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-   -   outlet boundary conditions (https://www.cfd-online.com/Forums/openfoam-solving/89090-outlet-boundary-conditions.html)

 Xhoven June 3, 2011 18:32

outlet boundary conditions

Hello,

I am very new to OpenFOAM but have spent time reading about it and playing around with it for hours. I am trying to model a simple dropbox, as shown in the picture. I am using interFoam (for the moment, I intend to use rasInterFoam eventually) and a free surface model. Right now the mesh is very course until I get it the way I want it. The end of the top pipe is the inlet, the end of the bottom pipe is the outlet, and the top of the box is open to the atmosphere. I have constructed the geometry using Salome and have gotten it to run in OpenFOAM without errors.

http://img684.imageshack.us/img684/2953/dropboxe.png

I am not sure if I have the boundary conditions right, however, especially at the outlet. I ran the simulation for 80 seconds, and from a vector plot in paraFoam, around 50 seconds the fluid in the outlet pipe actually seems to start coming back the other way. I don't believe this is right. (@55.5 seconds shown)

http://img827.imageshack.us/img827/4914/vecotrplot.png

This is what I have right now:

alpha1:
boundaryField
{
Inlet
{
type inletOutlet;
inletValue uniform 1;
value uniform 1;
}
Outlet
{
}
Atmosphere
{
type inletOutlet;
inletValue uniform 0;
value uniform 0;
}
Walls
{
}
}

p_rgh:
boundaryField
{
Inlet
{
}
Outlet
{
type buoyantPressure;
value uniform 0;
}
Atmosphere
{
type totalPressure;
p0 uniform 0;
U U;
phi phi;
rho none;
psi none;
gamma 0;
value uniform 0;
}
Walls
{
}
}

U:
boundaryField
{
Inlet
{
type fixedValue;
value uniform (3 0 0);
}
Outlet
{
}
Atmosphere
{
}
Walls
{
type fixedValue;
value uniform (0 0 0);
}
}

Can someone help me with my boundary conditions? The box is starting off empty (just air) and the fluid comes in from the inlet pipe. I really don't understand exactly what zeroGradient does, I just know I don't want to specify alpha at the outlet, it needs to be determined by the flow.;

 alberto June 4, 2011 17:04

Fix the pressure at the outlet and if you want to prevent back-mixing, use the inletOutlet BC.

Best,

 tH3f0rC3 June 5, 2011 16:17

Quote:
 Originally Posted by alberto (Post 310529) Fix the pressure at the outlet and if you want to prevent back-mixing, use the inletOutlet BC. Best,
I would suggest the same.
use:
Code:

```type inletOutlet; inletVaule uniform (0 0 0); value \$internalField;```
Best Regards,
tH3f0rC3

 Xhoven June 11, 2011 19:33

Thank you for your help, alberto and tH3f0rC3.

From your posts I take it I need to change the outlet boundary conditions to inletOutlet for pressure and velocity, 0 and (0 0 0). I will try that.

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