Aeroelastic Instability and Flutter for a 10 MW Wind Turbine

Abstract

The goal of this thesis is to evaluate if flutter is a challenge to a 10 MW wind turbine. Flutter is an aeroelastic instability which occurs due to the interaction between the aerodynamic forces and the elasticity of the blade. Torsional motions of the blade lead to variations in the aerodynamic forces due to changes in the angle of attack of the airfoil. The variation in aerodynamic forces creates flapwise vibration of the blade. When the vibrations of the blades are in an unfavourable phase with the aerodynamic forces, flutter occurs. Flutter may lead to rapidly increasing vibrations of the blade and failure of the blade. The 10 MW reference turbine from NOWITECH, Norwegian Research Centre for Offshore Wind Technology, was studied. An aeroelastic stability analysis was performed using the aeroelastic stability tool HAWCStab2. It was found that this wind turbine becomes unstable at approximately twice the operational speed of the turbine. The turbine does not experience flutter in normal power producing operation. A simulation in the time domain was also performed, using the aeroelastic tool HAWC2. In a run-away situation, the turbine was found to become unstable with flutter before it reached the run-away speed. The turbine was then analysed with other blades. A softer blade and a stiffer blade were studied. The soft blade was found to become unstable at 1.8 times the operational speed of the turbine. The stiff blade was found to become unstable at around 2.5 times the operational speed. The stiff blade was the only blade where the turbine was able to reach the run-away speed without experiencing instabilities.