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Old   January 29, 2013, 14:53
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Anonymous
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Hi All,

I am trying to get an LES case to converge, and I am having some difficulty. The case runs fine in FLUENT, but the pressure residual blows up in OpenFOAM. My mesh is great, timestep is really small (Co <0.05), so I believe my BCs are the problem. In FLUENT the outlet is set to outlet-vent. Can someone tell me how to set that in OpenFOAM. I jus want to double check what I've done.

Currently for U I have:

Outlet
type inletOutlet;
inletValue uniform (0 0 0);
value uniform (0 0 0);

For P I have:


OUTLET
{
type fixedValue;
value uniform 0;
}


Cheers
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Old   January 30, 2013, 05:55
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Tom Fahner
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Hello Anonymous,

It seems indeed your boundary conditions may lead to these divergence problems, similar problems where found here:

LES for wind over buildings

Maybe they have found a solution,

regards,
Tom
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Old   January 30, 2013, 10:52
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Anonymous
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Thanks Tom,

I had a look at the forum, and it helped. I had a question...what outlet Boundary condition in OpenFOAM would match an outlet-vent condition in FLUENT. In FLUENT, the outlet-vent condition allows backflow, but both the outflow and backflow have a pressure drop across the vent, proportional to the velocity (either in or out). Like a loss coefficient for a valve.

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Old   January 30, 2013, 11:30
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Eric R
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Quote:
Originally Posted by Industrial_CFD View Post
Thanks Tom,

I had a look at the forum, and it helped. I had a question...what outlet Boundary condition in OpenFOAM would match an outlet-vent condition in FLUENT. In FLUENT, the outlet-vent condition allows backflow, but both the outflow and backflow have a pressure drop across the vent, proportional to the velocity (either in or out). Like a loss coefficient for a valve.

Cheers
I'd take a look at this:

http://www.foamcfd.org/Nabla/guides/...ml#x32-1640293

Maybe pressureInletVelocity or something similar for U?
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Old   January 30, 2013, 11:32
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Tom Fahner
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I don't think there is a standard boundary condition available that is similar. I did experiment a bit with LES, but I am not an expert. Also I have no experience with Fluent, so I guess I am not the best person to give you more advice.

I only have one more suggestion:

The way you prescribe the Fluent boundary condition sounds like a sponge layer with backflow calculated based on pressure difference, maybe it helps to use

pressureInletOutletVelocity
instead of
inletOutlet with inletVelocity uniform(0 0 0)?

Regards,
Tom
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Old   January 30, 2013, 12:43
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Thanks Tom,

I will give that a try, they both sound similar. Boundary conditions for LES are very difficult, especially if they arent cyclical.

Thanks again,

Cheers
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