Characteristics of scalar dispersion in turbulent-channel flow

TL;DR

This study investigates scalar dispersion in turbulent channel flow using DNS data, revealing that large-scale structures dominate diffusion and that shear influences the anisotropic dispersion behavior.

Contribution

It provides new insights into the role of large-scale turbulent structures and shear effects on scalar dispersion in wall-bounded turbulence.

Findings

Large scales dominate turbulent diffusion.

Agreement with atmospheric boundary layer measurements.

Shear causes anisotropic dispersion patterns.

Abstract

The dispersion of a passive scalar by wall turbulence, in the limit of infinite Peclet number, is analyzed using frozen velocity fields from the DNS by our group. The Lagrangian trajectories of fluid particles in those fields are integrated and used to compute the first and second-order moments of the distribution of fluid-particle displacements. It is shown that the largest scales in the flow dominate turbulent diffusion, and the computed dispersions are in good agreement with measurements in the atmospheric boundary layer. This agreement can be understood noting that the life times of the large strucutures are much longer than the time scale of the transition from linear to Gaussian particle spreading in the cross-stream plane. Numerical experiments performed computing the Lagrangian trajectories in reference frames moving at different velocities suggest that this transition is controlled by the difference between the mean streamwise velocity and the phase speed of the large-scale structures of the cross-stream velocities. In the streamwise direction, the effect of the mean shear dominates and produces elongated scalar patches, with dispersion exponents which are different from the transverse ones.