# Grid Independence Study

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September 6, 2012, 09:18
Grid Independence Study
#1
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Vidit Sharma
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hello,
I am doing a simulation of film cooling and need to complete its grid independence study. I have completed four grids and the then plotted its effectiveness versus X/D. U can see the plots in the attached image.
So, can somebody tell me I need to make more grids or i can select the grid with minimum number of cell???

thank you.
Attached Images
 grid.png (10.3 KB, 39 views)

 September 6, 2012, 09:31 #2 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,099 Rep Power: 16 What means "effectivenes" ? You need to plot the differences between the cases, not the cases. Richardson Extrapolation can help to estimate the error.

September 6, 2012, 09:36
#3
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Vidit Sharma
Join Date: Aug 2012
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Quote:
 Originally Posted by RodriguezFatz What means "effectivenes" ? You need to plot the differences between the cases, not the cases. Richardson Extrapolation can help to estimate the error.
thank you sir for the reply..

So, u r saying that I hav to take the differences of the results and then plot them.? Like the difference between the results of Grid2 and Grid4 is about the order of 1*10^-3, then its acceptable to take the grid with minimum cells ?

 September 6, 2012, 09:41 #4 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,099 Rep Power: 16 No. You need to see the differences between your solutions to be able to judge about the grids. I can't see anything in your picture. What is the difference in your four grids? 20, 24, 28... is this the number of gridpoints?

 September 6, 2012, 09:44 #5 Member   Vidit Sharma Join Date: Aug 2012 Location: Delhi, India Posts: 32 Rep Power: 5 Grid 1 is of 20 Lakhs Grid 2 is of 24 lakhs Grid 3 is of 28 lakhs Grid 4 is of 37 Lakhs Is it fine to take these grids or i need more finer grids?

 September 6, 2012, 09:50 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,107 Rep Power: 19 Plot the efficiency at specific X/D ratios over the cell size (not the number of cells). Then you can judge grid dependency of your case. BTW: what is lakh?

 September 6, 2012, 09:53 #7 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,099 Rep Power: 16 I didn't know that "lakh" is an english word, up to now. My dictionary says, it is a "hundred thousand" in India, is that right? For grid refinement study, You need to refine your original grid in each dimension by some factor. If you can, use a factor of 2 in each dimension. That means your 2nd grid has 2*2*2 = 8 times the grid cells than your 1st grid. Your 3rd grid will have 64 times the grid points than your first grid. Than have a look at your values of interest. Is that possible to you?

 September 6, 2012, 10:01 #8 Member   Vidit Sharma Join Date: Aug 2012 Location: Delhi, India Posts: 32 Rep Power: 5 Sorry, grids are having 2, 2.4, 2.8, and 3.7 million cells. Yeah Its possible to do refine the grids with the factor u r suggesting but it will take more time to converge. If now u r able to under stand about no. of cells in the grids which i am using. What else I need to do?

 September 6, 2012, 10:07 #10 Member   Vidit Sharma Join Date: Aug 2012 Location: Delhi, India Posts: 32 Rep Power: 5 Thank U sir .. Now I get it ... Surely, I will read Richardson extraploation for CFD.

 September 6, 2012, 10:10 #11 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,099 Rep Power: 16 If you can, get a copy of "Computational Methods for Fluid Dynamics" by Ferziger and Peric. It's explained perfectly there.

September 6, 2012, 14:10
#12
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Vidit Sharma
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Quote:
 Originally Posted by flotus1 Plot the efficiency at specific X/D ratios over the cell size (not the number of cells). Then you can judge grid dependency of your case. BTW: what is lakh?
sory, grids are having 2, 2.4, 2.8 and 3.7 million cells...

 September 7, 2012, 02:50 #13 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,107 Rep Power: 19 again what learned

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