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Old   March 18, 2014, 07:32
Default Ideal gas problem
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Hi all.

I've a problem in simulating a Buoyant Ideal gas inside a closed domain.
Here is the problem.

If I use Air Ideal Gas I obtain a constant absolute pressure field, with a variable density and velocity field

If I use a constant density gas I obtain a Isochoric compression with no buoyancy.

How can I simulate a buoyant gas AND an increase in internal pressure due to temperature increase? Have I to use real gas model?

Thank you
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Old   March 18, 2014, 17:49
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Ideal gas will have a hydrostatic pressure gradient. You do not need real gas for this. Have a look in the documentation about hydrostatic pressure - the pressure variable has the hydrostatic component removed in buoyant simulations. But the absolute pressure should include the hydrostatic component.
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Old   March 19, 2014, 03:05
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Hi Glenn,

I've seen you were also answering me on another post Question from tutorial: Buoyant flow in a partitioned cavity

My problem is exactly the same (same tutorial), but in a steady state simulation. I use ideal gas but the average absolute pressure inside my domain is always 101325 Pa which is the reference pressure I set.

I tried to set Pressure level information but It give me error

STDOUT:

Fatal bounds error detected
---------------------------
Variable: Absolute Pressure

so probably I set something wrong
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Old   March 19, 2014, 06:52
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You will have problems running a closed domain with an incompressible fluid. You will get out of bounds errors on pressure...... Which is what you are getting

You have to either make the fluid compressible or put a pressure boundary in it to define the pressure.
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Old   March 19, 2014, 07:02
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Hi,

My fluid is compressible. Is Air ideal gas. I only get this error if I set pressure level information at 101350 Pa like the reference pressure inside my fluid_domain tab.

I can't insert a pressure boundary condition since my problem is a closed box ( like the tutorial )
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Old   March 19, 2014, 07:10
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Can you post an image of what you are modelling and your CCL?
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Old   March 19, 2014, 07:46
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I have just dowloaded the ansys tutorial "buoyant flow in a partitioned cavity". Than I have switched AIR at 25C (incompressible gas used in tutorial) with AIR IDEAL GAS and set the reference density for buoyancy as " volumeAve(Density)@Buoyancy2D"

Here I post the results for STEADY STATE and UNSTEADY STATE simulation. As you can see the pressure ( relative pressure ) is almost 11000 Pa in the unsteady simulation ( in agreement with a isochoric transformation for a ideal gas ). For the Steady simulation is almost 0 Pa

Setting are exactly the same

Domain Name : Buoyancy2D
+--------------------------------------------------------------------+
| Variable Name | min | max |
+--------------------------------------------------------------------+
| Density | 1.01E+00 | 1.27E+00 |
| Specific Heat Capacity at Constant Pressure| 1.00E+03 | 1.00E+03 |
| Dynamic Viscosity | 1.83E-05 | 1.83E-05 |
| Thermal Conductivity | 2.61E-02 | 2.61E-02 |
| Isothermal Compressibility | 9.87E-06 | 9.87E-06 |
| Static Entropy | -6.97E+01 | 1.56E+02 |
| Velocity u | -1.05E-06 | 1.09E-06 |
| Velocity v | -1.11E-06 | 1.06E-06 |
| Velocity w | 0.00E+00 | 0.00E+00 |
| Pressure | -4.35E-05 | 6.60E-05 |
| Temperature | 2.78E+02 | 3.48E+02 |
| Static Enthalpy | -2.01E+04 | 5.02E+04 |
+--------------------------------------------------------------------+
Domain Name : Buoyancy2D
+--------------------------------------------------------------------+
| Variable Name | min | max |
+--------------------------------------------------------------------+
| Density | 1.13E+00 | 1.42E+00 |
| Specific Heat Capacity at Constant Pressure| 1.00E+03 | 1.00E+03 |
| Dynamic Viscosity | 1.83E-05 | 1.83E-05 |
| Thermal Conductivity | 2.61E-02 | 2.61E-02 |
| Isothermal Compressibility | 8.85E-06 | 8.85E-06 |
| Static Entropy | -1.01E+02 | 1.24E+02 |
| Velocity u | -1.17E-06 | 1.22E-06 |
| Velocity v | -1.24E-06 | 1.18E-06 |
| Velocity w | 0.00E+00 | 0.00E+00 |
| Pressure | 1.17E+04 | 1.17E+04 |
| Temperature | 2.78E+02 | 3.48E+02 |
| Static Enthalpy | -2.01E+04 | 5.02E+04 |
+--------------------------------------------------------------------+
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Old   March 19, 2014, 17:34
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You should set the reference density to a constant value. I am not sure what CFX does if you set it to a potentially varying number like you have. It could do unexpected things.
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Old   March 20, 2014, 04:14
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Hi,

I asked to Ansys support service. They said It's a bug of the software. Basically to solve this situation they told me to insert a temperature variable " presure level information ", according to perfect law gas.

Hope this can help other people with same problem.

Thanks Glenn for you kindness
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Old   March 20, 2014, 06:13
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Sure, but still give it a constant value, not a function of the field variables. The whole point of a reference value is it is constant. If it changes it is not much of a reference.
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Old   March 20, 2014, 06:22
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Quote:
Originally Posted by Atze View Post
Hi,

I asked to Ansys support service. They said It's a bug of the software. Basically to solve this situation they told me to insert a temperature variable " presure level information ", according to perfect law gas.

Hope this can help other people with same problem.

Thanks Glenn for you kindness
What does it means: temperature variable " presure level information "? p = p(T) = Rho * R * T?
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Old   March 20, 2014, 06:46
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In solver control, under " pressure level informations " you have to write

P = P0/T0 * T , where P0,T0 are reference values for your gas and T is the mean temperature in your domain
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Old   March 20, 2014, 16:56
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Yes, but mean temperature at the initial condition (or some other condition) to return a constant value. Do not make it a function of the average temperature at that moment in time because then it is not constant.
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