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Old   February 8, 2008, 09:14
Default Y star too low
  #1
Dong Fu
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Go a question If my Y star is too low (<30), should I increse the mesh size of the first layer? because FLUENT User Guide says The logarithmic law for mean velocity is known to be valid for 30--300.

(Stander Wall Function, k-e Model)

Thanks in advance

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Old   February 11, 2008, 04:20
Default Re: Y star too low
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Ralf Schmidt
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Hi!

I only know about y plus value, but i thing, that is almost equal to y star in that region. For your settings, y plus should be between 30 and 300 - so, the answer to your question is YES - increase first layer of your mesh.

BUT - as i experienced, k-e, std wall function is not very good (depends of course on the problem).

choose sst k-w with a yplus bewenn 1 and 5! Seperation and reattachement can be displayed much more accuratly.

Ralf
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Old   February 11, 2008, 21:39
Default Re: Y star too low
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Dong Fu
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Deat Ralf Thank you for your answear

I have a case that have a y*=10 in most of the area. but I can not make the first layer mesh bigger because of the geometry. I found this in the FLUENT User Guide

"Although FLUENT employs the linear (laminar) law when y*<11.25, using an excessively fine mesh near the walls should be avoided, because the wall functions cease to be valid in the viscous sublayer"

Is it means I am also ok with a y*=10?

what dose excessively fine mesh mean? how fine is excessive?

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