On the Role of the Flow Permeability of Metal Foams on Trailing Edge Noise Reduction

Abstract

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.

3D-printed Perforated Trailing Edges for Broadband Noise Abatement

“…Above the cross-over frequency, MF inserts generate higher noise than the baseline case, with larger increase being measured for the MF with higher d c . This contribution is generally attributed to the roughness of the material [20,41,42]. For the perforated trailing edges, similar features are reported: with the exception of the frequency band with f c = 630 Hz -where the l h = 1.5 mm insert generates a tonal noise-higher permeability produces larger maximum noise attenuation levels (5 and 9.5 dB for perforated inserts with l h = 3 and 1.5 mm respectively).…”

“…These two parameters are obtained by least-squares fitting of Eq. (1) to 20 pressure drop data, measured for Darcian velocities ranging between 0 and 1.1 m/s [19,30]. The porosity σ of the perforated inserts, defined as the ratio of empty-to-solid volume, is also computed as σ = πd 2 h /(2l 2 h ).…”

“…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]. However, the permeability of most of these materials is altered, hence their capability to abate noise [20], when using conventional machining processes to shape them.…”

Temperature-Activated Change of Permeable Material Properties for Low-Noise Trailing Edge Applications

“…From Equation (1) it follows that the seepage velocity which can be expected for the permeable materials used in this study is low enough to neglect the inertia term. In a previous campaign [16], the physical properties of the metal foam were specified as given in Table 1. Table 1.…”

“…They proposed air flow resistivity as a key parameter to control the low-frequency noise attenuation. More recently, Rubio Carpio 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.…”