Time-domain in-situ characterization of acoustic liners in a flow duct via impulse response

An impulse method is proposed to measure in-situ liner characteristics in a flow duct with the immediate aim of obtaining the impedance boundary condition for use in time-domain acoustics. The proposed method is based on the assumption that impedance can be characterized by a sum of damped harmonic oscillators (DHO), which is advantageous for resolving the causality problem common in frequency-domain measurement techniques. The use of acoustic impulses of narrow space-time extents affords the proposed method an easy resolve of the influence of neighboring but irrelevant boundaries such as the duct exit, formidable in harmonic analyses where space-time equilibrium is assumed. The present method comprises an accurate CAA model for prediction of transient responses of a liner to a plane-wave source, an acoustic impulse generator capable of producing sharp, repeatable, millisecond-short sound pulses of desired waveforms, a partially lined duct in a wind-tunnel, and an optimization method for parametric identification of the embedded liner. The characterizing DHO parameters of the liner are inferred by minimizing the differences between the predicted and measured pressure responses. The reliability, effectiveness and potential benefits of the proposed time-domain impedance characterization method are studied and demonstrated. This method is expected to provide broadband inferences and insights into the mean flow effect on liners in various practical applications.