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Underrelaxation for steady state simulation in CFX 

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March 24, 2011, 10:20 
Underrelaxation for steady state simulation in CFX

#1 
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In Solver control, one can change the Timescale factor when Timescale control is set to 'Auto Timescale' option.
For many of my steady state simulations, I observe that the simulation shows lack of convergence (residuals stabilizing above convergence criteria) with the default value of Timescale factor (which is 1). However, when I reduce this factor, simulation converges. In some cases I had to reduce this factor to 0.1 to get convergence. Although CFX manual describes that one can specify physical timescale instead of auto timescale to get convergence, simply reducing the timescale factor for Auto timescale option seems more straightforward and better. Is it okay to do so? Also, does reducing this timescale factor to say 0.1 mean choosing an underrelaxation of 0.1? 

March 24, 2011, 16:20 

#2 
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If you make the timestep small enough then it's similar to using a very small relaxation factor. Basically you are freezing the solution, so the residuals might converge but it doesn't mean the solution is any good. I'd recommend you figure out why it didn't converge in the first place.


March 25, 2011, 05:28 

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@stumpy
Yes you are very right that there there may be some other issues and I am trying to find out those. Especially because the CFX manual says that the timescale chosen by the solver is on the conservative side. But isn't underrelaxation a valid way to slow down and hence help convergence for nonlinear equations? If the solution does converge with underrelaxation, would it not mean that the residuals (difference between partial differential equation and its discretized counterpart) have reduced to below the convergence criteria? I am not able to understand why the solution may be incorrect. It would be of great help if you could explain it in more detail. Thanks ! 

March 27, 2011, 19:12 

#4 
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Glenn Horrocks
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I do not understand your post. As stumpy says, CFX uses the time step size instead of under relaxation as the main method to stabilise the equations.
So yes, underrelaxation is a "valid way to .. help convergence", but CFX uses time step size. A converged solution is a converged solution, regardless of how you got there. No idea what you mean about "the solution may be incorrect". 

April 1, 2011, 06:47 

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Quote:
Ok, so reducing the timescale multiplication factor reduces the time step taken by CFX solver . I said 'solution may be incorrect' as stumpy replied that reducing the timescale factor may not be the right approach. But you commented that converged solution is a converged solution whatever may be the approach. So should I understand that that its ok to reduce the timescale factor by as much as one order of magnitude to get convergence? or even more? CFX support had once cautioned me against reducing the timescale factor so much and said that I should check if something else is not right in my simulation setup. I am not sure why they cautioned so. 

May 1, 2014, 01:30 
query

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Manpreet
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Hello Guys,
I need a help. Could anyone tell me physical significance of time scale factor / physical timescale. In my project. whenever I use physical timescale 1E5 or 6 then I get smooth curve for mass ,momentum etc equations and if I use 1E4.then its large variation in curve. Wt does it mean?? I did not get it. Through tutorials I got one formula that is for advection dominated flow; Timescale = delta t = L /U = length scale/ velocity scale I did not get its meaning exactly. Thanks Manpreet Singh manpreet_singh_er@yahoo.co.in 

May 1, 2014, 10:09 

#7 
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Glenn Horrocks
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The time scale in a steady state simulation determines how fast you are advancing the solution. A very small time scale means very slow advancement and slow convergence. A large time scale means fast advancement and faster convergence  as long as you are not too fast because then it goes unstable and diverges.


May 1, 2014, 13:44 

#8  
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Kevin Hoopes
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