# Rod-Airfoil Interaction Noise Reduction Using Gradient Distributed Porous Leading Edges
## 2 Experimental Set-Up and Data Processing  
### Test Facility and Model  
The acoustic tests of the rod-airfoil system were conducted in a small anechoic wind tunnel at the China Aerodynamics Research and Development Center (CARDC), i.e., in a 0.55 m \(\times\) 0.4 m aeroacoustic wind tunnel. The rectangular nozzle exit of this wind tunnel is 0.4 m high and 0.55 m wide. All six walls of the anechoic chamber are covered with fibreglass wedges, yielding a background noise level of less than 76 dB(A) for a flow velocity of 80 m/s (measured at a distance of 2 m perpendicular to the center of the nozzle). The maximum flow velocity is 100 m/s, and the incoming turbulence level is less than 0.2% at the centerline.  
A sketch of the experimental set-up is illustrated in Figure 1. A symmetric NACA 0012 airfoil was located downstream of a cylindrical rod. The rod diameter was \(d=20\) mm, while the airfoil chord length was \(c=200\) mm; thus, the dimensionless quantity \(d/c=0.1\) was the same as the benchmark set-up in [PERSON] et al. [5]. The rod-airfoil system was vertically installed between two parallel side plates that were flush-mounted with the nozzle exit. The downstream airfoil was placed at angles of attack ranging from \(\alpha=0^{\circ}\) to \(20^{\circ}\), with its fixed rotating axis located at a 30% chord length (i.e., 60 mm, the same as in [21]). In our experiments, the positions of the upstream rod were also changeable to enable adjustment of the streamwise gap \(L\) between the trailing edge point of the rod and the leading edge point of the airfoil at the angle of attack \(\alpha=0^{\circ}\). Three different gaps were set, i.e., \(L=100\), 140, and 200 mm. The last one produced \(L/d=10\), which is the same as the benchmark set-up in [PERSON] et al. [5]. Smaller gaps were not used as the downstream airfoil would have acted as a splitter plate, which would have prevented the formation of Karman vortices from the upstream rod and thus would have drastically reduced the noise radiation of the rod-airfoil configuration [12; 22; 23]. The range of the flow velocities under investigation was between \(V_{\infty}=20\) and 80 m/s, corresponding to Reynolds numbers based on a rod diameter and airfoil chord of \(Re_{d}=2.67\times 10^{4}-1.07\times 10^{5}\) and \(Re_{c}=2.67\times 10^{5}-1.07\times 10^{6}\), respectively.  
As shown in Figure 2, the tested airfoil was composed of an aluminum main body and three interchangeable leading edge strips (denoted as X, Y, and \(Z\)) with equal lengths in the chordwise direction (i.e., 10 mm or 5% of \(c\)). We used porous metallic foams (copper or iron-nickel) with a wide range of ppi values (20 to 120 ppi) and nonporous aluminum, whose ppi value was taken as 0, to manufacture the interchangeable strips. At present, we are unable to provide the detailed information of the porosity and the static permeability of the porous media, because neither we nor the material supplier had the dedicated devices to measure these parameters. In our tests, we used X**Y**Y**Z**\({}^{**}\) to label each leading edge configuration of the airfoil. For example, X80Y60Z40 corresponded to the configuration whose three strips were composed of 80, 60, and 40 ppi metallic foams from upstream to downstream.  
Figure 1: Sketch of the experimental set-up (not to scale).