Mechanism of flapwise vibration of a wind turbine airfoil under trailing edge windward states using aeroelastic reduced-order model

With the rapid increase in wind turbine size and the push for extreme weight reduction in blade design, the engineering community is confronted with the challenge of developing softer blades, alongside a growing need to mitigate blade loads. The emergency shutdown and active trailing edge windward (TEW) technique has emerged as a promising solution to alleviate wind turbine blade loads. However, under certain TEW conditions, the blade may experience unexpected aeroelastic vibrations in the flapwise direction. The underlying mechanism of this aeroelastic phenomenon is explored in this paper using numerical methods. First, notable flapwise vibrations are reproduced through a coupled CFD-CSD simulation of the DU91-W2-250 airfoil, which is confirmed by wind tunnel experiments. Subsequently, an aeroelastic reduced-order model (ROM) is developed by coupling the unsteady aerodynamics model with the structural dynamic equations. Using the ROM, the dominant subcritical fluid mode is identified, characterized by a pair of eigenvalues with a smaller stability margin, at certain angles of attack. The coupling between this subcritical fluid mode and the structural mode can lead to instability in the structural mode at lower reduced frequencies, thus triggering the observed flapwise vibration. This coupling mechanism is akin to stall-induced vibrations, where instability initiates at a specific wind speed (or reduced frequency) and persists as the reduced frequency approaches zero. Additionally, the influence of key parameters is examined, including the angle of attack, reduced frequency, mass ratio, and structural damping. These insights provide valuable guidance for turbine blade design, offering strategies for parameter selection and approaches to avoid aeroelastic vibrations under TEW conditions.