Suppressing Constraints on Lower Dimensions
Sometimes, boundary conditions on two adjacent objects can come into conflict on a shared object (point or edge). For the all the boundary conditions in the thermoviscous physics that are based on constraints, you have the possibility to select that certain objects of lower dimensions should be excluded from the main selection. To do this, you must first click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. In the settings a new sections named Excluded Edges and/or Excluded Points will then appear (when applicable). In these sections, you can select geometrical objects which should be excluded from the main selection when the constraint is applied.
Kutta Like Conditions for Linearized Euler
A Kutta condition can be set up to handle edge singularities that appear, for example, at the trailing edge of an airfoil or at the transition from a liner (modeled with an Ingard-Myers type impedance) to a hard wall. By using Exclude Edges and Exclude Points the appropriate Kutta condition can be set up. The problematic edges or points are excluded from the adjacent conditions and extra conditions can be set up here using the user defined Pointwise Constraint.
Velocity Condition for Linearized Navier–Stokes
When using the mechanical Prescribed Velocity condition, in a linearized Navier–Stokes model. you may get conflicts if it is located next to a no-slip condition (a wall). To remedy this, you can exclude the common edge or point.
Periodic Condition
When using the Periodic Condition, you may get conflicts or duplicates which makes the model either overconstrained, or problematic for the automatic constraint elimination algorithm. If you are aware of such situations, it is good practice to remove one of the potentially conflicting constraints. This could be in multiphysics models where you combine solid mechanics with linearized Navier–Stokes acoustics and have edges where you both apply a Periodic Condition and the Aeroacoustic-Structure Boundary multiphysics coupling.
Particularly if the geometry is curved, there is a risk that these constraints are not identical from a numerical point of view. In this case, excluding the conflicting edge from the selected boundary will make the behavior unique and fully predictable.
Another example where constraints will come in conflict is if you want to, for example, use periodic condition on parts of the geometry using weak constraints, while keeping the default pointwise constraints on other parts. If the same mesh node has both types of constraints, the solution will fail, so you must exclude any common geometrical objects from the selection in one of the constraints.
See also Excluded Surfaces, Excluded Edges, and Excluded Points in the COMSOL Multiphysics Reference Manual.