**5. Conclusions**

In this study, the possible benefits provided by Gurney flaps when applied to blades subject to continuous and non-sinusoidal variations of the angle of attack have been analyzed by means of an extended sensitivity analysis. These analyses are propaedeutic for understanding the possible use of GFs as power augmentation devices in Darrieus wind turbines. Symmetric NACA 4-digits airfoils were considered. The airfoil thickness which represents one of the investigated parameters had values of 12% chord (NACA0012), 15% chord (NACA0015), 18% chord (NACA0018), and 21% chord (NACA0021). The GF height and mounting (angle of inclination with respect to the chord and airfoil side with respect to the revolution axis) represented the other main variables. The different test cases were analyzed by means of unsteady CFD simulations. Then, radial basis functions were used to interpolate the results and provide detailed response surfaces describing the impact of the aforementioned variables.

Upon examination of the results, it was shown that:


Further developments of the present model could consider higher dimensionality of the model itself, viz., increasing the number of variables considered in the surrogate model. Additional variables that could be considered are, for example, the airfoil type, additional Gurney flap angles relative to the chord, or combination of differently shaped Gurney flaps in inward or outward pointing directions.

**Author Contributions:** Conceptualization, A.B. and F.B.; Methodology, A.B., F.B. and Z.B.; Software, F.B.; Validation, P.W.; Formal Analysis, P.W. and A.B.; Investigation, P.W.; Resources, A.B. and Z.B.; Data Curation, P.W. and A.B.; Writing-Original Draft Preparation, P.W.; Writing-Review & Editing, A.B.; Visualization, F.B.; Supervision, A.B. and Z.B.; Project Administration, Z.B.; Funding Acquisition, Z.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received the financial support by National Science Centre within the OPUS scheme under contract UMO-2017/27/B/ST8/02298.

**Acknowledgments:** Thanks are due to Giampaolo Manfrida from the Università degli Studi di Firenze for supporting the visiting period of MSc. Wi´sniewski in Firenze and to Giovanni Ferrara of the same university for providing the facilities for hosting him. All the authors would like also to thank Holst at TU Berlin for providing the experimental data for validation.

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