Effects of grazing flow profile on the acoustic and aerodynamic responses of a slit-resonator

Grazing mean flow has a significant influence on the acoustic and aerodynamic responses of acoustic liners or Helmholtz resonators. Great efforts have been made to model the relationship between the grazing flow velocity and the acoustic impedance of a liner. This paper considers the effects of boundary layer profiles on the acoustic response of a slit-resonator. Two-dimensional direct numerical simulations are conducted to reveal microscopic flow characteristics around the slit under grazing flow and grazing incident sound waves. Numerical results show that the grazing flow has a significant influence on the performance of the resonator. In general, the presence of a grazing flow has a detrimental effect on the sound absorption of the resonator. From the aspect of the aerodynamic response, The periodical inflow and outflow excited by the sound waves through the resonator neck become a source of perturbations in the boundary layer flow. It is found that a near-wall hydrodynamic mode would be excited under specific conditions. And the mode could be amplified under conditions where both high Mach number flow and high-intensity incident sound waves are present.