Experimental verification of a radial mode analysis technique using wall-flush mounted sensors

Sound fields in the inlet and outlet ducts of axial fans, compressors, and aircraft engines propagate as higher-order acoustical modes in a wide-frequency range. Decomposition of the sound field into azimuthal and radial modes permits direct conclusions on the sound generation processes and can lead to the identification of aerodynamic source areas. An established experimental method for assessing the sound field is to circumferentially traverse a radial microphone rake over 360 degrees, to measure the sound pressure in the duct at multiple circumferential and radial positions. On the inlet side of a turbo-machine this procedure cannot be employed because the wake of the rake would disturb the inlet flow conditions of the machine and alter its acoustic characteristics. Computer simulations have shown that the radial mode structure of the sound field can also be determined by measuring the sound pressure only at the inner duct wall, but at different axial locations. In this paper, experiments are described where the sound field in the outlet duct of an axial-flow fan is measured once with the conventional radial microphone rake and a second time with wall-flush mounted microphones. Subsequent radial mode analyses are carried out for both sets of measurement data to experimentally verify the new method.