Although boundary conditions can be set up for the porous matrix and fluid independently of each other, there exist a few common boundary conditions which deserve special attention. The following sections refer to the boundary conditions for the system written in Equation 3-11 and Equation 3-12. See derivation in Ref. 7, Ref. 8, and Ref. 9.

The Free, Sound Soft boundary condition defines a condition where the displacement of the porous matrix in Equation 3-11 is unconstrained, so it can move freely without experiencing any loads.

The sound soft boundary condition for acoustics creates a boundary condition for Equation 3-12 where the acoustic pressure vanishes, so it sets pf = 0.

For simulating a poroelastic medium bounded by a rigid impervious wall, impose a Fixed Constraint node for the porous matrix displacement in Equation 3-11, u = 0, and a sound-hard boundary condition for the pore pressure in Equation 3-12:

For a given fluid pressure p0 on the boundary, set the pressure in Equation 3-12 to pf = p0. Since the fluid pressure is set to p0, the normal stress on the porous matrix in Equation 3-11 reduces to

For a rigid porous matrix αB = εp, the load is equivalent to

and for a soft porous matrix αB = 1, there is no load since

For a prescribed displacement u0 at the boundary, set the displacement of the porous matrix in Equation 3-11 as u = u0 and assume a sound-hard (impervious) boundary for the fluid pressure in Equation 3-12:

For a prescribed velocity v0 at the boundary, set the displacement of the porous matrix in Equation 3-11 as

For a prescribed acceleration a0 at the boundary, set the displacement of the porous matrix in Equation 3-11 as

The normal displacement of the porous matrix in Equation 3-11 is constrained, but the porous matrix is free to move in the tangential direction

The impervious (sound hard) boundary condition for the fluid pressure in Equation 3-12 is obtained from

For a prescribed load FA at the boundary, suppose that one side of the septum is fixed to the porous matrix and the other side bears the load.

A septum is a very limp and thin impervious layer with surface density ρsep. Since the septum can be seen as a boundary mass density, this boundary condition is achieved by setting an effective load FS = FA + ρsepω2u on the porous matrix, so the normal stress in Equation 3-11 reduces to