Long Elastic Open Neck Acoustic Resonator in flow

ABSTRACT Passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a Quarter-wavelength behavior, thanks to perforated sheets backed to honeycombs (SDOF, DDOF). So, their acoustic absorption ability is naturally limited to medium and high frequencies because of constraints in thickness. The ratio "plate thickness / hole diameter" usually centred around 1 generates impedance levels dependent on the incident sound pressure level (SPL) and on the grazing mean flow (by a nonlinear dissipation mechanism of vortex shedding), which penalises the optimal design of liners. Moreover, their physical law is not suited to an absorption to the lowest frequencies, as needed for future Ultra High Bypass Ratio engines with shorter and thinner nacelles (frequencies around 500 Hz). A possible approach could be to link the perforated panel with tubes introduced in the cavity, to shift the resonance frequency to a lower frequency by a prolongation of air column length. Applied for an aeronautical liner, the resonance frequency decreases considerably compared to classical resonator (factor of about 1/5). The aim of this paper is to describe mathematically this type of concept called LEONAR ("Long Elastic Open Neck Acoustic Resonator) and to validate experimentally its linear behaviour according to SPL and flow.