Experimental Investigation And Numerical Simulation On Stall Flutter Of An Oscillating Wind Turbine Airfoil

The continuous development of wind energy technology gradually leads to wind turbines with larger,more slender and flexible blades with increasing aspect ratio and tip speed,while during everyday operation or extreme conditions the blade experiences stall induced vibrations – flutter,which creates the need for further study and understanding in order to ensure the safe operation and enhance the performance of wind turbines.In the context of this thesis,a wing model with NACA 64-418 airfoil was investigated both experimentally and numerically,especially the aerodynamics of the wing during the state of ‘Limit Cycle Oscillation' was analyzed.The main work accomplished includes the follows:1.Wind tunnel experiment of the oscillating airfoil.Sensors measured the time dependent pressure distribution,the torsional motion as well as the linear displacement,revealing details of the unsteady aerodynamics of the examined airfoil.Results include the dynamic response,the hysteresis characteristics of the lift coefficient and the pressure distribution.2.Flow field measurement of the oscillating airfoil.A PIV(Particle Image Velocimetry)system was employed to visualize the flow characteristics on the suction surface around the leading-edge area during limit cycle oscillation.The association between the flow change and the transient pressure distribution along the chord during upstroke and downstroke was analyzed.3.Numerical simulation of the oscillating airfoil.As a comparison and supplement of the experiment,Fluent solver was used to perform the two-dimensional transient simulation,and a UDF(User Defined Function)was written together with dynamic mesh method to simulate the flow and aerodynamic characteristics.The feasibility of the numerical method was verified in the context of this thesis,and it was found out that the three-dimensional effect will cause the lift measured lower than real value in experiment.The variation of pressure distribution on the upper surface of the oscillating airfoil is essentially due to the evolution of flow during upstroke and downstroke.Besides,the shedding of trailing edge vortices is the main reason for the periodic fluctuation of lift at conditions of large angle of attack and stall.