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-   -   Turbulent viscosity limited? (http://www.cfd-online.com/Forums/star-ccm/101340-turbulent-viscosity-limited.html)

Breivik May 1, 2012 19:18

Turbulent viscosity limited?
 
Hi,

I am modeleing a full skale ship in ccm+ and have done som 20000 iterations, but now in the later iterations i get the message: Turbulent viscosity limited in __ cells in region Region 1. Does anyone know why this occurs? If i stop the solver it also stands that the solution is in an inconsistant state.
And I'm also exsperiencing som fluctuating values of the drag coefficient.

Any respons would be much obliged

ryancoe May 2, 2012 08:24

This is a limit that you can set. If your simulation is of a particularly high Reynolds number, it may be necessary to increase the limit. There's some info on this in the help manual.

Breivik May 2, 2012 10:44

Ok
So it's under solver - K-Epsilon Turbulent Viscosity - Maximum Ratio?

The Reynolds number is 727361600

thank you for your answer.

LuckyTran May 2, 2012 13:52

I do not recommend increasing the limit for the viscosity ratio. The default limit is already exceedingly high and difficult to exceed for practical flows. Increasing the tolerance on this limit will increase the likelihood of a runaway (divergence) simulation. So we already know that viscosity ratio cannot be higher than this, why let it?

abdul099 May 2, 2012 14:44

Regarding the inconsistent state after stopping: This is just because you've stopped in the middle of a time step. So when you would postprocess in this state, the result depends on the number of inner iterations after starting the current time step. That's the inconsistency.
To get around this, just hit the "step" button. It will run again for the remaining number of inner iterations and then stop after the current time step.

Oscillations of drag coefficients is nothing unusual. Although many people worry about that, it's mostly nothing to worry about. Quite often that's just due to vortex shedding. When it oscillates around a constant value and the amplitude is not too high, don't worry. Average your drag coefficient and you're fine.

Breivik May 2, 2012 15:26

Thank you very much guys!
I let the simulation go for another 2000 iterations and it seems to oscillate around a fixed value.

Best regards
Breivik

SB123 July 15, 2013 10:19

Quote:

Originally Posted by LuckyTran (Post 358918)
I do not recommend increasing the limit for the viscosity ratio. The default limit is already exceedingly high and difficult to exceed for practical flows. Increasing the tolerance on this limit will increase the likelihood of a runaway (divergence) simulation. So we already know that viscosity ratio cannot be higher than this, why let it?

what to do if getting this message otherwise? just ignore the message and let the program limit the turbulence in ~200 cells in my case or is it a signal of something else wrong with the model?

what about the situation of a moderate Reynolds number 2.3E5?


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