.
The viscous fluxes are written in terms of the stress tensor .
In laminar flow, the stress tensor, 
is given by:
| (49) |
In turbulent flow, the complete stress tensor is given by:
| (50) |
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:
| (51) |
where the effective viscosity is 
, the sum of the laminar and turbulent viscosities.
The heat flux 
is computed as:
| (52) |