Results are presented from an experimental study into the fine-scale
structure of
generic, Sc≈1, dynamically passive, conserved scalar fields
in turbulent shear flows.
The investigation was based on highly resolved, two-dimensional imaging
of laser
Rayleigh scattering, with measurements obtained in the self-similar far
field of an
axisymmetric coflowing turbulent jet of propane issuing into air at local
outer-scale
Reynolds numbers Reδ≡uδ/v
of 11000 and 14000. The resolution and signal quality
of these measurements allowed direct differentiation of the scalar field
data
ζ(x, t) to
determine the instantaneous scalar energy dissipation rate field
(Re Sc)−1∇ζ·∇ζ(x,
t).
Results show that, as for large-Sc scalars (Buch & Dahm 1996),
the scalar dissipation
rate field consists entirely of strained, laminar, sheet-like diffusion
layers, despite the
fact that at Sc≈1 the scale on which these layers are folded
by vorticity gradients is
comparable to the layer thickness. Good agreement is found between the
measured
internal structure of these layers and the self-similar local solution
of the scalar
transport equation for a spatially uniform but time-varying strain field.
The self-similar
distribution of dissipation layer thicknesses shows that the ratio of maximum
to
minimum thicknesses is only 3 at these conditions. The local dissipation
layer thickness
is related to the local outer scale as
λD/δ
≡ΛRe−3/4δSc−1/2, with the average thickness
found to be 〈Λ〉=11.2, with both the largest
and smallest layer thicknesses following Kolmogorov
Re−3/4δ) scaling.
The effect of the liquid layer thickness on the evaporation intensity