# bounding of k and epsilon diverges solution

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October 10, 2017, 00:58
bounding of k and epsilon diverges solution
#1
New Member

Sagar
Join Date: Sep 2017
Location: Surat, India
Posts: 11
Rep Power: 8
Hello,

I am using multiphaseInterFoam to simulate one case. For this case i am using k-epsilon turbulence model.

I have created unstructured mesh and quality of mesh seems to be right. CheckMesh report is as given below.

Checking geometry...
Overall domain bounding box (-3.65028 0 -3.91767e-016) (6.37721 4.4 3.6496)
Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
Mesh has 3 solution (non-empty) directions (1 1 1)
Boundary openness (-3.1831e-017 1.41703e-015 8.34435e-017) OK.
Max cell openness = 3.08743e-016 OK.
Max aspect ratio = 8.96002 OK.
Minimum face area = 4.24312e-007. Maximum face area = 0.108377. Face area magnitudes OK.
Min volume = 1.50115e-010. Max volume = 0.0121032. Total volume = 90.7938. Cell volumes OK.
Mesh non-orthogonality Max: 68.5635 average: 17.1285
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.983603 OK.
Coupled point location match (average 0) OK.

Mesh OK.

When i run my case using laminar model it works well but when i switch to k-epsilon it diverges due to bounding of epsilon followed by bounding of k after some iteration.

My fvScheme and fvSolution files are attached with this thread.

For initialization of k and epsilon, i have calculated their values using standard formulas and their values are in the order of 10^-10(for k) and 10^-15 (for epsilon) at the inlet and at outlet zeroGradient condition is used.(I have also tried with different values but still diverge the solution)

So my Question is "What is the reason for divergence by adding turbulence model only and how to resolve it?"

Thanks for the help in advance.
Attached Images
 Residuals.jpg (41.0 KB, 204 views)
Attached Files
 fvScheme_and_fvSolution.zip (1.3 KB, 42 views)

 October 10, 2017, 06:24 #2 New Member   Join Date: Aug 2017 Location: Milan Area, Italy Posts: 10 Rep Power: 8 Hi, you could first try to reduce the max non-orthogonality of the mesh (at least below 60). If that is not an option you could start by adding non-ortho correctors to your solver (set nNonOrthogonalCorrectors to 2 or 3). When I have to work with max non-ortho around 70 I also change the schemes to something like this (and stick with first order accuracy for divSchemes of turbulence terms): Code: ```gradSchemes { default cellLimited Gauss linear 0.5; // or cellLimited leastSquares 1.0; grad(U) cellLimited Gauss linear 0.5; // or cellLimited leastSquares 1.0; } laplacianSchemes { default Gauss linear limited 0.5; } snGradSchemes { default limited 0.5; }``` If BC are consistent you should achieve stability. Please also consider uploading your log file for more info. breznak and YinJian like this.

October 12, 2017, 23:43
#3
New Member

Sagar
Join Date: Sep 2017
Location: Surat, India
Posts: 11
Rep Power: 8
Hi Teosim,

Thanks for your reply. I have incorporated your suggestions and try to simulate the case But it still diverges.

I am attaching residuals plot as well as log file after changing fvSchemes and fvSolution files as per your suggestions.

Main thing i have observed is that this case works well with laminar model and it diverges with k-epsilon model. It can also be seen from log file that initiation of divergence starts from k and epsilon which ultimately leads to divergence of velocity and pressure fields. Am I right ?

I have tried different values of k and epsilon in "0" directory but it not works. So what should be the bottleneck ?

I have tried to simulate same case with Fluent with same mesh and k-epsilon model. It works.

So, other question is "Is there any difference in convergence criteria or treatment of k and epsilon in Fluent and OpenFOAM ?"

All suggestion are welcome.
Attached Images
 Residuals_2.jpg (41.3 KB, 167 views)
Attached Files
 log.zip (11.1 KB, 25 views)

June 8, 2023, 09:52
#5
New Member

Březňįk
Join Date: Jun 2020
Location: Prag
Posts: 21
Rep Power: 6
These schemes work very nicely.

Quote:
 Originally Posted by Teosim When I have to work with max non-ortho around 70 I also change the schemes to something like this (and stick with first order accuracy for divSchemes of turbulence terms): Code: ```gradSchemes { default cellLimited Gauss linear 0.5; // or cellLimited leastSquares 1.0; grad(U) cellLimited Gauss linear 0.5; // or cellLimited leastSquares 1.0; } laplacianSchemes { default Gauss linear limited 0.5; } snGradSchemes { default limited 0.5; }``` If BC are consistent you should achieve stability. Please also consider uploading your log file for more info.

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