Constant density

gardian · November 8, 2010, 06:05

Hello

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, 16:34

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, 11:07

Yes
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"
thanks

ghorrocks · November 9, 2010, 16:43

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, 04:17

Hello

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, 16:36

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, 19:14

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

Regards

ghorrocks · November 11, 2010, 00:06

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.

November 8, 2010, 06:05	Constant density	#1
gardian New Member anton.gardi Join Date: Aug 2010 Posts: 21 Rep Power: 15	Hello is it possible in CFX to setup the density ( i want to be fixed at some point ) instead of the pressure? thank you

November 10, 2010, 04:17	tube	#5
gardian New Member anton.gardi Join Date: Aug 2010 Posts: 21 Rep Power: 15	Hello 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.

November 10, 2010, 19:14	sad situation	#7
happy Senior Member Safia Join Date: Oct 2010 Location: Australia Posts: 161 Rep Power: 15	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 Regards

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November 8, 2010, 16:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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?

November 9, 2010, 11:07		#3
gardian New Member anton.gardi Join Date: Aug 2010 Posts: 21 Rep Power: 15	Yes 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" thanks

November 9, 2010, 16:43		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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?

November 10, 2010, 16:36		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.

November 11, 2010, 00:06		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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.