, simply set to zero.
For slip walls, the viscous shear force at the wall boundary face, , simply set to zero.
For no-slip walls in laminar flow, Eqn. 356 is used to represent the velocity gradient tensor at the face:
| (53) |
where:
| (54) |
and 
is the specified wall tangential motion.
The stress tensor at the face is then given by:
| (55) |
For no-slip walls in turbulent flow, it is assumed that only the component of velocity parallel to the wall is of interest. A linear relationship between the wall shear force and the wall-parallel component of the difference in velocity between the wall and the cell is hypothesized:
| (56) |
Using the definitions of wall shear stress magnitude (
) and reference velocity (
), the coefficient of proportionality is:
| (57) |
The reference velocity 
is computed according to the specific turbulence model. The value of 
is obtained (as a function of 
) from the appropriate wall law.