# 4.12. 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 Regions Pane.

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 region(s) that will define the internal surface. Each region can only be used in a single internal surface.

All regions defined in the regions Pane 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.

## 4.12.2. 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

## 4.12.3. Fluid permeability¶

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

## 4.12.4. Internal resistance coefficient¶

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

## 4.12.5. Internal Surface Velocities¶

For each solid phase, specify the velocity of that solid through the surface (in meters/second).

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