Jet-installation noise reduction with flow-permeable materials

This paper investigates the application of flow-permeable materials as a solution for re- ducing jet-installation noise. Experiments are carried out with a flat plate placed in the near field of a single-stream subsonic jet. The flat plate is modular and the solid surface near the trailing edge can be replaced with different flow-permeable inserts, such as a metal foam and a perforated plate structure. The time-averaged jet flow field is character- ized through planar PIV measurements at three different velocities (Ma = 0.3, Ma = 0.5 and Ma = 0.8, where Ma is the acoustic Mach number), whereas the acoustic far-field is mea- sured with a microphone arc-array. Acoustic measurements confirm that installation ef- fects cause significant noise increase, up to 17 dB for the lowest jet velocity, particularly at low and mid frequencies (i.e. St < 0.7, with the Strouhal number based on the jet diameter and velocity), and mostly in the upstream direction of the jet. By replacing the solid trail- ing edge with the metal foam, noise abatement of up to 9 dB is achieved at the spectral peak for Ma = 0.3 and a polar angle θ = 40◦ , with an overall reduction in the entire fre- quency range where jet-installation noise is dominant. The perforated plate provides lower noise reduction than the metal foam (7 dB at the spectral peak for Ma = 0.3 and θ = 40◦ ), and it is less effective at low frequencies. This is related to the values of permeability and form coefficient of the materials, which are the major parameters controlling the pres- sure balance across the trailing edge and, consequently, the noise generated by the plate. However, despite having a high permeability, the plate with the metal-foam trailing edge still has a distinct noise production at mid frequencies (St ≈ 0.43 for Ma = 0.3). Based on the analyses of different treated surface lengths, it is conjectured that the solid-permeable junction in the plate acts as a new scattering region, and thus its position also affects the far-field noise, which is in line with analytical predictions in the literature. Nonetheless, both types of inserts provide significant noise reduction and are potential solutions for the problem of jet-installation noise