Turbulence modeling is the construction and use of a model to predict the effects of turbulence. Averaging is often used to simplify the solution of the governing equations of turbulence, but models are needed to represent scales of the flow that are not resolved.
Closure problem 
Closing the RANS equation requires modeling the Reynold's stress .
Eddy viscosity 
Joseph Boussinesq was the first practitioner of this (i.e. modeling the Reynold's stress), introducing the concept of eddy viscosity. In 1887 Boussinesq proposed relating the turbulence stresses to the mean flow to close the system of equations. Here the Boussinesq hypothesis is applied to model the Reynolds stress term. Note that a new proportionality constant , the turbulence eddy viscosity, has been introduced. Models of this type are known as eddy viscosity models or EVM's.
- Which can be written in shorthand as
- where is the mean rate of strain tensor
- is the turbulence eddy viscosity
- is the turbulence kinetic energy
- and is the Kronecker delta.
In this model, the additional turbulence stresses are given by augmenting the molecular viscosity with an eddy viscosity. This can be a simple constant eddy viscosity (which works well for some free shear flows such as axisymmetric jets, 2-D jets, and mixing layers).
Prandtl's mixing-length concept 
Later, Ludwig Prandtl introduced the additional concept of the mixing length, along with the idea of a boundary layer. For wall-bounded turbulent flows, the eddy viscosity must vary with distance from the wall, hence the addition of the concept of a 'mixing length'. In the simplest wall-bounded flow model, the eddy viscosity is given by the equation:
- is the partial derivative of the streamwise velocity (u) with respect to the wall normal direction (y);
- is the mixing length.
Smagorinsky model for the sub-grid scale eddy viscosity 
Spalart–Allmaras, k–ε and k–ω models 
The Boussinesq hypothesis is employed in the Spalart–Allmaras (S–A), k–ε (k–epsilon), and k–ω (k–omega) models and offers a relatively low cost computation for the turbulence viscosity . The S–A model uses only one additional equation to model turbulence viscosity transport, while the k models use two.
Common models 
The following is a list of commonly employed models in modern engineering applications.
- Ching Jen Chen, Shenq-Yuh Jaw (1998), Fundamentals of turbulence modeling, Taylor & Francis
- John J. Bertin, Jacques Periaux, Josef Ballmann, Advances in Hypersonics: Modeling hypersonic flows