is given by:
In laminar flow, the stress tensor is given by:
| (67) |
In turbulent flow, the complete stress tensor is given by:
| (68) |
where the turbulent stress tensor 
is also known as the Reynolds stress tensor. Unless a Reynolds stress transport model is being used, the Boussinesq approximation is invoked such that:
| (69) |
where the effective viscosity is 
, the sum of the laminar and turbulent viscosities.