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January 20, 2013, 15:54 
"Glitches" in k and omega

#1 
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Join Date: Jan 2011
Posts: 45
Rep Power: 14 
Hi,
I am simulating the flow in a channel using a modified simpleFoam and the komegaSST model. Every now and then, the solver can't solve for k or omega and their values just explode. Sometimes the simulation recovers from that state within a few hundred timesteps, sometimes it doesn't. Here is a sample output for a normal timestep and a problematic one: Code:
Time = 4180 DILUPBiCG: Solving for Ux, Initial residual = 0.002454946797335455, Final residual = 2.118201461652769e08, No Iterations 6 DILUPBiCG: Solving for Uy, Initial residual = 0.001256415853039685, Final residual = 1.037692770489594e08, No Iterations 6 DILUPBiCG: Solving for Uz, Initial residual = 0.003283733002456741, Final residual = 2.981279918575045e09, No Iterations 7 GAMG: Solving for p, Initial residual = 0.02201556239585755, Final residual = 1.282423077370965e07, No Iterations 16 GAMG: Solving for p, Initial residual = 0.008240116175764454, Final residual = 8.139580885669834e08, No Iterations 12 time step continuity errors : sum local = 1.531473863748542e06, global = 7.156743507425181e15, cumulative = 1.45393053629309e12 DILUPBiCG: Solving for omega, Initial residual = 8.626697921486998e06, Final residual = 8.156673087094997e11, No Iterations 5 bounding omega, min: 877.5914029924204 max: 9963507.932995282 average: 104575.2141635721 DILUPBiCG: Solving for k, Initial residual = 0.003874747711704138, Final residual = 5.92104344143439e09, No Iterations 7 bounding k, min: 0.0001332252168376633 max: 0.05713142005592581 average: 0.003205000635194682 S_PRO= 4.802711559863782e06W/K S_PRO,D= 2.028083115231596e06W/K S_PRO,D'= 2.774628444632186e06W/K S_PRO,C= 0W/K S_PRO,C'= 0W/K ExecutionTime = 1411.6 s ClockTime = 1434 s Time = 4181 DILUPBiCG: Solving for Ux, Initial residual = 0.002460388678483852, Final residual = 9.17931475513598e10, No Iterations 7 DILUPBiCG: Solving for Uy, Initial residual = 0.001256478971499746, Final residual = 1.16998685088727e08, No Iterations 6 DILUPBiCG: Solving for Uz, Initial residual = 0.003284937635407826, Final residual = 2.670988173110144e08, No Iterations 5 GAMG: Solving for p, Initial residual = 0.02191503352549878, Final residual = 1.226826542535508e07, No Iterations 16 GAMG: Solving for p, Initial residual = 0.008195653865403671, Final residual = 7.521263032788438e08, No Iterations 12 time step continuity errors : sum local = 1.415562053229773e06, global = 1.775764236376485e15, cumulative = 1.452154772056713e12 DILUPBiCG: Solving for omega, Initial residual = 8.719510075834142e06, Final residual = 4808428634545.903, No Iterations 1001 bounding omega, min: 2.164755144508585e+21 max: 3.598949521217116e+21 average: 1.308210762796994e+17 DILUPBiCG: Solving for k, Initial residual = 0.3147840750606549, Final residual = 1.995451694496316e16, No Iterations 1 bounding k, min: 4.418770186074049e05 max: 0.05208677946321078 average: 0.001778830031969948 S_PRO= 1262457298.893128W/K S_PRO,D= 2.02929169556324e06W/K S_PRO,D'= 1262457298.893126W/K S_PRO,C= 0W/K S_PRO,C'= 0W/K ExecutionTime = 1448.59 s ClockTime = 1471 s My fvSchemes for k and omega: Code:
ddtSchemes { default steadyState; } gradSchemes { default cellLimited Gauss linear 1; } divSchemes { default Gauss linear; div(phi,U) Gauss limitedLinearV 1; div(phi,T) Gauss upwind; } laplacianSchemes { default Gauss linear limited 0.333; } interpolationSchemes { default linear; } snGradSchemes { default limited 0.333; } fluxRequired { default no; p; } Has anyone some advice for me regarding what could be wrong and what I could do about this? Regards Christoph 

January 21, 2013, 14:08 

#2 
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Eric Robertson
Join Date: Jul 2012
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Try dropping your relaxation factor for omega and/or k to something less than 0.1, i.e. 0.05 or so.
I have recently run a kOmegaSST sim on a submarine and would have similar issues with the bounding omega exploding. Dropping the relaxation to 0.05 fixed it for me. 

January 21, 2013, 18:00 

#3 
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Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
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Also, you have your default divScheme set to Gauss linear...which is unbounded. You might want to explicitly set the divScheme for your turbulence parameters to something bounded.


January 22, 2013, 05:31 

#4 
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Join Date: Jan 2011
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Thanks to both of you, I will have to try a number of different things now.
A relaxation factor of 0.1 or even 0.05 sounds really low, but I'll give it a shot if all else fails. I tried a limited scheme: Code:
div(phi,omega) Gauss limitedLinear 0 1e9; I am currently using a smooth solver, which also seems to run without glithces in k and omega but it's slower than before. 

January 22, 2013, 16:08 

#5 
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Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
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I personally have an affinity to the upwind family. For 2.1.x, you could try
Code:
div(phi,omega) Gauss linearUpwind grad(omega); Code:
grad(omega) cellLimited leastSquares 1.0; Code:
cellMDLimited cellLimited faceMDLimited faceLimited 

January 23, 2013, 05:16 

#6 
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I have OF 2.0.1 here, but I can use your suggestion anyway (at least OF didn't complain). These are my fvSchemes now:
Code:
gradSchemes { default cellLimited Gauss linear 1; grad(omega) cellLimited leastSquares 1.0; } divSchemes { default Gauss linear; div(phi,U) Gauss limitedLinearV 1; div(phi,omega) Gauss linearUpwind grad(omega); } 

January 24, 2013, 13:51 

#7 
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Well, I'm not seeing any problems regarding the glitches I described, but as they only show up occasionally I can't be sure that the schemes you suggested actually did the trick.
One problem though: I have cyclic boundaries and it seems that the suggested schemes have amplified oscillations I already had in my solution. 

January 28, 2013, 10:33 

#8 
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Daniel P. Combest
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Location: St. Louis, USA
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you're still using Gauss linear as a default scheme for some of your variables. This linear scheme may be the source of your errors. I would change your default scheme to none, run your solver and then set each one of the schemes manually in your fvSchemes dictionary.


January 29, 2013, 03:57 

#9 
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Are you now referring to the glitches I described in the first post or to the oscillations I mentioned in my latest post? However, I now have all schemes set manually, but I have no experience regarding what might be suitable or not.


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