# "du/dy" by "CEL" expression

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 September 14, 2011, 19:44 "du/dy" by "CEL" expression #1 Member   sajad Join Date: Nov 2010 Posts: 51 Rep Power: 8 hi all. i want to calcuate derivative of u velocity with respect to normal cordinate.i mean that i want "du/dy". is it possible to use "CEL" expression to obtain this quantity? best regards.

 September 14, 2011, 23:25 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,452 Rep Power: 104 u.Gradient y

 September 18, 2011, 10:53 #3 Member   sajad Join Date: Nov 2010 Posts: 51 Rep Power: 8 hi Glenn can i use this formula "u.gradiant y" in pre processing? i want to set it as a boundary condition.i mean that my B.C is in dependent of "du/dy". so can i define CEL expression and use it as B.C? best regards.

 September 18, 2011, 18:24 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,452 Rep Power: 104 Can you explain what you are doing? Openings and outlets already set du/dy (normal to the interface) to zero.

 September 19, 2011, 10:07 #5 Member   sajad Join Date: Nov 2010 Posts: 51 Rep Power: 8 sure. i want to set slip velocity condition on walls and the formula is the maxwell-smoluchowski first order slip condition in this form: us-uw=((2-alfa)/alfa)*landa*du/dy so i need to have u gradient with respect of y on walls. is it possible to do this with writing "CEL" expression? thanks.

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