# grid independence

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 February 15, 2012, 07:19 grid independence #1 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 how do I conduct a grid independence test ,and what is the difference between grid independence and grid convergence test?

 February 15, 2012, 09:00 #2 Senior Member   Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 521 Rep Power: 16 All you have to do is run the same simulation on progressively finer grids (3 grids should be the minimum) by changing a global grid sizing parameter rather than local refinement, unless it takes too long to run/insufficient computer resources to see if the solution changes as you refine the grid. Hopefully, when the grid is fine enough the solution will not change (or change less than a tolerance you are happy with) so that the solution is 'grid independent'. This gives you an feel about reducing the discretization error inherent in CFD. There's no difference between the two, it's just a name.

 February 15, 2012, 09:09 #3 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 when you say finer grids ,do you mean by changing coarse,medium and fine mesh settings?will I see the changes in the form of a graph in the results(chart viewer) do grids and mesh mean the same ?

 February 15, 2012, 09:16 #4 Senior Member   Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 195 Rep Power: 10 When you do a grid-check you are making sure that the grid size is the correct value. Many times you can get a grid to converge but the answer is wrong. So, pick a parameter and double your grid size and record the parameter answer. Then, cut the grid in half and check the parameter answer. That will show you how your answer depends on grid size. This can also help in computational time, because you might find that you have a very fine mesh when one is not needed. rgd and hua like this.

 February 15, 2012, 09:26 #5 New Member   Join Date: Jan 2012 Posts: 9 Rep Power: 7 right , will I view these changes in the form of a graph in the results ? I set monitor points for certain parameters and viewed the results chart ,so there I tried choosing the number of nodes as my X axis ,which I could'nt as there were only mesh refinement factor ,expansion factor etc

 February 15, 2012, 09:29 #6 Senior Member   Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 195 Rep Power: 10 yes, as a graph. When I did this I changed the grid count until the answer changed. That shows the 'least' amount of refinement necessary for an adequate answer. So, your graph should be a horizontal line until it reaches the number of cells that are critical. With your parameter on the y-axis and grid number on the x-axis. rgd likes this.

 February 15, 2012, 13:45 #7 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,492 Rep Power: 40 I think you can distinguish two aspects: - a numerical solution is independent from the grid if after a refining in the grid the difference of the two solutions are (in some norm) lesser than a suitable tolerance. - a grid independent solution can be also "wrong", that means that you did not get convergence towards the solution you really want. To define convergence is necessary that for vanishing local truncation errors, the discretization error vanishes too. rgd likes this.

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