MAPPING CLOSURE AND NON-GAUSSIANITY OF THE SCALAR PROBABILITY DENSITY-FUNCTIONS IN ISOTROPIC TURBULENCE

An attempt is made to assess the amplitude mapping closure for turbulent scalar fields. The closure assumption is shown to be in very good agreement with the results obtained from direct numerical simulations (DNS) of passive scalars in isotropic turbulence. The amplitude mapping closure is then employed to investigate the non-Gaussian properties of one-point scalar probability density functions (pdf) in isotropic flows. It is shown that the general relaxation of a scalar pdf to a Gaussian distribution can only be expected when the scalar variance is very small. Some non-Gaussian characteristics imposed by initial and boundary conditions persist throughout the course of scalar evolution.