**4. Conclusions**

In this work, the effect of a wind gust of various shapes and intensities on a modern horizontal axis wind turbine was investigated. A detailed and high-fidelity aeroelastic model was employed, implicitly coupling a computational fluid dynamics (CFD) solver based on an overset technique and a computational structural mechanics (CSM) solver, loyally reproducing the characteristics of the composite material of each blade. The gusts were superimposed on the atmospheric boundary layer.

First, the effect of a wind gust introducing a zero net flow rate was analyzed. This gust was conceptually similar to the "extreme operating gust" from the 61400-IEC standards but introduced a higher velocity increase. Results showed that an initial decrease in the blade loads and displacement was a consequence of the negative velocity increase imposed on the border of the gust. When the positive core of the gust impacted on the blade, the inertia of the structure caused a delay in the tip movement. Furthermore, despite the high peak reached by the aerodynamic axial force on the blade, flow separation over the span affected by the gust prevented the blade from reaching extreme deflections. Increasing the gust intensity, this protective effect was magnified by the broader area affected by the separation.

Subsequently, a gust analogous to the "extreme coherent gust" from the 61400-IEC standards was tested, introducing a consistently positive velocity increase. In this case, the peak tip deflection was shown to be higher than in the previous case as a consequence of the more severe wind conditions. Flow separation was also observed and affected a broader portion of the blade suction side, resulting in a fast reaction of the blade, whose tip underwent a fast axial movement. It was therefore concluded that, for each gust tested, flow separation acted as a protection mechanism and prevented the blade from reaching extreme deflections. For all the tested gusts, different dynamics were observed for the torque and axial force, especially when separation occurred.

Lastly, it can be concluded that the presented methodology allowed for the detailed investigation of the interaction between the blade structural response and the occurring wind gust: such data can be useful in the design stage. Among others, results can be used to better estimate the loading of the blade with respect to the meteorological data about the frequency, size, and intensity of the wind gusts in the site selected for the installation of the analyzed wind turbine.

**Author Contributions:** Conceptualization, G.S., M.P., W.V.P., and J.D.; methodology, G.S.; software, G.S.; validation, G.S. and J.D.; formal analysis, G.S. and J.D.; investigation, G.S.; resources, J.D.; data curation, G.S. and M.P.; writing—original draft preparation, G.S.; writing—review and editing, G.S., M.P., W.V.P., and J.D.; visualization, G.S.; supervision, J.D. and W.V.P.; project administration, W.V.P. and J.D.; funding acquisition, W.V.P. and J.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Fonds Wetenschappelijk Onderzoek—Vlaanderen (FWO, grant no. G030414N). The computational resources (Stevin Supercomputer Infrastructure) and services used were provided by the VSC (Flemish Supercomputer Center), funded by Ghent University, the Hercules Foundation, and the Flemish Government, department of Economy Science and Innovation (EWI).

**Acknowledgments:** The authors want to thankfully acknowledge Jan Vierendeels for his contribution to this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
