[SharShabil]

Hi. I've been having a noob question, no case, just wanna understand something.
Linked image is the description for the inletOutlet boundary condition (from cfd-support.com):
https://imgur.com/a/XT5eLLR

Suppose we're taking a look at a single face at the outlet patch. The flow velocity is varying for some reason.

Fluid is flowing out >> velocity obeys a zeroGradient condition.
velocity decreases to reach 0. by the zeroGradient bc, the velocity is the same at the face we're looking at.

Velocity then continues decreasing, so that now fluid is flowing inwards.
The boundary condition now says that we're using a fixedValue, specified as inletValue. How is this dynamic? How can this patch act as an inlet then, if the velocity suddenly jumps to the fixed value of 5, or 0, or whatever? How about the difference between this face, and the one directly above it, still acting as an outlet and following a zeroGradient?

Also, what if you specify the inletValue as 0 and then water needs to flow in at that location and you prevent it? How is this face then still functioning as both an inlet and an outlet?

Thank you :-)

[SharShabil]

With the help of the OpenFOAM community on discord, I now kinda have an idea on what's going on.

This condition is a **Linear blend** of both the fixedValue and the zeroGradient conditions. It is not an _if_ function with strict outcomes, like I first thought.

inletOutlet is a kind of "mixed" boundary condition. An explanation there on can be found at: https://www.openfoam.com/documentati...bcs-mixed.html