.. _internal_surfaces:

Internal Surfaces
=================

Internal surfaces are surfaces defined within the overall boundaries of the
model.

To define an internal surface, there must be a region already defined in the
:ref:`Regions Pane <regions>`.

Add/Delete Internal Surfaces
____________________________

To add an internal surface, navigate to the ``ISs`` pane from the Modeler View,
and click the (+) button. A popup dialog will appear to select
:ref:`region(s) <regions>` that will define the internal surface. Each region can only be used in a single internal surface.

All regions defined in the :ref:`regions Pane <regions>` are listed in the popup, but only
the regions of the selected Surface Type are selectable.

- Plane Regions can be Impermeable internal surfaces.
- Box Regions can be X-Axis, Y-Axis, or Z-Axis Impermeable internal surfaces.
- Plane Regions can be Semi-permeable internal surfaces.
- Box Regions can be X-Axis, Y-Axis, or Z-Axis Semi-permeable internal surfaces.

To remove an internal surface, navigate to the ``ISs`` pane from the Modeler View, select
the internal surface to remove, and click the (-) button.

Internal Surface Type
_____________________

Planar internal surfaces can be of type:

- Impermeable
- Semi-Impermeable

Volumetric internal surfaces can be of type:

- X-Axis Impermeable
- Y-Axis Impermeable
- Z-Axis Impermeable
- X-Axis semi-permeable
- Y-Axis semi-permeable
- Z-Axis semi-permeable


Fluid permeability
___________________________

The permeability is only definable for semipermeable regions. The default value
is 1.0d32 m².

Internal resistance coefficient
_______________________________

The internal resistance is only definable for semipermeable regions. The default
value is 0.0 m⁻¹.

Internal Surface Velocities
___________________________

For each :ref:`solid phase <solids>`, specify the velocity of that solid through
the surface (in meters/second).

Disabled for Impermeable surface (since by definition the velocity would be zero).