## 1 Introduction
They came to the conclusion that a slightly curved and thick airfoil could improve the overall performance of the Darrieus rotor. Their study examined the interaction between the flow and the blades of Darrieus rotors with a high blade solidity ratio, featuring both asymmetric and symmetric blades, to evaluate their performanceunder low wind speed conditions using detailed CFD simulations [13]. The S815 airfoil was chosen for the study because it has a higher thickness and lower curvature percentage compared to other popular NREL S-series airfoils such as S1210, S809, and S818. The NACA 0018 airfoil, which is widely used for Darrieus rotors and has a higher power factor compared to other NACA series airfoils, was chosen as a comparison. In [14], the aerodynamic characteristics of the rotor were investigated in the form of blade interaction with the flow at different azimuthal angles, pressure coefficients, and changes in the lift-to-drag ratio. The effect of changes in low wind speeds on the blade interaction with the flow at different azimuthal positions of the rotor was also assessed. [PERSON] et al. [15] investigated the effect of positive and negative tilt angles on asymmetric blades. [PERSON] et al. [16] used the curtain arrangement and rib addition on blades to improve the performance of Savonius rotors. They conducted CFD simulations to study the impact of the curtain arrangement and rib addition on the rotor performance. They were able to improve the performance by about 42% versus a conventional rotor. The authors of study [17] analyzed key factors of aerodynamic design, as well as their advantages and disadvantages, and proposed a baseline VAWT design for further analysis. Particular attention was given to the potential applications of VAWTs for both large-scale offshore wind energy and small urban installations. However, the issue of aerodynamic optimization was not fully addressed. Further development of the proposed designs involves modifying the blade geometry. One such solution is the use of J-shaped blades, which enhance self-starting capability and exhibit unique aerodynamic characteristics. Other approaches include variable blade pitch angles [18; 19] and hybrid configurations that combine Darrieus and Savonius VAWTs [20; 21]. The high static torque generated by the Savonius turbine at a low TSR can be utilized to facilitate the self-starting of the Darrieus turbine. In addition to hybrid solutions aimed at improving the self-starting ability of VAWTs, other methods of blade geometry modification have also been explored. In [22], a 3D numerical analysis of J-type blades in VAWTs was performed. It was found that they form stronger vortices, reducing the torque at high TSRs, but they improve self-starting. Replacing straight blades with J-shaped ones increases the torque by 26.9% at 10 m/s and by 37.6% at 5 m/s, which makes them promising for low-speed wind in cities. Continuing the research on improving the aerodynamic characteristics of vertical-axis wind turbines, a new three-piece blade VAWT (3-PB VAWT) was proposed, combining the advantages of straight-bladed and screw turbines. Numerical modeling showed that this design provides more stable torque. In particular, the 3-PB VAWT increases the torque coefficient by 6.06% at a low TSR (0.44) and by 158.19% at a high TSR (1.77), combining high aerodynamic efficiency with low production cost [23]. In addition to blade shape optimization, an important area of research has been improving the starting performance of VAWTs. In this context, various methods have been proposed to address the low starting torque issue and improve the overall turbine efficiency. In addition, further research has shown that not only the starting performance but also the overall performance of VAWTs can be improved by changing the blade shape. In particular, using helical blades instead of straight blades has improved the turbine efficiency at high tip speed ratios (TSRs).