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May 25, 2013, 14:22 
Divergence Problem

#1 
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I am experiencing some divergence problems with my residuals using Fluent. I am using the komega 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" popup 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 

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misagh
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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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May 26, 2013, 00:26 

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刘峰
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I also wonder if lowering the refactors 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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I remember using incompressibleideal gas, polynomial, and 2 kinetic theory options. 

May 26, 2013, 17:04 

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Lefteris
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komega 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 komega.
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.cfdonline.com/Wiki/SST_komega_model)
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Lefteris Last edited by Aeronautics El. K.; May 28, 2013 at 13:31. 

May 26, 2013, 18:38 

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I am currently using SST komega. 

May 27, 2013, 00:12 

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刘峰
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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 idealgasmixinglaw, then when I ran, divergence problem occured. But when I revised it using massaveraged, 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 
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As you are using komega 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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Also, it appears that only my energy equation is experiencing divergence. 

May 28, 2013, 00:09 

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May 28, 2013, 03:31 

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generally, since you use the komega 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 pv 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 
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Yasir hayat
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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 multiphase 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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May 28, 2013, 13:31 

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Best of luck!!! 

May 28, 2013, 22:26 

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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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