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#1 |
Member
Join Date: Apr 2010
Location: Pisa / Italy
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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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#2 |
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Glenn Horrocks
Join Date: Mar 2009
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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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#3 |
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Hi Glenn,
I've seen you were also answering me on another post http://www.cfd-online.com/Forums/cfx...ed-cavity.html 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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#4 |
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Glenn Horrocks
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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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#5 |
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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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#6 |
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Glenn Horrocks
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Can you post an image of what you are modelling and your CCL?
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#7 |
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I have just dowloaded the ansys tutorial "buoyant flow in a partitioned cavity". Than I have switched AIR at 25°C (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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#8 |
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Glenn Horrocks
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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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#9 |
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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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#10 |
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Glenn Horrocks
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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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#11 | |
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Quote:
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#12 |
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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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#13 |
Super Moderator
Glenn Horrocks
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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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buoyancy, ideal gas |
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