Impedance Deduction Based on Insertion Loss Measurements of Liners under Grazing Flow Conditions

Abstract

This paper presents the results of insertion loss measurements and numerical impedance eduction of three dierent liner samples. An overview over the test rig and methodology is given and preprocessed results in terms of reection and transmission coecients as well as the energy dissipation are discussed. These coecients are calculated for discrete frequencies within the investigated frequency range. Subsequently, a numerical post processing is performed in the time domain and the educed impedance function for each sample and ow Mach number is presented. This post processing in the time domain uses an impedance model, which is based on the Extended Helmholtz Resonator with ve free parameters. The parameters of the model are tted via an optimization, which determines the whole frequency response by one optimization process. The comparison of measured and numerically evaluated energy coecients proves the usability of the tools for impedance evaluation under ow conditions. Finally the impedance results of the dierent samples are discussed, including a comparative study with Aermacchi data of the NLR ow tube and Aermacchi impedance tube experiments.