Acoustic liners for jet-installation noise reduction

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.