
[Sponsors] 
October 19, 2022, 09:13 
Wrong Solution for sensibleEnthalpy

#1 
New Member
Join Date: Oct 2022
Posts: 5
Rep Power: 3 
Hello,
I'm using OpenFoamv7. While trying rhoCentralFoam at the shock Tube, I realised that the Solution is different from the analytic Solution. I tracked the misstake to the thermophysicalproperties. When using sensibleEntahlpy for the energy the Soultion seems to be wrong, while using sensibleInternalEnergy results in the correct Solution. Now I'm searching for the reason, because I will have to use sensibleEnthalpy for a individual Solver (the reason I'm using OpenFoam v7). Anyone has an answer to my Problem? 

October 19, 2022, 09:49 

#2 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
The difference should likely result from your definition of cv / cp. Aside from that, they should be equal. There are numerical advantages, but the formal definition is identical, so if one has a significantly higher residual that should give it away. If one of them did not converge.
Hence check cv/cp and the residuals. 

October 20, 2022, 11:49 

#3 
New Member
Join Date: Oct 2022
Posts: 5
Rep Power: 3 
I double checked the values for cp and cv and everything seems to be ok....


October 20, 2022, 13:44 

#4 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
Are they the same or different? You might want to use the same value for both


October 21, 2022, 04:57 

#5 
New Member
Join Date: Oct 2022
Posts: 5
Rep Power: 3 
They are different. Why should using the same value for both result in a correct Solution? They are different by definiton?


October 21, 2022, 06:51 

#6 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
That is the question I am asking....make sure you use the correct values here for the medium, pressure etc you are solving for! With incompressible flow you should have cp=cv...if your medium does not behave like an ideal gas you might want to make sure to double check your thermodynamics. They are only different if some of the energy used to heat up the fluid goes towards thermal expansion...this might not be the case numerically and hence the difference.


December 3, 2022, 06:31 

#7 
New Member
Join Date: Oct 2022
Posts: 5
Rep Power: 3 
Sorry for the late Reply...
I tried it with cp=cv and I got the right Solution. But I still don't get why. I use a compressible Solver (rhoCentralFoam) with air, so cv and cp can't be the same. Why is the Solution still correct? 

December 5, 2022, 04:32 

#8 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
Full Energy equation formulated with internal energy:
Full Energy equation formulated with Enthalpy: You can transform both formulations into each other. In other words they are identical. If you ignore the different variable names h or e, you can see that the only difference is the pressure work term div(pv). That describes the work needed to change the volume. Let's assume this to be zero. I.e an incompressible fluid. You'd get different solutions if you had a difference between cp and cv in that case. One would need to consider h=cp T and e = cv T and still you wouldn't have the same solution. So it does not matter if you are using a compressible solver. If there is no compression because your flow is to slow you'll get a wrong solution. The reason being that your equation of state assumption is not valid. You assume that there is compression when there is likely none. 

December 12, 2022, 10:54 

#9 
New Member
Join Date: Oct 2022
Posts: 5
Rep Power: 3 
I'm looking at a shock tube with a 20 bar and a 1 bar side so there should be a compression involved...


Tags 
openfoam7, sensibleenthalpy, thermophysicalproperties 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Stable but wrong solution in VOF method  Ulixes  FLUENT  0  May 1, 2017 14:08 
Ansys CFD POST constructs a solution history from wrong file.  Wingman  ANSYS  2  November 28, 2016 17:01 
grid dependancy  gueynard a.  Main CFD Forum  19  June 27, 2014 21:22 
Naca 0012 (compressible and inviscid) flow convergence problem  bipulsaha  FLUENT  1  July 6, 2011 07:51 
Wall functions  Abhijit Tilak  Main CFD Forum  6  February 5, 1999 01:16 