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Problem with boundary conditions: Ideal Gas Model vs Boussinesq

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Old   June 9, 2017, 05:39
Default Problem with boundary conditions: Ideal Gas Model vs Boussinesq
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Nicholas
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Hello,

I have found a weird behavior with the software to which I can't find an explanation.

I was trying to see how different boundary conditions influenced the results in a simple case, using either the Ideal Gas model or the Boussinesq approximation.

My domain is a square with a Pressure Outlet on each side (I was using this condition since I have to use it further for a natural convection study in open atmosphere).

There are no driving force for the fluid other than the pressure differentials caused by the definition of the boundary conditions. Gravity is activated.

When defining the pressure on the boundaries as 'Constant' (equal to 0 on all the four sides), with the Ideal Gas model I get a flow toward the bottom, with Boussinesq toward the top of the domain.

When defining the pressure on the boundaries with a Field Function as -9.81*Density*height, with both the Ideal Gas and Boussinesq I obtain a flux toward the top.
In this case I was actually expecting no movement at all, since the pressure that I defined on the boundary should have avoided pressure differentials.

Finally, when defining the pressure on the boundaries with the Field Function 'pressure', I do get no significant movements, with both models.

So my questions are:

1) What causes the different behavior between Ideal Gas and Boussinesq when I use 'Constant' as boundary pressure?

2) Why aren't the pressure differential and the related fluid movements suppressed by the "-9.81*Density*height" Field Function?

3) What does the 'Pressure' Field Function exactly do?

Thank you in advance for your answers!

Attached you find screenshots of the vector fields obtained with the aforementioned settings.
Attached Images
File Type: jpg IdealGas_Constant.jpg (104.9 KB, 42 views)
File Type: jpg Boussinesq_Constant.jpg (103.0 KB, 38 views)
File Type: jpg IdealGas_FieldFunction.jpg (112.4 KB, 31 views)
File Type: jpg Boussinesq_FieldFunction.jpg (105.7 KB, 30 views)
File Type: jpg IdealGas_Pressure.jpg (87.1 KB, 27 views)
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Old   June 11, 2017, 21:30
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I believe STAR-CCM+ changes the definition of the pressure field function based on the presence of gravity. My mind is fuzzy on it, but it's in the user guide. Depending on what's activated certain functions represent the static pressure and others are static+piezometric.
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Old   June 12, 2017, 06:40
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On a first glance I missed the part of the user guide where it specified the definition of the 'pressure' Field Function in the presence of gravity.
It does indeed take into account the weight of the air column.
What I still don't understand is the strange behaviour that you can see in the low left corner of the domain, where all the fluid seems to concentrate in an exit.
Also, I couldn't find an explanation yet to points 1) and 2).

Last edited by Cobra; June 13, 2017 at 12:30.
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Old   June 16, 2017, 01:17
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What is the reference density set to and at what height does it occur?
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