On chirality and turbulent dispersion

Turbulent dispersion in the presence of chiral flow structures is investigated using Lagrangian stochastic models. Attention is focused upon modeled conditional Lagrangian accelerations that have quadratic dependencies upon Lagrangian velocities; a scenario that is consistent with mean accelerations derived from the Navier–Stokes equations and with the results of direct numerical simulations (i.e. with solutions of the Navier–Stokes equations). Chirality endows fluid-particle trajectories with a velocity-dependent preferred sense of rotation and is closely associated with the widely-utilized Eulerian diagnostic, turbulence-helicity. A non-vanishing mean chirality statistic is shown to be associated with oscillating Lagrangian velocity autocorrelation functions, oscillatory growth of dispersion, a significant reduction in dispersion for fixed turbulence kinetic energy and dissipation, vanishing mean angular momentum and the production of bi-modal plume structures. It is shown that two-point mean helicity statistics, characterizing the correlation between the velocity of one particle and the rotation of a second particle, can discriminate between two-particle Lagrangian stochastic models in the well-mixed class. A model is constructed for which this non-physical correlation vanishes.