How to change/set different boundary condition for particular cell region?

sunagra27 · April 8, 2021, 06:30

Dear all,

I am using OpenFoam 5.0 and working on heat transfer simulation using chtMultiRegionSimpleFoam solver.

1. I have a temperature data file which equals to the face center length of a particular region. This temp file has "0" temp in some regions and rest with variable temperature.
(say out of 1000 temp values, 400 are 0s and rest are varying). This is a Dirichet BC.

2. I have used this temperature data and simulated it.

But now, I need this "zero" temperature value to be changed to convection BC. I know the parameters for convection BC. But, how do I change the zero temperature cells to different boundary condition?

3. I checked with SetFields (FieldToCell), but I can only give scalar value there and not BC.

I hope my question is understood. Any leads will be appreciated.

Regards,
Sunag R A.

Robin.Kamenicky · April 8, 2021, 13:06

Hi Sunag,

So if I understand right, you need to change Dirichlet BC to Robin BC at patch faces where 0 temperature exists.

Are these faces geometrically neighbouring to each other so you can easily make a new patch from them and assign a different BC type?

Kind regards,
Robin

sunagra27 · April 8, 2021, 23:36

Dear Robin,

Thank you very much for your reply.

As the first part, yes you have understood correctly and yes I need to change BC where 0 temperature exists.

As for second part, the faces are geometrically neighbouring, but the geometry takes arbitrary shape.

I have attached two images of the images, one from top and isolated view. The blue color shows the 0 temperature where BC need to be changed.

Isolated view: https://drive.google.com/file/d/13I6...ew?usp=sharing

Top view: https://drive.google.com/file/d/1Jwr...ew?usp=sharing

Regards,

Sunag R A.

Nomis · April 9, 2021, 03:16

Hi Sunag,

maybe the codedMixed bc does the job for you...

e.g for the T file changing fixed value to zeroGradient at t = 10 s:

Code:

boundaryField
{

    patchName
    { 
    type codedMixed;

    refValue        uniform 0;
    refGradient        uniform 0;
    valueFraction        uniform 0;
    value             uniform 0;

    name mixedBCname;

    code
    #{
        const scalar t = this->db().time().value();
        
        scalar Tair = 305; 
        //scalar PI = Foam::constant::mathematical::pi;
        scalar w = 1; // w = 0 only gradient applies , w = 1 only fixedValue applies. 
            
        if(t >= 10)
        {
            w=0;            
        }
            
        this->refValue() = Tair;
        this->refGrad()=0; // zeroGradient
        this->valueFraction()=w;
    #};
    }    

}

sunagra27 · April 9, 2021, 03:36

Dear Robin,

This looks good and advanced. I will definitely work with this and update you. But, I have few questions based on this code.

1. So, in this, w = 0 means it applies Robin function and w = 1, applies Dirichlet function from an external source implemented.?

2. As you have mentioned as example, after t=10s, it will be switched to w=0 meaning Robin funciton. I understand that. But, how to keep this robin funciton only for temp = 0, rest should be with Dirichet function only..!!

3. Also, how to run this code? I mean, should I include this in system/region or system folder?

4. Main Convection BC I am intended to give in place of temp = 0, is as below:

type externalWallHeatFluxTemperature;
mode coefficient;
Ta constant 22;
h uniform 10;
kappaMethod solidThermo;
value uniform 30;

Thanks and Regards,

Sunag R A.

Nomis · April 9, 2021, 05:04

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