Neumann boundary conditions
 

Hi,

I want to change my boundary conditions during runtime. For the Dirichlet conditions I have found a way to set up the conditions in the code:

I have choosen fixedValue for the boundary type and I updated it in the code using:
U.boundaryField()[patchI]== mynewScalarField;

I have tried the same with fixedGradient type for a Neumann Condition but it doesn't update the gradient value. (I have reviewed it with .snGrad()).

Maybe someone can give me a useful hint,
thanks, m.

Hi Martin,

you have to do a refCast. Example:
After this you can edit gradient values in gradUField.

Of course you have to include "fixedGradientFvPatchFields.H" in your header.

Regards,
Stefan

Hi!
snGrad() has nothing to do with the gradient which you want to impose with your boundary condition, it is some geoemetrical quantity.

The fvPatchField is a field of quantities, which are applied as fixed gradients. Therefore, you have to write the wanted values to the field the same way you do it for the fixedValue stuff.

I hope you get the point...
Cheers, Andreas.

Hi Stefan,
I have already got the same answer from somebody else but thank you very much - this is exactly what I was seeking for. [.gradient() is not a member function of "volVectorField" but of "fixedGradientFvPatchVectorField"].

Hi Andreas,
I don't think u r right, U.boundaryField()[patchI].snGrad() in that case should show me the current boundary condition of the fixedGradient bc object.

B.r.,
Martin.

HAve u guys got the solution how to update the value of gradient, I think my problem is also similar to yours.

that is expressed below:

volScalarfield field
(
IOobject(fieldName,runtTime.timeName(),mesh,IOobje ct::NO_READ,IOobject::AUTO_WRITE),
mesh,
dimensionedScalar(fieldName,dimTemperature, 200),
"fixedGradient"
);
forAll(field.BoundaryField(),patchID)
{
field.boundaryField()[patchID]==30;
}

As an output of this code am getting something different.
that is.

boundaryField
{
Inlet
{
type fixedGradient;
gradient uniform 0;
}

and so on;
}

but I am interested to get output as below:

boundaryField
{
inlet
{
type fixedGradient;
gradient uniform 30;

}
}

Martin,
at the end, its the same.

What herbert is proposing is to copy (with the refCast) the reference of the fvPatchField to a new vectorField and editing THAT. Because of the reference, that still modifies the original field.

But you could write to your fixedGradientFvPatchVectorField directly, without giving it a new name. Just try it :-)

And - snGrad is definitely returning some geometrical stuff from the mesh, take a look at doxygen.

As long as it works there is no need to worry... ;)
Have fun,
Andreas.

Thanks, surely it works, I read this thread little bit late thoroughly and atlast i did it.

Help Neumann boundary condition in fluent
 

Hi!!
I need to define an output Neumann boundary condition using fluent. the condition is dc/dn=0, where c is the concentration of a certain species and n is the normal versor to the output face.
How can I do this? Thanks!

Hi Stefan and everybody,

I trying to implement neumann BC with your method but I had a fail during the compilation. I have merely copy your piece of code in my main.C file and I included "fixedGradientFvPatchFields.H" in my header.

I get the following error :


Do you have any idea where I am wrong ?

Regards,
Cyp

It should be:


vectorField& gradUField = gradUPatch.gradient();

(in case U is a volVectorField)

M.

Ok. Thank you very much. It compiles now.


But in case U is a volScalarField (a pressure field for example)? The code of Hebert should works, shouldn't it ?

No, i think in that case (for a volScalarField) you have to modify the first expression of Stefan's code in an appropriate way.
M.

I have a problem of understanding: the gradient of a scalar should be a vector. So the following snippet should work:

where M is a tensor, U the velocity field (vector), rho a volScalarField, and g the vector acceleration.

In fact, it compiles but fails while running:

I can't see my mistake..

You define the outer normal gradient (grad p * n) [n is the surface outer normal direction]

let p be a scalarF and V a vectorF then:

grad(p) is a vectorF -> (grad(p)) *n is a scalarF
grad(V) is a tensorF -> (grad(V)) *n is a vectorF

I hope it helps, M.

Thank you very much Martin. It helps me much in the understanding !

So I changed the code as :

Now, according to your definition, if I understand correctly, I need to get the projection of my field -(inv(M)&U)+rho*g on the patch : (-(inv(M)&U)+rho*g ) *n. Is it possible to do such a thing ?

I think it could work with :

The normal surface vector field on the boundarypart patchID is:
vectorField n = mesh.Sf().boundaryField()[patchID]


Take care that the left part of your calculation is finally also a vectorField on the appropriate domain (boundaryField()[patchID]).
M.

Hi Martin!

Thank you very much for your answer. I am not sure to understand very well...

I tried:

and either
or
or
but it does not compile... Do you see my mistake ??

Hi!


I succed in the compilation using this piece of code :


I set up a fixedValue at the inlet for my U_beta field but when I plot the results, the U_beta value is different that the one I chose...

Did I miss something ??

Regards,
Cyp

Please realize that you are editing the BC for p with your code
If you want to edit BC for U, you have to change it in the following way
Regards,
Stefan