一様な流れのあるフローダクト内の音響モード

抄録

Eduction of acoustic impedance of a sound absorbing material in grazing flow is often performed by using a flow duct. To understand the sound filed in the flow duct is effectual for more precise eduction. We investigated how the sound field in the flow duct whose top wall consists of a sound absorbing material with constant acoustic impedance changes with Mach number of the uniform flow. For various acoustic impedance values, propagation constants in the axial direction corresponding to 0th and 1st modes were calculated from the eigenvalue equation for a fixed value of the ratio of the flow duct height to the wave length (=1.21/2π). The value was determined to satisfy the cut off condition. The propagation constant represents the attenuation coefficient and the phase change rate in the axial direction. The following were obtained. The attenuation coefficient in the 0th mode decreases as the Mach number increases when the reactance is negative. This tendency is apparent when the resistance is small. In the 1st mode, the attenuation coefficient decreases in most calculated cases. As the Mach number increases, the phase in the axial direction changes more rapidly when the reactance is negative, and it changes more slowly when the reactance is positive. As for the case in which the attenuation coefficients of the 0th and 1st modes are comparative, the attenuation coefficient in the 0th mode decreases with the increase in the Mach number, but that in the 1st mode increases. Consequently, the 0th mode becomes dominant.