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Old   March 22, 2016, 10:11
Angry fluent divergence for no reason
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hello everyone,

i am working on a cfd simulation of a combustion chamber.
The mesh passed the Mesh Check in Fluent and Icem and the quality is good.
I use a Delaunay Mesh and profiles as Inlet Boundaries.
The Case initialiazed and works pretty good. The residuals seem to converge. But I have a big problem. It happens sometimes when I save my case and data that fluent diverges for no reason.

For example:
I saved a case and data file at iteration 500 and one at iteration 1000.
After Saving the Case and Data files of iteration files 1000 the solver does not work anymore and says:

# Divergence detected in AMG solver: x-momentum -> Increasing relaxation sweeps!
# You may try the enhanced divergence recovery with (rpsetvar 'amg/protective-enhanced? #t)

absolute pressure limited to 5.000000e+10 in 249 cells on zone 72

turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 12954 cells

Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: x-momentum
Divergence detected in AMG solver: y-momentum
Divergence detected in AMG solver: z-momentum
Divergence detected in AMG solver: pressure correction
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: species-1
Divergence detected in AMG solver: temperature
Error at host: floating point exception

Error at Node 0: floating point exception

Error at Node 1: floating point exception

Error at Node 2: floating point exception

Error at Node 3: floating point exception

Error at Node 4: floating point exception

Error at Node 5: floating point exception

Error at Node 6: floating point exception

Error at Node 7: floating point exception

Error at Node 8: floating point exception

Error at Node 9: floating point exception

Error at Node 10: floating point exception

Error at Node 11: floating point exception

Error: floating point exception
Error Object: #f


The weird thing is, that the case and data files of iteration 500 do not work as well! It seems like fluent does not want to work anymore and I have to start my calculation from the beginning without saving any case and data files.

Does anyone have a solution for that problem?

I use Fluent 16 in parallel mode with 12 cores and double precision.

Thank you very much

sufjanst
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Old   March 23, 2016, 10:06
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hello sufjanst,

i have/had a similar problem and was not able to find a solution. Does this happen with all your calculations or just one? Every time this happened to me i had to reset the setup in order to get it to work again.

cheers
peterpete
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Old   March 23, 2016, 17:08
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Lucky Tran
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I've also had a similar issue where it starts doing all sorts of stupid things after you save a file but will run forever if you do not save. Maybe this is the same as your problem.

I was only able to solve it by completely starting over a fresh new simulation (importing a new mesh, re-doing all my settings, and just everything). I tried using a settings file to import the old settings onto a new mesh and that also did not work. Re-initializing, and patching also did not fix anything. It seems like there is a way for the settings to get corrupted and cause the simulation to crash that is not easy to fix.

The only real way to avoid it is to save backup .cas files in case this happens (which I never do).
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