[mahzad]

Hi every one
As you know, there are two kind of forces acting on the cylinder surface,
pressure force and viscous force.
pressure forces act inward normal to the surface ,
but my question is about the direction of viscous forces, I know that they are tangential to the surface but I want to know about the direction? I have this question even about the airfoil case.
Does anybody know the answer to my question? or does anybody know any book or references which can help me with my problem?

Thanks in advance