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Mesh guidelines for K-omega turbulence model

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Old   January 17, 2008, 16:18
Default Mesh guidelines for K-omega turbulence model
  #1
Gautam
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Hello,

I am have a small doubt regarding near wall mesh density when modelling heat transfer using K-omega SST model. FLUENT suugests that if you are not modelling low reynolds number flows (as is my case) the near wall mesh density should be such that the first cell centroid has to be in the buffer layer with y+ ~ 30-300 and it goes on to advice not have the y+ values below the lower bound of 30.

My doubt is what will happen if the y+ value is way less than say even 5. Does FLUENT still go onto use wall functions and neglect the cells till y+=30 or does it affect the whole solution.

It would be really helpful if someone could help me in this regard.

Cheers, Gautam.
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Old   January 19, 2008, 18:44
Default Re: Mesh guidelines for K-omega turbulence model
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Sham
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When using k-w model, if you activate the transitional flow option meaning you are using Low Re hence you should use Enhance Wall Treatment where the Y+ values should be small whereby Y+<5. If you deactivate transitional flow option meaning you are using High Re whereby Standard Wall Function will be used and your 30<Y+<300.

Hope it helps. This is well-explained in the manual.

Sham.
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Old   January 19, 2008, 18:55
Default Re: Mesh guidelines for K-omega turbulence model
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Gautam..
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Thanks for your time Sham. I already mentioned in my question that I am NOT using transitional flows as mine is a high Reynolds number flow so I will using the same guidelines as standard wall function approach. But my doubt is what happens if my y+ is between 5 and 30 as fluent suggests NOT to have between these two limits for standard wall funtions? What will happen if I have a cell in the buffer layer? Will FLUENT continue to use the wall functions discarding the cells in the buffer layer or will it discard the wall functions and use the cells giving me erroneous results.

Cheers, Gautam.
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Old   February 8, 2008, 17:31
Default Re: Mesh guidelines for K-omega turbulence model
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Carlos
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Hi Gautam,

I see your problem. I have a similar problem. I model airflow around vehicles where the Re is high and I have no option but to use standard wall functions. However, when I analyse a grid independent solution, often I get say 5-10% of the surface area of the vehicle with y+ of less than 30.

I don't think Fluent switches the wall treatment in selected zones unless the y+ is below 5 and ideally less than 1, to model turbulence all the way to the wall. If you get cells with y+ of 15-30 then I don't think Fluent will alter the wall treatment and hence the solution shouldn't be much different. Unless you adapt the grid to y+ values then you'll almost ALWAYS get a solution with y+ below 30 in places because y+ depends on the solution.

Hope this helps,

Carlos.
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