Spallart-Allmaras model is a one equation model for the turbulent viscosity.
The turbulent eddy viscosity is given by
The constants are
According to Spalart it is safer to use the following values for the last two constants:
Modifications to original model
DES (1999) 
Model for compressible flows
There are two approaches to adapting the model for compressible flows. In the first approach the turbulent dynamic viscosity is computed from
where is the local density. The convective terms in the equation for are modified to
where the right hand side (RHS) is the same as in the original model.
Freestream: Ideally , but some solvers can have problem with that so can be used.
Outlet: convective outlet.
- Spalart, P. R. and Allmaras, S. R. (1992), "A One-Equation Turbulence Model for Aerodynamic Flows", AIAA Paper 92-0439.
- Spalart, P. R. and Allmaras, S. R. (1994), "A One-Equation Turbulence Model for Aerodynamic Flows", La Recherche Aerospatiale n 1, 5-21.