Zero Wall shear stress at porous interface
 

Hi all,
I am modeling a tube of incompressible flow with porous wall (artery) having the plasma infilterated into the wall. The velocity looks correct to me both inside the artery and wall. However, the wall shear stress is zero on the interface. I assume that it is because of normal infiltration having no gradient near the wall, but in practice we should have wall shear stress, right?
No slip boundary definitely gives you WSS, totally makes sense. I am confused if with infiltration CFX is doing something weird on the interface and I need to calculate WSS with CEL instead of trusting WSS values of CFD-post.