ON IMPROVEMENT OF THE IMPEDANCE EDUCATION ACCURACE BY ACCOUNT OF IMPEDANCE VARIABILITY ALONG THE ACOUSTIC LINER

An impedance eduction method considering impedance variability along the acoustic liner is proposed. Impedance eduction is based on minimization of the residual functional, which is the discrepancy between the experimental and computed values of acoustic pressure at the interferometer microphones. The computed values are obtained by solving the Helmholtz equation, convected Helmholtz equation and linearized Navier-Stokes equations for a two-dimensional duct with an impedance wall and flow based on finite-element method. The cases of constant and variable impedance along the liner sample are considered. It is established that when taking into account the variable impedance, the residual functional decreases more than twofold against the case of constant impedance.