Non-linear behavior of the transfer impedance of a Micro-Slit-Plate: effect of slit length in presence of a bias flow

The influence of the slit length and interactions between slits in a micro-slit plate (MSP) are investigated in the high amplitude regime. MSPs are plates with arrays of slit-shaped perforations, with width of the order of the acoustic viscous boundary layer thickness. The geometry discussed in this work is typical for fabrication by cutting and bending the plate, without removing material. Micro-slit plates designed with optimal acoustic properties in the linear regime will not be optimal when non-linear effects become significant resulting in vortex shedding. Impedance tube measurements of the transfer impedance on accurately manufactured micro-slit plates are compared to prediction by 2D incompressible Navier-Stokes model and to a quasi-steady theory. At moderately high amplitudes, the vortices remain close to the edges of the slit. The impedance of the plate is predicted within 20% by the model of a single isolated slit. At higher amplitudes, the vortices are traveling far away from the slit and a complex behavior. In this region, three-dimensional effects become important. For very high amplitudes, the vortices shed at neighboring slits start to interact, which affects the impedance.