Effect of source direction on liner impedance eduction with consideration of shear flow

Acoustic lining treatments are widely used in modern aircraft engines for noise reduction. The key to the successful application of this technique is to establish a link between the geometry parameters (thickness, hole diameters, etc.) and the acoustic impedance. To achieve this goal, the impedance eduction technique is widely used by the aircraft engine noise research community. The technique iteratively finds an impedance value that leads to the best fit between the predicted and experimental results. More recently, comparative studies have shown that, in the presence of flow, the acoustic impedance educed from a liner test-rig with an upstream acoustic excitation is different from that of the same liner subject to a downstream acoustic excitation. This discrepancy is surmised to be attributed to the Ingard-Myers boundary condition. The current work attempts to provide more information about the influence of boundary layer effect on the educed impedance when the acoustic source is exerted either upstream or downstream. A duct acoustic model for a non-uniform flow with a thin but finite-thickness sheared boundary layer over the liner is developed for impedance eduction. An alternative boundary condition is derived from the Pridmore-Brown equation using the perturbation method that gives rise to an effective impedance to be enforced at the interface between the sheared boundary layer and core flow region carrying a uniform mean flow. Then, a modified impedance eduction technique procedure is developed and used to obtain the liner impedance using a flow duct facility and the result is compared with that based on the Ingard-Myers condition. Results show that considering the sheared flow effect in the impedance eduction makes no significant improvement to the educed impedance for liners used in the experiment. (C) 2021 Elsevier Ltd. All rights reserved.