# Divergence Problem

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 May 25, 2013, 14:22 Divergence Problem #1 Member   Join Date: Jan 2013 Posts: 39 Rep Power: 5 I am experiencing some divergence problems with my residuals using Fluent. I am using the k-omega turbulence model running under steady state conditions. I refined my mesh as best as I can. The qualities (orthogonal qulity, aspect ratio, skewness, etc...) are around suggested values, but for some reason my residuals still diverge. I do not think this is a mesh quality problem. Also my solution actually reaches the "residuls have converged" pop-up when i run the case, but if I turn convergence criterion off, it would converge then diverge. I read that I could change the relaxation factors lower, but isn't that kind of cheating the system? My experience with lowering the factors is that it only "delays" the divergence, but not "eliminate" the divergence. Any help would be greatly appreciated. P.S I am pretty positive my setup is correct. In solution methods, I am mainly using second order solvers, except for the dissipation rate solver (using 1st order for this one).

 May 25, 2013, 14:39 #2 Member   misagh Join Date: Apr 2012 Posts: 63 Rep Power: 6 the convergence pattern must be steady.It may reach the criteria you want to ,but it shouldn't oscillate. By the way lowering the factors is not cheating.It is factors to ease the equation solution process.And in some special cases(that actually i am not familiar with those ones) you need to increase the factors. bests

May 25, 2013, 15:36
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 Originally Posted by misagh the convergence pattern must be steady.It may reach the criteria you want to ,but it shouldn't oscillate. By the way lowering the factors is not cheating.It is factors to ease the equation solution process.And in some special cases(that actually i am not familiar with those ones) you need to increase the factors. bests
I have tried lowering the factors, but all it seems to do is delay the divergence, not actually eliminate it from my simulation.

 May 26, 2013, 00:26 #4 Member   刘峰 Join Date: Mar 2013 Posts: 31 Rep Power: 5 I also wonder if lowering the re-factors can help eliminate divergence... Besides, pyroknife, I want to know whether your simulation involves gas mixture? If you do, what's your option about settong the properties of gas mixture. Regards

May 26, 2013, 15:30
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 Originally Posted by Liufeng_ustb I also wonder if lowering the re-factors can help eliminate divergence... Besides, pyroknife, I want to know whether your simulation involves gas mixture? If you do, what's your option about settong the properties of gas mixture. Regards
My simulation is currently involving methane (the one FLUENT has in it's material library). Does that count?
I remember using incompressible-ideal gas, polynomial, and 2 kinetic theory options.

 May 26, 2013, 17:04 #6 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 199 Rep Power: 7 k-omega turbulence model is sensitive and it depends greatly on the initial values of k and omega. If the values of these variables are not ok, the solution will most probably diverge. Try the SST k-omega. As for the underrelaxation factors of course it's not cheating! Don't forget that turbulence model's equations are stiff and make convergence even more difficult... EDIT ----- The free stream values of the turbulence variables I meant to say, not the initial values of k and omega. My bad, sorry. (http://www.cfd-online.com/Wiki/SST_k-omega_model) __________________ Lefteris Last edited by Aeronautics El. K.; May 28, 2013 at 13:31.

May 26, 2013, 18:38
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 Originally Posted by Aeronautics El. K. k-omega turbulence models is sensitive and it depends greatly on the initial values of k and omega. If the initials values of these variables are wronge, the solution will most probably diverge. Try the SST k-omega. As for the underrelaxation factors of course it's not cheating! Don't forget that turbulence model's equations are stiff and make convergence even more difficult...
Oh I see. Can you set the initial values of k and omega? If so, how?
I am currently using SST k-omega.

May 27, 2013, 00:12
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 Originally Posted by pyroknife My simulation is currently involving methane (the one FLUENT has in it's material library). Does that count? I remember using incompressible-ideal gas, polynomial, and 2 kinetic theory options.
Hi, friend.

I once experienced a divergence problem caused by the selection of the properties of mixture gas.

My mixture gas is composed of two species which are hydrogen and water vapor. Initially, I selected ideal-gas-mixing-law, then when I ran, divergence problem occured. But when I revised it using mass-averaged, this problem disapperaed.

Your simulation is a single gas species. But you can try to modify ideal gas option.

Wish you luck!

 May 27, 2013, 03:01 #9 New Member   Muhammad Aqib Chishty Join Date: May 2013 Location: Islamabad, Pakistan Posts: 23 Rep Power: 0 As you are using k-omega turbulence model with under steady state conditions, it is sensitive to free stream conditions. What time step size you are using? Try to reduce the time step size. Wish you best of luck!

May 27, 2013, 16:57
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 Originally Posted by MAC88 As you are using k-omega turbulence model with under steady state conditions, it is sensitive to free stream conditions. What time step size you are using? Try to reduce the time step size. Wish you best of luck!
I am running under steady state. Isn't time step only for unsteady?

Also, it appears that only my energy equation is experiencing divergence.

May 28, 2013, 00:09
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 Originally Posted by pyroknife I am running under steady state. Isn't time step only for unsteady? Also, it appears that only my energy equation is experiencing divergence.
I think that was my typing mistake.
Have you tried that?

May 28, 2013, 03:31
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 Originally Posted by pyroknife I am experiencing some divergence problems with my residuals using Fluent. I am using the k-omega turbulence model running under steady state conditions. I refined my mesh as best as I can. The qualities (orthogonal qulity, aspect ratio, skewness, etc...) are around suggested values, but for some reason my residuals still diverge. I do not think this is a mesh quality problem. Also my solution actually reaches the "residuls have converged" pop-up when i run the case, but if I turn convergence criterion off, it would converge then diverge. I read that I could change the relaxation factors lower, but isn't that kind of cheating the system? My experience with lowering the factors is that it only "delays" the divergence, but not "eliminate" the divergence. Any help would be greatly appreciated. P.S I am pretty positive my setup is correct. In solution methods, I am mainly using second order solvers, except for the dissipation rate solver (using 1st order for this one).
Many factors could cause instability problems. I guess you should show the history of residual as well as Cd and so on to provide more detail information.

generally, since you use the k-omega model, so the y+ should be about 1.
And, be careful, a too fine mesh for RANS simulation may also cause divergence problems for fine mesh may capture more instable flow phenomena.

What's more, for high order discretization scheme, lower relaxation factors is necessary, especially for pressure (set to 0.1 maybe, it depends on your p-v coupling scheme). You can also enable High order Term Relaxation, and have a try, it may works.

Please correct me if I am wrong.

 May 28, 2013, 10:26 #13 New Member   Yasir hayat Join Date: May 2013 Posts: 4 Rep Power: 5 Hi all . I am facing same problem. I want to simulate cryogenic boiling in fluid film bearings. I am using k omega sst model , with multi-phase mixture model but some where in the iteration my solution diverges and i get floating point error. i have noted that when i use energy equation it diverges and i have noticed the residua graph of temperature shoots up and i get floating point error. Can any one guide me why this is happening and any possible approach to tackle this problem. Regards Yasir hayat

May 28, 2013, 12:24
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 Originally Posted by MAC88 I think that was my typing mistake. You have to run your simulation in an unsteady mode. Have you tried that?
I have yet to try unsteady. My simulation is a steady case, however.

Quote:
 Originally Posted by ringtail Many factors could cause instability problems. I guess you should show the history of residual as well as Cd and so on to provide more detail information. generally, since you use the k-omega model, so the y+ should be about 1. And, be careful, a too fine mesh for RANS simulation may also cause divergence problems for fine mesh may capture more instable flow phenomena. What's more, for high order discretization scheme, lower relaxation factors is necessary, especially for pressure (set to 0.1 maybe, it depends on your p-v coupling scheme). You can also enable High order Term Relaxation, and have a try, it may works. Please correct me if I am wrong.
Thanks. That is (too fine of a mesh) what I think may be causing the problem. I did not save the data, but if i experience the divergence again, i'll post it on here. I am basically simulating pipe flow, but a bit more complicated than the normal 1 inlet/outlet, pipe flow problem. I ran a successful simulation with .4million elements using cutcell mesh. However, when I increased the length of my pipe, I used a 1.2million cutcell mesh and for some reason this failed. FYI, the .4 million element mesh had WORSE quality than the 1.2 million one, so I don't think the divergence is caused by poor element quality.

May 28, 2013, 13:31
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 Originally Posted by yasir_hayyat Hi all . I am facing same problem. I want to simulate cryogenic boiling in fluid film bearings. I am using k omega sst model , with multi-phase mixture model but some where in the iteration my solution diverges and i get floating point error. i have noted that when i use energy equation it diverges and i have noticed the residua graph of temperature shoots up and i get floating point error. Can any one guide me why this is happening and any possible approach to tackle this problem. Regards Yasir hayat
Reduce the under relaxation parameters.

Best of luck!!!

 May 28, 2013, 22:26 #16 Member   Join Date: Dec 2009 Location: China Posts: 79 Rep Power: 8 Hi, pyroknife More details is needed to solve your problem. Such as Y+ for the all computation domain, the discretization scheme, your PC setting and so on. If it is possible, post the geometry even the case file will be greatly helpful. Regards

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