What value shall I set for the Convergence criteria?

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 February 25, 2010, 08:07 What value shall I set for the Convergence criteria? #1 New Member   Steven Tay Join Date: Jan 2010 Posts: 28 Rep Power: 7 I am currently working on the sensitivity analysis on the meshes and time steps. However, I have a question on the convergence criteria. According to the Ansys training notes, we need to set the convergence criteria of minimum of 1e-4, if we need it to be more accurate, we have to reduce it. The question is, for my application, it is enough to set it at 1e-4? If it is not enough, which value should I set and how to decide which value is the most suitable value? Is it correct that we should set the default value first (1e-4) and after getting the best mesh and time step, run this setting and compare results with my experiment results? If the results tally, then the convergence criteria set was ok, if it does not tally, then I should reduce the convergence criteria till I get the best results. Does that make sense?

February 25, 2010, 08:12
#2
New Member

Arvind
Join Date: Mar 2009
Posts: 15
Rep Power: 8
Quote:
 Is it correct that we should set the default value first (1e-4) and after getting the best mesh and time step, run this setting and compare results with my experiment results? If the results tally, then the convergence criteria set was ok, if it does not tally, then I should reduce the convergence criteria till I get the best results. Does that make sense?

U r absolutely correct!!!! The convergence criteria is specific for a particular problem.For better accuracy u can set it to 1e-6.

 February 25, 2010, 08:25 #3 Member   SanS Join Date: Mar 2009 Posts: 42 Rep Power: 8 Apart from residuals alone, its a good practice to ensure that your variables of interest have also converged. You could do this by monitoring your variables using monitor points.

 February 25, 2010, 08:48 #4 New Member   Steven Tay Join Date: Jan 2010 Posts: 28 Rep Power: 7 Thanks for the prompt respond! I have a clearer directions now!

 February 25, 2010, 08:48 #5 New Member   Arvind Join Date: Mar 2009 Posts: 15 Rep Power: 8 @ sans Pls can u explain hw can we monitor the variable using monitor points

 February 25, 2010, 08:54 #6 New Member   Steven Tay Join Date: Jan 2010 Posts: 28 Rep Power: 7 I think sans is referring to adding user define monitor points, like inlet and outlet temperatures, pressures and so on, depending on which variables you are interested in. I hope I am right.

 May 14, 2010, 12:44 #8 Senior Member   Join Date: Feb 2010 Posts: 145 Rep Power: 8 Arvind, In CFX-Pre, right click on Output Control and select Edit. Click Monitor tab. Under Monitor Points and Expressions, click Add New Item. Enter expressions like massFlow()@Inlet or areaAve(Pressure)@Inlet or areaAve(Temperature)@Outlet, where Inlet and Outlet are Named Selections which have been created in Mesh. See manuals for syntax on additional expressions.

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