Airfoil Trailing-Edge Noise Reduction by Application of Finlets

Abstract

This experimental study investigates the effect of bio-inspired finlets on the reduction of trailing edge noise of a NACA 0012 airfoil. At a chord-based Reynolds number of 400,000, the far-field noise measurements show that the finlets are capable of reducing the trailing edge noise effectively from 1,000 Hz to 4,000 Hz with up to 6 dB reduction, for effective angles of attack from zero to eight degree. By correlating the far-field noise to the boundary layer measurements, an optimal finlet height to boundary layer thickness ratio to achieve the highest noise reduction is discussed. Static pressure and dynamic pressure fluctuations are examined, particularly within the finlet-treated area, to capture the flow dynamics through the treatments.The unsteady surface pressure fluctuation spectra show strong reduction at frequencies higher than 1,000 Hz in the finlet wake, complying with the far-field measurements. Both the auto-and cross-correlation results of the unsteady surface pressure reveal the formation of large-scale turbulence at the entrance and toward the exit of the finlets. Moreover, they suggest that the formation and subsequent mixing of these large-scale structures with the boundary layer can be beneficial to reducing the unsteady surface pressure fluctuations. The findings will be useful to potential optimization of the finlets.

Finlet Rails for the Reduction of the Trailing-Edge Noise

“…of T1 and T2, and also those at which the noise is reduced. The conclusion that finlet rails reduce the spectrum of the wall-pressure fluctuations would be consistent with what found in previous experimental studies on finlet fences, from wall-pressure measurements with flush-mounted microphones, particularly in Gstrein et al [14,54]. In those measurements, the frequency range of noise reduction overlaps well with the frequency range of attenuation of the wall-pressure fluctuations, thus evidencing the prominent role played by the attenuation of the wall-pressure fluctuations in the aeroacoustic behavior.…”

“…1. Having the finlet rails directly on 3d-printed inserts avoids the use of a substrate to connect them to the airfoil surface, which had to be employed in the work of Clark et al (2017) [12] as well as in later experimental studies on finlet fences [14,28].…”

“…In the field of rotorcraft and urban air mobility, TBL-TE noise constitutes a significant contributor to the emitted broadband noise [4,5]. With the aim of attenuating the TBL-TE noise, several passive strategies were recently investigated, including trailing-edge brushes [6], trailing-edge serrations [7][8][9], porous materials [10,11], streamwise finlets [12][13][14], and shallow dimples [15]. Lee and co-authors [16] recently provided a comprehensive review on the problem of TBL-TE and on the mitigation strategies that have been proposed.…”

“…The treatment was observed to produce a ''channeling effect'' on the flow, which increases the energy content of the large scales and enhances the spectral density of the low-frequency wall-pressure fluctuations. Recently, Gstrein et al [14] applied finlet fences on a NACA 0012 airfoil and found the best noise reduction at mid-to high-frequencies, for an optimal ratio of 0.7 between the fences height and the boundary-layer thickness.…”

Trailing-edge noise reduction through finlet-induced turbulence

“…Indeed, Gstrein et al. (2022) earlier found that the most effective trailing-edge noise reduction on a NACA 0012 aerofoil by applying the finlet treatments upstream of the trailing edge was achieved with a finlet height of approximately of the boundary-layer thickness. This suggests that the finlets modify the boundary-layer characteristics upstream of the trailing edge, which eventually leads to the suppression of the trailing-edge noise.…”