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-   -   Decrease of internal energy (https://www.cfd-online.com/Forums/fluent/90415-decrease-internal-energy.html)

 Tobard July 9, 2011 09:57

Decrease of internal energy

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!

Tobard

 nikhil July 9, 2011 11:32

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.

 Tobard July 9, 2011 12:00

Hi,

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

Thanks!

 nikhil July 9, 2011 14:12

Hvae you modeled cp as a function of temperature?

 Tobard July 9, 2011 16:45

No, every fluid characteristic is constant... :-s

 nikhil July 11, 2011 20:54

You should specify fluid properties as a function of temperature. That may give you the desired outcome

 Tobard July 11, 2011 21:04

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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