Marine Atmospheric Turbulence: A wind energy perspective

Offshore wind energy is experiencing unprecedented expansion in global installed capacity. The marine atmosphere, unaffected by obstructions such as urban landscapes, forests, and mountains, offers a more reliable wind resource. Simultaneously, a rapid growth in wind turbine sizes is underway. However, there is a need to characterize the dynamics of the marine atmospheric flow and its interaction with large wind power systems. This is crucial as the offshore systems are influenced by significantly larger atmospheric flow scales than their onshore counterparts.
This thesis provides an enhanced understanding of the interaction between flow structures in the marine atmosphere and large offshore wind farms. In-situ measurements from a research aircraft flying above large offshore wind farms in the North Sea were utilized to study the breakdown of turbulence structures when interacting with wind turbines. Key findings reveal that the vertical flux of horizontal momentum is the primary source of energy recovery in large offshore wind farms. Additionally, the breakdown of turbulence structures and the energy dissipation rates are significantly higher during neutral stratification than during stable stratification. These insights hold the potential for optimizing wind farm layout designs and determining the optimal spacing between offshore wind farm clusters.
A model for low-frequency wind turbulence is presented in this thesis. This model is based on a two-dimensional velocity spectral tensor and incorporates a well-defined length scale and an anisotropy parameter. The model is validated against two sets of offshore measurements: point measurements from ultrasonic anemometers at the FINO1 test site and remote sensing measurements from a nacelle lidar at Hywind Scotland offshore wind farm. An excellent agreement was demonstrated between the modeled and measured auto-spectra, cross spectra, coherence, and phase angles. The primary application of the low-frequency wind fluctuations model lies in generating synthetic wind fields, particularly for evaluating loads on offshore wind turbines. A method to generate such wind fields is also detailed in this thesis.
In the last part of this thesis, the impact of low-frequency wind turbulence on the damage equivalent loads and dynamic response of a large offshore wind turbine, i.e., IEA 15 MW reference wind turbine, is analyzed. Aeroelastic simulations demonstrate that low-frequency wind turbulence has the most significant effect on the tower base fore-aft and blade root flapwise moments. Furthermore, in the floating turbine configuration, pronounced responses in pitch and surge motions, along with mooring line tension, were observed at low frequencies. More importantly, this thesis reveals the significant underestimation of these responses at low frequencies by standard turbulence models like the Mann turbulence model.