Use the Impedance or
Interior Impedance condition to specify an acoustic impedance on an exterior boundary or as a transfer impedance on an interior boundary. The impedance can be any expression and can, for example, be a function of the frequency (
freq). The condition can be used to model a porous lining in a duct or specifying an outlet impedance. The condition is based on the Ingard-Myers condition, see
Ref. 1 and
About the Impedance Boundary Condition located in the
Theory Background for the Aeroacoustics Branch section. The condition is a so-called low frequency approximation; the viscous boundary layer of the background flow is assumed infinitely thin at the impedance wall.
Enter the value of the Normal impedance Zn (SI unit: Pa·s/m). This can be an analytical expression or data provided through an interpolation function. Click to select the
Grazing flow conditions check box, if the impedance is at a boundary with no normal velocity component for the background flow,
u0·
n = 0. This simplifies the equations that are solved for in the boundary condition.