Add a Slip (perfect) node to define perfect slip (no-penetration) for the acoustic particle velocity defined by

where σ is the stress tensor. The last equation expresses that no tangential stress exists at that boundary. Add this node from the Mechanical submenu.

This results in a so-called no-penetration condition where no viscous boundary layer is created. This condition can be used at boundaries where it is not necessary to model the losses in the viscous boundary layer. Used together with the Adiabatic condition, no acoustic boundary layer is modeled.

When running a model with a perfectly matched layer (PML), it is recommended not to combine a Slip (perfect) condition with a rational scaling in the PML, as this will lead to a singularity and an error.

Per default, the Slip (perfect) condition uses a Nitsche formulation (also often referred to as discontinuous Galerkin or penalty formulation) to prevent locking problems on curved surfaces.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.

Select Weak constraints as the Constraint if you want to switch from the default Nitsche formulation (Automatic) to a Lagrange multiplier formulation for the perfect slip condition (this formulation is not suited for an iterative solver). The Lagrange multiplier formulation also prevents locking problems.

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.