Numerical-experimental assessment of 3D printed labyrinthine duct liners under grazing incidence: From nominal design to real performance

This study presents a comprehensive experimental and numerical investigation of 3D-printed, locally reacting acoustic liners that embed coiled quarter-wavelength resonators behind a multi-perforated plate. The liners are tested under grazing incidence in an impedance tube. Numerical benchmarking is first carried out to identify a modelling strategy that achieves an optimal trade-off between physical fidelity and computational cost. The validated strategy is then used to compare three liner designs against impedance tube measurements under grazing incidence. A reference configuration is subsequently analysed in detail to investigate the physical mechanisms governing the multiresonant response, and to investigate the manufacturing effects by comparing different 3D-printing technologies, materials, and wall thicknesses. The results indicate that resonance frequencies are primarily controlled by the geometry of the unit cell, according to theory, whereas transmission loss (TL) amplitude and bandwidth may be influenced by specimen dependent non-ideal effects associated with manufacturing and experimental realization. Finally, water related effects are assessed experimentally through two controlled scenarios: water intrusion into the labyrinth and a surrogate partial occlusion of the perforations aimed at reproducing a distributed reduction of effective open area. Both perturbations substantially alter the acoustic behaviour, leading to resonance shifts and performance degradation.