Ideal Gas vs Constant Density Solving

spelletier · July 3, 2019, 23:58

Hello all!

I have a quick question regarding the effects of using the ideal gas law vs constant density solvers.

From what I understand it is recommended to use the ideal gas law for flows above ~mach > 0.3 to assure accurate results and generally below that value the compressibility effects are negligible. If however you were to use the ideal gas law for low mach number flows or flows that could be considered incompressible are you producing incorrect results? If I am not sure whether my flow is effected by compressibility is it more safe to use the ideal gas law or to leave a constant/incompressible density?

sbaffini · July 4, 2019, 04:27

If the method you are using is capable of handling the intricacies of a compressible fluid model (as ideal gas is) working in the low Mach regime, then you are fine, at worst you are wasting some computational resources.

sbaffini · July 4, 2019, 04:29

For example, the solvers in Fluent are all capable of this, so no worry if you are using it. Still, some specific knowledge is required to properly use such solvers.

July 3, 2019, 23:58	Ideal Gas vs Constant Density Solving	#1
spelletier New Member Sebastian Pelletier Join Date: May 2019 Posts: 12 Rep Power: 7	Hello all! I have a quick question regarding the effects of using the ideal gas law vs constant density solvers. From what I understand it is recommended to use the ideal gas law for flows above ~mach > 0.3 to assure accurate results and generally below that value the compressibility effects are negligible. If however you were to use the ideal gas law for low mach number flows or flows that could be considered incompressible are you producing incorrect results? If I am not sure whether my flow is effected by compressibility is it more safe to use the ideal gas law or to leave a constant/incompressible density?

July 4, 2019, 04:27		#2
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,192 Blog Entries: 29 Rep Power: 39	If the method you are using is capable of handling the intricacies of a compressible fluid model (as ideal gas is) working in the low Mach regime, then you are fine, at worst you are wasting some computational resources. spelletier likes this.

July 4, 2019, 04:29		#3
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,192 Blog Entries: 29 Rep Power: 39	For example, the solvers in Fluent are all capable of this, so no worry if you are using it. Still, some specific knowledge is required to properly use such solvers. spelletier likes this.

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