Decrease of internal energy

Tobard · July 9, 2011, 09:57

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!

July 9, 2011, 09:57	Decrease of internal energy	#1
Tobard New Member Join Date: Mar 2011 Posts: 28 Rep Power: 15	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

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July 9, 2011, 11:32		#2
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	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.

July 9, 2011, 12:00		#3
Tobard New Member Join Date: Mar 2011 Posts: 28 Rep Power: 15	Hi, I'm sorry: I forgot to precise this important point: fluid is oil, and the flow remains laminar. Thanks!

July 9, 2011, 14:12		#4
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	Hvae you modeled cp as a function of temperature?

July 9, 2011, 16:45		#5
Tobard New Member Join Date: Mar 2011 Posts: 28 Rep Power: 15	No, every fluid characteristic is constant... :-s

July 11, 2011, 20:54		#6
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	You should specify fluid properties as a function of temperature. That may give you the desired outcome

July 11, 2011, 21:04		#7
Tobard New Member Join Date: Mar 2011 Posts: 28 Rep Power: 15	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!