## 4 Effect of Surface Grooves on Aerodynamic Characteristics of Airfoil
The transient changes of the standard NACA0012 airfoil at the angle of attack of \(14^{\circ}\) and the triangular concave grooves on the suction side of the NACA0012 airfoil at the angle of attack of \(19^{\circ}\) and the triangular concave grooves on the pressure side of the NACA0012 airfoil at the angle of attack of \(10\) are calculated respectively, and the time step is set to 0.001 s.  
Figure 8: Three airfoils. (**a**) Triangular concave grooves on the suction side, (**b**) triangular concave grooves on the pressure side, and (**c**) NACA0012 airfoil. Comparing the speed distribution of the three airfoils, as shown in Figures 12-14, it can be seen that the three airfoils are the same in speed distribution, but because of the triangular concave grooves on the suction side, the boundary layer of the suction side becomes thicker. The boundary layer on the pressure side is different from the standard airfoil. Comparing Figures 15-17, it can be found that the pressure distributions are the same. The only difference is that the pressure difference of the NACA0012 airfoil with triangular concave grooves on the suction and pressure sides is smaller than that of the standard NACA0012 airfoil, which explains why the lift coefficient of the standard NACA0012 airfoil is better than the NACA0012 airfoil with triangular concave grooves on the suction side and the pressure side when the angle of attack is small. As can be seen from Figures 18-20, vortices are formed inside the triangular concave grooves. These vortices form \"air bearings\", which convert sliding friction into rolling friction. Figures 21-23 are the calculations of the three airfoils at their respective stall angles of attack. It can be found that, compared with the standard airfoil, the stall angle of attack of the airfoil with grooves on the suction side is significantly increased to \(19^{\circ}\), while that of the airfoil with grooves on the suction side is significantly increased to \(19^{\circ}\). The results of the simulations are shown in Figures 24-25. Figure 11: Variation of lift–drag ratio with angle of attack. Figure 9: Variation of lift coefficient with angle of attack. the pressure side is reduced to 10\({}^{\circ}\). Figure 21 shows the entire vortex development process on the suction side of the NACA0012 airfoil with triangular concave grooves at an angle of attack of 19. From Figure 22, it can be seen that the initial conditions of the initial vortices of both are similar, and they both begin to develop from the starting position of the leading edge of the suction side of the airfoil, but during the development process, the situation is completely different. On the suction side of the standard airfoil, its vortex is developed as a whole. For the NACA0012 airfoil with triangular concave grooves on the suction side, due to the existence of the triangular concave grooves, the vortex on the suction side is divided into a small vortex in the rear of the triangular concave grooves and a large vortex behind the rear of the triangular concave grooves. The airflow can maintain a normal flow on the surface of the triangular concave grooves. After the vortex was formed, the entire area of the suction side of the standard NACA0012 is covered by the vortex, which means that the boundary layer is separated completely, while for the NACA0012 airfoil with triangular concave grooves on the suction side, due to the existence of the triangular concave grooves, the vortex only covers the area behind triangular concave grooves. In other words, due to the existence of triangular concave grooves on the suction side, it changes the formation process of the airfoil vortex, and the boundary layer is locked in the grooves area, which can ensure the normal flow of airflow in some areas and reduce the influence of the vortex.