to model the Reynolds stress tensor as a function of mean flow quantities. The most common model is known as the Boussinesq approximation:
Eddy viscosity models use the concept of a turbulent viscosity to model the Reynolds stress tensor as a function of mean flow quantities. The most common model is known as the Boussinesq approximation:
| (141) |
where 
is the strain tensor:
| (142) |
and 
is the turbulent kinetic energy.
While some simpler models rely on the concept of mixing length to model the turbulent viscosity in term so mean flow quantities (similar to the Smagorinsky subgrid scale model used in LES), the eddy viscosity models in STAR-CCM+ solve additional transport equations for scalar quantities that enable the turbulent viscosity 
to be derived. These include Spalart-Allmaras, K-Epsilon and K-Omega models.
Since the assumption of Eqn. 141 that the Reynolds stress tensor is linearly proportional to the mean strain rate is known to be flawed, some two-equation models extend the linear approximation to include non-linear constitutive relations.