Error when using variable's value in variable's boundary value

Pled · September 6, 2018, 09:34

I have modeled an additional variable, let's call it 'A'.
The model that describes the flow of this variable is a transport model.
The domain has two walls one outlet and one inlet.
In the outlet the A's value is known.
In the inlet a zero flux is needed. But cfx does not allow it.
So I input A as A's value. But again, error are made during solution.
How can I model this ?

ghorrocks · September 6, 2018, 21:42

Your zero flux inlet is simply modelled by saying the value of A at the inlet is zero. Then the flux of A through the inlet will be zero. But I suspect that was not what you are asking - so you better ask your question again making sure you ask exactly what you intend.

Pled · September 7, 2018, 07:15

From a physics perspective having a zero flux does not mean having a value of 0.

The component-additional variable I am modelling is accumulated in that region but does not flow out of it. Think of it as a wall for that component, althought fluid is fllowing out of that region.

ghorrocks · September 7, 2018, 08:37

I think I get what you are talking about now. If the diffusion speed of your additional variable is faster than the inlet velocity then your additional variable will diffuse upstream and out the inlet. Is this what you are trying to stop?

To do this you are trying to impose a zero normal gradient on the additional variable at the inlet. The documentation suggests there is a zero flux option for additional variables (https://www.sharcnet.ca/Software/Ans.../i1305141.html).

September 6, 2018, 09:34	Error when using variable's value in variable's boundary value	#1
Pled Member Dimitrios S. P. Join Date: Jan 2018 Posts: 63 Rep Power: 8	I have modeled an additional variable, let's call it 'A'. The model that describes the flow of this variable is a transport model. The domain has two walls one outlet and one inlet. In the outlet the A's value is known. In the inlet a zero flux is needed. But cfx does not allow it. So I input A as A's value. But again, error are made during solution. How can I model this ?

September 6, 2018, 21:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Your zero flux inlet is simply modelled by saying the value of A at the inlet is zero. Then the flux of A through the inlet will be zero. But I suspect that was not what you are asking - so you better ask your question again making sure you ask exactly what you intend. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

September 7, 2018, 08:37		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	I think I get what you are talking about now. If the diffusion speed of your additional variable is faster than the inlet velocity then your additional variable will diffuse upstream and out the inlet. Is this what you are trying to stop? To do this you are trying to impose a zero normal gradient on the additional variable at the inlet. The documentation suggests there is a zero flux option for additional variables (https://www.sharcnet.ca/Software/Ans.../i1305141.html). __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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September 7, 2018, 07:15		#3
Pled Member Dimitrios S. P. Join Date: Jan 2018 Posts: 63 Rep Power: 8	From a physics perspective having a zero flux does not mean having a value of 0. The component-additional variable I am modelling is accumulated in that region but does not flow out of it. Think of it as a wall for that component, althought fluid is fllowing out of that region.