On the small-scale statistics of turbulent mixing in electrochemical systems

Using the electrochemical method, measurements were made in a plane channel liquid flow, for which the Reynolds number (Re ≡ U ∗ a/ν, U is the mean velocity and a is the hydraulic diameter) is variable between 3000 and 40,000. The Schmidt number of the salt (here, ferri-ferrocyanide) diffusing in water is typically about 1400, consequently the diffusion layer is located inside the viscous sublayer (y+ < 0.5, where superscripts + denote quantities dimensionless with respect to wall variables). The scalar field fluctuations are maintained by a mean concentration gradient, imbedded in a strongly sheared velocity field. The skewness and flatness factors were used to investigate the concentration field in the near-vicinity of the wall (in terms of anisotropy and internal intermittency). Thus, the concentration PDFs are very nearly Gaussian, with a slight dissymmetry that diminishes with increasing Reynolds numbers. On the direction x parallel to the wall, the skewness factor of ∂ c ∂ x is about 1, independently on the Reynolds number. This property is very similar to that of a Pr ∼ 1 passive scalar mixing in a sheared flow, already studied in the literature.