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-   -   Degassing Boundary Condition (https://www.cfd-online.com/Forums/openfoam-programming-development/166566-degassing-boundary-condition.html)

Lokati March 22, 2016 06:01

so it is like this:

valueFraction()=
sqr(n) = n*n

so n*n leads to a valueFraction=(1 0 0 1 0 1) .

I am really bad in mathematics so here is my question?

If you multiply the normal vectors n*n , how does it change the valueFraction to (1 0 0 1 0 1)?? The valueFraction is a tensor, but the normal vector is a vector.

Best regards,
Timucin

bigphil March 22, 2016 07:21

Quote:

Originally Posted by Lokati (Post 591033)
so it is like this:

valueFraction()=
sqr(n) = n*n

so n*n leads to a valueFraction=(1 0 0 1 0 1) .

I am really bad in mathematics so here is my question?

If you multiply the normal vectors n*n , how does it change the valueFraction to (1 0 0 1 0 1)?? The valueFraction is a tensor, but the normal vector is a vector.

Best regards,
Timucin

Hi Timucin,

It seems that you are a bit confused.

Some info that might help clarify things for you:

n*n does not equal (1 0 0 1 0 1).

(1 0 0 1 0 1) is the identity tensor in symmetric tensor form i.e. (xx xy xz yy yz zz).

valueFraction is a symmetric tensor not a tensor, hence it has 6 independent components, not 9.

If the normal vector is (nx ny nz) then n*n (an outer product) is a symmetric tensor: (nx*nx nx*ny nx*nz ny*ny ny*nz nz*nz).

To set valueFraction to (1 0 0 1 0 1):
Code:

valueFraction() = symmTensor(1, 0, 0, 1, 0, 1);
I also suggest you read the other threads on the directionMixed boundary condition.

Philip

Lokati April 4, 2016 04:33

Hi Phil,

how can (nx*nx nx*ny nx*nz ny*ny ny*nz nz*nz) set the normal component to fixedValue and the tangential components to fixedGradient?

Best regards,
Timucin

Lokati April 4, 2016 07:26

Hi Phil,

do yo know how i could directly change the value of the velocity neighbour cells? I want to fix the neighbour cells to Zero.

Accessing to a neigbour value is not the problem...but how can i directly change the value?

Best regards,
Timucin

jp279 April 21, 2016 10:18

Hi Lokati,

I am also trying degassing outlet in bubble column. I don't have condition on alpha as you, so can simply use Slip BC for the liquid and pressureInletOutletVelocity BC for the gas at the degassing patch? Isn't combination of these two BCs indeed the degassing BC?

Lokati April 21, 2016 13:45

Hi Jigar,

i understand you're problem, but it is different from mine, because you do not have alpha. I do not know how you can specify the BC for you're problem.

My advice is that you just make a case with openFOAM and try out some things.
This thread is very good for you i think, but i really don't know more about this BC.

Best regards
Timucin

jp279 April 21, 2016 14:37

Dear Lokati,

Thanks for the reply. I'll make a test case and will get back with its details as well as the results (if any).

Jigar


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