Large eddy simulation of conventional and bias flow acoustic liners

This study deals with the acoustic behavior of acoustic liners and the flow field around the perforated plate by solving numerically the compressible Navier-Stokes equations. The difficulty of visualizing the flow around the small holes of the plate makes the numerical simulations very attractive in order to well understand the acoustic behavior of the liner. The chosen liner was previously studied by Tam et al. [1] experimentally and using the Direct Numerical Simulation for the case of a conventional acoustic liner under normal sound wave incidence. The results obtained by Tam et al. [1] serve for the validation of the results obtained in the present research. Moreover, in this study, the focus is on the numerical simulation of the influence of bias flow on the absorption performance of a slit liner using the large eddy simulation. Two different methods are used to calculate the absorption coefficient in the computational aeroacoustics (CAA) simulation of the resonator. The first method is the transfer function method, and it simulates the impedance acoustic tube used in the experiment. The second method is the viscous dissipation method, and it calculates the power at which the acoustic energy is converted in viscous dissipation. The viscous dissipation method gives good agreement for the calculated absorption coefficients at the sound source pressure level of 150 dB. Shed vortices are developed in this case, and are considered to be the main cause of sound dissipation. The transfer function and the viscous dissipation methods used to calculate the absorption coefficients give good results for frequencies higher and equal to 2 kHz when the sound source pressure level is set to 130 dB. Only at the frequencies of 1 kHz, the obtained values are different from the values obtained by Tam et al. [1]. Shed vortices are not observed when the resonator is under only normal sound wave incidence of 130 dB. The introduction of a bias flow passing through the aperture of the resonator is investigated. The acoustic performance of the liner in this case and a comparison of the flow behavior at the aperture with a conventional liner is highlighted. When the sound pressure level is equal to 130 dB, an increase in the absorption is obtained for frequencies above the resonance frequency when the bias flow passing through the aperture is introduced. On the other hand, the absorption is reduced for the resonance frequency. However, when the SPL is equal to 150 dB, the absorption coefficient is higher near the resonance frequency, while for higher frequencies the absorption coefficient is lower.