Gust buffeting response of a wind turbine tower in mild and severe wind conditions at Østerild Test Center

This work focuses on the full-scale gust buffeting response of an isolated wind turbine tower (without rotor-nacelle assembly) about 116 m tall placed at Østerild Test Center (Denmark). It aims to clarify the reliability of analytical models behind design standards through the analysis of combined measurements of wind and structural response. The tower was instrumented with strain gauges mounted close to the base, which indirectly measured the bending moment (net of the mean component), and an accelerometer at about the top. Wind data, mostly velocity and direction at different heights, were recorded by nearby meteorological masts, and they are used to characterize the wind environment in terms of mean velocity profile, turbulence intensity, power spectral density, integral length scale, and coherence of velocity fluctuations. Mild wind conditions, differently from more severe winds, result not well described by the formulations provided by design standards, whereas the coherence function exhibits a disagreement with theoretical models also at high wind velocities. The measured along-wind and across-wind response (in terms of bending moment) is compared with the values got from the direct integration of gust buffeting theory for line-like structures, by using the actual wind characteristics. While a good agreement is found for the along-wind response, especially for the upper bound of the estimated structural damping, the across-wind response significantly deviates from a pure gust buffeting response even at high wind velocity, well above the expected lock-in range. Clear nonlinear aeroelastic response features are highlighted in those cases. Moreover, a comparison with common design standards is developed for the along-wind response, which is significantly underestimated in mild wind conditions.