# grid convergence problem

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 April 24, 2013, 23:23 grid convergence problem #1 Member   sajeesh Join Date: Feb 2013 Posts: 52 Rep Power: 4 Hi My channel is 50 X 50 micrometer square channel having length 1000 micrometer.I am using the workbench for the analysis. I am giving different mesh size by changing the value in I am getting different number of elements after each change. I tried Number of elements ranging from 7000 to 3 lakhs. My inlet boundary condition is mass flow rate 8.333e-7 kg/sec Outlet boundary condition is outflow Fluid water No turbulence.Simple flow viscous laminar.No energy terms But when i find out the average inlet and outlet total pressure from post processor..and to find out pressure drop through the channel.When i am doing this the pressure drop value also increases from 3670 pa to 3980 pa as the number of elements increases from 7000 to 3 lakhs. So now i cant get what is the optimum number of elements required for the further proceedings.grid convegence is not happened. Thanks

 April 24, 2013, 23:34 #2 Super Moderator     Sijal Ahmed Memon (turboenginner@gmail.com) Join Date: Mar 2009 Location: Islamabad Pakistan Posts: 3,993 Blog Entries: 6 Rep Power: 39 when you change mesh from 7000 to 3 lacks you have change in pressure 3670 to 3980 pa which is 8% increase from initital value at 7000 nodes. What is the difference between mesh size of 150,000 and 300,000 nodes?

 April 25, 2013, 02:45 #3 Member   sajeesh Join Date: Feb 2013 Posts: 52 Rep Power: 4 the number of elements pressure drop in Pa 312500 3982.393 214896 3931.363 160000 3939.024 96237 3890.693 20000 3759.658 10688 3670.525 A linear increase as the number of elements increase. what to do which elemnt size is better..or any problem of meshing.... thanks for your reply

 April 25, 2013, 03:20 #4 Super Moderator     Sijal Ahmed Memon (turboenginner@gmail.com) Join Date: Mar 2009 Location: Islamabad Pakistan Posts: 3,993 Blog Entries: 6 Rep Power: 39 So in your last two cases (two fine meshes) the change is 1.3%!. If it is acceptable to you then you have achieved grid convergence. Even with lower mesh density you have grid convergence if error is acceptable to you. Plot your all cases in excel and see if you have almost straight line after some mesh density. ghost82 likes this.

 April 25, 2013, 04:39 #5 Member   sajeesh Join Date: Feb 2013 Posts: 52 Rep Power: 4 i tried in excel but it is linearly increasing trend not at all a constant trend...then what should i do...any problem with meshing..

 April 25, 2013, 05:45 #6 Super Moderator     Sijal Ahmed Memon (turboenginner@gmail.com) Join Date: Mar 2009 Location: Islamabad Pakistan Posts: 3,993 Blog Entries: 6 Rep Power: 39 If error is below your requirements then you can assume solution to be mesh independent...

 April 25, 2013, 09:34 #7 New Member   Join Date: Nov 2011 Posts: 27 Rep Power: 5 I use the Grid Convergence Index proposed by Roache to perform grid convergence studies You may find the description of the procedure here: http://www.google.com/url?sa=t&rct=j...45645796,d.eWU Through this, you can estimate grid uncertainty and if your grid solutions lie within the asymptotic convergence range regards ghost82 and metmet like this.

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