**6. Conclusions**

2D and 3D flat plates, in addition to the DU06-W-200 airfoil, have been tested in a wind tunnel equipped with a new external balance, designed for the measurement of aerodynamic forces. The inherent features of the balance make it suitable for an accurate characterization of VAWT airfoils, although specific validation has been required to ensure its performance. Furthermore, CFD simulations of the airfoil have been performed with different turbulence models and flow conditions, complementing the experimental results, and illustrating vividly the unsteady phenomena involved.

The experimental drag and lift coefficients obtained for both the flat plates and DU airfoil match the data available in the literature remarkably well, even at large angles of attack, when unsteady flow is rather relevant. The studied balance has proved its ability to capture fluctuating forces, although noise-filtering procedures must be implemented to obtain more reliable instantaneous data.

The new GEKO turbulence model used for this work offers accurate predictions of forces and moment, providing the best results when running unsteadily in the case of detached conditions, although the mesh requirements are significantly higher with respect to the steady simulations.

Finally, in light of the obtained results, the balance design as well as the CFD models have been successfully validated, and their integrated use has proved to be a highly recommendable and useful approach to test new airfoil geometries or airfoils featuring flow augmentation devices for VAWT applications.

**Author Contributions:** L.S., methodology, investigation, data curation, writing—original draft preparation; M.G.V., methodology, investigation; A.P., writing—original draft preparation; J.G.P., funding acquisition, resources; S.V.-S., supervision, resources; J.M.F.O., conceptualization, visualization, writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the "Agencia Estatal de Investigación" (AEI) of the Spanish Ministry of Science and Innovation, in the context of the State Program to Promote Scientific-Technical Research and its Transfer, through the Project "Optimization through flow control techniques of a vertical axis wind turbine for urban environments" (ref. TED2021-131307B-I00), included in the NextGenerationEU funds of the European Community. Additionally, the support given by the University Institute for Industrial Technology of Asturias (IUTA) and the City Hall of Gijón, through the financed project SV-22-GIJON-1-04, is also recognized.

**Acknowledgments:** The authors would like to acknowledge the contribution of Katia Argüelles for her supervision and her administrative and technical support, as well as Bruno Pereiras for his advice and support during the measurement campaign.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **References**

