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Air without Boussinesq approximation

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Old   February 12, 2020, 06:44
Post Air without Boussinesq approximation
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Sidharth K PIllai
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Hi,
I am doing a natural convection problem with imposed rotation in an annular space using ANSYS CFX as part of my academic work(yes I am new to this). My Temp difference (Ts-Ta) is around 20 degree which is above the boussinesq approximation limit(15deg.).

My problem is with material option.

When i choose ideal gas(in which i persume boussinesq is not used as i specify the reference temp), I am getting results very close to real situation.
When i choose "air at 25deg" option where density is treated as const property and bousinessq is applied by default, temperatiure is overpredicted by more than 10degree.

My doubt are:
1. how can i apply a fluid which is not treated as ideal and also boussinesq is not applies as well. The third option in materials eqn of state is defining it as 'real gas', but i don't know how to obtain all values that i have to specify in it.

2. Is there any other way in which I can choose a library fluid (air) by not choosing boussinesq approximation?
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Old   February 12, 2020, 07:15
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Glenn Horrocks
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1) Have a look in the CFX tutorial examples on real gas. You can get them from the ANSYS Customer site.. Not sure if the ANSYS Student Portal has them, it is a bit advanced for a student application.

2) Boussinesq approximation is the only constant density approach to bouyancy in CFX. The alternative is to define a density function versus temperature and then that will define the buoyant effects. A simple function would be ideal gas, but real gas and other EOS are also applicable.
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Old   February 14, 2020, 05:32
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Thank you for the reply.
I would try your suggestions and come back if anything troubles me
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