Introduction
The general subject of this paper is turbulent mixing,
chemical reactions, and combustion in fully developed shear
layers and jets. More specifically, the purpose is to review the
results of a series of experiments that seems to us to have
important implications for understanding and modeling of these
flows, with possible implications for dealing with tur-bulent
transport in general. A comprehensive review of all im-portant
and useful recent experiments on the subject will not be
undertaken; instead, attention is focused on some ex-periments
that have, in our opinion, revealed new features of the mixing
and combustion processes in turbulent shear flows.
The recognition of the importance of the large-scale organized
structure (e.g., Ref. I) in turbulent shear flows has given new
directions to research in this field. While there is no doubt
that the existence of the large structures has been recognized
for some time (see, for example, Refs. 2 and 3), it was believed
that their main effect on turbulent transport was the convolution
at low wavenumbers of the turbulent/nontur-bulent interface in
the flow, which could be accounted for through the notion of
intermittency. Much of the recent work, both theoretical
and experimental, has had the objective of clarifying the fluid
mechanics. For the purposes of the pres-ent discussion, our
interest is in the implications for turbulent transport and
diffusion of scalars, where the role of the large-scale organized
motions is perhaps clearer. A complete theory would, of course,
deal with the momentum and scalar equa-tions simultaneously, but
since it is likely to be some time before such a theory is
available, it seems worthwhile to see what can be deduced from
the evidence already at hand.