Numerical Analysis of Unsteady Aerodynamics for a Horizontal Axis Wind Turbine in Yawed and Shared Flow Fields

Abstract

The main trends of the wind energy industry are the growth of the wind turbine capacity and offshore wind turbines. To satisfy the economic efficiency in off-shore, the rotor blade of a wind turbine becomes lager. Because of these trends, the numerical analysis considering gust, rotor-tower interaction, wind shear, yaw angle and dynamic stall are getting emphasized.This thesis focuses on the investigation of the aerodynamic response of a HAWT(Horizontal Axis Wind Turbine) in yawed and shared flow fields after the evaluation of turbulence models in 2D airfoil aerodynamic analysis. NREL S809 airfoil experimental data which was tested in Delft University of Technology is used for the 2D airfoil aerodynamic analysis and NREL Phase Ⅵ experimental data is used for 3D blade aerodynamic analysis.In 2D airfoil aerodynamic analysis, numerical analysis results showed that k-ω SST model predicted the experimental data as well as transition SST model did. Although transition SST model indicated better agreement than k-ω SST model, transition SST model has limitation in use because much computer resource is needed to meet y+ requirement. With results of previous researches and this thesis, k-ω SST model was confirmed as reasonable turbulence model.In 3D blade aerodynamic analysis, the validations were carried out for inflow velocity 7m/s, 10m/s, 15m/s, 20m/s and 25m/s(uniform flow) with 0° yaw angle by using k-ω SST model. Then, yawed flow field conditions were validated for 7m/s(uniform flow, pre-stall condition) and 15m/s(uniform flow, post-stall condition) with 30° yaw angle. Finally, yawed and shared flow field conditions were analyzed for reference velocity(hub height velocity) 7m/s(wind shear, pre-stall condition) and 15m/s(wind shear, post-stall condition) with 30° yaw angle by using power law velocity profile to apply wind shear. For uniform flow cases with 0° yaw angle, results such as pressure coefficient, normal force coefficient and tangential force coefficient showed a good agreement with the experimental data. However some discrepancy was observed in transition region (10m/s, 15m/s). This is why k-ω SST model assumes flow fields as fully turbulent and pressure tap is not enough to measure surface pressure on blade surface correctly. This discrepancy was also observed under the yawed flow field condition. However this phenomenon is reasonable because the objective of this thesis is to investigate of a wind turbine’s aerodynamic response tendency and the aerodynamic response tendency showed a good agreement. For yawed and sheared flow fields, the yaw angle and wind shear made the cyclic load variation and the asymmetrical wake structure. Because the experiment data of the shared flow field condition is not provided, pressure coefficients are compared between upward blade and downward blade and moment variations as blades rotate are expressed. These load variations will make fatigue loads for a wind turbine. However the characteristics of the load variations are different between 7m/s(pre-stall condition) and15m/s(post-stall condition), because the stall region exists and varies as blades rotate in 15m/s. For 7m/s, shaft torque is higher when blades locate at ψ=0° than ψ=180°. But this characteristic occurred reversely for 15m/s. These load variation results of this thesis can be used for development of optimal load control strategies to archive 20 years of wind turbine's design life.