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Visualize "diverging" cells

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Old   March 26, 2014, 11:58
Default Visualize "diverging" cells
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Hi!

I am running a simulation with star-ccm 7 and the solution does not really converges (but also did not diverges). So i want to see the cells, which are responsable for this. How can i visualize the problem-cells?

Thanks a lot!
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Old   March 26, 2014, 15:03
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hi Flux

I am not sure if you can check the cells that could create convergence issues explicitly in STAR CCM+ explicitly, although you can check cell quality, face validity and cell skeweness in a scalar scene.

another way to visualize 'bad cells' is by right clicking regions, and then compute cell quality. STAR CCM+ will automatically list the bad cells based on the parameters I mentioned in the beginning.

For creating a reasonable quality mesh, you have to ensure the wall y+ lies in the range 30-50 and in extreme case 100. For this you have to run initial simulation with your problem's exact boundary condition and check the wall y+ in a scalar scene. If the values are not in the specified range, recompute the mesh accordingly.
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Old   March 27, 2014, 04:53
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thanks for your feedback, but i already know this things

in sccm+, i can activate "temporary storage retained" at the solver-settings. this gives me the residuals as field functions. But i don't understand the meaning of this residuals, because they are in a range between -1 and 1. Even perfect hex-cells show positive values. So, i need an explanation of this residuals. I can't find anything in the manual.

thx!
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Old   March 27, 2014, 05:42
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Residual is roughly how well your solution satisfy the discrete governing equation.
The one plotted is the RMS and also normalized. There is an explanation on residual in the manual in "Watching Residuals" section.
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Old   March 27, 2014, 07:53
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Thx, but i know what a residual is. It seems the ones i am talking about are some kind of other residuals???
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Old   March 27, 2014, 21:04
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And which residual you are talking about then?
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Old   March 28, 2014, 04:10
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i have activated "temporary storage retained" box at (e.g.) the k-omega turbulence solver. After running one iterating step, i created a new threshold and chose "turbulent kinetic energy residual" as scalar. This gives me values between -1 and 1 (but very close to zero, e.g. -3e-5 and 2e-5).

The residual of the turbulent kinetic energy, in the normal residuals plot, is about 1e-9... so i don't understand the meaning of the residuals of the temporary storage retained, which are displayed in the threshold. It's a bit hard to describe what i mean^^

thanks again!
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Old   March 28, 2014, 04:19
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read the help about how the residual monitors are calculated since this is very important to your understanding of convergence - they are a normalised sum to give you just one value at each iteration

whereas the field functions are the values in each cell
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Old   March 31, 2014, 04:29
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ok, does this mean, the closer the values are at zero the better for the solution?


but why can the residuals of each cell become negative?
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Old   April 1, 2014, 09:06
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the documented equation in the help for residuals shows an absolute sign around it so one would expect it to be positive but this could be an error in the help and without this residuals can be negative when you study the equation

the values seem pretty symmetric about zero from my quick inspection of a couple of cases in a few versions of starccm+

so yes getting residuals close to zero is better
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