## 2 Experimental Setup and Data Processing  
### Experimental Setup  
Noise measurements were conducted in an aeroacoustic open-jet wind tunnel (see Figure 1), which is situated in a 5.5 m \(\times\) 3.7 m \(\times\) 4.0 m anechoic chamber. The nozzle exit of this wind tunnel is rectangular, with dimensions of 0.55 m (width) \(\times\) 0.4 m (height). This tunnel has a turbulence intensity of less than 0.2% and a maximum free-stream velocity of 100 m/s for the open test section. The investigated airfoil model is a symmetrical 2D NACA 0012 airfoil, with a nominal spanwise length \(L\) = 398 mm and a nominal chordwise length \(c\) = 200 mm. The rear end of the airfoil has a thickness of 0.5 mm, to avoid significant vortex shedding tonal noise of a blunt airfoil trailing edge. The NACA 0012 airfoil is held by two parallel side plates, which are used to avoid the effect of tip vortex shedding noise and to adjust the geometric angle of attack of the model. For this investigation, experiments were performed at three geometric angles of attack of \(\alpha\) = 0\({}^{\circ}\), \(\alpha\) = 5\({}^{\circ}\), and \(\alpha\) = 10\({}^{\circ}\), and seven free-stream velocities from \(V_{\infty}\) = 10 m/s to \(V_{\infty}\) = 70 m/s (interval of 10 m/s), corresponding to chord-based Reynolds numbers \(Re_{c}\) ranging between 1.3 \(\times\) 10\({}^{5}\) and 9.3 \(\times\) 10\({}^{5}\). As depicted in Figure 2, the model contains an interchangeable trailing edge section on the aluminum main body. The nominal chordwise length of this interchangeable section is 30 mm. In order to cover structured pores, it is further divided into three subregions (denoted as L, M, and N, from upstream to downstream, respectively) with an equal nominal chordwise length of 10 mm. As reported by [PERSON] and [PERSON] [16] in their experimental investigation of turbulent broadband noise reduction by using 3D-printed porous trailing edges, pore diameter is the first-order parameter for acoustical responses, and typically, a pore diameter larger than 1 mm will generate more unwanted shedding-related tones, which may jeopardize the noise reduction performance. Therefore, sub-millimeter-scale pores are investigated in the present study. As the pore diameter is very small, to ensure the effectiveness of the structured pores, we first used 3D printing technology and multi-axis CNC (Computer Numerical Control) forming technology to produce several small test pieces, respectively. As shown in Figure 3a, 3D-printed pores are  
Figure 1: The 0.55 m \(\times\) 0.4 m open-jet wind tunnel (**a**) without the tested model and (**b**) with the tested model. not perfectly round (similar phenomena were observed by [PERSON] et al. [15]) and may even be blocked, especially when the pore depth is large and the pore size is small (due to the limitations of 3D printing and the impact of its cooling process). On the other hand, pores with a diameter of 0.6 mm manufactured via CNC in Figure 3b are satisfactory. Moreover, CNC-manufactured porous surfaces can provide sufficient structural strength for future technical applications in a full-scale wing. Therefore, one solid baseline trailing edge and seventeen structured porous trailing edges (see Table 1) are finally manufactured by using mechanical processing technology. Several studies [11; 12; 15; 16; 17] have shown that the pore diameter, pore coverage, pore position, and pore arrangement order have a significant impact on airfoil self-noise reduction. Thus, the pore design of this experimental process is depicted in Figure 4, and it is controlled by pore diameter \(d_{h}\), chordwise spacing \(c_{h}\), and spanwise spacing \(s_{h}\) between adjacent pores. Unless otherwise specified, the investigated porous trailing edges are configured with \(d_{h}\) = 0.6 mm, \(c_{h}\) = 2.0 mm, and \(s_{h}\) = 10.0 mm. The three subregions of the interchangeable porous trailing edge were covered with the same/different pore diameter, and L**M**N** was used to label each configuration.