Abstract
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 coming from the fan, the compressor, the turbine, and the jet. In this context, the aim of the present article is to use high-fidelity large Eddy simulation (LES) to validate a multi-degree-of-freedom (MDOF) extension of the single-degree-of-freedom (SDOF) and double-degree-of-freedom (DDOF) analytical model provided by Hersh for impedance eduction of acoustic liners. First, 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 (SDOF). Then the extension of the Hersh model to multicavity (MDOF) geometries by using a recursive formulation is presented. Finally, high-fidelity simulations are carried out for single-orifice and multicavity (MDOF) configurations to validate the method extension and to understand how resonant coupling and acoustic impedance are affected by multicavity resonant elements. The excellent agreement between the high-fidelity results and the analytical predictions for the single-cavity pattern confirms that the Hersh model is a useful formulation for a preliminary design of a SDOF acoustic liner. The model extension to MDOF configurations enables the designers to broaden the design space, and thus, a validated analytical method is strictly necessary to perform sensitivity studies to the multicavity geometrical parameters (i.e., facesheet thickness, cavities depth, porosity). Basically, a multicavity configuration makes the liner element resonate at different frequencies, leading to multiple absorption peaks in the audible range. In this way, the acoustic performance of the liner is extended to a wider frequency range, overcoming the limitations of a traditional SDOF configuration.
