Experimental Investigation on the Facesheet Perforation Geometry on SDOF Acoustic Liner Impedance

This work experimentally investigates the effects of facesheet perforation geometry on the acoustic impedance of Single-Degree-Of-Freedom (SDOF) liners. Four liner samples with the same nominal percentage of open area and cavity geometry were manufactured, differing only in the perforation shape: sharp-edged, outside chamfered, inside chamfered, and double chamfered, keeping the inner diameter constant. The actual perforation dimensions were assessed through optical characterization. The liner impedance was evaluated at the Liner Impedance Test Rig of the Federal University of Santa Catarina using two experimental approaches: inverse impedance eduction and the in-situ technique. Measurements were performed for centerline Mach numbers up to 0.50 and sound pressure levels of 130 and 145 dB over the frequency range from 500 to 3000 Hz. The results show that the liner reactance is only weakly affected by the perforation geometry for the conditions considered. In contrast, the resistance is sensitive to the perforation shape, specially under conditions in which nonlinear effects are relevant. In the absence of grazing flow, chamfered perforations mitigate the nonlinear resistance increase observed at high sound pressure levels. With grazing flow, the resistance becomes primarily dominated by flow-induced nonlinearities, and configurations with an outside chamfer tend to exhibit lower resistance than the sharp-edged configuration. Overall, the measurements indicate that perforation-edge geometry can affect the resistance of SDOF liners more significantly than the reactance within the range of conditions investigated.