COMMENTS ON SOME BASIC EXPERIMENTAL RESUL TS ON MIXING IN TURBULENT FLOWS

The aim of this paper is to discuss some basic experiments on mixing b y turbulent flows, which give a clear understanding of the main mechan isms entering into the mixing process. A very small number of such fun damental physical situations may be found in the literature, from the simplest one concerning mixing in homogeneous turbulence up to more co mplex situations such as those encountered in mixing layers, which are strongly inhomogeneous. These experimental results enable some defini te conclusions to be drawn about basic average mechanisms such as the transfer from large to small scales of the concentration field, and ab out some scaling laws derived from the understanding of these mechanis ms. Nevertheless, the delicate problem of the topology of the finest s cales of the concentration field remains open particularly for liquids where the Schmidt number is high, and, consequently these scales are very small. Only very recent experimental work, using image analysis, gives some answers to this question, which is essential for the modeli ng of molecular diffusion processes. The different theoretical results from turbulence theory needed for the discussion will be reviewed in this article, without any straight forward developments, but with thei r physical meaning.