# Aerodynamic Enhancement of Vertical-Axis Wind Turbines Using Plain and Serrated Gurney Flaps
## 1 Introduction  
Vertical-axis wind turbines (VAWTs) are regarded as the potential future of distributed wind energy [1] because of their low manufacturing cost, low operation noise, and lack of a need for piloting mechanisms [2]. Despite the development of various VAWTs, their applications are limited due to the lower unit power output than that of a horizontal axial wind turbine of the same size [3]. Symmetrical airfoils, such as NACA00, are commonly used in VAWT blade design because they are believed to generate consistent aerodynamic forces during blade rotation [4]. However, symmetrical airfoils have lower lift coefficients compared with cambered airfoils and are susceptible to dynamic stalling at high attack angles [5]. Therefore, it is crucial to apply effective flow control methods to enhance the lift force of VAWT blades. The concept of the Gurney flap was initiated by [PERSON] in 1971, who used it on his car to increase traction and improve handling. The device is a microtab (referred to as a plain Gurney flap) protruding perpendicular to the airfoil lower side near the trailing edge. The optimal height of the flap is recommended to be between 1%c and 5%c by [PERSON] [6]. [PERSON] [7] found that the flap height should be smaller than 7%c in order to keep the average power increment of the oscillating airfoil. [PERSON] [8] later applied the Gurney flap as a simple lift-enhancing device on commercial aircraft and rotorcraft. Studies have shown that a Gurney flap with a height of 2% of the chord length can yield an approximately 30% increase in airfoil lift [9]. The effectiveness of Gurney flap techniques has also been demonstrated in improving the performance of rotorcraft. The plain Gurney flap (FGF) has found widespread applications in VAWTs due to its ease of implementation and high efficiency [10; 11; 12; 13; 14]. [PERSON] et al. [10] applied the PGF on NACA0012 and raised the maximum energy capture and efficiency by 22% and 15% undera certain range of reduced frequency. [PERSON] et al. [11] investigated the wake behavior of a symmetric airfoil with a plain flap at a low Reynolds number. The plain flap was found to raise the airfoil lift and its fluctuating magnitude at high angles of attack. [PERSON] et al. [12] studied the PGF's impacts and the parameter of solidity on the performance of VAWTs in three-turbine arrays, and the maximum average torque of the downstream rotor could be reached by 36.5%. [PERSON] et al. [13] investigated the lift effectiveness of a PGF tab on an isolated NACA0018 airfoil. An H-type VAWT rotor used three blades with flap installation and achieved an increased power coefficient at a low tip speed ratio (TSR \(<\) 2.5). [PERSON] et al. [14] found that the maximum improvement could reach up to 21.32% by mounting the PGF. Due to the excellent aerodynamic improvement in the VAWT by the PGF, more flow control techniques have been applied to the blades. [PERSON] et al. [15] and [PERSON] et al. [16] tested a novel plasma GF on the aerodynamic characteristics of a NACA0012 airfoil and indicated that the effects were the same as those of the conventional GF. [PERSON] et al. [17] developed an active flow control technique of a flapped airfoil for a VAWT. The flap angle was specially designed to avoid the separated wake behind the trailing edge and to delay dynamic stall. [PERSON] et al. [18] attached an active GF to a three-blade VAWT to increase the power coefficient by 5.14% under the operation condition. [PERSON] and [PERSON] [19] optimized a combination of the PGF and a semi-circular inward dimple on a VAWT blade, resulting in an approximately 35% increase in the average tangential force under the steady-state condition and a 40% increase under oscillating conditions. The increased drag is unavoidable when the lift is increased by a PGF. A critical issue is to reduce the drag when designing turbine configurations [20]. Owing to the drag penalty, various efforts have been made to reduce the flap's negative influences. [PERSON] et al.