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gardian November 8, 2010 07:05

Constant density

is it possible in CFX to setup the density ( i want to be fixed at some point )
instead of the pressure?

thank you

ghorrocks November 8, 2010 17:34

In a incompressible flow the density is fixed by definition. So I am assuming you are talking about compressible flows where the density can vary.

You cannot fix the density at a point in the simulation. You may be able to use a CEL expression to make a pressure boundary give you a fixed density at an inlet but I reckon this would cause no end of numerical instabilities.

Are you trying to match an experimental result where you have density at a single point?

gardian November 9, 2010 12:07

I have a Helium 3 gas in a cold tube and I know the exact number of moles inside .
i m trying to find out how are they distributed according to the boundary conditions.
which is some warmer ends of the tube.
what do you mean by : " You may be able to use a CEL expression to make a pressure boundary give you a fixed density at an inlet"

ghorrocks November 9, 2010 17:43

If you have a closed cavity then forget about the boundary condition comment, it was a bad idea anyway.

Based on your description then you should be able to specify an initial condition, then do a simulation to get the final condition (or the time history), and post process it.

So back to your original question - why do you want to specify a fixed density?

gardian November 10, 2010 05:17


it is because i know exactly the number of moles i put in this tube, by the pressure that I can measure before in another vessel. so i know how many moles are in this tube but not how they are distributed.

ghorrocks November 10, 2010 17:36

In a closed cavity for a well converged solution CFX will conserve mass. This means the average density in the chamber will be constant. You don't need to set this, this is already done.

So why do you need to set the density? I think you better explain what you are trying to do more thoroughly.

happy November 10, 2010 20:14

sad situation
Hi ghorrocks...

I need help from U specifically....Could U plz reply by your pravite email? where my system need many looks to go forword....pls, by pravite email because of my system confidentiality.I'v created my geometry & mesh and there are no error except for CFX-pre & Solution Update.

Please, do not ignore me, U are last help for me :confused:


ghorrocks November 11, 2010 01:06

I don't help people by private messaging. If you have a question then post it on the forum (as a new thread, please). If you can't show your model due to confidentiality reasons then make a simple version which shows your issues but does not infringe confidentiality.

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