  We determine the timescales associated with turbulent diffusion and
isotropization in closure models using anisotropically forced and freely
decaying turbulence simulations and to study the applicability of these models.
We compare the results from anisotropically forced three-dimensional numerical
simulations with the predictions of the closure models and obtain the turbulent
timescales mentioned above as functions of the Reynolds number. In a second set
of simulations, turning the forcing off enables us to study the validity of the
closures in freely decaying turbulence. Both types of experiments suggest that
the timescale of turbulent diffusion converges to a constant value at higher
Reynolds numbers. Furthermore, the relative importance of isotropization is
found to be about 2.5 times larger at higher Reynolds numbers than in the more
viscous regime.
