Lagrangian History Effect on Turbulent Pair Dispersion

We study experimentally the turbulent pair dispersion, an essential mechanism in mixing and transport of fluids. A new time scale that reflects the longer turn-over time of large scale turbulent eddies arises due to an inherent bias in random sampling of pairs. We reveal that the bias affects the statistics of initial pair separation velocity and masks the intermittent, super-diffusive mechanism of pair separation. Conditionally sampled pairs, with the time scale shorter than a Batchelor time scale, are shown to separate with ⟨Δr2⟩∝τ3, at moderate Reynolds numbers.