Nomenclature

The acoustic dissipation mechanisms of resonant liners are strongly nonlinear. Usually set in the inlet nacelles to reduce fan noise, perforated honeycomb sandwich panels are excited by grazing-incidence high sound pressure level (SPL) acoustic waves and air flows. The purpose of this paper is to study and quantify the acoustic absorption of a resonant liner, through direct numerical simulations (DNS) performed with the Onera CEDRE code. The liner is modeled by a cylindrical Helmholtz resonator, with a 0.8 mm diameter opening. Its response to acoustic waves is computed for different frequencies, SPL and incidences (normal and grazing). For the lowest SPL, the viscous wall friction and the acoustic radiation are identified as the “linear ” dissipation mechanisms; for the highest SPL, the additional “nonlinear ” vortex shedding mechanism is highlighted. Independently on the SPL, the flowfields around the resonator hole are found to be mostly axisymmetric (along the resonator axis). As for the usual acoustic properties, the impact of a rise of the SPL is not as obvious as anticipated: for the frequencies of low linear absorption, the absorption gets better when the SPL increases, but for the frequencies of high linear absorption, this evolution is reversed beyond a given threshold. In the latter case, the absorption spectrum of the liner is flattened but wider. A simplified aeroacoustics excitation is finally approached: a grazing low Mach number inviscid air flow is coupled with the acoustic waves. Several effects are pointed out, the main of which being the convection of the vortices downstream the resonator