Impacts of acoustic impedance of a perforated plate on near-wall turbulence

In this paper, the impact of wall acoustic impedance on near-wall turbulence of boundary layers is presented. Different acoustic impedances of a perforated plate with a backing cavity are used to control near-wall turbulence. It was shown that the specific acoustic impedance of the perforated plate correlates with sweeps and ejections in the near-wall region. A reduction of up to 7.8% in sweep intensity and 7% in ejection intensity was observed at a normalized specific acoustic reactance of 69.6 and an impedance angle of approximately 0.5π, which is associated with a reduction of about 7% in the inner peak of the turbulence intensity profiles. At this impedance angle, the wall-normal velocity is in phase with the pressure at the wall leading to transpiration through the perforated plate. This transpiration decreases the near-wall cycles by interacting with sweep and ejection events and reducing their effects. Consequently, turbulence intensity, shear stress in the near-wall region, and local friction drag are all reduced.