## 1 Introduction  
Drag reduction of riblets is one of the most important issues in drag reduction of non-smooth surfaces. In the past two decades, research on reducing frictional resistance has received considerable attention. So far, attempts have been made in two directions: a method to delay the boundary layer transition and a method to modify the turbulent structure in the turbulent boundary layer. Among various techniques for controlling turbulent resistance, riblets have been extensively studied, especially for subsonic boundary layers. For the drag reduction and flow control of the riblet surface, a lot of research and analyses have been carried out on flat plates and aircraft wings. These experimentshave fully confirmed the effectiveness of non-smooth surfaces for drag reduction and flow control. [PERSON] [1; 2; 3; 4] achieved drag reduction at subsonic speeds through the V-riblet surface, reducing it by up to 8%, and by optimizing the geometry of the riblet, it can also reduce drag by up to 10%. [PERSON] [5] conducted a research summary on the application of riblet drag reduction in aircraft in 2002. [PERSON] and [PERSON] [6] affixed a film with riblets structure to the suction side of the LC 100D airfoil section at low wind speeds. It was measured using a wake measurement method in the range of 0-6\({}^{\circ}\) angle of attack: at 0\({}^{\circ}\) and 20\({}^{\circ}\) angles of attack. The total resistance decreases by about 2%, of which the viscous resistance decreases by about 7%. At larger angles of attack, there is no drag reduction effect. [PERSON] and [PERSON] [7] measured the turbulent flow of the wake area, which is near the trailing edge of the NACA0012 airfoil section, at a zero angle of attack through experiments. The drag reduction rate reached 13.3% when the riblet height (h) was h = 0.152 mm. When h = 0.076 mm and 0.023 mm, the drag reduction rates are 2.7% and 7.3%, respectively. [PERSON] et al. [8; 9] attached a riblets film to the NAC 0012 and GAW-2 airfoil section in the 0-12 \({}^{\circ}\) attack angle range at low wind speeds along the flow direction within 80% of chord length range of the suction and pressure surfaces, respectively. The drag reduction experiment was carried out using the wake measurement method, and the maximum drag reduction rate reached 10%. There are three representative explanations for the study of the characteristics of the riblet surface boundary layer and its drag reduction mechanism: the \"second vortex group\" theory, the \"protruding height\" theory, and the \"mini air bearing\" theory. The \"second vortex group\" theory proposed by [PERSON] and [PERSON] [10] suggests that the flow direction vortex in the opposite direction of rotation interacts with the secondary vortex formed on the top of the riblet. The formation of low-speed strips and the spreading motion weaken the interaction between adjacent riblets and reduce the motion instability. [PERSON] [11; 12] believes that the most important thing is that the riblets restrict the movement of the vortex along flow direction in the span direction, which results in the weakening of the burst on the wall and the reduction of the frictional resistance on the wall. [PERSON] [13] believes that low-speed, quiet fluid with low frictional resistance is retained at the bottom of the narrow riblets, thereby reducing the total resistance. [PERSON] and [PERSON] [14] confirmed this experimentally. They measured the flow velocity field at the bottom of the riblets with a hot-wire probe. Compared with a smooth surface with the same projected area, the riblets' surface resistance was reduced by 4%. [PERSON] et al. [15] measured the flow velocity distribution at the top and bottom of the riblets and found that the frictional resistance at the top of the riblets increased, while the frictional resistance at the bottom of the riblets decreased significantly. [PERSON] et al. [16] proposed the \"protruding height\" theory.