Three-dimensional Rotational And Dynamic Stall Of Horizontal Axis Wind Turbine Blades

Three-dimensional(3D)rotational effects and dynamic stall are two significant phenomena on horizontal axis wind turbines(HAWTs),because they are determining factors in the unsteady and nonlinear aerodynamic responses.These two phenomena can interact with each other and then make the blade flow highly complicated,thereby presenting a great challenge of predicting the HAWT aerodynamic loads accurately.This thesis is aimed at providing a deep understanding of the 3D rotational effects and dynamic stall on HAWTs using the CFD simulations,the blade element momentum(BEM)method,and the analytical models.Firstly,the independence study about computational mesh,time step,and CFD settings is conducted.The CFD simulations are validated against the wind-tunnel experimental data and are proved to be adequately accurate and reliable.Afterwards,the flow mechanisms of 3D rotational effects and dynamic stall on the NREL Phase VI blade are studied in detail.The boundary-layer flow with 3D rotational effects is revealed by analyzing the governing equations.Then,we give the reason why 3D rotational effects become strong on inboard blade,in separated flow,and at high rotor speed.The comparative study of aerodynamic characteristics between NREL Phase VI blade and NREL S809 airfoil is also carried out.The calculated results indicate that3 D rotational effects can effectively reduce the separation vortices in their length and height.This produces high suction values on the suction side,and thus increase the sectional aerodynamic forces and nose-down pitching moment.Interestingly,the theoretical analysis and CFD simulations show that flow separation is a sufficient but not necessary condition for radial flow.Dynamic stall of the NREL S809 airfoil undergoing pitch oscillation is also studied by URANS simulations.The flow mechanisms of aerodynamic hysteresis are analyzed,and the effect of different oscillation parameters is also studied.The equivalence between the dynamic-stall characteristics of the airfoil under pitch oscillation and oscillating freestream is detailedly analyzed,based on thin-airfoil theory and URANS simulations.These two motions lead to an opposing effect on the effective airfoil camber and the suction on the upper surface,particularly when the unsteadiness is high.Furthermore,the aerodynamic responses of the NREL Phase VI blade at different spanwise locations,wind speeds,and yaw angles are predicted by the dynamic stall model and the 3D rotational correction model.The obtained results indicate that 3D rotational effects play a significant role in the dynamic stall of a wind turbine blade.If the combined effect of 3D rotational effects and dynamic stall is properly considered,the prediction accuracy will be greatly improved.Then,the combined effect of 3D rotational effects and dynamic stall under yawed conditions is further analyzed using the CFD simulations,the BEM method,and NREL’s experimental data.The combined effect is found clearer on downwind side than on upwind side,and also clearer on retreating blade side than on advancing blade side.On one hand,the combined effect can accelerate the flow reattachment during the decreasing angles of attack process,thereby effectively reducing the aerodynamic hysteresis.On the other hand,the combined effect can favorably suppress the separated flow in the manner of changing the massive trailing-edge separated flow into the moderate leading-edge separated flow,avoid the bursting of separation bubbles,and thus delay the onset of stall.This thesis also includes the effect of vortex generators on 3D rotational effects and dynamic stall based on CFD simulations.Vortex generators are found to decrease the radial flow within the boundary layer of the NREL Phase VI blade,reduce the 3D rotational effects,and then make the local aerodynamic forces lower than the value without vortex generators.Consequently,vortex generators only lead to a limited improvement in the aerodynamic power of a rotating blade,although they can greatly increase the aerodynamic efficiency of the wind turbine airfoil.Vortex generators are also found to effectively suppress the separated flow of the oscillating NREL S809 airfoil,delay the onset of dynamic stall,and then considerably increase the maximum lift coefficient.For single-row vortex generators,the chordwise location plays a crucial role in dynamic stall characteristics of the airfoil and the flow reattachment.For double-row vortex generators,the second row of vortex generators can further accelerate the near-wall flow and suppress the flow separation,thereby further reducing the aerodynamic hysteresis.