Microfluid Dynamics and Acoustics of Resonant Liners

It is known that most of the acoustic dissipation associated with a resonant liner takes place around the openings of the resonators. However, because the openings are physically very small, there has not been any direct experimental observation of the flow and acoustic fields in this region. As a result, current understanding of liner dissipation mechanisms are either completely 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 inside 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 simulations. Numerical experiments reveal that at low sound intensity, acoustic dissipation comes mainly from viscous losses in the jetlike unsteady laminar boundary layers adjacent to the walls of the resonator opening. At high sound intensity, dissipation is due to the shedding of microvortices at the mouth of the resonator. The energy dissipation rate associated with the shedding of microvortices is found to be very high. Results of a parametric study of this phenomenon are reported.