Vortex-induced vibration suppression technology for flexible wind turbine towers

As wind turbine tower height increases proportionally with rotor diameter expansion, the tower's stiffness, constrained by economic costs, has not improved proportionally, resulting in significantly enhanced structural flexibility. This increased flexibility makes the system prone to substantial geometric deformation during operation. Such deformation intensifies the coupled vibration between the tower and blades, thereby weakening equipment stability. Therefore, suppressing tower vibrations under complex operational conditions, such as strong winds and turbulence, while ensuring stable turbine operation, has become a crucial research focus in wind turbine structural safety. Consequently, this study proposes a vortex-induced vibration suppression method designed for flexible wind turbine towers.

This study focuses on a 140-meter tower of a 3.3 MW wind turbine unit and establishes a finite element model. The wind turbine blades, rotor, drivetrain, and nacelle are simplified as concentrated mass blocks, with 19 concentrated mass points defined at 14 different heights from the tower base to the top. This approach simplifies the model and improves computational efficiency while ensuring the accuracy of the results. A modal analysis of the wind turbine tower was conducted, and the first two mode shapes were obtained and normalized. These results were compared with existing literature to validate the model's accuracy. This study proposes a vortex-induced vibration suppression scheme by designing spoilers on the outer wall of the tower. Three spoiler arrangement schemes are proposed: double-helical, spaced-helical, and vertical-spaced. Simulations of the three spoiler arrangement schemes were conducted under the same conditions and compared with a tower model without spoilers. The suppression effect of vortex-induced vibrations was evaluated based on the drag coefficient and its standard deviation around the tower wall. To further optimize the scheme, the angle and spacing of the spoilers were adjusted, and flow field velocity data at selected monitoring points were extracted. The optimal parameters of the spoilers were determined based on the vortex-induced vibration frequency, obtained by applying a Fourier transform to the velocity data at the monitoring points.

Computational fluid dynamics simulation results indicate that the double-helical spoilers and spaced-helical spoilers exhibit similar improvement effects, reducing the standard deviation of the drag coefficient by 79.85% and 77.50%, respectively, compared to the conventional tower. The vertical-spaced spoiler scheme shows a slightly lower improvement effect, with a reduction of 41.23%. Simulation results for varying the angle and spacing of the spoilers reveal that, for double-helical spoilers, the vortex-induced frequency of the tower remains lower than that of the conventional tower within the angle range of 40° to 80°. The lowest vortex-induced frequency occurs at 62°, which is 17.8% lower than that of the conventional tower. For spaced-helical spoilers, the vortex-induced frequency remains relatively low within the spacing range of 70 mm to 94 mm, with the lowest frequency occurring at 94 mm, representing a 16.5% reduction compared to the conventional tower.

Installing spoilers on the outer wall of a wind turbine tower can improve the drag coefficient and its standard deviation, demonstrating effectiveness in streamlining the flow field and suppressing vortex-induced vibrations. Optimizing parameters such as the angle and spacing of the spoilers can further reduce the frequency of vortex-induced vibrations and enhance the stability of flexible towers.