Some new features of the passive scalar mixing in a turbulent flow

We analyze experimentally the statistical properties of a turbulent mixing created in the gap between two counter-rotating disks at a Taylor Reynolds number R-lambda approximate to 400. Local isotropy is investigated fur the inertial and dissipative scales r, using two tests, one applied on C(r), the correlation coefficient between temperature increments and velocity increments, and the other one on S(r), the temperature increment skewness factor. When heating one of the disks and cooling the other one, either positive, negative or almost null values of C and S can be obtained at small scales as a direct result of the presence of several temperature sources. In particular, we emphasize the fact that null or small values for these quantities in the inertial range are an evidence of local isotropy of the temperature field. In these cases, we use the Vaienti et al. equation [Physica D 73, 99 (1994)] for the evolution of the temperature increments probability density functions (PDFs) to predict the inertial and dissipative range PDFs, using an initial PDF, and two measurable closure functions. The intermittent behavior quantified through these statistics is well reproduced by the numerical integration of this evolution equation. (C) 1999 American Institute of Physics. [S1070-6631(99)01703-1].