### Background of Savonius wind turbine

Vertical-axis wind turbines (VAWTs) include both a drag-type configuration, such as the Savonius rotor, and a lift-type configuration, such as the Darrieus rotor. The simplest type of vertical-axis wind turbine is the Savonius rotor, the operation of which depends on the difference in drag force when the wind strikes either the convex or concave part of its semi-cylindrical blades. Savonius rotors are good at self-starting and work independently of wind direction. However, its efficiency is relatively lower than that of the lift-type VAWTs. Due to its simple design and low construction cost, Savonius rotors are primarily used to pump water and generate wind power on a small scale, and its large starting torque makes it suitable for starting other types of wind turbines that have inferior starting characteristics, such as the Darrieus rotor and Gyro mill
[1]. Recently, some generators with high torque at low rotational speed, suitable for small-scale wind turbines, have been developed, suggesting that Savonius rotors may yet be used to generate electric power
[1].

Wind turbine aerodynamics must be designed for optimal output to exploit the wind energy in a specific location. Diaz et al.
[2] analyzed the drag and lift coefficients of a Savonius wind turbine to quantify the aerodynamic performance of the rotor. They found that maximum efficiency, in terms of power coefficient, occurs at a tip speed ratio of *λ* = 1, and the drag coefficient decreases sharply when the tip speed ratio increases or decreases from this value. They also found that the most important region of Savonius rotor operation occurs at a tip speed ratio around *λ* = 1, where the lift coefficient remains as a constant 0.5. Sawada et al.
[3] studied the mechanism of rotation of a Savonius rotor with two semi-cylindrical blades and found that a rotor with a gap ratio of 0.21 produces positive static torque at all angles. They also found that lift force contributes significantly to dynamic torque, while the rotor angle is between *α* = 240° and *α* = 330°. Aldoss and Obeidat
[4] used the discrete vortex method to analyze the performance of two Savonius rotors running side-by-side at different separations. They compared their computational results on torque and power coefficients with their experimental results for verification. Fujisawa and Gotoh
[5] studied the aerodynamic performance of a Savonius rotor by measuring pressure distribution on the blade surfaces at various rotor angles and tip speed ratios. Torque and power performance, evaluated by integrating the pressure, were in close agreement with direct torque measurements.

Rahman et al.
[6–8] experimentally studied aerodynamic characteristics, such as the torque and drag coefficients, of a three-bladed Savonius rotor model by measuring the pressure difference between the convex and concave surfaces of each semi-cylindrical blade of the stationary rotor at different rotor angles and the variation of the separation point with the increase of rotor angle. They used the static coefficients for dynamic prediction and compared the findings in terms of power coefficients for different tip speed ratios with experimental results for the two-bladed Savonius rotor. Rahman et al.
[9] conducted both experimental investigations and computational fluid dynamic (CFD) simulations to establish the feasibility of improving the performance of a simple, three-bladed Savonius VAWT. The normal drag coefficient, tangential drag coefficient, and torque coefficient were calculated both experimentally and numerically, and the results were compared. In each case, the calculations matched well. The numerical results were more accurate and gave positive values for combined drag coefficients and the total static torque coefficient.

Gupta et al.
[10] compared a three-bucket Savonius wind turbine with a three-bucket Savonius-Darrieus wind turbine. They found that the power coefficient of the combined turbine decreases as the overlap ratio increases. The maximum power coefficient of 51% was found where there was no overlap. They claimed that the combined rotor without overlap, which showed 51% efficiency, was the highest efficiency of a Savonius wind turbine at any overlap condition under these test conditions. Altan et al.
[11] did some experimental studies to improve the performance of the Savonius wind turbine using a curtain. They placed the curtain arrangement in front of the rotor in a configuration capable of preventing the negative torque that affects the convex blade surface of the Savonius wind turbine.

Sargolzaei and Kianifar
[12] simulated a Savonius wind turbine using artificial neural networks (ANNs) to estimate power ratio and torque. They experimentally investigated seven prototype Savonius wind turbines and compared the experimental results with their predicted ANN results. Their predicted results were in good agreement with their experimental results. They found that increased wind speed causes torque increase. For all their models, they found that maximum torque was at 60° and minimum torque was at 120°. Altan and Atilgan
[13] numerically simulated their experimental work using FLUENT 6.0 and GAMBIT 2.0. Their model was two-dimensional, and they used a standard *k*-*ϵ* turbulence model. To calculate pressure and velocity distribution, they used a semi-implicit method for pressure-linked equation (SIMPLE) analysis algorithm. By comparing the numerical and experimental results, they concluded that the curtain improved the performance of Savonius wind turbines.

Saha et al.
[14] fabricated a two-stage Savonius wind turbine by inserting valves on the concave side of the blades. They compared its performance to a conventional Savonius wind turbine and found that with valves on a three-bladed turbine, the power coefficient was higher compared to a two-bladed turbine for both semi-circular and twisted blades. Without valves, air strikes the blades and rotates them in a negative direction. Saha et al. also varied the number of stages in a Savonius wind turbine and found that while the power coefficient increased from one to two stages, it decreased from two to three stages due to increased inertia. They tested the twisted blades of one, two, and three stages and found that the three stages had a better power coefficient, and the twisted blades showed better performance.

To decrease the variation in static torque in conventional Savonius rotors at a 0° to 360° rotor angle, Kamoji and Kedare
[15] tested a helical rotor with a twist of 90°. They conducted experiments in an open-jet wind tunnel at gap ratios of 0.0, 0.05, and 0.08 to study the effect of gap ratio and Reynolds number on its performance and evaluated static torque, dynamic torque, and power coefficients. They compared its performance with and without a shaft between the end plates at different gap ratios. A helical rotor without a shaft was also compared with the performance of the conventional Savonius rotor. They found that all helical rotors have a positive coefficient of static torque at all rotor angles, but the rotors with a shaft had a lower power coefficient than those without. The coefficient of power of the rotor without a shaft with a 0.0 gap ratio was marginally less than the conventional Savonius rotor.

Gupta et al.
[16] investigated the performance of two-bladed Savonius turbine with five overlaps of 16.2%, 20%, 25%, 30%, and 35%. Among them, 16.2% overlap condition showed maximum power extraction. The pressure drop across the rotor from upstream to downstream as well as the maximum pressure difference across the returning bucket is displayed in the same condition. Qasim et al.
[17] worked with impeller scoop-frame type with movable vanes wind turbine (VAWT). The objective was to maximize the drag factor by closing the vanes on convex shape and opening when air hits the concave part. Due to the movement of vanes for and against the wind, a higher drag factor is worked on the impeller scoop-frame type with movable vanes and has higher efficiency than flat vanes.

Ghatage and Jyeshtharaj
[18] have done an experiment by changing the shape of the blade as well as the blade number. They have studied with both regular curved blade and twisted curved blade. The experiment concluded that the two blades with twist enhance the efficiency of turbine. In their experiment, the 30°-twisted two-bladed turbine gave the better power coefficient. It can be concluded that the twisted blade attributes relatively higher drag over the turbine surface.

Kumbernuss et al.
[19] studied two-staged Savonius-type turbines with different number of blades, the shape of the blades, the overlap ratio, and the phase shift angle. The wind turbine was tested under four different wind speeds of 4, 6, 8, and 10 m/s. There were three turbines with the overlap ratios of 0, 0.16, and 0.32. The overlap ratio of 0.16 produced the better performance among the three, followed by the 0.32 overlap ratio. At lower and higher air velocities, the larger and smaller phase shift angles, respectively, will produce better performance of the turbines.

Carrigan et al.
[20] had the objective to introduce and demonstrate a fully automated process for optimizing the airfoil cross section of a vertical-axis wind turbine. The objective was to maximize the torque while enforcing typical wind turbine design constraints such as tip-speed ratio, solidity, and blade profile. This work successfully demonstrated a fully automated process for optimizing the airfoil cross section of a VAWT.

Researchers from different parts of the world have been investigating the aerodynamic characteristics of Savonius wind turbines and trying to identify the optimum design in order to achieve better performance compared to horizontal-axis wind turbines. Although much research has been going on experimentally and numerically on Savonius wind turbine performance improvement, there are few to no comprehensive studies using both experimental and numerical methods for various gap ratios at different Reynolds numbers. The primary goal of the present study is to investigate the aerodynamic characteristics of three-bladed Savonius wind turbines in order to contribute to the performance improvement of vertical-axis Savonius wind turbines. To achieve this goal, the authors designed and fabricated Savonius wind turbine scale models with no overlap ratio and two different overlap ratios, measured the pressure distribution around the Savonius turbine rotor models, and calculated the drag coefficients. Static torque was measured using the subsonic wind turbine for all models at varying angles of rotation, the mesh was generated numerically around all turbine models using GAMBIT, and fluid flow fields around the models were solved using *k*-*ϵ* turbulence model of FLUENT. Pressure contours, velocity contours, and torque were determined at various Reynolds numbers. A detail of the experimental and computational procedure of this research work can be found in the thesis work done by one of the authors
[21].