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 Chander March 24, 2011 10:20

Under-relaxation for steady state simulation in CFX

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 under-relaxation of 0.1?

 stumpy March 24, 2011 16:20

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.

 Chander March 25, 2011 05:28

@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 under-relaxation a valid way to slow down and hence help convergence for non-linear equations? If the solution does converge with under-relaxation, 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 !

 ghorrocks March 27, 2011 18:12

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, under-relaxation 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".

 Chander April 1, 2011 05:47

Quote:
 Originally Posted by ghorrocks (Post 301170) So yes, under-relaxation 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".
Glenn,

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.

 manpreet May 1, 2014 00:30

query

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 1E-5 or 6 then I get smooth curve for mass ,momentum etc equations and if I use 1E-4.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

 ghorrocks May 1, 2014 09:09

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.

 khoopes May 1, 2014 12:44

Quote:
 Originally Posted by ghorrocks (Post 489290) 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.
This is exactly right. I would also suggest not judging convergence based on residuals alone. Create monitor points in CFX-Pre for things that interest you, drag, pressure drop, velocity and then monitor their evolution as you run the simulation. Ideally you should choose monitors based on the goal of the simulation as this is the data you will extract when its complete anyway. Using monitor points like this will tell you a lot about whether you are reaching steady state.

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