[vigneshkrish333]

What is the purpose of giving the fluid density as "IDEAL GAS"?

as I am working on a FSI problem where my working fluid is "AIR", I know the density as 1.225 kg/m3? what will happen if I select the density as IDEAL GAS instead of CONSTANT in the drop down menu of the density in the materials-->fluid-->air-->density(drop down menu)-->ideal gas? please explain the science behind this!!



Thanks in Advance sir!!

[LuckyTran]

Because "air" in general does not have a constant density, it depends on the temperature and pressure (and many other things) and ideal gas is a much better model for it. The ideal gas model has been used since the earliest days of thermodynamics.



Btw if you measure the density of the air you are breathing it's probably not 1.225 kg/m^3 (because that's the density of air at the reference temperature of 15 Celsius), it's a lot closer to 1.18.


So the first major change is that your density will change from 1.225 to something like 1.18, this much is like running with a different constant density setting. The next major change is you now have compressibility effects. Depending on what your flow is, this may change a lot of change nothing at all. You can estimate the Mach number of the flow, if the Mach number is low then the compressibility effects are low and there will hardly be any change. Probably you are in the low Mach number category because if compressibility effects were important, you wouldn't be ignorant of it in the first place.

[vigneshkrish333]

Hello sir...Thank you for your quick reply. I am dealing with the FSI of compressor piping system. I am considering a compressor of working range of 250 psi outlet pressure. In this case is it okay to consider the air as IDEAL GAS? The piping system I am considering is the pipe after the compression chamber sir.

[LuckyTran]

It's a lot better than assuming that it's a constant density, that's for sure.


Also the density of air at 250 psi is on the order of 30 kg/m^3.... not 1.225

[vigneshkrish333]

Originally Posted by LuckyTran

It's a lot better than assuming that it's a constant density, that's for sure.


Also the density of air at 250 psi is on the order of 30 kg/m^3.... not 1.225

Hello Sir!!
How you are saying that the density of air at 250 psi as 30 kg/m^3??
what is the formula sir....How you are calculating that?

[karachun]

Density formula can be derived from Ideal Gas Law.
https://en.wikipedia.org/wiki/Ideal_gas_law

I recommend you to read some thermodynamic book and improve your theory knowledge. At this point your theory is weak and without theory background you can not success in such complex problem as compressor.