論文ID: 19-00042
A conventional method for acoustic impedance eduction in a flow duct is proposed. This method is based on the straight forward method. Assuming that a single mode propagates in the duct and the flow is uniform, propagation constants for both forward and retreat waves are explicitly calculated from three transfer functions between four microphones aligned in a line parallel to the flow duct. Two values of acoustic impedance are separately obtained from either of the propagation constants. The test section of our flow duct has a square cross section whose three walls are rigid and the rest wall is the acoustic impedance wall of interest. To verify the present method, we performed numerical simulations and experiments. First, the boundary element method was employed to obtain the sound pressure field in the flow duct without grazing flow; the target acoustic impedance was prescribed. The acoustic impedance is successfully reproduced by the present method from the sound pressures at virtual four microphones even when the retreat wave is apparent. Then, the acoustic impedance of a perforated plate without grazing flow was experimentally educed. The value agrees with the value measured by the 2-microphone-method in an impedance tube. Third, the acoustic impedance of the perforated plate with grazing flow of Mach number around 0.12 was educed. The resistance increases and both of the reactance and the rate of change in the reactance with frequency decrease from those values without grazing flow. In consequence, when the grazing flow exists, the maximum value of the sound absorption coefficient and its frequency increase, and the frequency range where the sound absorption coefficient shows higher value expands.
