Modelling the effect of freestream turbulence on dynamic stall of wind turbine blades

Abstract Large-eddy simulations of flow over a pitching airfoil are conducted to study the effect of freestream turbulence on the aerodynamic characteristics. A primary field of applications of this study is wind turbine aerodynamics. The wind turbines operate in yaw in large scale variations of wind direction due to very large turbulence eddies, and the blades operate in a periodically oscillating condition. The pitching frequency of the airfoil corresponds to a typical rotating frequency of modern large wind turbines. A divergence-free synthetic turbulence inflow is applied at the upstream region of the pitching airfoil to investigate the effect of small-scale freestream turbulence on dynamic stall. Phase-averaged lift, drag and moment of the pitching airfoil show good agreement with experimental data in the literature. Characteristic phenomena of dynamic stall, such as leading edge vortex motions, are analysed and quantified. The effect of the small-scale upstream turbulence is significant on the lift coefficient during the downstroke. The power spectral density of the streamwise velocity sampled from one point in the wake shows that the inertial sub-range tends to extend towards the pitching mode for the turbulent inflow, while there is a distinctive spectral gap for the laminar inflow.

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