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y+ and airpak

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Old   November 10, 2005, 12:47
Default y+ and airpak
  #1
Drauss
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iam using a commercial code which is though based on Fluent, doesnt have the post-processing feature where we can compute y+. based on littreture work for indoor ventilation it is recommended to put the first grid at the distance of 10 mm from the wall. However, iam curious as what is the y+ for that location. therefore i need to compute y+ manually to see whats going on; by checking the net i found the following formulae:

y+ = sqrt(t/density)*(first grid distance/dynamic viscosity)

i used k-e model and please can someone clarify the following point. What does t represents in the formulae; how come y+ doesnt take into account the velocity??

many thanks for your time
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Old   November 11, 2005, 02:21
Default Re: y+ and airpak
  #2
D.Pavitran
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Hi,

t = wall shear stress,

sqrt of wall shear stress over density = frictional velocity.

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Old   November 11, 2005, 12:01
Default Re: y+ and airpak
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Drauss
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thanks

any idea how can i determine the wall shear stress; i ofcourse do have the viscosity (mui) but del-y/del-x; i do not have

is there an alternative to compute the y+

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Old   November 11, 2005, 13:14
Default Re: y+ and airpak
  #4
Jonas Larsson
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You can estimate y+ with one of the tools here:

http://www.cfd-online.com/Links/tools.html#yplus
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Old   November 11, 2005, 13:45
Default Re: y+ and airpak
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Drauss
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thank you for the link. my problem is a 8.4-by-7.2 meter room; can i quantify the length scale as 8.4; iam not sure i can use the diffuser's hydraulic diameter as a length scale for this problem

also, i want to ask how can i estimate the time-step for an unsteady simulation, using the referance length scale and referance velocity scale?? many thanks
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