The Spalart-Allmaras turbulence models  solve a single transport equation that determines the turbulent viscosity. This is in contrast to many of the early one-equation models that solve an equation for the transport of turbulent kinetic energy and required an algebraic prescription of a length scale.
The original model was developed primarily for the aerospace industry, and has the advantage of being readily implemented in an unstructured CFD solver, unlike the more traditional aerospace models such as Baldwin-Lomax  and Johnson-King . This has resulted in its popularity increasing as the use of unstructured CFD methods has grown more widespread in the aerospace industry.
The authors of the original Spalart-Allmaras turbulence model presented results for attached boundary layers and flows with mild separation (such as flow past a wing), and it is reasonable to expect that these are the types of flows for which the model will yield the best results. Wilcox  presents free-shear flow spreading rates for the model. While acceptable results are obtained for wake, mixing layer and radial jet flows, the predicted spreading rates for plane and round jets are quite inaccurate. Therefore, Wilcox concludes that the model is not well suited to applications involving jet-like free-shear regions. It is also likely to be less suited to flows involving complex recirculation and body forces (such as buoyancy) than two-equation models such as K-Epsilon and K-Omega or Reynolds Stress Transport.
Three variants of the model are available in STAR-CCM+:
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