Research progress on acoustic impedance eduction technology of aeronautical liner

This paper provides an overview of the impedance eduction techniques of liners in aeroacoustic environments, including the basic principles, experimental arrangements, research progress and features of various methods. In low-frequency plane-wave fields, the impedance tube method, the single mode method, the current mainstream Objective Function Method (OFM) and Straightforward Method (SFM) are available, the latter of which is recognized for high efficiency and accuracy. But all these methods are confined to low-frequency ranges for the modal component limitation. In high-frequency multimodal fields, the in-situ method, the three-dimensional (3D) OFM, the Multimodal SFM (M-SFM), the quasi-3D SFM and the Mirror-based Multimodal SFM (MM-SFM) are developed. All of them address the modal limitation and thus can extend the upper frequency limit to different degrees. The in-situ method suffers from invasive tests resulting in significant errors and limitations. The 3D-OFM suffers from an iteration of 3D numerical simulations resulting in extremely low efficiency. The M-SFM suffers from small transverse spacing between adjoint probes so that its accuracy is not enough. The quasi-3D-SFM suffers from large demand for measuring probes depending on the highest-order mode. The MM-SFM can effectively solve the above problems. It adopts a zigzag array and unfolds it into a mapping diagonal array in a multiple-width equivalent sound field, based on the field periodicity and the mirror symmetry, and then invokes the multimodal Prony algorithm of the M-SFM to decompose the field, so as to educe the acoustic impedance finally. This method breaks the previous upper frequency limit of 3 kHz and can cover the main frequency range of 0.2−10 kHz for aeroengine fan noises; takes only 1 minute to conduct a full-frequency eduction which is at least 3 orders of magnitude more efficient than the 3D-OFM; is of high accuracy and reliability; and decreases the probe number significantly. Generally, it possesses advantages of broadband, efficient, accurate, fewer-probes and non-invasive tests.