The hydrodynamic and acoustic fields for a NACA 0018 with solid and porous trailing edge inserts are investigated. The porous inserts, covering 20% of the chord, are manufactured with metal foams with cell diameters of 450 and 800 μm and permeability values of 6 × 10 −10 and 2.7 × 10 −9 m 2. The experiments are performed at a chord-based Reynolds number of 2.63 × 10 5 and an angle of attack of 0 •. The porous trailing edge with higher permeability provides up to 11 dB noise attenuation with respect to the solid case for frequencies below a cross-over Strouhal number St = 0.26. Lower noise abatement (up to 7 dB) takes place below St = 0.3 for the insert with lower permeability. Conversely, noise increase with respect to the solid case is measured above the previously defined St value. A decrease in turbulence intensity is reported (up to 3% of the free-stream velocity), with lower intensity being measured for the insert with lower permeability. It is also observed that the permeability of the insert is linked to the increase of the anisotropy of highly energetic turbulent motions, being stretched in the streamwise direction, and the reduction of the eddy convection velocity (up to 20% with respect to the baseline case). In view of the results, the reduction of the velocity fluctuations is proposed as one of the mechanisms for low frequency noise abatement, being more relevant for the metal foam insert with lower permeability.

The turbulent flow over a NACA 0018 airfoil with porous trailing edge inserts and the resulting scattered turbulent-boundary-layer trailing edge noise are studied to investigate the effect of the cross-flow through the material. The experiments are performed at a chord-based Reynolds number of 2.63×10 5 and an angle of attack of 0 •. Two different porous inserts, covering 20% of the chord, are manufactured with the same metal foam (cell diameter of 800 µm and permeability of 27×10 −10 m 2). In order to assess the effect of the flow permeability on the far-field noise open and closed inserts are used. In the first ones the cellular structure is kept open, in the second ones the symmetry plane is filled with adherent material to impede flow communication between the two sides. The fully permeable trailing edge insert reduces noise intensity up to 11 dB for Strouhal number based on the chord lower than 16 while a noise increase is measured at higher frequencies. The non-permeable insert, on the other hand, does not show noise reduction with respect to the solid trailing edge below that Strouhal number but only noise increase at higher frequencies, confirming that flow permeability through the insert is necessary to achieve noise attenuation. The intensity of the noise increase is similar to that of the fully permeable insert thus suggesting that it is generated by the rough surface. The analysis of the mean flow field shows that only minor differences are present between the open and closed porous inserts. However, evidence of cross-flow through the material is found in the analysis of turbulent statistics.

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Most acoustic imaging methods assume the presence of point sound sources and, hence, may fail to correctly estimate the sound emissions of distributed sound sources, such as trailing-edge noise. In this contribution, three integration techniques are suggested to overcome this issue based on models considering a single point source, a line source, and several line sources, respectively. Two simulated benchmark cases featuring distributed sound sources are employed to compare the performance of these integration techniques with respect to other well-known acoustic imaging methods. The considered integration methods provide the best performance in retrieving the source levels and require short computation times. In addition, the negative effects of the presence of unwanted noise sources, such as corner sources in wind-tunnel measurements, can be eliminated. A sensitivity analysis shows that the integration technique based on a line source is robust with respect to the choice of the integration area (shape, position, and mesh fineness). This technique is applied to a trailing-edge-noise experiment in an open-jet wind tunnel featuring a NACA 0018 airfoil. The location and far-field noise emissions of the trailing-edge line source were calculated.

Note: This paper is part of the Special Topic on Passive and Active Control of Turbulent Flows.

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In this paper, the performance of four acoustic imaging methods: conventional frequency domain beamforming (CFDBF), functional beamforming (FUNBF), enhanced high resolution CLEAN-SC (EHR-CLEAN-SC) and generalized inverse beamforming (GIBF), is investigated in terms of accuracy and variability. Three experimental test cases are considered: 1) a single speaker emitting synthetic broadband noise, 2) two speakers emitting incoherent synthetic broadband noise, and 3) trailing-edge noise generated by a tripped NACA 0018 airfoil. All the measurements were performed in the anechoic wind tunnel of Delft University of Technology. Overall, GIBF and EHR-CLEAN-SC offer the most accurate results when point sources (speakers) are present. They even achieve super-resolution by separating sound sources beyond the Rayleigh resolution limit. Repeating the measurements indicates a standard deviation in the results of less than 1 dB. When analyzing distributed sound sources, such as trailing-edge noise, CFDBF and FUNBF provide the best performance. This indicates that the acoustic imaging method needs to be selected based on the expected sound source configuration.

Turbulent boundary layer trailing-edge noise scattered by a NACA0018 airfoil equipped with 3D printed perforated trailing-edge inserts, i.e. with straight cylindrical channels connecting the two sides of the airfoil, is investigated. The inserts have different permeability in order to assess the effect of this property on broadband noise generation. Far-field noise is measured with a phased microphone array. The experiments are performed at free-stream velocities of 26 and 41 m/s, corresponding to chord-based Reynolds numbers of 3.4×10 5 and 5.4×10 5 , and at angles of attack of 0 and 4.8 •. The inserts, with permeability values of 1.5×10 −9 and 5.4×10 −9 m 2 , attenuate respectively up to 5 and 9.5 dB at 0 • and up to 4 and 7.5 dB at 4.8 • incidence. The noise abatement of inserts with straight passages is compared with that of inserts manufactured using metallic foams with a random pore distribution but similar permeability. It is found that to achieve similar overall noise attenuation levels, the perforated inserts require at least 3 times higher permeability than the metal foam inserts. From this we conclude that in order to maximize the noise attenuation potential of permeable inserts, the inner structure of the permeable trailing-edge insert must be considered.