Enhanced micro-perforated wall-treatments for reducing the boundary layer noise in a low-speed wind-tunnel: modelling, characterization and optimization studies.

Abstract

Micro-perforated aero-acoustic liners are robust non-fibrous wall-treatments able to achieve significant axial attenuation in low-speed ducted flows, if their input impedance is suitably optimized. They are also potential candidates for broadband reduction of the flow-induced noise, useful to increase the signal-to-noise ratio in wind-tunnels test section or for fan noise control when inserted in casing treatments. The current theoretical and experimental study determines under which range of frequencies, holes diameter and free stream flow velocity, micro-perforated partitions are able to efficiently reduce the boundary layer noise, either by absorption or transmission mechanisms, depending on the spectral content of the aero-acoustic excitation. The formulated analytical models, either modal- or wavenumber-based, are validated against wind-tunnel measurements of the overall power injected by a low-speed boundary layer into a micro-perforated partition and its aerodynamic transmission loss. A hole-based Strouhal number is found below which little back-scattering of the wall-pressure components occurs. Optimization studies are performed to determine the range of parameters that enhance the total absorption or transmission of boundary layer noise, with special emphasis on the low frequency content of the wall-pressures.