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-   -   What is that meatning?? (http://www.cfd-online.com/Forums/fluent/31746-what-meatning.html)

Neo June 30, 2003 20:53

What is that meatning??
 
Dear user:

when I run "MRF" model, I choose k-epsilon model. After fewer iterates, the control windown show that "turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 1 cells". Could you know what is that meaning ? Or it is just warning message ?

Thanks

neeu July 1, 2003 07:43

Re: What is that meatning??
 
if you search "turbulent viscosity limited" within this forum you will find detail answers

ap July 1, 2003 16:35

Re: What is that meatning??
 
The warning tells you that the ratio of turbulent viscosity and laminar (molecular) viscosity is higher than 10^5, which is the standard limit set in FLUENT.

Usually this warning is due to one or more of the following reasons:

- Poor grid quality. Maybe this is not your case because the problem happens in only 1 cell. Does it get worse proceeding with the calculation?

Plot a contour of the viscosity ratio (you have to define it as a Custom Field Function) to understand where the cell is located. Maybe there you have high velocity gradients. Try to refine the grid in that zone.

- Poor discritization scheme. If you use first order discretization scheme, try with second order or higher.

- Bad turbulence boundary conditions. Check them.

However, remember that the limit is just a way to obtain a stable solution. It may happen, in some cases, that the turbulent viscosity ratio has to be higher than the limit fixed by FLUENT. So, just test what happens if you increase the limit, for example to 10^6 or 10^7 (Solve->Control->Limits). Usually this doesn't solve the problem, but it's worth to try :)

Hi

ap

Neo July 1, 2003 20:20

Re: What is that meatning??
 
Hi ap:

Thanks for your response

I will try your suggestion

Neo

xiang July 2, 2003 00:46

Re: What is that meatning??
 
Dear ap,

I saw the same message in my work. But when I tried to plot the contour of the turb. viscosity ratio, all the values were much less than the limit 1e5. It seems whenever the ration exceeds the limit, fluent immidiately modify some parameters after issuing the warning message. Thus, I couldn't identify where the cells are located. Do you have any idea about this? Thank you very much! yours, Xiang RongBiao

ap July 2, 2003 08:59

Re: What is that meatning??
 
How did you plot the turbulent viscosity ratio?

I think FLUENT just limits the turbulent viscosity so that the turbulent viscosity ratio doesn't exceed its limit.

Hi

ap

xiang July 2, 2003 22:17

Re: What is that meatning??
 
Thanks for your reply. I think I clicked the "compute" button in the contour displaying panel, then i can see the minimum and maximum turbulent viscosity ratio. The max. ration is far below the limiting value in my case. So I don't know how to identify the cells causing the warning message. Did you identify those cells in Fluent or with other package? Thanks again! yours, xiang rongbiao

ap July 3, 2003 11:31

Re: What is that meatning??
 
I identified the affected cells with FLUENT.

I defined a Custom Field Function for the turbulent viscosity ratio (Turbulent visc/Laminar visc.) and plotted a contour plot of it.

Hi

ap

xiang July 3, 2003 21:39

Re: What is that meatning??
 
Thanks, sir.


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