Units with use of the Ideal gas law?

Chilli83 · April 8, 2009, 06:14

Hi all

I have a little question about how CFX get the units to match when I use the setting Air Ideal gas. I have defined following expression in CFX-PRE:
$\rho_{ref} = \frac{p_{ref} \cdot M_{air}}{R \cdot T}$
whit the values of:
$p_{ref}$ =1 atm
$M_{air}$ =28.96 g/mol

=293.65 K
and then I let CFX use the establish R in CFX and I get a density of the air in the unit kg/m^3. But I can not see how the units match, so could somebody give me a hint about how CFX get the units to match?

Thank you very much.

Kind regards
Michael

Lance · April 8, 2009, 07:11

R have units J K^-1 mol^-1 and simple dimensional analysis resolves your density to kg m^-3

Chilli83 · April 8, 2009, 08:05

Quote:

Originally Posted by Lance

R have units J K^-1 mol^-1 and simple dimensional analysis resolves your density to kg m^-3

Hi Lance

Thank you for your reply. I found out that I used a wrong value and unit for the gas constant but it gives the correct now.

Kind regards
Michael

April 8, 2009, 06:14	Units with use of the Ideal gas law?	#1
Chilli83 New Member Michael Join Date: Apr 2009 Posts: 20 Rep Power: 17	Hi all I have a little question about how CFX get the units to match when I use the setting Air Ideal gas. I have defined following expression in CFX-PRE: $\rho_{ref} = \frac{p_{ref} \cdot M_{air}}{R \cdot T}$ whit the values of: $p_{ref}$ =1 atm $M_{air}$ =28.96 g/mol =293.65 K and then I let CFX use the establish R in CFX and I get a density of the air in the unit kg/m^3. But I can not see how the units match, so could somebody give me a hint about how CFX get the units to match? Thank you very much. Kind regards Michael

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April 8, 2009, 07:11		#2
Lance Senior Member Lance Join Date: Mar 2009 Posts: 669 Rep Power: 22	R have units J K^-1 mol^-1 and simple dimensional analysis resolves your density to kg m^-3