Broadband performance of multi-degree-of-freedom septum liners under high-velocity grazing flows

Developing advanced acoustic treatments, such as the Multi-Degree-of-Freedom (MDOF) septum liner, to realize the broadband noise reduction is critical for silent aeroengines. This study investigates experimentally the MDOF septum liner and its impedance model on the Beihang Grazing Flow Duct (BGFD) setup, over a wide frequency range under grazing flows up to 0.5 Mach number and Sound Pressure Level (SPL) up to 150 dB, typically encountered in aeroengine nacelles. Several specimens varying in the numbers, types, and depths of septa among units are designed, fabricated, and measured. Their impedances and Transmission Losses (TL) are obtained using the mirror-based multimodal straightforward method and the mode decomposition technique, respectively. Generally, the model predictions show good agreement with the educed impedances in all cases, and such liners with a large-porosity facesheet exhibit low acoustic nonlinearities in the presence of high SPL, especially under high-velocity grazing flows. Moreover, a MDOF liner we delicately designed, compared with a conventional broadband three-layer perforated liner as the reference, is close to the resonant state at more frequencies and thus has higher and wider measured TL spectra almost from 1 kHz up to 10 kHz at studied Mach numbers, under the premise of saving 22.7 mm in the thickness. These show that, the MDOF septum liner, if well designed, can achieve an ultra-broadband efficient sound attenuation using more limited spaces in complex aeroacoustic environments.