Numerical study on sound absorption characteristics of a non-locally reacting liner

For acoustic liners used in commercial nacelles, due to the drainage slots which cut into the partition walls of closely packed cavities, the Helmholtz resonator liners are inevitably not simply locally reacting. Previous investigations confirm that the drainage slots introduce a non-locally reacting behavior which has a significant effect on the absorption spectrum comparing to the equivalent locally reacting liner design. How- ever, the resulting effects of these drainage slots are currently not taken into account during the acoustic liner design process. Therefore, a numerical investigation based on computational aeroacoustic methods (CAA) is carried out to investigate the influence of drainage slots on acoustic attenuation characteristics of liner. Different slot configurations are designed and simulated. Both 2-D and 3-D direct numerical simulations are carried out. Numerical results show that comparing with the locally reacting liner configuration the cell interaction causes an additional dissipation peak which is higher than the Helmholtz resonance and it increases with the size of the slot. Under high sound pressure level incoming waves, the basic characteristics of the spectrum remain. However, vortex shedding transpires at the resonance around both the slits and the drainage slot which generally increases the dissipation and makes the absorption procedure much more intricate. Vorticity contours indicate the connection of two coupled cavities decreases the strength of vortex shedding around the Helmholtz resonance and significantly increases the vorticity magnitude near the additional peak frequency. Finally, an empirical model to predict the extra attenuation peak frequency is derived based on the numerical results.