Physical parameters of water in CFX

aximefu · March 21, 2012, 03:00

In CFX what's the differences between physical parameters of water and water at 25℃. I want to simulate the phase change phenomenon between water and vapor, when choosing working fluids I don't know chich ones should be chosen.

If I choose water as a working fluid, the physical properities of which are the same with that of water at 25℃ when temperature is 25℃and different when temperature is not 25℃. Am I right?

ghorrocks · March 21, 2012, 05:20

If you are doing phase change stuff then if you use a constant properties material like water at 25C you will have an error, something like 10-20% at a guess, due to properties change. If you want to be more accurate than that you need to use variable properties - IAPWS is most accurate for water, it is a very accurate for water properties over just about any condition most people encounter - but it is much harder to converge.

If you are happy with a bit of an error then stick with fixed properties and your simulation will converge much easier. And definitely do all the simulation development (ie getting the phase change stuff working) using constant properties, and only flick over to IAPWS when it is working reliably.

aximefu · March 21, 2012, 05:38

Actually I met overflow problems very usually, but I don't sure if those were related to the fluid physical parameters' choosing.

ghorrocks · March 21, 2012, 05:42

That's why you should do development work with constant properties, and only switch to variable properties when everything else is working properly.

aximefu · March 21, 2012, 06:12

I just had a try with variable properities, and overflow appeared. It's hard to say which factor lead to the emergence of the problem.

ghorrocks · March 21, 2012, 16:37

Exactly. That's why you do development work on constant properties, and only when everything is working well do you turn on variable properties.

happy · March 21, 2012, 20:48

Quote:

Originally Posted by ghorrocks

Exactly. That's why you do development work on constant properties, and only when everything is working well do you turn on variable properties.

Does this work even with air with variable properties. I used the air with subsonic mode. However, I have convergance issue when I used ideal air gas.
Regards

ghorrocks · March 21, 2012, 21:14

By constant or variable properties I mean things like viscosity and specific heat. Variable density is not regarded as variable properties, that is regarded as incompressible flow versus compressible flow. But compressible flow also has its own stability issues and increases the complexity of a simulation. Again, do not use a compressible solver unless the flow needs it.

March 21, 2012, 03:00	Physical parameters of water in CFX	#1
aximefu New Member K.M.Yang Join Date: Feb 2012 Location: JiNan China Posts: 22 Rep Power: 14	In CFX what's the differences between physical parameters of water and water at 25℃. I want to simulate the phase change phenomenon between water and vapor, when choosing working fluids I don't know chich ones should be chosen. If I choose water as a working fluid, the physical properities of which are the same with that of water at 25℃ when temperature is 25℃and different when temperature is not 25℃. Am I right? Last edited by aximefu; March 21, 2012 at 03:54.

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March 21, 2012, 05:20		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If you are doing phase change stuff then if you use a constant properties material like water at 25C you will have an error, something like 10-20% at a guess, due to properties change. If you want to be more accurate than that you need to use variable properties - IAPWS is most accurate for water, it is a very accurate for water properties over just about any condition most people encounter - but it is much harder to converge. If you are happy with a bit of an error then stick with fixed properties and your simulation will converge much easier. And definitely do all the simulation development (ie getting the phase change stuff working) using constant properties, and only flick over to IAPWS when it is working reliably.

March 21, 2012, 05:38		#3
aximefu New Member K.M.Yang Join Date: Feb 2012 Location: JiNan China Posts: 22 Rep Power: 14	Actually I met overflow problems very usually, but I don't sure if those were related to the fluid physical parameters' choosing.

March 21, 2012, 05:42		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	That's why you should do development work with constant properties, and only switch to variable properties when everything else is working properly.

March 21, 2012, 06:12		#5
aximefu New Member K.M.Yang Join Date: Feb 2012 Location: JiNan China Posts: 22 Rep Power: 14	I just had a try with variable properities, and overflow appeared. It's hard to say which factor lead to the emergence of the problem.

March 21, 2012, 16:37		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Exactly. That's why you do development work on constant properties, and only when everything is working well do you turn on variable properties.

March 21, 2012, 21:14		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	By constant or variable properties I mean things like viscosity and specific heat. Variable density is not regarded as variable properties, that is regarded as incompressible flow versus compressible flow. But compressible flow also has its own stability issues and increases the complexity of a simulation. Again, do not use a compressible solver unless the flow needs it.