Too large temperature decreasing above sound speed

Roland R · December 5, 2009, 17:44

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

ghorrocks · December 6, 2009, 05:34

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.

Roland R · December 7, 2009, 12:28

Quote:

Originally Posted by ghorrocks

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.)

Roland R · December 7, 2009, 14:46

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...

ghorrocks · December 7, 2009, 18:17

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?

December 5, 2009, 17:44	Too large temperature decreasing above sound speed	#1
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	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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December 6, 2009, 05:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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.

December 7, 2009, 14:46		#4
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	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...

December 7, 2009, 18:17		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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?