Study Of Dynamic Stall Model And Flow Control Mechanism Of Wind Turbine Airfoil

The blades of large-scale wind turbines are more flexible and operate under highly unsteady aerodynamic conditions,which causes blades under dynamic stall phenomenon.Dynamic stall can result in aerodynamic fatigue loads and a discrepancy in the aerodynamic performance between design and operation.It is necessary to develop an unsteady aerodynamic evaluation model to suit the conditions.However,the models used to evaluate the unsteady aerodynamic characteristics of the airfoil originated from the researches on helicopter propellers,and have not been fully verified in the application of wind turbines.Meanwhile,the flow control devices,vortex generators,play an important role in improving the environmental adaptability and efficiency of the blade and become the standard configuration of the blade in some designs.There is a lack of researches on the influence of vortex generators in unsteady conditions.For a further study of the dynamic stall process of wind turbine airfoils and the mechanism when the vortex generator is installed,and in order to establish a prediction model suitable for unsteady aerodynamics of large wind turbine blades,experimental and numerical simulation methods are adopted in this paper.Based on the fundamentals of aerodynamics and the dynamic stall semi-empirical model,a comprehensive evaluation system for control effects of vortex generators and an improved dynamic stall semi-empirical model are proposed to be suitable for wind turbine airfoil which undergoes trailing-edge stall and with vortex generators.An experimental platform for the oscillatory motion of airfoils was designed and built independently to realize the dynamic oscillation motion of airfoil.The flow field information was collected by synchronous triggering of the motor pulse signals and the signals acquisition equipment,which could realize the accurate oscillatory motion of the airfoil under different test conditions.When the attack angle of the airfoil is locked,the information can be statically tested as a base for comparison.The DU91-W2-250 airfoil was chosen as the research object,and tests of smooth airfoil and that with vortex generators were carried out in different conditions.The numerical simulation method is also used to supplement the analysis of the flow field and high-frequency signals.Firstly,the static characteristics of stall and flow control of vortex generators are studied.In the static cases,it is found there are twice stalls with the increase of attack angle of the airfoil,which is light stall and deep stall.Deep stall causes a sudden increase in drag and sudden drop of lift-to-drag ratio and moment coefficient,with the change of the pressure center of the airfoil.Vortex generators can effectively delay the occurrence of light stall on the airfoil and increase the contributing range of the airfoil with a high lift coefficient.But vortex generators no longer have the effect of delay separation in the deep stall zone of the airfoil.In addition,the installation of vortex generators will increase the parasitic drags.The additional drag should be reduced by optimizing the shape and installation positions in the design and applyment of vortex generators.Then,as for dynamic conditions of the smooth airfoil,with the reference of static results,it is found that the dynamic stall mainly shows the delay effect.When the attack angle changes instantaneously,the pressure change of the flow field has a time lag,and the transition and separation on the airfoil are obviously later than the static conditions.The development process of dynamic stall mainly includes:fluid is attached to the airfoil;flow separates from the trailing edge of the airfoil and gradually develops to the leading edge;the vortex gradually moves to the trailing edge and is shed in the wake;the fluid reattaches back to the airfoil.The growth of the counterclockwise vortex leads to a decrease in the lift,and the growth of the clockwise vortex leads to an increase in the lift.With the increase of reduced frequency,the unsteady aerodynamic characteristics of the airfoil is more obvious,and the aerodynamic efficiency in the upstroke pitching process is lower.The aerodynamic damping of the airfoil is different in the different range of attack angles.The negative aerodynamic damping may lead to dynamic instability of airfoil.Furthermore,vortex generators are used for dynamic flow control.As for dynamic conditions of the airfoil with vortex generators,it is found that the vortex generators can also effectively suppress the dynamic stall.The essential reason is that the induced vortex generated by the tip of the vortex generators causes the disturbance of the boundary layer,which increases the energy in the airfoil boundary layer and makes the boundary layer be able to withstand the adverse pressure gradient.However,the installation of the vortex generator should be carefully designed.Improper installation position and shape will not only increase the airfoil resistance,but also increase the possibility of the dynamic instability of the airfoil.The hysteresis effect of the dynamic stall process is a three-dimensional effect.The installation of a vortex generator will reduce the non-uniformity of the flow field and suppress the spanwise movement of the vortex.On the basis of the results above,six factors are proposed in order to evaluate the aerodynamic and control performance of the vortex generators.They are extracted from static aerodynamic efficiency,dynamic aerodynamic efficiency,aerodynamic stability within a large angle of attack before stalling,aerodynamic stability maintenance after stalling,and safety factors that prevent stall flutter.The 6 parameters constitute an evaluation system for evaluating the control effect of the vortex generators.It can comprehensively evaluate the aerodynamic performance and control effect of the vortex generators.These factors cover different conditions and different attack angles,and can provide a reference for the optimization of the vortex generators.At last,according to the characteristics of the airfoil in the dynamic stall,the semi-empirical dynamic stall model is improved.It mainly includes the critical condition of the pressure,the separation point prediction at the attack angle nearby change of the oscillation direction,and the installation of vortex generators.It is shown that the improved model,Mod-BLs predicts lift coefficients with better accuracy.It can be used for unsteady aerodynamic calculations of wind turbine airfoils and aerodynamic devices.In summary,the aerodynamic performance and flow field mechanism of the smooth wind turbine airfoil and that with vortex generators in the dynamic stall are deeply studied.Besides,a reliable unsteady aerodynamic prediction method of wind turbine airfoil and that with vortex generators is proposed.