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-   -   write out nusselt number (gradient of T, respectively) for timesteps (http://www.cfd-online.com/Forums/openfoam-programming-development/69832-write-out-nusselt-number-gradient-t-respectively-timesteps.html)

 sven November 5, 2009 23:42

write out nusselt number (gradient of T, respectively) for timesteps

I need to calculate a nusselt number for a body in a crossflow for different time steps. Does OpenFOAM provide a function which can do that? If not, can I somehow use the probing function to write out a temperature gradient at a wall, like I can write out fields at certain points? Has anyone ever done something like this? Thanks a lot!

 santos November 6, 2009 06:38

Hi Sven,

You can see how I modified simpleFoam and turbFoam (now its pisoFoam) for determination of Sherwood number:

http://openfoamwiki.net/index.php/Co...mpleScalarFoam
http://openfoamwiki.net/index.php/Co...turbScalarFoam

There is some coding and recompilation involved, but not too difficult. Just make sure you use the adequate dimensions for temperature in place of mass fraction.

Regards,
Jose Santos

 sven November 6, 2009 23:01

Hey Sanots,

thanks for your answer. I had a look at your source Code files, especially at simpleScalarFoam.C. I think, what I need is something similar to the following lines in your code:

Code:

```// Calculates kc and Sh on each patch         Info<< "Calculating kc and Sh" << endl; label patchi = mesh.boundaryMesh().findPatchID("electrode1"); label patchj = mesh.boundaryMesh().findPatchID("electrode2"); // kc = 1 / Tb * D * (dT/dy)_wall                 kc.boundaryField()[patchi] =                   1/Tb.value()*DT.value()*-T.boundaryField()[patchi].snGrad();                 kc.boundaryField()[patchj] =                   1/Tb.value()*DT.value()*-T.boundaryField()[patchj].snGrad();```
I just want to get the gradient, so I guess it should look like this:

Code:

```label patchi = mesh.boundaryMesh().findPatchID("myPatch"); gradT.boundaryField()[patchi]=T.boundaryField()[patchi].snGrad();```
Where "myPatch" is the name of the patch for which I want to get the Temperature gradient.
and at the end of the code I need something like

Code:

```volScalarField output           (                 IOobject                 (                     "Sh",                     runTime.timeName(),                     mesh,                     IOobject::NO_READ,                     IOobject::AUTO_WRITE                 ),         output=gradT           );         runTime.write();```
Is it correct like this? Thanks a lot!

 santos November 7, 2009 08:48

Yes, you are on the right track. Look below for my suggestion. Regards!

Code:

```volScalarField gradT             (                 IOobject                 (                     "gradT",                     runTime.timeName(),                     mesh,                     IOobject::NO_READ,                     IOobject::AUTO_WRITE                 ),                 mesh,                 dimensionedVector                 (                     "gradT",                     T.dimensions()/dimLength,                     0                 )             ); label patchi = mesh.boundaryMesh().findPatchID("myPatch"); gradT.boundaryField()[patchi]=T.boundaryField()[patchi].snGrad(); runTime.write();```

 sven November 7, 2009 21:55

Thanks a lot Santos, I got it working. The solver now writes out the gradT for myPatch in every time directory. However, I want the solver to write all these data only in one file instead of several files. I think I can perhaps use the probe function, but I dont know how to probe on a patch. Do you know how this works or do you have another idea? Thank you very much! I really appreciate your help!

Sven

 santos November 8, 2009 06:03

I am not familiar with the probe function, sorry. I normally extract gradT values in one file in other way:

1 - Make your solver write gradT on the screen with the patch name, normally you want an area-averaged value. Look in the solvers I mentioned above for guidance.

2 - Launch the simulation and redirect your output to a log file
Code:

`simpleScalarFoam > log &`
Code:

`grep <your_patch> <log_file> | awk '{print \$8}' > gradTvalues`
This line will look for 'your_patch' in you 'log_file' and will extract the 8th entry in that line (in my case its the gradT value, please change it accordingly).

Regards,
Jose Santos

 vilop6 February 1, 2010 13:59

hi all

i m intersseted by this topic and i will try it to calculate nusselt number, my quastion is how integrate "gradT" over the patch using any intagration method availble or not in openfoam "trapez, simpson, RkX, ..."

regards

 Goutam February 29, 2012 14:53

Dear Santos,

Thanks for your help. I have written the code in the file buoyantBousseinsqSimpleFoam.C. After that when I set wmake, then I am getting error. Is this code is ok? I want to calculate GradT.

 aaron_lan March 14, 2012 08:51

Hi Goutam,

Have you solved your problem of calculating GradT in buoyantBousseinsqSimpleFoam? I am also very interested in this case.

 Goutam March 14, 2012 08:58

No. But I have calculated local and Average Nusselt number. You can see my post on this. I gave the code.

 giovanni10 April 6, 2012 12:16

average Nusselt number

Quote:
 Originally Posted by sven (Post 235294) I need to calculate a nusselt number for a body in a crossflow for different time steps. Does OpenFOAM provide a function which can do that? If not, can I somehow use the probing function to write out a temperature gradient at a wall, like I can write out fields at certain points? Has anyone ever done something like this? Thanks a lot!
Dear Sven,
Did you finally find a solution to your problem? Unfortunately, I`m not an expert in C++. So, It is too difficult for me. I would like to calculate the average Nusselt number. I am working on a 2D laterally and volumetrically heated square cavity. I have seen your discussion here. Could you help me? Thanks in advance!

 liuchaofu May 8, 2013 10:06

Hi Goutam.Can you tell me how to calculated local and Average Nusselt number.Thank you .

 nwpukaka August 31, 2014 20:04

Quote:
 Originally Posted by santos (Post 235428) Yes, you are on the right track. Look below for my suggestion. Regards! Code: ```volScalarField gradT             (                 IOobject                 (                     "gradT",                     runTime.timeName(),                     mesh,                     IOobject::NO_READ,                     IOobject::AUTO_WRITE                 ),                 mesh,                 dimensionedVector                 (                     "gradT",                     T.dimensions()/dimLength,                     0                 )             ); label patchi = mesh.boundaryMesh().findPatchID("myPatch"); gradT.boundaryField()[patchi]=T.boundaryField()[patchi].snGrad(); runTime.write();```

is T.boundaryField()[patchi] temperature value at boundary field?

but what is the definition of snGrad()?

Regards,
Kan

 ssss September 1, 2014 13:31

Why not just:

Code:

```label patchi = mesh.boundaryMesh().findPatchID("myPatch"); surfaceVectorField TGrad = fvc::interpolate(fvc::grad(T)); vectorField TsurfGrad = FvolGrad.boundaryField()[patchi]; vectorField normal = patch().Sf()/patch().magSf(); scalarField snGradT = TsurfGrad & normal;```
Gets the patch ID from its name. Calculates the gradient of T in all surfaces. Then gets the gradient only in the surfaces.

After that it calculates the normal in the surface and the last one gets the gradient in the normal direction of the surface(dT/dn = grad(T)*vectorNormal). Hope that is what your looking for

 AliMahmoodi June 16, 2015 14:54

Nusselt Number in OpenFOAM

Hi every body

i add energy equation to icoFoam and it was compiled successfuly.
Now i d like to calculate Nusselt Number in duct .
Has any one known how it should be possible<

 wenxu August 7, 2015 05:08

Dear Jose Santos,

I searched some of your threads, and I found that your are familiar with the OF. Now I want to calculate the gradient explicitly in OF using upwind scheme. I want to calculate the sign distance function: mag(Grad(phi)) = 1.

It is not difficult to implement this on structured mesh based on FDM, but it is not easy for me to implement it on unstructured mesh based on FVM, such as OF.

Could you give me some hints? Thank you in advance!

Best regards,
Wen

 Garfield November 4, 2015 12:57

Hi, Santos
I have a question about the bulk temperature in your files, in your files you said that we need to define the bulk temperature (Tb) in the transportProperties dictionary, but the Tb will change with location, could you tell me how to handle that?
Thanks a lot
Best
Garfield

 santos November 5, 2015 09:30

Hi Garfield,

My example does not relate to temperature, rather relates to an arbitrary scalar T (http://openfoamwiki.net/index.php/Co...mpleScalarFoam). Tb is in this case a reference value for the scalar (say reference mass fraction of a component) that is used to determine the mass transfer coefficient at the walls.

Regards,
Jose

 Garfield November 5, 2015 10:50

Thanks a lot!

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