Experience with 9 coefficient Janaf model modeling specific heat capacity

CFDger · August 7, 2020, 03:39

Hello,

I am modeling hypersonic air flow at Ma=7.9 and expect the total temperatures to be around 12000K (without considering chemical reactions). I always used the 7 coefficient polynomials for modeling my specific heat cp(T). But these polynomials are only valid up to 6000K. I am not sure whether FLUENT extrapolates values outside of this area or if it assumes a constant cp.

I have seen that the newer 9 coefficient polynomials can be fit up to 20000K. For the species O2, N2, O, N, NO I have gotten the coefficients which are valid for a max. temp of 20000K. But the one for air is apparently still only valid for a max temp of 6000K.

This is the link which provides the 9 coefficient polynomials:
https://www.grc.nasa.gov/WWW/CEAWeb/ceaThermoBuild.htm

Does anyone have experience with modeling air in such flight conditions?

Later on I plan to implement chemical reactions with which the maximum stagnation/total temperatures shoud be no higher than 7000K. So I don't think this should be an extreme problem at that point. But for initialising my problem I would like to start without chemical reactions and with air as fluid.

Fluid conditions:
Ts=1113K, ps=1824Pa, rho=5.71E-3kg/m^3

Thank you very much!

AlexanderZ · August 7, 2020, 04:09

do you expect to get these properties in open source?

use linear (or any other) extrapolation for range above 6000K

CFDger · August 7, 2020, 05:15

Well, yes until now I did. Why should I not expect to get those properties in open source?

Could you or sb else explain me how to set this up in the FLUENT settings (linear extrapolation for range above 6000K)?

Thanks!

AlexanderZ · August 7, 2020, 08:29

in fluent there is an option to use piecewise-linear properties.
you can use it.
For values above 6000K look at graph trend and based on your expectations extrapolate curve there

CFDger · August 19, 2020, 02:56

Another question which you or somebody else might be able to answer:

The standard 7 coefficient cp(T) polynom has the form of a classic polynom:

cp/R = a1+a2*T+a3*T^2+a4*T^3+a5*T^4
This is easy to implement in FLUENT as you mentioned.

But the 9 coeffcient cp(T) polynom has a different form:

cp/R = a1/T^2+a2/T+a3+a4*T+a5*T^2+a6*T^3+a7*T^4

Do you have to write an UDF to be able to implement this kind of "polynom" or is there a different way in ANSYS FLUENT?

Thanks

AlexanderZ · August 19, 2020, 03:41

I think, UDF is the only option.

on the other hand, you may use piecewise-linear option

Seyed Saeid Nabavi · May 4, 2022, 14:17

hi dude
sorry, can i have that 7 coeff polynomial for air cp? i need cp(T) up to 6000k.

August 7, 2020, 03:39	Experience with 9 coefficient Janaf model modeling specific heat capacity	#1
CFDger Member Join Date: May 2020 Posts: 42 Rep Power: 6	Hello, I am modeling hypersonic air flow at Ma=7.9 and expect the total temperatures to be around 12000K (without considering chemical reactions). I always used the 7 coefficient polynomials for modeling my specific heat cp(T). But these polynomials are only valid up to 6000K. I am not sure whether FLUENT extrapolates values outside of this area or if it assumes a constant cp. I have seen that the newer 9 coefficient polynomials can be fit up to 20000K. For the species O2, N2, O, N, NO I have gotten the coefficients which are valid for a max. temp of 20000K. But the one for air is apparently still only valid for a max temp of 6000K. This is the link which provides the 9 coefficient polynomials: https://www.grc.nasa.gov/WWW/CEAWeb/ceaThermoBuild.htm Does anyone have experience with modeling air in such flight conditions? Later on I plan to implement chemical reactions with which the maximum stagnation/total temperatures shoud be no higher than 7000K. So I don't think this should be an extreme problem at that point. But for initialising my problem I would like to start without chemical reactions and with air as fluid. Fluid conditions: Ts=1113K, ps=1824Pa, rho=5.71E-3kg/m^3 Thank you very much!

August 7, 2020, 04:09		#2
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	do you expect to get these properties in open source? use linear (or any other) extrapolation for range above 6000K __________________ best regards ****************************** press LIKE if this message was helpful

August 7, 2020, 08:29		#4
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	in fluent there is an option to use piecewise-linear properties. you can use it. For values above 6000K look at graph trend and based on your expectations extrapolate curve there __________________ best regards ****************************** press LIKE if this message was helpful

August 19, 2020, 03:41		#6
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	I think, UDF is the only option. on the other hand, you may use piecewise-linear option CFDger likes this. __________________ best regards ****************************** press LIKE if this message was helpful

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August 7, 2020, 05:15		#3
CFDger Member Join Date: May 2020 Posts: 42 Rep Power: 6	Well, yes until now I did. Why should I not expect to get those properties in open source? Could you or sb else explain me how to set this up in the FLUENT settings (linear extrapolation for range above 6000K)? Thanks!

August 19, 2020, 02:56		#5
CFDger Member Join Date: May 2020 Posts: 42 Rep Power: 6	Another question which you or somebody else might be able to answer: The standard 7 coefficient cp(T) polynom has the form of a classic polynom: cp/R = a1+a2T+a3T^2+a4T^3+a5T^4 This is easy to implement in FLUENT as you mentioned. But the 9 coeffcient cp(T) polynom has a different form: cp/R = a1/T^2+a2/T+a3+a4T+a5T^2+a6T^3+a7T^4 Do you have to write an UDF to be able to implement this kind of "polynom" or is there a different way in ANSYS FLUENT? Thanks

May 4, 2022, 14:17		#7
Seyed Saeid Nabavi New Member Join Date: Jul 2020 Posts: 7 Rep Power: 5	hi dude sorry, can i have that 7 coeff polynomial for air cp? i need cp(T) up to 6000k.