creating a new field by volume averaging of an existing field and reuse it in Solver
 

Hello Foamers,

Using VolfieldValue, I created a new field based on volume averaging of an existing field. Now, I don't know how to ask Solver to call and read the new field and use it for its next time step.

Your help is appreciated.

Thanks in advance!

We're going to need some more information to help you. What solver/application is this? What code do you already have? How do you want to use the new field that you've created?

Caelan

Hi Caelan,

Thanks for your help.

I have a code based on PimpleFoam, modified for solidification. The soldification temperature (Tsol) is a function of total liquid fraction (alphAvg). Therefore, I defined a new field (using volFieldValue in controlDict) that calculates the volume average of alpha. The output file can be seen properly in my postprocessing.

Now, my problem is how to read (call) this output FROM Solver (heattransfer/Pimplefoam) to be used for the next time step.
As an option, I just simply created Tsol as another volScalarfield in CreateFields.H. Then tried to relate Tsol to alphAvg there. but it seems Tsol is NOT UPDATED by alphAvg at each time step and only uses the initial alphAvg. somehow I think Solver can't read alphAvg in postprocessing folder.

Thanks for your time.

Ok, I think I'm getting it. My first thought is this : creating a field or doing field manipulation in createFields will only happen once because createFields is only called once at the beginning to initialize fields. If you want the field to be updated, you'll need to put code inside of the runtime loop.

Caelan

I think you are absolutely right.


what I did in controlDict for defining the new field alfAvg is:

functions
alfAvg
{
type volFieldValue;
libs ("libfieldFunctionObjects.so");
enabled true;
writeFields no;
valueOutput true;
regionType all;
writeControl timeStep;
writeInterval 1;
operation volAverage;
fields
(
alpha
);
}


and in createField.H, I tried to call this new field by:

Info<< "Reading field alfAvg\n" <<endl;
volScalarField alfAvg
(
IOobject
(
"alfAvg",
runTime.timeName(),
mesh,
IOobject::MUST_READ,
IOobject::AUTO_WRITE
),
mesh
);

then I tried to use alfAvg in energy equation (TEqn) for next steps of the solution...and as you mentioned its called only once at the beginning.

How can I define something similar in the run loop? basically where is run loop? Energy equation solver (TEqn)?

any change needed in pimpleFoam.c or elsewhere?

your reply is highly appreciated.

cheers

I see. So using the function object here can be thought of doing something "on top" of the base solver -- it doesn't know to communicate with the rest of the solver (ie both ways -- it is a one way communication). So what we can do is create the field like you have in createFields and then compute an average inside the loop (eg each timestep).

The run loop is inside pimpleFoam.C. You'll see a while statement. Then, after the time integration we can put something like :

This will create a scalar alfAvg that is the average alpha in the domain (scaled by cel volume). Then, you can use alfAvg in Teqn.H. Beware that the specifics of the scalar (ie scalar vs dimensionedScalar, etc) and getting it to work with the rest of the solver can be an issue, but it's a solvable one.

Caelan

Dear Caelan,

Thanks to your post, I could modified the code. However, still I think the alfAvg is not engaged with other fields.

What I did is this:

for volume fraction, alfAvg, I removed its functionObject of volFieldValue in controlDict and also from createField.H.

instead, as you suggested, I added this line in pimpleFoam.C:

scalar alfAvg = gSum(alpha*mesh.cells().size())/(mesh.V().size())

from the outputs, I can see that the alfAvg is calculated correctly. However, I defined a new scalar Tsol (solidification temperature) based on alfAvg as:

dimensionedScalar Tsol = 0.5*alfAvg;

later on, in Teqn file, Tsol will be used in energy equation solving as I wish. but from outputs, I can see that Tsol is not updated based on alfAvg since it remains equal to its initial value!!

as an option, I tried to define Tsol=0.5*alfAvg inside Teqn (and not in pimpleFoam), but it didn't change.

meanwhile after compiling the solver, I get this warning... unused variable'alfAvg'....

ps. I added the line of scalar alfAvg = gSum..... to runTime loop at different locations of pimpleFoam (inner loops, outer loop) but I didn't help.

I don't know how to call and use alfAvg inside Solver.
:confused:

by the way, in postProcessing, I have an output file for alfAvg. Is it proper to ask Solver to read alfAvg from their for Tsol?

Regards,
Gbaz

It is incredibly hard to follow what you've done without the code tags/ providing more code (eg the Teqn). Also providing output/ error from a log file helps. Additionally, info statements are helpful in general for debugging.

Caelan

Thanks for your advise. I rewrite it as this. "Tsol" is in TEqn.

You are right. I forgot !

This is runTime loop:


and this is part of TEqn, where I need Tsol:


I hope this helps otherwise please let me know what should I add.

For future reference, "code tags" are used to isolate chunks of code within a comment,

The # button will insert them as you create a comment, and you place your code in between them.

As for your problem, I don't even see Tmelt (I'm assuming this is your quantity of interest) is used in Teqn.

Caelan

with your guidance, I edited my previous thread. Hope you can see it in a better way now. Appreciate your comment.

I mean:

Much clearer, thanks. I see now where Tsol is going. I'd recommend putting the code to calculate Tsol inside Teqn.H. Then creating a volScalarField (or uniformDimensionedField) of Tsol (name it something else). Then you ensure computation of alfAvg/Tsol right before you need it. You might need to create a volScalarField or uniformDimensionedField of Tsol to subtract it from T.

Caelan

Thanks Linda. I did so and now applying Caelan's comment.

Caelan:I'd recommend putting the code to calculate Tsol inside Teqn.H.
Gbaz: I removed it from pimplefoam.c and added it to TEqn as this:


Caelan:Then creating a volScalarField (or uniformDimensionedField) of Tsol (name it something else).

Gbaz: In CreateFields.H, I added this:

Name it as Tmelt is correct?:confused:

Caelan:You might need to create a volScalarField or uniformDimensionedField of Tsol to subtract it from T.

Gbaz: Is this something more than what I did in createfields.H ?

Applying above comments, outputs are still showing that Tsol is not updated. This means alfAvg is not also updated and remains equal to initial value of alfAvg.

by the way, I tried to remove initial value file of alfAvg in 0 direcroty. but running failed with this message:

FOAM FATAL ERROR:
cannot find file "C:/PROGRA~1/BLUECF~1/ofuser-of5/run/SolidiFoam/0/alfAvg"

:confused:

Sorry for the confusion -- I didn't realize you'd created a volScalarField named Tsol. So, you don't need "dimensionedScalar" before Tsol in Teqn.H. ...unless Ts and Tl volScalarFields, which it looks like they could be.

If they are volScalarFields, then your Teqn.H looks like it'll be :

If not, a messy/rudimentary solution would be :

Or a cleaner version, using Tsol as a dimensionedScalar (and removing its declaration as a volScalarField from createFields.H)

Note that I quickly tested all options and they compiled using a the dev branch. I've not tried running anything, though.

Also note that for your calculation of alfAvg you dropped the mesh.V() volume normalization part.

Replying to your most recent post, this is because createFields pulls intial conditions from your 0 directory to create the fields needed for the solver to run. It does not write files. Additionally, when we create the scalar alfAvg, we are just creating it for use within the solver -- it is a singular value that is not written. You'll likely want to add it to the Teqn.H file as well to make sure it's updated before Tsol. Otherwise you'll need to initialize it to some value (eg in craeteFields).

Caelan

Thanks Caelan for your help.
:)

Applying all your comment, now I want to trace Tsol values over the the domain. basically it should be changing as alfAvg is changed as


the output file of postprocessing/alfAvg/0/volFieldValue.dat shows that alfAvg is updating properly (from initial value of 1 to final value of 0.84) but Tsol is always equal to 0.5*(Tl+Ts) means alfAvg have been taken as 1 which is the initial value! does it mean Tsol is not updated!

by the way, what's the easiest way to monitor alfAvg and Tsol on screen (or log file) as they fly?

cheers,
Gbaz

Like you said, this means that alfAvg is not being updated. Where did you place the alfAvg code? And do you declare it in createFields?

An easy way to see values is as follows :

Place those lines after you compute alfAvg/Tsol (eg in Teqn.H, if you put the calculations there), and the information will be printed to the terminal or the log file.

Caelan

Caelan: Where did you place the alfAvg code? And do you declare it in createFields?

Gbaz: alfAvg is in pimpleFoam, at the end of runTime loop as:

and declared in createFields.H as:





Caelan: Place those lines after you compute alfAvg/Tsol (eg in Teqn.H, if you put the calculations there), and the information will be printed to the terminal or the log file.


Gbaz: I did so as you see ablve, but still can't see them on screen. This is my screen: