Direct impedance eduction of acoustic liners in multimodal ducts with shear flows

• An impedance eduction method for multimodal sound fields and sheared mean flow. • An improved post-processing procedure for educing impedance from wavenumbers. • An assessment of different liner configurations for impedance eduction. • A new microphone array to eliminate spurious mode effects. • The influence of different incident modes on the educed impedance. Impedance eduction of acoustic liners with flow is usually performed under the assumption of a uniform mean flow and in the presence of an incident plane wave. However, recent studies have demonstrated that shear flow significantly influences the propagation of sound waves and their interaction with the liner. Consequently, it is crucial to characterize acoustic treatments in flow and acoustic conditions that closely resemble their target application, such as aircraft nacelles. This paper investigates and validates experimentally a direct method of impedance eduction in multimodal sound fields with shear flows on the MAINE Flow facility, which was validated only through simulations. Besides the double-liner configuration proposed to enhance the eduction robustness, additional developments are implemented, including a dominant mode selection algorithm to address the complexity introduced by multimodal fields and a 2-line microphone array design to avoid spurious mode effects. Different high-order incident modes (in the direction normal to the liner) are also considered. Finally, the recommended setup, with two types of liner samples, are used to investigate the effects of flow and incident acoustic field on liner impedance. It is also observed that using antisymmetric high-order modes as incident field significantly enhances the robustness of the eduction process.