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Old   December 5, 2009, 16:44
Default Too large temperature decreasing above sound speed
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Roland Rakos
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Hello CFX Users,

I am often engaged in compressible flow and I have a problem:
If the Much number is larger than 1, so the temperature decreases overly. (For example: In a very thin gap -150K (!) has developed) I know that this temperature decreasing comes from the energy equation but in my opinion the wall friction should compensate this large temperature decreasing....at least partially. (So it seems to me that the CFD does not take into consideration the effect of the friction properly...)

Did anybody experience this problem?

Thanks;
Roland
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Old   December 6, 2009, 04:34
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Glenn Horrocks
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I seem to remember answering this before....

Firstly - can you explain how you got a negative absolute temperature? This is an obvious boundedness problem so you have a major problem there.

Secondly - Can you explain why you think wall friction has much of an effect on the temperature?

Thirdly - If you want to include viscous heating just activate the option. It is built into CFX but I strongly doubt it is what you are looking for.
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Old   December 7, 2009, 11:28
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Roland Rakos
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Quote:
Originally Posted by ghorrocks View Post
I seem to remember answering this before....

Firstly - can you explain how you got a negative absolute temperature? This is an obvious boundedness problem so you have a major problem there.

Oh, I'm sorry, I have written wrong. It is not -150 K instead -150 C.

Secondly - Can you explain why you think wall friction has much of an effect on the temperature?

In my opinion the fricton (at the walls) always should go with a little temperature increasing...

Thirdly - If you want to include viscous heating just activate the option. It is built into CFX but I strongly doubt it is what you are looking for.
Did you think at the "Incl. Viscous Work Term" ? This option was activated.
Besides I tried an other approach too: I defined outside temperature and heat transfer coefficient at the wall, but the temperature was very low still (about -80 C). (The theme of my experimentation is a simple 2D laval nozzle.)
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Old   December 7, 2009, 13:46
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Consequently I understand that the effect of the wall friction is not too large in terms of the temperature increasing, but I don't think that -80 C is able to develope in the reality...
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Old   December 7, 2009, 17:17
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Why not? Have you looked at the temperature an ideal gas will get to at the pressures you have through a gap of the size you have?
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