Effects of anisotropy on the geometry of tracer particle trajectories in turbulent flows

TL;DR

This study investigates how flow anisotropy influences the geometry of tracer particle paths in turbulent flows by analyzing curvature and torsion PDFs, revealing the impact of large-scale motions on trajectory shapes.

Contribution

It introduces the curvature vector to quantify anisotropy effects and links trajectory geometry with flow velocity fluctuations, providing new insights into flow anisotropy impacts.

Findings

Flow anisotropy affects tracer trajectory geometry.

Large-scale motions cause meandering trajectories.

Results align with previous studies for certain flow types.

Abstract

Using curvature and torsion to describe Lagrangian trajectories gives a full description of these as well as an insight into small and large time scales as temporal derivatives up to order 3 are involved. One might expect that the statistics of these properties depend on the geometry of the flow. Therefore, we calculated curvature and torsion probability density functions (PDFs) of experimental Lagrangian trajectories processed using the Shake-the-Box algorithm of turbulent von K\'arm\'an flow, Rayleigh-B\'enard convection and a zero-pressure-gradient turbulent boundary layer over a flat plate. The results for the von K\'arm\'an flow compare well with previous experimental results for the curvature PDF and numerical simulation of homogeneous and isotropic turbulence for the torsion PDF. Results for Rayleigh-B\'enard convection agree with those obtained for K\'arm\'an flow, while results for the logarithmic layer within the boundary layer differ slightly, and we provide a potential explanation. To detect and quantify the effect of anisotropy either resulting from a mean flow or large-scale coherent motions on the geometry or tracer particle trajectories, we introduce the curvature vector. We connect its statistics with those of velocity fluctuations and demonstrate that strong large-scale motion in a given spatial direction results in meandering rather than helical trajectories.