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-   -   Air ideal gas wrong density (http://www.cfd-online.com/Forums/cfx/116707-air-ideal-gas-wrong-density.html)

Badi April 23, 2013 11:49

Air ideal gas wrong density
 
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


Regards

cdegroot April 23, 2013 12:55

I have never noticed this issue. Are you using CFX?

singer1812 April 23, 2013 16:44

Check your relative pressure. I bet you set it to 1atm and you didnt account for that in your BC's.

Badi April 23, 2013 17:18

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.

singer1812 April 23, 2013 17:28

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.

Badi April 23, 2013 17:33

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

Badi May 2, 2013 12:49

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

ghorrocks May 2, 2013 18:52

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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