EXAMINATION OF DIFFUSION MODELING USING ZERO-MEAN-SHEAR TURBULENCE

An examination of diffusion modeling in second-moment turbulence closu res is presented. The main objectives are to gain a better understandi ng of the processes that are grouped into the net diffusive transport of the Reynolds stresses, D-ij, and to assess existing models for D-ij . The benchmark case of zero-mean-shear (ZMS) turbulence is used as a guide. The analysis of ZMS turbulence shows that the pressure-velocity processes in D-ij play a central role in establishing the anisotropy level in diffusive turbulence. Existing models for D-ij based solely o n the triple velocity correlation are shown to be inadequate in the di ffusive limit. The present evaluation of diffusion models using ZMS tu rbulence and subsequent analysis of ZMS turbulence show that the Lumle y (Lumley; J. L., ''Computational Modelling of Turbulent Flows,'' Adva nces in Applied Mechanics, Vol. 18, Academic, New York, 1978, pp. 123- 176) model is the most viable existing diffusion model. A modification to Lumley's model that enables exact predictions in the diffusive lim it is suggested.