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Divergence detected in AMG solver: k, epsilon, pressure coupled

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Old   January 4, 2014, 14:30
Default Divergence detected in AMG solver: k, epsilon, pressure coupled
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Tanjina Afrin
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Hello everyone,

Hope everyone is doing fine. I am modeling a 3D model of underdrain pipe flow. I am using parallel computing. Details about the model is :

Inlet: pressure inlet with hydrostatic pressure
outlet: pressure outlet
wall and symmetry

Solver: Pressure based
Model: Standard k-e model
Solution Method: coupled
Least square cell based gradient and 2nd order upwind momentum

Time : transient

URF was all 1 except k and e. Those were 0.8

I started with 0.0015 time step. After running 5 second ( almost 35000 iteration) it gave the error " divergence detected in AMG solver: k"

so I reduced all URF to 0.7 . Then it gave the error " divergence detected in AMG solver: epsilon ". I reduced the URF to 0.5~0.6. Then it gave several error after 2.7 sec run time.

# Divergence detected in AMG solver: pressure coupled --> Decreasing coarsening group size!
# Divergence detected in AMG solver: pressure coupled --> Increasing relaxation sweeps!
# Divergence detected in AMG solver: pressure coupled --> Changing to W-cycle!

turbulent viscosity limited to viscosity ratio of 10^5 in 93000 cells.

Please find the following image for error and residuals. I tried with Flow courant number 100, but no improvement.

Before showing error it gives same mass flow rate in outlet for long time, which I assume as converged.

I am not an expert in CFD field. Trying to learn these things for my thesis. Hope someone can help me how can I get rid of these error. Thanks in advance

Regards,
Tanjina
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File Type: jpg error.jpg (66.8 KB, 14 views)
File Type: jpg residuals.jpg (88.0 KB, 20 views)
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Old   January 8, 2014, 05:59
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sina
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Dear Tanjina

Do you use mesh adaption?
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Old   January 8, 2014, 17:06
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Tanjina Afrin
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Quote:
Originally Posted by sina.sh View Post
Dear Tanjina

Do you use mesh adaption?
Hello Sina,

Thanks for your reply. I tried to use Mesh Adaption, but couldn't use to solve the skewness. Finally I found where skewness exceed 0.98. Its in between two cell zone. Since I didn't find any way to refine the mesh on a certain zone, I remesh the whole model, for which the elements number get double. But the skewness is below the 0.98 and model is running properly. I am still trying to find refine the mesh on a certain portion of model.

Would you please help me in this regards ? Thanks in advance.

Best Regards,
Tanjina
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Old   January 11, 2014, 01:35
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sina
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Dear tanjina

Mesh adaption in fluent not work properly! you can use CFX to solve your case.

Regards
sina
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Old   January 12, 2014, 21:56
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Nguyễn Khang Ninh
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sometime i also encountered this problem. I solved by reduced number of elements mesh
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Old   January 13, 2014, 11:46
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Tanjina Afrin
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Quote:
Originally Posted by sina.sh View Post
Dear tanjina

Mesh adaption in fluent not work properly! you can use CFX to solve your case.

Regards
sina
Hi Sina,

Thanks for your reply and I am sorry for late reply. CFX is a solver just like fluent, right? I have to use Fluent. Is there any way that I can use mesh adaption from CFX and then solve in Fluent ? ( Pardon me if it's an unrealistic question)

Hi Ntsphong2, thanks for your reply also. Could you please explain what do you mean by reduced number of element mesh ?? Thanks in advance.

Regards,
Tanjina
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