# wall shear stress formulation

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 June 27, 2011, 12:42 wall shear stress formulation #1 New Member   Rakesh Join Date: May 2011 Posts: 21 Rep Power: 8 Dear All! what is the correct equation for finding wall shear stress in X and Y direction in ANSYS FLUENT FOR WRITING UDF? I assume that for X direction wall shear stress=mu*du/dy and for Y DIRECTION =mu*dv/dx .please confirm.

June 28, 2011, 07:51
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Amir
Join Date: May 2009
Location: Montreal, QC
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Quote:
 Originally Posted by rakadit Dear All! what is the correct equation for finding wall shear stress in X and Y direction in ANSYS FLUENT FOR WRITING UDF? I assume that for X direction wall shear stress=mu*du/dy and for Y DIRECTION =mu*dv/dx .please confirm.
Dear Rakesh,
Unfortunately, there is a big bug in computing wall shear stress in FLUENT. To write a UDF for this purpose, use normal vectors of each element to project tensor elements.
PS:
T=2.0*mu*D ; D=0.5*(del(v)+trans(del(v)))

 July 7, 2011, 01:40 UDF help #3 New Member   Rakesh Join Date: May 2011 Posts: 21 Rep Power: 8 Dear Mr. Amir, Thanks for your suggestion.I tried this concept for writing udf for wall shear but I could not implement del(v) and trans( v) functions. Actually in the previous UDF , which you had corrected for wall shear, results are coming but I am not getting boundary layer velocity gradients.I am getting velocity gradients just in the flow direction . Now ,I have written new UDF with suggestions from people on net,where dudy MACRO is replaced by f-u(f,t)-c-u(c,t) etc. concept.I have posted this UDF on this site. I am getting some problem with it. please help to rectify the problem. My aim is to model wall shear in a way that ,instead of no slip condition,non-zero velocity is obtained in the transverse direction(prep. to flow). Thanks for all your help. Rakesh

August 17, 2011, 12:48
Dear Amir
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Ali
Join Date: Aug 2011
Location: Milwaukee
Posts: 34
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Quote:
 Originally Posted by Amir Dear Rakesh, Unfortunately, there is a big bug in computing wall shear stress in FLUENT. To write a UDF for this purpose, use normal vectors of each element to project tensor elements. PS: T=2.0*mu*D ; D=0.5*(del(v)+trans(del(v)))
salam amir jan
agha man ye moshkele kheili bozorg ba hamin ghaziyeye wall normal and shear stress daram mitunid komakam konid plz?

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