CFD Online Logo CFD Online URL
Home > Forums > OpenFOAM Running, Solving & CFD

Generalized Neumann BC q f nc grad f%3dg

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   August 17, 2008, 09:51
Default Good day everyone! Are gene
Posts: n/a
Good day everyone!

Are generalized Neumann boundary conditions available in OpenFOAM?

In case the expression "q f + n.(c grad f)=g" is not quite clear, what I mean is:
"q f" is the field f scaled by a factor q
"n.(c grad f)" is the scalar product of the surface normal n with the gradient of f scaled by c. (c may be tensorial).

If this BC is not available, maybe someone could be so kind to give a rough idea of how it could be implemented? Which classes are best used as a basis, which files do I need to modify?

I have seen the mixedFvPatchField, but it seems not to implement the generalized Neumann BC, instead it is a "linear transition" between fixedValue and fixedGradient BC. Am I right?

I don't see which physical meaning this mixed BC has, and I didn't find it in any of the tutorials. Can you suggest any demonstrative situation that would require the mixed BC? Just for curiosity.

  Reply With Quote

Old   August 18, 2008, 20:52
Default If this generalized Neumann BC
Posts: n/a
If this generalized Neumann BC is not available, do you have a suggestion how to model the temperature T_b of a boundary (conductivity k_b) which is connected to a temperature T_ext through a thin heat conducting layer (thickness L, conductivity k_ext)?

Equating the interior and exterior fluxes
n.(k_b (grad T)_b) = k_ext (T_ext-T_b)/L
I end up with the type of BC I had asked about in the previous posting.

I am sure this kind of BC is used frequently, but how is it called in OpenFOAM?

With kind regards
  Reply With Quote

Old   August 19, 2008, 04:44
Default There is a boundary that kind
Senior Member
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 14
eugene is on a distinguished road
There is a boundary that kind of does what I think you want. It is called wallHeatTransfer.

Found here:

eugene is offline   Reply With Quote

Old   August 19, 2008, 05:30
Default Dear Eugene, that is exactly w
Posts: n/a
Dear Eugene, that is exactly what I was looking for.

After a quick look into wallHeatTransferFvPatchScalarField.C I think that it simply adjusts the refValue, refGrad and valueFraction of a mixedFvPatchScalarField to produce the gen. Neumann BC. I'll study it in more detail in order to see if I can modify it for tensorial c in "q f + n.(c grad f)=g".

I find it a bit frustrating that such general purpose functionality is allocated to special fields like thermophysical modelling, because users who need it for other purposes will never find it unless they study almost all of the existing OpenFOAM code.

In quite old forum postings I read that the work on improving the documentation is mostly frozen until sponsoring is available. Are any guesstimates around about the payment required to get at least a basic description of all available solvers, turbulence models and boundary types? It should be possible to collect 10-100$ each from some 50 forum users who value a more complete documentation. Just an idea. If there are any comments, we might move to another thread.

A big thank you!
  Reply With Quote

Old   July 7, 2009, 11:27
Default Heat loss at wall
Senior Member
Markus Rehm
Join Date: Mar 2009
Location: Erlangen (Germany)
Posts: 184
Rep Power: 10
markusrehm is on a distinguished road
Hi all,

I need to implement wall heat loss into a combustion solver. As mentioned before the BCs at src/thermophysicalModels/basic/derivedFvPatchFields/ can do that. I think gradientEnthalpy would be most appropriate.

I tried it with the buoyantSimpleFoam-hotRoom-case modifying the fixedWalls in the temperature file:

        //type            zeroGradient;
        type            gradientEnthalpy;
        gradient        uniform -1000.0;
and it results in

DILUPBiCG:  Solving for Ux, Initial residual = 1, Final residual = 8.06253e-07, No Iterations 6
DILUPBiCG:  Solving for Uy, Initial residual = 1, Final residual = 7.71905e-07, No Iterations 6
DILUPBiCG:  Solving for Uz, Initial residual = 0.999994, Final residual = 7.96709e-07, No Iterations 6
"Speicherzugriffsfehler" should be a memory allocation error.

So is it a problem with the thermos or did I only apply the BC in a wrong way?

What is the unit of the gradientEnthalpy, maybe [W/m2]?

Regards, Markus.
markusrehm is offline   Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Grad evaluation cfdmarkus OpenFOAM Bugs 2 July 16, 2009 18:54
Best Grad School ramboy Main CFD Forum 4 October 6, 2008 14:35
Generalized Orr-Sommerfeld Hector Carrasco Chamblas Main CFD Forum 0 March 30, 2006 15:46
Zero-Grad Pressure Shazlan Siemens 3 March 15, 2005 04:48
Generalized eigenvalue problem z.zeng Main CFD Forum 2 September 18, 2000 00:09

All times are GMT -4. The time now is 13:21.