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

#1 
New Member
Claudio
Join Date: Jul 2009
Posts: 9
Rep Power: 8 
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 
Member
Jules Bell
Join Date: May 2009
Posts: 31
Rep Power: 8 
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  
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,565
Rep Power: 90 
Quote:
Quote:
Quote:
Quote:
Quote:
Quote:


September 14, 2009, 10:31 

#4 
New Member
Claudio
Join Date: Jul 2009
Posts: 9
Rep Power: 8 
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  
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,565
Rep Power: 90 
Quote:


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Time step size and max iterations per time step  pUl  FLUENT  31  August 21, 2015 04:46 
Changing deltaT of current time step  olwi  OpenFOAM Running, Solving & CFD  1  April 30, 2009 04:55 
false time step implementation for steady state  Kushagra  CFX  1  June 22, 2008 19:06 
Time step  Turbulence  Convergence  Camilo Costa  CFX  0  March 13, 2008 15:23 
Convergence moving mesh  lr103476  OpenFOAM Running, Solving & CFD  30  November 19, 2007 15:09 