On the Use of Experimental Methods to Improve Confidence in Educed Impedance

Abstract

Results from impedance eduction methods developed by NASA Langley Research Center are used throughout the acoustic liner community. In spite of recent enhancements, occasional anomalies persist with these methods, generally at frequencies where the liner produces minimal attenuation. This investigation demonstrates an experimental approach to educe impedance with increased confidence over a desired frequency range, by combining results from successive tests with different cavity depths. A series of tests is conducted with three wire-mesh facesheets, for which the results should be weakly dependent on source sound pressure level and mean grazing flow speed. First, a raylometer is used to measure the DC flow resistance of each facesheet. These facesheets are then mounted onto a frame and a normal incidence tube is used to determine their respective acoustic impedance spectra. A comparison of the acoustic resistance component with the DC flow resistance for each facesheet is used to validate the measurement process. Next, each facesheet is successively mounted onto three frames with different cavity depths, and a grazing flow impedance tube is used to educe their respective acoustic impedance spectra with and without mean flow. The no-flow results are compared with those measured in the normal incidence tube to validate the impedance eduction method. Since the anti-resonance frequency varies with cavity depth, each sample provides robust results over a different frequency range. Hence, a combination of results can be used to determine the facesheet acoustic resistance. When combined with the acoustic reactance, observed to be weakly dependent on the source sound pressure level and grazing flow Mach number, the acoustic impedance can be educed with increased confidence. Representative results of these tests are discussed, and the complete database is available in electronic format upon request. Note: An e iωt time convention is used throughout this paper, and all impedances are normalized by ρ 0 c 0 , the characteristic impedance of air.

Numerical investigation of a honeycomb liner grazed by laminar and turbulent boundary layers

“…The grazing flow Mach number can be controlled up to Mach 0.6. Additional details on the GFIT and its data acquisition system are available in Jones & Watson (2011). M. G. Jones & W. R. Watson (2012, private communication) provided a baseline experimental dataset based on a Mach 0.5 grazing flow over a single degree-of-freedom conventional liner (liner details will be presented later) that included Pitot profiles at locations away from the test liner and impedance values taken under single frequency, 130 dB conditions.…”

Uncertainty Analysis of the Grazing Flow Impedance Tube

“…These flow results are not predicted well by most impedance models, and are believed to be due to limitations in the impedance eduction process caused by the assumption of uniform flow. 9 This is expected to be the focus of a future investigation. Nevertheless, the current results demonstrate that the current impedance eduction process is repeatable, as indicated by the small confidence intervals.…”

On a stabilization of the Ingard-Myers impedance boundary condition and its time domain implementation

“…In this section, we show an application of the proposed TIMIBC to a duct acoustics problem. The numerical results will be compared with the NASA Langley Grazing Flow Impedance Tube (GFIT) experimental dataset , 23 which is commonly used as a benchmark for testing the accuracy of an impedance boundary condition implementation. A diagram of the computational domain, modeled after the GFIT test rig, is shown in Figure 10.…”