Experimental characterization of the turbulent boundary layer over a porous trailing edge for noise abatement

Abstract

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.

3D-printed Perforated Trailing Edges for Broadband Noise Abatement

“…7 (b), where positive values refer to noise abatement with respect to the baseline case. As reported previously [15,39,40], metallic foams reduce noise only below a material-dependent cross-over f c . For the two metal foams used in the present study, namely d c = 450 µm and d c = 580 µm, the cross-over is respectively found at f c = 2.5 and 2 kHz approximately.…”

“…The application of permeable materials at the trailing edge also represents a promising TBL-TE noise abatement technique. Previous experiments report up to 11 dB noise reduction with respect to a reference (solid) case [14][15][16] for materials that establish flow communication between suction and pressure sides. This communication requirement could be fulfilled by applying homogeneous permeable materials with different micro-structures at the trailing edge: fiber felts [17], polyurethane foams [15], micro-perforated plates [18] or open-cell metal foams [19].…”

Sound quality metrics applied to aircraft components under operational conditions using a microphone array

“…The obtained source maps were integrated over different regions of integration (ROI) [50,51] to exclude the influence of other sources outside of these areas. This technique is analogous to the source power integration (SPI) [17,22,[50][51][52][53][54][55][56][57][58][59] but considering the formulation of functional beamforming [21,22]. This integration process provides the sound emissions for each source.…”

Self Noise Reduction and Aerodynamics of Airfoils with Porous Trailing Edges

“…There are different methods to reduce this noise by modifying the blade, which includes the use of flow permeable materials. Several studies on this passive modification exist, including experimental studies on brush-like or slitted trailing edge extensions [1][2][3], on fully porous airfoils [4,5] and on airfoils with trailing edges modified by metal mesh sheets [6] or porous foams [7][8][9]. In addition, the availability of 3D printing technologies enables the use of porous trailing edges manufactured via rapid prototyping [10].…”