About Regions

A region is a contiguous volume where a fluid flows.

See Also
Defining a Fluid Domain

You specify a region by selecting the surface that contacts the fluid. A blue cubical glyph indicates which side of the surface you defined as the fluid region; you might have to flip the direction of the glyph to establish which side contacts the fluid. The example below shows two scenarios involving the same surface. In the top image, the fluid flows inside the cylinder; in the bottom image, fluid flows around the cylinder.