Effects of Acoustic Liner Length on the Educed Impedance

The efficacy of impedance eduction in a grazing flow environment may be affected by the degree of sound attenuation achieved during testing. The SPL attenuation achieved depends on the test conditions, liner performance, and axial length of the sample. Previous studies have shown that an attenuation of at least 3 dB is required for accurate impedance eduction. This investigation considers liner samples of varying axial lengths to validate these previous findings and determine a suitable sample length that minimizes manufacturing costs while maintaining eduction accuracy. For nonlinear designs, changing the liner length will change the average SPL across the liner, resulting in length-dependent impedance values. This effect can be dealt with by splitting the acoustic liner into segments for the eduction process, such that the impedance variability due to changing SPL between the respective segments is determined. Three additively-manufactured liner samples are tested in the NASA Langley Grazing Flow Impedance Tube at various flow speeds (Mach ≤ 0.30) and SPL values (≤ 150 dB). These three samples vary in sensitivity to SPL. Impedance results are compared to identify differences solely due to liner length. The results confirm that for accurate impedance results, at least 3 dB of attenuation is recommended. For nonlinear liners, differences in average SPL for various liner lengths influence the impedance; however, this effect is minimized when grazing flow is present. This is confirmed by results of the multizone analysis, where educed impedance in the upstream zone matches an equivalent length standalone liner, while the downstream zone has a different educed impedance.