The interface condition establishes the continuity between the two sides of an intersection between different sub-tasks. For the momentum equation, the interface condition guarantees continuity of the velocity field and of the contact forces. The interface condition is available only for the boundary section that is defined by an intersection with an adjacent subdomain.
Important: If you prescribe an interface condition for one sub-task, you must also prescribe an interface condition for the sub-task on the other side of the boundary.
Note that it is also acceptable to define a condition other than the interface condition at the intersection between two sub-tasks. For example, you can impose a zero velocity field to model a wall of zero thickness between the two sub-tasks.
Interfaces can be fixed or moving. For a moving interface, a kinematic condition is added to the system of equations, in order to solve for the position of the interface. Moving interfaces are described in detail in Free Surfaces and Extrusion. Note that a fixed interface condition defines a linear problem, unless a nonlinearity arises from another parameter or equation.
To define an interface condition between two flow sub-tasks, simply choose the Interface menu item in the list of boundary condition choices.
Interface
To define an interface condition between a flow sub-task and a porous (Darcy) flow sub-task, simply choose the Interface with porous media menu item in the list of boundary condition choices.
Interface with porous media
See Porous Media for more information about defining a porous flow sub-task.