**4. Summary and Conclusions**

An idea of the Savonius turbine with a variable geometry of blades was proposed. Blades made of an elastic material were continuously deformed during the rotor revolution in order to increase a positive torque of the advancing blade, and, at the same time, to decrease a negative moment of the returning blade. The main outcomes of the performed investigations are outlined below:

• An elaborate two-dimensional numerical model was developed to simulate a transient flow in the variable-geometry rotor in order to assess its aerodynamic performance. The shape and position of the rotor blades were subject to continuous changes according to the constraints defined in the structural analysis. The rotational motion and deformations of the blades were transferred to the fluid flow (CFD) analysis, where deformations of grid elements and remeshing options were applied. This method yielded a satisfactory agreement with a typical method of simulations, which consists in rotation of the internal domain surrounding the fixed-shape Savonius rotor.


A significant increase in the aerodynamic performance of the Savonius turbine with continuously deformed blades was confirmed. The power coefficient exceeded *Cp* = 0.3 and it reached almost *Cp* = 0.4 for the highest eccentricity magnitude. Thus, this design is at least comparable to Savonius turbines equipped with augmentation systems presented in the literature [11,26]. However, additional mechanisms applied to deform blades and locate the rotor in the proper position with respect to the incoming wind are required. They will make the design of the turbine more complex than the original Savonius and they will consume a part of the energy generated by the turbine. Thus, all turbine elements need to be carefully designed to be resistant and efficient. Additionally, as the turbine performance depends considerably on the blade deformation, it is necessary to select an easily deformable material with high fatigue resistance. The numerical model of the deformable Savonius rotor needs to be further developed to limit the numerical instabilities during the solution. The mechanical losses due to friction and blade deformations are to be included. Also the problem of time consuming simulations has to be addressed.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/14/3717/s1: Video S1: NonDeformable\_Savonius.wmv; Video S2: Deformable\_Savonius.wmv.

**Author Contributions:** Conceptualization, K.S. and D.O.; methodology, K.S., D.O., P.R.; investigation, K.S., D.O., P.R., E.M.; writing—original draft preparation, K.S. and D.O.; All authors have read and agreed to the published version of the manuscript.

**Funding:** The investigations have been financially supported by the research project POWR.03.02.00-00-I042/16-00 of the National Centre for Research and Development and Innovation Incubator 2.0 project MNISW/2019/157/DIR of the Ministry of Science and Higher Education.

**Acknowledgments:** We would like to thank Malgorzata Jozwik for her significant linguistic help during the preparation of the manuscript.

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