科研之友
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摘要
In the study of in-duct aero-acoustic phenomena two-port analysis based upon measurements has become an important method for the plane wave region. However even at moderate Mach numbers (0.2-0.3) the errors in the results can be hard to suppress. Ways of dealing with this include the use of over-determination methods and methods of obtaining more accurate wavenumbers. Most of the previous published work on this subject deals with the passive (scattering) part of the two-port. In this paper, both the passive and active (sound generating) parts of the two-port are addressed, and different methods for the determination of the source data and the scattering matrix are evaluated. For the source data in the form of a cross spectrum matrix an over-determination method is introduced. Additionally, a method of obtaining the mean Mach number from experimentally determined wavenumbers is described. In order to evaluate the methods measurements are conducted at a mean Mach number of 0.2 for two test cases an empty duct and a mixer plate inside the duct. The main improvements in the scattering matrix results are achieved by discarding measurements from an abundant set, based upon the measured coherence between excitation and fluctuating pressures. For the source part, it is shown that the error in the magnitude of the source cross spectrum matrix can be significantly suppressed by having additional (> 2) reference microphones on each side of the two-port. The mean Mach number obtained from an experimentally determined wavenumber yields more accurate scattering matrix results in both phase and magnitude, than those based upon flow velocity measurements at one point and an assumed flow profile.
- 出版日期2011-12-19
