Systematic testing of the microstructural and aeroacoustic properties of porous metals applicable as low-noise trailing-edge (TE) treatments has been initiated within the Collaborative Research Center SFB 880-Fundamentals of High-Lift for Future Civil Aircraft. Generic TE noise experiments were performed at Re = 0.8 × 10 6 to 1.2 × 10 6 in DLR's open-jet AWB facility. Complementary flow measurements in the closed test section MUB wind-tunnel of the TU Braunschweig served to quantify the induced aerodynamic effects. The presented database forms part of an ongoing cumulative effort, combining experimental and numerical methods, to gain a deeper understanding of the prevalent TE noise reduction mechanisms. For the large variety of porous materials tested herein a clear dependence of the achieved broadband noise reduction (reaching 2-6 dB at maximum) on the flow resistivity was identified. Basic design recommendations for material resistivity and pore sizes, the latter to minimize high-frequency self-noise contributions, were deduced for low-noise TE applications. An acoustic nearfield pressure release across the porous region, adversely coupled with a loss in lift performance for porous TE replacements, appears as the major noise-reduction requirement.

This paper gives an overview about prediction capabilities and the development of noise reduction technologies appropriate to reduce high lift noise and propeller noise radiation for future low noise transport aircraft with short takeoff and landing capabilities. The work is embedded in the collaborative research centre SFB 880 in Braunschweig, Germany. Results are presented from all the acoustics related projects of SFB 880 which cover the aeroacoustic simulation of the effect of flow permeable materials, the characterization, development, manufacturing and operation of (porous) materials especially tailored to aeroacoustics, new propeller arrangements for minimum exterior noise due to acoustic shielding as well as the prediction of vibration excitation of aircraft structures, reduced by porous materials.

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This paper presents the experimental results of a study conducted at DLR, ONERA and NASA on the shielding of sound by an NACA 0012 airfoil. The work presented was done in the context of the AVT-233 working group of the Science and Technology Organization (STO) of NATO. The experiments were conducted in the DLR Acoustic Wind Tunnel Braunschweig (AWB), the ONERA F2 tunnel and the NASA Quiet Flow Facility (QFF), with the goal of investigating facility-to-facility effects on the collected data. Two impulsive source concepts were used in the course of these experiments, DLR's laser sound source and ONERA's electric discharge source (SPARC). The collected data reveal that the different tunnel environments do not strongly affect the results obtained with either source. The laser sound source is found to deliver consistent results in all three wind tunnels, for the 7, 14 and 28 kHz octave bands at M=0.0 and M=0.16. In the highest octave band considered (56 kHz), the results are found to be very sensitive to the choice of operating parameters. The SPARC source also delivered consistent results in the low frequency range, in both the F2 tunnel and the AWB, for both Mach numbers tested. This joint effort has led to the development of a highly valuable database for the validation of shielding prediction tools.