Diffusion layer thickness in turbulent flow

Average thickness of diffusive layers in a turbulent flow is described using
an idea of Lagrangian meso-scale element convected by mean flow and large
scale turbulence. This idea enables a formulation of a simple model for the
diffusive layer thickness assuming that its evolution is determined by the
diffusive growth and two components, compressive normal and tangential, of
the turbulent strain rate tensor. Analysis of the possible effects of the folding
action of the turbulence leads to the conclusion that the folding becomes
significant only at the scales far superior to the considered dimensions of the
meso-scale elements, thus it may be neglected in the present formulation. The
evolution equation for the meso-scale element thickness is derived and put
to test against experiments conducted in plane and round jets. The model
proved capable of producing, using the same values of two model constants,
values of the diffusive layer thickness in good qualitative agreement with the
measurements.
While the present numerical simulations of the turbulent jets are made
using very simple, perhaps simplistic, flow and turbulence description, they
nonetheless allow a fairly accurate estimation of turbulence microscales at
different locations in a jet. It turns out that neither Kolmogorov nor Taylor
scale provides a good universal reference scale for the diffusive layer thickness
and it is local turbulence conditions and history of the meso-scale element
determining the latter.