The Early Stages of Turbulent Micromixing

The aim of this work is to investigate some fundamental aspects of small scales scalar mixing in high Reynolds numbers (Re = u’L/u = 5 × 103 - 1 × 104) turbulence, in the generic situation where the injection scale is small compared to the stirring (integral) scale, L. We observe the first steps of the time and space evolution of a small quantity of a scalar introduced in the far field of a water turbulent jet, where the velocity fluctuations are known to be nearly homogeneous, isotropic and constant. We inject the scalar on the centerline of the jet, 20 diameters downstream the outlet (see Fig.l) throught a small tube, whose diameter d, can be varied (0.04 < d/L < 0.125). In order to investigate the role played by molecular diffusion in the mixing evolution, two different types of scalar are used: disodium fluorescein in water (Pr = 2000) and temperature in water (Pr = 7). The local concentration fluctuations of the scalar are detected by an optical probe (spatial resolution 150µm) for the fluorescein and by a cold film (spatial resolution 250µm) for the temperature. The probe is placed at different distances x, 0 < x < L, from the injection-tube outlet and so, using Taylor hypotesis, we can have informations about the time evolution of the mixing for 0 < t < t(L) = L/u’.