3D-printed Perforated Trailing Edges for Broadband Noise Abatement

Abstract

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.

An alternative permeable topology design space for trailing-edge noise attenuation

“…Distinctively, the 3D-printed perforated material with the Kevlar sheet is far different from this trend; for a relatively low value of r, the value of C is around the same order of the metal foam sample with d c ¼ 0.45 mm, which has the highest r. This means that the application of the Kevlar sheet on a highly permeable 3D-printed perforated topology creates a rather unconventional combination of r and C. The present topology of the 3D-printed perforated material sample with and without the Kevlar sheet give lower values of r than the lowest value of r found in the collection of 3Dprinted photopolymeric materials. In the literature, 28 tonal noise increase by the airfoil equipped with the 3D-printed TE insert with the lowest r has been reported, suggesting that there exists a threshold of r below which the tonal noise would occur. The location of this threshold lies somewhere between the case with the lowest r and the case next to it, where the tonal noise was no longer found.…”

“…This suggests that the ranking according to r may not be a good indicator of the ranking of noise attenuation when permeable materials with different topologies are considered altogether. 28 In the upcoming sections, dependency of noise attenuation magnitude on different material characteristics is further discussed to identify a better indicator for noise attenuation based on the permeable material characteristics.…”

“…13,15,18 The permeable materials are usually characterized by their resistivity (inversely proportional to the permeability) and form drag coefficient. 24–27 Since the form drag coefficient usually increases gradually with the increasing resistivity, 20,28 only the latter has mostly been sufficient to be related to the aeroacoustic characteristics. 13,15 Generally, it has been found that decreasing the resistivity, potentially by increasing the pore size of the porous materials, increases the noise attenuation.…”

“…More recently, simplified perforated topologies comprising of an array of straight channels linking the suction and the pressure sides of the airfoil have also been employed as another alternative to the porous (metal) foam materials. 17,28 The investigation by Herr et al. 18 discussed previously also featured a micro-perforated TE insert which could achieve a comparable noise attenuation of approximately 6 dB to more conventional porous topologies such as the porous aluminium.…”

“…Nevertheless, care has to be taken when reducing the resistivity of such a perforated structure as tonal noise may appear. Herr et al 18 and Rubio Carpio et al 28 have commonly reported tonal noise increase found in their highly permeable variants of the perforated structure. It has been hypothesized that the tonal noise is caused by vortex shedding across regions of regular perforation, 18,31 yet this issue has not further been addressed.…”