Mitigating Grazing Flow Impedance Eduction Errors in Additively Manufactured Porous Liners

Additively manufactured porous metamaterials, including triply periodic minimal surface (TPMS)
cores, are promising candidates for aeroengine liners but often behave as extended-reacting
treatments under grazing flow. Standard impedance eduction procedures assume locally-reacting
behavior and can yield physically inconsistent results for such materials. This study evaluates
internal partitions (thin solid walls inserted at regular intervals along the liner’s streamwise extent)
as a means of suppressing internal streamwise propagation to improve eduction consistency. Six
TPMS configurations were tested in the NASA Langley Grazing Flow Impedance Tube (GFIT) at
partition spacings of 4′′, 2′′, and 1′′, as well as unpartitioned, and at Mach numbers up to 0.3.
For most geometries, 2′′ partition spacing eliminated impedance discontinuities and substantially
improved Prony correlation, though effectiveness varied with core topology. Partitions also altered
the acoustic response of the liners: resistance increased, resonance frequencies shifted downward,
and peak attenuation decreased while bandwidth broadened in some cases. Because partitions
modify the liner itself, the educed impedance characterizes the partitioned system rather than the
original unpartitioned core. These results establish partitioning as a practical pathway toward liners
that are more compatible with standard eduction methods, while demonstrating that the intervention
is itself an acoustic design change.