Wall
The Wall condition is the most common condition at solid surfaces. 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.
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 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. In some applications like for example, in large muffler systems, it is often not necessary to model these loss effects and thus switching to slip and adiabatic can be a good approximation. This also means that the mesh does not need to resolve the acoustic boundary layer which can save DOFs.
Mechanical
Select a Mechanical condition — No slip (the default) or Slip. See the No Slip and Slip conditions for further details.
Thermal
Select a Thermal condition — Isothermal (the default) or Adiabatic. See the Isothermal and Adiabatic conditions for further details.
Constraint Settings
To display this section, click the Show button () and select Advanced Physics Options.
If Slip is selected for the Mechanical condition and Use weak constraints is enabled then the default discontinuous Galerkin (penalty like) formulation is switched to a Lagrange multiplier formulation instead. See the Slip condition for details. For the No-slip and Isothermal selections a weak formulation is used instead of a pointwise constraint.