Accurate experimental two-port analysis of flow generated sound

• In the study of in-duct aero-acoustic phenomena two-port analysis based upon in-duct measurements has become an important method for the plane wave region. However already at moderate Mach numbers (0.2 - 0.3) the errors in the results can be hard to suppress. Ways of dealing with this includes the use of over-determination methods and methods of obtaining more accurate wave numbers. In this paper 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. In addition, a method of obtaining the mean Mach number from experimentally determined wave numbers 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 wave number is observed to yield more accurate scattering matrix results in both phase and magnitude, than that based upon flow velocity measurements at one point and an assumed flow profile.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

Nyckelord

Acoustics

Akustik

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