# Additional Variable with only source term

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 November 14, 2018, 15:46 Additional Variable with only source term #1 Member   Ftab Join Date: Sep 2011 Posts: 87 Rep Power: 14 Dear all, As I am trying to solve an ODE for additional variable which looks like this: dX/dt=aX+b I tried with diffusion equation with Zero diffusivity, but Kin_Diff should be between 0 and 1. What should I do? Put Kin_Diff equal to 1e-100? Is there a better way? By the way, the source term was modeled as a subdomain and source. Was it correct? Thanks

 November 14, 2018, 16:17 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,748 Rep Power: 143 That should work. Yes, you can turn the diffusion off by setting a very low diffusivity. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

November 15, 2018, 09:42
#3
Member

Ftab
Join Date: Sep 2011
Posts: 87
Rep Power: 14
Quote:
 Originally Posted by ghorrocks That should work. Yes, you can turn the diffusion off by setting a very low diffusivity.

Thanks a lot Glenn!
The simulation gave this error, but ran and converged:
Bounds error detected
---------------------
Variable: X.Dynamic Diffusivity
Locale : wall

Bounds error detected
---------------------
Variable: Y.Dynamic Diffusivity
Locale : wall

And this was for D=1e-40

Last edited by ftab; November 15, 2018 at 14:45.

 November 15, 2018, 16:30 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,748 Rep Power: 143 Looks like 1e-40 is too low and triggers a bounds warning. Try 1e-10. And if you want to be sure do a sensitivity check to ensure that this value is low enough that it is effectively zero (that is, run 1e-9 and compare the results. If no significant difference then it is effectively zero). __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

November 16, 2018, 09:01
#5
Member

Ftab
Join Date: Sep 2011
Posts: 87
Rep Power: 14
Quote:
 Originally Posted by ftab Thanks a lot Glenn! The simulation gave this error, but ran and converged: Bounds error detected --------------------- Variable: X.Dynamic Diffusivity Locale : wall Bounds error detected --------------------- Variable: Y.Dynamic Diffusivity Locale : wall And this was for D=1e-40

Update:
This strategy works only for D up to 1e-15, as the truncation error of 1e-16 returns zero and bound error for the Diff coefficient

 November 16, 2018, 17:17 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,748 Rep Power: 143 A minor point, but I don't think this is a truncation error. I think simply the bounds of that variable are set to 1e-15 and if you go below that it triggers the bounds error. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.