Scalar fluctuation PDF’s and kinetics of turbulent mixing

Experiments are conducted aiming at understanding the kinetics of turbulent mixing and its sensivity to the intrinsic diffusive molecular properties of the scalar being mixed. The concentration fluctuations PDF resulting from the transient turbulent mixing of a scalar blob exhibits a rapidly decreasing exponential shape, sharpening in time. It is parametrised by a mixing time, which depends on the injection time of the blob in the medium, on the Schmidt number Sc and is found to be independent of the Reynolds number. The transient turbulent mixing of scalars is thereby shown to “bypass ” the hierarchy of scales of the Kolmogorov cascade, the reduction process of the transverse scalar sheet thickness peeled off from the blob just after its formation and pertaining initially to the dissipative range of scales involving a strong, but constant stretching rate.