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September 12, 2009, 15:56 
Question Regarding Convergence and Time Step Selection

#1 
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Claudio
Join Date: Jul 2009
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Hi,
I've been reading practical tutorials and seen that a reasonable target value for RMS residual for steady state simulations is 0.0001. However, I'm not quite sure for transient problems what is an appropriatte target value. I understand transient simulations are solved by computing a steady state solution for many discrete points in time. Each steady state solution is iterated through a number of inner loops called “coefficient loops”. What is a reasonable RMS target for each time step? I've read that 35 loops should allow you to reach convergence. I've noticed CFX by default monitors the RMS for each timestep. If the RMS value for that timestep is above the target, does that mean the solution is not converged? Should I wait until all monitored RMS quantities are below the target for all time steps from there on to be confident about my solution? What other quantities are good practice to monitor in order to trust the obtained solution? Thanks! 

September 12, 2009, 16:40 

#2 
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Jules Bell
Join Date: May 2009
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I usually use a target value of 1e4 for the MAX Residual.
And yes, if the residual is higher than your set convergence limit, then the solution for that timestep has not converged within the maximum number of coefficient loop iterations. 

September 13, 2009, 01:45 

#3  
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Glenn Horrocks
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September 14, 2009, 10:31 

#4 
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Claudio
Join Date: Jul 2009
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Hi,
Thank you very much for your replies. Glen, I'm not exactly sure how to perform that sensitivity analysis. What you mean is that I should see how my answer varies as I use different target values for RMS? As long as the answer doesn't change significantly as I increase the target RMS, I could use that higher value (or lower, eventually), right? 

September 14, 2009, 18:39 

#5  
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Glenn Horrocks
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