A Helmholtz resonator with a curved cavity is studied for reducing jet-installation noise in a configuration comprised by a subsonic jet and a nearby flat plate. The face-sheet and cavity are designed with the Guess method and the impedance is verified through an experiment and a simulation of an impedance tube. Both single and double degree-of-freedom liners, the latter with a perforated septum inside the cavity, are studied. A good agreement is obtained for the impedance curves from the design method and the experimental and numerical impedance tube, particularly at frequencies close to the absorption peak. Numerical simulations of the installed jet are performed with an array of resonators placed inside the plate with the face-sheet on the lower side, targeting noise reduction at a ground observer. Far-field spectra show that noise reduction in the order of 7 dB is obtained with respect to the baseline solid plate, at the resonance frequency of the single degree-of-freedom liner. Moreover, there is a broad frequency range around the resonance in which the sound pressure levels of the lined plate are lower. For the configuration with the double degree-of-freedom resonator, further noise reduction (approximately 3 dB) is obtained at higher frequencies, around the second peak in the absorption coefficient curve. The results also show that a slight noise reduction (2 - 3 dB) occurs for an observer on the shielded side of the plate, but in a significantly narrower band around the resonance frequency. This is attributed to the absorption of acoustic waves from the jet itself prior to their scattering at the trailing edge, coupled with a less abrupt impedance discontinuity at the trailing edge. Consequently, the resonators also act by reducing the strength of the acoustic source at the designed resonance frequency.

Porous materials have been widely investigated as a mean for noise reduction. Numerical simulations can be used to investigate the physical mechanisms responsible for noise reduction; however, a correct modeling of the porous medium through an equivalent fluid model is essential to minimize the computational costs. This paper reports a detailed review of a few applications of the equivalent fluid model based on a three-layer approach, a method that is particularly useful to account for the variation of porous material thickness in aerospace applications. The multilayer approach has been applied in three relevant aerodynamic noise issues: leading-edge impingement noise, turbulent boundary-layer trailing-edge noise, and jet installation noise. Comparison with experiments is used to validate the simulation approach.

This paper investigates the application of flow-permeable materials as a solution for reducing 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 characterized through planar PIV measurements at three different velocities (M_a=0.3, M_a=0.5 and M_a=0.8, where M_a is the acoustic Mach number), whereas the acoustic far-field is measured with a microphone arc-array. Acoustic measurements confirm that installation effects 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 trailing edge with the metal foam, noise abatement of up to 9 dB is achieved at the spectral peak for M_a=0.3 and a polar angle θ=40^∘, with an overall reduction in the entire frequency 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 M_a=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 pressure 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 M_a=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.