# Grid independency test

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 July 17, 2013, 02:05 Grid independency test #1 New Member   Join Date: Jul 2013 Posts: 16 Rep Power: 5 Can anybody tell me what is the reason for not achieving grid independency?

 July 17, 2013, 03:11 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,309 Rep Power: 22 There is a multitude of possible reasons. If you describe more in detail what you did, what you expected and what you got...

 July 18, 2013, 06:27 grid independency #3 New Member   Join Date: Jul 2013 Posts: 16 Rep Power: 5 I want to simulate swirling flow in a cylinder and just wanted to do the grid independency test. But I got different plots of swirl and axial velocities near wall and axis region. I just wanted to know the reason for this behaviour. Does this happens or is there any mistakes in my simulation?

 July 18, 2013, 06:54 #4 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 12 Is the flow laminar or turbulent? What is your resolution in terms of the smallest scales in the flow? Do you use any type of model? Tell us more about the numerics you are using to solve this flow. Has the code been verified? There are so many possible explanations for this behavior. You need to provide the parameters of your physics and your numerics for us to help you. FMDenaro likes this.

 July 19, 2013, 01:06 grid independency #5 New Member   Join Date: Jul 2013 Posts: 16 Rep Power: 5 I am simulating turbulent swirling flow using CFX in a cylinder using standard k-e model. meshing is done using icem cfd. I just increase the number of nodes on inlet face by multiples of 5 to find out grid independency.

 July 19, 2013, 03:10 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,309 Rep Power: 22 Decreasing the cell size in only one region of the mesh will not reduce the discretization error in the other (constant-size) regions of the mesh.

 July 26, 2013, 06:43 #7 New Member   Adrian Join Date: Jul 2013 Location: Dublin Posts: 21 Rep Power: 6 The k-e model can tend to give strange grid convergence results if you are decreasing the first cell height at the walls with increasing mesh refinement. Check to see that your wall functions are valid, or switch to a more appropriate turbulence model. __________________ www.idacireland.com

August 18, 2013, 11:13
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Quote:
 Originally Posted by prom Can anybody tell me what is the reason for not achieving grid independency?

This kind of tests are very important to show that the solution you obtained
is accurate enough. You should know that your solution is dependent on the size of your grid cells. The smaller is your cells, the more accurate your solution will be. But below a given grid size the solution becomes independent of this size. It means that you stop to gain accuracy below this given grid size. Then it is not anylonger necessary to decrease the grid size.
So you have to perform this test to be sure that you have run your computation with the right grid size.
Keep in mind that these tests may concern spatial grid size (dx, dy) or time step (dt)

To my mind these tests should be always performed before delivering any solution.This is a must.

 August 19, 2013, 06:59 #9 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 10 Keep in mind, as suggested by Adrian, that some models, such as wall functions, are dependent on the mesh/time-step size. Other models to look out for includes: solidification, melting, evaporation... Another aspect is that when doing mesh/time-step refinement, the accuracy of the solution increases (and the dissipation goes down). This creates a "conflict" where you are able to achieve convergence for coarse mesh, but fine mesh may exhibit transient behaviour... __________________ --- Julien de Charentenay

 August 19, 2013, 10:06 #10 Member   Sheng Join Date: Jun 2011 Posts: 58 Rep Power: 7 Hello prom, I just recomend a book verification and validation. There's a very detailed discussion on this topic. Other replies in this forum are also valuable. This book is, Roache, P. J., Fundamentals of Verification and Validation, 2nd eds, Hermosa publishers, 2009.

 September 9, 2013, 06:40 Gid #11 New Member   steven Join Date: Mar 2013 Posts: 3 Rep Power: 5 Dear GID is varies from case to case , you need to conduct a timestep independence study. Reagrds

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