Add a Slip node to define slip 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 condition with a rational scaling in the PML, as this will lead to a singularity and an error.

Per default, the Slip condition uses a so-called 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 Use weak constraints if you want to switch from the default discontinuous Galerkin (penalty like) formulation to a Lagrange multiplier formulation for the 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.