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Massage of turbulent viscosity limited to viscosity ratio

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Old   December 21, 2015, 15:59
Default Massage of turbulent viscosity limited to viscosity ratio
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When i am running my simulation, the following message is appeared. Why?

"turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 18987 cells 33 3.7137e-02 1.5782e-02 6.7494e-02 8.9973e-03 1.8689e-02 4.5216e-02 -2.9683e+01 1874:07:22 4999967"

Is there any problem?

I change maximum turb viscosity ratio to high values in Solve>control>limit and this above massage doesn't appear. Am I right?


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Old   December 21, 2015, 16:09
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It may or may not be a problem, but the limiter is there for a reason. In by far the majority of simulations, such high turbulent viscosities will not be realistic (maybe you could encounter them in high-density fusion plasmas or so, but they are not present in anything I've encountered). So although changing the limiter removes the message, the question is still, are your turbulence values realistic?

In many cases where I encountered the warning, the simulation was soon to diverge - in some cases the warning went away with time though and was in that respect an artifact of the solution process. So what you can try is give it some time, if the warning disappears with time, it's fine. If not, you will need to find the underlying reason. Which probably means you need some mesh refinement, maybe a smaller timestep, etc.

Good luck!
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Old   December 21, 2015, 17:46
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Ceesh is right. It indicates a non physical solution in existence. However, I would like to add something to that.

From my experience, since you are the one who is managing the solution and it's limits, you may be able to increase and decrease the limits of any parameter to a limit that you would like. Therefore, this will eliminate the error message and thus make you happy. However, this is violating one of the major concepts of cfd which is boundedness of the solution.

A lot of the times, people use the wrong Turbulent viscosity ratios and wrong Turbulent Intensities( too high or too low) to initialize the solution and therefore the resulting simulation is not physical.

For instance, if you are repeating a wind tunnel experiment, the best practice would be to find out, estimate or research about typical Turbulent Viscosity ratios at the inlet, and Turbulent intensities input too( or Length scale etc..) . This will lead to a better solution and can make the solution lie in the physical limits which makes more sense.

Hope this was helpful.


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