# Improvement of Aerodynamic Performance of Savonius Wind Rotor Using Straight-Arc Curtain
## 1 Introduction  
Over the past few years, owing to the rapid development of the world economy and the increasing population, the demand for traditional energy sources-such as coal, oil, and natural gas has increased dramatically, making traditional energy reserves increasingly scarce. This has gradually become an important inhibiting factor for the sustainable development of society. Also, the excessive use of traditional energy sources has produced severe environmental pollution. Therefore, it is necessary to find renewable energy. As a renewable energy source, wind energy has many advantages. It is inexhaustible, widely distributed, clean, and pollution-free. The rapid improvement and considerable advancement of wind energy systems have made wind power an indispensable energy source [1, 2]. Therefore, reasonable and efficient use of wind power is an effective way to ease the energy crisis. Wind turbines, the most important part of wind power stations, have gradually attracted the interest of many scholars. According to the axis of rotation, wind turbines can be divided into two types: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). HAWTs are well known to have higher efficiency than VAWTs and have been mostly used for electricity production [3]. However, a HAWT farm is land-consuming and needs to be connected with an integrated power system, which limits its development to some extent. In contrast, VAWTs have lower installation and maintenance costs, less wear on the moving parts, and are independent of the wind direction. These advantages make their application more convenient and flexible [4, 5, 6]. Therefore, VAWT is considered as a better choice for meeting the requirements of the distributed energy strategy. The Savonius wind rotor is a kind of VAWT [7]. This rotor has a shape similar to an \"S\" and consists of two semi-cylindrical surfaces with a small overlap between them. Its main advantages are as follows:* It has the ability to accept wind from any direction;
* It has good starting torque characteristics [8; 9; 10];
* It can be operated in a wide range of wind conditions;
* It has a simple and cheap construction as well as compact size;
* The electrical equipment of this wind rotor can be placed at ground level and have low noise emission [11]. However, the power performance of Savonius wind rotors (\(Cp\approx 15\)-\(19\%\)) is rather low when compared with that of wind rotors with a horizontal axis (\(Cp\approx 45\%\)) and a Darrieus-type wind rotor with a vertical axis (\(Cp\approx 35\%\)) [12]. Therefore, it is necessary to improve the aerodynamic performance of Savonius wind rotors. In recent years, many studies have been carried out to improve the performance of Savonius wind rotors. The improvement mainly relies on optimizing the basic design parameters of wind rotors, changing the shape of wi nd rotor blades, or adding some deflectors. [PERSON] (1992) found that the optimum size of the bucket overlap is around 15% of the size of the bucket chord by experiment [13]. [PERSON] and [PERSON] (1994) studied the aerodynamic performance of a Savonius wind rotor by measuring the pressure distribution on the blade surfaces at various rotor angles and various tip-speed ratios (TSRs) [14]. [PERSON] and [PERSON] (2008) carried out wind tunnel tests of Savonius wind rotor models in an open-jet wind tunnel to study the effect of the stage aspect ratio, the rotor aspect ratio, and the number of stages on the wind energy utilization rate [15]. They found that the performance of two stage and three stage rotors remains the same after increasing the stage aspect ratio and the rotor aspect ratio by a factor of two and three, respectively. [PERSON], [PERSON] and [PERSON] (2008) studied the efficiency of a three-bucket Savonius-Darrieus wind turbine and a three-bucket Savonius wind turbine [16]. They found that the combined turbine has a high power coefficient.