A nonsequential turbulent mixing process

We study the relaxation of an initially segregated scalar mixture in a randomly stirred medium, aiming at describing the overall concentration distribution of the mixture, its shape, and its rate of deformation as it evolves toward uniformity. An ever dispersing mixture is realized by releasing a plume of scalar in a large scale, sustained turbulent medium on the axis and in the far field of a turbulent jet. The fluid particles constitutive of the plume are soon resolved into a set of stretched sheets whose rates of diffusive smoothing build up the overall mixture concentration distribution. The randomness of the particle's net elongation at a given instant of time induces a distribution of the mixing time from which molecular diffusion becomes effective in erasing the concentration differences. This ingredient is shown to rule the composition of this dispersing mixture, providing a detailed analytic description of the overall concentration distribution. It compares favorably with experiments using three different passive scalars, suggesting that the mixture composition results from a nonsequential, one-step lengthening process distributed among the sheets. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3319821]