# Physic of the field function

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 June 12, 2012, 09:03 Physic of the field function #1 Member   David L Join Date: May 2012 Posts: 43 Rep Power: 7 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.

 June 12, 2012, 14:18 #2 Member   David L Join Date: May 2012 Posts: 43 Rep Power: 7 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.

 June 13, 2012, 08:52 #3 Member   Ryan Coe Join Date: Jun 2010 Location: Albuquerque, NM Posts: 98 Rep Power: 9 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. __________________ Ryan

 June 14, 2012, 18:59 #4 Member   David L Join Date: May 2012 Posts: 43 Rep Power: 7 thanks so much

 June 15, 2012, 15:58 #5 Senior Member   Join Date: Oct 2009 Location: Germany Posts: 637 Rep Power: 15 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. LexMedina likes this. __________________ We do three types of jobs here: GOOD, FAST AND CHEAP You may choose any two!

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