Aeroacoustic characterization of micro-perforated liners under a low-speed turbulent boundary layer

Micro-perforated liners in contact with a grazing airflow are efficient solutions for the attenuation of sound waves in turbofan inlets, in exhaust silencers and in power plant duct systems. Most of the resonators are locally-reacting, e.g. backed by a cavity filled with a honeycomb core. However, bulk-reacting resonators are also promising solutions, especially to enhance the attenuation of axial modes in flow ducts. The present study presents costefficient aero-acoustic characterization of a number of micro-perforated liners, either locallyor bulk-reacting, set in a wind tunnel test section beneath a low-speed turbulent boundary layer. Their performance is compared in terms of attenuation of the acoustically-induced, but also flow-induced sound pressure levels without external noise source. A dual nosecone microphone system enables to estimate profiles of the axial and transverse intensity vectors above the micro-perforated liners. A decorrelation technique is used to extract the acoustic fluctuations from the measured pressure signals. It is shown how the attenuation of the sound pressure levels in the flow test section nearby the wall-treatment can be substantially modified when shielding the resonator cavity by weakly or highly porous micro-perforated panels.