On the mechanisms of jet-installation noise reduction with flow-permeable trailing edges

Abstract

This paper investigates the mechanisms by which flow-permeable materials provide noise reduction in an installed jet configuration. Numerical simulations are carried out with a flat plate placed in the near field of a single-stream subsonic jet (M_a=0.3 and M_a=0.5). The trailing-edge region of the plate is replaced by three different permeable structures, with different properties: a metal foam, a perforated plate and a diamond-shaped structure. Due to its complex geometry, the metal foam is modeled with an equivalent region governed by Darcy's law. The perforated plate has the highest resistivity among all investigated configurations, whereas the metal foam and diamond inserts have similar properties. Far-field spectra show significant noise reduction when the solid trailing edge is replaced with the permeable materials, with a maximum decrease of 12 dB at St=0.12, for M_a=0.5. Beamforming results show that the dominant acoustic source is located at the solid–permeable junction for the metal foam and diamond structures, whereas the perforated one has the source positioned near the trailing edge, similarly to the solid case. A breakdown of the far-field noise generated by the plate is also performed, where the contributions from the solid and permeable sections are computed separately. The former has distinct regions in the noise spectrum, which are dominated either by surface pressure fluctuations or trailing-edge scattering. However, for the permeable region, the results point to a significant mitigation of the noise due to scattering, which is no longer the dominant mechanism in any frequency range. This is confirmed by lower values of spanwise coherence, computed from the surface pressure, for the permeable trailing edge, compared to the solid case. Therefore, the clear dominant mechanism in the permeable region of the plate is the unsteady loading to pressure wave impingement. This is verified for all investigated configurations so that even with a low permeability structure, significant noise reduction can still be achieved.