Fluid particle dispersion in homogeneous turbulent shear flow

The dispersion of Lagrangian fluid particle pairs in homogeneous turbulent shear flow is studied by a direct numerical simulation using 256×1282 grid points. It is emphasized that anisotropy requires the effects of both magnitude and orientation of the initial separation vector on two-particle statistics to be considered separately, especially at early times when these statistics are primarily determined by the Eulerian structure of the velocity gradient fluctuations. The effects of initial particle positions and pair separation on displacement and dispersion statistics are examined in relation to classical theory. Particle-pair dispersion is found to be most effective in the streamwise direction, and especially so for particle pairs initially separated in the direction of the mean velocity gradient. Similarly, two-particle velocity correlations are anisotropic, being strongest in the streamwise component but weakest in the cross-stream component. The particle-pair separation distance probability density function differs significantly from a Gaussian form used in most dispersion models for air-quality applications. It is hoped that the new results will be helpful for extending two-particle stochastic models to anisotropic turbulent flows.