The impact of quarter wave tube (QWT) arrangements in terms of axial location, circumferential distribution and number of cavities on damping of azimuthal modes is investigated both experimentally and numerically in an atmospheric annular test rig under isothermal conditions. Well-established measurement techniques are applied to characterize the damping potential for azimuthal modes of different quarter wave tube (QWT) arrangements. For additional insight into the considered configurations eigenfrequency studies are conducted using linearized Euler equations (LEE). Measured boundary conditions are used for inlet and outlet. The reflection coefficient of a single quarter wave tube (QWT) is measured on an impedance rig using longitudinal wave excitation. It is shown that this reflection coefficient can be used for the eigenfrequency analysis of the annular rig in which the QWT is mounted in radial direction.
The effects of different quarter wave tube configurations on the spatial mode shape of the first azimuthal mode and the corresponding change in modal dynamics are analyzed. This provides guidance for the circumferential and axial arrangement of damping devices to most effectively attenuate an annular acoustic system. It is illustrated that the complete acoustic system including the resonators has to be considered to properly dimension the acoustic characteristics of a damping device.