科研之友
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摘要
A new impedance eduction method is presented and validated against a benchmark method, and the effects of measurement uncertainty errors on the impedances educed with this new method are assessed. Unique features of the new method include the following: 1) the upstream and downstream boundary conditions contain higher-order duct modes, 2) the impedance spectra of unique nonuniform test liners on opposite walls may be educed simultaneously, and 3) the measured data for the impedance eduction are acquired only at the source and duct termination planes. The validation exercise is performed with a rigid-wall insert and a conventional liner over a range of frequencies and flow Mach numbers in the NASA Langley Research Center's grazing flow impedance tube. The primary conclusions of the study are that the impedance spectra of the rigid-wall insert and the conventional liner that were educed from the new method are in very good agreement with those that were educed by using the benchmark method. However, the effects of measurement uncertainty on the educed impedance are greater at the lower frequencies and the higher Mach numbers for the new method. All indications are that this occurs because the new method 1) uses significantly less data to perform the impedance eduction than the benchmark and 2) is currently based on a rather crude approximation to the measured pressure gradient, which is more sensitive to the refractive effects of the boundary layer than the measured lower-wall pressure that is required in the benchmark method.
- 出版日期2011-10
