[antonella.longo@ingv.it]

I am trying to compute the drag and lift coefficients for the flow past a fixed cylinder. For this I need to compute the viscous and pressure forces at the wall of the cylinder. How to compute the viscous stress forces at the surface of cylinder? Since at the surface of cylinder the velocities are set to zero does it mean that viscous stresses are zero?

[agd]

The viscous stresses are related to the velocity gradient tensor. The viscous stresses on the surface will be given by the dot product of the surface normal with this tensor. Zero velocity at the wall is not the same thing as zero velocity gradient at the wall. Perhaps the Wiki on this site would be of help?

[FMDenaro]

Originally Posted by antonella.longo@ingv.it

I am trying to compute the drag and lift coefficients for the flow past a fixed cylinder. For this I need to compute the viscous and pressure forces at the wall of the cylinder. How to compute the viscous stress forces at the surface of cylinder? Since at the surface of cylinder the velocities are set to zero does it mean that viscous stresses are zero?

no, not at all ... I don't know if you are working in 2d or 3d, however you have to integrate the deviatoric stress tensor which is 2*mu*Grad v. You can see it involves derivatives along the normal direction to the wall that do not vanish