Multi-particle Lagrangian statistics in homogeneous rotating turbulence

TL;DR

This study uses direct numerical simulations to analyze how rotation influences the Lagrangian properties of turbulence, revealing anisotropic dispersion and the alignment of structures with the rotation axis.

Contribution

It provides new insights into the effects of rotation on Lagrangian turbulence, including anisotropic dispersion and the geometric distortion of particle groups.

Findings

Vertical pair dispersion is faster than horizontal.

Rotation increases the flattening of tetrads.

Triads and tetrads align with the rotation axis.

Abstract

Geophysical flows are often turbulent and subject to rotation. This rotation modifies the structure of turbulence and is thereby expected to sensibly affect its Lagrangian properties. Here, we investigate the relative dispersion and geometry of pairs, triads and tetrads in homogeneous rotating turbulence, by using direct numerical simulations at different rotation rates. Pair dispersion is shown to be faster in the vertical direction (along the rotation axis) than in the horizontal one. At long times, in Taylor's regime, this is due to the slower decorrelation of the vertical velocity component as compared to the horizontal one. At short times, in the ballistic regime, this result can be interpreted by considering pairs of different orientations at the release time, and is a signature of the anisotropy of Eulerian second-order functions. Rotation also enhances the distortion of triads and tetrads also present in homogeneous and isotropic turbulence. In particular, at long times, the flattening of tetrads increases with the rotation rate. The maximal dimension of triads and tetrads is shown to be preferentially aligned with the rotation axis, in agreement with our observations for pairs.