Turbulence models by definition are additional approximate equations so that the RANS equation set ( 5.10 ) can be mathematically closed. Every turbulence model involves some uncertainty. It can not be an exact representation of the flow physics. Notwithstanding this realization, the computational fluid dynamics (CFD) community does need turbulence models. At the same time, any arbitrary relationship between the secondary and the primary unknowns cannot be accepted by the community as a viable turbulence model. There are, after all, features based on which a turbulence model can be deemed acceptable.