## Fluids

#### Choosing a Real Gas Model

• CFDGer
Subscriber

Hello,

for my thesis I am currently calculating/deciding whether I need to implement a real gas model or if it's sufficient whether I use the ideal gas law.

Case 1:

h=30km, Ma=8

p= 1,197kPa, p_t=1,16e7 Pa

T= 226,5K, T_t=3125,7 K

Case 2:

h=50 km, Ma=12

p=79,77 Pa, p_t=1,15e7 Pa

T=270,6K, T_t=3734,28 K

Now in the Fluent User Guide I discovered the two terms in which the ideal gas model can be used.

p/p_c<<1

or

T/T_c>2 and p/p_c<1

Now my question is which pressure and temperature I should use to determine the required gas model. If I use the ambient (static) pressure (as the "p" in the above equations) I should use the ideal gas law (this was done in similar final papers aswell for Ma=. If I use the total pressure I should use a real gas model.

In another final year paper the flow in a height of 80 km with a Ma=23 was simulated. This person used the Redlich-Kwong Real Gas model. My above mentioned calculations/observations apply to this case aswell which confuses me on which model I should use...

Best regards

• DrAmine
Ansys Employee

Absolute pressure is the pressure you should consider. As real gas calculation are much more difficult than simple ideal gas, one would not use real gas where ideal gas assumption is valid.

• CFDGer
Subscriber

Hello Amine,

But then how can I explain that at Ma=23 at h=80km the real gas model should be used? For this case (Ma=23, h=80km, p=1,05Pa, T=199K) according to the Fluent User Guide the Ideal Gas Model should be used. But to my understanding that's wrong.

Could you maybe elaborate your explanation (or somebody else)?

Regards

• DrAmine
Ansys Employee

There is nothing against using CEOS but Absolute Pressure is the one you should consider for your judgement. Real Gas contains more physic and I assume that is the way to when we have flows with Ma>30 (coupled with MAxwsell equations: plasma) for Flows which are hyper-sonic (3.5 to 30 no real gas effects noticeable but you can use real gas models)-

• CFDGer
Subscriber

just for my understanding, can you maybe name one explicit case in which real gas models *need* to be used? Even for Ma>30 the condition p/p_c<< is fulfilled (at least to my understanding) as we are just looking at the ambient pressure which is independent of the Mach number. Or do you mean we have to look at the maximum absolute pressure which occurs at our geometry (at the stagnation point)?

Sorry for all these questions but I really want to understand this topic.

best regards

• DrAmine
Ansys Employee

For Ma>30 you might plasma and dissociation effects: Example: Reentry of capsule..

Look after the "operating" pressure in your domain (mean) and use that as criterion. Is not magic: you can even compare the runs with ideal EOS or real EOS afterwards.

Ask you prof. for more theoretical background.

• CFDGer
Subscriber

My idea was to add dissociation effects anyways. But I wanted to model the gas as an ideal gas with temperature dependent cp, viscosity, etc.

Comparing both ideal and real EOS with each other is a good idea which I will probably do, thank you!

One last question: How can I check the (mean) operating pressure in my domain? The only one I know is the one I set to 0 before solving my problem.

Best regards

• DrAmine
Ansys Employee

Volume Average of Absolute Pressure in the domain.