Use the Wall node to model the most common conditions at solid surfaces. This is the default boundary condition. This condition contains both a mechanical and a thermal selection. The default is a no-slip and isothermal condition applicable in most cases. More advanced conditions at boundaries can be set up by combining any of the Mechanical and Thermal conditions available. To model a wall on an interior condition, use the Interior Wall condition.

For very small dimensions or at very low ambient pressures (in the Knudsen number regime from 0.001 to 0.1), the wall conditions will behave nonideally; for this purpose, use the Slip Wall boundary condition. This is the regime known as slip-flow in CFD.

The Wall condition is also the default condition added when the physics interface is set up. Note that the selections in this default condition can be changed (they are not locked to all boundaries). When used together with a perfectly matched layer (PML), it is recommended to use the no-slip option at the outer boundary of the layer. Specifically, using the Slip (perfect) condition in combination with a rational scaling in the PML will lead to a singularity and an error.

The no-slip condition is the origin of the viscous boundary layer and the isothermal condition is the origin of the thermal boundary layer. It is within these acoustic boundary layers that the main dissipation happens. See the Theory Background for the Thermoviscous Acoustics Branch section for more details.

Select a Mechanical condition — No slip (the default) or Slip (perfect). See the No Slip and Slip (Perfect) conditions for further details.

Select a Thermal condition — Isothermal (the default) or Adiabatic. See the Isothermal and Adiabatic conditions for further details.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. If Slip (perfect) is selected for the Mechanical condition and Weak constraints is selected for the Constraints, then the default Nitsche (penalty like) formulation (Automatic) is switched to a Lagrange multiplier formulation instead. See the Slip (Perfect) condition for details. For the No-slip and Isothermal selections a weak formulation is used instead of a pointwise constraint.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. See Suppressing Constraints on Lower Dimensions for details.