The unsteady aerodynamic characteristics and serious time-lag of wind turbine blades during dynamic stall made the actual measured value of rotor power seriously deviate from its static prediction value. Therefore, ignoring the acceleration term of downwash flow caused by low-order added mass and undergoing state variable transformation, a novel dynamic stall prediction model including airfoil attachment flow and dynamic separation flow of trailing edge was proposed based on Theodorsen theory and Kirchhoff potential flow theory. The model was used to analyze the unsteady dynamic stall characteristics of six airfoils from NREL 5 MW offshore wind turbine blades. The conclusions were as follows. The airflow through the airfoil continuously varied between the fully attached flow and the fully separated flow. Due to the dual effects of the dynamic downwash flow induced by the shedding trailing edge vortex of the attached flow and the pressure lag caused by the dynamic separation of the boundary layer, the variation curve of the dynamic lift coefficient and the static lift phenomenon curve deviated greatly. The dynamic lift coefficient curves of six airfoils showed a very obvious time-lag phenomenon. The dynamic lift coefficients of DU40, DU35, DU30, DU25, DU21 and NACA64 airfoils increased significantly, reaching 17.6%, 60.9%, 60.7%, 55.1%, 63.7% and 40.8% respectively. The dynamic stall angle of attack greatly exceeded the static stall angle of attack, increasing to 36.53°, 21.40°, 20.20°, 17.68°, 16.97° and 21.42° respectively. The prediction values of six airfoils were consistent with the conclusions of public experimental data, which proved that the calculation results of the proposed model were accurate, reliable and universal.
Key words
Theodorsen theory /
airfoils /
wind turbine blades /
lift coefficient /
prediction models
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