The Thermoviscous Acoustic–Structure Boundary coupling () is used to couple a Thermoviscous Acoustics interface to any structural component. The feature couples to Solid Mechanics, Shell, Layered Shell, Membrane, and Multibody Dynamics interfaces.

where ut,fluid is the total fluid velocity (including a background component if applicable), usolid is the solid displacement, and n is the surface normal. The first equation is in both cases in the frequency domain and the second is in the time domain. This coupling results in the stress (or normal stress) also being continuous across the boundary. The condition for the total temperature Tt can be set to either isothermal or adiabatic. In the case where a shell or membrane is interior to the thermoviscous acoustics domain, a slit is automatically applied to the pressure p and temperature T degrees of freedom (DOFs).

See Settings for further details about Label and Name.

The default Name (for the first multiphysics coupling feature in the model) is tsb1.

This section defines the physics involved in the multiphysics coupling. The Thermoviscous Acoustics and Structure lists include all applicable physics interfaces. See the Coupled Interfaces in Acoustic–Structure Boundary for details.

Select the Mechanical condition as No slip (the default) or Slip (perfect). For the no-slip option all the structure and fluid velocity/displacement components are coupled. This condition also results in the generation of a viscous boundary layer (which should be resolved by the mesh). For the slip (perfect) option, the velocity/displacement is only coupled in the normal direction to the surface; this condition is equivalent to the coupling in pressure acoustics and no viscous boundary layer is modeled. The slip (perfect) condition can be used for larger systems that have a geometric scale large then the viscous boundary layer thickness.

Select the Thermal condition that applies on the solid surface to be either Isothermal (the default selected) or Adiabatic. The isothermal condition results in the generation of a thermal boundary layer, while the adiabatic does not. Use the adiabatic option for larger models that have a geometric scale larger than the thermal boundary layer thickness.

These settings are shown when the Advanced Physics Options is selected. It controls how the constraints (continuity in displacement and the thermal condition) are handled. For the Mechanical constraints type, select either Study controlled (the default), Weak constraints, Nitsche constraints, or Pointwise constraints. For the Thermal constraint type, select either Weak constraints, Pointwise constraint (the default) or Nitsche constraints. Finally, select the Constraint method to apply to the pointwise constraints, either Elemental or Nodal (the default).

When the Mechanical constraints type is set to Study controlled, the weak constraints are automatically selected when an eigenfrequency study is performed. This type of coupling is necessary as the eigenvalue (the angular frequency ω) enters the coupling expression. This will create extra variables at the boundary (so-called Lagrange multipliers), ensuring the correct behavior and solution. For a normal frequency domain study, the pointwise constraint is automatically selected. In the time domain there are no options, and the weak formulation is always used.