Validation of an Analytical Model for the Acoustic Impedance Eduction of Multi-Cavity Resonant Liners by a High-Fidelity LES Approach

The massive growth of the air traffic during the last years is leading to stricter limitations on the noise emission levels radiated from aircraft engines. To face this issue, the installation of acoustic liners on the intake duct and the exhaust nozzles is a common strategy adopted to properly abate noise emissions. In this context, the aim of the present paper is to use high-fidelity LES simulations to validate a MDOF (multi-degree of freedom) extension of the SDOF (single-degree of freedom) analytical model provided by Hersh for impedance eduction of liners. Firstly, the results of the original Hersh model are compared with LES calculations performed with the OpenFOAM suite on a single-orifice and single-cavity layout. Then the extension of the Hersh model to MDOF geometries by using a recursive formulation is presented. Finally, high fidelity simulations are carried out for SDOF and MDOF configurations to validate the method extension. The excellent agreement between the high-fidelity results and the analytical predictions for the single-cavity pattern confirms the validity of the Hersh model for preliminary designs. The model extension to MDOF configurations enables the designers to broaden the design space, and to perform sensitivity studies to the multi-cavity geometrical parameters. A multi-cavity configuration makes the liner element resonate at different frequencies leading to multiple absorption peaks in the audible range. Doing so, the acoustic performance of the liner is extended to a wider frequency range, overcoming the limitations of a traditional SDOF configuration.