**4. Conclusions**

In this study, a typical Savonius turbine with a constant diameter of the rotor was studied, where the variation of the overlap ratios in both horizontal and vertical directions was considered. In this paper, simulations were carried out by using the computational fluid dynamics (CFD) and solving the URANS equations and realizable K-ε turbulence model. The results showed that at near zero and positive horizontal overlap ratios, a relatively higher efficiency than that of the other states could be achieved. In fact, for lower tip speed ratios, the rotor with an overlap ratio of +0.15 produces a higher torque ratio and is more efficient in a wider range of tip speed ratios. Therefore, it was selected as the optimal horizontal overlap. In the next step, at this optimal point, vertical overlaps were investigated. Among these overlaps, the value of −0.1 showed the best torque and power coefficients. As a result, a rotor with a horizontal overlap ratio of +0.15 and vertical overlap ratio of −0.1 was selected as the optimal model. The improvement percentage in the average Cm in the horizontal overlap state, was 11.5%, and in the vertical overlap state, was 16%, when compared to the base state with zero overlap. Additionally, the improvement percentage in the average Cp in the horizontal overlap state was 3.7%, and in the vertical overlap state, was 7.5%, when compared to the same base state.

This article is a part of an ongoing project. In the next step, this optimal blade application will three-dimensionally examine an oscillating water columns (OWC) system. Finally, the effect of tensile forces on the rotor's structure will be investigated by using FSI simulation so that the information obtained can be used to select the raw materials and manufacture the rotor.

**Author Contributions:** Conceptualization, M.E., R.S., and R.A.; methodology, M.E., R.S., R.A., and M.S.S.; software, M.E.; validation, M.E.; formal analysis, M.E., R.S., R.A., and M.S.S.; investigation, M.E.; resources, R.S. and R.A.; data curation, M.E., R.S., and R.A.; writing—original draft preparation, M.E., writing—review and editing M.E., R.S., R.A., and M.S.S.; visualization, M.E., R.S., and R.A.; supervision, R.S., R.A., and M.S.S.; project administration, R.S. and R.A.; funding acquisition, R.S.

**Funding:** This research received no external funding.

**Acknowledgments:** The Sea-Based Energy Research Group at Babol Noshirvani University of Technology is acknowledged for providing technical and administrative assistance.

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