of 1e5 in 140 cells?

i am not sure what this means but this was a message fluent gave me when i ran a simulation for drag using the k-e model. reynolds number is about 30 million. if you have ideas please let me know.

Frank

The turbulent viscosity ratio is defined as following:



turbulent viscosity
---------------------
laminar viscosity




FLUENT limits this ratio to a fixed value in order to obtain a stable solution.

Usually this limitation happens when you use a poor quality grid, or you specify wrong boundary conditions for turbulence variables.

Plot a contour of turbulent viscosity ratio. You'll identify where the problem happens. Try to refine the grid there. Also check turbulence boundary conditions.

Hi

ap

thanks i found the section where the turbulent viscosity ratio is high should i refine the grid there?

frank

It happened to me in the beginning of iteration. As the solution get converging, it simply goes away.

Yes. Try to refine the grid.

Also, if you use first order discretization scheme, try to switch to a higher order one.

As Lee said, it's possible that you'll have this warning message during the first iterations. If it will disappear, don't consider it as a problem.

Hi

ap

It happened to me too, but went away after 22 iterations. 22 is very few, but is this something I should worry about?

[Ama]

hi,

do you know how to refine the grids where tubulant viscosity is limited ??

when i run my program it does give me as : "viscosity ratio is limited to viscosity ratio of 100000000 in ........... cells" and also "reversed flow in ... face on outflow ...."

how can i resolve this?

Please i need your help.

Many thanks !!

Ama

[eng_s_sadeghi]

Originally Posted by Ama

hi,

do you know how to refine the grids where tubulant viscosity is limited ??

when i run my program it does give me as : "viscosity ratio is limited to viscosity ratio of 100000000 in ........... cells" and also "reversed flow in ... face on outflow ...."

how can i resolve this?

Please i need your help.

Many thanks !!

Ama

It is better to correct your mesh in gambit, else you can refine it in fluent, refine the places which turbulence is higher than a value.

[RodriguezFatz]

Originally Posted by johnnyb
;108196

It happened to me too, but went away after 22 iterations. 22 is very few, but is this something I should worry about?

No, don't worry. If it vanishes during calculation, it is just due to a bad initial guess. This is not a problem for your final results.

[cfd seeker]

any idea how to set turbulence boundary conditions?

[RodriguezFatz]

Once you chose a turbulence model, new checkboxes and input fields become available in the boundary conditions of your inlets and outlets. Here You can set the turbulence boundary conditions as well.

[shk12345]

I want to know how to and when to use which turbulence models.
Kindly give your comments.
Regards
shk

[RodriguezFatz]

Originally Posted by shk12345

I want to know how to and when to use which turbulence models.
Kindly give your comments.
Regards
shk

Your question is much too open to be answered in a forum. I don't think anybody will have the patience to give you an adequate answer. I strongly recommend to read several books about that.

[shk12345]

Suggest me name of some books

Regards
shk

[RodriguezFatz]

Originally Posted by shk12345

Suggest me name of some books

Regards
shk

All books I have only have small chapters about exactly that topic. But this one is really one of the best CFD books:
"An Introduction to Computational Fluid Dynamics - The Finite Volume Method", H K Versteeg, W Malalasekera
with some pros and cons about important models. (But again, just a small chapter)

I got several other superb books, unfortunately for You: They are on German...

Also this paper:
"Hybrid LES/RANS methods for the simulation of turbulent flows", Jochen Fröhlich, Dominic von Terzi
is brilliant - if You are interested in unsteady turbulence modelling.

What kind of flow are You interested in?

[Marion]

Hi,
On CFD online you have a section called "books" and a subsection "turbulence" http://www.cfd-online.com/Books/brow...?category_id=4
That's where I found the reference to "Turbulence modeling for CFD" - I find it very interesting, even though I havent finished it yet :-)
Marion.

[shk12345]

Originally Posted by Marion

Hi,
On CFD online you have a section called "books" and a subsection "turbulence" http://www.cfd-online.com/Books/brow...?category_id=4
That's where I found the reference to "Turbulence modeling for CFD" - I find it very interesting, even though I havent finished it yet :-)
Marion.

Thanks for the reference.

[D_SH]

Hello, my friends!
I model ispernie water droplets in contact with the flow of highthermal (steam). After several iterations of the message ' turbulent viscosity limited to viscosity ratio ' Solver takes off. The reason?
Eulerian model and turbulence to the k-omega.

[shivakumar]

Can anyone explain me about free stream laminar and free stream turbulent viscosity?

[davidwilcox]

Shivakumar,
In the Navier-Stokes equation, you do not see a turbulent viscosity. It comes about when you do a Reynolds Average on the N.S equations. This is to account for the Reynolds stresses which is a product of the advection terms. The eddy viscosity is also used in the formulation of the gradient diffusion term to account for the triple correlations and pressure diffusion. NOTE: these are modelled terms and not explicit. So coming back to your question, to what is laminar and what is turbulent viscosity, the answer is this laminar viscosity is just the constant of the diffusion term in the N.S equation while turbulent viscosity is the "constant" used in RANS to model the reynolds stresses, etc. Hope this helps