[wyldckat]

Greetings to all!

@Ali: I've had your post in my to-do list for some time now and I finally managed to look into this today:

Originally Posted by Alish1984

1. In channelFoam tutorial, what are the contents of the 0 file? how are they produced? are they from a long time run of the simulation without any perturbed initial condition?

Possibly used a utility similar to PerturbU: http://www.cfd-online.com/Forums/ope...-perturbu.html - more specifically, the one on this post: http://www.cfd-online.com/Forums/ope...tml#post187619 post #34
Coincidentally, someone else also named Ali asked a similar question back in 2005 and got this answer: http://www.cfd-online.com/Forums/ope...tml#post187629 post #44

Originally Posted by Alish1984

2. what is Ubar or magUbar in channelFoam source? I read the source and I guess that it is averaged velocity over entire volume. then what does it mean? if it was averaged velocity over a cross area then ok, it makes sense. but why does channelFoam tend to meet such a weird condition?

In OpenFOAM 2.1.x, file applications/solvers/incompressible/channelFoam/readTransportProperties.H
has this code:

    //  Read centerline velocity for channel simulations
    dimensionedVector Ubar
    (
        transportProperties.lookup("Ubar")
    );

    dimensionedScalar magUbar = mag(Ubar);
    vector flowDirection = (Ubar/magUbar).value();

Therefore, "Ubar" is the "centerline velocity for channel simulations". The likely reason: if a channel was filled with a fluid going at laminar flow speeds, almost the whole flow speed would be identical; if it's in the turbulent regime, then the speed at the centerline should be the on average similar to the laminar speed... for example, similar to a parabolic flow profile.

Originally Posted by Alish1984

3. I compared results of a channel395 case solving by the pisoFoam and channelFoam for some different conditions
a) channelFoam with initial conditions in 0.org file: no turbulence
b) channelFoam without cyclic BC (turbulentInlet velocity replaced) with IC in 0 file (perturbed IC): turbulence decays

c) pisoFoam with initial conditions in 0.org file without cyclic BC (turbulentInlet velocity replaced): no turbulence
d) pisoFoam without cyclic BC (turbulentInlet velocity replaced) with IC in 0 file (perturbed IC): turbulence decays

then I have this question, it seems that these are cyclic BC and perturbed initial condition both that run the turbulence, is it right?
if it is right then what can we do if we dont want to use cyclic BC in a case?

I think I found a paper a few minutes ago that state that pimpleFoam or channelFoam modified to use PIMPLE would give the best results... ah, it's this one: http://congress.cimne.com/eccomas/cf...pers/01454.pdf
Just in case the link gets broken in the future, the paper specifics:

• OPENFOAM SIMULATION OF MASS TRANSFER IN SPACER-FILLED CHANNELS
• José L.C. Santos, João G. Crespo, Vítor Geraldes
• V European Conference on Computational Fluid Dynamics, ECCOMAS CFD 2010

As for the tests and questions you made on this point:

1. cyclic BCs were used with an initial field value, because it's trying to quickly simulate how a section of fluid inside a channel would behave. For example, if the channel is as big as 10km by 500m, and you only want to study how 100m x 100m section of fluid behaves over a period of time, it's not necessary to mesh the whole channel of 10km by 500m, it's just a matter of meshing only the 100m by 100m and using cyclic BCs.
2. I'm not sure I understood your 2 questions. I say this because if I understood you correctly, it seems you have not tried/studied any other tutorial example in OpenFOAM's "tutorials" folder, which many exemplify how to set-up and use turbulence modelling. Perhaps you can detail a bit more what you're asking?

Best regards,
Bruno