## 1 Introduction  
Interest in improving the performance of wind turbines by optimizing the methods used for blade design and flow control devices has grown in the last few decades. In particular, several research efforts have focused on refining these methods to increase the efficiency of wind turbines [1, 2, 3, 4, 5]. Wind turbine manufacturers have attempted to increase the blade length not only to improve the annual energy production (AEP), but also to lower the cost of energy [6]. Traditionally, the wind industry has performed aerodynamic blade design and performance analysis based on Blade Element Momentum Theory (BEMT). Aerodynamic blade design methods for wind turbines were reviewed previously [7, 8, 9, 10]. The design method based on BEMT applies various types of loss-correction models [11, 12, 13] to the tip and hub of the blade, where the three-dimensional flow field is dominant. However, the problem of aerodynamic reliability with high accuracy persists in the various loss-correction models based on BEMT, owing to the enlargement of the blade length and the taper of the blade tip. To overcome the reliability problem, a 3D steady-flow simulation using CFD was performed after preliminarily performing a blade design based on BEMT.  
The aerodynamic performance and load of the blade, and the complex 3D viscous flow field around the blade tip and hub were evaluated using the previous simulation results [14; 15].  
In this study, the use of a vortex generator (VG) as a passive performance-enhancing device for a large wind turbine was studied. A VG is a passive flow control device in the form of a small vane and is mainly used to delay the stall occurrence of an airplane blade [16; 17; 18] or even to improve the heat transfer performance by forming longitudinal vortices [19]. In the wind energy industry, the VG has been studied to improve the aerodynamic performance of wind turbines installed on flat terrain, as well as to extend their lifespan through the wind-kinetic energy conversion of wind turbines installed on complex terrain [20]. The effectiveness of the VG has been proven through academia-industry joint research, and it has been installed and operated in thousands of wind turbines since 2013 [21]. VGs are installed toward the root and suction surface of the blade of a wind turbine to mitigate airflow separation. Many CFD analyses and wind tunnel tests have been carried out to assess the feasibility of VG adoption with wind turbine blades. However, few CFD studies focused on full-scale wind turbines equipped with Gurney flap VGs other than airfoil base simulation and wind tunnel testing or CFD using virtual porous models [22; 23; 24; 25; 26; 27]. There was a CFD analysis study of the VG-mounted wind turbine blade by RANS, SST k-omega equation, but the blade length was as short as 35 m, and a cuboid VG was investigated for the convenience of lattice formation [28]. Although there have been studies that calculate power increase via BEM using the lift and drag coefficient, calculations using BEM are less accurate than RANS CFD analysis results [29]. The aerodynamic characteristics of multi-MW horizontal-axis wind turbine blades have been analyzed using the commercial CFD software ANSYS CFX [30]. The results of the CFD analysis were validated with GH-Bladed design data [31]. These researchers optimized the shape and layout of the VG to maximize the performance of the wind turbine in terms of power generation.