Along-Wind Aerodynamic Damping of Wind Turbine Towers: Determination by Wind Tunnel Tests and Impact on Tower Lifetime

As wind turbines become larger and their towers more slender, aeroelastic effects play a bigger role in the wind turbine’s dynamic behavior. This study focuses on the along-wind aerodynamic damping of wind turbine towers, which has been determined by wind tunnel experiments using the forced oscillation method according to Steckley’s approach. Reynolds number scale effects have been considered through surface roughness modifications using sand paper and a dimple pattern, which have been described in detail. The wind tunnel measurements are performed in sub-critical, critical and trans-critical flow regimes, as well as in low- and high-turbulence conditions, which allows for an accurate description of the required relative roughness and Reynolds numbers for achieving trans-critical conditions. The resulting along-wind aerodynamic damping values according to Steckley’s and Holmes’ approaches are compared, and an analytical relation between them is established. Both approaches are then used in aeroelastic multi-body-simulations of an onshore 6 MW reference wind turbine and their impact on the wind turbine lifetime is evaluated through fatigue proofs at the tower base section. Holmes’ approach seems more appropriate for the application in aeroelastic multi-body simulations. A lifetime extension for the wind turbine tower of approximately 0.4% is achieved for the reference wind turbine tower, which roughly corresponds to 1 to 2 months for 20 years of operation. An analytical expression is given for the estimation of the tower’s aerodynamic damping in parked and operating conditions.