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Boundary conditions in OF

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Old   June 10, 2013, 07:48
Arrow Boundary conditions in OF
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James
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Hi everyone,

I am trying to simulate internal airflow thorugh a complex geometry using OpenFoam. I think that simpleFoam for stationary, incompressible flow would be enough for the first step.

The problem is setting the boundary conditions file. I only have information about the value of pressure at the inlet an outlet (and I use fixedValue on this boundaries), but I donīt have any information about velocity values, except at wall (no slip condition). How can I set up the velocity file for inlet and outlet? And what about pressure at wall?

Thanks in advance for the answers

Last edited by Tensian; June 24, 2013 at 14:42.
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Old   June 10, 2013, 11:00
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Lorris
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Actually you don't have to fix your velocity value for inlets and outlets if you know the pressure values.

The motion of the fluid is precisely caused by the pressure difference, so openFoam will calculate the velocity field.

The pressure on the walls can be set as zeroGradient. It's derivated from he navier-stokes and continuity equations. You can take a look here if you want to understand why (not my work)

And I am interested by your simulation, what hypothesis do you use for you wall friction ?
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Old   June 10, 2013, 11:25
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James
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Thanks for your fast reply Yunilo.

I understand that fluid motion is due to pressure differences. The problem is Iīm a beginner in OpenFoam.

There is no doubt about what conditions I have to impose at walls:
velocity file, fixed value=(0,0,0)
pressure file, zerogradient

At the outlet I think that I should write:
velocity file, zerogradient
pressure file, fixed value

But in the inlet I am not sure what I have to suppose, because I know the value (atmospheric pressure, so Gauge pressure=0), so I use fixed value for pressure. What happens with velocity file at inlet? Because I understand that itīs not a zerogradient condition...

Now I am realizing that I have written Gauge pressure values (as in Fluent set up). Is that right for OpenFoam?

At this first stage, I am using no slip condition at walls, without no roughness and also a rigid wall without deformation. If you need it, I can refer you to some documentation on the web about that issue.

Thank you so much.

Last edited by Tensian; June 21, 2013 at 07:57.
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Old   June 10, 2013, 12:07
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Sorry for not being clear !

Actually you can use zeroGradient velocity condition on both inlet and outlet, as you can consider that on a small distance upstream and downstream there is no section variation.

Actually I'm wondering how you manage to simulate the influence of nasale pilosity () as it's a very small geometry.

Good luck for your continuation.
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