Menu

Discussion Forum

New Discussion
查找

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Problem with gas in Non-Isothermal Flow (nist)

Posted 2011年12月18日 GMT-5 08:16 Fluid & Heat, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD) Version 4.2 3 Replies

Herve Vince
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

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 2011年12月19日 GMT-5 08:04
Lechoslaw Krolikowski
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2011年12月18日 GMT-5 20:21
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2011年12月18日 GMT-5 23:44
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2011年12月19日 GMT-5 08:04
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.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content