Implicit Large-Eddy Simulation on Acoustic Liner with Combined Acoustic Forcing and Flow Convection Effects

This study investigates the vortex dynamics of an acoustic liner under the combined effects of external acoustic forcing (reaching 150 dB) and high-speed flow convection. The geometrical structure comprises eight modular units, integrating a cavity and a millimeter-level slit, mounted within a straight tube. To resolve the fully coupled flow–acoustic fields, we employed two-dimensional implicit large-eddy simulation (iLES), with the compressible Navier–Stokes equations discretized via the spectral/hp element method. The computational framework incorporated spectral vanishing viscosity for implicit high-wavenumber filtering and a de-aliasing technique for numerical stability. The simulated results achieved good agreement with the experimental results (Tam et al., 2014, J. Sound Vib., 333:13). Beyond the general performance of overall noise reduction, a counterintuitive localized noise amplification appears just after each slit, introducing periodic oscillation of the acoustic wave along the liner surface. Subsequently, vortex dynamics around the microslit are clearly resolved and identified as an antiphase interaction between cavity-directed vortex shedding and a separation bubble driven by upwind flow. The final cavity-by-cavity analysis of acoustic trends and coherent vortices shows that vortex shedding beneath the slit primarily drives noise reduction, while separation bubble formation above the slit constitutes a secondary noise source. These findings provide theoretical guidance for optimizing acoustic liner noise-reduction designs.