For no-slip wall conditions, FLUENT uses the properties of the flow adjacent to the wall/fluid boundary to predict the shear stress on the fluid at the wall. In laminar flows this calculation simply depends on the velocity gradient at the wall, while in turbulent flows one of the approaches described in Section 12.10 is used.
For specified-shear walls, FLUENT will compute the tangential velocity at the boundary.
If you are modeling inviscid flow with FLUENT, all walls use a slip condition, so they are frictionless and exert no shear stress on the adjacent fluid.
Shear-Stress Calculation in Laminar Flow
In a laminar flow , the wall shear stress is defined by the normal velocity gradient at the wall as
When there is a steep velocity gradient at the wall, you must be sure that the grid is sufficiently fine to accurately resolve the boundary layer. Guidelines for the appropriate placement of the near-wall node in laminar flows are provided in Section 6.2.2.
Shear-Stress Calculation in Turbulent Flows
Wall treatments for turbulent flows are described in Section 12.10.