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Old   April 23, 2013, 11:49
Default Air ideal gas wrong density
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Sebastian
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Hi guys.
I have a little problem concerning the air ideal gas. The gas should have a density of 1.2 kg/m^3 (calculated by hand, pressure 1 bar, T = 348.4K)
Somehow Ansys always gives me a value of 2 kg/m^3

Has anyone a clue why this could be happening? Its a pretty simple case but I cant fix this problem


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Old   April 23, 2013, 12:55
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Chris DeGroot
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I have never noticed this issue. Are you using CFX?
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Old   April 23, 2013, 16:44
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Edmund Singer P.E.
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Check your relative pressure. I bet you set it to 1atm and you didnt account for that in your BC's.
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Old   April 23, 2013, 17:18
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Hey i checked everything, i did in fact set it to 1 bar on both domains.
my setup looks like this:

- 1d problem
- first 50 cell ideal gas with 1 bar and ~350K
- Domain interface
- second 50 cells 0.1 bar and ~280K

so my reference pressure is 1 bar on both sides and my initialization pressure is 1/ 0.1 bar.

I dont get how the "air ideal gas" actually works.
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Old   April 23, 2013, 17:28
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Edmund Singer P.E.
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Your BCs (and initialization) is relative to the 1bar relative pressure you are specifying. So, in essence, you have a 2 bar region, and the denisty is correct for that pressure.

Either alter your relative pressure to 0 bar (make sure you are really specifying BCs to 0bar relative pressure) or change all your BCs and intializations to account for the 1 bar relative pressure.
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Old   April 23, 2013, 17:33
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ah alright thanks I see. I thought its really referring to the pressure not adding up on it.

gonna try it out tomorrow thanks for the help

edit: it works with a relative pressure of 0 bar. thanks for that

Last edited by Badi; April 24, 2013 at 07:23.
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Old   May 2, 2013, 12:49
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Hey im having a strange problem again.
Im basically still having the same setup but now with water as fluid.

On my left side I have 0 bar reference and 1 bar relative pressure.
On the right side I have 0 Pa reference and 5000 Pa relative pressure.

When i start my transient simulation ill get strange results for for my very first timestep (0).

It shows 0 Pa on the left side and -9.5e4
The range is correct but the numbers are wrong. I dont really know where the problem could come from

Last edited by Badi; May 2, 2013 at 13:09.
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Old   May 2, 2013, 18:52
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Models often need a small timestep to start with if you are modelling the starting transient. So you need a smaller timestep - at least to begin with.
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