Impedance eduction experiments covering higher frequencies based on the multimodal straightforward method

Abstract

The multimodal straightforward method is developed further, validated and applied experimentally. The trust-region algorithm is introduced to reduce the eduction time, a flow duct setup with a cross section of 51 × 51 mm2 is built for the impedance eduction, and several single- and multi-layer perforated liners are tested from 0.5 kHz up to 6.0 kHz at two average Mach numbers under several types of modal sound sources. It is validated that, this method, by utilizing a diagonal microphone array and improving Prony’s algorithm, enables the automatic match between axial and transverse wavenumbers, and thus realizes the full decomposition to three-dimension multimodal sound fields. By introducing the trust-region algorithm, it can educe the impedance with a good consistency and a higher time efficiency. Generally, this method is applicable to the practical impedance eduction in multimodal aeroacoustic environments on a real flow duct, with the upper frequency extended from 3.0 to 6.0 kHz covering the main frequency range of aeroengine fan noises. For multilayer liners, the educed impedance spectrum is considerably flatter than the prediction by an existing impedance model in some cases especially near the antiresonance, and the grazing flow magnifies such a flatting trend accompanied by more evident predicting deviations. The spectra of educed impedance and transmission loss show that the multilayer liner possesses more resonant frequencies within the concerned range and broadband absorption potentials if well designed.