Comparison of Impedance Eduction Test Rigs with Different Boundary-Layer Profiles

The experimental characterization of acoustic liners has garnered significant attention within the research community. Typically, such characterization involves measuring the acoustic impedance of liners using various techniques under conditions similar to those found in turbofan engines. This study aims to offer a comparative analysis of educed acoustic impedance for two liner samples, manufactured by the same provider and tested in the UFSC Impedance Test Rig and the NASA GFIT. Leveraging the distinct geometries of these test rigs, the investigation considers the impact of different grazing flow profiles in terms of the boundary-layer displacement thickness on the educed impedance. Notably, impedance measurements exhibit congruent outcomes in the absence of grazing flow but diverge in terms of resistance values when grazing flow is present, with the UFSC Impedance Test Rig consistently indicating higher values. The postprocessing algorithms of both institutions are uniform. In addition, a comparison involving a semiempirical predictive model has been conducted. The findings suggest that the observed disparities relate to the grazing flow boundary-layer profile. In summary, it is important to report both the mean flow Mach number and the displacement thickness to enable proper comparison between impedances educed from data acquired in different test rigs.