Equivalent Fluid Modelling of a Honeycomb Acoustic Liner using the Acoustic Porous Medium Formulation

Abstract

Over the past decades, numerous methods have been used to study acoustic liners through their impedance, of which numerical methods are of particular interest. Despite their advantages, numerical methods are bounded by certain drawbacks. One such drawback stems from the need to accurately discretize the liner geometry, which can lead to large simulation wall-clock time. Another limitation is caused by the absence of a stable and robust impedance boundary condition to account for the liner effects. With such limitations, there is a need to develop new methodologies which can effectively mimic the effects of an acoustic liner. A possible approach could be to model them as equivalent fluids. Thus, the primary objective of this project is to investigate the efficacy of the equivalent fluid approach to reproduce the liner effects. To satisfy this objective, the Acoustic Porous Medium (APM), which is an equivalent fluid formulation available in the flow-solver PowerFLOW® is employed in the current thesis. For reproducing the liner effects with an APM, two different methodologies are explored. In the first approach, the APM model parameters are derived through an inverse algorithm which uses a priori known impedance as an input and gives the corresponding model parameters as an output. In the second approach, the APM parameters are derived through the pressure drop values, measured across the liner facesheet under a steady laminar flow. With both these approaches, APM faces some limitations when compared with the reference results. The shortcoming is primarily cause by the inability of the APM formulation to correctly account for the mass reactance effects of the reference liner. Moreover, the results have also raised doubts over the use of the APM formulation for low input porosity values. Thus, some improvements are required in the current implementation of APM to improve its efficacy for reproducing the acoustic effects of liner used in this thesis.