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-   -   'Air Ideal gas' density problem (https://www.cfd-online.com/Forums/cfx/120795-air-ideal-gas-density-problem.html)

rameezib July 15, 2013 05:49

'Air Ideal gas' density problem
 
I had run a simulation of a stator-rotor combination, for which I had chosen the medium as Air Ideal Gas. After the simulations were done, when i generated report, i was surprised to see the considered density to be 2.23 kg/m-3. Shouldn't it have been 1.2256 kg/m-3? Also how do i rectify it to make it as 1.2256 kg/m-3

ghorrocks July 15, 2013 20:31

This looks like an error in your reference pressure. If the reference pressure is 1atm and you have a pressure of 1atm then you have 2atm absolute and densities about 2.2kg/m3.

rameezib July 15, 2013 23:34

Thanks a lot for your valuable reply. Could you please respond to the following questions too.
Should i take the reference pressure as 0 atm?
So now do i have to re run the simulations?
And how do i rectify it?

ghorrocks July 15, 2013 23:45

You should put the reference pressure as a typical pressure in your domain. The documentation explains this.

And yes, you will have to redo the simulation as your current results are wrong.

rameezib July 16, 2013 00:24

Thanks again. I would like to explain my problem definition-
I had specified the reference pressure as 1 atm

Boundary condition: mass flow inlet to static pressure outlet
So accordingly i plugged in the values as
Inlet: Normal speed as 8.49 m/s
Outlet: 1 atm as my turbomachine opens into atmosphere.
So to rectify the problem of density, should i change the reference pressure as 0 atm or the outlet pressure as 0 atm or some other changes should be done.

evcelica July 16, 2013 05:51

Outlet pressure should be zero, reference pressure should be 1 atm.
Either will work fine, as long at they add up to 1atm, but this way is better for round off errors and things of that nature.

rameezib July 16, 2013 12:45

Thanks for your help.


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