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December 30, 2016, 08:07 
Grid Independence Study  Acceptable?

#1 
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William
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Hi,
I'm doing a CFD Analysis on an airfoil. I'm trying to make a Grid Independence Study. My instructor told me to have the tolerances for lift, drag and momentum 1e2, 1e4 and 1e2 respectively. I'm using fixed velocity (15 m/s), fixed angle (0 deg), fixed farfield size (100c), fixed wall spacing (6e6), max y+ (0.47), e.t.c. but change of mesh density.  I'm using the kw SST model for the grid convergence study. The first figure attached is the lift and drag as function of the cell count. The second figure is how many tolerances the previous results are from the densest grid results. The graph below (in both figures) says how long time the run took. I will have to make it 130 times  So I really need to know when I have acceptable grid. How can I be sure I have converged results?  Data is available on the attached Excel file. Thanks in advance. 

December 31, 2016, 15:37 

#2 
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Hamid Zoka
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Hi
Practically thete is not any grid independent solution. As you move on with smaller grids the results vary at some extend. The selected mesh size and the parameter which is used to qualify a mesh independent solution usualy depends on the phenomenon to be studied. Whatever it is, 130 runs for a grid independce study does not make scence. It seems pressure distribusions on airfiol sides will be a good indication of convergence when there is no flow separation. Another important issue is computational costs. You should answer that what computational costs are affidable at your side. 

January 1, 2017, 04:40 

#3 
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Alex
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A common problem when searching for a "grid independent" solution lets rather call it a grid sensitivity analysis is that the solutions on different grids are not obtained with the same level of accuracy.
This might be the case in your analysis because initially the results seem to stabilize and then begin to deviate for very high cell counts. Have you made sure that the iterative error is negligible throughout your analysis? Did you use some fixed residual limit to stop the iterations? It might be advisable to simply run the iterations until the residuals level out for the mesh sensitivity analysis. Additionally, you should run at least one similar analysis for a case with high AoA. What does your grid look like? Where exactly did you put the additional cells for your analysis? Feel free to share some pictures. Btw: which solver are you using? 

January 1, 2017, 17:00 

#4  
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William
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Quote:
I'm actually expecting flow separation due to geometry, so I'm not sure I can count on the Cp plot to find convergence? Quote:
I'm using Matlab to run ICEM CFD and Ansys Fluent through batch mode. I'll leave some pictures of some of the meshes. (4585 cells, 77077 cells and 1952085 cells) 

January 1, 2017, 17:01 

#5 
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William
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More Figures (Reached Maximum attachments)


January 1, 2017, 17:07 

#6 
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William
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More Figures (Reached Maximum attachments)


January 2, 2017, 05:30 

#7 
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Alex
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My initial guess would have been that fluent does not converge to a sufficient level within 2000 iterations with these kinds of meshes, especially the finer ones. There are quite a few high aspect ratio cells and volume jumps could be high.
Do you use double precision for fluent? What is the output of fluent when you click on "mesh>check" with the finest mesh loaded? 

January 2, 2017, 05:37 

#8  
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Arjun
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Quote:
That is quite possible for various reasons. 

January 2, 2017, 06:02 

#9 
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Filippo Maria Denaro
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I agree, using any iterative method on via via refined grids requires to be careful in the convergence threshold. Double precision is also mandatory in such case.


January 6, 2017, 13:13 

#10  
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William
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Quote:
But what do you mean by 'these kind of mesh'? Most tutorials I've seen do similar meshes? Is there some other mesh technique I should go for? I've been trying to minimize the aspect ratio, but when aiming for y+ < 1, the aspect ratio increases. Here is the mesh check comment from Fluent on the densest mesh. (A mesh I could never use for my analysis) Quote:
I've also attached an result example from previous model. 

January 7, 2017, 03:57 

#11  
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Alex
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Quote:
And just to be on the safe side you should do what the fluent mesh warning tells you. You will still be left with the third point on the list and there is obviously no way around it for a mesh sensitivity analysis. It just takes more iterations to obtain the solutions on finer meshes. From my experience, 2000 Iterations are simply not enough for 2D meshes with more than 1e6 cells when you need to be sure that the iterative error is negligible. When you say your residuals "appear to be flat" how exactly does this look like for finer grids? The magnitude of the residuals should drop below 1e14 with high quality quad meshes and double precision. In addition to checking the residuals you should also check the trend of lift and drag coefficients over the iterations. 

January 9, 2017, 21:29 

#12 
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I think you are mixing to many things at the same time. Although these papers are "old" I strongly encourage you to read them:
1. Further discussion of numerical errors in CFD; Ferziger, Joel H, PERIĆ, MILOVAN 1996 2. Perspective: a method for uniform reporting of grid refinement studies Roache, Patrick J It is important to reduce the other source of error before claiming grid independent solution. If you are using a time dependent solution then you need to make sure that you are comparing them all at the specific time. In contrast if you are using a steady state approach then, the iterative error must be small enough (O) 10^14. double precision is also encourage... 

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convergence check, grid convergence study, grid dependency 
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