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Additional Variable with only source term

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Old   November 14, 2018, 16:46
Default Additional Variable with only source term
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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
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Old   November 14, 2018, 17:17
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That should work. Yes, you can turn the diffusion off by setting a very low diffusivity.
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Old   November 15, 2018, 10:42
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Quote:
Originally Posted by ghorrocks View Post
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 15:45.
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Old   November 15, 2018, 17:30
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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).
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Old   November 16, 2018, 10:01
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Quote:
Originally Posted by ftab View Post
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
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Old   November 16, 2018, 18:17
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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.
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