shape of the blade to obtain the bend-twist coupling,and reduce the effective angle of attack to alleviate the loads.At present,sufficient research has been carried out on the load reduction ability of the swept-back blade.However,the aerodynamic evolution characteristics,spanwise characteristics and aeroelastic stability of the swept-back blade are not very clear.It is necessary to conduct in-depth research on the aerodynamic and aeroelastic characteristics of the swept-back blade to further promote the commercial application.Due to uncontrollability and high costs,full-scale blade field experiments still face many challenges.For this reason,takeing wind turbine airfoil DU93-W-210 as the research object,a experimental platform with two degrees of freedom for airfoil model was designed based on the wind tunnel environment and reasonable simplification.Aerodynamic and aeroelastic experiments were carried out and the effects of sweep and other structural parameters on the aerodynamic and aeroelastic characteristics of airfoil model were analyzed and discussed.In the static aerodynamic experimental study,the effects of sweep on the spanwise flow characteristics,the Reynolds number effect and the static hysteresis effect were investigated,which laid the foundation for the research work of the full text.It can be found that the swept-back airfoil model has a significant three-dimensional flow phenomenon.As the angle of attack increases,the surface flow successively undergoes three stages of uniform attachment flow,mixed separation flow and fully separation flow.Moreover,the flow separation in the downstream section is more serious in the mixed separation flow region.Meanwhile,compared with the results of the straight airfoil model,the slope of the C_L curve reduces and the C_Lmax of the upstream section rises with the increase of the sweep angle,but the C_Lmax of the midstream and downstream sections show opposite pattern.Sweep has a delay effect on light stall,while it can also cause full stall to occur in advance.Besides,increasing the Reynolds number can expand the range of the mixed separation flow region of the swept-back airfoil model and sweep can suppress the static hysteresis effect to some extent.In addition,the method of correcting the aerodynamic force by decomposing the the inflow velocity has been experimentally verified,and the applicable range of the sweep correction model has been clarified.In the experimental study of dynamic stall,the effects of sweep and pitch oscillation parameters on the dynamic stall characteristics of the airfoil model were discussed.The results reveal that the degree of dynamic stall of the swept-back airfoil model has a prominent difference in spanwise direction.In the light stall region,the aerodynamic hysteresis of the swept-back airfoil model gradually increases from upstream to downstream.Compared with the straight airfoil model,sweep has a mitigation effect on the dynamic stall and increases aerodynamic damping,which could improve stability of the airfoil model.Meanwhile,it can be found from the pressure distributions on the upstream section that on the one hand,sweep suppresses the separation of the trailing edge of the suction surface in the positive stroke and has a delay effect on stall.On the other hand,it promotes the reattaching process of the surface flow in the negative stroke which alleviates the dynamic stall.Additionally,the effect of pitch motion control parameters on the dynamic stall characteristics of the swept-back airfoil model is basically the same as that of the straight airfoil model.Based on the study of aerodynamic characteristics,an aeroelastic similarity criterion parameter Ae was proposed.Rigid airfoil model with elastic support was used to simulate the aeroelastic response of flexible blades,and the effects of structural stiffness,torsion center position and sweep on the aeroelastic characteristics of the airfoil model were explored.As a result,a total of four aeroelastic response types are found,including static balance,negative stall flutter,decay oscillation and positive stall flutter.And their physical characteristics are revealed through the aerodynamic evolution process and phase plane analysis.Furthermore,the time-domain analysis method was employed to obtain the stability boundary of the stall flutter for straight airfoil model.Subsequently,the structural parameters of the airfoil model were changed and some phenomena were found as follow.First,increasing the structural stiffness can increase the critical wind speed of negative stall flutter,which is conducive to enhancing the aeroelastic stability.At the same time,the Ae number of critical conditions of negative stall flutter under different stiffnesses has a high consistency,which shows the effectiveness of the aeroelastic similarity criterion.Secondly,moving the position of the torsion center to the leading edge can suppress the occurrence of stall flutter,otherwise it will reduce the aeroelastic stability of the airfoil model.In addition,the aeroelastic response of the swept-back airfoil model is statically balanced,so the sweep can improve the torsional stability.