Investigations on vortex-induced vibration of a wind turbine airfoil at a high angle of attack via modal analysis

Abstract

Vortex-induced vibration of a forced oscillating wind turbine airfoil at 90° angle of attack is numerically investigated with the aid of the dynamic mode decomposition technique. This situation may be encountered during parking or idling operations where the yaw angle is engaged due to a failure of the control system. The airfoil might induce vortex-induced vibration, which accompanies with the “lock-in” phenomenon. In this phenomenon, the shedding frequency will “jump” into the structure natural frequency that could cause limit cycle oscillations. For a series of forced oscillation calculations with constant oscillation amplitude, the airfoil will become unstable before entering the locked-in region and regain stable state before leaving the locked-in region with a frequency ratio from 0.8 to 1.2. Additionally, in this range, a smooth phase shift between the vibration-induced aerodynamic force and the airfoil motion is found, which mainly influences the characteristics of energy exchange on the airfoil surface. Finally, dynamic mode decomposition is employed to identify different flow features in the flow field to illustrate vortex-induced vibration. Dominant pressure modes are obtained, and the mechanism of energy exchange on the airfoil surface is illustrated.