It is known that most of the acoustic dissipation associated with a resonan
t liner takes place around the openings of the resonators. However, because
the openings are physically very small, there has not been any direct expe
rimental observation of the flow and acoustic fields in this region. As a r
esult, current understanding of liner dissipation mechanisms are either com
pletely theoretical or are based on experiments using much larger physical
models. Inasmuch as large openings were used in these experiments, the true
Reynolds numbers were unfortunately not reproduced. The flow around and in
side a typical liner resonator under the excitation of an incident acoustic
field is investigated by direct numerical simulation (DNS). There are two
distinct advantages in using DNS. First, by the use of a carefully designed
grid, even very small-scale features of the flowfield can be resolved and
observed. Second, the correct Reynolds number can be imposed in the simulat
ions. Numerical experiments reveal that at low sound intensity, acoustic di
ssipation comes mainly from viscous losses in the jetlike unsteady laminar
boundary layers adjacent to the walls of the resonator opening. At high sou
nd intensity, dissipation is due to the shedding of microvortices at the mo
uth of the resonator. The energy dissipation rate associated with the shedd
ing of microvortices is found to be very high. Results of a parametric stud
y of this phenomenon are reported.