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.