“…In these older empirical models, which typically were obtained by simple tensor invariance arguments, the Reynolds stresses were taken to be nonlinear polynomial functions of the mean velocity gradients. In recent years, nonlinear Reynolds stress models of this type have been obtained within the context of two-equation turbulence modeling by more formal expansion techniques incorporating, for example, the Direct Interaction Approximation (DIA) and the Renormalization Group (RNG) (see Yoshizawa 1984, Speziale 1987, Rubinstein and Barton 1990, and Yakhot et al 1992. These models, which are characterized by an explicit relationship between the Reynolds stress tensor and the mean velocity gradients (and possibly their time derivatives) have come to be referred to as "anisotropic eddy viscosity models.…”