1. free opening:
A free opening represents an area on the surface of a solid object (e.g., block, plate), or an area on a planar object such as a wall, through which a fluid is free to flow in any direction. In most cases, an opening represents a hole on the boundary of the cabinet where the model fluid is exposed to the external environment.
2. Recirculation opening
Recirculation openings model recirculation devices such as heating or cooling units