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

A novel method for measuring liner characteristics in situ in a flow duct is proposed with the immediate aim of obtaining the impedance boundary condition for use in time-domain acoustics. This method as an integrated system in the present demonstration employs an accurate computational aeroacoustics model for prediction of the transient response 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 physical setup of a partially lined duct in a wind tunnel; and an optimization method for parametric identification of the embedded liner. Based on the assumption that impedance can be characterized by a sum of damped harmonic oscillators, the characterizing parameters of the liner are inferred by minimizing the differences between the model-predicted and the in situ measured pressure responses at strategically and conveniently placed sensor locations and are immediate applicable as causal broadband timedomain impedance-equivalent boundary conditions for the computational aeroacoustic model. The use of acoustic impulses of narrow space-time extents affords this method an easy resolve of the influence of neighboring but irrelevant boundaries such as the duct exit. The reliability, effectiveness, and potential benefits of this method are studied and demonstrated. This method is expected to provide broadband inferences and insights into the mean flow effect on liners in a wide range of practical applications with the present setup. It is believed that the feasibility demonstrated here opens a new dimension for theoretical, experimental, and numerical studies in the time domain.