## 1 Introduction  
During the operation of wind turbines, flow separation occurs at the blade root, which reduces the wind turbine efficiency [1]. Flow control technologies, such as VGs [2], plasma flow control [3], microflaps [4], microtabs [5], blowing and suction [6], synthetic jets [7], and flexible walls [8], have been increasingly applied to the design or optimization of wind turbine blades, aiming to improve their aerodynamic performance. [PERSON] [9] and [PERSON] [10] compared these flow control methods, while [PERSON] [11] and [PERSON] [12] found that VGs are one of the most effective devices for improving the aerodynamic performance of blades. In 1947, [PERSON] [13] was the first to apply VGs to wing flow control. The basic principle of flow separation control using VGs is that when the fluid passes through a VG, a concentrated vortex will be generated downstream. Under the action of the concentrated vortex, the high-energy fluid outside the boundary layer will enter the bottom of the boundary layer, increasing the mixing of the fluids inside and outside the boundary layer. This will increase the kinetic energy of the fluid at the bottom of the boundary layer and delay its separation [14; 15]. Due to their simple structure and convenient installation, VGs have been widely applied in fields such as flow control [16; 17; 18] and heat transfer [19; 20]. In 1995, [PERSON] et al. [21] conducted two-dimensional wind tunnel tests on the National Advisory Committee for Aeronautics (NACA) 4415 airfoil in the subsonic wind tunnel of the Aerospace Research Laboratory. The aim of these tests was to record the cross-sectional lift and bending moment characteristics under different airflow conditions. The effect of the airfoil leading-edge roughness and VGs on the aerodynamic performance of the airfoil was investigated. In 2006, [PERSON] and [PERSON] [22; 23; 24] experimentally studied the control effect of triangular VGs on the boundary layer flow separation on a curved plate. Their experimental set-up could change the installation angle of VGs and measure the wake structure of small wings. In 2010, [PERSON] et al. [25] performed numerical simulations to study a blunt trailing edge airfoil with and without VGs, and the performance of the airfoil was analyzed. The interaction between vortices generated by the VGs and wake vortices or separation vortices was investigated, while a mechanism for suppressing the boundary layer separation using VGs was revealed. In 2011, [PERSON] et al. [26] studied the effect of VGs on the aerodynamic performance of airfoils using numerical simulations, where the Spalart-Allmaras (SA) turbulence model and the standard wall function were employed. Their study results demonstrated that when the VGs were close to the separation point, the airfoil had a high maximum lift coefficient, and the performance of the rectangular and curved edge VGs was better than that of the triangular VGs. [PERSON] et al. [27] designed two types of VG combinations and conducted wind tunnel experiments in order to investigate airfoil flow control in depth. Their results revealed that the reasonable combination of a flap and VGs can significantly improve the aerodynamic performance of the airfoil, providing an ideal form of combined control. In 2012, [PERSON] et al. [28] experimentally investigated the effects of triangular VGs on airfoil aerodynamic performance. The VGs were located 10% and 20% away from the airfoil leading edge. The results showed that VGs can improve the airfoil aerodynamic characteristics, and the effect is better when the VGs are arranged 20% away from the leading edge. In 2013, [PERSON] and [PERSON] [29] performed wind tunnel experiments using three-dimensional particle image velocimetry to observe the effect of VGs on the near-stall flow behavior of the DU91-W2-250 airfoil. Their aim was to study the flow structures induced by the VGs on the wing.