An investigation of the suitability of an alternative boundary condition for an impedance eduction technique

Acoustic lining treatments are universally employed on modern aero engines to suppress the excessive noise to meet the restrictive noise regulations and also to improve the ride comfort of passengers. In the design of an acoustic liner for realistic engine noise problems, it is important to establish a link of its geometry parameters (porosities, hole diameters, etc.) to its acoustic property (acoustic impedance) so that the noise control performance can be predicted and optimized with reasonable fidelity. An appealing approach to achieving this goal is the impedance eduction technique in which an experimentally designed environment is made to approximate as realistic as possible the environment to which the liner is exposed. The impedance value that leads to the best fit between prediction and measurement results is reckoned to be the liner acoustic impedance. However, recent experimental evidence shows that the acoustic impedance educed from a liner test-rig with the acoustic excitation exerted upstream is different from that of the same liner subject to a downstream acoustic excitation. Such a phenomenon occurs when the flow is present in the duct. The Ingard-Myers boundary condition, which assumes a vanishingly thin boundary layer, is one of the causes for this discrepancy. To overcome the ambiguity issue encountered in the eduction technique, the suitability of an alternative boundary condition, which accounts for a thin but finite-thickness sheared boundary layer over the liner, is examined. The boundary condition is derived from the Pridmore-Brown equation that gives rise to an effective impedance to be enforced at the edge of a uniform mean flow in the core part of the duct. Then standard modeling techniques such as the mode-matching method can be used to model the acoustic propagation through the duct. The developed sound propagation model is integrated to an impedance eduction technique to obtain the liner impedance using a flow duct facility and the result is compared with that based on the Ingard-Myers condition.