# Questions about the inletOutlet and outletInlet boundary conditions

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# Questions about the inletOutlet and outletInlet boundary conditions

Posted August 2, 2011 at 22:27 by tfuwa

Quote:
Originally Posted by Fransje
Ok.

Let's try to explain inflowOutflot and outflowInflow simply. Let's consider the following code:
Code:
```U:
outlet
{
type inletOutlet;
inletValue uniform (-2 0 0);
value (0 0 0);
}```
With this boundary condition, we have that:
• If the velocity vector at the outlet points out of the domain, then the boundary condition will be of the Neumann type, ie, zeroGradient.
• If the velocity vector points into the domain, then we can imagine that the outflow is no longer an outflow, but an inflow, and that we therefore would like to specify a Dirichlet boundary condition, ie, with constant value. In our case the boundary condition would take the value (-2 0 0), ie, Ux = -2, Uy = 0, Uz = 0.
From what I've read, the value (0 0 0) option is a dummy option needed for paraFoam/paraview and should not further be considered. Please correct me if I'm wrong.

Similarly, if, for some reason, we were to specify:
Code:
```k:
outlet
{
type outletInlet;
outletValue uniform 5;
value 0;
}```
we would have that
• If the velocity vector at the outlet was to point out of the domain, then the boundary condition would be of the Dirichlet type, with value 5.
• If the velocity vector at the outlet was to point into the domain, then the outflow conditions would switch from fixedValue to zeroGradient, and become a Neumann boundary condition.

I hope this was helpful!

this seems very good.
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