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Old   January 28, 2008, 10:50
Default Turbulence viscosity ratio
  #1
Haitham
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Dear Experts

I am running a highly turbulence flow with Turbulence intensity of about 100%.

The simulation is for wind flow around and through a porous structure with hollow interior. The simulation seems to be converging, however, i am getting the message of "Turbulence viscosity is limited to the viscosity ratio of 1x 10e5 in 116 cells".

Would you please advice me on the implication of this? and what do you recommend for me to change in order to eliminate this problem?

Your help is greatly appreciated

regards

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Old   January 28, 2008, 11:26
Default Re: Turbulence viscosity ratio
  #2
Phil
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Search the user manual for the string "Turbulence viscosity is limited"
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Old   January 28, 2008, 11:40
Default Re: Turbulence viscosity ratio
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haitham
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Hi

i did try to search the manual but no luck!
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Old   January 28, 2008, 15:30
Default Re: Turbulence viscosity ratio
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AAA
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Hi

This message is a result of either a poor mesh or a bad property at the BCs. Try adaption first to resolve the meshing problem.

Regards
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Old   January 29, 2008, 07:34
Default Re: Turbulence viscosity ratio
  #5
gotang
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Actually, it is not necessarily either (though it could be). Occasionally, this problem can crop up without obvious fault - I have had it occur to me with very good meshes and robust BCs.

I think the first thing to do is to take a look at where you are getting excessive turbulent viscosity - draw up cut-planes in your grid and plot contours of turbulent viscosity ratio. You will likely find that they are clustered in one region - if it is by the outlet (for example), you may be able to flush it out by altering your outlet BC for a few hundred iterations and then returning it to your original BC.

I would also advise you to check the quality of your mesh, but not by adaption - take it back into your meshing software and look at cell skewness/cell determinants. If these are poor, then remesh. Else, I would not bother.

The other thing to do (if the other things fail) would be to re-initialise and then use artificial but gentle BCs to gradually establish the flow - for example, start with pressure-inlet and pressure-outlets with only a small pressure difference, gradually increase the pressure difference until it is representative of your real BCs and then finally change to whatever BCs you wanted to use.
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Old   February 5, 2008, 05:49
Default Re: Turbulence viscosity ratio
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haitham
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Thank You Gotang

That was very useful, the problem with my input condition where the maximum viscosity ratio was because i used an exagerated wind profile at the beginning when i refined to better suit the field accouterments the problem disappeared
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