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April 25, 2011, 15:04 
Grid refinement study for Order of accuracy and GCI

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I am trying to ascertain the numerical accuracy of my simulations in CFX by following the procedure mentioned in the editorial policy of Journal of engineering.
My simulations are 3D turbulent flow . I take the overall pressure drop from inlet to outlet as the solution variable to calculate GCI and I use a representative mesh size for each mesh which I calculate as (total volume/total number of cells)^(1/3). I have generated results for many meshes and tried to calculate GCI for 3 meshes at any time. I have ensured that the grid refinement ratios r21 and r32 are both above 1.3 for any of the mesh set that i consider. However, I find that for the different mesh sets that i consider: a)either the the parameter GCI_fine*(r21^p)/GCI_course is not close to one. This parameter should be close to one for the grids to be in asymptotic range. b)when this parameter is indeed close to one for a set of meshes, (and GCI_fine is also is small..less than 2 percent), the observed order of accuracy p that I calculate comes out to be around 4 . Now I think it is incorrect and should be less than 2 (as the formal order of accuracy of CFX is 2...please correct me if I am wring). I have tried many fine and coarse meshes. Is the way I am calculating GCI correct? Is the value of p>2 indeed correct? For the results to be considered accurate what is the maximum acceptable value of GCI? Last edited by Chander; April 25, 2011 at 17:43. 

April 26, 2011, 22:27 

#2 
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Glenn Horrocks
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Firstly well done on having a good look into mesh refinement. I reckon about half of the weird questions on this forum are from people who have not done a proper check of whether their simulation is accurate so I am glad you are doing it properly.
Yes, CFX is a second order code for much of its numerics but in my experience that rarely equates to a order of convergence of 2 due to numerical issues, nonlinearities etc. So if it is converging nicely I am happy regardless of the number. If the parameter is not in the asymptotic range then you generally have to use a much tighter mesh for convergence, or you have a model which does not converge. Turbulence models with wall functions often do this as if they refine below y+<11 the model is not valid. 

April 28, 2011, 11:39 

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Secondly regarding the turbulence models, I am using the standard komega model in CFX. As you know, it uses automatic wall treatment which as per CFX manual should work for any mesh refinement. I could not use the commonly recommended SST model as it simply does not converge. Do you think something could be wrong here? And lastly, I have obtained the converged results by reducing the automatically determined timestep for steady state simulations by one order of magnitude. I have checked my setup again and again and it seems that it is fine. I have also played with boundary conditions for turbulence but convergence seems impossible without such large reduction in timestep. I remember that you had mentioned once before that convergence is fine irrespective of the timestep used as long as we get convergence. Still, it would be great if you could also have a look at another thread of mine here: Time step dependence of convergence behavior of steady state simulations in CFX 

April 28, 2011, 18:39 

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Glenn Horrocks
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May 3, 2011, 12:42 

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I am a bit concerned after your reply. I am not able to get convergence with SST. I am attaching a crosssection of my mesh that I have used for SST. n the first pic showing the mesh, the fine mesh is the fluid part and the surrounding coarse mesh is the solid part. I am also attaching the convergence plots. The convergence criteria was rms residual of 1e6 Do you see any obvious mistake/issue here? 

May 3, 2011, 19:13 

#6 
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Glenn Horrocks
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It is common to have steady state convergence problems as you refine the grid. Some tips are here:
http://www.cfdonline.com/Wiki/Ansys...gence_criteria 

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gci, grid refinement 
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