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postProcessing "grad(U")

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Old   September 5, 2017, 10:12
Default postProcessing "grad(U")
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Livia Adinolfi
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Dear Foamers,

I would like to calculate the shear stresses in a laminar simulation of an open channel flow. I am using the interFoam solver with OF 4.1
I was trying to postProcess and calculate grad(U). But I have this error:

Executing functionObjects
--> FOAM Warning :
From function bool Foam::functionObjects::regionFunctionObject::store (Foam::word&, const Foam::tmp<T>&, bool) [with ObjectType = Foam::GeometricField<Foam::Tensor<double>, Foam::fvPatchField, Foam::volMesh>]
in file /home/ubuntu/OpenFOAM/OpenFOAM-4.1/src/OpenFOAM/lnInclude/regionFunctionObjectTemplates.C at line 61
Cannot store cache-able field with the named used in the cache.
Either choose a different name or cache the field and use the 'writeObjects' functionObject.
--> FOAM Warning : functionObject grad: Cannot find required field U


I know that there are many threads on this topic and I have read something, but still I did not solve this problem.

Can anyone help me ?
Thanks in advance.
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Old   September 5, 2017, 11:30
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Kevin van As
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Which command exactly did you type?

Did you include something like "-field U" or "-fields (U)" in the argument list, such that the utility knows that it should read U from file?
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Old   September 5, 2017, 12:35
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after the simulation:

interFoam -postProcess -func "grad(U)".

I tried also to do it in runtime by typing in the ControlDict file:


type grad(U);
libs ("");
fieldName "U";
resultName "gU";


but I have always the same error.
What would you suggest ?

Thanks a lot!
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Old   April 6, 2020, 07:06
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Gazi Yavuz
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this is too late answer. though it would be helpful for others. the function should be like;


gradient /could be anything you name
type grad;
libs ("");
field U;
writeControl writeTime;

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Old   October 11, 2021, 04:20
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just type:

postProcess -func "grad(U)"

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