## 2 Materials and Methods
### Aeroacoustic Measurements  
Acoustic measurements are carried out in the anechoic wind tunnel of Delft University of Technology. The rectangular contraction outlet of the vertical, open-jet wind tunnel measures \(40\times 70\,\mathrm{cm}^{2}\). TBL-TE noise is evaluated for a NACA0018 airfoil with chord length \(c=20\,\mathrm{cm}\). An interchangeable trailing edge with a length of 4 cm allows for the use of different solid and porous inserts. The airfoil is placed between two wooden side plates with a height of 1.2 m and a distance between the contraction outlet and the leading edge of 0.5 m. All experiments are carried out for a geometrical angle of attack of \(0^{\circ}\) adjusted based on a digital inclination measuring device. Experiments are performed for flow speeds between \(U=15\,\mathrm{m}\,\mathrm{s}^{-1}\) and \(U=25\,\mathrm{m}\,\mathrm{s}^{-1}\). For the given airfoil setup and flow conditions, these flow speeds correspond to chord-based Reynolds numbers of \(Re_{1}=1.88\times 10^{5}\) and \(Re_{2}=3.14\times 10^{5}\), respectively. To ensure turbulent flow past the  
Figure 1: Experimental test rig used for specifying porous material properties at varying temperatures (**a**). Pressurized air is forced through a cylindrical material sample (**b**) and the static pressure drop is recorded. The porous material is metallic foam with a nominal pore diameter of \(d_{p}=800\,\mathrm{\SIUnitSymbolMicro m}\) (**c**). trailing edge, transition is forced at \(x/c=0.2\) using carborundum grains with a diameter of 0.84 mm. Turbulent state of the boundary layer is verified by placing an external microphone at different span- and streamwise locations close to the airfoil surface. A solid trailing edge insert as well as a porous one made from P800 metal foam is used during the experiments. The porous inserts are manufactured through spark erosion (EDM) machining. Each porous trailing edge is composed out of three individual pieces with spanwise dimensions 13 cm (2x) and 14 cm (1x). Figure 2 shows the geometry of the trailing edge inserts and gives an overview over the different porous versions that are tested. For activation of the porous material via temperature control, the porous trailing edge inserts are equipped with electrical heating wires. A total of 12 heating wires, with an outer diameter of 1.1 mm each, are aligned in the spanwise direction and distributed across the trailing edge cross-section according to Figure 2. Each individual heating wire consists of a conductive inner wire with a diameter of 0.6 mm, covered with an electrical insulation. The core is manufactured out of a copper-nickel alloy (CuNi44) with a high length-specific electrical resistance of \(R_{s}=1.73\,\Omega\,\mathrm{m}^{-1}\). For insulation from the metallic foam, a polymeric (PVDF), high-temperature shrinking tube is used. The dissipated heating power \(P_{diss}=RI^{2}\) depends on the wire resistance \(R\) and is a quadratic function of the electric current \(I\). The resistance of a single wire spanning the entire trailing edge  
Figure 2: Aeroacoustic measurement setup in the vertical, anechoic wind tunnel of TU Delft (**a**). The airfoil can be equipped with exchangeable trailing edge inserts (**c**) and heating of the porous metal foam is achieved by means of electrical heating wires (**b**). width of 0.4 m amounts to \(R_{w}=0.4R_{s}=0.69\,\Omega\) and with a maximum current of \(I=5\) A, the available heating power results in \(P_{el}\approx 210\) W.  
The surface temperature of the trailing edge is analyzed using a CEDIP Titanium 530L IR infrared camera with a resolution of 320 \(\times\) 256 pixels. The camera lens is positioned at a distance of 0.5 m away from the airfoil plane. The heating wire temperature is measured using a type-K thermocouple placed inside the porous material close to the wire surface as shown in Figure 2. The measured surface temperature distribution is plotted in Figure 3 for three different flow speeds.