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Physic of the field function

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Old   June 12, 2012, 09:03
Default Physic of the field function
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David L
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Hi guys

I look for exactly that do a field function of Star ccm+ for calculating WALL Y+, I want to know what? the reason? for using this field fucntion from Star ccm+ ... maybe you could said where I find the definition of the WALL Y+, because I do not find in the help it.
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Old   June 12, 2012, 14:18
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David L
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WALL Y+ is value*u*y/v *where*

u**is the reference velocity,*
y is the normal distance from the centroid to the wall in wall-adjacent cells*
v***is the kinematic viscosity.
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Old   June 13, 2012, 08:52
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Ryan Coe
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If I understand your question correctly (not sure if I do):

In general, y+ is used a measure of distance in which parts of the boundary layer can be generally defined (see http://en.wikipedia.org/wiki/Law_of_the_wall).

The wall y+ field function gives the y+ value at the center of the cell closest to a wall. It is a useful measure of how well you mesh is resolving the boundary layer (lower number = better resolution).

Hopefully this helps.
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Old   June 14, 2012, 18:59
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David L
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thanks so much
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Old   June 15, 2012, 15:58
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I'm sorry Ryan, I have to partly disagree.
A lower number does not automatically mean you better resolve the boundary layer. That's also dependent on the number of layers.
The wall Y+ just let you know if your first cell center is within the viscous layer or in the logarithmic range of the log wall law (or anywhere in between).
A low Y+ doesn't give any advantage when you don't have enough cells to resolve the boundary layer.
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