Broadband Low-frequency Sound Attenuation by Composite Meta-liner under Grazing Flow

For modern aviation turbofan engines, a high bypass ratio is a design trend. The efficient attenuation of broadband noise resulted from high bypass ratio remains challenging for conventional acoustic liners. Here, we propose a composite meta-liner, in which a coiled-up cavity coupling array with nonuniform cross-sections provides excellent low-frequency acoustic attenuation performance, while perforated plates and porous materials, covering the upper surface of the coupling array, effectively consume the mid- and high-frequency noise. Specifically, the meta-liner can efficiently attenuate sound energy within a frequency range from 400Hz to 4000Hz under a wide range of grazing flow Mach numbers from 0 to 0.26, but its thickness is only 60.5mm (< λ/14 at 400Hz). The characteristics of the meta-liner are investigated theoretically and numerically. This work provides an avenue for designing new kind of compact acoustic liners.