# Problem with grid convergence for turbulent flow around cylinder

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November 7, 2018, 06:23
Problem with grid convergence for turbulent flow around cylinder
#1
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A
Join Date: Jun 2016
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Dear OpenFOAMers,

I have trying to achieve grid convergence for the mean drag coefficient and RMS of the lit force coefficient on a stationary, fixed cylinder at Re = 3.6*10^6. Yet, it seems that the drag and lift force coefficient keep on increasing with relatively big jumps with every mesh refinement. I have used the following meshes:

1. Coarse mesh: N = 69.069
2. Regular mesh: N = 98.730
3. Fine Mesh: N = 176.635

All the meshes were done in 100Dx100D domain. The mesh was divided into multiple blocks, where the cylinder was surrounded by the typical O-grid block. The turbulence model that I am using is k-w SST without wall models. Y+ was smaller than 1 and equal in all three meshes. I have seen in literature another numerical study (title: Numerical simulation of flow around a smooth circular cylinder at very high Reynolds numbers. M. Ong et al., 2009) where grid convergence was reached for a 2D stationary cylinder at Re =3.6*10^6 with approx. N = 50.000. Yet, the paper I am referring to used the k-epsilon model with wall functions and they used a smaller domain.

I have also worked with smaller computational domains, however, this did not necessarily lead to different results compared to the larger domains (esp. because I kept the topology of the mesh the same).

Personally, I think the meshes I have created are fine and should produce GC. I have uploaded the mesh topology of the fine mesh as a screenshot in the attachment. The regular and coarse mesh have the same topology, yet the amount of cells allocated to the all the edges are decreased with a factor of 1.25. Btw; I created the mesh in ANSYS ICEM and converted it with the utility fluentMeshToFoam (which seemed to work).

A summary of my Mesh properties/CFD settings:

- Flow is resolved in 2D
- Re = 3.6*10^6
- Turb. model: k-w SST without wall function
- yPlus < 1
- I use standard BC for a cylinder (Uniform inlet velocity, zero ref. pressure at the outlet, fixedValue of U = (0 0 0) around cylinder wall.
- I use a fixed timestep. Al the disc. schemes used are 2nd order. The final residuals have been put on a tolerance of 1e-08. For the p-U coupling I use the PIMPLE family, with 20 outer and 3 inner Correctors and some PIMPLE tolerances.
- The case has been run in parallel on four processors.
- I know the Courant number in all three cases was well below Co < 0.7
- The case files have been uploaded in the attachment.

Things I have tried so far:
- I have tried a different mesh topology
- I Used less tight PIMPLE settings
- I have tried to run with a smaller domain size
- I have played a bit with the disc. schemes (tried some 1st order as well)
- I tried to use the adjustable timestep scheme as well (Co < 0.7)

The problem is that especially the lift coefficient keeps on increasing with big numbers, which makes the grid convergence very hard. I have tried to already run with N = 400.000 cells, but still the lift seemed to go to even higher numbers. It looks like the finer the mesh, the higher the lift force (without reaching a ceiling for grid convergence). I dont believe N>400.000 would be required to achieve grid convergence. Personally, I would think that the meshes are fine (esp. since y+ < 1 and the O-grid is very fine) but there might be a problem with the CFD settings.

I am out of ideas at this moment and I am wondering if some of you has faced a similar problem before with achieving grid convergence for a turbulent flow. Please let me know if you have any advice/tips/tricks on this matter. Thanks in advance! And feel free to ask for more information
Attached Images
 TurbMeshFine.png (22.4 KB, 23 views) TurbMeshFineZoom.png (15.7 KB, 20 views)
Attached Files
 TurbCase.zip (7.8 KB, 1 views)

 November 12, 2018, 04:23 Bump! #2 Member   A Join Date: Jun 2016 Posts: 30 Rep Power: 7 Bump! I am very eager to hear your opinion!

 November 13, 2018, 04:19 #3 Senior Member     Jan Join Date: Jul 2009 Location: Hamburg Posts: 102 Rep Power: 17 Hi, have you tried kLowReWallFunction for k and nutLowReWallFunction for nut? No idea, if this helps, but maybe worth to check. Best, Jan

November 13, 2018, 05:39
#4
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A
Join Date: Jun 2016
Posts: 30
Rep Power: 7
Quote:
 Originally Posted by JNSN Hi, have you tried kLowReWallFunction for k and nutLowReWallFunction for nut? No idea, if this helps, but maybe worth to check. Best, Jan
Hi Jan,

Thanks for your reply. Is maybe not a bad idea, since I am not using a typical wall function for BL prediction but I am resolving up to the viscous sublayer. This could indeed be seen as a 'Low-Re' approach. Thanks!

 Tags cylinder, grid convergence, grid convergence study, k omega sst, turbulent