How to use real gas as fluid domain in CFX ?

badboyz31 · September 23, 2014, 22:05

First of all, I would like to thank all of CFD-Online members for the solutions in my previous posts.

Currently, I want to create a hypersonic vehicle simulation with real gas (air). I've noticed that air ideal gas model is readily available, but the real gas simulation should need a gas model first. I've read about something like redlich-kwong and peng-robinson, and some other equations of state, but I don't really understand which model is the best for hypersonic flow.

What I want to ask is, which model is the most suitable and how do I create the model in CFX ?

I've tried to create a new material in CFX-Pre, but I don't really know if I am already doing the right thing. If there is an available tutorial, just drop it here. Thank you very much.

ghorrocks · September 23, 2014, 22:44

In hypersonic flow you get dissociation of the gas molecules into plasma. You cannot account for plasma using the real gas models in CFX to my knowledge as plasma is not an equation of state effect. CFX's real gas models are usually used to account for the non-linear material properties as you approach the liquid phase.

If you are just going to ignore the plasma then you are likely to be in the low density gas, moderate temperature regime, and ideal gas is as good as anything there.

singer1812 · September 24, 2014, 08:01

I concur with Glenn. One thing you might want to add to your model for hypersonic speeds is change the ideal gas visocity and conductivity from constant to another function. Sutherlands formula is documented for these and you can look that up.

badboyz31 · September 24, 2014, 08:59

So that's how it is. I believe that I am looking for the non-equilibrium effect of gas at very high temperature. Perhaps, I am out of luck here, but thanks anyway.

badboyz31 · September 24, 2014, 18:29

One more thing, what if I were to model the air as a mixture of oxygen and nitrogen as ideal gas. Perhaps it could be a reaction going on in the mixture. How do I make this material model ?

And about the boundary condition at the wall, I believe that I should replace adiabatic wall with another boundary condition. If I used wall temperature, which temperature should I input ?

ghorrocks · September 24, 2014, 18:37

You could use CFX's chemistry modelling to do this I guess. Have a look at the tutorials for examples.

But I am not convinced that this is heading in the right direction - I am no expert in hypersonics but there is lots of literature out there about it so I strongly recommend you find out what models are used in hypersonics modelling to account for these effects. Your comment "Perhaps it could be a reaction going on in the mixture." does not inspire me with confidence that you understand the basic physics of what is going on (I certainly don't understand it, hypersonics is not my field).

September 23, 2014, 22:05	How to use real gas as fluid domain in CFX ?	#1
badboyz31 Member Join Date: May 2013 Posts: 45 Rep Power: 12	First of all, I would like to thank all of CFD-Online members for the solutions in my previous posts. Currently, I want to create a hypersonic vehicle simulation with real gas (air). I've noticed that air ideal gas model is readily available, but the real gas simulation should need a gas model first. I've read about something like redlich-kwong and peng-robinson, and some other equations of state, but I don't really understand which model is the best for hypersonic flow. What I want to ask is, which model is the most suitable and how do I create the model in CFX ? I've tried to create a new material in CFX-Pre, but I don't really know if I am already doing the right thing. If there is an available tutorial, just drop it here. Thank you very much.

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September 23, 2014, 22:44		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	In hypersonic flow you get dissociation of the gas molecules into plasma. You cannot account for plasma using the real gas models in CFX to my knowledge as plasma is not an equation of state effect. CFX's real gas models are usually used to account for the non-linear material properties as you approach the liquid phase. If you are just going to ignore the plasma then you are likely to be in the low density gas, moderate temperature regime, and ideal gas is as good as anything there.

September 24, 2014, 08:01		#3
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	I concur with Glenn. One thing you might want to add to your model for hypersonic speeds is change the ideal gas visocity and conductivity from constant to another function. Sutherlands formula is documented for these and you can look that up.

September 24, 2014, 08:59		#4
badboyz31 Member Join Date: May 2013 Posts: 45 Rep Power: 12	So that's how it is. I believe that I am looking for the non-equilibrium effect of gas at very high temperature. Perhaps, I am out of luck here, but thanks anyway.

September 24, 2014, 18:29		#5
badboyz31 Member Join Date: May 2013 Posts: 45 Rep Power: 12	One more thing, what if I were to model the air as a mixture of oxygen and nitrogen as ideal gas. Perhaps it could be a reaction going on in the mixture. How do I make this material model ? And about the boundary condition at the wall, I believe that I should replace adiabatic wall with another boundary condition. If I used wall temperature, which temperature should I input ?

September 24, 2014, 18:37		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You could use CFX's chemistry modelling to do this I guess. Have a look at the tutorials for examples. But I am not convinced that this is heading in the right direction - I am no expert in hypersonics but there is lots of literature out there about it so I strongly recommend you find out what models are used in hypersonics modelling to account for these effects. Your comment "Perhaps it could be a reaction going on in the mixture." does not inspire me with confidence that you understand the basic physics of what is going on (I certainly don't understand it, hypersonics is not my field).