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-   -   Grid independence study (https://www.cfd-online.com/Forums/fluent/34290-grid-independence-study.html)

 prasanth July 28, 2004 19:59

Grid independence study

Dear friends,

To study the grid independency/dependency, how should the mesh size be varied in order to check the solution at different grid sizes and get a range at which there is no variation in the solution.

Thanking you for a kind response!

prasanth

 ozgur July 30, 2004 08:43

Re: Grid independence study

Hi,

You can adapt your grid and continue you simulation with the refined grid to see whether there is a significant change in the parameters.

regards

 wxl July 30, 2004 10:59

Re: Grid independence study

Do you mean that we don't have to redo the meshing in gambit, instead, we only need to adapt grid in fluent? Thanks.

 prasanth July 30, 2004 15:42

Re: Grid independence study

Hello Ozgur

but I am talking about grid independence studies!! you will take 3-4 different grid sizes and like that, repeat the simulation until we get no change in solution!

Thanks prasanth

 Chetan Kadakia August 4, 2004 16:57

Re: Grid independence study

I believe you can modify the grid based on different methods (not just high gradients), and when you are convinced that adapting the grid anywhere will not change your outcome, then you are grid independent. Your solution is 3D, the continuous adaption may not be practical. You can also refine the grid in Gambit as it seems you are about to do, but do decide where you want to make your changes based on what you are studying in the flow. May I ask what you are simulating, what model you are using, and how many cells you are going to have in your various meshes.

 ajb August 9, 2004 19:01

Re: Grid independence study

A good rule of thumb I use is 15 to 25% difference between subsequent grids. Sometimes it is necessary to refine grids in order to achieve values of pressure, velocity, turbulent reynolds number, y+ (if turbulent flow), etc that are appropriate. Then once those values are "believable" or close to experiment, refine grid with something like 3 or 4 or more densities. If you are in hexahedrals, you can refine in one direction at a time to ensure there is no effect from grid. Or if in tetrahedrals, a gross refinement of total cell sizes would be O.K. Finally, if tetrahedrals are use, it is always a good idea to try to create hex grid that is very close to the density of the tet grid so you can see whether there is effect of grid type. Hope it helps.

 gafoorcp August 16, 2014 10:53

hii
after doing some solutions with different grid sizes..am gettting matching result in some parameter and different result in some other parameter.....
is this a grid independent solution?

 lehoanganh07 August 16, 2014 21:40

You can make the number of cell in order 1.5 time incresing, and do calculation with same condition, if the solution is unchange to much, so you can get grid independent and chose the smallest grid fir your calculation

 gafoorcp August 17, 2014 02:10

Quote:
 Originally Posted by lehoanganh07 (Post 506296) You can make the number of cell in order 1.5 time incresing, and do calculation with same condition, if the solution is unchange to much, so you can get grid independent and chose the smallest grid fir your calculation
thanks sir....
is this 1.5 times increasing is a rule of thumb or can we take any other ratio for increment?

 lehoanganh07 August 17, 2014 23:43

No, It is just my work, you can do whatever you want

 gafoorcp August 18, 2014 02:26

Quote:
 Originally Posted by lehoanganh07 (Post 506386) No, It is just my work, you can do whatever you want

 gafoorcp August 19, 2014 02:21

Quote:
 Originally Posted by lehoanganh07 (Post 506386) No, It is just my work, you can do whatever you want
sir...
after completing some run with different number of cells....am getting a matching result within a range of number of cells....and a different result after that range.....so can we take the range at which we r getting same solution for our actual testing....is that a valid method......
regards
gafoor

 lehoanganh07 August 19, 2014 11:36

So it is not conv yet, I think need more grid point

 gafoorcp August 19, 2014 14:00

Quote:
 Originally Posted by lehoanganh07 (Post 506676) So it is not conv yet, I think need more grid point

 ringtail August 20, 2014 01:31

FYI http://www.grc.nasa.gov/WWW/wind/val.../spatconv.html

In my opinion, grid independence is almost impractical, especially for engineering

 gafoorcp August 20, 2014 06:42

Quote:
 Originally Posted by ringtail (Post 506738) FYI http://www.grc.nasa.gov/WWW/wind/val.../spatconv.html In my opinion, grid independence is almost impractical, especially for engineering
thanks sir.......for your valuable suggestion.....and can you suggest me that if am getting same solution in a range of 60000 cells(maximum number of cells are only 1.2 lakh)...can I go further with my solution with this grid setup...pls reply...

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