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Problem with gas in Non-Isothermal Flow (nist)

Posted Dec 18, 2011, 8:16 a.m. EST Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD) Version 4.2 3 Replies

Herve Vince
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Hello

I have a problem with gases in the Non-isothermal flow module (Comsol 4.2)
To illustrate that problem, I will use the official model "Free Convection in a Water Glass" (www.comsol.com/showroom/gallery/195/) because I have created a model based on that and if one's solves the problem on that model, it will solve my problem too.

So, in the example "Free Convection in a Water Glass", if I replace the water by steam or air (from the material library), the simulation crashes and it says that no initial solution has been found. If I deactivate the Volume Force in the nist physics model (which is "-nitf.rho*g_const") it works (but it is not interesting because the gas doesn't move).

Have you any idea about the origin of the problem ?

Thank you very much.
3 Replies Last Post Dec 19, 2011, 8:04 a.m. EST
Lechoslaw Krolikowski
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Posted: 1 decade ago Dec 18, 2011, 8:21 p.m. EST
Hi,

If you replace the water (density independent on pressure) by air (density dependent on pressure) and do not change the relation p=nitf.rho*g_const*(0.1[m]-z) in the Initial Values node then the circular dependency appears, because to calculate the pressure you need to know the density but to calculate the density you need to know the pressure.

Best regards,
Andrzej
Hi, If you replace the water (density independent on pressure) by air (density dependent on pressure) and do not change the relation p=nitf.rho*g_const*(0.1[m]-z) in the Initial Values node then the circular dependency appears, because to calculate the pressure you need to know the density but to calculate the density you need to know the pressure. Best regards, Andrzej
Herve Vince
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Posted: 1 decade ago Dec 18, 2011, 11:44 p.m. EST
Of course, the compressibility !
Thank you very much.

But ... what would be the right equations/initial conditions for this problem with a compressible gas ?
Of course, the compressibility ! Thank you very much. But ... what would be the right equations/initial conditions for this problem with a compressible gas ?
Lechoslaw Krolikowski
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Posted: 1 decade ago Dec 19, 2011, 8:04 a.m. EST
In the Initial Values node you can use p = rho_0*g_const*(0.1[m]-z) + p_0 ,
where rho_0 = p_0*M/(R*Tinit),
M - molar mass of the gas phase,
R - gas constant,
p_0 - pressure in the vessel,
Tinit - initial temperature.
In the Initial Values node you can use p = rho_0*g_const*(0.1[m]-z) + p_0 , where rho_0 = p_0*M/(R*Tinit), M - molar mass of the gas phase, R - gas constant, p_0 - pressure in the vessel, Tinit - initial temperature.

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