Compact segmented meta-liners for enhanced acoustic absorption with grazing flow

This work demonstrates the inherent limitations of conventional acoustic liners in achieving efficient low-frequency absorption in one-dimensional transmission problems with grazing flow. We show that absorption bounds generally exist when the acoustic treatment is modeled by a uniform impedance. Additionally, flow-induced non-reciprocity makes the design of absorbers more challenging for incident waves propagating with the flow compared to those propagating against the flow. To address these challenges, we propose a segmented meta-liner and a corresponding design methodology. The meta-liner consists of perforated faceplates backed with Helmholtz resonators, which incorporate embedded tilted necks. Wiremesh is placed at the neck openings to introduce additional acoustic losses. Our design methodology combines a numerical model with experimental impedance data. This method avoids errors introduced by theoretical or empirical impedance models and simplifies geometric design for practical implementation, thereby providing robust solutions for sound absorption under grazing flow. Experimental results confirm the deep subwavelength absorption of 3D-printed samples. All designs surpass the absorption limits of uniform impedance boundaries, a common assumption in conventional liner design. Furthermore, experimental results indicate that these designs exhibit robust absorption across low flow Mach numbers between 0 and 0.2.