A numerical investigation on sound absorption mechanism of micro resonator with offset slits

Acoustic liners are widely used in commercial aero-engine to suppress noise. In theoretical investigations, the liner geometry is often assumed as an array of symmetric micro resonator with orifice or slit at the center. However, in real application, orifices or slits distributed in micro resonator are offset. For better understanding of sound absorption mechanism of micro resonator with offset slits under high incident sound pressure level (SPL), direct numerical simulations (DNS) using high order low dispersion and low dissipation computational aeroacoustics (CAA) method are carried out. The simulations are first validated by experimental data, showing good agreement and establishing the relevance of the simulation methodology. Numerical simulations of resonators with single offset slit or two slits are then conducted. The two sound absorption mechanisms, namely viscous dissipation and vortex shedding, are discussed with detailed numerical data and analysis, which lead to quantitative parametric description of the energy partition between the two mechanisms as a function of both frequency and geometry. It is shown that offset slit can reduce vortex shedding and results in less sound absorption. The effects of more than one slit are, however, opposite; more vortex shedding occurs with more slits so that sound absorption is enhanced. This may potentially help guide liner design in practical applications.