Validation of an inverse analytical technique to educe liner impedance with grazing flow

Abstract

This paper is based on a previously developed analytical technique to educe the acoustic impedance of a liner sample, placed inside a rectangular duct in grazing incidence. This technique uses the measurement of complex acoustic pressure at four positions inside the duct, upstream and downstream the lined section, and educing the result using the mode matching method. In the mode matching code, the flow was simplified to be uniform, which is not usually the actual case. This technique was opposed by several doubts regarding the mean flow assumption and the possible pressure discontinuity at the hard-soft and soft-hard steps. It was believed that the pressure field away from this discontinuity is representative of the real case, and the code would be capable to educe the correct liner impedance. In this paper, the code is validated against benchmark data from NASA. First the code was tested to be able to reproduce the pressure field for a given impedance. Second, it was tested to educe the correct impedance for a given pressure distribution. The results of the mode matching code were in excellent agreement with the experimental data. The effect of shear flow was investigated and found out that the differences are quite small for the chosen duct size and frequency of interest. On the other hand, the mode matching formulation was re-written based on the pressure instead of the velocity potential. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.