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-   -   boundary condition "calculated" (https://www.cfd-online.com/Forums/openfoam-solving/115086-boundary-condition-calculated.html)

nick.17 March 23, 2013 10:09
boundary condition "calculated"
 
hi foamers. I'd like to understand this kind of boundary conditions. It is quite a silmple question, but I didn't find a sure answer.

In the boundary field for certain variables (eg eddy viscosity "nut" in a RASModel running a realizable k-epsilon, as i'm doing) i put:

Code:
   
box_outlet
    {
        type            calculated;
        value          uniform 0;
    }
Why i have to put a "value", if I want a calculated value. It will be 0 or it will be calculated?

thanks a lot!

fumiya March 23, 2013 10:48
The value has no effect on the results.
It only affects how the field nut looks like in paraview at time "0".

Best regards,
Fumiya

nick.17 March 24, 2013 06:49
so, my field nut is simply calculated and I state with "value" the initial condition? isn't it redundant? because I state also the following in \case\0\nut :
Code:
internalField  uniform 0;
I thought this was the initial condition.

fumiya March 24, 2013 10:34
For the ordinary variables(e.g., U, p), the internalField defined in the files in 0 directory is the initial condition
at each cell centers. On the other hand, for the field nut, the internalField is not used for the calculations
(it is merely a placeholder). The initial value of nut at each cell centers and face centers of boundary faces
are calculated by the turbulence model you selected.

If you use the kEpsilon model, the nut value at each time step is calculated by

Code:
nut_ = Cmu_*sqr(k_)/epsilon_;
nut_.correctBoundaryConditions();
Hope this helps,
Fumiya

Miner March 24, 2013 11:47
I have been trying to understand BC's better too. I see BC's defined in changeDictionaryDict, and again the same BC's defined in the 0 folder. The BC type is identified in each case. Why are the same things specified in multiple places? What happens if the BC types don't match from file to file?

nick.17 March 24, 2013 11:51
Thanks a lot for your explanation!
I have another question: if I calculate, after a simulation, the components of the velocity,
I have some files (MagU, Ux, Uy and Uz) in folder 0/ where, for each boundary condition i have the following:

Code:
boundaryField
{
    box_inlet
    {
        type            calculated;
        value          uniform 0;
    }
    box_outlet
    {
        type            calculated;
        value          uniform 0;
    }
    box_ground
    {
        type            calculated;
        value          uniform 0;
    }
the airflow it is set up to 60m/s on freestream, so, if I calculate the components anywhere on my domain, it is correct that it is "type calculated", but, the meaning of
"value uniform 0" there isn't, or better, I can't find it... Is it another placeholder?


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