Vortex-induced vibrations and post-lock-in cross-wind oscillations of wind turbine tower based on field measurements

Development of vortex-induced vibration (VIV) prediction models for application to full-scale structures benefits from validation with field measurements. This study presents comprehensive field response measurements carried out on two large wind turbine towers in Østerild, Denmark. Measurements on a tower-only configuration (tower without nacelle) were carried out over a considerable period of time during which many VIV events were recorded. In addition, the second mode VIV vibration was observed on the fully-built wind turbine tower. Considering the complexity of the field measured cross-wind response, a classification algorithm was developed to identify a predominant VIV process. Significant cross-wind oscillations were observed not only during lock-in, but also at higher wind speeds, termed here as "post-lock-in (cross-wind) oscillations", which are associated with lateral gust buffeting. A method is presented for estimating the standard deviation of the cross-wind displacement response over the entire measured wind speed range. This approach accounts for both VIV and post-lock-in oscillations and results to a good estimate compared to the measured data.