Investigation of Nonlinear Multi-Tone Excitation Interaction Effects for an IFAR 4 Liner

This paper is focused on analysis of the acoustic properties of one of the liners defined under the framework of the International Forum for Aviation Research (IFAR), challenge four. Challenge four had the topic of exploring the effects of high sound pressure amplitudes on the measured impedance of liners. The effect of different source fields including: broadband random, tonal and multi-tone excitation was to be studied. Originally four liner samples of varying geometric properties were defined by NASA and should be produced by additive manufacturing. Since the study focuses on high level but no flow conditions measurements can be made in normal incidence impedance tubes. The present paper is only concerned with one of the four liner designs defined for the challenge, since this liner has already been shown to exhibit stronger nonlinear effects compared to the other three, which makes it the most interesting for the purpose of the study. Less focus is also on benchmarking with results from the other partners participating but some comparisons is made with published results from NASA and DLR. Results is presented investigating nonlinear interaction effects mostly for multi-tone excitation conditions but also a combination of broadband random and tonal excitation.