A Comparison of Impedance Eduction Test Rigs with Different Flow Profiles

The experimental characterization of acoustic liners applied for turbofan engines has been in the spotlight of the community for the last few decades. In general, such characterization is done by measurements of the liner acoustic impedance using different techniques in conditions as close as possible to those encountered in turbofan engines. Although a great amount of work has been published related to these techniques, few comparisons between different experimental setups using identical samples are available. The goal of the present study is to provide a comparison between educed acoustic impedances for two nominally identical liner samples in the UFSC Impedance Test Rig and the NASA Langley Research Center Grazing Flow Impedance Tube (GFIT). Due to the geometrical differences between the test rigs, it is possible to consider the effect of different grazing flow profiles on the educed impedance. Impedance measurements between the two facilities show similar results in absence of grazing flow, and different results when the grazing flow is present. Results are presented with both test rigs targeted to two different conditions: (i) same centerline Mach number and; (ii) same average Mach number. Both comparisons suggest a higher acoustic resistance obtained with the UFSC Impedance Test Rig. A comparison using semiempirical predictive models was also conducted. The results suggest that the main source for the observed difference is the grazing flow profile, represented by its boundary layer displacement thickness