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Decrease of internal energy

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Old   July 9, 2011, 09:57
Default Decrease of internal energy
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Hello everybody!

Does anyone can explain that phenomenon?

I'm studying a transient flow inside a closed rotating cylinder. There is no heat flux at the wall.
As expected, the average internal energy increases because of viscosity, but after a few time steps it starts to clearly and regularly decrease!

I don't see any physical reason to explain that fall... Any ideas?

Thank you so much!

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Old   July 9, 2011, 11:32
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Hi,

What kind of fluid are you simulating?

Because for an ideal gas/fluid ,(u=Internal energy) u=u(T); However for a real gas u=u(P,T).

If the flow is turbulent, pressure drop will be significant as compared to the temperature drop. Thus dependence of u on pressure is significant.

That may be the reason why your internal energy is decreasing.
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Old   July 9, 2011, 12:00
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Hi,

I'm sorry: I forgot to precise this important point: fluid is oil, and the flow remains laminar.

Thanks!
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Old   July 9, 2011, 14:12
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Hvae you modeled cp as a function of temperature?
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Old   July 9, 2011, 16:45
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No, every fluid characteristic is constant... :-s
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Old   July 11, 2011, 20:54
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You should specify fluid properties as a function of temperature. That may give you the desired outcome
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Old   July 11, 2011, 21:04
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Are you sure? The expected elevation of temperature (analytically evaluated) should be around 0.002 K...
That's very few, but that is that "very few" that interests me...

By the way, do you know if the "coupled" pressure-velocity coupling (instead of "SIMPLE") can have an influence on that weird fall? It's the only one that give me satisfactory convergence.

Thanks!
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