Non-local dispersion and the reassessment of Richardson's t3-scaling law

TL;DR

This paper challenges the classical Richardson t3-scaling law for turbulent pair dispersion by showing the significant role of non-local effects and large-scale shear layers, proposing a new interpretation of the dispersion regimes.

Contribution

It introduces a non-local dispersion model that accounts for large-scale shear effects and reinterprets the t3-scaling law as a transition regime rather than a universal law.

Findings

Pair dispersion inside shear layers is highly non-local.

t3 scaling is not observed within the inertial range for initial separations.

The proposed model explains the transition from Batchelor to Taylor dispersion regimes.

Abstract

The Richardson scaling law states that the mean square separation of a fluid particle pair grows according to t3 within the inertial range and at intermediate times. The theories predicting this scaling regime assume that the pair separation is within the inertial range and that the dispersion is local, meaning that only eddies at the scale of the separation contribute. These assumptions ignore the structural organization of the turbulent flow into large-scale shear layers, where the intense small-scale motions are bounded by the large-scale energetic motions. Therefore, the large scales contribute to the velocity difference across the small-scale structures. It is shown that, indeed, the pair dispersion inside these layers is highly non-local and approaches Taylor dispersion in a way that is fundamentally different from the Richardson scaling law. Also, the layer's contribution to the overall mean square separation remains significant as the Reynolds number increases. This calls into question the validity of the theoretical assumptions. Moreover, a literature survey reveals that, so far, t3 scaling is not observed for initial separations within the inertial range. We propose that the intermediate pair dispersion regime is a transition region that connects the initial Batchelor- with the final Taylor-dispersion regime. Such a simple interpretation is shown to be consistent with observations, and is able to explain why t3 scaling is found only for one specific initial separation outside the inertial range. Moreover, the model incorporates the observed non-local contribution to the dispersion, because it requires only small-time-scale properties and large-scale properties.