## 1 Introduction  
Turbulent boundary layer trailing edge (TBL-TE) noise can be a major contributor to airfoil self-noise in aerospace [1; 2; 3] and wind energy [4; 5] applications. Far-field noise is emitted by scattering of convected turbulent boundary layer pressure fluctuations due to the jump in impedance at the trailing edge [6]. A promising concept for TBL-TE noise reduction, firstly investigated in the 70's [7; 8], is the use of \"variable impedance\" airfoils for which the solid trailing edge is replaced with permeable materials. Later studies confirmed noise reduction capabilities of porous treatments applied to both, leading [9] and trailing edges [10]. The recent analytical work of [PERSON] and [PERSON] [11] proved possible noise reduction through finite porous extensions with a focus on low-frequency gusts and the permeable-impermeable junction. In comparison to other trailing edge modifications, [PERSON] et al. [12] concluded from their experiments that porous materials offer the best noise control solution. They used a liner-type porous material [13] as an alternative to more complex geometries such as porous metal foams. [PERSON] et al. [14] experimentally found that fully permeable SD7003 airfoils with different resistivity reduced far-field noise levels by up to 8 dB with respect to solid airfoils. Porous trailing edge treatments fitted to a DLR F16 airfoil were tested by [PERSON] et al. [15] who reported broadband noise reduction up to 2-6 dB depending on the material topology. They proposed air flow resistivity as a key parameter to control the low-frequency noise attenuation. More recently, [PERSON] et al. [16] used permeable metallic foams to replace the rigid trailing edge of a NACA0018 airfoil which led to far-field noise reduction of up to 11 dB. Comparison with porous but non-permeable inserts showed that no noise reduction is achieved when flow communication is neglected. This justifies the use of the material's airflow resistivity as a characterizing parameter for potential noise reduction.  
Replacing parts of a solid airfoil with permeable materials has been proved effective in reducing airfoil self-noise; however, it introduces an aerodynamic penalty due to increased surface roughness and pressure balance between suction and pressure side. In experiments with poro-serrated trailing edge configurations, [PERSON] et al. [17] reported decreasing lift with respect to the solid reference case while no clear trend towards increasing drag was shown. For partly porous SD7003 airfoils operating at angles of attack between \(-12^{\circ}\) and \(24^{\circ}\), [PERSON] and [PERSON] [18] recently performed balance measurements of lift and drag in addition to acoustic measurements. In agreement with [PERSON] et al. [19], they identified the flow resistivity as characterizing parameter for aerodynamic performance and observed a shift of the drag polar with increasing length of the permeable surface towards higher drag and reduced lift. The present report investigates the possibility to actively control the resistivity of permeable materials by heating of a porous metal foam. The goal is to switch between trailing edge configurations which are in favor of maximum noise reduction and minimum aerodynamic penalty, respectively.  
The content of this paper is structured as follows: Section 2 is divided in two parts and first addresses the methods and experimental setup used to establish a relation between fluid temperature and air flow resistivity. The second part issues the aeroacoustic microphone array measurements with a focus on the trailing edge heating capabilities and the acoustic post-processing technique. Experimental results are presented and discussed in Section 3 and a concluding summary of the main findings can be found in Section 4.