The Spalart–Allmaras model was designed specifically for aerospace applications involving wall-bounded flows and has been shown to give good results for boundary layers subjected to adverse pressure gradients.
In its original form, the model is effectively a low-Reynolds number model, requiring the viscosity-affected region of the boundary layer to be properly resolved ( y+ ~1 meshes).
In addition, it cannot be relied on to predict the decay of homogeneous, isotropic turbulence.
It solves a transport equation for a viscosity-like variable
The turbulent eddy viscosity is given by The rotation tensor is given by where d is the distance from the closest surface and
is the norm of the difference between the velocity at the trip (usually zero) and that at the field point we are considering.
There are several approaches to adapting the model for compressible flows.
In all cases, the turbulent dynamic viscosity is computed from where
are modified to where the right hand side (RHS) is the same as in the original model.
[citation needed] The third approach involves inserting the density inside some of the derivatives on the LHS and RHS.
The second and third approaches are not recommended by the original authors, but they are found in several solvers.