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Wrong temperature/pressure under constant gravity

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Old   January 29, 2012, 21:22
Default Wrong temperature/pressure under constant gravity
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jpo
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Hello,

I am trying to demonstrate the influence of gravity on a vertical air column. I did this simulation:

--------
Constant gravity against z, gravity = -981 m/s^2 (very high, but good to visualize the effect)

Fluid: 15m vertical air column (adiabatic walls), no flow

Initialize at 300K and 1 atm
volume is closed, no inlets/outlets


Residuals drop down but not smoothly
Temperature and pressure rise in the volume

perhaps someone has done this before and can give advice?
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Old   January 29, 2012, 21:50
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JPO,

Pressure won't be uniform in the volume, nor will temperature at first. The gravity, which is 100 times higher than normal, should cause pressure at the base to be:

\rho \cdot g \cdot h
or

density of the air (assume you're using ideal gas) * 981 * 15


Temperature should also change as the gas at the base is compressed, but given that it's adiabatic, should begin to become more uniform as thermal diffusion/convection make everything even.

Note that closed boundary problems are often quite difficult to converge, especially when you have variable temperature/pressure properties and (I'm guessing) initialization with 0 velocity.

A better way to demonstrate this is to make the top of your domain have a pressure boundary of 0 Pa gauge, and the bottom to have a mass flow of something like 0.00001 kg/s. The rate of flow is likely to be smaller than the rate you'll get due to natural convection, and you'll see the influence of gravity very well on your system (pressure at the bottom should be equal to what I've written above)

Enjoy!
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Old   January 29, 2012, 22:24
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Thank you ComputerGuy, I will try this... If a sealed domain (almost) converged, this one should be easier to converge

Trying it now...
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Old   January 30, 2012, 14:18
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Yes, I see the pressure difference (unchecked whether it conforms to the hydrostatic equation, but it should) and a temperature and density differences as well

This should be seen at any outlet pressure though... only the density/pressure/temp at the bottom should readjust to accomodate the change, if I understand this correctly
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